What is valve overlap
How does valve overlap affect emissions?
During the exhaust stroke, as the piston pushes the burned gases out of the cylinder, the flow develops a velocity. Since the gas flow has mass, it develops an impulse. This impulse follows Newton's first law that a body in motion tends to keep moving. Indeed, as the cylinder empties, the exhaust gas flow pulls the cylinder slightly.
- If there is no overlap, the outlet valve closes before the inlet valve opens and most of this suction is lost.
- If there is an overlap, the outlet valve closes while the inlet valve opens. This uses the suction power and begins drawing in the intake mixture before the intake stroke begins. Most cars take advantage of this action.
- If there is a large overlap, the outlet valve is fully open while the inlet valve opens. With this overlap, the suction effect can draw part of the intake mixture into the outlet before the outlet valve closes.
There is a tradeoff between overlap and emissions. At low speeds, the overlap is the main contributor to emissions. At high speeds, the overlap contributes to the performance by using the suction power to start the suction mixture. Lots of overlap is why super high performance cars tend to have very crappy idle. If the overlap is extreme, the exhaust valve will still be open during the intake stroke, drawing some exhaust back into the cylinder.
A variable valve control is the compromise. By dynamically adjusting the amount of overlap, the correct amount of overlap can be set for each speed. Some vehicles take advantage of the ability to suck some exhaust gas back into the cylinder to eliminate the need for an EGR valve. (GM Inline 6 in the Trailblazers)
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