From the perspective of 50-something years of tinkering with Diesels, a jake brake is a totally different animal than compression braking with a gas engine. A Jacobs brake turns the engine into an air compressor- - - -a compression stroke, without a power impulse following. It does so by opening the exhaust valve just as the fuel injector would normally spray, and letting the compression pressure escape from the exhaust system. On a 4-stroke engine such as a Cummins, that happens as the center rocker arm that operates the injector plunger just begins to move- - - -it triggers an oil pressure solenoid valve that forces the exhaust valve(s) open. On a 2 stroke Detroit, there are four exhaust valves and NO intakes, so the compression gets released RIGHT NOW! That's why a Detroit with a Jake really rattles windows! Very little fuel gets wasted, because the throttle valve or the fuel rail in a Detroit is at the "idle" position when the Jake is working.
There's no way to relieve the compression on a gas engine, so during a closed throttle slowdown a little fuel/air mix is drawn into the exhaust while coasting. With the right exhaust temperature, some of that fuel gets burned in the exhaust pipe and muffler instead of all of it getting consumed in the cylinders.
Jerry
