Jacob Brake Info

How a compression brake works: 

When the driver releases the accelerator on a moving vehicle powered by a diesel engine,

the vehicle's forward momentum continues to turn the engine's crankshaft, drawing air

into the cylinders as the pistons move down and compressing that air as the pistons move

back up. The pressure of the compressed air pushes back on the up-going piston, tending
 
to slow the vehicle.

 But, without a compression release mechanism, as the piston passes through top dead

center and heads back down again, the compressed air in the cylinder acts as a spring

and pushes the piston down, returning most of the work done in compression back to

the crankshaft, creating very little braking effect. The engine turns freely and the vehicle

coasts. 

When a compression release engine brake is active, a valve releases the pressure from

the cylinder before the piston starts back down, so the slowing effect is present on the up

stroke, but no accelerating effect is present on the down stroke and the net effect is the

vehicle slows down. 

With a gasoline engine, the mechanics are different and a special valve is not necessary

for engine braking to happen when the driver releases the accelerator. In the gasoline

engine, with the accelerator released, a throttle prevents the free flow of air into the

cylinders, so there is little pressure to release at the top of the compression stroke. The

throttle itself provides engine braking through friction in the air flowing through it. But a

diesel engine does not have a butterfly valve to limit air on the intake side. 

 

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 as described above. 

 

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 driver

has turned on the compression release engine brake, it will activate when the driver

releases the accelerator. There are also switches on the clutch and accelerator pedals that

deactivate the compression brake when the driver disengages the clutch or presses the

accelerator.

 

Darren Williams