The crankshaft of the engine is rotated by the following components: Clutch, Transmission, Drive shaft and Differential. A braking torque can be applied to the drive train of the vehicle in order to reduce the speed of the vehicle.
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 Cat 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 | g01113757 |
(1) Solenoid valve (2) Valve connector (3) Master cylinder (4) Slave cylinder | |
The compression brake consists of three identical housing assemblies. 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. The rocker arm and the exhaust bridge assembly is used to transfer force from slave piston (3) to the exhaust valves. The brake logic signal for the Cat compression brake is carried to solenoid valve (1) by the lead wire. This is done in order to activate the Cat compression brake on the two cylinders of the engine.
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.
Operation of the Compression Brake
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| Illustration 2 | g01308841 |
Schematic for master-slave circuit (1) Check valve (2) High pressure oil passage (3) Slave piston adjustment screw (4) Master piston (5) Actuator valve (6) Oil drain passage (7) Slave piston (8) Master piston spring (9) Spring for the slave piston (10) Exhaust rocker arm (11) Exhaust bridge (12) Fuel injector rocker arm (13) Rocker arm shaft oil passage (14) Engine oil pump (15) Engine oil pan (16) Exhaust valve (A) Actuation port (T1) Drain port (T2) Drain port (P) Actuator spool for supply port | |
The compression brake is operated by engine oil from engine oil pan (15). The engine oil is pressurized by engine oil pump (14). The engine oil supply for the compression brake is supplied through rocker arm shaft oil passage (13). Actuator valve (5) controls the flow of the supply oil in the compression brake housing.
When the actuator valve is activated by a signal from the ECM, low pressure oil passes from the actuator spool supply port (P) to actuation port (A). The oil flow opens check valve (1) and flows into high pressure oil passage (2). Oil is supplied to slave piston (7) and master piston (4) .
This engine oil pressure overcomes master piston spring (8). The piston is forced downward toward fuel injector rocker arm (12). Oil fills the cylinder for the master piston and the high pressure oil passage between the master piston and the slave piston. The master piston follows the movement of the fuel injector rocker arm. The master piston moves upward with the fuel injector rocker arm. The movement of the master piston causes a flow of high pressure oil that closes the circuit's check valve. The closed check valve causes pressure to increase in the hydraulic circuit of the master piston, the slave piston, and the high pressure oil passage.
As the oil pressure in the high pressure oil passage is increased by the movement of the master piston, the slave piston is forced downward in the bore. This compresses spring (9) for the slave piston. As the slave piston moves downward, the slave piston makes contact with exhaust valve rocker arm (10). The slave piston continues to apply a force to the exhaust valve rocker arm. This force causes exhaust valves (16) to open. As the exhaust valves open, the cylinder pressure that is created by the engine's piston during the compression stroke is relieved through the open exhaust valves.
During compression brake operation, the ECM disables the signal to each of the fuel injectors that are selected for braking. The power stroke of the engine will not occur for the selected cylinders during the application of the compression brake. This creates a net braking force at the flywheel.
As the fuel injector rocker arm moves downward, the master piston is allowed to retract into the bore. As the master piston retracts, the hydraulic pressure in the circuit is decreased. This allows the slave piston to retract and control of the exhaust valves is returned to the engine's valve train.
De-energizing the actuator valve allows the oil to drain from port (T1) and (T2) to the engine oil pan through oil drain passage (6) .