Variable Cam Timing: Description

There are four possible types of Variable Cam Timing (VCT) systems. The 2003 ZX2 are Exhaust Phase Shifting (EPS) system. The exhaust cam is the active cam and is being retarded. The 2003 LS, Thunderbird and Focus SVT vehicles have Intake Phase Shifting (IPS) system. A intake phase shifting system will move the intake cam in the advance direction. The other two possible systems are Dual Equal Phase Shifting (DEPS) both intake and exhaust cams are phase shifted equally as well as Dual Independent Phase Shifting (DIPS) where the cams are shifted independently. The systems have three operational modes; idle, part throttle, wide open throttle and a default mode. At idle and (low engine speeds with closed throttle) the phase angle is controlled by air flow 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 enhance engine power, fuel economy and idle quality. IPS systems have the added benefit of improve torque. In addition a VCT system will eliminate the need for an external Exhaust Gas Recirculation (EGR) system. The elimination of EGR system is accomplished by controlling the overlap in valve opening between the intake valve opening and exhaust valve closing. Increased vehicle reliability is achieved with the elimination of the EGR system.

The VCT system consists of an electric hydraulic positioning control solenoid, a CMP 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 provides the PCM with crankshaft positioning information in 10 degree increments.

The following list of components and their specific operation corresponds to numbers in illustration. See Fig 1.

1. The PCM receives input signals from the IAT, ECT, CMP, TP, MAF 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. VCT system is enabled by PCM when the proper conditions are met. PCM disables VCT system if a fault is detected.
3. The CKP signal is used as a reference for CMP positioning.
4. 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.
5. Oil flows to either side of the piston chamber in the VCT assembly, which changes the linear piston motion to a rotational motion that advances or retards the camshaft.