Jake brakes are trivial to understand: you’re compressing a spring and “magically” losing that energy instead of allowing it to be returned. But I don’t get how gasoline engine brakes work: from what I can tell the manifold holds a vacuum which resists the piston downstroke, but the vacuum returns the same energy in the piston’s upstroke (minus friction which is negligible). Furthermore, once all cylinders have undergone one full cycle, they all hold a vacuum so if one cylinder is being retarded, it’s opposite is being actuated meaning the force balance is pretty close to neutral. So where’s the energy loss here?
In: Engineering
Jake brakes are mechanically blocking a port to create vacuum, engine braking in a normal car is simply the engine compressing air on one of every four strokes each piston but not doing anything with it.
That’s why jake brakes exist, because while engine braking will slow down a heavy truck, it’s not enough drag to prevent acceleration on it’s own, even in a diesel which has incredibly high compression.
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