Variable valve timing (VVT) is a system for varying the valve opening of an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...

. This allows the engine to deliver high power, but also to work tractably and efficiently at low power. There are many systems for VVT, which involve changing either the relative timing, duration or opening of the engine's inlet and exhaust valves. One of the first practical VVT systems used a variator to change the phasePhase refers to the relative timing between the inlet and exhaust camshafts, expressed as an angular measure. of the

camshaft A camshaft is a shaft that contains a row of pointed cams, in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition systems ...

and valves. This simple system cannot change the duration of the valve opening, or their lift.'Lift' is the distance by which the valves open. Later VVT systems, such as the helical camshaft or the movable fulcrum systems, could change these factors too. Despite this limitation, the variator is a relatively simple device to add to an existing engine and so they remain in service today. As the benefit of the variator relies on changing the relative timing between inlet and exhaust, variator systems are only applied to double overhead camshaft engines. A variator system that moved a single camshaft for both inlet and exhaust would be possible, but would have no performance benefit.

Alfa Romeo system

Alfa Romeo was the first manufacturer to use a variable valve timing system in production cars (US Patent 4,231,330). The 1980 Alfa Romeo Spider 2.0 L had a mechanical VVT system in SPICA fuel injected cars sold in the US. Later this was also used in the 1983 Alfetta 2.0 Quadrifoglio Oro models as well as other cars. The technique derives from work carried in the 1970s by Alfa Romeo engineer Giampaolo Garcea and in Italian the device is termed ''variatore di fase''. The Alfa Romeo Twin Spark engine, introduced in the 1987 Alfa Romeo 75, also uses variable valve timing. The Alfa system varies the phase (not the duration) of the cam timing and operates on the inlet camshaft.

Applications

* 1980 Alfa Romeo Spider * 1987 Alfa Romeo 75

Twinspark Alfa Romeo Twin Spark (TS) technology was used for the first time in the Alfa Romeo Grand Prix car in 1914. In the early 1960s it was used in their race cars ( GTA, TZ) to enable it to achieve a higher power output from its engines. And in the ea ...

* 1991 Alfa Romeo 155 1.8l and 2.0l petrol engines * 1997

Alfa Romeo 156 The Alfa Romeo 156 (Type 932) is a compact executive car produced by the Italian automobile manufacturer Alfa Romeo. It was introduced at the 1997 Frankfurt Motor Show as the replacement for the Alfa Romeo 155. The 156 received a positive rec ...

with the 1.6l, 1.8l and 2.0l petrol engines * 1998 Alfa Romeo GTV and Spider with the 1.8l and 2.0l petrol engines * 2000 Alfa Romeo 147 with the 1.6l and 2.0l petrol engines, except for the model with the 105 bhp engine *2004 Alfa Romeo GT with the 1.8l and 2.0l petrol engines * 2001 Fiat Stilo with the 1.8l or 2.4l engines

Volkswagen system

Volkswagen use a variator system with two variators, one for each camshaft. Like the Alfa Romeo system, these are electrically-controlled hydraulic units, mounted in the camshaft's timing belt pulley. These systems are fitted to the Volkswagen VR5 and

VR6 engine VR6 engines are V6 piston engines with a narrow angle between the cylinder banks and a single cylinder head covering both banks of cylinders. Volkswagen Group introduced the first VR6 engine in 1991 and VR6 engines currently remain in product ...

s, and also to the W8 and W12 engines. The multiple-

bank A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets. Becau ...

W engines have four variators in total, one for each camshaft. The Volkswagen variator is referred to as a 'fluted variator', owing to the shape of the hydraulic components. Unlike the Alfa Romeo system with its helical splines and indirect actuation,Hydraulic action in the Alfa Romeo is axial, using helical splines to then cause a rotation of the variator. the Volkswagen system has a direct rotational action. The internal components of the variator resemble a paddle wheel inside a loose casing, so that it is free to move a few degrees from side to side. By applying hydraulic pressure on one side of these paddles, a phase shift is achieved. The hydraulic fluid is engine oil, controlled by a solenoid valve mounted on the cylinder head and controlled by the ECU. A Hall effect sensor also monitors the camshaft position.

Other variator-based VVT systems

Variable Cam Timing Variable Camshaft Timing (VCT) is an automobile variable valve timing technology developed by Ford. It allows for more optimum engine performance, reduced emissions, and increased fuel efficiency compared to engines with fixed camshafts. It uses ...

, Ford * VANOS, BMW

References

{{Reflist, colwidth=35em Variable valve timing Engine technology