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Why do you leave a petrol car in gear but a diesel car in neutral for additional breaking?

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Answered 2007-10-22 18:26:35

Engine braking is caused by the vaccum of the engine attempting to pull air through the closed throttle valve. Nearly all petrol (or gas for us in the US) engines use a throttle valve. Diesel engines on the other hand do not have a throttle. They are controlled solely by the amount of fuel injected into the cylinders. Not having a throttle valve, there is no restriction to the engine provide a vaccum when deccellerating, and hence, no engine braking. On large trucks (or lorries) the diesel engines will often have installed a valve in the exhaust system to provide the same effect, or they will have a compression release built into the valve train to do the same thing (often referred to as a "Jake Brake" as they were originally manufactured by the Jacobs company).

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