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Volumetric efficiency?

General engine tech -- Drag Racing to Circle Track

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Danglerb
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Volumetric efficiency?

Post by Danglerb » Fri Jul 31, 2009 5:10 pm

Sorry for the basic question, but I've been looking at various engine models, and I started wondering about volumetric efficiency. As I understand it, its how well a motor pumps air.

A 5.0L motor that pumps through 4.0L of air would be 80%.

My question is if the practical upper limit for volumetric efficiency would be a flow of the displacement plus the volume above the piston at TDC?

Using the 5.0L with a head volume of 60cc's and a flat top piston to make it simple, total volume above the pistons would be 480cc's, so max volumetric efficiency would hit a wall about 5.48/5.0 = 110%?

Without boost I am having a hard time guessing how it could be more.

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Post by SWR » Fri Jul 31, 2009 6:16 pm

You have 680fps air coming in, slow that down to zero and you're - possibly,if all else is working in conjunction - looking at 25-some % pressure increase over ambient. I belive F1 was a fair bit over 130% now...
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Post by beth » Fri Jul 31, 2009 8:52 pm

This is the way I look at volumetric efficiency.

Volumetric efficiency percentage is the amount of volume captured by the cylinder at the moment the intake valve closes divided by the displacement volume. The captured volume is typically less than the volume that enters the cylinder because some is lost out the exhaust valve during overlap.

The piston is already on it's way up the cylinder by the time the intake valve closes and the ingested volume has been heated by the valves, piston top and chamber so the cylinder is pressurized considerably even at 100% volumetric efficiency.

Volumetric efficiency is dependent on good breathing during peak flows, pressure wave tuning so the pressure waves arrive at helpful times and high velocity flow with it's resulting high pressure during intake valve closing.

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Post by FastFourierTransportation » Fri Jul 31, 2009 9:09 pm

Can-o-worms here. Just make sure you don't bring up the notion of VE with forced-induction motors. No one seems to be able to agree on exactly how to define it because of the non-static readings for air "pressure", and difficulty in measuring density, or what would be 100% "target" density. :shock:

It's much simpler to define in a naturally aspirated engine, thank god. :lol:
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Post by MadBill » Fri Jul 31, 2009 10:19 pm

-But even NA, you can get arguments! The simplest to grasp (and measure) is [(dyno air meter reading)/(engine displacement in cubic feet/2 x RPM.)] x 100. Thus a 350 c.i. engine running at 6000 RPM and consuming 600 CFM would have [600/(350/1728/2) x 6000] x 100 or 98.7% V.E.

Beth describes another definition: The percentage of trapped CFM Vs. the engine's capacity and RPM. This discounts any volume blowing ineffectively (or relatively so) on through the cylinders and out the exhaust. Unfortunately, this is hard to measure outside of a well-equipped laboratory, so most builders must fall back on B.S.A.C. , Brake Specific Air Consumption, for clues.

For example, let's say you have an engine on the dyno equipped with a pretty basic set of headers and have run a few cams and index settings through it to optimize power. Now you add a set of triple step dyno and PipeMax-optimized headers, in the expectation of great results. Instead, mid range is up a bit, but top end is nothing special. Looking at the dyno data, V.E. is up substantially at the top end, but B.S.A.C. is too! What's wrong? Answer: The cam has too much overlap for the 'super sucker' exhaust, and substantial fresh mixture is blowing right on through the engine, adding little or nothing to the power output.
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Post by #84Dave » Fri Jul 31, 2009 10:53 pm

I agree with FFT. Volumetric Efficiency determination/definition is a mess. Gordon Blair says even SAE has it wrong. That's why he uses 'Delivery Ratio'. The way I look at it, one can't have a V.E. over 100% unless the air/mixture is referenced to some standard pressure & density. My simple mind tells me....... if one has a 355 cubic-inch swept volume SBC with a 37 cubic-inch total chamber volume, for [8] cylinders, the max capacity is 392 cubic-inches. Period. That's the total volumetric capacity of the engine. Whether you're filling it with air/fuel or liquid concrete! Trapped volume is another story. Again, that's why Blair uses Delivery Ratio.
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Post by Danglerb » Sat Aug 01, 2009 4:52 am

What sort of things are going to characterize the differences between a motor with 80% and a motor with 120% volumetric efficiency?

Ideal intake and exhaust I would think would be similar, just tuned to different rpm ranges as needed?

Is RPM a factor, or could I build a 120% motor at any RPM that suited the application?

Is it mainly valve event timing and how the intake and exhaust are tuned?

Is the amount of overlap a huge factor?

Thanks for all the info so far.

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Post by beth » Sat Aug 01, 2009 12:32 pm

I agree volumetric efficiency as I define it is impractial to measure and is not a useful tool for the engine builder. Volumetric efficiency as measured on the dyno is useful as Bill has illustrated.

I think the concept of volumetric efficiency is useful as a way to visualize how port flow, CSA, port volume, velocity, taper, temperature and cam timing events work together to help or hinder engine performance.

An engine with high VE will have all these things working together at the desired rpm.

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Post by David Redszus » Sat Aug 01, 2009 1:01 pm

The problem with Volumetric Efficiency is simply its name. It is incorrectly named.

A two liter engine will always take in two liters of air. Always.

But... the density of the air that is taken into the engine is what makes a diference. Since volume (two liters) times density equal mass, the proper term for engine efficiency should relate to Air Mass efficiency.

But the air mass that is sent through the induction system is not necessesarily the amount that remains in the engine after the inlet valve is closed. That is the trapped air mass or as Blair calls it Charge Mass.

Now it is very possible for an engine to be charged with a higher air mass than would be normal for its displacement x air density. In fact, some NA engines can exceed 125% efficiency with proper gas exchange tuning.

Forced induction systems do not increase the volume of air to the engine. They do increase the pressure and therefore the air density and therefore the Trapped Air Mass. Which of course, is where the increased power comes from.

If we know the temperature and pressure of the inlet air, and the parameters of the engine, we can calculate the air mass very easily. Now we know how much fuel to give the engine (if we know which fuel we are using).

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Post by MadBill » Sat Aug 01, 2009 3:36 pm

Danglerb wrote:What sort of things are going to characterize the differences between a motor with 80% and a motor with 120% volumetric efficiency?

Ideal intake and exhaust I would think would be similar, just tuned to different rpm ranges as needed?

Is RPM a factor, or could I build a 120% motor at any RPM that suited the application?

Is it mainly valve event timing and how the intake and exhaust are tuned?

Is the amount of overlap a huge factor?

Thanks for all the info so far.
Not easy to get a simple answer here, is it? :)
One reason is that a whole book could be (and many have been) written on the subject, but briefly: Engine V.E.s above 100% require that pretty well all the design variables are optimized for power with a minimum of compromises. Minimum throttle and intake manifold restrictions and correctly chosen runner lengths, very efficient flowing ports, cam specs matched closely to the port flow characteristics, intended RPM range and intake exhaust system dimensions, highest possible compression ratios for the fuel used and more.
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Post by Danglerb » Sat Aug 01, 2009 5:44 pm

I'm reading Phillip Smith's scientific intake and exhaust design book, but despite having a good technical background in physics its slow going for me (works wonders for falling asleep at night though). Strict definition doesn't bother me, Charge Mass is fine, as long as it relates back to standard temperature and pressure CFM of air outside the system going in.

What sticks in my mind as a basic truth is that engines are air pumps. Nobody can make HP without the air flow into the motor to support it. More specifically torque is directly related Volumetric Efficiency, and since peak torque is significantly lower in rpm than peak HP, I am making the guess that flow constraints are small factors, and torque can have a pretty wide band, so also guessing manifold frequency tuning is less of a factor and flow rates and mass in the tubes are larger factors?

I think I like my original questions, and suspect that when I find the answers I will have learned something. ;)

**************
What sort of things are going to characterize the differences between a motor with 80% and a motor with 120% volumetric efficiency?

Ideal intake and exhaust I would think would be similar, just tuned to different rpm ranges as needed?

Is RPM a factor, or could I build a 120% motor at any RPM that suited the application?

Is it mainly valve event timing and how the intake and exhaust are tuned?

Is the amount of overlap a huge factor?

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Post by ibmorjamn » Sun Aug 02, 2009 11:27 am

this subject sort of ties in to my cam phasing question . One thing I have noticed about V.E. is most of the time I have heard a number it was derived from a sim program so if that were the case it would be theoretical just like theoretical HP from a flow bench. the problem I have with that is it rarely equates to real #

I have heard cylinder pressure is very important to get the best results and cylinder pressure would be running or dynamic and regulated by valve events. so going with the "air pump" as described the most amount of air/fuel you can trap , compress and ignite. of course the amount of air going in is relative to flow and velocity and I simply look at mcsa or choke point. I know it is more complicated but I can only see the valves and the cross section limiting the flow. I think velocity would increase with rpm. so more overlap would blow intake charge out of the exhaust at low rpm but at higher rpm become more efficient as the air speed picks up.
feel free to correct me on any of this !
I have a short distance to go and even shorter time to get there !

My single is 108mm x 70.6mm 114.47mm rod or 4.251" x 2.77" rod 4.5" also makes 45 ft. lbs @ 6800 rpm
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Post by BrazilianZ28Camaro » Sun Aug 02, 2009 12:00 pm

I think that VE is directly relationed with the correct intake/exaust/valve timing to get advantege from the correct intake PULSE at desired rpm and use the complete pulse lenght to fill the cylinders using the total inertia from the charge to create a higher than atmosferic internal cylinder pressure.

To me, the exaust excevenge associated to this move can, efectivally fill the cylinder with a bigger AF volume, and therefore the engine breathes more air than displaces.

Tests show incredibly 5-7 psi above atm at intake head port in highly developed NA engines that are correctly pulse-tunned acting like a supercharger.

A good indicator of VE is especific power, like hp per CI. as seen on F1 and PS engines.

Just my $.02
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Post by David Redszus » Tue Aug 04, 2009 1:35 am

Lets suppose we were to run an engine with the head entirely removed. What would be the VE of the air flowing into the cylinder? If you said 100% you would be correct. Now all that matters is the density of the air in order to determine the air mass flow. Once we know the mass air flow we can determine the amount of fuel that can be burned and the heat energy that can be produced by this "engine".

But if we spin the engine fast enough, even without the head, at some point we will no longer be able to maintain a VE of 100%.

Now lets attach a cylinder head with valves, ports, cams, inlet runners, and exhaust pipes. How easy will it be to get back to our VE 100% figure? Anything that gets in the way of an air molecule will reduce the VE.

There is only one way to reach or exceed our VE 100% value; we must increase the inlet pressure. And that is exactly what tuned induction and exhaust systems do; they increase the inlet pressure. The air flow will increase with the square root of the pressure differential.

But air is compressible; it stretches and compresses, which makes our task very much more difficult. Reducing friction will not make more power. It will reduce the power lost to friction.

The only real way to make more power is to burn more fuel, either per stroke or per unit time.

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Post by Danglerb » Wed Aug 05, 2009 1:57 am

I don't know how you meaningfully discuss flow if standard density, or scfm isn't implied, so for the sake of my sanity lets assume flow is scfm or at least based on it.

No head, just a piston moving up and down in a cylinder flush with a flat deck maybe? Nothing is trapping the air, but it might be educational to consider the maximum mass of air "inside" the open end of the cylinder in a cycle.

Very slow the Ve is 100%.
Very fast it approaches 0% (piston much faster than super sonic).
Someplace in the middle its going to be more than 100%
More than that and I need some paper and a book to figure out.

Amount of fuel burnt can be tuned rich or lean, so I prefer to think of engine performance as the amount of air it can pump, not including blow thru to exhaust from overlap. Charge mass maybe.

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