Variable Valve Timing: How Does This Technology Work?

BMW Vanos
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Variable valve timing, or variable timing distribution, is a technology that allows optimizing the parameters of a four-stroke internal combustion engine, thereby increasing its performance and reducing fuel consumption.

With variable valve timing, it is possible to control the lift, valve opening moment or valve opening time, or a combination of the mentioned parameters, independently of the crankshaft position. However, the valve's control depends on the revolutions, engine load, and other factors.


How does variable valve timing work?

With a standard distribution, the timing is given by its geometry, and the movement of the valves is tightly linked to the position of the crankshaft. The opening and closing of the valves are thus immutable and dependent on the movement of the pistons.

However, the moment of opening and closing of the valves significantly affects the quality of filling the cylinders depending on the engine speed. Thus, with variable timing, the camshaft setting changes depending on engine speed and load.

At idle and high rpm, the intake camshaft is set to close the intake valve a little later than normal, which helps the engine to run smoothly at idle and make good use of power at high engine speeds.

At low and medium speeds, the camshaft is set to close the intake valve a little earlier than usual, which results in greater filling of the cylinders and improved torque flow.

Valve timing adjustment effect

1. Delayed closing of the intake valve

If the intake valve stays open a little longer than normal, the piston pushes air out of the cylinder and back into the intake manifold during the compression stroke. The air that is pushed out fills the intake pipe with higher pressure, and during the following strokes, it sucks this air back into the combustion chamber.

Delayed valve closing reduces suction pumping losses by 40% during load and reduces nitrogen oxide emissions by 24%. Hydrocarbon emissions remain unchanged.

2. Premature closing of the suction valve

Another way to reduce the pumping losses associated with low engine speed is to create a high vacuum by closing the intake valve earlier than usual. This involves closing the intake valve halfway through the intake stroke.

At low speeds and loads, the engine's fuel and air requirements are low, and the work required to fill the cylinder is relatively high, so premature intake valve closure greatly reduces pumping losses. Premature closing of intake valves reduces pumping losses by 40% and fuel consumption by 7%. Nitrous oxide emissions are also reduced by 24%.

3. Premature opening of the intake valve

Another way to reduce emissions is to open the intake valve prematurely. By opening the intake valve earlier than usual, some burnt exhaust gases are forced out of the cylinder through the intake valve.

In the intake manifold, these exhaust gases are cooled by the surrounding air and sucked back into the cylinder space during the next stroke, which helps to regulate cylinder temperature and nitrogen oxide emissions.

4. Early/late closing of exhaust valves

With the help of the exhaust valve, we can also reduce emissions. When the exhaust valve opens, the piston pushes the exhaust gases outward from the cylinder into the exhaust manifold. We can control how much exhaust gas is left in the cylinder by manipulating the exhaust valve timing.

If the exhaust valve is open longer than usual, the cylinder is emptied more and thus ready to be filled with more fuel and air during the intake stroke, allowing the engine to make more power. If the exhaust valve is closed a little earlier, more exhaust gases remain in the cylinder, which reduces the formation of emissions.

Advantages of variable valve timing

Variable valve timing technology is used to improve cylinder head replacement in a reciprocating internal combustion engine, resulting in a higher power, lower fuel consumption, lower emissions, and high torque across a wide range of engine speeds.

Variable valve timing is mainly used in spark-ignition engines. This is because these engines work in a wider range of revolutions, which is why the use of variable valve timing technology is more efficient and logical. The fundamental disadvantage of gasoline engines is throttle regulation, which causes a decrease in their efficiency at low loads.

Thanks to the variable timing of the valves, it is possible to reduce or completely remove the throttle valve, which reduces the pneumatic resistance-pumping losses in the intake manifold and thus increases the filling efficiency of the engine, especially at low loads.

In addition to gasoline engines, variable timing technology is also starting to be applied to diesel engines, mainly due to the ever-tightening emission standards. The first diesel engine for passenger cars with variable valve timing was developed by Mitsubishi in 2010.

The use of variable valve timing can bring

  • 10-30% reduction in fuel consumption
  • 10-15% increase in effective power and torque
  • 20-25% reduction in exhaust gas emission production

Variable valve timing design

Different manufacturers use different technologies to implement variable valve timing. Structurally, variable valve timing can be achieved, for example, in the following ways:

  • mechanically controlled camshaft
  • hydraulic camshaft movers
  • hydraulic valve control
  • electromagnetically controlled valves

Designation of engines equipped with variable valve timing:

In addition to different technologies, car companies also use different designations for their engines, which are equipped with variable timing. Here are some examples:

AVCS (Subaru)

AVLS (Subaru)

CVTCS (Nissan, Infiniti)

CVVT (Alfa Romeo, Citroën, Hyundai, Kia, Peugeot, Renault, Volvo)

DCVCP (General Motors)

MIVEC (Mitsubishi)

MultiAir (Fiat)

N-VCT (Nissan)

S-VT (Mazda)

Ti-VCT (Ford)


VarioCam (Porsche)

VCT (Ford)

VTEC, i-VTEC (Honda)

VVL (Nissan)

Valvelift (Audi)

VVEL (Nissan)

VVT (Chrysler, General Motors, Suzuki, Volkswagen Group)

VVT-i, VVTL-i (Toyota, Lexus)

VTVT (Hyundai, Kia)

A short video demonstration of how variable VVT ​​valve timing works: