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
Air in the cylinder behaves like a perfect spring only while the valves are closed. When the valves are open, the energy is not conserved.
Strokes:
1 – lose energy to suck air in the cylinder
2 – lose energy by compressing air
3 – expand air and recover energy from step 2
4 – lose energy by pushing air outside the cylinder.
Steps 1 and 4 are where the engine brake is effectly slowing down the vehicle.
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