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. 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. 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 1.8 2ZR-FE V6, used in Toyota's next generation of compact vehicles such as the Scion XD. 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.
Click here for a Video Animation of Dual VVT-IClick here for a Video Animation of VVT-IBill Sherwood wrote an article on the later generation VVT-I used on the Beams engines which is another great read. Click below to see the article.
Toyota VVTL-i Variable Valve Timing & Lift - IntelligentSource:
http://www.beams-redtop.com/vvti.htm 
