Continuously Variable Valve Timing System: Description and Operation

Description

The Continuously Variable Valve Timing (CVVT) system advances or retards the timing of the intake and exhaust valves according to the control signal from the ECM, calculated from engine load and speed. CVVT control creates valve overlap, which improves fuel economy and reduces exhaust emissions (NOx, HC), improves combustion stability, increases volumetric efficiency and improves engine output by reducing pumping loss, internal exhaust gas recirculation effect and increasing trigger operation. The system includes the CVVT oil control valve (OCV), which supplies oil to the camshaft phaser or holds it in position according to the PWM (pulse width modulated) control signal from the ECM, the CVVT oil temperature sensor (OTS), which measures the engine oil temperature, and the camshaft phaser, which changes camshaft phase using hydraulic pressure from engine oil. Engine oil leaving the CVVT oil control valve changes the camshaft phaser in the direction of engine rotation (Intake Advance/Exhaust Retard) or in the opposite direction (Intake Retard/Exhaust Advance) by rotating the rotor connected to the camshaft in the phaser.

Principle of Operation

CVVT has a mechanism that rotates the rotor valve using hydraulic force generated by engine oil supplied to the advance or retard chamber according to the CVVT oil control valve control.

[CVVT System Mode]

(1) Low Speed/Low Load	(2) Partial Load
(3) Low Speed/High Load	(4) High Speed / High Load

Operating Condition	Exhaust Valve		Intake Valve
	Valve Timing	Effect	Valve Timing	Effect
(1) Low Speed / Low Load	Fully ADVANCE	* Valve overlap * Improved combustion stability	Fully RETARD	* Valve overlap * Improved combustion stability
(2) Partial Load	RETARD	* Improved relaxation work * Reduced pumping loss at load changes * Reduced HC	RETARD	* Reduced pumping loss at load changes
(3) Low Speed / High Load	RETARD	* Improved relaxation work	ADVANCE	* Prevention of reverse intake flow (improved volumetric efficiency)
(4) High speed / high load	ADVANCE	* Reduced pumping loss at load changes	RETARD	* Improved volumetric efficiency