During downhill operation or any slow down condition, the engine crankshaft is turned by the rear wheels through the following components:
- Differential
- Drive shaft
- Transmission
- Clutch
To reduce the speed of the vehicle, an application of a braking force can be made to the pistons of the engine.
As the engine compression brake is activated, the braking power occurs by opening the exhaust valves near the top of the compression stroke. This releases highly compressed air and energy into the exhaust system. Therefore, no combustion will occur because the engine brake only works under one condition:
- The engine is in a "No Fuel" mode.
Since the compressed air is released through the exhaust system during the power stroke, a positive power is not generated. The momentum of the vehicle returns the piston to the exhaust stroke. This is a two-step process:
- The cylinder vents the compressed air.
- The piston relies on the momentum of the vehicle for movement.
This two-step process creates the retarding effect for the engine brake. This results in a loss of energy because the work that is done by the compression of the cylinder charge is not returned by the expansion process. This loss of energy comes from the rear wheels which provides the braking action for the vehicle.
Components of the Compression Brake
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| Illustration 1 | g00318128 |
Installed Compression Brake (1) Rear housing. (2) Front housing. (3) Stud. (4) Support bracket. | |
The compression brake consists of two different housings. Both brake housings are installed in the compartment for the valve mechanism above the rocker arms and the rocker arm shaft. Each brake housing is positioned over three cylinders. Each brake housing is mounted on a support bracket (4) at the end of the rocker shaft brackets. Special adjustment screws for the exhaust rocker arm and exhaust valve bridges are necessary.
Note: The operation of the compression brakes only uses the exhaust side of the engine valve mechanism.
A spacer is used between the cylinder head and the valve cover for the installation of the valve cover. With an installed compression brake, this increase in height is 90 mm (3.5 inch).
The front compression brake housing and the rear compression brake housing consist of the following parts:
- Three master pistons
- Three slave pistons
- Three control valves
- One solenoid valve
Engine Compression Brake's Operation
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| Illustration 2 | g00322183 |
Master-Slave Circuit Schematic (1) Solenoid valve (2) Spring (3) Control valve (4) High pressure oil passage (5) Slave piston adjustment screw (6) Master piston (7) Spring (8) Rocker arm adjustment screw (9) Rocker arm (10) Slave piston (11) Exhaust pushrod (12) Exhaust valves (13) Ball check valve (14) Oil passage from manually operated control switch (15) Low pressure oil passage | |
The compression brake operates with engine oil. The engine oil is supplied from the rocker arm shafts. Solenoid valve (1) controls the oil flow in the brake housing.
When the solenoid is activated, solenoid valve (1) moves down. The downward movement of the solenoid valve closes the oil drain passage to the oil pan. At the same time, solenoid valve (1) opens low pressure oil passage (15) to three control valves (3). As low pressure passage (15) is filled with engine oil, control valves (3) are pushed up in the chamber against the force of spring (2). At this position, a groove in control valve (3) should align with high pressure oil passage (4). This oil passage supplies high pressure oil to slave piston (10) and master piston (6).
The engine oil pressure will now lift ball check valve (13). The engine oil pressure also fills high pressure oil passage (4) and the chambers behind the pistons (master-slave circuit). This pressure moves the pistons downward. Then, the pistons can contact the engine valve mechanism.
When the oil pressure is the same pressure through all the oil passages, the small spring will force ball check valve (13) against the seat of the ball check valve. The system is completely pressurized, and the system is ready for operation with the engine valve mechanism. When the solenoid is activated, the compression brake is ready to operate in approximately 1/5 of a second.
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| Illustration 3 | g00318164 |
Oil Passage Schematic (Front Housing) (1) Solenoid valve. (3) Control valves. (4) High pressure oil passage. (6) Master pistons. (10) Slave pistons. (15) Low pressure oil passage. | |
When engine pushrod (11) for the exhaust valve begins to move up on the normal cycle of the exhaust valve, the following two components can make contact with master piston (6) :
- Rocker arm (9)
- Rocker adjustment screw (8)
As master piston (6) begins to move up, the oil pressure increases in passage (4). The pressure increases because ball check valve (13) will not allow the oil to escape. Since there is a constant increase in pressure with the rocker arm movement, slave piston (10) is pushed down with enough force to open exhaust valves (12).
The master-slave circuit is designed so that master piston (6) is moved only by an engine cylinder on the exhaust stroke. At the same time, slave piston (10) opens only the exhaust valves of an engine cylinder on the compression stroke. This occurs just before the cylinder arrives at the top center position. The braking force is constant, and the sequence is the same sequence as the firing order of the engine. See Table 1.
| Relationship of the Master-Slave Circuit | |
| Location of Master Piston | Location of Slave Piston |
| Sequence for Activation | |
| No 1 Pushrod | No 3 Pushrod |
| No 5 Pushrod | No 6 Pushrod |
| No 3 Pushrod | No 2 Pushrod |
| No 6 Pushrod | No 4 Pushrod |
| No 2 Pushrod | No 1 Pushrod |
| No 4 Pushrod | No 5 Pushrod |
When solenoid valve (1) is in the OFF position, the engine oil supply passage is closed, and the oil drain passage to the oil pan is opened. This drains the oil beneath the control valve (3). Spring (2) pushes control valve (3) to the bottom of the chamber. This position allows the oil from high pressure oil passage (4) to drain into the chamber above the control piston.
Spring (7) now moves master piston (6) up to the NEUTRAL position of the master piston and away from rocker arm adjustment screw (8). The time that is necessary for the system to stop operation is approximately 1/10 of a second. The compression brake will not be able to operate now until the solenoid is activated again.
Controls for the Compression Brakes
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| Illustration 4 | g00727330 |
Schematic of the Control Circuit (1) Connection to solenoid valve (2) Compression brake control switch (3) Diode (4) Clutch switch (5) Circuit breaker (6) Fuel pump switch (7) Battery | |
The compression brake is activated electrically with three different switches that are connected in a series in the circuit. A manually operated control switch (2) is located on the dash of the vehicle. This is a three-position switch, and this switch allows three braking options:
- 100 %
- 50 %
- 0 % or no retardation (braking force)
Fuel pump switch (6) permits the operation of the compression brake only when the throttle is at the IDLE position. Any increase in fuel (more throttle) will stop the current flow, and the compression brake will not operate.
The next switch in series is clutch switch (4). Clutch switch (4) is set to permit brake operation only when the clutch is engaged. This prevents the engine from stalling by the compression brake when the drive line is not engaged with the engine.
Fuel pump switch (6) and clutch switch (4) work automatically after operator control switch (2) is manually positioned. This control circuit permits any one of the three switches to prevent operation of the brake, but any operation of the compression brakes requires all three of the switches to be closed before operation can begin.
A small diode (3) is connected between the power source and the ground of the terminal. This protects the switch contacts from arcing.