How Does Variable Valve Timing Work?

Many Car Makers Have Ways of Making Engines More Efficient

Jan 17, 2010
Sam Domett

Honda's latest generation i-VTEC - Honda UK

Variable valve timing optimizes an engine's performance by ensuring it gets the exact amount of air and fuel in, and exhaust out

Air enters and exits a conventional internal combustion engine through poppet valves, which are usually activated by a camshaft which pushes on the valve to open and close them.

But as an engine increases in revolutions per minute a constantly changing pattern of opening and closing the valves is more efficient and produces more power for a given fuel consumption.

There is also valve lift, which describes how far the valve opens to let gas pass through. The more advanced valve timing systems can adjust both valve timing and lift.

Honda V-TEC Engine Valve Timing

Alfa Romeo actually produced a variable valve timed engine first, but it was Honda that made it famous. Honda's original VTEC uses camshafts with two different profiles, the first for low to mid rpm, and the second is essentially a race profile with high lift and duration. When engine rpm rises high enough a locking pin is pushed by oil pressure to connect the high rpm cam follower and the valves operate through the race profile cams.

The latest V-TEC system is i-VTEC, which continuously adjusts valve timing and lift, which means performance is optimised over the entire the rev range, and so rather than a turbo-like sudden boost in power the engine merely produces the maximum power at any rpm.

Mitsubishi MIVEC Valve Timing

This is Mitsubishi's answer to VTEC, and like VTEC it uses two different cam profiles, but kicks in at a much lower rpm to boost torque rather than produce high RPM performance.

Nissan Engine Variable Valve Timing

You will find this on many Nissan engines, including the RB25DE and RB25DET in previous generation Skylines. This adjusts the valve timing by accelerating or decelerating the camshafts, so lift and duration are not altered.

Toyota VVT-I Engine Valve Timing

Toyota's Variable Valve Timing with Intelligence has two stage hydraulically controlling cam phasing, adjusting only the timing of the valves. Dual VVT-i, acting on both the intake and exhaust valves, was introduced on the Altezza RS200.

Toyota VVTL-I Engine Valve Timing

This is the next generation of Toyota's variable valve system, which adds valve lift control to the system, and like Honda's VTEC this uses camshafts with two separate lobes for each valve, and the engine switches from one lobe to another through a rocker arm and slipper follower which moves between each lobe.

Engine Valves of the Future

The next step in valve control technology is electrically operated valves, which will allow virtually an unlimited range of both valve timing and lift. Many car makers are currently working on this technology, both to produce optimum power and reduce both fuel consumption and exhaust emissions.

Sam Domett - I have been a motoring journalist for over 15 years, beginning with my own website. I then moved on to the Manukau Daily News. My next job ...