Variable Valve Timing Actuator: Description and Operation
VARIABLE CAM TIMING SYSTEMOverview
Variable Cam Timing (VCT) enables rotation of the camshaft(s) relative to the crankshaft (phase-shafting) as a function of engine operating conditions. There are four types of VCT systems.
- Exhaust Phase Shifting (EPS) system - the exhaust cam is the active cam being retarded.
- Intake Phase Shifting (IPS) system - the intake cam is the active cam being advanced.
- Dual Equal Phase Shifting (DEPS) system - both intake and exhaust cams are phase shifted and equally advanced or retarded.
- Dual Independent Phase Shifting (DIPS) system - where both the intake and exhaust cams are shifted independently.
All systems have four operational modes; idle, part throttle, wide open throttle and default mode. At idle and low engine speeds with closed throttle, the phase angle are controlled by air flow, engine oil temperature and engine coolant temperature. At part and wide open throttle the PCM controls cam timing based on engine RPM, load and throttle position. VCT systems provide reduced emissions and enhanced engine power, fuel economy and idle quality. IPS systems also have the added benefit of improve torque. In addition, on some applications a VCT system can eliminated the need for an external Exhaust Gas Recirculation (EGR) system. The elimination of the EGR system is accomplished by controlling the overlap in valve opening between the intake valve opening and exhaust valve closing.
Currently for the 2004 model year, Ford Motor Company uses the PS and DEPS systems. The IPS system is on Lincoln LS, Thunderbird and Focus SVT and the DEPS system is on the F150 5.4L 3V.
Variable Cam Timing
Variable Cam Timing System:
The VCT (variable cam timing) system consists of an electric hydraulic positioning control solenoid, a CMP (camshaft position sensor) and trigger wheel. The CMP trigger wheel has a number of equally spaced teeth equal to the number (n) of cylinders on a bank plus one extra tooth (n+1). Four cylinder and V8 engines use a CMP 4+1 tooth trigger wheel. V6 engines use a CMP 3+1 tooth trigger wheel. The extra tooth placed between the equally spaced teeth represents the CMP signal for that bank. A CKP (crankshaft position sensor) provides the PCM with crankshaft positioning information in 10 degree increments (Figure 115).
1. The PCM receives input signals from the IAT (intake air temperature), ECT (engine coolant temperature), EOT (engine oil temperature), CMP, TP (throttle position), MAF (mass air flow) and CKP to determine the operating conditions of the engine. At idle (low engine speeds and closed throttle) the PCM controls camshaft position based on air and coolant temperatures. During part and wide open throttle, camshaft position is determined by engine RPM, load and throttle position. The VCT system will not operate until the engine is at normal operating temperature.
2. The VCT system is enabled by the PCM when the proper conditions are met.
3. The CKP signal is used as a reference for CMP positioning.
4. The VCT solenoid valve is an integral part of the VCT system. The solenoid valve controls the flow of engine oil in the VCT actuator assembly. As the PCM controls the duty cycle of the solenoid valve, oil pressure/flow advances or retards the cam timing. Duty cycles near 0% or 100% represent rapid movement of the camshaft. Retaining a fixed camshaft position is accomplished by dithering (oscillating) the solenoid valve duty cycle.
The PCM calculates and determines the desired camshaft position. It will continually update the VCT solenoid duty cycle until desired positioning is achieved. A difference between the desired and actual camshaft position represents a position error in the PCM's VCT control loop. The PCM will disable the VCT and place the camshaft in a default position if a fault is detected. A related DTC will also be set when this fault is detected.
5. When the VCT solenoid is energized, engine oil is allowed to flow to the VCT actuator assembly which advances or retards the cam timing. One half of the VCT actuator is coupled to the camshaft and the other half is connected to the timing chain. Oil chambers between the two halves couple the camshaft to the timing chain. When the flow of oil is shifted from one side of the chamber to the other, the differential change in oil pressure forces the camshaft to rotate in either a advance or retard position depending on the oil flow.