just a DA question regarding dp engine brakes

Peralko wrote:
The reason a gas engine brakes with the throttle closed is that it is pulling a vacuum with the throttle closed. A diesel engine always has a wide-open throttle, so no vacuum and no resistance. Modern DPs typically have one of three different engine braking systems. The first is an exhaust brake, which closes off the exhaust to create back pressure. It is the least effective. The Jake brake opens the exhaust valve for each cylinder at the top of the compression stroke, so the energy used to compress the air in the cylinder is not released on the downstroke-and the piston pulls a vacuum going back down. Jake brakes typically have two or three stages, depending on how many cylinders the Jake activates. The transmission retarder uses the transmission as a braking medium. The retarder is continually variable, and is as effective as a Jake. With any of the systems, the transmission is programmed to progressively go to lower gears when the engine brake is activated.