Average air consumption of an engine.

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Post Tue Jul 22, 2008 6:35 pm

Is there a way to determine how many CFM of air an engine will consume?

If we are talking a 3.5L V6 with a 7000RPM limit, what other variables need to be plugged in to get the CFM usage?

Any info, URL, Calculators, formulae or comments are very welcome!

Thanks!

Chris
Conceptual
 
Joined: 15 Nov 2007

Post Tue Jul 22, 2008 11:24 pm

Is the CFM usage of an engine a secret? 3 hours of searching, and I cannot find any type of calculator, or even a range number...

Any help would be appreciated!

Chris
Conceptual
 
Joined: 15 Nov 2007

Post Tue Jul 22, 2008 11:43 pm

Well here's some of the constants:

Swept volume: 3500cc
Air-fuel ratio: 14:1 (roughly)
Max revs: 7000rpm
# of intake strokes: 7000/4 = 1750

Dammit, I knew taking this gap year would ruin my engineering knowledge :lol:

Sorry I can't be of any more use but try playing with those numbers?
Image
'10-'11 Head of Powertrain - Glasgow University Formula Student
Scotracer
 
Joined: 22 Apr 2008
Location: Edinburgh, Scotland, UK

Post Wed Jul 23, 2008 1:46 am

Forget looking at the displacement numbers, look at the rate of combustion.

Take power, assume thermal efficiency and air/fuel ratio (actually, assume a range).

You should be getting a mass flow of air value.

You can convert that to volume flow for any atmospheric conditions.
SZ
 
Joined: 21 May 2007

Post Wed Jul 23, 2008 5:29 am

Chris, I also have no idea about how to figure how much air an engine uses.

Then I typed "air consumption engine".

In the first attempt I got a table for what I'd call "pollution calculation" purposes. I'd guess we don't know if air is at atmospheric pressure when it enters the cylinder, so probably this kind of calculation is wrong. Anyway, this is a first estimate for your engine:

First formula, if you only know the displacement
Air used = Displacement/2 * rpm

Air used = 3.5 liters/2 * 7000 rpm = 12250 liters/minute

As a liter/minute is like 0.0353 cfm, then that's like 433 cfm.

However, in the second attempt I found that you could use this approach:

You know that air weighs 1.2 grams/liter and a gallon of gasoline weighs 6 pounds (that's like 2700 grams). So, if you have a fuel:air ratio of 1:14, for every gallon of gas you use 14 * 2700 = 37800 grams of air.

If you divide by 1.2 grams that weighs one liter of air, that's 31500 liters of air you have to use per gallon, which means you get more or less 1100 cubic feet of air per gallon of gasoline.

Second formula if you know the fuel you used
Air used (cfm) = fuel used (in gallons/minute) * 1100

I don't know. Something's wrong here. You have to use 0.38 gallons/minute to get the 433 cfm of the first approach. If you check somehow how much fuel your engine uses (you can download the information from the ECU of your car at your local workshop, btw) you can find how much air you need.

So, for the third attempt, let's go with SZ approach (welcome, SZ!), which seems to be right (unless your car uses 23 gallons/hour at that rpm regime!). I take the approach "for-dummies", step by step.

Power of a V6, 3.5 liters at 7000 rpm. Shut. Let's see.

My first "googling" tells me that Ford offers a V6, 3.5 liters rated at 265 hp. Honda gives you the same V6/3.5 at 275 hp. Give or take, that's like 200 Kwatts. You have peak power at 5000 rpm if you get a Ford, or at 6200 rpm in the case of Honda.

I see a problem here: according to SZ assumptions, we should use less fuel at 7000 rpm, when we have less power, but let's move on over the "little problem" of calculating thermal efficiency for every speed of the engine... ehem.

I assume a thermal efficiency of 30%, that gives me 666 Kwatts (nice number! :)).

If you run your engine for one hour you have 200 Kwatts-hour, that's like 720 Mjoules (at 3.6 Mjoules per Kwatt-hour).

In one minute you get 12 Mjoules of work (that is, 720/60).

Knowing that gasoline gives you 47 Mjoules per kilogram, you have to use .255 Kg of gasoline (that I get from 12/47). That's a little over half a pound of gas per minute, which means less than 0.09 gallons/minute (dividing by 6 pounds per gallon), a figure very different from the "volumetric" answer I got in the first attempt.

To sum up:

Fuel used (gallons/minute) = Power (HP) * 0.000351

As I know already (from the second attempt) that you have to multiply the fuel used by 1100, I get

Third formula if you know the power of your engine
Air used (cfm) = Power (HP) * 0.39

In your case:

Air used = 275 HP * 0.39 = 105 cfm (give or take)

That's like 1/4th of the first, "volumetric" answer I got.

Can anyone explain where I made a mistake? Or, in the improbable case I made no mistakes (in the 15 minutes I spent on this), why the air seems to be at 1/4th of the "normal" density inside the cylinder?
Ciro
Ciro Pabón
 
Joined: 10 May 2005

Post Wed Jul 23, 2008 5:56 am

*from the top of my head*

Scotracer is trying to calculate volume of air that the engine aspires, supposing that volumetric efficiency equals 100%.

Let´s define volumetric efficiency as: vol aspired / displacement (in spanish is "rendimiento volumetrico" and I dont know the exact translation)

Volumetric effciency varies with rpm, engine load (its not the same eff at constant 5000 rpm in 1st or 5th gear) among others (these two the more importants)

So, if you get to know vol eff at a determined engine speed and load condition you only will know volume of aspired air.

Then, if you wanna know how much of that air oxigen´s is combusted you will have to take into account fuel composition, air/fuel mixture ratio and fuel consumption

As I said before, all these is from the top of my head, I may be forgetting something.

A way to learn for yourself is to enter this page and download the engine simulation demo. You´ll have to model the engine (only 1 cilinder allowed in demo, full version available if you are rich or work for a BIG engine manufacturer) and for that you need to input A LOT of parameters, but there is a tutorial with an example to follow :wink:

http://www.lesoft.co.uk/
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti
 
Joined: 10 Jul 2007
Location: Argentina

Post Wed Jul 23, 2008 8:24 am

Belatti;

If you have a good idea of power output and overall (thermal) efficiency at that point, then you won't need volumetric efficiency, RPM or load for a quick calc as power output is dependent of all of them.

Thermal efficiency for an IC engine in a car is yet to pass 33% AFAIK (and that's for a Honda F1 turbo V6... so street numbers should be put in perspective).

Engine bore, stroke and cylinder count can give you a good guide to efficiency characteristics (VE particularly, though to be accurate you need valve, port and intake/exhaust system geometries), but they don't tell you how much fuel you're combusting. This (which requires the air volume flow you're trying to calculate) is independent of the engine configuration. How much energy you're releasing in combustion from a fuel mass flow perspective - once you consider efficiency - therefore determines your air demand.

Lesoft (Lotus Engine Simulator) and related 1-D gas dynamics programs (GT Power, Wave) are great at modeling the VE side of things quite accurately; the rest is just simulated. The calcs aren't hard to do by hand, but I'd quit focussing on exact volumetric values... you engine requires a mass of air and conditions are variable. You'll get a more accurate appraisal of what's going on by looking for a mass flow of air at a given A/F.

On that, 14:1 is optimisic for an A/F on a street car. Likely a little richer for an OEM road car.

Focus on a range of values, e.g. A/F, air density, thermal efficiency.

There is a much easier way.

Spend $30 or so on a cheap OBD-II cable set, connect it to a post-96 US car (or post-01 European car), and log the MAF sensor input. Calibrate the sensor against it's range (pretty easy to simulate) and you've got your air demand (and a lot else to have fun with).

Ciro, it's not uncommon to have fuel maps that require less fuel at higher RPM beyond the peak but it's not very common either. It's far more important to ensure that fuel mapping on the way to peak power (e.g. with increasing RPM) is always monotonic increasing... nasty implications for power delivery otherwise.
SZ
 
Joined: 21 May 2007

Post Wed Jul 23, 2008 2:36 pm

Would it be better to simply purchase a $20 flow gague and hook it to the motor?

I actually found another site where they were doing the same thing, and the numbers they were getting out of a 2.4L 4 cylindar was 253CFM@6000RPM, so the 400 number would probably be pretty close.

It amazes me how high these numbers are. My first thought was that it would be around 50CFM, but WOW, do these engines consume ALOT of air!!!

I appreciate everyones contributions! I spent an aweful long time searching for this answer, and I thought that it would be a readily available calculator...

Thanks again!

Chris
Conceptual
 
Joined: 15 Nov 2007

Post Wed Jul 23, 2008 2:51 pm

Conceptual wrote:Would it be better to simply purchase a $20 flow gague and hook it to the motor?
Chris


Most engines have this already, a mass air flow (MAF) meter.

With an OBD-II dongle and a PC you'll be reading that, manifold pressure, RPM, throttle position... and whatever other live data your engine puts out. Fault codes too.

Conceptual wrote:I actually found another site where they were doing the same thing, and the numbers they were getting out of a 2.4L 4 cylindar was 253CFM@6000RPM, so the 400 number would probably be pretty close.

It amazes me how high these numbers are. My first thought was that it would be around 50CFM, but WOW, do these engines consume ALOT of air!!!
Chris


The first number sounds about right.

Remember - street engines are (often) comparatively inefficient against racing designs (thermal, VE, etc) - they're going to chew air for similar power levels by comparison.
SZ
 
Joined: 21 May 2007

Post Wed Jul 23, 2008 11:59 pm

SZ, you are right, but only to get an idea, not precise values.

I was following Scotracer´s "volumetric" approach in order to tell how difficult things may turn when you go that way.

The LEsoft aproach is good to understand how many things are involved in engine eff (thermal, volumetric, overall) but maybe Chris question was pointing to know something about a specific engine.

SZ wrote:Engine bore, stroke and cylinder count can give you a good guide to efficiency characteristics (VE particularly, though to be accurate you need valve, port and intake/exhaust system geometries), but they don't tell you how much fuel you're combusting. This (which requires the air volume flow you're trying to calculate) is independent of the engine configuration.


To know VE precisely all you need is displacement and a P-v (specific volume "v" not "V") diagram for the effective cicle at a given load and speed. A modern dyno can get you this graphic. No need to know valve, port and intake/exhaust system geometries or the air volume flow.

SZ wrote:How much energy you're releasing in combustion from a fuel mass flow perspective - once you consider efficiency - therefore determines your air demand.


No:
1- not all fuels have the same stechiometric A/F ratios
2- unburnt fuel can go out
3- air humidity can go in and out without even reacting
4- air can go in and out without even reacting (poor mix case)

I would not say that fuel mass flow determines your air demand.
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti
 
Joined: 10 Jul 2007
Location: Argentina

Post Thu Jul 24, 2008 1:26 am

I was reading a short feature on the current 2.4 L V8 Formula engines - 650 liters per second (23ft@3) at 19000RPM - no other variables mentioned.

http://www.f1complete.com/content/view/1283/283/
http://en.wikipedia.org/wiki/Formula_One_car


EDITED
Carlos
 
Joined: 2 Sep 2006
Location: Canada

Post Thu Jul 24, 2008 2:15 am

Belatti

Down here we say "quit measurebating"! You won't get a precise answer from a simple hand calc (and nothing more) on this one.

1D analysis is as precise as you'll get for simulating gas dynamics in an engine, and in that, in capturing factors particular to VE. It's a common tool in modern engine development - used properly a good operator an achieve accuracy in VE within 1-2%. This is what it takes to simulate VE accurately (the approach being discussed), to which all intake and exhaust path geometries play a part.

Measuring it is a different story altogether. Dynamometers (at least those I've used) don't give p-v (indicator) graphs unless you've an in-cylinder pressure transducer and a crank reference signal. Even then the are under such a graph gives you work per cycle, not volumetric efficiency (which takes simpler equipment - VE is just measured airflow over pumping potential, and has nothing to do with work related to combustion). There are smarter ways of measuring it than (very expensive) cylinder pressure transducers.

Whilst VE does go a long way to explaining your engine's air demand as an air pump, it's just as valid - possibly more so - to look at the demand it has for combustion requirements, for which the rest is excess regardless. If you know the efficiencies involved, you can calculate how much air total goes through. Nothing underhand about it.

Your four points make no sense. Unburnt fuel, air and vapour (all common occurances, though you tend to avoid the excess air scenario) factor in as efficiencies, though these are small compared to products combusted. Stoichiometry you multiply though. If I was marking a student on estimating how much air an engine consumes at peak power, I'd suggest working out air demand is an easier approach, and that estimating VE with similar accuracy is very difficult.

No simple equation is going to give you an exact result, though I'd suggest (I did suggest) that starting off with what your engine needs and going from there is a good bottom end estimate. If you want a reasonable top end (assuming your engine doesn't pass 100%VE, and some do) then simply do an air pump calc. Which was done, and the figure was not-unexpectedly high. Air demand gives you a reasonable bottom end estimate if you're realistic with your efficiencies (which Ciro's calc's weren't, but a good first attempt). Armed with that knowledge you have a good range and a good basis to take your question further. I'd have estimated mid-to-high 300's (CFM) for the engine in question at the engine speed quoted, though that can be refined knowing more about it. A good indicator of VE is to look at a fuel mapping strategy (if you can get a copy of one, or log it accordingly).

If you want precise numbers without doing detailed simulation, either get yourself some VE data or go measure air flow using a calibrated reference (as most modern engines do). Furthermore if Chris wants exact numbers he should avoid a volumetric flow definition, as engines demand air by mass flow for combustion.
SZ
 
Joined: 21 May 2007

Post Fri Jul 25, 2008 10:40 am

Ciro,

By the time I got to the end of your answer I had forgotten what the question was! #-o
John Stitch
 
Joined: 22 Apr 2008

Post Sun Aug 03, 2008 11:31 pm

Image

From a model I have in Lotus Software and use to play with.

I know from the people who made that Model, that with that graphics, its perfectly possible to calculate air consumption as I said in a previous post.

Hope Professor Knowsalot won´t come again and criticize my method :roll:
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti
 
Joined: 10 Jul 2007
Location: Argentina

Post Mon Aug 04, 2008 12:42 am

Belatti wrote:Image

From a model I have in Lotus Software and use to play with.

I know from the people who made that Model, that with that graphics, its perfectly possible to calculate air consumption as I said in a previous post.

Hope Professor Knowsalot won´t come again and criticize my method :roll:


Watch your criticisms, they may come back to bite you.

If you're using Lotus Engine Simulator - which is a 1D gas analysis package - what I say earlier applies:

1D analysis is as precise as you'll get for simulating gas dynamics in an engine, and in that, in capturing factors particular to VE. It's a common tool in modern engine development - used properly a good operator an achieve accuracy in VE within 1-2%. This is what it takes to simulate VE accurately (the approach being discussed), to which all intake and exhaust path geometries play a part.


Particularly that bit about a good operator... 120% VE for a NA street engine at 7kRPM? You're beyond a bit optimistic - I'd revise your understanding of VE.
SZ
 
Joined: 21 May 2007

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