Valve overlap

[408 Nova]

Easy. Air is flowing from higher pressure to lower pressure.
As long as the pressure above the exhaust valve, is lower then the pressure below the exhaust valve, air is flowing out of the combustion chamber, into the exhaust port.
As long as the pressure below the intake valve is lower then the pressure above the intake valve, air is flowing out of the intake port, and into the combustion chamber. It doesn't matter where the piston is.

This description sounds more like normal operation, without taking into account the affects of overlap on the engine.

All that applies with overlap as well. An engine works off of pressure differentials, no matter what part of the valve timing curve it’s at. If none of the above exist, you get reversion / charge contamination, overlap or not.

Reversion/charge contamination during overlap is what causes the motor to run rough until it "comes up on the cam". Coming up on the cam is when reversion/charge contamination is no longer occuring, and the column of exhaust gases exiting the exhaust port and header tube has enough momentum to start clearing the combustion chamber/intake port of exhaust gases, and starts bringing in a fresh air/fuel mixture into the chamber and cylinder. The question asked was about overlap. Camking didn't specifically say that the column of exhaust gas exiting through the exhaust port and header tube drawing on the intake port when both valves are open, is overlap.

[Krooser]

Could we discuss what is happening in the intake port and combustion chamber etc during the overlap cycle?

Easy. Air is flowing from higher pressure to lower pressure.
As long as the pressure above the exhaust valve, is lower then the pressure below the exhaust valve, air is flowing out of the combustion chamber, into the exhaust port.
As long as the pressure below the intake valve is lower then the pressure above the intake valve, air is flowing out of the intake port, and into the combustion chamber. It doesn't matter where the piston is.

Perfect , simple terms explanation Mike. It also shows how a piston in motion alters the dynamic and can cause reversion. The only other thing that "can" hurt flow at overlap is a giant dome on a piston , especially if it is an opposing valve ( hemi) head.
See you next week "to pick up a sticker " LOL

You and me both... plus I have to pitch my bomber sponsorship proposal....

[Warp Speed]

This description sounds more like normal operation, without taking into account the affects of overlap on the engine.

All that applies with overlap as well. An engine works off of pressure differentials, no matter what part of the valve timing curve it’s at. If none of the above exist, you get reversion / charge contamination, overlap or not.

Reversion/charge contamination during overlap is what causes the motor to run rough until it "comes up on the cam". Coming up on the cam is when reversion/charge contamination is no longer occuring, and the column of exhaust gases exiting the exhaust port and header tube has enough momentum to start clearing the combustion chamber/intake port of exhaust gases, and starts bringing in a fresh air/fuel mixture into the chamber and cylinder. The question asked was about overlap. Camking didn't specifically say that the column of exhaust gas exiting through the exhaust port and header tube drawing on the intake port when both valves are open, is overlap.

Overlap is exactly as Mike stated, pressure differentials.
That is really how the whole engine operates.

[plovett]

Does anybody except me get excited about the Ideal Gas Law? PV=nRT?

It, along with pressure differentials, explains SO MUCH about what is going on in an internal combustion engine.

paulie

[408 Nova]

[quote=hoffman900 post_id=702610 time=1512117681 user_id=20313


All that applies with overlap as well. An engine works off of pressure differentials, no matter what part of the valve timing curve it’s at. If none of the above exist, you get reversion / charge contamination, overlap or not.

Reversion/charge contamination during overlap is what causes the motor to run rough until it "comes up on the cam". Coming up on the cam is when reversion/charge contamination is no longer occuring, and the column of exhaust gases exiting the exhaust port and header tube has enough momentum to start clearing the combustion chamber/intake port of exhaust gases, and starts bringing in a fresh air/fuel mixture into the chamber and cylinder. The question asked was about overlap. Camking didn't specifically say that the column of exhaust gas exiting through the exhaust port and header tube drawing on the intake port when both valves are open, is overlap.

Overlap is exactly as Mike stated, pressure differentials.
That is really how the whole engine operates.
[/quote]

A basic 4-stroke engine like we’re talking about here works off of pressure differentials created by a piston. Overlap is an entirely different thing.

[GARY C]

LSA is just an average is is relatively meaningless.

All that applies with overlap as well. An engine works off of pressure differentials, no matter what part of the valve timing curve it’s at. If none of the above exist, you get reversion / charge contamination, overlap or not.

You need the separation between the lobe to achieve the proper 4 valve events and overlap to properly deal with those pressure differentials so is it really meaningless?
Some engines need more some need less but they are still things that need to be understood when building an engine.
There have been 2 stroke engine that don't use rings and they don't need valves so should I throw out learning about rings, valves and valve seat angles for my V8?

[CamKing]

A basic 4-stroke engine like we’re talking about here works off of pressure differentials created by a piston. Overlap is an entirely different thing.

That's where you're wrong.
It's all about pressure differentials.
The piston motion is just partially responsible for the pressure differentials.
We have positive flow before TDC, because the exiting exhaust causes a low pressure area below the intake valve during overlap.
We also have positive flow after BDC, because the velocity of the flow in the port, and the closing intake valve, keep the pressure above the intake valve higher then the pressure below it, even though the piston is on it's way back up.

[CamKing]

Overlap is exactly as Mike stated, pressure differentials.
That is really how the whole engine operates.

Shush. That's top secret.

[David Redszus]

Overlap is exactly as Mike stated, pressure differentials.
That is really how the whole engine operates.

Shush. That's top secret.

If you keep giving away valuable information there will be little left for us to sell.

[RevTheory]

That's kind of hard to dispute, isn't it?

[CamKing]

That's kind of hard to dispute, isn't it?

It's really as simple as watching water flow down hill, into a pond.

[piston guy]

Engines "can" run without overlap because of pressure differential caused by piston motion alone. Taking advantage of pressure differential created by residual motion ( exhaust gas flow /vacuum) with the use of overlap in valve timing can be very beneficial additionally.
Is that "scholarly" enough LOL May be what 408 nova was getting at.

[ptuomov]

Engines "can" run without overlap because of pressure differential caused by piston motion alone. Taking advantage of pressure differential created by residual motion ( exhaust gas flow /vacuum) with the use of overlap in valve timing can be very beneficial additionally.
Is that "scholarly" enough LOL May be what 408 nova was getting at.

Even if there’s zero valve overlap, at many rpms the engine intake will still breathe based on exhaust scavenging. If you pull a vacuum in the combustion chamber and close the exhaust valve, the vacuum is still there when the intake valve opens.

[kirkwoodken]

If you forget about 4 strokes and read up on 2 stroke exhaust systems, you will see the real meaning of "coming on the pipe". The exhaust pulsing may pull in intake mix while the piston is at the bottom of the stroke.

https://search.yahoo.com/yhs/search;_yl ... &fr2=12642

I've seen small 2 stroke engine HP claims as high as 6 HP per cubic inch, normally aspirated, achieved without valves, and never any concerns about valve float.

[GARY C]

If you forget about 4 strokes and read up on 2 stroke exhaust systems, you will see the real meaning of "coming on the pipe". The exhaust pulsing may pull in intake mix while the piston is at the bottom of the stroke.

https://search.yahoo.com/yhs/search;_yl ... &fr2=12642

I've seen small 2 stroke engine HP claims as high as 6 HP per cubic inch, normally aspirated, achieved without valves, and never any concerns about valve float.

I tend to think a lot of future 4 stroke power will come from the exhaust side as we learn how to better develop it.
It seems over the years of 4 stroke development overlap has decreased...