wolfe10 Report post Posted June 19, 2009 There are three different "auxiliary" braking systems used on different diesel engines.: Exhaust Brakes, Engine Compression Brakes and Variable Geometry Turbo Brakes. Exhaust brake: literally a "flap" which closes off exhaust flow just downstream of the turbo in the exhaust system. This causes back pressure (55 PSI on ours) which generates braking power. With the Allison transmission, the exhaust brake is usually tied with downshifting of the transmission to the "pre-selected" gear (usually 2nd or 4th). Think of it as a potato stuffed in the tailpipe. Engine Compression Brake (aka Jake brake): The exhaust valves are opened as the pistons reach TDC (Top Dead Center) on the compression stroke after the engine has done the "work" of compressing about 18 volumes of intake air to 1 volume. If the exhaust were not let to escape by the compression brake's opening, the exhaust valves (i.e. coasting with brake off), the "compressed air" would mostly be returned as power to the engine, forcing the piston back down. With the Jake brake on, the engine works to compress air in the cylinder, and then the air is let out. This generates quite a lot more braking force than an exhaust brake. The smallest engines to offer an engine compression brake are the Caterpillar C9 and Cummins ISL. Variable Geometry Turbo: The vanes reverse or aperture closes (depends on engine manufacturer) to create back pressure with much the same effect as an exhaust brake. Brett Wolfe Share this post Link to post Share on other sites
