<P> A compression release brake (also known as a Jacobs brake or "jake brake"), is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, so the large amount of energy stored in that compressed air is not returned to the crankshaft, but is released into the atmosphere . </P> <P> Normally, during the compression stroke, energy is used as the upward - traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down . However, with the jake brake in operation, the compressed air is suddenly released just before the piston begins its downward travel . (This sudden release of compressed air creates audible sound waves similar to the expanding gases escaping from the muzzle of a firearm .) Having lost the energy stored within the compressed air, there is no "spring back" so the engine must expend yet more energy pulling the piston back down again . </P> <P> This type of brake is banned or restricted in many locations where people live because it creates a machine gun like sound loud enough to disturb the peace, including waking people at night . It is very effective, however, and creates immense amounts of braking force which significantly extends friction brake life--a 565 hp (421 kW) diesel engine can produce up to 600 hp (450 kW) of braking power at 2,100 RPM . </P> <P> An exhaust brake works by causing a restriction in the exhaust, much like the intake throttle causes in a gasoline engine . In simple terms, it works by increasing the back - pressure of the exhaust . Nearly all of these brakes are butterfly valves similar to a throttle valve, mounted downstream of the turbocharger if there is one . </P>

When is the braking effect of the engine the greatest