When an engine is at TDC are any of its valves supposed to be in the "open" position?

 

When a particular cylinder has a piston at TDC, all the valves in that cylinder are closed.

At TDC for a particular cylinder, the cams lobes are pointing upwards and away from the motor in opposite directions.

 

yep! that's the way.

 

question #2

is there any point where all of the valves in the engine (exhaust and intake) are all supposed to be closed?

 

No

 

Question #1- at TDC... during the end of the compression stroke only (leading into the power stroke). TDC just after the exhaust stroke (leading into the intake stroke) will generally have either exhaust, intake or both sets of valves open.

Question #2- Not in multi-cylinder engines. A single cylinder works as stated above, but any engine with more than 1 cylinder will have at least one other cylinder in a position requiring valve lift.


[Modified by texan, 8:50 PM 1/22/2003]

 

If you're referring to all the cylinders (4 in this case) in the engine, then the answer is no.

The only time that the valves in any given cylinder are closed is at TDC. In a four cylinder engine, the #1 and #4 cylinders are in the same point position, and #2 and #3 cylinders are in the same physical postion.

Firing order alternates b/w pairs (#1 & #4) and (#2 & #3). 1-3-4-2

Think of like this.....#1 fires at TDC. Then either #2 or #3 can fire (but not #4) b/c each pair of cylinders is 180 degrees out of phase with the other. They chose #3 to fire instead of #2 b/c of better engine harmonics. After #3 fires, then it has to alternate back to the #1/#4 pair....and so on and so forth.

 

TDC <u>leading into the power stroke</u>.

 

oh yeah...shoulda extrapolated.

 

ok this is a question from "IN VTEC"'s response. So if #1 and #4 are at TDC and their valves are close. #2 and #3 are at the bottom of their stroke. Would be possible to have two valves in cylinder #2 to be open and 0 valves open in cylinder #3?

Thanks for clearing this up for me.

 

Yes.

Both would be on their way up. #3 would be on the compression stroke leading into TDC where the intake valves are just starting to close, and #2 is 180 out of phase with #3...putting it in the exhaust stroke leading into TDC, where the exhaust valves are just starting to open. I should also say that the #3 intake valves wil continue to close and the #2 exhaust valves will continue to open...leaving you with a little bit of 2 open / 2 open overlap, but it'll eventually get you to #3 closed / #2 exhaust valves open.


[Modified by IN VTEC, 11:49 PM 1/22/2003]

 

If #1 is about to ignite, then yes, it's valves are closed, #4 will be at the top of the exhaust stroke so the exhaust valve will still be open.

 

Back to basics:
Four strokes of your average four stroke motor are-intake,compression,power,exhaust. During the intake stroke, the piston is going down and the intake valve is open. During the compression stroke, intake and exhaust valves are closed, piston is coming up. Power stroke-the spark plug has fired, the mixture is burning and expanding, pushing the piston down, intake and exhaust valves are closed. Exhaust stroke-piston is coming back up, pushing the burnt mixture out of the open exhaust valve. The cycle starts over.
Note: there is a point between the exhaust stroke and the intake stroke where both the intake and exhaust valves are open. The piston is approaching TDC on the exhaust stroke, and the exhaust valve is closing. The intake valve is just beginning to open in preparation for the intake stroke. This is known as "overlap" the point at which both the intake and exhaust valves are open. Theoretically, the exhaust gasses going out of the exhaust valves cause a slight vacuum and draw a fresh charge of air and fuel in through the intake valves.
Honda N/A motors seem to like a lot of overlap to produce bigger HP at high rpm.
Edit:answer to question-yes. Intake and exhaust valves are both open at TDC between exhaust and intake strokes.


[Modified by b19coupe, 4:36 AM 1/23/2003]