When the compression brake is activated, braking power is accomplished by opening the engine's exhaust valves. The exhaust valves are opened near the top of the compression stroke in order to release the highly compressed air into the exhaust system. The compression brake can only be activated when the engine is in the no-fuel position. Thus, combustion does not occur and no positive force is produced on the piston. The compressed air pressure that is released to the atmosphere prevents the energy from returning to the engine piston on the power stroke. The result is a loss of energy since the work that is done by the compression of the cylinder charge is not returned by the expansion process. This loss of energy is taken from the rear wheels. The rear wheels provide the braking action for the vehicle.
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| Illustration 1 | g00501265 |
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Compression brake (1) Control valve (2) Lead wire (3) Bolt assembly (4) Slave piston (5) Solenoid valve (6) Master piston (7) Stud and nut | |
The compression brake consists of three identical housing assemblies. A housing assembly is installed in each of the valve mechanism compartments above the rocker arms and above the rocker arm shaft. Each housing assembly is positioned over two cylinders. The housing assembly is mounted to the supports for the rocker arm shaft with studs and nuts (7). The housing assemblies are supported on the cylinder head with bolt assembly (3). The exhaust bridge assembly is used to transfer force from slave piston (4) to the exhaust valve. The brake logic signal for the compression brake is carried to solenoid valve (5) by lead wire (2). This is done in order to activate the compression brake on the two cylinders of the engine.
A spacer is used on the top of the valve cover base in order to provide space for the installation of the compression brake and of the valve cover. When the compression brake is installed, the height increases approximately
The control circuit for the compression brake permits the operation of either one, two, or all three of the compression brake housing assemblies. This provides progressive braking capabilities with the retarding effect of two cylinders, of four cylinders, or of all six cylinders in the engine.
Performance of the Compression Brake
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| Illustration 2 | g00427859 |
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Performance of the compression brake | |
The performance of the compression brake, which is shown in the graph, represents an engine with the three housing assemblies that are activated in order to stop the vehicle. The compression brake should not be activated when the engine rpm is above 2300. The maximum engine power rating is 2100 rpm. At this level, the amount of braking that is produced by the compression brake is approximately 315 to 325 horsepower.
Operation of the Compression Brake
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| Illustration 3 | g00501985 |
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Schematic for master-slave circuit (1) Rocker arm shaft oil passage (2) Solenoid valve (3) Lead wire (4) Spring (5) Control valve (6) High pressure oil passage (7) Slave piston adjustment screw (8) Master piston (9) Engine oil pump (10) Spring (11) Engine oil pan (12) Fuel injector rocker arm (13) Fuel injector pushrod (14) Slave piston (15) Oil drain passage (16) Low pressure oil passage (17) Ball check valve (18) Exhaust valve rocker arm (19) Exhaust valve (20) Exhaust bridge assembly | |
The compression brake is operated by engine oil that is distributed around the studs. This oil is supplied through the supports for the rocker arm shaft. Solenoid valve (2) controls the oil flow in the compression brake housing.
When the solenoid is activated by a signal from the logic for the compression brake, solenoid valve (2) moves downward. This causes oil drain passage (15) to engine oil pan (11) to be closed. At the same time, low pressure oil passage (16) to control valve (5) is opened. As low pressure oil passage (16) is filled with engine oil, the control valve is pushed upward in the chamber against the force of spring (4). At this position, a groove in control valve (5) is in alignment with high pressure oil passage (6). Slave piston (14) and master piston (8) are supplied by the high pressure oil passage. Engine oil pressure will now lift ball check valve (17). High pressure oil passage (6) and the chambers behind the slave pistons and behind the master pistons will be filled with engine oil pressure. This pressure moves the master piston downward until contact is made with fuel injector rocker arm (12). When upward motion is initiated on the master piston, the pressure increases above the current level of the engine supply pressure. This causes ball check valve (17) to seat. The system is now operating in conjunction with the exhaust valve and with the injector rocker mechanism. When the solenoid is activated, the compression brake could be operable within 1/5 of a second.
Fuel injector pushrod (13) will begin to move upward on the pumping stroke of the electronically controlled unit injector. When this occurs, fuel injector rocker arm (12) makes contact with extending master piston (8). As the master piston begins to move upward, the oil pressure increases in high pressure oil passage (6). This happens because ball check valve (17) will not allow oil to exit. The upward movement of the fuel injector rocker arm creates a constant increase in pressure. This forces the slave piston downward against the screw assembly in exhaust bridge assembly (20). The slave piston moves downward with enough force to open exhaust valve (19).
Note: Only one of the two exhaust valves for each cylinder on this engine is used in the operation of the compression brake.
This master-slave circuit is designed to move the master piston (8). The master piston is moved when the engine is on the compression stroke. The master-slave circuit is designed so that slave piston (14) opens one exhaust valve of the same cylinder only on the compression stroke. This occurs slightly before the piston reaches the top center position. The braking force is constant. The operation of the compression brake of a cylinder is caused by the motion of the valve mechanism of that cylinder. This causes the firing sequence of the valves to be identical to the firing order of the engine.
When solenoid valve (2) is in the OFF position, the engine oil supply passage is closed and oil drain passage (15) is opened. This allows oil to drain from underneath control valve (5). This also allows spring (4) to push the control valve to the bottom of the chamber. This position allows oil from high pressure oil passage (6) to drain into the chamber above the control valve's piston. This chamber vents to the outside of the compression brake housing. Spring (10) now moves master piston (8) to the retracted position away from fuel injector rocker arm (12). 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 until solenoid (2) is activated again.