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
The energy loss is occurring at the throttle body/throttle plate. Fundamentally, an automobile engine is a pump that moves air from the intake manifold to the exhaust manifold. When you engine brake you close the throttle and as a result more work is required to pull air through the closed throttle.
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