Details of Variable Valve Timing-intelligent, How it works?

Dual Variable Valve Timing with intelligence (VVT-i) optimizes both the intake and exhaust valve timing over a broad range of engine speeds. This flexible engine technology increases torque at lower speeds and horsepower at higher speeds to improve engine performance and fuel economy, as well as help reduce emissions.

HOW TOYOTA VVTi ENGINE WORKS?

VVT-i, or Variable Valve Timing with intelligence, is an automobile variable valve timing technology developed by Toyota, similar to the i-VTEC technology by Honda. The Toyota VVT-i system replaces the Toyota VVT offered starting in 1991 on the 4A-GE 20-Valve engine. Perodua use this technology and convert the name to DVVT(Dual Variable Valve Timing). The VVT system is a 2-stage hydraulically controlled cam phasing system.

VVT-i, introduced in 1996, varies the timing of the intake valves by adjusting the relationship between the camshaft drive (belt, scissor-gear or chain) and intake camshaft. Engine oil pressure is applied to an actuator to adjust the camshaft position.

Engine designers have known for a long time that they could get better performance out of an engine under certain circumstances by allowing the intake valve to open slightly before the exhaust valve closes. This increases the time for the fuel/air mixture to enter the cylinder during the intake stroke. In this condition the exhaust and intake valves are open at the same time; this is called ‘valve overlap’. In conventional engines ‘valve overlap’ timing is fixed.

Fixed valve overlap allows the engine to perform well within a certain rev range, however there are three main undesirable side effects.

1) Fuel is wasted – the fuel/air mixture is not always efficiently burned and this causes unburnt fuel to pass through the engine.

2) Higher levels of undesirable exhaust emissions are produced.

3) Power output potential is not fully realised.

Variable valve timing allows the relationship between the separate inlet and exhaust camshafts to vary the valve timing overlap. In doing so it overcomes the side effects described above by using a computer to continuously vary the intake valve timing and overlap. The valve timing and overlap are adjusted through a series of simple mechanisms to ensure the optimum conditions apply across all the working rev range. The advantages are lower fuel consumption, lower exhaust emissions and higher power output. Because the system is continuously variable, an ‘i’ for ‘intelligent’ has been added to the acronym.

In 1998, “Dual” VVT-i (adjusts both intake and exhaust camshafts) was first introduced in the RS200 Altezza’s 3S-GE engine. Dual VVT-i is also found in Toyota’s new generation V6 engine, the 3.5L 2GR-FE V6. This engine can be found in the Avalon, RAV4, and Camry in the US, the Aurion in Australia, and various models in Japan, including the Estima. Dual VVT-i is also used in the Toyota Corolla (1.6 dual VVT-i 124bhp).

Other Dual VVT-i engines include the 1.8L 2ZR-FE I4, used in Toyota’s next generation of compact vehicles such as the Scion XD. It is also used in the 2JZ-GE and 2JZ-GTE engines used in the Lexus IS300 and in the Toyota Supra. By adjusting the valve timing engine start and stop occurs virtually unnoticeably at minimum compression. In addition fast heating of the catalytic converter to its light-off temperature is possible thereby reducing hydrocarbon emissions considerably.