Variable valve timing is controlled through a hydraulic system that takes instruction from a system of electronic controls.
Electronic control is by the engine management computer, which uses input from a number of engine sensors to determine the ideal position for the camshafts. The sensors include those that measure airflow into the intake system, coolant temperature, throttle position and camshaft position.
The control unit then actuates changes through an oil control valve positioned at each intake camshaft sprocket. The oil control valve uses oil pressure from the engine to advance and retard the intake camshafts via the AVCS actuator.
Mounted in the chain- or belt-driven drive sprocket, the actuator is fitted directly to the camshaft. Chambers in the actuator allow oil pressure to advance or retard it within the timing-belt sprocket. The oil fills the chambers and pushes against three lobes to turn the actuator and the camshaft on its axis.
The intake valves open just after the piston reaches the top of the cylinder (called “top dead center” or TDC; BDC refers to “bottom dead center”) at the end of the exhaust stroke, as the piston begins the intake stroke. The exhaust stroke creates negative pressure within the chamber, and intake air enters the cylinder with positive pressure “to fill the void.” There is very little or no overlap between the exhaust and the intake strokes.
Retarding valve timing improves the smoothness of engine operation at idle, which tends to be a problem area in high-performance engines without variable valve control. (If you remember the muscle cars of the 1960s and 1970s, you may recall how roughly they idled.)
From idle through medium engine loads, AVCS advances the intake valves to begin opening during the last part of the exhaust stroke, when the exhaust valves are still slightly open. Some of the pressure created during the exhaust stroke flows into the intake manifold, having the effect of exhaust gas recirculation (EGR). The intake valves also close earlier during the intake stroke.
Advancing valve timing for some overlap helps reduce the level of harmful oxides of nitrogen in the exhaust. It also improves volumetric efficiency, which is an indication of how well air flows through the engine. The greater the efficiency, the stronger the engine’s performance.
When the engine is used aggressively for greatest performance, AVCS advances the intake valves further to open even sooner during the exhaust stroke. This produces a scavenging effect – that is, intake airflow helps clear the cylinder of exhaust gas. It also closes the intake valves sooner on the compression stroke.
This results in improved volumetric efficiency and helps to generate higher power output.
Overall, varying valve timing helps the engine to develop power more evenly between low and high speeds. At the same time, it improves engine idle and lowers exhaust emissions.