How an engine works.Waldron1943 t1_iwerj2m wrote
Reply to comment by evilleppy87 in How an engine works. by ooMEAToo
They do open before BDC. Imagine a graph with two curves plotted. One curve is the "benefit" you get from leaving the valve closed and using every last bit of pressure. The other curve is the "benefit" you get from opening the valve early and starting exhaust flow. Those two graphs cross before BDC; there's more "benefit" to opening the valve than there is to keeping it closed.
Also, it's actually open past TDC. Right at TDC you've invested energy in establishing flow; if you slam the exhaust valve right then that flow just "piles up" against the valve, which doesn't help you. As a matter of fact, the intake valve opens (and stays open a surprisingly long time) before the exhaust valve closes...that's called "valve overlap". Less overlap = low RPM power, longer overlap = high RPM power. Overlap uses that flow through the exhaust valve to pull more fuel & air mixture into the cylinder.
myawesomeself t1_iwgmva8 wrote
I think this is what everyone expects, that the exhaust should be open near TDC so it might overlap with intake a little, however in the model it clearly closes completely somewhere around halfway up and there is a noticeable gap between the exhaust valve closing and intake opening which is the unusual timing people are talking about.
Waldron1943 t1_iwhkl0j wrote
Oh yeah, I mean in real engines. The model is all fuxored up; check out the valve clearances!
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