In this question, it is suggested that valve overlap can affect emissions. In what way can valve overlap affect emissions?
During the exhaust stroke as the piston is pushing the burnt gases out of the cylinder the stream develops speed. Since the stream of gas has mass it will develop momentum. This momentum follows Newtons first law a body in motion tend to stay in motion. As the cylinder empties the exhaust stream will actually pull a slight suction on the cylinder.
- If there is no overlap the exhaust valve closes before the intake valve opens and most of that suction is lost.
- If there is some overlap the exhaust valve is closing while the intake valve is opening. This takes advantage of the suction and starts pulling in the intake mixture even before the intake stroke begins. Most cars take advantage of this action.
- If there is a lot of overlap then the exhaust valve is completely open while the intake valve is opening. With this amount of overlap the suction action may pull some of the intake mixture into the exhaust before the exhaust valve closes.
There is a compromise between overlap and emissions. At low RPM overlap mostly contributes to emissions. At high RPM overlap contributes to performance by taking advantage of the suction to jump start the intake mixture. Lots of overlap is why super high performance cars tend to have a very crappy idle. When overlap is taken to the extreme the exhaust valve is still open during the intake stroke which will pull in some exhaust back into the cylinder.
Variable valve timing is the compromise. By adjusting the amount of overlap dynamically the correct amount of overlap can be set for every RPM. Some vehicles will take advantage of sucking in some exhaust back into the cylinder to eliminate the need for an EGR valve. (GM inline 6 in the Trailblazers)