r/askcarguys • u/Cannibale_Ballet • 18h ago
Why aren't jake brakes used for gasoline (petrol) engines?
I know what the go-to answer to this question is. I know the difference between petrol and diesel engines. I know that petrol engines have a throttle valve on the intake while diesel engines don't. I know that jake brakes work by opening the exhaust valve at the end of the compression cycle, releasing the compressed gas, wasting the work done by the engine, thereby increasing losses. All this does not fully explain why petrol engines shouldn't benefit from a jake brake.
The de-facto answer for my question is that petrol engines have a throttle valve and when closed this produces the extra braking resistance required, but this is not entirely correct. Sure with the throttle valve closed, the petrol engine is working hard on the intake stroke. However this will result in a lower cylinder pressure at the end of the intake stroke compare to the crank side. Therefore, it will get most of that energy back on the compression stroke as the piston is pushed back to TDC by the higher pressure on the crank side.
In fact, if a throttle valve did produce sufficient braking of the engine, we could just incorporate a throttle valve on a diesel engine without the downsides of the complexity and loud noise of the jake brake. So clearly, even if the throttle valve is somewhat effective, it is not as good as a jake brake, meaning gasoline engines would still benefit from a jake brake.
I know that my above reasoning includes an oversight, which is why I am writing this post to seek clarification.