8
INSTALLATION MANUAL FOR 490A
Section 6: Brake Maintenance
Theory of Operation
Energizing the engine brake effectively converts
a power-producing diesel engine into a pow-
er-absorbing air compressor. This is accomplished
through motion transfer using a master/slave
piston arrangement which opens cylinder exhaust
valves near the top of the normal compression
stroke, releasing the compressed cylinder charge
to exhaust.
The blowdown of compressed air to atmospher-
ic pressure prevents the return of energy to the
engine piston on the expansion stroke. The effect
is a net energy loss, since the work done in com-
pressing the cylinder charge is not returned during
the expansion process.
Exhaust Blowdown
Referring to Fig. 11, exhaust blowdown occurs as
follows:
1. The energized solenoid valve permits engine
lube oil to flow under pressure through the
control valve to both the master piston and the
slave piston.
2. Oil pressure causes the master piston to move
down, coming to rest on the exhaust rocker
arm adjusting screw.
3. The exhaust rocker arm adjusting screw begins
upward travel (as in normal exhaust cycle),
forcing the master piston upward and direct-
ing high pressure oil to the slave piston. The
ball check valve in the control valve imprisons
high-pressure oil in the master/slave piston
system.
4. The slave piston, under the influence of the
high-pressure oil moves down, momentarily
opens one exhaust valve while the engine
piston is near its top dead-center position, re-
leasing compressed cylinder air to the exhaust
manifold.
5. Compressed air escapes out to the atmo-
sphere, completing a compression braking
cycle.
FIG. 11
OIL IN
SOLENOID
VALVE
CONTROL
VALVE
ADJUSTING
SCREW
HIGH
PRESSURE
OIL
MASTER
PISTON
EXHAUST
PUSHROD
SLAVE
PISTON
BALL
CHECK
VALVE
EXHAUST VALVES
EXHAUST
ROCKER
LEVER
GUIDELESS
CROSSHEAD
LOW
PRESSURE
OIL
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