Rob
As I was driving through a mountain pass on my way home from vacation, a transport truck passed me heading downhill. As it neared my open window, the big rig made a sound like a .50-calibre machinegun. What is this sound and where is it coming from?
Brad
Brad, the machinegun sound is caused by a Compression Release Engine Brake, or Jake Brake, which is really an accepted slang term for the original company name; Jacobs Vehicle Systems. The Jake, or more accurately, the Jacobs Engine Brake is a device mounted to the valve train of a diesel engine that mechanically actuates the combustion chamber exhaust valve. When this valve is forced open, the sound that emanates is created by the release of compressed air that has been trapped inside the combustion chamber.
So, that's the Reader's Digest version - which creates more questions than it answers. Why a Jake Brake in the first place? The Compression Release Engine Brake was the result of a near-death experience by its inventor, Clessie L. Cummins, the founder of the Cummins Engine Company. In 1931, he miscalculated what it would take to slow down a (then) large transport truck while descending a hill. Applying the brakes and gearing down didn't work and he almost drove into the side of a freight train that had crossed his path. Cummins started thinking of solutions but it wasn't until he retired that he started a skunkworks project that would create a means for a diesel engine to "work" in reverse.
Brad, the reason a diesel engine does not have the inherent ability to provide engine braking like that of a gasoline engine is because it does not have a throttle plate. When a gasoline engine is put into the condition of "closed throttle deceleration" - coasting down a hill with your foot off the throttle - the closed throttle provides resistance to air flow into the engine. In essence, the gasoline engine is trying to suck itself inside out.
Because a diesel doesn't have a throttle plate or restrictor, it can't do the same trick. Cummins figured that trapping and releasing the compression stroke would perform the same function. Smart guy.
By fitting his invention to his diesel engine, Cummins was able to trap the ingested air of each cylinder and before the piston was able to cross over top dead centre and cause this compressed air to now work like an engine, the engine brake opened the exhaust valve, which literally spit the compressed air into the exhaust system - wasting the compression energy. This is the noise you hear, Brad. What Cummins had done was create a very large compressor out of a diesel engine. This resistance of compressing the air in each cylinder and allowing it to escape into the atmosphere proved to be effective in slowing down a large vehicle while descending a hill. There is speculation that this braking power equals or exceeds the normally developed horsepower of the engine.
Another question: Why a Jacobs Engine Brake when it was developed by Cummins? It wasn't until Cummins retired that he started work on his invention. He wanted to start his own company actually, but through a twist of fate (and a marriage), the Cummins family joined the Englund family. The father of the bride, Bob Englund happened to be vice president of the Jacobs Manufacturing Company. At that time, it manufactured drill chucks - and still does. Discussions ensued at the dinner table and the Clessie L. Cummins Division of Jacobs Manufacturing was born. The division later became Jacobs Vehicle Systems.
Whew! Long way around, but there it is Brad - noise and history folded together.