VARIABLE VALVE TIMING Submitted by: DIVYA CHAUDHARY 09ITMG1128ECE

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. Two-stroke engines use a power valve system to get similar results to VVT.

HISTORY First used as a technology through experimental aircraft of the 1940s, It wasn't until 1982 when the first automotive use of VVT was found inside the engine of an Alfa Romeo Spider. Becoming the launch pad for the technology, Honda introduced the VTEC system in Japan and Europe in It was the 1990 Acura NSX that brought variable valve control to the United States, eventually sparking a flourishing interest in VVT to the mainstream auto market of today

WORKING OF VVT Piston engines normally use poppet valves for intake and exhaust. These are driven (directly or indirectly) 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. VVT allows the cam timing to change, which results in greater efficiency and power, over a wider range of engine RPMs.

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 Honda Civic The VTEC system equipped on the Honda Civic has been adding excitement to 4- cylinder engines.

ADVANTAGES OF VVT Variable valve timing is not a horsepower booster like a turbocharger. Instead, VVT is designed to make use of the engine's existing power in the most effective way possible. Assisting right from engine start up, timing would configure the valves for a shorter opening period aiding the initial warming up of the engine. When the engine reaches higher performance, valve staging is adjusted for a longer opening time allowing more air/fuel to flow into the combustion chambers. Ultimately, engine efficiency is greatly improved which also means less fuel is needed to run a VVT engine. Vehicles including VVT engines would expect fuel saving benefits of 5-10%.

ADVANTAGES (contd.) VVT systems are also an effective component for emission control. Since the 1970s, automobile engines would reuse exhaust gases through the next combustion cycle to reduce NOx, an major smog- creating pollutants. For this, an EGR (Exhaust Gas Recirculation) valve was developed. VVT technology uses a process called internal EGR where a small amount of exhaust gas leaks back into the combustion chamber.

Well into the future, valve timing may be accomplished more easily as engine's camshaft may be completely discarded and replaced with electrical solenoids controlling each valve.

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