overlap

[Walter R. Malik]

You can not put a concrete figure upon the affects of INERTIA as it never remains the same unless it is constant.

[nitro2]

I've never seen the pressure wave lower the pressure above the intake valve during overlap.

I'm not clear on what you are saying. Are you saying that you haven't seen the pressure wave in the intake at overlap go negative or were you saying something else?

[CamKing]

I've never seen the pressure wave lower the pressure above the intake valve during overlap.

I'm not clear on what you are saying. Are you saying that you haven't seen the pressure wave in the intake at overlap go negative or were you saying something else?

I'm talking about the pressure right above the intake valve.
It's the pressure differential at the valve, I'm concerned with.

Maybe you have more accurate data then what I've seen.
When testing between peak torque and peak HP, I have not seen the pressure right above the intake valve drop below the pressure in the chamber during overlap.
I'd love to see any data you have. Maybe I'm missing something.

BTW, I don't like to use terms like "negative pressure", because it can confuse people. That's why I look at absolute pressure only.
If the pressure in the port in only 25", that looks like a negative pressure compared to atmospheric pressure(29.92"), but it's still a possitive pressure compared to the 15" in the chamber.

[lorax]

I'm talking about the pressure right above the intake valve.
It's the pressure differential at the valve, I'm concerned with.

Maybe you have more accurate data then what I've seen.
When testing between peak torque and peak HP, I have not seen the pressure right above the intake valve drop below the pressure in the chamber during overlap.
I'd love to see any data you have. Maybe I'm missing something.

BTW, I don't like to use terms like "negative pressure", because it can confuse people. That's why I look at absolute pressure only.
If the pressure in the port in only 25", that looks like a negative pressure compared to atmospheric pressure(29.92"), but it's still a possitive pressure compared to the 15" in the chamber.

Speaking in anything other than "A" pressures can make the above seem very confusing, and misleading.
I deal with a similar issue at work, with "heat" at temperatures well below freezing, where many don't think about "heat" even existing.
When you deal in Torr and microns, inches seems huge. But its all positive compared to 0 psia and 0 Kelvin

[CamKing]

Speaking in anything other than "A" pressures can make the above seem very confusing, and misleading.
I deal with a similar issue at work, with "heat" at temperatures well below freezing, where many don't think about "heat" even existing.
When you deal in Torr and microns, inches seems huge. But its all positive compared to 0 psia and 0 Kelvin

Yep, I get customers saying "oh my, I've got 7" of vacuum in the manifold", and I respond, "no you've got 22.92" of pressure".

[BigBro74]

You can not put a concrete figure upon the affects of INERTIA as it never remains the same unless it is constant.

Walter, I like that you are bring the term Inertia into the discussion. I know we are talking about air(fuel), but I try to keep the fact that it does have inertia in my thought process (even thought it may be misguided ). with that in mind, there must be a miniscule amount of time where inertia of the gasses in the exhaust and incoming intake mixture have some time lapse or discontinuity with some of the other forces acting upon them (?) (this is a question)

[CamKing]

that's why we use overlap to gwet the air in the port moving before the piston starts moving down.
Air has mass, and it's elastic. It takes time to get it moving, and it takes time to stop it.

[BigBro74]

Mike, that is how I always have pictured it.
But how long does it take to get the inertia going and how long for it to stop? how does the inertia change with the density in the intake? how can we come to know this part better. this is the $64,000 question to me (I seem to get it wrong often LOL).

Edit: My own ignorance just made me a pro LOL

[nitro2]

CamKing, I think the whole "what is the pressure in the intake port at overlap and how does in change with rpm" thing has been covered in a few threads, and everytime it has been a bit of a mess with some ill feelings. The OP has started a similar post in the Advanced section but no takers last time I checked, and for good reason.

Basically think of it like this, when the intake valve is closed the pressure in the intake port oscillates back and forth, back forth, back and forth, say +/-5 psi from atmospheric pressure. Each oscillation takes a certain fixed amount of time to go between + 5 and -5, which is dictated by intake length, speed of sound etc. However the engine speed is not fixed unless it is a generator. So at some particular rpm, lets say the rpm for TQ peak, the oscillation will be timed to generate say +5 psi in the intake port at IVO as an example, but when the rpm is changed (either higher or lower) IVO will no longer be timed to the +5 point on the oscillation (oscillation speed is fixed but engine speed has changed). It doesn't take too much shift in rpm to have IVO correspond with 0 psi or -5 psi on the oscillation, especially for short intake runners. The flow at overlap goes down the toilet when there is no inlet pressure, just exhaust suction, and worse yet when there is inlet suction at overlap.

Before someone points it out, the oscillation is renewed on each cycle and decreases in magnitude slightly for each bounce back and forth. It doesn't just bounce back and forth +5 to -5 or we'd have perpetual motion LOL, it starts out higher maybe +10 psi then gets a little smaller with each bounce due to friction and so forth, then gets renewed on the next cycle.

As a side note it is possible to essentially eliminate the intake oscillation by the time IVO rolls around so that the intake port is basically sitting at atmospheric pressure irregardless of rpm. However, this doesn't happen by chance, it has to be done on purpose, and since the process to get rid of the oscillations messes up other things, I think I can safely say that no one making any kind of power has a non-oscillating port LOL.

[nitro2]

BTW, I don't like to use terms like "negative pressure", because it can confuse people. That's why I look at absolute pressure only.
If the pressure in the port in only 25", that looks like a negative pressure compared to atmospheric pressure(29.92"), but it's still a possitive pressure compared to the 15" in the chamber.

I would agree with that for some things, however when it comes to port pressures on an NA engine, that would be like quoting gas mileage in litres per 100 km to the average US citizen LOL. I still have to convert that nonsense into miles per gallon to relate to it. Telling someone the exhaust pressure at 10 BTDC is 15"Hg will mean nothing to them but telling them it is 7 psi suction is something that most people can relate to.

In any event absolute pressures are useful for doing proper calculations, but the software does all that good stuff. Stating pressures in absolute units instead of relative units won't change the flow, its just whatever is convenient, so I'll let you convert my numbers .

[CamKing]

Telling someone the exhaust pressure at 10 BTDC is 15"Hg will mean nothing to them but telling them it is 7 psi suction is something that most people can relate to.

But that's just it, it's not suction. It's still positive pressure.
You wouldn't tell someone living in Denver, that there's suction outside his front door.
Those pulses are not possitive to negative pressure. They pulse from more pressure to less pressure, but they're always possitive pressure.

[nitro2]

Telling someone the exhaust pressure at 10 BTDC is 15"Hg will mean nothing to them but telling them it is 7 psi suction is something that most people can relate to.

But that's just it, it's not suction. It's still positive pressure.
You wouldn't tell someone living in Denver, that there's suction outside his front door.
Those pulses are not possitive to negative pressure. They pulse from more pressure to less pressure, but they're always possitive pressure.

Of course and most people know this, nonetheless, suction is a word and people relate to it. It is expected that one knows that there is no such thing as negative absolute pressure, just as there is no such thing as negative absolute temperature. However, people don't go around saying its 420 degrees outside (absolute) when its -40 F outside, though it would be correct to do so. I will continue to call it suction, or negative pressure, and just assume that anyone that is going to get anything out of this thread knows WTH it is referencing.

[Walter R. Malik]

You can not put a concrete figure upon the affects of INERTIA as it never remains the same unless it is constant.

Walter, I like that you are bring the term Inertia into the discussion. I know we are talking about air(fuel), but I try to keep the fact that it does have inertia in my thought process (even thought it may be misguided ). with that in mind, there must be a miniscule amount of time where inertia of the gasses in the exhaust and incoming intake mixture have some time lapse or discontinuity with some of the other forces acting upon them (?) (this is a question)

What, in your thoughts, is a "miniscule amount of time" when at a simple 5,400 RPM, the time of "overlap" is happening 45 times a second.

[BigBro74]

Walter- I was just saying that my thought process is sometimes a little misguided. (You know it has to be true!)


by time I wasn't really thinking about milliseconds, probably crank degrees would be more appropriate, not really sure because as you were saying it is changing and you cant put a real concrete number on it. I was really trying to get at how someone like Mike decides what trade-offs to make in speccing out overlap area, which he pretty well answered in one his earlier posts.

Thanks, Jason

[lorax]

Telling someone the exhaust pressure at 10 BTDC is 15"Hg will mean nothing to them but telling them it is 7 psi suction is something that most people can relate to.

But that's just it, it's not suction. It's still positive pressure.
You wouldn't tell someone living in Denver, that there's suction outside his front door.
Those pulses are not possitive to negative pressure. They pulse from more pressure to less pressure, but they're always possitive pressure.

Of course and most people know this, nonetheless, suction is a word and people relate to it. It is expected that one knows that there is no such thing as negative absolute pressure, just as there is no such thing as negative absolute temperature. However, people don't go around saying its 420 degrees outside (absolute) when its -40 F outside, though it would be correct to do so. I will continue to call it suction, or negative pressure, and just assume that anyone that is going to get anything out of this thread knows WTH it is referencing.

People don't go around saying 420 degrees, (which BTW is 233+ K) but the majority of the world does say -2.22 C when the U.S. say +28F. You are comparing scales. Its why Kelvin and Pascal and even BAR are easier to work with in engineering. Even Hg is OK, if you keep in mind that anything less than 29.9 is still a positive pressure.

Put 6 Joe Plumbers in a room with a hole in the wall and air passing thru it to the other side, where you have 6 engineers. The plumbers say the air is being SUCKED thru the hole from the other side, and the engineers say its being PUSHED from the other side.
Are you telling me the plumbers are right?