Like, what is the mechanism that slows the car considering the hydraulic brakes aren’t involved at all? How does it vary by transmission type (manual, regular auto, CVT)?
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How this works is subtly different in gasoline engines v.s. diesels.
In gasoline cars, if you want to engine brake, you reduce the gear which, when you let out the clutch causes the engine to rotate faster. This does a couple of things. Because most of the rotating parts in a car engine are lubricated with oil, at high RPMs they experience significant fluid friction. Particularly the transmissions gears which are partially submerged in oil, the crackshaft which also sloshes around in a pool of oil, the pistons are also lubricated by a thing Layer of oil on the cylinder walls which results in hydrodynamic drag it high rpm.
The oil pump and coolant pump also produce significantly more drag on the drivetrain at high RPMs for physics reasons beyond the scope of this post.
Lastly, during engine braking the throttle is closed. This results in an increased throttle vacuum whichnthe engine has to fight against during each intake stroke. Usually the crackcase space under the piston is at slightly higher pressure than atmospheric pressure, which pushes up against the cylinders durning the intake stroke.
Diesel engines don’t have a throttle valve in the intake, because it wouldn’t benefit them and would impair starting. However this means that since they don’t work against a throttle vacuum they produce less parasitic drag in this scenario by comparison. Although fluid friction losses are still in play, like I described earlier.
For this reason many larger diesel powered trucks are equipped with either an Exhaust Brake or a Compression brake. The exhaust brake is a throttle valve, but it’s placed on the exhaust manifold instead. This causes the exhaust to be compressed which creates resistance during the exhaust stroke.
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