1D
Camshaft Position Control (VVT: Variable Valve Timing) System Description
System Description
The VVT system is an electronic control system which continuously vary and optimize the intake valve timing in response to the engine operating condition.
The optimized intake valve timing produce such an air intake with high efficiency that both the higher power generation and lower fuel consumption can be attained in the whole engine speed range from low to high. In the area of the average engine load, low emission of nitrogen oxides (NOx) and high fuel efficiency can also be attained by making the valve opening overlap between the intake and exhaust valves longer.
For the brief of the system operation, the intake valve timing is varied by the cam timing sprocket (1) which varies the rotational phase between the intake camshaft (3) and sprocket. The rotor (2) in the cam timing sprocket is actuated by switching or adjusting the hydraulic pressure applied to the chambers for the timing advancing (7) and/or retarding (6). To switch or adjust the hydraulic pressure appropriately, ECM operates the oil control valve (12) with detecting the engine speed, intake air value, throttle opening, engine coolant temperature and camshaft position (angle).
4. Oil passage to chamber for timing retarding 8. Oil filter 10. Oil pan 13. Oil return
5. Oil passage to chamber for timing advancing 9. Oil pump 11. Control signal from ECM 14. Oil flow
Oil Control Valve
The oil control valve switches and adjusts the hydraulic pressure applied to the cam timing sprocket by moving the spool valve (1) according to the duty pulse signals output from the ECM. By this operation, the intake valve timing is varied continuously. Signals output from the ECM are the duty pulse of about 240 Hz.
Cam Timing Sprocket
The cam timing sprocket is equipped with the chambers for timing advancing (2) and retarding (3) which are separated by the rotor (5). The rotor rotates receiving the hydraulic pressure applied to both the chambers. The sprocket (1) is installed on the housing (4) and the rotor is secured on the intake camshaft by fastening the bolts. Therefore, the actuation of the rotor makes the phase difference between the sprocket and intake camshaft.
6. Seal
Timing Advancing
When the duty ratio of the signal output from the ECM is heavy, the spool valve (4) of the oil control valve moves to the left (opposite direction against the coil (5)). By this spool valve movement, the pressurized oil (1) is led into the chambers for timing advancing and the oil in the chambers for timing retarding is drained.
This operations actuate the rotor (3) and result in the advanced timing of the intake valve.
2. Drain
Timing Holding
When the duty ratio of the signal output from the ECM shows that of holding, the spool valve of the oil control valve is located at hold position. Because this condition generates no oil pressure changes in both chambers, the rotor is fixed at a target position.
Timing Retarding
When the duty ratio of the signal output from the ECM is light, the spool valve of the oil control valve moves to the right (head for the coil). By this spool valve movement, the pressurized oil is led into the chambers for timing retarding and the oil in the chambers for timing advancing is drained. This operations actuate the rotor and result in the retarded timing of the intake valve.
Targeted Timing Varying Operation
Driving condition Valve timing Target of control Effect
Engine running at idle speed Most retarded To shorten the valve opening overlap in order to prevent the exhaust gas counterflow to intake manifold. Stabilization of the engine rotation at idle speed.
Average engine load range To the advanced side To lengthen the valve opening overlap in order to enhance the internal exhaust gas recirculation and reduce the pumping loss. Improvement of the fuel efficiency.
Lowering of the exhaust emission.
Light engine load range To the retarded side To shorten the valve opening overlap in order to prevent the exhaust gas counterflow to intake manifold. Keeping of the engine stability.
Low or average engine speed range with heavy engine load To the advanced side To advance the closing timing of the intake valve in order to improve the volumetric efficiency. Improvement of generating the engine torque at low and average engine speed.
High engine speed range with heavy engine load To the retarded side To retard the closing timing of the intake valve in order to improve the volumetric efficiency. Improvement of generating the engine power.
Low engine coolant temperature Most retarded To shorten the valve opening overlap in order to prevent the exhaust gas counterflow to intake manifold and reduce the fuel increasing.
To slow the fast idle speed of the engine as a result of stabilizing the engine idling. 		Stabilization of the fast idling of the engine.
Improvement of the fuel efficiency.
At engine starting and stopping Most retarded To shorten the valve opening overlap in order to prevent the exhaust gas counterflow to intake manifold. Improvement of start ability