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
> So where’s the energy loss here?
The gasoline engine is still exhausting at the top of the stroke. It blows out high pressure exhaust and then needs to work again to draw in more air, being slowed by the resulting vacuum.
You say the energy is being “returned” in the form of the upstroke compressing the air drawn in by the manifold vacuum, but it isn’t returned to the crankshaft because the pistons aren’t pushed down by that compression for the next stroke.
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