Variable Valve Timing

This is the second in our series of seven fuel economy technologies Detroit could have pursued but did not. Although almost all of the world's automakers have made at least one engine with variable valve timing within the last 10 years, before that time they were rare. Even today the majority of engines have fixed timings.

The video illustrates the idea. Basically, the fuel-air intake valves and the exhaust valves open a certain distance and stay open for a certain time. Also the location in the piston cycle at which they are open is important. At a given RPM, the engine has to open the valves different amounts at different positions for different times to get maximum efficiency. If the valves have fixed timings, the engine will only be at its top efficiency at one narrow RPM band. However, if the valves can modify their timing, the engine can reach high efficiency over a wider band of RPM.

In normal operation, there is a moment near the end of the exhaust stroke when the exhaust valve and the intake valve are both open. Also the exhaust valve stays open a little while into the intake stroke. This time when both valves are open is known as the overlap, and is one of the most important variables to control. At low RPM, the overlap should be low. This is because at low RPM the airflow is fast relative to the engine speed. At high RPM, the engine is moving so fast relative to the air that it is better to open the intake valve early so that the air has a better chance to enter.

The first vehicles with variable valve timing technology were not introduced to the US market by Detroit. Instead Alfa Romeo, Nissan and Honda blazed the trail. One more chance missed by Detroit.

No comments:

Post a Comment