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Function
When the accelerator is released on moving vehicle, its forward momentum continues to turn the Diesel engine's crankshaft and compress air in the cylinders. As the piston passes through top dead center, the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning most of the energy expended in compression back to the crankshaft. Therefore the engine does not effectively aid in slowing the vehicle.
In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum, the balance coming from internal friction of the engine itself. Diesel engines, however, do not throttle air intake and hence do not provide engine braking from throttling losses.
A compression release engine brake uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. The stem of this valve telescopes during normal operation so the valve remains closed, but is locked at full length by a solenoid when the engine brake is engaged so that the valve opens as directed by the cam. This releases the compressed air in the cylinder, thus preventing it from returning its energy back to the piston. Accordingly, engine drag increases and the vehicle speed is reduced.
The driver controls consist of an on/off switch and, sometimes, a multi-position switch that controls the number of cylinders on which the brake is active. When the compression release engine brake is turned on, it will activate when the driver releases the accelerator. There are also switches on the clutch and accelerator pedals that will deactivate the compression brake when the clutch is disengaged or the accelerator is pressed.
Legislation
The use of compression release engine brakes may cause a vehicle to make a loud chattering or "machine gun" exhaust noise, especially vehicles having high flow mufflers, or no mufflers at all, causing many communities in the United States to prohibit compression braking within municipal limits. Drivers are notified by roadside signs with legends such as "Brake Retarders Prohibited," "Engine Braking Restricted," "Jake Brakes Prohibited," or "Compression Braking Prohibited," and enforcement is typically through traffic fines. Such prohibitions have led to the development of new types of mufflers and turbochargers to better silence compression braking noise.
See also
Engine braking
compression release
Exhaust brake
Retarder (mechanical engineering)
External links
Jacobs Vehicle Systems
Pacbrake Engine & Exhaust Brakes
Banks Power exhaust brake page
Categories: Vehicle braking technologiesHidden categories: Articles lacking sources from September 2009 | All articles lacking sources



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