Anti lock Braking Systems - ABS

Stop without skidding, and maintain control of the vehicle. That's the premise of ABS. It was first introduced in the 1980's and has been undergoing constant refinement ever since. The system is typically comprised of 4 ABS rings, 4 sensors, an ABS computer and a number of pressure-management circuits in the brake lines. The ABS rings are attached either to the wheels, or more often, to the brake discs. They look like a notched ring - see the image to the right.

The sensors are magnetic field sensors which are held very close to the ABS rings and can detect the slight change in magnetic field as the teeth on the ring pass them. The pulsing field tells the ABS computer that the wheels are spinning, and how fast they're spinning.
When you brake, the wheel rotation starts to slow down. The ABS computer "listens" to the input from the sensors and can detect if one wheel is slowing down much quicker than the others - the precursor to the wheel locking up. (This all happens in milliseconds, by the way). When the computer detects this condition, a pressure regulator in the brake circuit interrupts the pressure in the brake lines by momentarily reducing it so that the brakes release just enough to give the wheels a chance to keep spinning rather than locking up. The computer then instructs the regulator to re-apply full pressure and again measures the wheel rotation. This on/off/measure cycle happens around 15 to 30 times a second. If the ABS kicks in, you'll feel it through the brake pedal as a vibration because the pulsing in the brake circuit affects all the components.

Newer generation ABS systems

As technology marches on, so does the control / feedback system used in ABS. It used to be the case that any single wheel approaching lockup would cause the ABS system to pulse the brake pressure for all the wheels. With the latest vehicles, the ABS computer is connected to 4 pressure regulators instead of just the one. This means it can selectively apply pulsed braking only to the wheel(s) that need it. So if three of the tyres are gripping well, but the front-left is beginning to skid, the ABS can unlock the front-left brake and pulse it to try to regain grip. It's called three- or four-circuit ABS and it's all very James Bond. When hooked up to the traction control system, this type of multi-circuit ABS can also be used to influence the overall traction of a car in extreme maneuvers, such as helping to prevent rollover and inside-wheel-lifting.

ABS and skid control

So how to talk about the biggest misconception about ABS - that it will make you come to a stop more quickly? This is a prickly subject to talk about. In one camp you have drivers like me who just can't stomach the idea of a computer breaking the physical connection between my right foot and the brake system. Whilst in the other camp you have people who believe that ABS is the best thing since sliced bread. It's these people in the second camp who have the all-out belief that ABS will help you stop faster, and in certain conditions, this is true. On a wet or greasy road surface where the traction is severely reduced, an ABS system can pulse the brakes and prevent lockup much better than a human can. But why? The whole point of brakes is to slow you down. To do that they rely on friction in two places - between the brake pads and the rotors, and between the tyres and road surface. If one of those factors is taken out of the equation, the brakes become useless. The most typical situation is that a driver will panic-react to something and step on the brakes with as much power as they can muster. The brake system amplifies this power, grabs hold of the brake rotors and the wheels stop turning almost instantly. This causes the tyres to now skid across the road surface, and as they do so, they become subject to dynamic attrition. In other words, if a tyre is rotating and gripping the road, the "stick" factor is much higher than if the wheel is locked and skating across the same surface. So that's what ABS does - in an emergency, it ensures that the wheels don't lock up but instead keep spinning so that the tyres maintain grip with the road. (That's where ABS gets its name - Anti-Lock Brakes.) This is where the real benefit of ABS comes into play. If you're going to attempt to avoid an accident, the best thing to do is to try to steer around it. If your tyres are skidding on the road surface, you can point your wheels pretty much wherever you want because the actual direction you end up going will have nothing to do with the wheels and everything to do with the direction you were travelling, combined with the camber of the road. Once the tyres lose grip, all bets are off. With ABS, if those wheels keep turning and the tyres keep gripping, then when you ham-fistedly grab the steering and yank it to one side, the car will still turn and you might be able to avoid the accident. So that's the true essence of ABS - to maintain control over the direction of the car.

So why the negativity, Chris?

My bone of contention with ABS is not so much to do with the technology as the placebo effect is has on drivers. ABS is widely misunderstood and if you ask most drivers, they'll tell you that ABS helps them to stop more quickly, and as I illustrated above, in certain conditions this is true. But even the most well-trained driver is going to be subject to panic in an emergency, and more often than not, will lock their arms on the steering wheel bracing for the coming impact. Once you do this, you're no longer steering so the ABS is trying to give you control over your car but you're not taking advantage of it. Given that this is the most natural human instinct, people accept this as "the way of crashes" but somehow believe that if they have ABS, they'll be able to stop before they get to the point of impact, and that's simply not true. I believe too many people think ABS gives them a license to drive faster, because they mistakenly believe that it will get them out of any situation. It's yet another technical placebo that has been put into vehicles which is making the standard of driving worse. The more gadgets and "driver aids" that get put into a car, the worse the drivers become because they live in a rose-spectacled world where they believe that it's the car's responsibility to get them out of any sticky situation that might arise. It bothers me so much I have a "rant" page dedicated to it here : Nanny Cars.

Political correctness and the push for ABS in every vehicle

It's a widely perpetrated myth that speeding is the cause of most accidents, so it follows that if you can develop a method of helping drivers to bring their vehicles to a stop in a more controlled fashion, you'll help to reduce the number of accidents. Good idea, but it doesn't have a lot of substance to it. If you check my page with studies on the facts vs. the fiction of speeding, you'll see that only 4% of all accidents are caused by loss of control of the vehicle with excessive speed as the primary contributing factor. So ABS wasn't really designed for that - it's difficult to reduce the incidence of the already lowest cause of motoring-related accidents. In truth, distracted drivers (like I mentioned above, driving in their cosetted mobile living rooms), their actual ability to drive properly (training and advanced driver courses) and their ability to have some form of spatial awareness are much bigger factors than speed itself and none of those can be overcome by clever braking systems. Shouldn't we be pushing for more driver training programs to attempt to treat the real cause of the accidents rather than simply putting a bandage on the result?
So what about the emotive issue of pedestrian accidents? What if you, the driver, could stop quicker? It's a staggering fact that 84% of vehicle-pedestrian accidents are actually the pedestrian's fault and in most of those cases, even if you could have stopped on a dime, the accident would not have been prevented. Seriously. Read the the facts vs. the fiction of speeding page - you'll be astonished. I'm not condoning running over pedestrians - that would be stupid. I know first-hand what it's like - I had one of those 84% jog out in front of me using his cellphone when I was riding my motorcycle some years ago. I hit him square in the back despite being hard on the brakes, and threw him a good 10 metres down the road. He survived with some scrapes and bruises but I still think about it to this day. I can't begin to imagine what it would have been like if the stupid bugger had actually died.

ABS in snow and ice, and on gravel

Ah yes. The subject of a good 75% of the emails I get about ABS. The two camps for this argument are split almost exactly 50/50. In one camp, those like me who from experience would rather have their tyres lock up in deep snow to give me at least a fleeting chance of having them dig through the snow to find some road. Those who have anecdotal evidence that ABS is total crap in snow and ice. Whilst in the other camp, those who again believe ABS will somehow magically stop them from crashing in the same conditions. Those who have similar anecdotal evidence disproving all those in the first camp.
ABS by its very nature is designed to stop the wheels from skidding by allowing them to keep turning. On deep packed snow and ice, that's exactly what they're going to do - skid, so ABS effectively removes a considerable amount of your braking in an emergency in these conditions. It's why some cars have ABS disable systems for snow and ice, and it's why ice racers yank the fuse to the ABS system before they even get in a car to race.
The ABS Education Alliance, a group aiming to help educate drivers on how ABS will best benefit them, has this to say on the subject:
Even in fresh snow conditions, you gain the advantages of better steerability and stability with four-wheel ABS than with a conventional system that could result in locked wheels. In exchange for an increased stopping distance, the vehicle will remain stable and maintain full steering since the wheels won't be locked. The gain in stability makes the increase in stopping distances an acceptable compromise for most drivers.
So the short answer to this debate is that ABS is worse in snow and ice for overall stopping distance, but better for controlability.

The hidden gremlin of ABS - what they don't advertise.

If you look at the statistics for crashes, a large percentage of them are "fender benders" - low-speed impacts that only do a little damage and so slow that the vehicle occupants are in no danger; normally about 10mph. I'll give you one guess what the typical "minimum activation speed" is for ABS. That's right. On a lot of vehicles, the ABS is useless much below about 10mph. Seriously. Try it yourself. Find an empty road on a slight downhill grade - even better if its on a dewy morning. Run your ABS-equipped car up to about 10mph and jam on the brakes as hard as you can. The car will skid to a stop and the ABS system will remain totally silent.

Aftermarket ABS systems

To the best of my knowledge, there's no such thing. A few years back a couple of companies tried to market what they called ABS systems that could be retrofitted to any vehicle. The product was a cylinder with a pressure-relief valve in it. The idea was that you inserted this system into the brake circuit somewhere. When you stomped on the brakes - symptomatic of locking up the wheels - the pressure relief valve opened and bled off some brake fluid into the cylinder, thus lowering the braking pressure being sent to the wheels. The idea was to take the "spike" off the initial push of the brake pedal so it wasn't ABS at all. The whole idea of putting something like this into a brake circuit makes me shudder - I wouldn't want to be the person trying to get their insurance and medical claims through after an accident when the investigators found one of these contraptions in their brake line!

A final thought on ABS

Consider this: if you're in an accident and your ABS works perfectly, you'll leave no skidmarks on the road surface. An inspection of the car will show the brakes and ABS system are working perfectly but the absence of skidmarks could lead the police accident scene investigator to believe you didn't brake at all. That in turn could lead to you being the "at fault" driver with all the consequences that involves. Think about it. This exact scenario happens many times every day. Amongst all those ABS-related emails I get, at least one a week is telling me about someone who's had this problem.....

Remember : ABS attempts to ensure that your car stops in the shortest distance possible for most road surfaces. It is not a substitute for you, the driver, paying attention to the road and your driving.

Brake-assist and collision warning systems

Picture credit: Volvo

By 2006, brake-assist and accident warning systems were starting to find their way into consumer cars. I for one just don't like the idea. The manufacturers are reinforcing the misconception that the driver is no longer responsible for their actions. Volvo's collision warning system (CWS), for example, constantly monitors your speed and uses a radar with a 15° forward field of view to determine the distance to any object in front of you. If the distance begins to shrink but you don't slow down, the system sounds a buzzer and flashes a bright red light in a heads-up display to alert you. The brake pads are automatically placed against the discs and when the driver finally does use the brakes, the system monitors the pedal pressure. If the pressure is determined to be too light, the braking power is amplified by the system.
It's a great idea, but the TV commercials for this system need some serious attention. Volvo's commercials actually show a woman driving a Volvo, arranging papers on her passenger seat and talking on a cellphone. When the collision warning system activates and she looks up, bemused, then applies the brakes to avoid running into a truck in front of her - a truck that she would have seen and presumably slowed down for had she been paying attention. I know it's not meant to be taken this way, but that Volvo commercial actually appears to be promoting distracted driving - Volvo will attempt to save you from your own ineptitude because apparently it's just too inconvenient now to be paying attention to the road ahead.
Rather than train drivers to understand that they need to be responsible for their actions, that they need to be alert to their surroundings and that they need to pay attention when they're driving, collision warning systems essentially attempt to treat the symptoms rather than trying to cure the problem itself.
Brake-assist and auto-brakes go one step further. In some high end vehicle now (top end BMWs and Mercedes' for example), the collision-detection system is linked into the brakes like it is with the Volvo system, but it's also been given the flexibility to do all the braking for you. Adaptive cruise control, for example, will control the throttle just like a normal cruise control system, but will also apply the brakes if it determines that you're getting too close to the vehicle in front. Full auto-brakes will actually stop the car for you if you fail to respond. All these systems work in essentially the same way - they monitor the brake use and distance to the vehicle in front. If the computer thinks you're not braking hard enough, it will assist you.
These systems are all very clever but they tread the thin ethical line. Just because engineers can make their vehicles do this doesn't mean they should. Consider this: with in-vehicle monitoring and tracking systems like OnStar, and the impending satellite-tracking systems for road tolling, it's not too hard to imagine all those systems chained together in such a way that the vehicle will literally prevent you from speeding by limiting the throttle availability and controlling the brakes. If you really want to be driven like that in a vehicle over which you have no control at all, take the bus.

Now don't misunderstand me here - I think a lot of what Volvo do in vehicle safety is a good idea - the transparent A-pillars, the blind-spot assist and things like that - they all go towards eliminating problems inherent with the design of cars. But I believe putting systems into a car that attempt to compensate for the ineptitude of the person behind the wheel is a mistake. But that's just my opinion.

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Other Brake Technologies

There are other brake technologies that are becoming available in vehicles now, and a lot of them are gathered together in the 2006 / 2007 BMW models. They're the rolling embodiment of clever brake engineers just showing off. Three of the more notable features are:

• Brake Drying. The X3 has rain-sensing windscreen wipers. When they sense rain, they also send information to the onboard computer. In turn, it goes into a cycle of occasionally bringing the pads into light contact with the brake rotors. This generates enough friction to eliminate any film of water that might be on the surface of the rotors, but not enough that it slows the car down or is even detectable by the driver.
• Brake Stand-by. This is a pre-emptive system that attempts to detect when sharp braking is about to happen. Potentiometers attached the accelerator can detect when the driver takes their foot off it very quickly. That would normally be followed by the brake being applied very quickly. When the onboard computer senses this condition, it moves the brake pads right up to the rotors using the same mechanism that the brake drying system uses. Ultimately, if the driver does jump on the brakes, they're ready to work the millisecond the driver's foot touches the pedal. It may not sound much but that tiny difference in distance moved, translates into a saving in time between putting your foot on the brake and the car actually slowing down. That in turn translates into forward distance - or less of it.
• Brake Fade Compensation. Right up at the top of the page I explained what brake fade was. If the brake rotor temperature begins to rise, this system increases the hydraulic pressure used to press the pads against the rotors without requiring any more pressure on the brake pedal. I'm not sure if this system has a warning light or not, but it should otherwise drivers could end up driving on horribly faded brakes without realising it, and eventually, even the extra hydraulic pressure isn't going to help.

All the above devices fall into that ethical grey area again, but unlike the brake-assist and collision-detection systems outlined earlier, these three brake technologies don't actually attempt to compensate for any wrongdoing on the driver's behalf. They simply help prepare the car for when the driver chooses to use the brakes. From that point of view, I would regard these as better technologies than those which go the whole hog and interfere with your driving.

HYBRID VEHICLES

Why do hybrid vehicles make sense?

 Latest blog entry

09/23/2013 07:00 AM

Finding the right part in a hurry

If you tinker with your own car, you'll know how difficult it can be sometimes to find the right parts quickly. Do you go to the local store? Do you look online? When you're online, how do you know that company A has a better price than company B? Who actually has it in stock? If you go the internet route, it's easy to waste half an hour or more banging through search sites and dealers trying to get to the bottom of that particular can of worms. But I recently came across a site that's a bit more clever than most. It's like an aggregated list of many online vendors. You put in the part you want (from the list of available vehicles and parts) and the site then shows you a whole load of online vendors who have it in stock, with the price. So you can click through and buy the part pretty easily from there. There might be other sites out there that do the same thing but I've not see one before, so this is still a novelty for me. It's American-market only right now. The site has an odd name but I think it's worth checking out - OEMcats.com

You've no doubt heard of hybrid cars by now, most likely the Toyota Prius. More manufacturers are jumping on the hybrid bandwagon, but just what is a hybrid car? Simply put, it's a vehicle that uses a combination of two technologies to drive it. Why does it make sense? Easy - as simple a change as a pure series hybrid configuration can cut the amount of fuel you use by half, and emissions by 90% - something which was proved ages ago - see aXcessaustralia. GM proved the same thing but then actively killed both the EV1 full electric vehicle and the EV1 series hybrid which at the turn of the millenium could break 100 mpg: The EV1 hybrid. I'm not going to get into the bribes and political maneuvering that caused the GM decision - if you're interested, the film Who Killed The Electric Car has more than enough information in it to make you boil with anger.

Helping to solve the mpg problem

The real limitations on gas-mileage in today's cars are:

1. Internal friction in the vehicle. This can be helped by swapping to electric, or at least hybrid electric, systems and drive-by-wire, with motors in the wheels)
2. Friction on the vehicle caused by the ground on the tyres. This can be helped by swapping to low-profile low-rolling resistance tyres, as mandated by law in California, and by reducing the vehicle mass using more carbon fibre, aluminium and plastics)
3. Friction on the vehicle caused by air resistance/drag. This can be helped by altering the vehicle profile to better match a teardop shape or airfoil, as has been done to a certain extent with the Prius and more so with Mercedes' Boxfish concept car). See the mini section on aerodynamics later on for more info on this.

If you do consider the three factors above, then even the clunkers of today could manage over 100 mpg, and minor improvements in the future could easily yield a 200 mpg family car (actually, an affordable 100+mpg family car could have been manufactured by the big companies and on sale at a reasonable cost back in 2002, with 150-200 mpg cars hitting the market by 2009). So we know what the problems are, and things are moving ahead albeit slowly. Here then is some information on the heart and soul of a hybrid car : the engine.

Hybrid Engines

The most common hybrid cars are petrol-electric, like the Prius. Petrol-electric hybrid cars use a normal petrol engine, just like you'd find in any other car, but in addition, there are one or two high-torque electric motor-generators. The motor-generator(s) draw power from a bunch of car batteries stored either in the floorpan of the car (for a low centre of gravity) or in the rear (for convenience). With power supplied to the motor-generator, it behaves like an electric motor. When no power is supplied but the shaft is turning, it becomes a generator to create power. In this mode, you get regenerative braking, where the energy required to slow the vehicle down is all taken up in the motor-generator to re-charge the battery packs. Both the petrol engine and the motor-generator(s) are connected to an onboard computer system which has been programmed by men in white coats to work as efficiently as possible. There are three mainstream technologies in the hybrid market at the time of writing, each championed by a different company or group of companies. Note: the diagrams below all show rear wheel drive for ease of explanation, but hybrid drives can be any of the standard drivetrains from front wheel only to 4-wheel drive.

IMA - integrated motor assist (Honda)

The motor-generator (electric motor and regenerative generator) is in-line with the petrol engine, typically built into the bell-housing in front of the gearbox. The motor-generator is used to assist the petrol engine, thus reducing the load on it and allowing it to be smaller than it would otherwise be for a vehicle of the same weight. For example the Civic hybrid uses a 1.3l engine where the non-hybrid uses a 1.8l engine. The motor-generator cannot turn without turning the petrol engine too. First-generation systems didn't have enough power to be able to run the car on electric alone. Current generation ones do through higher powered motors and the ability to shutoff the petrol engine when coasting. Because the motor-generator is in-line, the regenerative braking works very simply - as you start to brake, the motor becomes the generator. Conversely it is also used as the primary starter motor for spinning the petrol engine up quickly after it has been turned off, for example at traffic lights. There is also a backup 'regular' starter motor for cold-starts and emergencies. Of the three mainstream hybrid technologies, IMA is by far the simplest to implement, maintain and repair. In the following images, red is the battery pack, green is motor-generator 1 and blue or purple is motor-generator 2.

Hybrid Synergy Drive (Toyota)

Toyota's take on hybrid drive has a pair of motor-generators, one in-line like the Honda IMA design, one not. The key to its success is the compound planetary gearset in the transmission. In the Toyota system, the petrol engine and one motor-generator are connected to one of the inputs, the second motor-generator to the second input and the wheels to the third. Through a clever use of electronics, the planetary gearbox can be locked and unlocked in various configurations dependent on what is required. For example under modest acceleration, the petrol engine drives the planetary gearbox as well as the first motor-generator. The output from that is fed to the second motor-generator along with the output from the gearbox to drive the wheels. In pure electric mode, the first motor-generator freewheels, the petrol engine is turned off and all the electric power is fed to the second motor-generator. Under regenerative braking the second motor-generator becomes the generator as it does in the IMA system above. The difference is that if the battery pack is full, the energy derived from the second motor-generator is redirected to the first motor-generator which in turn uses it to induce drag in the petrol engine to slow the vehicle down. As a result, the actual brakes in a Toyota Hybrid car do not wear very quickly at all because most of the braking is provided by the motor-generators. Only in severe cases do the brake pads actually engage the brake rotors. This is all made possible by the central engine computer and throttle-by-wire / brake-by-wire system.

Dual-mode or 2-mode Hybrid (GM).

The third hybrid system comes from GM and has two operating modes as oppose to the single mode of IMA or HSD. It again uses two motor-generators. In first and second gears, the first motor-generator sends power to the second motor-generator, and that coupled with the petrol engine provide the power to the wheels. In higher gears or under heavier loads, the petrol engine always runs (as oppose to the IMA and HSD systems where it can be turned off or have cylinders deactivated). The difference is in how the motor-generators work in cooperation with it. As speed increases, the first motor-generator gets to the point where it's providing no useable input to the drivetrain. At this point it begins to freewheel and the second motor-generator begins to act as a generator. As speed increases further, the first motor-generator begins to act as a generator again and at this point its power is once again fed to the second motor-generator which now becomes a motor. Coupled with variable intake timing, direct common-rail injection and a host of other technologies, these all come together to give GM's take on hybrid technology.

Most hybrids have an energy display screen mounted either in the instrument cluster or in the centre console. This is a small LCD which gives you, the driver, information about what mode you're driving in, and where the power is going. Again, the most recognisable and famous of these displays to date is that from the Toyota Prius (see right). The only real problem with these displays is the fascination they provide to the novice hybrid driver. Watching the animations spin around and the energy arrows scroll here and there as you drive is certainly informative but not really conducive to safe driving. One benefit however is the constantly-updated gas-mileage chart. Many Prius owners report that this spurs them to attempt to get videogame-like high scores in their cars, driving them in such a fashion as to get the highest recorded mpg from their cars. If nothing else, the energy display affects most drivers in terms of educating them as to how their driving style directly impacts their gas-mileage.

The battery question

At the time of writing, the estimated lifespan for the batteries in a hybrid car is about seven years. The cost of doing this for the Toyota hybrids is about US$10,000 which is a sizeable percentage of the cost of the entire car. The original theory was that you would have driven enough distance to recoup the extra cost via fuel savings but with the price of petrol where it is now, that is becoming harder and harder to achieve. So far there hasn't been a large recall for batteries for any of the hybrid manufacturers and I've not yet heard of anyone kicking up a stink about the cost. That means one of three things. (1) The batteries are lasting longer than expected, so people haven't had to swap them out yet. (2) They're paying the money but nobody has complained in the press. (3) The manufacturers are doing it free for good publicity.

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Plug-in hybrids

It's been said that the reason the all-electric car failed in America is because if people forgot to plug it in overnight, they couldn't drive it the next day. The real reason had more to do with the politics of Big Oil, the California clean air act and GM's unwillingness to promote electric vehicles. Regular petrol-electric hybrids are an excellent choice for people wanting to be more frugal in their gas mileage, but the all-electric mode will only run for a couple of miles before the battery pack is completely drained. In fact, in the US, the Prius has been hobbled by the removal of the all-electric mode completely at the behest of Big Oil. The ideal solution to the pure-electric problem, and the petrol-electric problem is to have a plug-in hybrid. Essentially the idea is very simple. You drive the car as you would normally but you plug it in overnight. And extra set of deep-cycle marine batteries is charged up and can be used to drive in pure electric mode the following day. If the batteries run down, the car reverts to the behaviour of a normal petrol-electric hybrid. If you forget to plug it in overnight, again it behaves like a normal petrol-electric hybrid. In other words, if you choose to plug it in overnight, you buy yourself 30 or 40 miles of driving without using a single drop of petrol. If you forget, no biggie - you can still drive.
Famously, CalCars have converted a regular Prius to be a 100mpg+ vehicle with their plug-in conversion. Now to be fair, their vehicle doesn't actually do 100 miles by burning a single gallon of gas - that's a bit misleading. Using the gas-mileage figure is a convenient way of telling the consumer how their vehicle compares to other vehicles in a unit of measure that people understand. So how is this possible? Well the average commuter typically doesn't drive more than 30 miles a day. With the plug-in conversion, that entire distance is covered on pure electric mode, with the petrol engine only kicking in on a low charge or when it's needed for a burst of acceleration. Because the petrol engine is used so rarely, by the time you fill up, you can easily have covered more than 100 miles and only used a single gallon of petrol because most of that mileage was actually done in pure electric mode. CalCars will turn any hybrid into a plugin for you, for a price.

Diesel-electric hybrids

In 2011 Volvo launched the first diesel-electric hybrid - the V60 plugin - although it works just like a current-generation petrol-electric hybrid. Nobody yet manufactures a true diesel-electric hybrid - one that works just like a diesel-electric train. The small diesel engine is directly connected to a generator. The generator produces electricity on-demand, which is fed to wheel motors (electric motors built into the wheels) to drive the vehicle forwards. Volvo got close to this idea in 2007 with their ReCharge concept car, but never turned it into a full production vehicle. The benefits of this type of hybrid are obvious - no transmission, no driveshafts - a huge reduction in weight and complexity. In fact, these vehicles, if built, could almost be considered full EVs - fully electric vehicles. See the EV Bible for more information.

Picture credit: Volvo

The cost of hybrids

Because hybrid engine technology is still relatively new, it cost you more to buy a hybrid car than the equivalent petrol-engined car. Some countries, cities and states have incentives to do this, like energy grants, or paying the price difference. Ultimately, if you're willing to write-off the initial extra cost, owning a hybrid is definitely cheaper. If you include the extra cost up front and factor it across the lifetime of the vehicle, you'd need to own a hybrid for about 7 years covering about 15,000 miles a year to break even, given the rising cost of petrol compared to the mpg savings of operating the car. If you choose to go the plug-in hybrid route, you'll be paying even more for a company like CalCars to convert your car for you, but again, over the lifetime of ownership, you can probably recoup the cost within 5 years.

Renting hybrids

In July 2007, Hertz started to offer hybrids as an option for rental. Some Hertz locations allow you to specify the exact vehicle you want when you rent. There is of course a price premium, but for example if you were to rent from Hertz in England, because the cost of petrol over there is so prohibitively expensive, renting a hybrid will save you money as soon as you go over the 250 mile mark. Up to 250 miles, it's cheaper to rent a regular compact vehicle and fill it with petrol. Over 250 miles, the extra cost of the hybrid is negated by the fuel-saving and you're on your way to a cheaper overall rental.

First responder safety and hybrids

With the amount of electrical energy stored in Hybrid vehicles now, first responders need to be aware of the differences when they're attending crashes involving hybrids. The biggest safety concern is the high voltage battery pack and the high voltage lines that run along the underside of the car. If a rescue worker cuts through these by mistake, they'll be electrocuted, and that would be A Bad Thing. Most manufacturers have high voltage cutoffs on their battery supplies now, and all appear to have adopted a common colouring scheme for their wiring. Blue is intermediate voltage and orange is high voltage. The location and operation of the various systems differs from vehicle to vehicle and because of this, there are companies springing up that specialise in providing information and training courses specifically to address these issues. If you're interested in reading more, or you're a first responder who might be asked to deal with wrecked hybrids, Hybrid Hazards is one such company.

Are Hybrids really "green"?

This question gets asked a lot. As you drive the vehicle down the road, the answer is absolutely "yes" - a hybrid vehicle is far more green. But it's not just the car and your everyday driving that you have to consider. If you have a plug-in hybrid for example, the initial reaction is normally "of course it's green - I'm using electricity and that produces no exhaust". This is only true when you consider the vehicle in isolation. When you take into account the big picture, at some point a power station generated the electricity that was used to charge your plug-in hybrid. The power station might or might not have been clean - it could have been a coal-burning dinosaur, or it could have been a solar farm out in the desert. The assumption is that the chance of being able to control and regulate the pollution from a single powerplant is far greater than the chances of everyone keeping their emissions systems 100% in check.
What about the batteries themselves? They cost a lot to manufacture both in monetary terms and in terms of consumable items. Worse still, if they're not disposed of properly, they pollute landfills with toxic waste. That's not very green. Again some assumptions have to be made though - mass producing the batteries brings the cost down, whilst proper recycling of them ensures they don't pollute at the end of their useful life.
Taking it one step further - what about the plastics used in every car - not just hybrids. Are they recycled plastics? Can they themselves be recycled? Until the late 90's, most car plastics couldn't be recycled and would end up in landfills. Plastic isn't know for it's biodegradability.
So it's difficult to give a straight answer to the question of whether or not hybrids are really green. I think the best answer is that taken as a whole, hybrids are greener than most vehicles built to-date, but there's still a lot more that can be done.

It's not all about the powerplant - aerodynamics are important too

There's a good reason why the Prius looks the way it does. Toyota didn't deliberately design a nerd-mobile. They deliberately designed something which was aerodynamically clean. The unfortunate side-effect is that compared to more traditional car design, "aerodynamically clean" = "looks nerdy". So what are the key factors affecting how a car moves through the air and why is this important? Well the same air that you and I breathe is pretty thick stuff. You'd never know it to breathe it, but in the right situation, it makes for supersonic aircraft that grow in length and films about Apollo mishaps that have cliffhanger endings. Compared to a vacuum, air is very thick indeed and so anytime you push something through it, it generates drag. Drag is easy to understand - stick your arm out of the window next time you're driving. With the palm of your hand facing oncoming traffic, the aerodynamic drag will try to snap your arm off once you get to speed. But twist your hand so it's palm-down and suddenly it's much easier to keep your arm in place. The cross section of the side of your hand presents a much smaller area to the oncoming air which means less drag. Supposing you could now drive faster and faster, you'd eventually feel heat buildup on your hand because of the drag, and if you could go fast enough, you'd end up with a bloody stump which, because of the heat, would also be neatly cauterised at the same time. Best not try that then.

Reducing aerodynamic drag

Aerodynamic drag is an odd thing in that it's not a linear function. In other words, the amount of drag on an object at 40mph is not double the drag experienced on the same object at 20mph. Because we're not writing a physics paper here, it's simplest to think of frictional drag as being roughly proportional to the square of the velocity:

where p = air density, A is the cross sectional area, C is the coefficient of drag and v is the speed.
This means that you get more drag with:

• denser air
• larger cross section
• higher coefficient of drag
• more speed

Well there's not a car designer can do about air density so that part of the equation is entirely down to mother nature. The cross section is something that can be altered though - making a car have a smaller frontal area means it induces less drag. It's why the hand experiment I talked about above works like it does - when you turn your hand palm-down, you're massively reducing 'A' in the equation above, so you get less drag. Coefficient of drag is a complicated topic, but suffice to say it can be affected by anything from the slope of the radiator and windscreen to the size of the door handles to wheel wells to aerials. See the sections on reducing the coefficient of drag below. The only thing you, the driver, can control is v - your speed. The faster you go, the more drag on your vehicle. The more drag, the more energy the engine has to spend to push you through the air. Slow down and you get better fuel economy.
So the key to all this is reducing frictional drag - anything that can be done to reduce this value will naturally increase the fuel economy of a vehicle because the engine will expend less energy (and thus burn less fuel) to move the car through the air. And this is why the Prius looks like it does - because the designers went for a smaller frontal cross section combined with a lower coefficient of drag. So the best way to think of this is the Drag Area of a vehicle:

Drag Area

The drag area of a vehicle is determined by multiplying its cross sectional area by its coefficient of drag. The table below shows a couple of examples that most people will be familiar with - the Toyota Prius (second generation) and the Hummer H2. (figures from Rüdiger Cordes' Opel GT page)

	Prius	Hummer
Drag coefficient	0.26	0.57
Frontal area	2.23m²	4.29m²
Drag Area	0.579	2.44

If you learn nothing else here, then understand that table above and you'll understand why vehicles perform how they do. The Hummer has a much larger frontal area, because of it's size, and a much larger coefficient of drag, because of (amongst other things) the vertical radiator grille and near-vertical windscreen. Combined, it means that for the same size of engine, a Hummer expends over four times the amount of energy to move it through the air.

Reducing the coefficient of drag

Picture credits: www.hondauk-media.co.uk, evworld.com, Citroën

There are all sorts of things that can be done to a vehicle to reduce the coefficient of drag. If you're into cars, most of you will recognise the 1999 Honda Insight's faired-in rear wheels. They looked odd but were designed like that for a reason. By being faired-in, the rear wheel arch was removed so air was no longer forced to pass a spinning wheel in a gaping hole in the bodywork. That caused turbulence, which caused drag, which increased the drag coefficient. By making smooth body panels that covered the wheels, Honda created a more streamlined shape, less likely to cause turbulence. It's worth pointing out that Honda didn't get there first though. GM re-introduced this particular styling cue in 1996 on their fully electric EV1. I say 're-introduced' because in 1955, Citroën got there a full 44 years earlier with their DS model.
Anything that creates turbulence will induce drag, and so increase the coefficient of drag. For example removing the rear view mirrors does two things - it removes two items that poke out from the side of the car which create turbulence, as well as reducing the frontal cross section. Both of these affect the overall aerodynamic drag formula above, but it's not really wise for designers to simply remove rear view mirrors. Instead, they make smaller, more streamlined mirrors designed to present less frontal cross section, and importantly, to create less drag.
Another styling and design cue mimicked from the DS is the overall shape of the car - wider at the front and tapering to a narrower rear, both in side view and plan view. Why is this? Well it's because that is how car designers begin to approach the most aerodynamic shape there is.

What's the most aerodynamic shape?

If you think of air as a fluid, then think of what nature created to best cut through water, it won't surprise you to know that the most aerodynamic shape looks like a fish or a teardrop - the shape water naturally forms when it falls through air. A wide, circular front moves the air aside, compressing it with minimal fuss, whilst a long sculpted tail allows the air to expand again in the low pressure area behind the shape without creating turbulence. If you've ever been to Speed Week on the Bonneville Salt Flats, you'll have seen drop-tank or belly-tank racers. These guys take old world war 2 era aircraft drop tanks and build cars out of them. Those old drop tanks were designed to be added to aircraft to increase range (by carrying more fuel) without a massive increase in drag (which would have negated the whole excercise). They're the perfect aerodynamic shape. So vehicles like the EV1, the Citroën DS and the Honda Insight all attempt to get to a usable version of this shape as best they can.

Picture credits: www.so-calspeedshop.com

Did Citroën really get there first?

For the history buffs, here's a nugget of trivia : Romanian engineer Aurel Perşu actually came to the conclusion that the teardrop was the best shape in 1922 and built a vehicle to prove it. He patented the shape in 1924 and both Ford and GM expressed an interest in buying the patent at the time, but since they didn't want to commit to also building the car, Perşu refused their offers. That vehicle had a drag coefficient of 0.22 - lower than most vehicles today - and it's still fully functional and on display at the Romanian National Technical Museum in Bucharest. Since 2006, the museum has awarded an annual Aurel Perşu Aerodymanic Automobile prize to the car manufacturer that produced a vehicle in the previous model year with a drag coefficient lower than 0.3. In 2006 the Mercedes S-Class took the prize and in 2007 the prize was given to Toyota Corolla.

What the heck was that all about?

You might get to this point and wonder why there's a whole bunch of guff about aerodynamics on a page dedicated to hybrid vehicles. Simple - whilst the hybrid drivetrain is important, other factors play a role too. One of the more important ones is the shape of the car. The more a designer can do to make a car cut through the air more cleanly, the less engine capacity required to do so, or the more efficient a given size of engine can become. It's pointless designing a Hybrid Hummer or SUV (even though GM have already done so) without taking other factors in to consideration. Without a change to the aerodynamic shape, drag-inducing appendages and excess weight, a hybrid SUV is about as effective as a fart in a hurricane.

TRANSMISSION SYSTEM OF CAR

Transmission, or gearbox?

 Latest blog entry

09/23/2013 07:00 AM

Finding the right part in a hurry

If you tinker with your own car, you'll know how difficult it can be sometimes to find the right parts quickly. Do you go to the local store? Do you look online? When you're online, how do you know that company A has a better price than company B? Who actually has it in stock? If you go the internet route, it's easy to waste half an hour or more banging through search sites and dealers trying to get to the bottom of that particular can of worms. But I recently came across a site that's a bit more clever than most. It's like an aggregated list of many online vendors. You put in the part you want (from the list of available vehicles and parts) and the site then shows you a whole load of online vendors who have it in stock, with the price. So you can click through and buy the part pretty easily from there. There might be other sites out there that do the same thing but I've not see one before, so this is still a novelty for me. It's American-market only right now. The site has an odd name but I think it's worth checking out - OEMcats.com

That question depends on which side of the Atlantic you're on. To the Europeans, it's a gearbox. To the Americans, it's a transmission. Although to be truthful, the transmission is the entire assembly that sits behind the flywheel and clutch - the gearbox is really a subset of the transmission if you want to split hairs.
Either way, this page aims to deal with the whole idea of getting the power from your engine to the ground in order to move your car (or bike) forwards.

Manual gearboxes - what, why and how?

From the Fuel & Engine Bible you know that the pistons drive the main crank in your engine so that it spins. Idling, it spins around 900rpm. At speed it can be anything up to 7,500rpm. You can't simply connect a set of wheels to the end of the crank because the speed is too high and too variable, and you'd need to stall the engine every time you wanted to stand still. Instead you need to reduce the revolutions of the crank down to a usable value. This is known as gearing down - the mechanical process of using interlocking gears to reduce the number of revolutions of something that is spinning.

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A quick primer on how gears work

In this case I'm talking about gears meaning 'toothed wheel' as oppose to gears as in 'my car has 5 gears'. A gear (or cog, or sprocket) in its most basic form is a flat circular object that has teeth cut into the edge of it. The most basic type of gear is called a spur gear, and it has straight-cut teeth, where the angle of the teeth is parallel to the axis of the gear. Wider gears and those that are cut for smoother meshing are often cut with the teeth at an angle, and these are called helical gears. Because of the angle of cut, helical gear teeth have a much more gradual engagement with each other, and as such they operate a lot more smoothly and quietly than spur gears. Gearboxes for cars and motorbikes almost always use helical gears because of this. A side effect of helical gears is that if the teeth are cut at the correct angle - 45 degrees - a pair of gears can be meshed together perpendicular to each other. This is a useful method of changing the direction of movement or thrust in a mechanical system. Another method would be to use bevel gears.

The number of teeth cut into the edge of a gear determines its scalar relative to other gears in a mechanical system. For example, if you mesh together a 20-tooth gear and a 10-tooth gear, then drive the 20-tooth gear for one rotation, it will cause the 10-tooth gear to turn twice. Gear ratios are calculated by divinding the number of teeth on the output gear by the number of teeth on the input gear. So the gear ratio here is output/input, 10/20 = 1/2 = 1:2. Gear ratios are often simplified to represent the number of times the output gear has to turn once. In this example, 1:2 is 0.5:1 - "point five to one". Meaning the input gear has to spin half a revolution to drive the output gear once. This is known as gearing up.

Gearing down is exactly the same only the input gear is now the one with the least number of teeth. In this case, driving the 10-tooth gear as the input gear gives us output/input of 20/10 = 2/1 = 2:1 - "two to one". Meaning the input gear has to spin twice to drive the output gear once.

By meshing many gears together of different sizes, you can create a mechanical system to gear up or gear down the number of rotations very quickly. As a final example, imagine an input gear with 10 teeth, a secondary gear with 20 teeth and a final gear with 30 teeth. From the input gear to the secondary gear, the ratio is 20/10 = 2:1. From the second gear to the final gear, the ratio is 30/20 = 1.5:1. The total gear ratio for this system is (2 * 1.5):1, or 3:1. ie. to turn the output gear once, the input gear has to turn three times.
This also neatly shows how you can do the calculation and miss the middle gear ratios - ultimately you need the ratio of input to output. In this example, the final output is 30 and the original input is 10. 30/10 = 3/1 = 3:1.

Collections of helical gears in a gearbox are what give the gearing down of the speed of the engine crank to the final speed of the output shaft from the gearbox. The table below shows some example gear ratios for a 5-speed manual gearbox (in this case a Subaru Impreza).

Gear	Ratio	RPM of gearbox output shaft when the engine is at 3000rpm
1st	3.166:1	947
2nd	1.882:1	1594
3rd	1.296:1	2314
4th	0.972:1	3086
5th	0.738:1	4065

Final drive - calculating speed from gearbox ratios. It's important to note that in almost all vehicles there is also a final reduction gear. This is also called a final drive or a rear- or front-axle gear reduction and it's done in the differential with a small pinion gear and a large ring gear (see the section on differentials lower down the page). In the Subaru example above, it is 4.444:1. This is the final reduction from the output shaft of the gearbox to the driveshafts coming out of the differential to the wheels. So using the example above, in 5th gear, at 3000rpm, the gearbox output shaft spins at 4065rpm. This goes through a 4.444:1 reduction in the differential to give a wheel driveshaft rotation of 914rpm. For a Subaru, assume a wheel and tyre combo of 205/55R16 giving a circumference of 1.985m or 6.512ft (see The Wheel & Tyre Bible). Each minute, the wheel spins 914 times meaning it moves the car (914 x 6.512ft) = 5951ft along the ground, or 1.127 miles. In an hour, that's (60minutes x 1.127miles) = 67.62. In other words, knowing the gearbox ratios and tyre sizes, you can figure out that at 3000rpm, this car will be doing 67mph in 5th gear.

Making those gears work together to make a gearbox

If you look at the image here you'll see a the internals of a generic gearbox. You can see the helical gears meshing with each other. The lower shaft in this image is called the layshaft - it's the one connected to the clutch - the one driven directly by the engine. The output shaft is the upper shaft in this image. To the uneducated eye, this looks like a mechanical nightmare. Once you get done with this section, you'll be able to look at this image and say with some authority, "Ah yes, that's a 5-speed gearbox".

So how can you tell? Well look at the output shaft. You can see 5 helical gears and 3 sets of selector forks. At the most basic level, that tells you this is a 5-speed box (note that my example has no reverse gear) But how does it work? It's actually a lot simpler than most people think although after reading the following explanation you might be in need of a brain massage.
With the clutch engaged (see the section on clutches below), the layshaft is always turning. All the helical gears on the layshaft are permanently attached to it so they all turn at the same rate. They mesh with a series of gears on the output shaft that are mounted on sliprings so they actually spin around the output shaft without turning it. Look closely at the selector forks; you'll see they are slipped around a series of collars with teeth on the inside. Those are the dog gears and the teeth are the dog teeth. The dog gears are mounted to the output shaft on a splined section which allows them to slide back and forth. When you move the gear stick, a series of mechanical pushrod connections move the various selector forks, sliding the dog gears back and forth.

In the image to the left, I've rendered a close-up of the area between third and fourth gear. When the gearstick is moved to select fourth gear, the selector fork slides backwards. This slides the dog gear backwards on the splined shaft and the dog teeth engage with the teeth on the front of the helical fourth gear. This locks it to the dog gear which itself is locked to the output shaft with the splines. When the clutch is let out and the engine drives the layhshaft, all the gears turn as before but now the second helical gear is locked to the output shaft and voila - fourth gear.

Grinding gears. In the above example, to engage fourth gear, the dog gear is disengaged from the third helical gear and slides backwards to engage with the fourth helical gear. This is why you need a clutch and it's also the cause of the grinding noise from a gearbox when someone is cocking up their gearchange. The common misconception is that this grinding noise is the teeth of the gears grinding together. It isn't. Rather it's the sound of the teeth on the dog gears skipping across the dog teeth of the helical output gears and not managing to engage properly. This typically happens when the clutch is let out too soon and the gearbox is attempting to engage at the same time as it's trying to drive. Doesn't work. In older cars, it's the reason you needed to do something called double-clutching.
Double-clutching, or double-de-clutching (I've heard it called both) was a process that needed to happen on older gearboxes to avoid grinding the gears. First, you'd press the clutch to take the pressure off the dog teeth and allow the gear selector forks and dog gears to slide into neutral, away from the engaged helical gear. With the clutch pedal released, you'd 'blip' the engine to bring the revs up to the speed needed to engage the next gear, clutch-in and move the gear stick to slide the selector forks and dog gear to engage with the next helical gear.

The synchromesh - why you don't need to double-clutch.

Synchros, synchro gears and synchromeshes - they're all basically the same thing. A synchro is a device that allows the dog gear to come to a speed matching the helical gear before the dog teeth attempt to engage. In this way, you don't need to 'blip' the throttle and double-clutch to change gears because the synchro does the job of matching the speeds of the various gearbox components for you. To the left is a colour-coded cutaway part of my example gearbox. The green cone-shaped area is the syncho collar. It's attached to the red dog gear and slides with it. As it approaches the helical gear, it makes friction contact with the conical hole. The more contact it makes, the more the speed of the output shaft and free-spinning helical gear are equalised before the teeth engage. If the car is moving, the output shaft is always turning (because ultimately it is connected to the wheels). The layshaft is usually connected to the engine, but it is free-spinning once the clutch has been operated. Because the gears are meshed all the time, the synchro brings the layshaft to the right speed for the dog gear to mesh. This means that the layshaft is now spinning at a different speed to the engine, but that's OK because the clutch gently equalises the speed of the engine and the layshaft, either bringing the engine to the same speed as the layshaft or vice versa depending on engine torque and vehicle speed.

So to sum up that very long-winded description, I've rendered up an animation - when you see parts of a gearbox moving in an animation, it'll make more sense to you. What we have here is a single gear being engaged. The layshaft the blue shaft with the smaller helical gear attached to it. To start with, the larger helical gear is free-spinning on its slip ring around the red output shaft - which is turning at a different speed because it's connected to the wheels. As the gear stick is moved, the gold selector collar begins to slide the dog gear along the splines on the output shaft. As the synchromesh begins to engage with the large helical gear, the helical gear starts to spin up to speed to match the output shaft. Because it is meshed with the gear on the layshaft, it in turn starts to bring the layshaft up to speed too. Once the speed of everything is matched, the dog gear locks in place with the output helical gear and the clutch can be engaged to connect the engine to the wheels again.

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What about reverse?

Reverse gear is normally an extension of everything you've learned above but with one extra gear involved. Typically, there will be three gears that mesh together at one point in the gearbox instead of the customary two. There will be a gear each on the layshaft and output shaft, but there will be a small gear in between them called the idler gear. The inclusion of this extra mini gear causes the last helical gear on the output shaft to spin in the opposite direction to all the others. The principle of engaging reverse is the same as for any other gear - a dog gear is slid into place with a selector fork. Because the reverse gear is spinning in the opposite direction, when you let the clutch out, the gearbox output shaft spins the other way - in reverse. Simple. The image on the left here shows the same gearbox as above modified to have a reverse gear.

Crash gearboxes or dog boxes.

Having gone through all of that business about synchromeshes, it's worth mentioning what goes on in racing gearboxes. These are also known as crash boxes, or dog boxes, and use straight-cut gears instead of helical gears. Straight-cut gears have less surface area where the gears contact each other, which means less friction, which means less loss of power. That's why people who make racing boxes like to use them.
Normally, straight-cut gears are mostly submerged in oil rather than relying on it sloshing around like it does in a normal gearbox. So the extra noise that is generated is reduced to a (pleasing?) whine by the sound-deadening effects of the oil.

But what is a dog box? Well - motorbikes have been using them since the dawn of time. Beefing the system up for cars was the brainchild of a racing mechanic who wanted to provide teams with a quick method of altering gear ratios in the pits without having to play "chase the syncro hub ball bearings" as they fell out on to the garage floor.
Normal synchro gearboxes run at full engine speed as the clutch directly connects the input shaft to the engine crank. Dog boxes run at a half to a third the speed of the engine because there is a step-down gear before the gearbox. The dog gears in a dog box also have less teeth on them than those in a synchro box and the teeth are spaced further apart. So rather than having an exact dog-tooth to dog-hole match, the dog teeth can have as much as 60° "free space" between them. This means that instead of needing an exact 1-to-1 match to get them to engage, you have up to 1/6th of a rotation to get the dog teeth pressed together before they touch each other and engage. The picture on the right shows the difference between synchro dog gears and crash box dog gears.
So the combination of less, but larger dog teeth spaced further apart, and a slower spinning gearbox, allegedly make for an easier-to-engage crash box. In reality, it's still quite difficult to engage a crash box because you need exactly the right rpm for each gear or you'll just end up grinding the dog teeth together or having them bounce over each other. That results in metal filings in your transmission fluid, which ultimately results in an expensive and untimely gearbox rebuild.
But it is more mechanically reliable - it's stronger and able to deal with a lot more power and torque which is why it's used in racing.
So in essence, a dog box relies entirely on the driver to get the gearchange right. Well - sort of. Nowadays the gearboxes have ignition interrupters connected to them. As you go to change gear, the ignition system in the engine is cut for a fraction of a second as you come to the point where the dog teeth are about to engage. This momentarily removes all the drive input from the gearbox making it a hell of a lot easier to engage the gears. And when I say 'momentary' I mean milliseconds. Because of this, it is entirely possible to upshift and downshift without using the clutch (except from a standstill). Pull the gear out of first, and as you blip the throttle to get the engine to about the right speed, the ignition is cut just as the gears engage.
Even the blip of the throttle isn't necessary now either - advanced dog boxes can also attempt to modify the engine speed by adjusting the throttle input to get the revs to the right range first.
Of course even with all this cleverness, you still get nasty mechanical wear from cocked up gear changes, but in racing that doesn't matter - the gearbox is stripped down and rebuilt after each race.

Before the gearbox - the clutch

So now you have a basic idea of how gearing works there's a second item in your transmission that you need to understand - the clutch. The clutch is what enables you to change gears, and sit at traffic lights without stopping the engine. You need a clutch because your engine is running all the time which means the crank is spinning all the time. You need someway to disconnect this constantly-spinning crank from the gearbox, both to allow you to stand still as well as to allow you to change gears. The clutch is composed of three basic elements; the flywheel, the pressure plate and the clutch plate(s). The flywheel is attached to the end of the main crank and the clutch plates are attached to the gearbox layshaft using a spline. You'll need to look at my diagrams to understand the next bit because there are some other items involved in the basic operation of a clutch. (I've rendered the clutch cover in cutaway in the first image so you can the inner components.) So here we go.

In the diagram here, the clutch cover is bolted to the flywheel so it turns with the flywheel. The diaphragm springs are connected to the inside of the clutch cover with a bolt/pivot arrangement that allows them to pivot about the attachment bolt. The ends of the diaphragm springs are hooked under the lip of the pressure plate. So as the engine turns, the flywheel, clutch cover, diaphragm springs and pressure plate are all spinning together.
The clutch pedal is connected either mechanically or hydraulically to a fork mechanism which loops around the throw-out bearing. When you press on the clutch, the fork pushes on the throw-out bearing and it slides along the layshaft putting pressure on the innermost edges of the diaphragm springs. These in turn pivot on their pivot points against the inside of the clutch cover, pulling the pressure plate away from the back of the clutch plates. This release of pressure allows the clutch plates to disengage from the flywheel. The flywheel keeps spinning on the end of the engine crank but it no longer drives the gearbox because the clutch plates aren't pressed up against it.
As you start to release the clutch pedal, pressure is released on the throw-out bearing and the diaphragm springs begin to push the pressure plate back against the back of the clutch plates, in turn pushing them against the flywheel again. Springs inside the clutch plate absorb the initial shock of the clutch touching the flywheel and as you take your foot off the clutch pedal completely, the clutch is firmly pressed against it. The friction material on the clutch plate is what grips the back of the flywheel and causes the input shaft of the gearbox to spin at the same speed.
Burning your clutch
You might have heard people using the term 'burning your clutch'. This is when you hold the clutch pedal in a position such that the clutch plate is not totally engaged against the back of the flywheel. At this point, the flywheel is spinning and brushing past the friction material which heats it up in much the same was as brake pads heat up when pressed against a spinning brake rotor (see the Brake Bible). Do this for long enough and you'll smell it because you're burning off the friction material. This can also happen unintentionally if you rest your foot on the clutch pedal in the course of normal driving. That slight pressure can be just enough to release the diaphragm spring enough for the clutch to occasionally lose grip and burn.
A slipping clutch
The other term you might have heard is a 'slipping clutch'. This is a clutch that has a mechanical problem. Either the diaphragm spring has weakened and can't apply enough pressure, or more likely the friction material is wearing down on the clutch plates. In either case, the clutch is not properly engaging against the flywheel and under heavy load, like accelerating in a high gear or up a hill, the clutch will disengage slightly and spin at a different rate to the flywheel. You'll feel this as a loss of power, or you'll see it as the revs in the engine go up but you don't accelerate. Do this for long enough and you'll end up with the above - a burned out clutch.

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Motorcycle 'basket' clutches

It's worth spending a moment here to talk about basket clutches as found on some Yamaha motorbikes. Even though the basic principle is the same (sandwiching friction-bearing clutch plates against a flywheel), the design is totally different. If nothing else, a quick description of basket clutches will show you that there's more than one way to decouple the a spinning crank from a gearbox.
Basket clutches need to be compact to fit in a motorbike frame so they can't have a lot of depth to them. They also need to be readily accessible for mechanics to be able to service them with the minimum amount of fuss, something that's near impossible with regular car clutches. A basket clutch has a splined clutch boss bolted to the shaft coming from the engine crank with strong springs. Metal pressure plates slide on to this shaft, in alternating sequence with friction material clutch plates. The clutch plates are splined around the outside edge, where they fit into slots in an outer basket - the clutch housing. The clutch housing is bolted on to the layshaft which runs back through the middle of the whole mechanism and into gearbox. Clever, but as usual, not much use without a picture, so here you go.

In operation, a basket clutch is simplicity itself. A throw-out bearing slides around the outside of the layshaft and when you pull the clutch lever, the throw-out bearing pushes against the clutch boss. The clutch boss compresses the clutch springs and removes pressure from the whole assembly. The friction plates now spin freely in between the pressure plates. When you let the clutch out, the springs push the clutch boss in again and it re-asserts the pressure on the system, crushing the friction and pressure plates together so they grip. And there you have a second type of clutch.
You should now feel proud that with all your newfound (and somewhat geeky) understanding of clutches, you can go about your business safe in the knowledge that you sort of understand how all this spinning, geared-and-splined witchcraft works.

COMPLETE ENGINE OIL GUIDE

How much do you value the engine in your car? The life of your engine depends in no small part on the quality of the oil you put in it - oil is its lifeblood. People typically don't pay much attention to their oil - oil is oil, right? In the bad old days, maybe, but engine oil underwent something of a revolution in the 80's and 90's when hot hatches, 16-valve engines and turbos started to become popular. Combined with the devastating problems of black death the days of one oil catering for everyone were over.
Take Castrol for example. They led the field for years with their GTX mineral oil. This was eventually surpassed by semi-synthetic and fully synthetic oils, including GTX2 and GTX3 Lightec. Those were surpassed by Formula SLX and most recently, Castrol GTX Magnatec. All manufacturers have a similar broad spectrum of oils now - I just mention Castrol in particular as they're my oil of choice for my own cars.

What does my oil actually do?

Your engine oil performs many functions. It stops all the metal surfaces in your engine from grinding together and tearing themselves apart from friction, and it transfers heat away from the combustion cycle. Engine oil must also be able to hold in suspension all the nasty by-products of combustion like silica (silicon oxide) and acids. Finally, engine oil minimises the exposure to oxygen and thus oxidation at higher temperatures. It does all of these things under tremendous heat and pressure.

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How do I read the numbers around the 'W'? For example 5W40?

As oils heat up, they generally get thinner. Single grade oils get too thin when hot for most modern engines which is where multigrade oil comes in. The idea is simple - use science and physics to prevent the base oil from getting too thin when it gets hot. The number before the 'W' is the 'cold' viscosity rating of the oil, and the number after the 'W' is the 'hot' viscosity rating. So a 5W40 oil is one that behaves like a 5-rated single grade oil when cold, but doesn't thin any more than a 40-rated single grade oil when hot. The lower the 'winter' number (hence the 'W'), the easier the engine will turn over when starting in cold climates. There's more detail on this later in the page under both viscosity, and SAE ratings.

A quick guide to the different grades of oil.

Fully Synthetic	Characteristics
0W-30 0W-40 5W-40	Fuel economy savings Enhances engine performance and power Ensures engine is protected from wear and deposit build-up Ensures good cold starting and quick circulation in freezing temperatures Gets to moving parts of the engine quickly
Semi-synthetic	Characteristics
5W-30 10W-40 15W-40	Better protection Good protection within the first 10 minutes after starting out Roughly three times better at reducing engine wear Increased oil change intervals - don't need to change it quite so often
Mineral	Characteristics
10W-40 15W-40	Basic protection for a variety of engines Oil needs to be changed more often

What the heck was Black Death?

Black Death first appeared in the early 80's when a sticky black substance was found to be the cause of many engine seizures in Europe. It was extremely frustrating for vehicle owners because dealers and mechanics had no idea what was going on. Black Death just wasn't covered under insurance - if your engine had it, you paid to fix it yourself. Many engines were affected but Ford and Vauxhall (GM) suffered the most. Faster roads, higher under-hood temperatures, tighter engineering tolerances and overworked engine oils turned out to be contributors to the problem. The oils just couldn't handle it and changed their chemical makeup under pressure into a sort of tar-like glue. This blocked all the oil channels in the engines, starved them of lubrication and caused them to seize. I don't recommend this but you can reproduce the effect with a frying pan, cooking oil and a blowtorch. The cooking oil will heat up far quicker than it's designed to and will turn to a sticky black tar in your pan. Either that or it will set fire to your kitchen, which is why I said "don't do this".
Anyway, burning kitchens aside, Black Death was the catalyst for the production of newer higher quality oils, many of them man-made rather than mineral-based.

Black death for the 21^st century

There's a snappy new moniker for Black Death now: sludge. The cause is the same as Black Death and it seems to be regardless of maintenance or mileage. The chemical compounds in engine oils break down over time due to prolonged exposure to high temperatures and poor maintenance habits. When the oil oxidises, the additives separate from it and begin to chemically break down and solidify, leading to the baked-on oil deposits turning gelatinous, like black yoghurt. What doesn't help is that due to packaging, modern engines have smaller sumps than their older counterparts, and so hold less oil. This lower volume of oil can't hold as much crap (for want of a better word) and that can lead to earlier chemical breakdown.
The most common factor in sludge buildup is a combination of mineral oils, a lack of maintenance by the car owner and harsh driving conditions. However, a 2005 Consumer Reports article discovered that some engines from Audi, Chrysler, Saab, Toyota, and Volkswagen appear prone to sludge almost no matter how often the oil is changed.

What does sludge look like?

I was contacted by a BMW driver who had been having a particularly harsh time with sludge and was discussing it on the Bimmerfest forums. He posted some images of his problem and other readers posted similarly-framed images of the same engine components in "normal" condition. Here are two of those photos. On the left is what the cam case should look like in a well maintained engine when photographed through the oil filler cap. On the right is what the same type of engine looks like when suffering sludge buildup.

In this example, the consensus was that the sludge buildup was caused by an overheating engine, oil that hadn't been changed for 20,000 miles of stop-go city driving, a lot of cold starts and a period of about 12 months in storage without an oil change.

Picture credit: Ketchup at the Bimmerfest forums

Curing sludge

There are no hard and fast rules for curing an engine of sludge buildup. If it's really bad, flushing the engine might be the only cure, but that could also cause even more problems. If flushing the engine results in bits of sludge getting lodged where they can do more damage, you're actually worse off.
It's interesting to note that some race techs have reported sludge buildup in race engines as a result of aftermarket additives being used in conjunction with the regular oil. The chemical composition of the additives isn't as neutral as some companies would lead us to believe, and combined with particular types of oil and high-stress driving, they can cause oil breakdown and sludge to appear. The lesson from them appears to be "don't use additives".

When is sludge not sludge?

Easy; when it's an oil and water emulsion from a leaking or blown head gasket. If this happens, you get a whitish cream coloured sludge on the inside of the oil filler cap that looks like vanilla yoghurt or mayonnaise. The cap is typically cooler than the rest of the cam case and so the oil/water mix tends to condense there. If the underside of your filler cap has this sort of deposit on it, chances are the engine has a blown head gasket. A surefire way to confirm this is if your oil level is going up and your coolant level is going down. The coolant gets through the breaks in the head gasket and mixes with the oil. When it gets to the sump it separates out and the oil floats on top. A more accurate way to check for this condition is to use a combustion leak tester, or block tester. If you're in America, NAPA sell them for about $45 (part #BK 7001006). If you're in England, Sealey sell them for about £70 (model number VS0061). Combustion leak testers are basically a turkey baster filled with PH liquid, with a non-return valve at the bottom. To use one, run your engine for a few minutes until its warm (not hot) then turn it off. Use a protective glove (like an oven glove) and take the radiator or reservoir cap off. Plug the bottom of the combustion leak tester into the hole and squeeze the rubber bulb on top. It will suck air from the top of the coolant through the non-return valve and bubble it through the PH liquid. If the liquid changes colour (normally blue to yellow), it means there is combustion gas in the coolant which means a head gasket leak.

Note:

There is one other possible cause for the mayonnaise: a blocked scavenger hose. Most engines have a hose that comes off the cam cover and returns to the engine block somewhere via a vacuum line. This is the scavenger hose that scavenges oil vapour and gasses that build up in the cam cover. If it's blocked you can end up with a buildup of condensation inside the cam cover, which can manifest itself as the yellow goop inside the filler cap.

VW / Audi sludge problems

While the the 1.8T engines in Audi A4's, Audi TT, VW Passat, Jetta, Golf, New Bettle, are all very prone to sludge build-up, Audi/VW does not have an extended warranty for them from the factory. The factory warranty is 4 year/50,000 miles but it can be extended if purchased.
Although Audi/VW now has 10,000 mile service intervals, oil changes can be done between "services", and should be done if the vehicle is driven in heavy traffic, offroad, and non-highway use. Also, Audi/ VW will only warrant an engine if the customer has proof of all their oil changes. As of 2004 I belive all 1.8T engines must use synthetic oil.
So if you own one of these sludge-prone engines, what can you do? Obviously, Volkswagen Audi Group (VAG) states that you use only VW/Audi recommended oil. You should also keep up on your oil changes, making them more frequent if you drive hard or haul a lot of cargo. The most important thing for the VW or Audi owner is this: if the oil light comes on and beeps the high pitch beep that almost everyone ignores, pull over and shut the engine down immediately. Many VAG engines can be saved by this procedure. Have the vehicled towed to a VAG dealer. Their standard procedure is to inspect the cam bearings; if they're not scored, the oil pan will be removed and cleaned out and all the crankcase breather hoses and the oil pickup tube will be replaced. They'll do an oil pressure test with a mechanical gauge, and hopefully will also replace the turbo lines. Finally, the turbo will be checked for bearing free-play. The VAG turbos run really hot even with proper oil and coolant supply - that's why you need a good quality synthetic in them.

Toyota sludge problems

For their part, Toyota have the dubious honour of having the most complaints about sludge buildup in their engines - over 5,000 in 2008 alone. At the time of writing there is a class action suit going on against them. Details can be found at www.oilgelsettlement.com

Saab sludge problems

For an example of sludge in a Saab 9 5 Aero with only 42,000 miles on it, you might be interested to read my case study on this engine, put together with the help of a reader. Our sludge case study.

Like the site? The page you're reading is free, but if you like what you see and feel you've learned something, a small donation to help pay down my car loan would be appreciated. Thank you.

Mineral or synthetic motor oil?

Mineral oils are based on oil that comes from dear old Mother Earth which has been refined. Synthetic oils are mostly concocted by chemists wearing white lab coats in oil company laboratories. The only other type is semi-synthetic, sometimes called premium, which is a blend of the two. It is safe to mix the different types, but it's wiser to switch completely to a new type rather than mixing.

Synthetics

Despite their name, most synthetic derived motor oils (ie Mobil 1, Castrol Formula RS etc) are actually derived from mineral oils - they are mostly Polyalphaolifins and these come from the purest part of the mineral oil refraction process, the gas. PAO oils will mix with normal mineral oils which means you can add synthetic to mineral, or mineral to synthetic without your engine seizing up (although I've heard Mobil 1 is actually made by reformulating ethanol).
These bases are pretty stable, and by stable I mean 'less likely to react adversely with other compounds' because they tend not to contain reactive carbon atoms. Reactive carbon has a tendency to combine with oxygen creating an acid. (As you can imagine, in an oil this would be A Bad Thing.) They also have high viscosity indices and high temperature oxidative stability. Typically a small amount of diester synthetic (a compound containing two ester groups) is added to counteract seal swell too. These diesters act as a detergent and will attack carbon residuals. So think of synthetic oils as custom-built oils. They're designed to do the job efficiently but without any of the excess baggage that can accompany mineral based oils.

Pure synthetics

Pure synthetic oils (polyalkyleneglycol) are the types used almost exclusively within the industrial sector in polyglycol oils for heavily loaded gearboxes. These are typically concocted by even more intelligent blokes in even whiter lab coats. These chaps break apart the molecules that make up a variety of substances, like vegetable and animal oils, and then recombine the individual atoms that make up those molecules to build new, synthetic molecules. This process allows the chemists to actually "fine tune" the molecules as they build them. Clever stuff. But Polyglycols don't mix with normal mineral oils.

While we're on synthetic oils, I should mention Amsoil. They contacted me and asked to point out the following:
Amsoil do NOT produce or market oil additives and do not wish to be associated with oil additives. They are a formulator of synthetic lubricants for automotive and industrial applications and have been in business for 30+ years. They are not a half-hour infomercial or fly-by-night product, nor have they ever been involved in a legal suit regarding their product claims in that 30+ year span. Many Amsoil products are API certified, and ALL of our products meet and in most cases exceed the specifications of ILSAC, AGMA etc. Their lubricants also exceed manufacturers specifications and Amsoil are on many manufacturers approval lists. They base their claims on ASTM certified tests and are very open to anyone, with nothing to hide.

Amsoil recommend engine oil additives are NOT to be used with their products. They have a pretty good FAQ on the Amsoil website: Amsoil FAQ (external link). There is also a particularly good page talking about testing Amsoil in taxis.

If I put new, fully synthetic oil in my older engine, will the seals leak?

This question comes up a lot from people who've just bought a used vehicle and are wanting to start their history with the car on fresh oil.
The short answer: generally speaking, not any more. The caveat is that your engine must be in good working order and not be leaking right now. If that's the case, most modern oils are fully compatible with the elastomeric materials that engine seals are made from, and you shouldn't have any issues with leaks.
The longer answer:

Mixing Mineral and Synthetic oils - current thinking

Here's the current thinking on the subject of mixing mineral and synthetic oils. This information is based on the answer to a technical question posed on the Shell Oil website:
There is no scientific data to support the idea that mixing mineral and synthetic oils will damage your engine. When switching from a mineral oil to a synthetic, or vice versa, you will potentially leave a small amount of residual oil in the engine. That's perfectly okay because synthetic oil and mineral-based motor oil are, for the most part, compatible with each other. (The exception is pure synthetics. Polyglycols don't mix with normal mineral oils.)
There is also no problem with switching back and forth between synthetic and mineral based oils. In fact, people who are "in the know" and who operate engines in areas where temperature fluctuations can be especially extreme, switch from mineral oil to synthetic oil for the colder months. They then switch back to mineral oil during the warmer months.
There was a time, years ago, when switching between synthetic oils and mineral oils was not recommended if you had used one product or the other for a long period of time. People experienced problems with seals leaking and high oil consumption but changes in additive chemistry and seal material have taken care of those issues. And that's an important caveat. New seal technology is great, but if you're still driving around in a car from the 80's with its original seals, then this argument becomes a bit of a moot point - your seals are still going to be subject to the old leakage problems no matter what newfangled additives the oil companies are putting in their products.

Flushing oils

These are special compound oils that are very, very thin. They almost have the consistency of tap water both when cold and hot. Typically they are 0W/20 oils. Their purpose is for cleaning out all the gunk which builds up inside an engine.

Note:

Some hybrid vehicles now require 0W20, so if you're a hybrid driver, check your owner's manual. Also I believe Honda switched to recommending 0W20 in 2011 to meet their CAFE ratings (thinner oil gives less drag on engine parts which improves - fractionally - the mpg). If you look at 2010 models vs 2011, you'll see things like the Element and CR-V getting a tiny mpg boost in the official figures despite being the exact same car. They achieved this by remapping the gearbox shift points and dropping the cold viscosity rating on the oil. In reality unless you live in northern Alaska, or do an above average number of cold-start short journeys, 5W20 ought to be more than suitable.

Do I need a flushing oil?

Unless there's something seriously wrong with your engine, like you've filled it with milk or shampoo, you really ought never to need a flushing oil. If you do decide to do an oil flush, there's two ways of doing it. You can either use a dedicated flushing oil, or a flushing additive in your existing oil. Either way it's wise to change the filter first so you have a clean one to collect all the gunk. (This typically means draining the oil or working fast). Once you have a new filter in place, and the flushing oil (or flushing solution) in there, run the engine at a fast idle for about 20 minutes. Finally, drain all this off (and marvel at the crap that comes out with it), replace the oil filter again, refill with a good synthetic oil and voila! Clean(er) engine. For the curious amongst you, looking in the oil filter that was attached when you did the flush will be an educational exercise in the sort of debris that used to be in your engine.
Of course, like most things nowadays, there's a condition attached when using flushing oils. In an old engine you really don't want to remove all the deposits. Some of these deposits help seal rings, lifters and even some of the flanges between the heads, covers, pan and the block, where the gaskets are thin. I have heard of engines with over 280,000km that worked fine, but when flushed, failed in a month because the blow-by past the scraper ring (now really clean) contaminated the oil and ruined the rod bearings.

Using Diesel oil for flushing

A question came up some time ago about using diesel-rated oils to flush out petrol engines. The idea was that because of the higher detergent levels in diesel engine oil, it might be a good cleaner / flusher for a non-diesel engine. Well most of the diesel oil specification oils can be used in old petrol engines for cleaning, but you want to use a low specification oil to ensure that you do not over clean your engine and lose compression (for example). Generally speaking, an SAE 15W/40 diesel engine oil for about 500 miles might do the trick.

Which oil should you buy? (the short version)

That all depends on your car, your pocket and how you intend to drive and service the car. All brands claim theirs offers the best protection available - until they launch a superior alternative. It's like washing powders - whiter than white until new Super-Nukem-Dazzo comes out. For most motorists and most cars, a quality mainstream oil is the best, like Castrol GTX. Moving up a step, you could look at Duckhams QXR and Castrol Protection Plus and GTX3 Lightec. The latter two of these are designed specifically for engines with catalytic converters. They're also a good choice for GTi's and turbo engines. Go up a step again and you're looking at synthetic oils aimed squarely at the performance market like Mobil-1.
To help you through the maze of oils available, there's a site available now (the motor oil evaluator) that aims to lessen the confusion with a relatively balanced scoring system based on published specifications such as viscosity and pour point. It's a good starting point if you're looking for even more in-depth info.

Which oil should you buy? (the long version)

Quality Counts! It doesn't matter what sort of fancy marketing goes into an engine oil, or how many naked babes smear it all over their bodies, or how bright and colourful the packaging is, it's what's written on the packaging that counts. Specifications and approvals are everything. There are two established testing bodies. The API (American Petroleum Institute), and the European counterpart, the ACEA (Association des Constructeurs Europeens d'Automobiles - replaced CCMC in 1996). You've probably never heard of either of them, but their stamp of approval will be seen on the side of every reputable can of engine oil.

The API

The API classifications are different for petrol and diesel engines:

• For petrol, listings start with 'S' (meaning Service category, but you can also think of it as Spark-plug ignition), followed by another code to denote standard. 'SN' is the current top grade but 'SH' is still the most popular.
• For diesel oils, the first letter is 'C' (meaning Commercial category, but you can also think of it as Compression ignition). 'CJ' is the highest grade at the moment, (technically CJ-4 for heavy-duty) but 'CH' is the most popular and is well adequate for passenger vehicle applications.

Note:

Castrol recently upgraded all their oils and for some reason, Castrol diesels now use the 'S' rating, thus completely negating my little aid-memoir above. So the older CC,CD,CE and CF ratings no longer exist, but have been replaced by an 'SH' grade diesel oil. This link is a service bulletin from Castrol, explaining the situation.

The CCMC/ACEA

The ACEA standards are prefixed with an 'A' for petrol engines, 'B' for passenger car diesel, 'C' for diesel with particulate filter, or 'E' for heavy-duty diesel. (The older CCMC specifications were G,D and PD respectively). The ACEA grades may also be followed by the year of issue which will be either '04 or '07 (current). Coupled with this are numerous approvals by car manufacturers which many oil containers sport with pride.

The full ACEA specs are:

• A1 Fuel Economy Petrol †
• A2 Standard performance level
• A3 High performance and / or extended drain
• A5 Fuel economy petrol with extended drain capability †
• B1 Fuel Economy diesel †
• B2 Standard performance level (now obsolete)
• B3 High performance and / or extended drain
• B4 For direct injection passenger car diesel engines
• B5 Fuel economy diesel with extended drain capability †

† Not suitable for all engines - should ONLY be used in engines specifying this fuel efficient grade. Refer to the manufacturer handbook of contact your local dealer if you're not sure.

Mineral oils:

• E1 Non-turbo charged light duty diesel
• E2 Standard performance level
• E3 High performance extended drain
• E5 (1999) High performance / long drain plus American/API performances. - This is ACEAs first attempt at a global spec.
• E7 Euro 4 engines - exhaust after treatment (EGR / SCR)

Part / full synthetic oils:

• E4 Higher performance and longer extended drain
• E6 Euro 4 specification - low SAPS for vehicles with PDF (see below)

Low SAPS diesel (Sulphated Ash, Phosphorous, Sulphur)
For diesel engines fitted with a diesel particulate filter (DPF) - a filter unit in the exhaust that takes out the microscopic soot particles. Regular diesel oils used in engines that have a DPF can cause the filter to become blocked with ash.

• C1 Low SAPS (0.5% ash) fuel efficient
• C2 Mid SAPS (0.8% ash) fuel efficient, performance
• C3 Mid SAPS (0.8% ash)

Many OEM are now using their own specifications to capture these specifications. eg. Mercedes 229.31/51, BMW Longlife 04, VW 507 00 etc.
There is also a trend now towards manufacturers requiring their own specifications - in this case the OEM specification is the one that needs to be adhered to. If it says BMW Longlife 04, the oil must say this on the pack to be suitable for use.

Typically, these markings will be found in a statement similar to: Meets the requirements of API SH/CD along the label somewhere. Also, you ought to be able to see the API Service Symbol somewhere on the packaging:

Beware the fake API symbol

Some unscrupulous manufacturers (and there's not many left that do this) will put a symbol on their packaging designed to look like the API symbol without actually being the API symbol. They do this in an effort to pump up the 'quality' of their product by relying on people not really knowing exactly what the proper API symbol should look like. To the left is an example of a fake symbol - it looks similar but as long as you remember what to look for, you won't get taken by this scam.
Amsoil are one of the biggest inadvertent offenders of the fake API symbol. Take a look at one of their labels here on the right. See that little starburst that says "Fuel efficient formula SL-CF"? It's actually not an API-certified SL or CF oil. (To be fair, some Amsoil products are API certified and they do have the correct labelling, but their top-tier products do not). The issue of their lack of API certification on these products caused such a stir at Amsoil that they had to generate a FAQ to answer the most commonly-asked questions. You can find a copy of that here : Amsoil & API Licensing. It explains everything logcially and clearly, and it's not scientific doublespeak. Which is nice.

A Brief History of Time API ratings
Some people have asked about the old standards, and although they're not especially relevant, some rampant plagiarism from an API service bulletin means I can bring you all the API ratings right back from when the earth was cooling. the table below to see the ratings.

Petrol Engines			Diesel Engines
Category	Status	Service	Category	Status	Service
			CJ-4	Current	Introduced in 2006 for high-speed four-stroke engines. Designed to meet 2007 on-highway exhaust emission standards. CJ-4 oils are compounded for use in all applications with diesel fuels ranging in sulphur content up to 500ppm (0.05% by weight). However, use of these oils with greater than 15ppm sulfur fuel may impact exhaust aftertreatment system durability and/or oil drain intervals. CJ-4 oils are effective at sustaining emission control system durability where particulate filters and other advanced aftertreatment systems are used. CJ-4 oils exceed the performance criteria of CF-4, CG-4, CH-4 and CI-4.
SN	Current	For all automotive engines presently in use. Introduced in the API service symbol in November 2010	CI-4	Current	Introduced in 2002 for high-speed four-stroke engines. Designed to meet 2004 exhaust emission standards implemented in 2002. CI-4 oils are formulated to sustain engine durability where exhaust gas recirculation (EGR) is used and are intented for use with diesel fuels ranging in sulphur content up to 0.5% weight. Can be used in place of CD, CE, CF-4, CG-4 and CH-4
SM	Current	For all automotive engines presently in use. Introduced in the API service symbol in November 2004	CH-4	Current	Introduced in 1998 for high-speed four-stroke engines. CH-4 oils are specifically designed for use with diesel fuels ranging in sulphur content up to 0.5% weight. Can be used in place of CD, CE, CF-4 and CG-4.
SL	Still current but nearly obsolete	For all automotive engines presently in use. Introduced in the API service symbol in 1998	CG-4	Current	Introduced in 1995 for high-speed four-stroke engines. CG-4 oils are specifically designed for use with diesel fuels ranging in sulphur content less than 0.5% weight. CG-4 oil needs to be used for engines meeting 1994 emission standards. Can be used in place of CD, CE and CF-4.
SJ	Still current but nearly obsolete	For all automotive engines presently in use. Introduced in the API service symbol in 1996	CF-4	Current	Introduced in 1990 for high-speed four-stroke naturally aspirated and turbo engines. Can be used in place of CD and CE.
SH	Obsolete	For model year 1996 and older engines.	CF-2	Current	Introduced in 1994 for severe duty, two stroke motorcycle engines. Can be used in place of CD-II.
SG	Obsolete	For model year 1993 and older engines.	CF	Current	Introduced in 1994 for off-road, indirect-injected and other diesel engines including those using fuel over0.5% weight sulphur. Can be used in place of CD.
SF	Obsolete	For model year 1988 and older engines.	CE	Obsolete	Introduced in 1987 for high-speed four-stroke naturally aspirated and turbo engines. Can be used in place of CC and CD.
SE	Obsolete	For model year 1979 and older engines.	CD-II	Obsolete	Introduced in 1987 for two-stroke motorcycle engines.
SD	Obsolete	For model year 1971 and older engines.	CD	Obsolete	Introduced in 1955 for certain naturally aspirated and turbo engines.
SC	Obsolete	For model year 1967 and older engines.	CC	Obsolete	Introduced in 1961 for all diesels.
SB	Obsolete	For older engines. Use this only when specifically recommended by the manufacturer.	CB	Obsolete	Introduced in 1949 for moderate-duty engines.
SA	Obsolete	For much older engines with no performance requirement. Use this only when specifically recommended by the manufacturer.	CA	Obsolete	Introduced in 1940 for light-duty engines.

Grade counts too!The API/ACEA ratings only refer to an oil's quality. For grade, you need to look at the SAE (Society of Automotive Engineers) ratings. These describe the oil's function and viscosity standard. Viscosity means the substance and clinging properties of the lubricant. When cold, oil can become like treacle so it is important that any lube is kept as thin as possible. Its cold performance is denoted by the letter 'W', meaning 'winter'. At the other end of the scale, a scorching hot oil can be as thin as water and about as useful too. So it needs to be as thick as possible when warm. Thin when cold but thick when warm? That's where MultiGrade oil comes in. For ages, good old 20W/50 was the oil to have. But as engines progressed and tolerances decreased, a lighter, thinner oil was required, especially when cold. Thus 15W/50, 15W/40 and even 15W/30 oils are now commonplace.

The question of phosphorus and zinc.

Phosphorus (a component of ZDDP - Zinc Dialkyl-Dithio-Phosphate) is the key component for valve train protection in an engine and 1600ppm (parts per million) used to be the standard for phosphorus in engine oil. In 1996 the EPA forced that to be dropped to 800ppm and then more recently (2004?) to 400ppm - a quarter of the original spec. Valvetrains and their components are not especially cheap to replace and this drop in phosphorus content has been a problem for many engines (especially those with flat-tappet type cams). So why was the level dropped? Money. Next to lead, it's the second most destructive substance to shove through a catalytic converter. The US government mandated a 150,000 mile liftime on catalytic converters and the quickest way to do that was to drop phosphorous levels and bugger the valvetrain problem. Literally.
In the US, Mobil 1 originally came out with the 0W40 as a 'European Formula' as it was always above 1000 ppm. This initially got them out of the 1996 800ppm jam and knowledgeable consumers sought it out for obvious reasons. Their 15W50 has also maintained a very high level of phosphorus and all of the extended life Mobil synthetics now have at least 1000ppm. How do they get away with this? They're not classified as energy/fuel conserving oils and thus do not interfere with the precious government CAFE (corporate average fuel economy) ratings. (See my section on the EPA and fuel economy in the Fuel and Engine Bible for more info on this). This also means that they don't get the coveted ratings of other oils but they do protect your valvetrain. The same rule of thumb is true for racing oils like Royal Purple - because they're not classified as energy / fuel conserving, it would seem they still contain good quantities of ZDDP.
In fact, as a general rule-of-thumb, staying away from XX-30 oils and going to 10W-40 or higher might be the way to go if you have an older engine. 10W-40 and above is generally also not considered to be 'gas saving' and like the Mobil example above, doesn't mess with the CAFE rating.
If you live in England, Castrol market a product with ZDDP in the product description - 'Castrol Classic Oil With ZDDP Anti-Wear Additive' although it's not mainstream enough to be available everywhere. You'll have to find a specialist dealer. Castrol Classics. In the US, Rislone manufacture an oil supplement to boost the ZDDP content of your existing oil. Rislone Engine Oil Supplement.

API rating backward compatibility and 2V engines

This section contains information from Bruce Dance, Brian over at bigcoupe.com and LN Engineering and their combined experience with API ratings and 2 valve engines

If you own a two-valve spark ignition engine or certain diesel engines (which do not have to meet recent emission standards) the only sensible (ie widely available) oil to put in right now is synthetic or semisynthetic to meet API SL/CF and not a higher rating. As I touched upon above, oils with a CG and higher rating typically don't contain enough ZDDP, and the replacement friction modifiers don't work in highly loaded valve trains (generally older engines especially those with 2V design). If you try to compensate by adding a ZDDP additive into a newer oil it still might not work because of interactions with other additives in the oil.
Why the discrepancy in the ratings? The API no longer include a valve train wear test that accurately simulates 2V cam follower loading. They do perform a test that simulates 4V loading and then they allow a lot of wear to occur and still 'pass'. The ACEA tests are a lot tougher but still not tough enough. Whilst the newer CG, CH and higher API oil standards should be 'better in every way', they are really just 'improved in some ways'. Hence the increasing use of manufacturer-specific standards.
There is a lot of info kicking around on the web on this topic because it has caused a LOT of problems with some engines especially Porsche aircooled units.
One of my readers found out when he went to buy oil for his (modern 4V common rail diesel) Nissan that they expressly prohibit the use of CG or higher rated oils. Nissan mandate that owners use CF oils in these engines. It's worth noting that the CF spec was already out of date when these engines were built but Nissan did not use the latest API spec because it wasn't good enough!
The fact that API have dropped the CF tests/standard does not in any way improve the later oils that do not meet this standard.

Marine Diesels and other special considerations.

Inland Marine Diesels (and certain road vehicles under special conditions) can (and do) glaze their bores due the low cylinder wall temperatures causing the oil (and more importantly the additive pack) to undergo a chemical change to a varnish-like substance. The low temperature is caused by operating under light load for long periods.
This is related to engine design, some engines being nearly immune to it and others susceptible. The old Sherpa van diesel engines were notorious for this problem. The "cure" (such as it is) is to use a low API specification oil, such as CC. Certain engine manufacturers/marinisers are now marketing the API CC oil for this purpose under their own name (and at a premium). You'll find some modern engines where its industrial/vehicle manual states API CF and the marinised manual states API CC/CD. {Thanks to Tony Brooks for this information.}

Marine Oils.

I sometimes get asked "why are marine engine oils so expensive and why can't I just use regular motor oil in my marine engine instead?". Well, the National Marine Manufacturers Association Oil Certification Committee (click here for more info) introduced a four-stroke engine oil test and standard called the 4T certification. This specification is meant to assist boaters and manufacturers in identifying four-stroke cycle engine oils that have been specially formulated to withstand the rigors of marine engine operation. The certification was prompted by the growing influence of four-stroke engines in the marine market and their unique lubrication demands. So the simple answer is that regular road-based engine oil products don't contain rust inhibitors and won't pass the 4T certification. Lakes, waterways and the sea are a lot more aggressive an environment for an engine to operate around than on land.
Note : the NMMA have long had a similar specification for 2-stroke oils destined for marine use, called the TC-W3® certification.

The eBay problem

This paragraph may seem a little out of place but I have had a lot of problems with a couple of eBay members (megamanuals and lowhondaprelude) stealing my work, turning it into PDF files and selling it on eBay. Generally, idiots like this do a copy/paste job so they won't notice this paragraph here. If you're reading this and you bought this page anywhere other than from my website at www.carbibles.com, then you have a pirated, copyright-infringing copy. Please send me an email as I am building a case file against the people doing this. Go to www.carbibles.com to see the full site and find my contact details. And now, back to the meat of the subject....

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Engine oil / Motor oil Shelf Life.

I couldn't decide whether to put this in the FAQ or the main page, so it's in both, because I get asked this question a lot. Typically, the question is along the lines of "GenericAutoSuperStore are having a sale on WickedlySlippy Brand synthetic oil. If I buy it now, how long can I keep if before I use it?"
In general, liquid lubricants (ie. oils, not greases) will remain intact for a number of years. The main factor affecting the life of the oil is the storage condition for the products. Exposure to extreme temperature changes, and moisture will reduce the shelf life of the lubricants. (an increase of 10°C doubles oxidation which halves the shelf life) ie. don't leave it in the sun with the lid off. Best to keep them sealed and unopened.

Read more: http://www.carbibles.com/engineoil_bible.html#ixzz2g4kOIQzg