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| Illustration 1 | g00996889 |
Flow of oil through the electrohydraulic system (typical example) (1) Electrohydraulic actuator (throttle actuator) (2) Oil manifold (3) Oil filter (4) Oil tank (5) Oil pump (6) Oil pressure switch (7) Pressure relief valve | |
The electrohydraulic system has a self-contained supply of oil that is separate from the engine lubrication system.
Oil tank (4) is a reservoir for most of the oil. The tank sight gauge indicates the amount of oil in the tank. Oil pump (5) draws oil from the tank through tubing. The oil pump is driven by a gear in the front housing. The oil begins circulating when the engine is cranked. The oil continues to circulate during engine operation.
The oil is pumped upward through tubing and through oil filter (3) . If the oil filter gets plugged, the oil filter differential pressure rises. An indicator on the oil filter base indicates the need to change the filter.
The oil is pumped upward through tubing and through oil manifold (2) . The manifold has an oil pressure switch (6) that is normally open. When the hydraulic oil pressure rises to the switch rating during cranking, the switch closes. The Electronic Control Module (ECM) detects closure of the switch, which indicates that the hydraulic oil has achieved operating pressure.
If the oil pressure switch does not close, the engine will not start. If the hydraulic oil pressure becomes insufficient during operation, the oil pressure switch opens and the ECM activates an engine shutdown.
Pressure relief valve (7) is also installed in the oil manifold. The pressure relief valve is adjusted in order to maintain the correct hydraulic oil pressure at full load.
The oil flows through electrohydraulic actuator (1) . Hydraulic oil pressure is used to help move the electronically controlled throttle actuator. After the oil flows through the electrohydraulic actuator, the oil returns to oil tank (4) .
Electrohydraulic Actuator
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| Illustration 2 | g00997351 |
Flow of oil in the electrohydraulic actuator (1) Housing (2) Spring (3) Piston (4) Oil passage (5) Cage (6) Spool (7) Solenoid (8) Plunger | |
The electrohydraulic actuator housing (1) is always filled with oil. The force of hydraulic oil pressure and spring (2) press piston (3) to the fully extended position. This is the minimum fuel position.
The oil also fills oil passage (4) which has an opening on the side of the piston flange that is opposite from the housing main cavity. The pressure from the oil passage into the opening opposes the pressure in the cavity.
During engine operation, oil is constantly flowing through the actuator housing. The oil is allowed to flow through holes in the valve assembly stationary cage (5) and the oil flows around spool (6) . The oil also flows out of the housing.
Pulse Width Modulation (PWM) - This is a digital signal. The frequency is constant: the widths of the signal pulses are varied within a fixed interval (frequency). The variable widths are also called a duty cycle. The variable widths enable the signal to be interpreted as a range of values. This provides more accurate control than a signal that can only be ON or OFF.
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| Illustration 3 | g00998181 |
Examples of various PWM signals | |
During engine operation, the Electronic Control Module (ECM) detects the actual engine speed via a signal from the engine speed/timing sensor. The ECM compares the actual engine speed to the desired engine speed.
When the ECM determines that more fuel is needed, the ECM increases the width of the PWM signal to solenoid (7) . The solenoid moves plunger (8) in order to move spool (6) . When the spool is pushed, the spool diverts more oil from the main cavity to the housing outlet.
When more oil is draining from the main cavity, the force of the oil pressure from oil passage (4) becomes greater than the force of the oil pressure and the spring in the cavity. The greater force pushes the piston flange in order to retract the piston into the housing. As more oil is drained from the housing, the piston is pushed further into the housing. The fully retracted position is the maximum fuel position.
The piston is connected to the throttle plate with a linkage. For adjustment of the linkage, refer to the engine Specifications manual. Illustration 4 shows the relationship of the piston and the linkage.
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| Illustration 4 | g00998141 |
Positions of the piston (A) Minimum fuel position (B) Maximum fuel position | |
The ECM continuously modulates the PWM signal to the solenoid in order to match the actual engine speed to the desired engine speed.
There is no feedback for the throttle position. The ECM issues a throttle command that represents a percent of the position. The output can be viewed on the Caterpillar Electronic Technician (Cat® ET). The actual throttle position can be viewed on a mechanical pointer that is built into the mechanism of the throttle.