How an engine works.evilleppy87 t1_iwdssjt wrote
Reply to comment by drsbd in How an engine works. by ooMEAToo
Shouldn't it be open til the piston reaches TDC? If so that means with those cams it should open later in the exhaust stroke. It seems like they start opening just before the piston reaches BDC, and close half way up the exhaust stroke.
drsbd t1_iwdur3t wrote
Now that I look again you're right they are only open for roughly half the exhaust stroke. Good eye
Waldron1943 t1_iwerj2m wrote
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!
Snufflepuffster t1_iwe3wrq wrote
often valves close or open early to take advantage of acoustics (shock waves) to ram air into the cylinder or throw it out. So timing is very nuanced and can be adjusted drastically from engine to engine. In general the exhaust valve isn’t going to match the stroke, it might be early or late, extended or short due to design.
evilleppy87 t1_iwe8c32 wrote
I'm aware that there's going to be some variability, it just seemed to me that closing 90° before TDC was a little extreme, especially considering in most cases the exhaust valve closes a little after TDC
Drunken-samurai t1_iwge1ej wrote
Bringing up variability, there is VVT (variable valve timing) which takes advantage of this in real time to use different valve timings at different RPM's to provide more efficiency or power.
evilleppy87 t1_iwgrf7o wrote
Like Honda VTEC, for example?
Drunken-samurai t1_iwkt2fq wrote
Yes your right, this is part of what the VTEC acronym is (Variable Valve Timing & Lift Electronic Control).
Wayed96 t1_iwgmkik wrote
And even a little longer to improve intake speed
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