Variable valve timing

In Camshafts and valve timing you saw that no single cam is best at every engine speed. A cam that shuts the intake valve early fills the cylinder well at low revs but chokes the engine at the top. One that shuts it late breathes well at the redline but idles lumpily and feels weak low down. A fixed cam has to be a compromise.

Variable valve timing (VVT) gets round this by letting the engine move its camshafts while it runs. The ECU turns each camshaft a few degrees earlier or later against its pulley or sprocket, choosing the best position for the engine speed and load at that moment. The lobes stay the same shape; what changes is when the valves open and shut.

Almost every modern petrol engine has it, usually on the intake cam and often on the exhaust cam too. Makers give it their own names: VVT-i (Toyota), VANOS (BMW), VTC (Honda), CVVT (Hyundai and Kia), Ti-VCT (Ford) and VarioCam (Porsche) are all the same idea.

The part that moves the cam is a cam phaser, built into the cam sprocket. The demo shows one end-on:

• The outer housing is the sprocket, driven by the belt or chain. It has lugs reaching in, with oil chambers between them.
• The rotor, in Mustard, is bolted to the camshaft. Each of its vanes sits in one of the chambers.
• Engine oil pumped into the chamber on one side of each vane pushes the rotor round, turning the camshaft ahead of the sprocket. That’s advancing the cam. Oil on the other side pushes it back, retarding it.
• An oil control valve, a solenoid the ECU switches many times a second, decides which side gets the oil. The cam position sensor tells the ECU where the cam actually is, so it can correct any difference.

When the engine is switched off, a spring or the drive pushes the rotor back to its home position, fully retarded for an intake cam, and a small locking pin holds it there until the oil pressure builds after a start. The two Paper marks line up at home, so the gap between them shows how far the phaser has moved.

The graphs show what that does. Top right is the lift of each valve through one cycle, as in the last demo: the intake in Mustard and the exhaust in Coral, with the overlap shaded. The graph along the bottom shows how full the cylinder gets at full throttle across the rev range, with the phaser in Mustard and a fixed cam with the same lobes in Coral.

• At Low revs, the ECU advances the intake cam 45° of crank turn. Watch the Mustard hump slide left: the intake valve now shuts only 37° after the bottom, before the rising piston can push the slow-moving charge back out. The cylinder fills to 90%, against 86% for the fixed cam.
• Drag the engine speed up, or press Next 1,000 rpm. The ECU retards the cam step by step, so the valve shuts later to suit the faster air.
• At the Redline, the cam is back almost at home and the intake shuts 79° after the bottom. The air rushing in packs the cylinder to 94%, while the fixed cam, shutting too early for these revs, manages 88%.
• Press Fixed cam and compare. In the middle of the rev range, around 4,000 rpm, the fixed cam is just as good: that’s where its compromise is set. It loses at both ends.

Torque follows how full the cylinder gets, so VVT gives an engine a flatter torque curve: more pull low down and more power at the top, as Horsepower vs torque explains. It also means the engine can use slightly longer cams than a fixed-cam engine could, since the phaser hides their bad manners at low revs.

The model is simplified, and the demo shows full throttle only. Real engines also move the cams with load, as the next section explains.

The ECU has a map of cam positions for every combination of engine speed and load, much like its fuel and ignition maps. Broadly:

• At idle, it holds the intake cam at home, where the overlap is smallest. With little time for the valves to share, exhaust can’t leak back into the intake, so the idle is smooth and steady.
• At low revs and full throttle, it advances the intake cam so the valve shuts early and traps the most air. That’s where most of the extra low-down torque comes from.
• When cruising at part throttle, it often adds overlap on purpose. A little exhaust stays in the cylinder or is drawn back into it, which cools the burn and cuts the nitrogen oxides (NOx) it makes. With the cylinder partly filled with exhaust, the throttle can open further for the same power, so the engine wastes less effort pulling air past it and uses less fuel. Engineers call this internal EGR (exhaust gas recirculation).
• At high revs, it retards the intake cam so the valve shuts late and the fast-moving air keeps packing in.

Dual VVT adds a phaser to the exhaust cam too, so the ECU can set when the exhaust shuts and how much overlap there is separately from the intake. Most modern twin-cam engines have it. Single-cam (SOHC) and pushrod engines can still use a phaser, but it moves the intake and exhaust lobes together.

A phaser moves the same lobe earlier or later. Some systems go further and change how far and for how long the valves open.

• Honda VTEC gives each pair of valves an extra, bigger lobe. At low revs the valves follow the mild lobes. At a set engine speed, oil pressure pushes a pin through the rocker arms and locks them to the big lobe, so lift and duration jump up together. The “VTEC kicking in” feeling is the change from one cam profile to the other. Mitsubishi’s MIVEC and Toyota’s VVTL-i work in a similar way.
• BMW Valvetronic varies the intake lift smoothly, from barely open to fully open, using an electric motor and an extra lever between the cam and each valve. The engine can control how much air it takes in with the valves themselves, so the throttle stays almost fully open and the engine wastes less effort pulling against it.
• Fiat MultiAir puts a small oil chamber between the cam and each intake valve. A solenoid can let the oil out early, so the valve shuts sooner or opens less than the lobe asks for, cylinder by cylinder and stroke by stroke.

These systems are more complex than a phaser, and they’re often used alongside one.

Most VVT faults come down to oil, because the phasers are moved and held by it.

• Low, old or wrong oil. Thick, dirty or low oil makes the phasers slow to respond or unable to hold position. The engine feels flat, may idle roughly, and the ECU turns on the engine warning light with a cam timing fault code: on most cars P0011 or P0012 for the intake cam (“over-advanced” or “over-retarded”) and P0014 or P0015 for the exhaust. Check the oil first, and use the exact grade the maker specifies; Why engines need oil explains why.
• Blocked oil control valve. Many have a fine mesh screen that clogs with sludge, or the solenoid itself sticks or fails. The symptoms and fault codes are the same as above. On many engines the valve is easy to reach and fairly cheap to replace.
• Worn phaser. A worn locking pin or vanes can let the rotor flap about for a second or two after a cold start, before the oil pressure builds, giving a short clatter or rattle from the end of the engine. On engines with a timing chain, a stretched chain gives a similar start-up rattle, so both are usually checked together, as Timing belts vs timing chains explains.
• Phaser stuck at home. Press Stuck phaser in the demo. The engine is fine at high revs but feels flat and lazy low down, where the cam should be advanced, and the ECU flags the fault. Stuck advanced is the opposite: a rough, lumpy idle that can stall, as there’s too much overlap.
• Faulty cam or crank position sensor. The ECU can’t tell where the cam is, so it can’t control it. It may hold the phaser at home and run in a limp mode with less power.

VVT changes how cams are tuned, because the cam position is now part of the ECU’s software.

• A remap can change the cam timing tables as well as fuel, ignition and boost. Moving where the cam advances and retards can fill out the torque curve, but the factory map is usually close to the best already; most of a remap’s gain on a turbo engine comes from boost.
• Performance cams on a VVT engine need the cam tables retuned to suit them. Without that, the ECU puts the new lobes in the wrong place, and the car can idle badly and set fault codes.
• Phaser limiters and lock-outs. A big performance cam can bring the valves close to the pistons when the phaser is fully advanced or retarded. Limiters reduce the phaser’s travel, and on race engines some builders lock the phaser in one place, giving up the low-rev benefits for a simpler, stronger setup.
• VTEC engagement point. On Honda engines, tuning software can change the speed at which the big lobes cut in, to suit a new intake or exhaust.

Any change to the cam timing should be checked on a rolling road, where the power and torque curves show whether it’s actually helped.