A variable valve timing (VVT) phaser sits on the end of a camshaft and can twist it a few degrees while the engine runs. Use the buttons to look inside: a chain-driven stator, a rotor bolted to the camshaft, and an oil control valve that fills one side or the other.
A variable valve timing (VVT) phaser is a hydraulically adjustable hub on the end of a camshaft. Its job is to let the engine change when the valves open and close — their timing — without changing how long they stay open. Do that well and the engine can produce more torque low down, breathe better high up and pollute less.
It manages this with two parts that can rotate a little relative to each other. The outer stator is turned by the timing chain, exactly like an ordinary sprocket. Inside it, a rotor is bolted to the camshaft. Oil pumped into one side or the other nudges the rotor around within the stator, so the camshaft leads or lags the chain.
Externally a phaser is a round metal hub with an oil line. Cut open, it becomes a clear machine: a ring of sealed chambers, vanes that divide them, and a solenoid valve that decides which chambers fill. It is a rotary hydraulic actuator driven by engine oil.
The single governing idea is that oil pressure on one side and then the other swings the rotor. Everything else — the sprocket, stator, vanes and channels — exists to contain that oil and guide the push.
The chain that drives the camshaft gears onto the phaser's outer sprocket. It is bolted to the stator housing, so the whole outer part turns at chain speed as one solid piece with the rest of the valve train.
Inside the stator sits the rotor, bolted firmly to the camshaft. Under normal running the rotor turns along with the stator, so the camshaft simply follows the chain just as it would in a fixed engine.
Vanes on the rotor reach out to the stator and divide the ring-shaped gap between them into separate, oil-tight chambers. Each chamber has an advance side and a retard side, and keeping them sealed is what lets oil pressure act on the rotor.
Engine oil arrives at the oil control valve, whose spool slides under command from the engine computer. Its position connects the pump only to the advance channel, only to the retard channel, or holds the rotor in between.
Oil flowing through the advance channel fills the advance chambers and pushes the rotor forward relative to the stator. Oil through the retard channel fills the retard chambers and pushes it back. The size of the chambers lets the rotor move through only a few degrees — but a few degrees of camshaft is a real change in valve timing.
Because the rotor is bolted to the camshaft, any swing of the rotor swings the camshaft too. It now runs slightly ahead of or behind the chain, so the cam lobes open the valves earlier or later than before.
Low-speed running may want the valves advanced for torque and a steady idle; high-speed running may want them retarded for power and lower emissions. The computer keeps sliding the spool to reach whichever the engine needs at that moment.
It lets the engine shift when the valves open and close relative to the crankshaft. The phaser sits on the end of a camshaft and can twist the camshaft forward or back within a range, changing valve timing while the engine runs.
The phaser has two parts: an outer stator driven by the timing chain, and an inner rotor bolted to the camshaft. Oil pressure between them swings the rotor relative to the stator, so the camshaft is rotated a few degrees ahead or behind the chain.
The vanes on the rotor divide the space inside the stator into sealed chambers. Filling the advance chambers on one side pushes the rotor ahead; filling the retard chambers on the other side pushes it back.
It is the solenoid that decides which chambers get oil. The engine computer slides its spool one way or the other to send pressurised oil to the advance or the retard side, which is how the timing is commanded.
Different running conditions want different timing. Advancing it at low speed can improve torque and idle, while retarding it at high speed can improve power and reduce emissions, so the phaser keeps adjusting as conditions change.
No. A phaser shifts the timing, not the duration: the valve opening stays the same length, but it starts and ends earlier or later. Changing the duration is done by a different mechanism called variable valve lift or duration.