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.
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.
