Every week, Exhaust Notes will pick out an industry, technical or obscure term from the automotive world and give you the lowdown on what it means and how it’s used. Already know the answer? Congratulations, you’re ahead of the game.

If you’d like an explanation of an automotive term, e-mail us at autosblog@live.com

This week’s term: variable valve timing (VVT).

To get down to basics: Piston engines use valves for intake and exhaust; those valves are controlled by the cam lobes (cams) on the engine's camshaft. The cams control how long the valves stay open, which directly correlates to the amount of airflow into the engine and, in turn, dictates the rate at which fuel is burned.

 

The shape (profile) of the cam shaft -- the position and size of the cams -- is built specifically for an optimal engine rpm; which is to say, it's built specifically for an optimal speed. Because engines need different amounts of air at different speeds (more air at higher speeds, less at lower), a fixed valve system means that a vehicle is being perfectly efficient at only one specific speed. At higher-than-optimal speeds, for example, the valve closes before all the air has had a chance to flow in, which reduces performance. At lower speeds, the valve stays open too long, allowing fuel to escape, which reduces efficiency and increases emissions.

 

A variable valve timing system solves this problem (via a number of different mechanical operations) by allowing for changes to how long the cam stays open, or the angle at which it opens, during engine operation. Therefore, at slower speeds the cams close the valves faster, and at higher speeds the cams allow them to stay open longer, thereby increasing engine efficiency and performance.