Jake Brake

You are confusing combustion with compression. On a Jacobs engine brake, there is no butterfly valve in the exhaust. That is an exhaust brake, not a Jake brake. A Jacobs engine brake assembly is fully contained under the valve cover on top of the valve train and is a mechanism that opens the exhaust valve at or near the top of the compression stroke. The compression stroke absorbs the energy, then when the exhaust valve is opened at the top of that stroke prematurely by the Jake, that energy is expelled out the exhaust before the compressed air can return the energy to the drive train on the "fire" stroke. Sorta like jumping on a Pogo stick. As you land, the spring compresses, but instead of the spring shooting you back up, the spring breaks.

And fuel injection only occurs when you are on the throttle. The injector still actuates, but without your foot on the throttle, no fuel is supplied to the injector so there is no fuel to inject, it just goes thru the motion. And without fuel, there is no combustion. The "jake brake" sound you hear is simply the sudden release of the compressed cylinder's air pressure.

 

I see what you're saying. I guess I combined both types to make one imaginary super brake in my head.

 

I just learned a lot - and nobody got their feelings hurt!