Volumetric Efficiency

General engine tech -- Drag Racing to Circle Track

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Re: Volumetric Efficiency

Post by mk e »

swampbuggy wrote: Thu Sep 20, 2018 10:11 pm Holy crap, i did not expect for my O.P.to end up starting a class/discussion/arguing/session on physics LOL. ....
That's the way these things always go..."simple question" which is actually FAR from simple followed by many pages of what can only be called argument.

An engine simulation program can be very helpful in letting you "experiment" with different setups and understanding why they work or why they don't.....its all about energy conservation and to ability to velocity and pressure in the intake and exhaust into pressure in the cylinder at the moment the intake valve closes.
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Re: Volumetric Efficiency

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swampbuggy wrote: Thu Sep 20, 2018 10:11 pm can we give answers to what choices when building an engine help make a higher V.E.
#1 Who does the cylinder heads.
#2 See #1

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Re: Volumetric Efficiency

Post by SchmidtMotorWorks »

If you want to see the state of the art manifolds for American V8s, look at the Ford Coyote manifolds and Chrysler dual length runners.

The Cobra Jet (short runner) and 2018 Mustang GT (long runner) are highly optimized.

Image


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The Chrysler dual length manifold is an extraordinary piece.

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Re: Volumetric Efficiency

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As an enthusiast drag racer, a stickler for terminology and admittedly a bit of a nerd, I've always hated the term "volumetric efficiency".

It may have already been said but..

An engine ALWAYS consumes 100% of its cylinder volume. The real question(s) are:
At what density? Or, how much MASS air/fuel flow?
And most importantly... How efficiently was the fuel AND air utilized?
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Re: Volumetric Efficiency

Post by 68corvette »

Except under peak hp, where piston pushes some of the potential volume back to intake track, or if previous cycle left exhaust gases to cylinder, or if scavening is working and you can also fill swept volume with fresh charge?

I guess "always" is a strong word and I am not sure what "consumes" means, but I assumed incoming volume through intake valve :D
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Re: Volumetric Efficiency

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86_regal wrote: Fri Sep 21, 2018 12:32 pm As an enthusiast drag racer, a stickler for terminology and admittedly a bit of a nerd, I've always hated the term "volumetric efficiency".

It may have already been said but..

An engine ALWAYS consumes 100% of its cylinder volume. The real question(s) are:
At what density? Or, how much MASS air/fuel flow?
And most importantly... How efficiently was the fuel AND air utilized?
I'd agree what and how much we can get into the cylinder and how its utilized is important. I think a cylinder spends a majority of its RPM range with less than 100% air/fuel mixture in its volume. Extending that RPM range where we do is key.
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Re: Volumetric Efficiency

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I think this thread just made another full loop back to the beginning :wink:
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Re: Volumetric Efficiency

Post by SchmidtMotorWorks »

mk e wrote: Fri Sep 21, 2018 1:37 pm I think this thread just made another full loop back to the beginning :wink:
Calling Vannik, to sort this out.
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Re: Volumetric Efficiency

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86_regal wrote: Fri Sep 21, 2018 12:32 pm As an enthusiast drag racer, a stickler for terminology and admittedly a bit of a nerd, I've always hated the term "volumetric efficiency".

It may have already been said but..

An engine ALWAYS consumes 100% of its cylinder volume. The real question(s) are:
At what density? Or, how much MASS air/fuel flow?
And most importantly... How efficiently was the fuel AND air utilized?
If you think of it as referencing atmospheric pressure volume, V.E. makes sense, even if it's far from the complete story.
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Re: Volumetric Efficiency

Post by 86_regal »

68corvette wrote: Fri Sep 21, 2018 12:53 pm Except under peak hp, where piston pushes some of the potential volume back to intake track, or if previous cycle left exhaust gases to cylinder, or if scavening is working and you can also fill swept volume with fresh charge?

I guess "always" is a strong word and I am not sure what "consumes" means, but I assumed incoming volume through intake valve :D
In colloquial terms, yes "VE" does change when the piston pushes some of volume back into the intake or if the previous cycle exhaust gases in the cylinder.

My ONLY point was, In LITERAL terms, a 350ci engine "always" displaces (rather than "consume") 350 cubic inches of air volume every 2 revolutions, irrelevant of whether the throttle blades are cracked, at WOT, the inlet air temp is 500* or -90*; something was taking up the space that was made available when each piston was descending toward BDC (of course this doesn't address that some of the displaced volume does decrease when the piston ascends from BDC resulting in a reduced trapped volume, but I digress). The volume this engine displaces doesn't change.

Unfortunatley many of us have been trained or "indoctrinated" into the use of "VE" making it more difficult to cross over to the much more applicable UOM, Mass air flow.

Maybe I'm wrong, but I can't help but conclude that if more dyno/dyno sheets measured MAF, you'd see far fewer skewed correction factors.
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Re: Volumetric Efficiency

Post by 86_regal »

MadBill wrote: Fri Sep 21, 2018 2:26 pm
86_regal wrote: Fri Sep 21, 2018 12:32 pm As an enthusiast drag racer, a stickler for terminology and admittedly a bit of a nerd, I've always hated the term "volumetric efficiency".

It may have already been said but..

An engine ALWAYS consumes 100% of its cylinder volume. The real question(s) are:
At what density? Or, how much MASS air/fuel flow?
And most importantly... How efficiently was the fuel AND air utilized?
If you think of it as referencing atmospheric pressure volume, V.E. makes sense, even if it's far from the complete story.
I understand there are CFs to equate VE to sea level air density standards and the other things missing from the complete story.

As per my last post, why use a metric like volume, which is fixed, unchangeable and having only very little to do with the actual mass of air "consumed" by an engine and then apply CFs which are commonly distorted, to determine an engines breathing efficiency when we could simply use a mass air flow without CF?
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Re: Volumetric Efficiency

Post by mk e »

SchmidtMotorWorks wrote: Fri Sep 21, 2018 2:18 pm
mk e wrote: Fri Sep 21, 2018 1:37 pm I think this thread just made another full loop back to the beginning :wink:
Calling Vannik, to sort this out.
I don't think there is any real question about what it is or how it works.....the issue seems to be that many wish there was a silver bullet to make hp.
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Re: Volumetric Efficiency

Post by swampbuggy »

Randy 331...I think i am setting pretty good as far as cylinder heads are concerned, its the parts i still have to buy that i have to be concerned with. Mark H. :)
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Re: Volumetric Efficiency

Post by David Redszus »

My ONLY point was, In LITERAL terms, a 350ci engine "always" displaces (rather than "consume") 350 cubic inches of air volume every 2 revolutions, irrelevant of whether the throttle blades are cracked, at WOT, the inlet air temp is 500* or -90*;
A 350 cubic inch engine will displace (or ingest) 717cc of air per revolution, regardless of rpm or other conditions.
Using the SAE standard J1349, which is 29.24 inHg at 77F deg, the ingested air mass would be .00092 lbs per revolution.
This would occur at the engine torque peak, not the power peak.

If Standard Temperature conditions are used (29.92 inHg and 32F deg) the air mass would become .00102 lbs per revolution.
This would result in an increase of Volumetric Efficiency of 10.8%.

The air volume has not changed, nor can it change; but the air density has changed, causing a change in air mass and therefore in VE. Obviously, changes in pressure and temperature will cause substantial changes in VE and performance.

Since, for our engine the peak VE occurs at the torque peak, we can call that OUR 100% VE point, and every other torque value, above or below, will represent the relative change in VE. In effect, the torque curve of the engine becomes the VE curve for
that engine; only the units are relative to it peak torque value.

Take any engine rpm vs torque curve, and divide each value by the peak torque and graph the result against rpm. That is the VE curve for your engine. It will change from day to day, perhaps even hour to hour.

But the trick is to improve or raise the curve. How?
We can raise the inlet pressure using inlet tuning or forced induction. Or we can lower the inlet air temperature.
Or we can try to trap the ingested air and prevent its escape due to flow reversion.

Of course, we can change the way the engine burns the fuel. But that is no longer relevant to VE; it then slides into the realm of CE, Combustion Efficiency.
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