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. This conversion is made possible by a master piston to slave piston arrangement. Movement of the rocker arm from one exhaust valve is transferred hydraulically to the exhaust valve of another cylinder. This opens the exhaust valves of the cylinder with the compressed air. The valves are opened near the top of that cylinder's normal compression stroke cycle. The compressed cylinder charge is now released into the exhaust manifold.
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.
Components of the Compression Brake (Model 346D)
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| Illustration 1 | g00534221 |
Installed Compression Brake (1) Rear housing (2) Front housing (3) Stud (4) Support bracket | |
The compression brake consists of two different housings. One housing is for the first three cylinders. The other housing is for the last three cylinders. The housings are mounted on support brackets (4) and on two studs (3) that are at the end of the rocker shaft brackets. Special exhaust rocker arm adjusting screws and exhaust valve bridges are necessary. Later engines have only one support bracket (4) for each housing.
Note: Only the exhaust valve mechanisms are used in the operation of the compression brake.
A spacer is used on top of the valve cover base to permit installation of the valve cover. The compression brake increases the height of the engine. The height increase is less than 50.8 mm (2.0 inch).
Both the front and rear compression brake housings consist of the following parts:
- three master pistons
- three slave pistons
- three control valves
- one solenoid valve
Compression Brake Operation (Model 346D)
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| Illustration 2 | g00534226 |
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) Rocker arm shaft oil passage (8) Engine oil pump (9) Ball check valve (10) Slave piston (11) Rocker arm (12) Spring (13) Rocker arm adjustment screw (14) Oil drain passage (15) Low pressure oil passage (16) Exhaust valve bridge (17) Push rod for the exhaust (18) Engine oil pan (19) Exhaust valves | |
The compression brake operates with engine oil which is supplied from the rocker arm shafts. Solenoid valve (1) controls the oil flow in the housing.
When the solenoid is activated, the solenoid valve (1) moves down. The solenoid valve closes the oil drain passage (14) to the oil pan (18). At the same time, the solenoid valve opens the low pressure oil passage (15) to the three control valves (3). As the low pressure passage (15) is filled with engine oil, the control valves (3) are pushed up in the chambers against the force of spring (2). At this position, a groove in control valve (3) is in alignment with high pressure oil passage (4) that supplies slave piston (10) and master piston (6). Engine oil pressure will lift the ball check valve (9). This fills the high pressure oil passage (4) and the chambers behind the slave pistons and master pistons. The oil pressure causes the master piston to move down. The master piston comes to rest on the corresponding exhaust rocker arm adjustment screw.
When the push rod (17) for the exhaust valves begins to move up on the normal exhaust stroke, the rocker arm (11) and the adjustment screw (13) move the master piston (6) upward. As the master piston (6) begins to move upward, the oil pressure increases in passage (4). This causes the ball check valve (9) to close. Since there is a constant increase in pressure with the rocker arm movement, the slave piston (10) is forced down against the exhaust valve bridge (16) of a different cylinder with enough force to open exhaust valves (19). 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 for Front Housing (1) Solenoid valve (3) Control valves (4) High pressure oil passages (6) Master pistons (10) Slave pistons (15) Low pressure oil passage | |
This master-slave circuit is designed so that the master piston (6) is only moved by the exhaust rocker arm for a cylinder that is on the exhaust stroke. The slave piston (10) opens only the exhaust valves of a cylinder that is on the compression stroke. The valves are opened just before the piston reaches the top center position. The order is shown in the following table:
| Master Piston-Slave Piston Relationship | |
| Location of Master Piston | Activates Slave Piston For |
| No. 1 exhaust pushrod | No. 3 exhaust valve |
| No. 5 exhaust pushrod | No. 6 exhaust valve |
| No. 3 exhaust pushrod | No. 2 exhaust valve |
| No. 6 exhaust pushrod | No. 4 exhaust valve |
| No. 2 exhaust pushrod | No. 1 exhaust valve |
| No. 4 exhaust pushrod | No. 5 exhaust valve |
When solenoid valve (1) is in the off position, the engine oil supply passage is closed, and oil drain passage (14) to the oil pan is opened. This allows the oil that is under the control valve (3) to drain. Spring (2) pushes the control valve (3) to the bottom of the chamber. This position allows the oil from the high pressure oil passage (4) to drain into the chamber above the control piston. The chamber vents to the outside of the housing. Spring (12) now moves master piston (6) up to the master piston's neutral position that is away from rocker arm adjustment screw (13). 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.
Components of the Compression Brake (Model 349A)
The compression brake consists of two different housings. One housing is for the first three cylinders. The other housing is for the last three cylinders. The housings are mounted on a support bracket and on studs that are at the end of the rocker shaft brackets. Special exhaust rocker arm adjusting screws and exhaust valve bridges are necessary.
Note: Only the exhaust valve mechanisms are used in the operation of the compression brake.
A spacer is used on top of the valve cover base to permit installation of the valve cover. The compression brake increases the height of the engine. The height increase is less than 50.8 mm (2.0 inch).
Both the front and rear compression brake housings consist of the following parts:
- three master pistons
- three slave pistons
- three control valves
- three trigger valves
- three charge valves
- three delay pistons
- one solenoid valve
Compression Brake Operation (Model 349A)
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| Illustration 4 | g00542785 |
Master-Slave Circuit Schematic (1) Solenoid valve (2) Plenum chamber (3) Charge valve (4) Control valve (5) Delay piston (6) Trigger valve (7) Master piston (8) Slave piston (9) Exhaust rocker arm for the No. 3 cylinder (10) Exhaust rocker arm for the No. 1 cylinder (11) Exhaust valve bridge for the No. 3 cylinder (12) Exhaust valve bridge for the No. 1 cylinder | |
The braking cycle is accomplished by utilizing the motion of an exhaust valve pushrod of another cylinder. For example, the No. 1 cylinder's exhaust pushrod opens the exhaust valves of the No. 3 cylinder.
When the solenoid (1) is energized, engine oil flows into the brake housing. The engine oil flows through the control valve (4) to the following components:
- master piston (7)
- slave piston (8)
- trigger valve (6)
- delay piston (5)
- charge valve (3)
The oil flows through the charge valve in order to fill the plenum (2). The oil pressure causes the master piston to move downward. The master piston comes to rest on the exhaust rocker arm (10) .
As the exhaust valve pushrod begins to open the cylinder's exhaust valves, the exhaust valve rocker arm begins to move the master piston upward. This creates a high pressure oil flow to the delay piston. This causes the delay piston to move and the delay piston compress the plenum oil to a high pressure. The delay piston and the plenum act as a high pressure spring in order to activate the slave piston at the appropriate time.
At the appropriate time, the master piston opens the trigger valve. This allows the high pressure oil to flow from the delay piston through the trigger valve to the slave piston. The slave piston moves downward and the slave piston contacts the exhaust valve bridge. This opens the exhaust valves. The compressed air that is inside the cylinder is released into the exhaust manifold.
When the solenoid is deactivated, the oil flow is stopped. The oil that is between the master piston and the slave piston is allowed to escape above the control valve. The plenum pressure is slowly bled off until the engine oil pressure is reached.
| Master Piston-Slave Piston Relationship | |
| Location of Master Piston | Activates Slave Piston For |
| No. 1 exhaust pushrod | No. 3 exhaust valve |
| No. 5 exhaust pushrod | No. 6 exhaust valve |
| No. 3 exhaust pushrod | No. 2 exhaust valve |
| No. 6 exhaust pushrod | No. 4 exhaust valve |
| No. 2 exhaust pushrod | No. 1 exhaust valve |
| No. 4 exhaust pushrod | No. 5 exhaust valve |
Compression Brake Operation Control
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| Illustration 5 | g00537001 |
Compression Brake Control Circuit (A) ECM (B) Compression Brake (1) Clutch switch (2) Brake switch (3) Cruise control on/off switch (4) Throttle position sensor (5) Compression brake enable line (6) Compression brake on/off switch (7) Compression brake low switch (8) Compresion brake high switch (9) Signal that is used to energize the front housing (10) Signal that is used to energize the rear housing | |
The compression brake is activated by a signal from the compression brake operation control in order to energize the solenoids in the compression brake housings. The compression brake operation control receives the input from the retarder enable and the compression brake switches that are located in the cab of the vehicle. The compression brake had the following switches:
- on/off
- high
- low
The status of the retarder enable is determined by the ECM with the information from the following sources:
- clutch switch
- brake switch
- cruise control on/off switch
- throttle position sensor
The retarder enable prevents the operation of the compression brake under improper engine operating conditions. The correct operating conditions are the following conditions:
- Engine speed is greater than 950 rpm.
- The clutch is engaged.
- no throttle
- Compression brake switch is ON.
The compression brake has three modes of operation. The modes are set according to the customer specified parameters. The modes are the following modes:
- Manual mode
- Coast mode
- Latch mode
In the manual mode, the compression brake is activated whenever the previous operating conditions are met.
In the coast mode, the compression brake operates under the following conditions:
- Engine speed is above 950 rpm.
- The clutch is engaged.
- No throttle
- Compression brake switch is ON.
- Service brakes are applied.
In the latch mode, the compression brake operates under the following conditions:
- Engine speed is above 950 rpm.
- The clutch is engaged.
- No throttle
- Compression brake switch is ON.
The compression brake comes on when the service brakes are applied. The compression brake will stay on even if the service brakes are released. The compression brake will shut off if one of the following conditions occur:
- Engine speed drops below 950 rpm.
- The clutch is disengaged.
- The throttle is turned on.
When the compression brake low switch is closed, the compression brake operation control sends a signal in order to energize the rear compression brake. When the compression brake high switch is closed, the compression brake operation control sends a signal in order to energize both the front and the rear compression brakes.