Variable Valve Control (VVC)
Variable valve timing is used on the V8 engine to enhance low and high speed engine performance and idle speed quality.
For each inlet camshaft the VVC system comprises:
- VVC unit
- Valve timing solenoid
The VVC system alters the phase of the intake valves relative to the fixed timing of the exhaust valves, to alter:
- The mass of air flow to the cylinders.
- The engine torque response.
- Emissions.
The VVC unit uses a vane type device to control the camshaft angle. The system operates over a range of 48 degrees and is advanced or retarded to its optimum position within this range.
The VVC system is controlled by the ECM based on engine load and speed along with engine oil temperature to calculate the appropriate camshaft position.
The VVC system provides the following advantages:
- Reduced engine emissions and improved fuel consumption which in turn improves the engines internal EGR effect over a wider operating range.
- Enhanced full load torque characteristics.
- Improved fuel economy through optimized torque over the engine speed range.
The VVC unit is a hydraulic actuator mounted on the end of the inlet camshaft. The unit advances or retards the camshaft timing to alter the camshaft to crankshaft phase. The ECM controls the VVC timing unit via a oil control solenoid. The oil control solenoid routes oil pressure to the advance or retard chambers either side of the vanes within the VVC unit.
The VVC unit is driven by the primary drive chain and rotates relative to the exhaust camshaft. When the ECM requests a retard in camshaft timing the oil control solenoid is energized which moves the shuttle valve in the solenoid to the relevant position allowing oil pressure to flow out of the advance chambers in the VVC unit whilst simultaneously allowing oil pressure into the retard chambers.
The ECM controls the advancing and retarding of the VVC unit based on engine load and speed. The ECM sends an energize signal to the oil control solenoid until the desired VVC position is achieved. When the desired VVC position is reached, the energizing signal is reduced to hold the oil control solenoid position and consequently desired VVC position. This function is under closed loop control and the ECM can sense any variance in shuttle valve oil pressure via the camshaft position sensor and can adjust the energizing signal to maintain the shuttle valve hold position.
VVC operation can be affected by engine oil temperature and properties. At very low oil temperatures the movement of the VVC mechanism will be slow due to the high viscosity of the oil. While at high oil temperatures the low oil viscosity may impair the VVC operation at low oil pressures. The oil pump has the capacity to cope with these variations in oil pressure while an oil temperature sensor is monitored by the ECM to provide oil temperature feedback. At extremely high oil temperatures the ECM may limit the amount of VVC advance in order to prevent the engine from stalling when returning to idle speed.
VVC does not operate when engine oil pressure is below 1.25 bar. This is because there is insufficient pressure to release the VVC units internal stopper pin. This occurs when the engine is shut down and the VVC unit has returned to the retarded position. The stopper pin locks the VVC unit to the camshaft to ensure camshaft stability during the next start up.
Valve Timing Solenoid
The valve timing solenoid controls the position of the shuttle valve in the bush carrier. A plunger on the solenoid extends when the solenoid is energized and retracts when the solenoid is de-energized.
When the valve timing solenoids are de-energized, the coil springs in the bush carriers position the shuttle valves to connect the valve timing units to drain. In the valve timing units, the return springs hold the ring pistons and gears in the retarded position. When the valve timing solenoids are energized by the ECM, the solenoid plungers position the shuttle valves to direct engine oil to the valve timing units. In the valve timing units, the oil pressure overcomes the force of the return springs and moves the gears and ring pistons to the advanced position. System response times are 1.0 second maximum for advancing and 0.7 second maximum for retarding. While the valve timing is in the retarded mode, the ECM produces a periodic lubrication pulse. This momentarily energizes the valve timing solenoids to allow a spurt of oil into the valve timing units. The lubrication pulse occurs once every 5 minutes.