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
This is what I gathered from the wikipedia article on the subject:
When you suck, you suck on a vacuum, which requires energy (because the other side of the piston is at atmospheric pressure).
When you squeeze, you squeeze that vacuum, getting your energy back.
When you bang, there’s no bang, and you’re required to suck on the vacuum again, which requires energy.
When you’re about to blow, air from the exhaust manifold is allowed to enter the cylinder, killing the vacuum and ensuring that you won’t get that energy back. Then you blow this air out again, and the cycle starts anew.
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