Variable Valve Timing (VVT)

Variable Valve Timing

In internal combustion engines, variable valve timing, often abbreviated to VVT, is a generic term for an automobile piston engine technology. VVT allows the lift, duration or timing (in various combinations) of the intake and/or exhaust valves to be changed while the engine is in operation. Variable Valve Timing & Lift improve engine efficiency by optimizing the flow of fuel & air into the engine for various engine speeds. Two-stroke engines use a power valve system to get similar results to VVT.

Working of a Non-VVT Engine

Piston engines normally use poppet valves for intake and exhaust. These are driven by cams on a camshaft. The cams open the valves (lift) for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing is also important. The camshaft is driven by the crankshaft through timing belts, gears or chains.

 

The profile, or position and shape of the cam lobes on the shaft, is optimized for a certain engine revolutions per minute (RPM), and this tradeoff normally limits low-end torque, or high-end power. In a non-VVT engine, as the RPM increases, the time the valve stays open also becomes less (so in a way the open time is variable with RPM but opens and closes at a faster rate. At high engine speeds, an engine requires large amounts of air. However, the intake valves may close before all the air has been given a chance to flow in, reducing performance. On the other hand, if the cam keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions.

 

Working of VVT Engine

In VVT, the time the intake valves stay open is controlled by other means in addition to that based on the engine's RPM. VVT allows the cam timing to change, which results in greater efficiency and power, over a wider range of engine RPMs. With VVT a valve may be made to stay open longer with increasing RPM. The overall effect is that the engine can breathe better and along with fuel injection can give better performance/mileage.

 

VVT is achieved by having more than one cam profile per valve and selecting a cam based on the RPM ranges (or load conditions), such that at the lowest range (or load condition) the cam profile chosen will be the one that ensure minimum air required to maintain the RPM. The ECU will still inject fuel based on the amount of air sucked but this time the amount of fuel will be less as compared to the non-VVT engine. Hence we can see that the engine will burn lesser fuel and hence give better mileage.

 

Need of VVT

Pressure to meet environmental goals and fuel efficiency standards is forcing car manufacturers to turn to VVT as a solution. Most simple VVT systems advance or retard the timing of the intake or exhaust valves. Others (like Honda's VTEC) switch between two sets of cam lobes at a certain engine RPM. Furthermore Honda's i-VTEC can alter intake valve timing continuously.

 

Alternative to VVT

It should not be taken that a non-VVT engine is bad. These too have techniques to ensure efficiency. Consider the low RPM example I gave, the ECU can detect the load conditions/RPM and ensure a lean or ultra-lean burn (i.e., inject lesser fuel than that required by the ideal air-fuel ratio). This will also improve efficiency and is a technique commonly used.

 

Convert car from non VVT to VVT:-

It would be impractical to try converting a non-VVT engine to VVT - at least where reasonable budget it concerned, not to mention the time/effort required. Better would be to replace the engine itself (though this too won't be cheap)

 

Systems with VVT Implementation

As I told earlier VVT is a generic term for an automobile technology. This allows the lift, duration or timing of the intake and/or exhaust valves to be changed while the engine is in operation

 

There are various systems developed by Automobiles manufacturers to implement VVT in there system such as VVT-i, VVTL-i by Toyota, VTEC by Honda, CVVT by Volvo, VANOS by BMW etc.

Some can be only use to control one valve (single VVT) while other are to control both valve (dual VVT).

I will explain about few of them.

VVT-i  (Variable Valve Timing with Intelligence)

VVT-i is an implementation of VVT technology in an intelligent way using microprocessors to control the VVT functionality using some actuators.  VVT-i was developed by Toyota and came into implementation from 1996 which bring variations in the time of the intake and exhaust valves. This is an automobile variable valve timing technology which is quite similar to BMW's VANOS and was aimed at replacing the Toyota's VVT technology introduced in 1991 for 4A-GE engines.

The technology is responsible for variations in the timing of the intake valves by making adjustments in the mechanisms between the camshaft drive(belt, chain etc) and the intake camshaft. The medium of these adjustments is the Engine Oil pressure which is applied to the actuator for adjusting the camshaft position. Adjustments made in the overlap time in between the opening of intake valves and closing of exhaust valves is responsible for higher efficiency of the engine. Several variants of this system have been introduced since the introduction of VVT-i including VVTL-i, Dual VVT-i, VVT-iE and Valvematic.

VVTLi (Variable Valve Timing and Lift Intelligent System)

VVTL-i is a system whose principle of functionality has been derived from VVT-i but it is different in a way that it alters the valve timing as well as the valve lift(duration).

Dual VVT-i

Dual VVT-i as the name suggests would carry out the same function but on two valves at the same time. Dual VVT-i was introduced in 1998 in 3S-GE engines which not only alters the timing of the intake valves but also of the exhaust valves camshafts.

VVT-iE
VVT-iE stands for Variable valve timing – intelligent by electric motor. Being a variant of the Dual VVT-i technology this technology adjusts and maintains the intake camshaft timing with the help of electrically operated actuator.

VTEC
Honda also developed a technology of its own called the VTEC(Variable Valve Timing and Lift Electronic Control) which not altered the camshaft timings but also made an engine to have multiple camshafts rendering it with better performances.

Application of VVT

 

VVT technology is utilized in all types of internal combustion engines. It can be implemented for Gasoline, Diesel & CNG engines etc.

 

VVT Vehicles in India

 There are many cars where VVT is implemented worldwide. I would like to mention few where VVT technology is implemented. Although it may be be implemented in All cars of below series but some particular models only.

 

Vehicles in India

Maruti Suzuki SX4

Toyota Corolla

TATA Safari

 

Future vehicle with VVT in India

Toyota Fortuner

Maruti Cervo*

Tata Nano**

 

* Maruti cervo is expected to be competitor of Tata Nano, Its already launched in Japan, but no official news if it will be launched in India

* I heard one of high end model of Tata Nano is planned to have VVT, but not sure when)