3406E and 3456 Engines for Caterpillar Built Machines Caterpillar

• The compression brake reduces the vehicle speed on grades and on curves.

• The compression brake will help control the vehicle better on grades and on curves.

• The compression brake will give the operator more control when a reduction in speed is necessary.

• The compression brake will give the operator more control when long applications of the service brakes are not desired.

• Use of the compression brake will increase brake life of the service brakes.

The crankshaft of the engine is driven by the wheels of the vehicle during deceleration. The crankshaft of the engine is also driven by the wheels of the vehicle during downhill operation. When the crankshaft is being driven by power from the wheels a compression brake can be used to help slow the vehicle.

The engine uses energy from the crankshaft during the compression stroke of the engine. When the compression brake is activated, the braking power is accomplished by opening the exhaust valves near the top of the compression stroke. This releases the highly compressed air and energy into the exhaust system. No fuel is injected while the compression brake is activated. No combustion takes place because no fuel is injected in the cylinder. Consequently, the engine does not have a typical “power” stroke. The result is loss of energy, which provides the braking action for the vehicle.

Illustration 1	g01021887
Typical example (1) Brake housing assembly (2) Bolts (3) Slave piston (4) Solenoid valve (5) Exhaust valve bridges (6) Stud, spacer and nut

The compression brake consists of three identical housing assemblies (1). A brake housing is installed in each of the valve mechanism compartments. The brake is located above the rocker arms and the rocker arm shaft. Each housing assembly is positioned over two cylinders. The housing is mounted to the support for the rocker arm shaft with bolts (2). The housing is supported on the cylinder head with one stud, one spacer, and one nut (6). The exhaust rocker arm is used to transmit force from slave piston (3) to the exhaust valve. A lead wire carries the signal of the compression brake to solenoid valve (4). Each lead wire activates the compression brake on two cylinders of the engine.

Note: Only the valve mechanism that is for the exhaust is used in the operation of the compression brake.

Note: The slave piston pushes on the rocker arm for the exhaust valves. The rocker arm depresses BOTH exhaust valves of each cylinder during the operation of the compression brake.

The control circuit for the compression brake provides progressive braking capability. The control circuit for the compression brake permits the operation of one, two, or all three of the compression brakes. Therefore, the compression brake can be activated on two cylinders, four cylinders, or six cylinders of the engine.

Compression Brake Operation

Illustration 2	g01023115
Master-slave circuit schematic (1) Ground wire (2) Solenoid valve (3) Lead wire from the ECM (4) Lead wire to the sensor for the ECM (5) Spring (6) Control valve (7) High pressure oil passage (8) Slave piston adjustment screw (9) Slave piston (10) Master piston (11) Oil passage in the rocker arm shaft (12) Engine oil pump (13) Spring (14) Rocker arm for the electronic unit injector (15) Engine oil pan (16) Oil drain passage (17) Low pressure oil passage (18) Ball check valve (19) Exhaust valve bridge (20) Exhaust valves (21) Rocker arm for the exhaust valve (22) Camshaft

The compression brake operates on engine oil. The engine oil is supplied around the bolts through the rocker arm shafts. Solenoid valve (2) controls the oil flow in the compression brake housing assembly.

The solenoid is activated by a signal from the ECM. Solenoid valve (2) moves down when the compression brake is activated. Solenoid valve (2) closes oil drain passage (16) to engine oil pan (15). Solenoid valve (2) also opens low pressure oil passage (17) to control valve (6). As low pressure oil passage (17) is filled with engine oil, the control valve is pushed up in the chamber against the force of spring (5). In the up position, a groove in control valve (6) is in alignment with high pressure oil passage (7) that supplies slave piston (9) and master piston (10). The engine oil pressure will now lift ball check valve (18). High pressure oil passage (7) is filled with the engine oil from passage (17). The chambers behind the slave piston and the master piston are also filled. This pressure moves the master piston downward until contact is made with electronic unit injector rocker arm (14). Ball check valve (18) is seated as the upward motion is initiated on the master piston. Ball check valve (18) is closed by an increase in pressure from the movement of the master cylinder. The system is now in operation. When the solenoid is activated, the compression brake is ready to operate in approximately 1/5 of a second.

When camshaft (22) begins to move up on pumping stroke of the electronically controlled unit injector, electronic unit injector rocker arm (14) makes contact with extending master piston (10). As the master piston begins to move up, the oil pressure increases in high pressure oil passage (7). The pressure increases because ball check valve (18) will not allow the oil to escape. The slave piston is forced down against the exhaust rocker arm because of a constant increase in pressure on the upward movement of the electronic unit injector rocker arm. The force is great enough to open exhaust valves (20).

Master piston (10) is moved only when the engine cylinder is on the compression stroke. Slave piston (9) opens the two exhaust valves of the same cylinder only on the compression stroke. The exhaust valves are opened immediately before the top center of the compression stroke. The braking force is constant. The operation of the compression brake is caused by the motion of the valve mechanism for the same cylinder. The sequence for the operation of the compression brake is the same sequence as the firing order of the engine.

When solenoid valve (2) is in the off position, the engine oil supply passage is closed, and the oil drain passage (16) is opened. The oil will drain from control valve (6). Spring (5) will then push the control valve to the bottom of the chamber. The oil from high pressure oil passage (7) will now drain into the chamber that is above the piston for the control valve. Spring (13) now moves master piston (10) up to the retracted position. The master piston will no longer be in contact with electronic unit injector rocker arm (14). The time that is necessary for the system to stop the operation is approximately 1/10 of a second. The compression brake will not be able to operate now until solenoid valve (2) is activated again by the ECM.

Slave piston adjustment screw (8) is used to set the clearance (lash) between the compression brake and the rocker arm assemblies. Refer to Testing and Adjusting, "Compression Brake" for information on the adjustment of the compression brake. The correct adjustment will optimize the brake performance without overstressing the engine components.