Why is valve overlap necessary?

Question

I've been working with cars for almost 3 years now, I've rebuilt an L28 (i can make a lego house in terms of engine skill ;p ) I understand all the basic and "advanced" engine concepts but why is valve overlap necessary? From what I can understand, you don't want to chip or crack your exhaust valves with such high temperatures but wouldn't no valve overlap seem to be the most economical type of setup, especially with boosted manifold pressure? Im looking at my valve overlap and i have 102 degrees from BDC (exhaust open 58* to intake closes 44*)

I would just like to learn more about the concepts, and how lifter lash and chain slack make that HP difference.

http://i81.photobucket.com/albums/j223/thetremendoustim/valveoverlap.jpg

Answer 1

Overlap actually has to do with inertia of the flow of the fuel/air mixture (from my days in school in the 1960s). As fuel is drawn into the cylinder, there is some inertia in its flow. Just as the piston starts its compression stroke (a few degrees after bottom dead center), the intake valve is allowed to close (camshaft grind of the cam lobe determines when that occurs). This allows just a little more fuel/air mixture into the cylinder, giving a few more HP points.



It's similar on the exhaust stroke. Burned gasses have inertia as they flow out the exhaust port. So the exhaust valve stays open for a few degrees after the piston reaches top dead center of the exhaust stroke as determined by the camshaft grind of the cam lobe.

Answer 2

Valve Overlap

Answer 3

It has to do with the cylinder filling efficiency: by allowing some fresh mixture past the exhaust valve during overlap, it lets more fuel-air mixture into the compression chamber which means more power output from each power stroke. Improved cylinder filling means the same fuel quantity is used in a more efficient manner. This translates into better fuel economy.



Imagine filling an empty, sealed vessel with water using a pipe at its side. At one point, the filling will slow down and eventually stop with some air trapped at the top, therefore it will be filled, say 95 percent. If a hole is made at the top, the trapped air blows out and empty vessel will be filled to the top (100 percent) in less time and with less effort (no counterpressure), with some water spilling over. That's what valve overlap is about.

Answer 4

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Let me think. That would be whichever cylinder is at the top of it's exhaust stroke. Based on the firing order, that should be cylinder 6 - the cylinder directly opposed to cyl 1 in the firing order. Similarly, 5 would overlap when 8 was firing, 7 would overlap while 4 was firing, 2 would overlap while 3 was firing, and so on. This is true because each cylinder fires once every two rotations. In an 8-cylinder engine, that makes for 4 firings each rotation. Most engines (this included) are even-firing, which means that these events are evenly spaced. That makes for one firing every quarter turn (90 degrees) of the crankshaft. This means that the fourth cylinder to fire (6) must be at TDC at the same time as cylinder 1. However, they are at opposite points in their cycle. Cyl 1 has compressed and is about to fire, and cyl 6 has pushed out exhaust and the valves are overlapped, preparing for intake.

Answer 5

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RE:

Why is valve overlap necessary?

I've been working with cars for almost 3 years now, I've rebuilt an L28 (i can make a lego house in terms of engine skill ;p ) I understand all the basic and "advanced" engine concepts but why is valve overlap necessary? From what I can understand, you don't want to chip or...

Answer 6

Yes its important, because if it is not there you will have an exhaust leak under the hood (bad), and it will throw off the EGR system and you will have a MIL light burning up the dash (check engine light).