Volvo D12 - Page 17

Group 25 Intake and Exhaust System Design and Function
Rocker Arm Plunger
The purpose of the rocker arm plunger is to eliminate all
valve clearance during the compression braking.
Engine Operation
When the engine is operating (compression brake not
activated), there is reduced oil pressure — approx. 100
kPa (14.5 psi) — via the control valve to the rocker arm
shaft and the rocker arm non-return valve (1) is open. Oil
can flow freely through the non-return valve in both di-
rections. As a result, no oil pressure is built up between
the rocker arm plunger (2) and the rocker arm.
The set valve clearance is great enough to prevent the
camshaft induction and decompression lobes from open-
ing the exhaust valves.
The valve mechanism operates the same as on an en-
gine without a compression brake; in other words, only
the exhaust lobe opens the exhaust valves.
T2006828
Fig. 8: Rocker Arm Plunger
1 Non-return valve
2 Rocker arm plunger
Compression Braking
During compression braking, the control valve does not
reduce the oil pressure, so an oil pressure of at least
200 kPa (29 psi) is delivered to the rocker arm shaft.
The pressure in the rocker arm non-return valve (1) be-
comes so great that the plunger in the non-return valve
is moved out of its rest position, and the ball now func-
tions as a non-return valve. Pressure is built up between
the rocker arm plunger (2) and the rocker arm. The
plunger is pressed out and presses the rocker arm roller
against the lobes on the camshaft. In this way, the valve
clearance is eliminated and the lifting height on the in-
duction and decompression lobes is sufficient to open
the exhaust valves.
The rocker arm plunger is fitted with a pressure limiting
valve (3). When the oil pressure between the rocker arm
plunger and the rocker arm becomes too great, the pres-
sure limiting valve opens and oil can exit through the
hole in the bottom of the plunger. The opening pressure
of the pressure limiting valve is governed by the force of
the valve spring.
T2006836
Fig. 9: Rocker Arm Plunger
1 Non-return valve
2 Rocker arm plunger
3 Pressure limiting valve
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