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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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Wiki User
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


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