obviously the Mass flow rate is greater in FI, but assuming equal engines, which one moves a greater velocity of air.

I always figured FI since the greater pressur differential will create a greater velocity.


arugment i've been hearing are

"
A piston engine is a positive displacment pump. It delivers a constant volume of air as a function of displacment and frequency (rpm)." which i dismiss because it assumes constant VE, which is incorrect
_________________


and there are some other comments stating volume flow is equal or greater for NA, which i dont see.

 

That's a good question! If I were more of an engineer I could probably figure it out. I suspect that both the density and the velocity of the air charge are increased, but would certainly like to learn more about it.

Rich

 

Rich said it well in 15 words. "I suspect that both the density and the velocity of the air charge are increased."

With the same amount of valve open time (approximately) the FI engine has to move more molecules of air (mass) in the same time, so it either moves them faster or packs them closer together. Probably ( I hate "prolly", BTW) both.

Isn't "pressure" basically defined as how many molecules are contained in a given volume?

I suspect a centrifugal supercharger/turbocharger might accelerate the molecules to a higher velocity than the positive displacement type which pumps a given volume for every revolution.

In any event, more molecules of air (mass) injested during the intake cycle = higher VE, right? The volume that the molecules occupy depend upon the temperature and pressure, so VE might be defined as the volume of air at standard temperature and pressure (STP) which the engine actually injests compared to the swept volume (displacement) at STP.

In my mind "volume flow" isn't really a descriptive term. Assume volume in cubic feet and flow in cubic feet per minute (cfm). "Volume flow" would then be "cubic feet cubic feet per minute" which isn't very intuitive.

As for a piston engine being a positive diaplacement pump, that's not quite right if VE changes with rpm which it almost always does.

FWIW, velocity isn't volume nor mass, just like speed isn't distance, so we are more concerned with the amount or mass of air moved than it's velocity. Sure at a given pressure difference, which is necessary to move air, higher velocity with the same density means more (mass) flow. If the density was less, however, higher velocity might mean the same or even less flow. An example might be a port on an NA engine that was too small so that the velocity was very high, but the density was low because the molecules were farther apart, and total airflow (mass) was less, and therefore power was down. Remember Bernoulli's thoughts on pressure and velocity.

Damn, but compressible fluids are difficult to deal with.

 

I thought pressure was the force acting on an object and the number of molecules in a given container the density.

Not being a wiseass. Just reality checking.

 

speaking with our VP of engineering (very qualified man, designed some very impressive motors for some teams), I asked him about velocity NA vs SC.


His answer was that it just depends too much to give a yes or no answer. Runner cross sections are usually tuned to application so in any performance oriented vehicle. both are pretty much close to max velocity (mach 1?)

But if velocity is damn close to equal, Boosted heads usually run much larger ports so i guess they move larger volumes.

but your answer begs the question, why is volume flow "cubic feet, cubic feet per second.

Area * velocity (in standard unutsi) comes out to be Ft^3/s

pretty easy to visualize that.


I know mass flow is "whats important," this just came up in discussion.

 

I guess the correllation of pressure and density in what he said is more of a hint than anything......

Velocity is an issue in any motor and there are always limits that affect the potential of the motor in HP and RPM and in a positive atmosphere motor it has to be an issue also. Couple that with the higher temps you see in blower motor the density of the air has to be a big part of the equation.

Think about an alcohol motor, it has 3 times the fuel volume in the ports so that has to be taken into account when you work on port volume and cross section. When you are at 12:1 fuel takes up very little but 4:1 the fuel is 20% of the port. Does that mean the port is 20% larger to account for that? It's probably around there (don't know I only drink alcohol not burn it) but a wet flow bench would say a lot here, and some density calculations of both fuels at each given AFR.

The best solution instead of enlarging the port in an Alky motor is to put down nozzles in the head to introduce the fuel in the bowl area of the port and not compromise the rest of the port. Audi and their direct injection Lemans cars is the next step from down nozzles. I think is a good idea since you only have to worry about fuel flow and movement in the chamber rather than the wet flow thru the port. Wet flow developments are good for carbed motors obviously but I would expect the future to be in direct injection where it's allowed, as long as the technology is there it's probably cheaper development work to develop dry flow ports rather than wet.

Going into boosted motors we need to know the VE's were are looking at, 100-110%VE is one thing (NA), 150-170%VE is another (Boosted), so obviously there is 50-60% more volume going thru the port in a boosted motor or is there?

Now to pull in what the old man said.....
Gotta go into the VE formula....

VE = Actual Mass of Air in Cylinder / Potential Mass of Air in Cylinder

VE = (Volume of Air Actually in Cylinder x Density of Air Actually in Cylinder) / (Volume of Air Cylinder can hold x Density of Air based on atmospheric conditions)

NOTE---> We don’t have to account for the fuel here cause we are going to assume the AFR is the same for what the cylinder can hold and what goes into it. They cancel out.

Eureka! Volume and Density are important here. So how much can density actually change? Read on

So let’s use what we know:

Now for example a 400 cube motor..... 50 cu in per hole

Then we need to know the air density.....

To make it simple go here

Fun thing is that you have to convert everything for that....

F = (C-32)/1.8
mb to "Hg
lbs/cu in = kg/m^3 x 0.00003613

So for STD = (29.92 "hg, 60 Deg F and 32 Deg DP) = (1013.21 mb, 15.5 Deg C, ) deg C DP)= 1.22 Kg/m^3 = 0.000044lbs/cu in Air Density

Then...

Cubes (49.94) x Air Density (0.000044lbs/cu in) = .0022 lbs of air OR as the old man puts it (mass) flow

That gives us the potential for the cylinder being filled....

Now let’s look at what fills that....

First the NA motor

NA Motor 100% VE, Intake Port Temp of 95degs F (35 C), Dew Point 0 Degs C, 1.0 "Hg of Vacuum (978.896mb because (-1.0)"Hg - Atmosphere 29.92"Hg = 28.92"Hg)

Air Density .0000412lbs/cu in Less Dense Air

So VE 100% (100% / 100) x potential mass (.0022) = .0022

.0022/ Air Density (.0000398) = 55.276 Cubes of Air

So it takes 55.276 Cubes of Air to get 100% VE in a 50 Cube Cylinder or 10% more volume than what its potential is.

Now a Supercharged motor


13 psi ((Boost(psi)+14.7) x 68.97 = mb) into the cylinder...

That makes the mb 1910.47

A typical intake port temp is probably around 130degs F (54.4 C) (This is a air/water intercooled motor)

Dew point is still 0 C

That gives us .000073lbs/cu in of air, much denser air!

Let's say the motor is at 170% VE

So VE 170% (170% / 100) x potential (.0022) = .00374

.00374 / Air Density (.000073) = 51.23 Cubes of air or 2.5% more than it's potential. (which doesn't suprise me because we have no intake tuning pulses going on here)

So when you increase the air density of the charge you can decrease the volume of air going thru the port and achieve higher VE's.

Now does it make sense that it's called Volumetric Efficiency? It should really be called Mass Flow Efficiency.

So I guess that in a blower motor the volume of the intake air charge is NOT increased, because the density of the charge is increased at a much higher rate and the volume can actually be decreased. (maybe because this is a case by case basis) The key is the density is much greater in a boosted motor.

We have linked velocity to volume (rather than density) so if the volume is the same or less then the velocity is the same or less. To keep it under .55 MACH. Basically this info is really usefull to guys in Mustang Racing Classes with "stock" type small port heads and blowers more than anything.

This also shows the tremendous effects of intake port temp on VE and therefore HP. A colder charge means a denser charge and less volume that is needed to achieve the same VE, but if you can pull that same volume into the port then you can have an even higher VE.

Once you get all these formulas you start to see how DA or Air Density plays on things like carbs and compression ratios, ESPECIALLY Dynamic Compressions Ratios..... (This last thing came to me while talking with Jon Kaase at this year's Engine Masters, which is funny because when I asked him “what’s the dynamic compression ratio of your motor this year?” he responds with an all too innocent “what’s dynamic compression ratio?” a little later he told me he was looking at the barometer for the whole summer at the dyno site, now what did he need that for? lol )

So to answer the question.......


I hope that explains it all now.



BTW I wrote this because I know all this info and understanding is in my head, but as in anything if you explain and teach what you know you will get a greater understanding of the material/skill itself. Works in teaching Race Driving Skills, Engine Building and I would assume in Medicine right Rich? I would say this helped me as much as anything.

Bret

 

Good question.

Maybe I read "volume flow" wrong the first time. I took "volume flow" to mean volume (in cubic feet, or Ft^3 for example) times flow which is volume per time ( in Ft^3/s or /min). Doesn't that come out to be Ft^3 * Ft^3/min? or Ft^6/min?

Area * velocity is easier to visualize, as you said.

 

The only confussion seem to be with the term "Volume Flow". I don't beleive this is a recoginzed scientific or engineering term, as Flow includes the volume.
As in, a flow of 100gal/hr, or 5lb/m, etc.
I'm sure it's just me, but I've never seen "FI" represent boosted engines. Would you'll be so kind as to tell me what it stands for.

 

I think FI comes from the "Forced Induction" forum found here in CamaroZ28.com. For a long time I thought it meant Fuel Injection like lit did in the old days, when everything was carbureted except "fuelie" Corvettes.

Forced Induction engines were "Blower Motors" but I suspect the Administrator didn't want us calling them BMs.

 

Ah so. Thank you master, I am unworthy.
Fuel Injection!!! Couldn't get passed it.

 

so bret, in your post with the formulas, are you saying that with FI you can have higher VE with less cubes of air per cylinder? just doesnt make sense to me...you would think that in order to have a higher VE you would have to have more cubes of air instead of less.

maybe im just reading it wrong? i have been drinking, so bear with me if any of this doesnt make sense!

 

Well it's more about the content of the air.

There are two parts of the equation. VOLUME = Cubes of Air and Density which is the pounds per cubic inch. So for ever cubic inch of volume you have X amount of mass.

The mass of the air is what tells the story here and what we are worried about is how much we have to play with. A denser charge means that the motor will have more oxygen to burn per cubic inch of volume which makes more power.

GAS COMPOSITION OF DRY AIR (%)
Nitrogen 78.09
Oxygen 20.95
Argon 0.93
Carbon dioxide 0.03
All others <0.03

So if you now have:

NA w/ 100% VE = .0000412lbs/cu in and 55.276 Cubes of Air = .000477 lbs of oxygen in the cylinder ready to be burned

(BTW this is a rough estimate saying that oxygen and nitrogen are equal in mass which they are not but it's for example only)


FI w/ 170% VE = 000073lbs/cu in and 51.23 Cubes of Air = .000783 lbs of oxygen.

That's 64% more oxyegen in the cylinder to be burned which will make much more power!

It's just like N2O, or Nitromethane you are adding more oxygen into the cylinder, nitro actually relases more oxygen once you get it's whole chemical reaction started right.

If you reread that post and go thru it you will get it. It took me 4 hours to write so it's not just going to bang turn on a light.

Bret

 

ok...i think i see now. i was just thrown off by the less cubic inches of air statement. i was think about the FI motor flowing more air and about volume as in cfm of air moving through the motor...not vloume in the cylinder.

hell, it took me a couple hours to sober up and reread it for it to make any sense to me!