Clarke gave an excellent description of the use of the Jake Brake. Diesels not equipt with an exhaust brake or a Jake brake have significantly less inheritant compression braking than gasoline engines. Let off a gasoline engine and you can feel it slowing you down due to the butterfly or the throttle plate closing off the air intake into the cylinders. Let off a diesel engine, particularly a two-stroke diesel engine, and you don't feel as much slowing affect since there is no butterfly/throttle plate to close. As a matter of fact, the Detroit Diesel two strokes have a low pressure blower that is gear driven off the engine that continues to pump air into the cylinders even though the fuel is shut off to the cylinders. A true Jake Brake (engine compression brake) provides engine braking by turning the engine into a giant air compressor. A Jake Brake works by closing the exhaust valves on all or certain cylinders. It is important to note that the higher the engine rpm, as long as you are within the engines maximum speed range, the more braking power the Jake has. Therefore you want to gear down to keep the engine up close to redline to maximize the effectiveness of the Jake. It will help at lower rpms but typically, on an engine that redline's at say 2100 rpm, most of the effectiveness is above 1800 rpm.
EDIT:
The Jake brake on a four-stroke diesel engine has more braking capacity than a two-stroke diesel engine. Example: Series 60 (4 stroke) Jake has about 550 braking HP and an 8V92 (2 stroke) Jake has about 260 braking HP.
Using just the service brakes on a long grade is a recipe for disaster in a heavy vehicle as you will have to stay on the brakes to the point that they overheat and become ineffective. Use of gearing down the transmission alone may get you to the bottom safely but you might have to go down in 1st gear without the Jake to maintain a constant controlled speed as opposed to maintaining a constant controlled speed in 2nd or 3rd with the Jake Brake.
Some newer coaches have two speed Jake Brakes. With those you can switch between low and high to vary the number of cylinders that are aiding the braking effort.
ALWAYS be safe and obey the truck speed limit on long declines. It doesn't matter how fast you get to the bottom, it matters how safely you get to the bottom of the mountain. Avoid use of the Jake brake on wet roads.
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