The system improves engine performance at low and high speeds, at idle, and also leads to a decrease in the content of harmful substances in exhaust gases.
The VVT system changes the intake valve timing relative to the fixed exhaust valve timing to change:
- the mass of air entering the engine cylinders;
- engine torque and the content of harmful substances in the exhaust gases.
The VVT mechanism uses a lamellar device that allows you to adjust the angular position of the camshaft (see section "Operation of the VVT system"). The maximum adjustment range is 48°, and the angular position of the camshaft can change in this range both in one direction and in the other.
The ECM controls the VVT system by calculating the optimum camshaft position based on the following signals: engine speed, load, engine shaft temperature, throttle position. For more information refer to Electronic Engine Controls (303-14B Electronic Engine Controls - 4.4L).
The VVT system has the following advantages:
- Reduced exhaust emissions and fuel savings by optimizing valve timing, which improves internal exhaust gas recirculation (EGR) over a wider operating range.
- Full load torque improvement by optimizing valve timing at any engine speed to improve volumetric efficiency.
- Reduced fuel consumption by optimizing torque over the entire engine speed range.
This system also has additional advantages over a non-VVT system, as it operates with lower oil pressure and provides faster response times.
Operation of the valve timing control system (VVT)
Item name | Spare part number | Description |
A | - | Lag |
B | - | Advance |
1 | - | Engine oil pressure |
The VVT mechanism is a hydraulic actuator mounted at the end of the intake camshaft and changes the angular position of the camshaft relative to the crankshaft. A control solenoid valve, controlled by the ECM, supplies pressurized oil to the advance or retard chambers located on either side of the three vanes inside the mechanism housing.
The VVT mechanism is driven by the primary circuit and rotates relative to the exhaust camshaft sprocket. When the ECM sends a request to decrease the retard angle, the control solenoid valve is energized, causing the spool to move to the appropriate position and pressurized engine oil through the filter to the VVT advance chamber. When a request to increase the retard angle is sent, the control solenoid valve is energized, causing the spool to move to the appropriate position, and pressurized engine oil exits the advance chamber and simultaneously enters the VVT retard chamber.
Upon receiving a command from the ECM, the VVT mechanism is set to the optimum position between full advance and retard for a given engine speed and load. When this operation is performed, the ECM sends a voltage signal to the control solenoid valve until the predetermined position is reached. After that, the voltage supply signal is reduced to a value necessary to hold the electromagnetic mechanism, and therefore the spool in a stationary position. This function is controlled with feedback: the ECM evaluates the decrease in spool oil pressure from signals from the camshaft position sensor. The ECM increases the value of the voltage supply signal in such a way as to maintain the spool in the desired position.
The properties and temperature of the engine oil can affect the ability of the VVT mechanism to change the angular position of the camshaft. At very low oil temperatures, the increased viscosity of the oil slows down the movement of the VVT mechanism, and at high oil temperatures, low viscosity can affect the operation of the mechanism with insufficient oil pressure. To maintain good performance of the VVT mechanism, a high displacement oil pump and an engine oil temperature sensor are used to allow the ECM to monitor the operation of the mechanism. Normally, the VVT system is controlled in a closed loop, except for extreme temperature conditions such as cold starts below 0°C. At extremely high oil temperatures, the ECM may limit the advance of the VVT mechanism so that the engine does not stall when returning to idle.
The VVT mechanism does not operate when the oil pressure is below 1.25 bar, because this pressure is not enough to release the internal locking pin of the VVT mechanism. This usually occurs when the engine is stopped and the VVT mechanism returns to the retarded position. A locking pin secures the VVT mechanism to the camshaft to stabilize the shaft the next time the engine is started.
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