Date: Sun, 8 Dec 1996 19:54:37 -0800 (PST) From: Mike West Subject: Re: Valves: Overlap on cam >I don't understand. You mean, that while the piston is moving away from >the top, creating vacuum, the intake valve is open, and the exhaust valve >is open? Pulling in exhaust? Is the intake valve open when the piston is >moving toward the top, and the exhaust is being pushed back into the intake >manifold? > >Also, help me out here. This is a 2 stroke engine. One stroke sucks in >air/fuel (piston moving down)(valve(s?) open) and then compresses mixture >(piston moving up)(valves closed), second stroke is igniton and expansion >(piston moving down)(valves closed) then expulsion of exhaust (piston >moving up) (valve(s?) open). Is this right? What then is a 4 stroke >engine? And is my weedeater a 2 or 4 stroke engine? > >Thanks for letting me bother you, Eric Rainey >VW busses, Mountain Bikes and Macintosh. What else is there? - - - - - - - - - - - - - I could cop out here and just point at the text but I'll give it a try. In the case of the simultaneous open, which is caused by "cam lobe overlap" there is an period of time at top dead center of the piston prior to the intake stroke when both valves are, shall we say "partially open". What you don't see is how fast this happens and that the exhaust cycle hasn't competely ended and that those gasses have inertia of their own. They are doing some sucking here at the end of cycle. Also the magic shit about having back-pressure on the exhaust comes into play here. Just enough but not too much. And RESONANT FREQUENCIES !! you wonder how magic got started! There should be just enough overlap that the fuel/air mix starts towards the exhaust valve and gives it that brief cooling and then the exhaust is shut before any fuel/air can get out the exhaust. Without this slight draw by the exhaust gases on the fuel/air, the poor damn exhaust valve will never get any cooling. This all happens while the piston is at top dead center and hasn't started to draw the air/fuel in yet. Just that "inertia" magic. If your exhaust is without back pressure and the cam is designed to have some then yes, it is conceivable that the exhaust gas inertia will actually pull fuel/air out the exhaust valve. This all happens while the piston is sitting at TDC and the engine is turning 4500 rpm. Now the exhaust closes completely, the intake valve is still opening, and the piston starts down drawing the fuel/air away from both valves and into the cylinder itself. That's how brief that overlap is. Then to site an experience we've all had, "thru no fault of our own", everything get's out of whack and the car "backfires". So exhaust can come out your nose too! Just don't do it again. You realize I'm divulging secrets of the "Cam Grinders Guild" here. If I disappear some night, you'll know why. If I tell all, you guys will be out in the shop filing your own "bump sticks" out of bar stock. Hahhh! :-) The 2 stroke - 4 stroke thing: See the post titled "strokin' and engines", then go look in any cheap dictionary for the definition of "two cycle and four cycle". Definitions are mans way of agreeing on a subject. If you don't agree, it's ok . Just means we can't talk to each other on that subject. Doesn't affect that 1600 engine out there one way or another. Let me be clear on one thing. I am not a "master mechanic". I'm a very ex-engineer who is trying to dispell a lot of bullshit that some grease monkey is trying to sell me in lieu of services. I've given up on him and will build my own from now on but I'm a "doubting Thomas". I have to know why part "A" goes in part "B". Not just because someone said so. So I read some books. . . . and some Sermons. If you want to build engines check with Bob, he's a mechanic. He may or may not know about the "Rhomboid lattice structure of crystaline materials" but he does know "it breaks when you do that". This is way too long. west I can't believe I'm talking to a "Macintosh Puke" :-)