Variable Valve Timing, or VVT, is an engine technology that changes when the intake and exhaust valves open and close. These valves control fresh air entering the cylinder and burned gases leaving it, so their timing strongly affects power, fuel use, and emissions. A fixed valve schedule is a compromise, but an engine needs different timing at idle, cruising speed, and hard acceleration.
VVT helps the same engine behave smoothly at low speed while still breathing well at high speed.
In many VVT systems, a cam phaser rotates the camshaft slightly ahead of or behind its normal position while the engine is running. The engine control unit uses sensors for speed, load, throttle position, and temperature to command oil pressure or an electric actuator inside the phaser. Advancing or retarding the cam changes valve opening, valve closing, and valve overlap, which is the short time when intake and exhaust valves are open together.
By adjusting this valve schedule, the engine can improve torque, reduce pumping losses, and lower exhaust emissions.
Key Facts
- VVT means Variable Valve Timing, which changes valve opening and closing times while the engine runs.
- A camshaft controls valve motion because each cam lobe pushes a valve open at a specific crankshaft angle.
- Crankshaft angle measures engine position in degrees, with one four-stroke cycle lasting 720 degrees.
- Valve overlap is the period when intake and exhaust valves are open at the same time.
- At low rpm, less overlap usually improves smooth idle and low-speed torque.
- At high rpm, more overlap or later intake closing can help the cylinder fill better and increase power.
Vocabulary
- Camshaft
- A rotating shaft with shaped lobes that open and close the engine valves.
- Cam phaser
- A device that rotates the camshaft slightly relative to the timing chain or belt to change valve timing.
- Valve overlap
- The interval when the intake valve and exhaust valve are both open near the end of the exhaust stroke.
- Engine control unit
- The computer that reads engine sensors and commands systems such as fuel injection, ignition, and VVT.
- Combustion chamber
- The space above the piston where the air fuel mixture burns and produces pressure.
Common Mistakes to Avoid
- Thinking VVT changes how far the piston moves is wrong because piston travel is set by the crankshaft and connecting rod geometry.
- Assuming advanced timing is always better is wrong because the best valve timing depends on rpm, load, temperature, and emissions needs.
- Confusing valve timing with ignition timing is wrong because valve timing controls airflow while ignition timing controls when the spark occurs.
- Ignoring valve overlap is wrong because overlap can help high-speed airflow but can also cause rough idle or exhaust gas mixing at low speed.
Practice Questions
- 1 A four-stroke engine completes one cycle in 720 degrees of crankshaft rotation. If the intake valve opens 10 degrees before top dead center and closes 40 degrees after bottom dead center, for how many crankshaft degrees is the intake valve open?
- 2 An engine is running at 3000 rpm. How many complete four-stroke cycles does one cylinder complete per second?
- 3 Explain why a VVT system might use less valve overlap at idle but more valve overlap at high engine speed.