Quote:
Originally Posted by Christ
OK, I just ed-jama-cated muhself. Apparently, it opens the exhaust valve at the top of compression to keep the compressed air from acting like a spring and forcing the crankshaft back down, so the energy bleed happens just from the initial compression of the air intake in the first place.
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That's correct. A compression brake (Jake Brake being a manufacturer/brand name of compression brakes) is generally a device that sits on top of the valve train. On engines with EUI in injectors (electronic injectors that have their own dedicated camshaft lobe, push rod and/or rocker arm for injection), there is a master piston sitting on top of the injector rocker and a slave piston sitting on top of the exhaust valve of the same cylinder. Since you want the exhaust valve to open near the top of the compression stroke, you can use the injector can lobe to achieve this, as the injector plunger will be beginning movement for a possible injection event. On engines without EUI injectors (PLN, common rail, or HEUI systems), the master piston sits above an intake rocker of a different cylinder - a cylinder that will be beginning its intake stroke at around the same time the cylinder in question is nearing the top of compression.
When an engine braking event is requested by the ECM, a solenoid valve is energized, allowing engine oil pressure to fill the cavity between the slave and master pistons through a check valve, pushing the master piston down against its rocker arm. When the rocker arm is pushed by the camshaft, the oil in this cavity between the two pistons is locked in by the check valve, and a hydraulic link is created which pushes the slave piston and its exhaust valve(s) down. This pressure can reach up to 5000psi during high RPM/high boost pressure braking events. When the ECM wants to stop the braking event, it de-energizes the solenoid valve which blocks the flow of oil pressure to the cavity between the pistons, and vents this cavity to sump so the pistons can return to their normal positions.
The Cummins ISX engine actually has a dedicated camshaft lobe for the compression brake, consisting of two bumps: one that opens the exhaust valve near the top of the compression stroke, and another that opens the same valve at the bottom of the intake stroke. This allows the blast of released compression from one cylinder to scavenge to another cylinder through the exhaust manifold, thus charging the cylinder with 40% more air than normal, increasing braking and reducing noise levels substantially.