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 vacuum is being pulled in the expansion stroke and in the injection stroke, because no fuel has been injected there is no explosion.
in the upwards cycle after this the exhaust valve will be open so the vacuum will not pull the pistons upwards.
even in cars without direct injection this works because the car can simply stop igniting the fuel, which is also why engine braking happens so quickly and why some sports cars have popping sounds coming from the exhaust as the left over fuel gets into the exhaust and ignites there.
Latest Answers