Calculating air-fuel ratio?

mew905

mew905

New Member
There's got to be a way to do this without an oxygen sensor.

Wikipedia says the optimal ratio of fuel to air is 14.7:1, or 14.7 gallons of air to 1 gallon of gasoline.

When I do the calculations, engine CFM is SUPPOSED to be (Displacement (CI) x RPM x VE)/3456 and that will get you a CFM rating. Well assuming a 65% VE on a reed-valved engine tested at 80MPG @ 40MPH (giving you a run time of 2 hours per gallon), that means that my engine should be sucking in that full 14.7 gallons of air over 2 hours. Safe to assume, right? Well, that formula says my engine should be sucking in about 4.5CFM at 6000 RPM (4 CI). Not much, right? Well remember 4.5 CFM is cubic feet per minute. So over two hours that gives us a number of 540 cubic feet over the course of 2 hours. Guess how many gallons that is? about 4040... That's right, over the course of using 1 gallon of gasoline, every calculation I can find says my engine is sucking in 4 THOUSAND gallons of air. That's 4000:1, waaaayyyyyyyy off from 14.7:1, the motor shouldn't even come close to starting.

So the question is... where am I going wrong? It should be possible to check your air-fuel ratio using this method (or at least get close). So unless our engines have a volumetric intake efficiency of 28%, this has to be way, way way wayy off. Unless my engine is just really poorly tuned... Average 2-stroke volumetric efficiency is about 55% according to a chart I saw.

I realize this calculation would change because I'm assuming I have a perfect ratio, which I know I dont, but it has to be close enough to even fire, whereas my calculations say I'd be way too far off.

Another way I would imagine is to use the compression ratios to calculate how much air the engines are getting (13.8 PSI atmospheric to 100 PSI combustion = 7:1 ratio), assuming reed valves and no losses through the exhaust...

Cmon guys, there's gotta be a way to do this.

EDIT: apparently that CFM calculation is for 4-strokes, and very wrong. Donaldson Engine Intake System Catalog says 2-strokes is (CID x RPM / 1728) x VE.

So that gives me 9 CFM... still way, way way way too much. I should be seeing 0.0159 CFM... or a VE of about 0.11% according to that calculation. Dafuq??

EDIT 2: Apparently it is actual WEIGHT for 14.7:1, so 14.7 grams of air vs 1 gram of gasoline. so 2.83KG of gasoline to 41.6KG of air, or 1120 CF of air over 2 hours. assuming I have the perfect ratio (which I don't), gives me 9.3 CFM. Seems I've got it! Horay!

So now, instead of a much more expensive AFR meter (AKA an oxygen sensor) which can cost upward of $300 for a handheld one (but give you real-time accurate results), you can calculate it based on your milage and intake air flow, which an air flow meter only costs about $15 for a cheap one.
 
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Seems like a lot of math and then the numbers won't always be exactly what works best.
I've never had a need to know what the exact ratio of fuel to air is. I solder and drill my jets to give me the best plug reading and performance. I would bet that if I had a way to actually measure my ratios they would be darn close to what is considered optimum.

Tom
 
I guess that I'm overly simplistic on some things......

If it runs to my satisfaction, I just put in fuel mix when the tank is low........heck, I just put in fuel mix even if it's not running quite right. Then I repeat.


(Sorry, just had to do it.....)
 
I also do just like 2door, never had a need for big complicated math, nothing wrong with all the math or numbers but when I know through simple testing my engine is running at its best, thats all I need to do... no calculators needed.....lol!

Map
 
Mew, that really is fascinating. I really enjoy the math and deep consideration/do the home work sort of stuff. I honestly don't have a clue how it could be bench marked.

I am with Tom and Map on the "if the plug is happy, so am I" thing But you got me thinking, how could air flow be measured and the failings of QC in our 2 smokes?

In practice, the plug is probably the best test but you raise a cool thunk. Well above my paygrade and capibilities but really looking forward to seeing what you come up with.
 
I'm with everyone else here. Why would you want to calculate the exact AFR for your motor without a full EFI setup?

Carburetors are never going to account for every variable so to be safe, it's best to jet them a bit rich. I'm going to use an Ecotrons small engine EFI kit for my XR80 build because it's possible to keep a nearly-optimal afr at all times. The ECU literally makes thousands of adjustments a minute to keep the AFR at an optimal point (which isn't always 14.7 anyway ;) ).

If you are using a carburetor, you might find that your actual AFR is around 12:1 or so. Of course, this will vary depending on engine temperature, outside temperature, elevation, etc. All of these variables make calculating AFR on a carbureted engine fairly pointless imo.
 
Yeh, I totally agreed with everyone here on not too worry about cfm/afr etc, especially when it come to the chinese made engine. I rather look at the spark plug to tell me if I'm rich or lean and go from there. Anyhow, I really don't know much about carbs tuning so I'm gonna have some frustration tuning it.

Now, on my turbo project, I would not run it w/o a wideband as things get expensive quick. So yes, I do have a wideband that isn't installed on the car yet, and it wouldn't take long to stick it into the muffler on the predator for a quick reading though...
 
Dan said:
Mew, that really is fascinating. I really enjoy the math and deep consideration/do the home work sort of stuff. I honestly don't have a clue how it could be bench marked.

I am with Tom and Map on the "if the plug is happy, so am I" thing But you got me thinking, how could air flow be measured and the failings of QC in our 2 smokes?

In practice, the plug is probably the best test but you raise a cool thunk. Well above my paygrade and capibilities but really looking forward to seeing what you come up with.
Click to expand...

It's more of a thought project than anything, I realize nothing can really compare to an actual oxygen sensor (unless there's cylinder-based sensors, like the one soup mentioned). I just got thinking (really, really bored at work and I love math)... there's got to be a way to calculate it without needing a $400 piece of equipment. If I had to tune many engines within a week (say one a day or even one every two days), I might buy a handheld oxygen sensor, but in terms of the few engines I have done, it's just not worth the cost, but if I can figure it out for even one engine, it could help.

But the other question is how do we calculate the actual VE of our engines? haha. Obviously if 9cfm goes in, chances are 9cfm will come out.... waaaait a second, the air flow meter can do that too! 100% VE for a 66cc at 8000 RPM comes out to about 18.5CFM... so the ratio of what you really get, versus what you see, gives you your VE.

I love keeping my mind occupied with such useless nonsense. Like what would your gear ratio have to be in order for the pitch of your engine via the doppler effect to stay the same as you accelerate away from someone?
 
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Techbiker said:
The ECU literally makes thousands of adjustments a minute to keep the AFR at an optimal point (which isn't always 14.7 anyway ;) ).
Click to expand...

Sorry to call you out, but when isn't 14.7 optimal? 14.7 gives you the greatest flame speed, thus giving maximum power for the minimum amount of fuel, anything else reduces flame speed. This means if 14.7 ratio isn't giving you maximum power, and jetting up or jetting down is, it's not your ratio that needs changing, it's your spark timing. Turbo guys see more power from adjusting the ECU to provide more gas because their flame speed is much higher than it was. This means the piston is fighting the pressure rather than working with it, causing detonation, bearing failure, blown heads, etc. Reducing the flame speed by adding more gas nets them more power because peak pressure occurs on the piston downstroke again, when they could have just retarded their spark timing to account for the greater flame speed and gained more power AND better milage.
 
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