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Fuel/Air Stoichiometry vs Peak EGT?

1001001

Well Known Member
I'm not 100% sure this belongs here but is a fuel related question...

Is anyone aware of a general chart or formula showing EGT vs actual stoichiometric air to fuel ratio in internal combustion engines? I am attempting some calculations about theoretical fuel burn and keep coming up with less fuel flow than expected for a given MAP.

My assumptions are that at a given MAP, the combustion chamber fills with ambient air and that the fuel metering device, be it a carburetor or fuel injection system, is in turn metering Fuel in based on the air flow. It seems that if I use the MAP as the basis for calculating the molar composition of the precombustion mixture, I get lower fuel flows than expected at a given engine speed.

I know that I could calculate this by playing around with adiabatic flame temperatures in the combustion chamber but quite frankly I am not trying to write an undergraduate engineering thesis, just understand the relationship of fuel:air mixture to EGT a little better.

Anyone know when we talk about peak EGT, just how close that is to the actual stoichiometric air requirement?
 
Remember from the 100% of the BTU?s that the fuel makes, you are only going to get about 30% of that energy that actually makes HP. From studies done at Lycoming .065 is around peak EGT. I believe .062 is stoichiometric.

Don
 
Remember from the 100% of the BTU’s that the fuel makes, you are only going to get about 30% of that energy that actually makes HP. From studies done at Lycoming .065 is around peak EGT. I believe .062 is stoichiometric.

Don

Don, thanks for the response. I think I figured out the error in my calculation and now am getting much more reasonable numbers when I use a molar ratio of 12.5:1 O2 to fuel (assuming isooctane as the fuel, which I know is not quite right). 12.5:1 happens to be the exact stoichiometric requirement for combustion of isooctane. When I adjust to a leaner or richer mixture, my calculations follow as one would expect from a fuel usage curve as found in an engine operator's manual. My numbers result in a calculated overall efficiency of about 28% relative to the heat of combustion of isooctane.

Now what I'm trying to do is figure out what stoichiometric ratio is implied by a "full rich" mixture setting. Obviously for a normally aspirated engine this will change based on altitude.

Again thanks for the response! What are the units on your numbers above or are they just fuel:air versus air:fuel (i.e. 1/the ratio I am using)? 1/12.5 = 0.08
 
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From studies done at Lycoming .065 is around peak EGT.

Lycoming, peak at 0.065:



C.F. Taylor, from previous studies, peak about 0.066



The recent (relatively speaking) Swift fuel study done at the FAA's Hughes Technical Center is one of the few windows for folks like us, given that manufacturers like to label everything Top Secret.

It lists the calculated stoichiometric ratio for 100LL at 1/14.9, or 0.067. The graphs for baseline runs at various power settings (IO-540K) have peak EGT right around 1/15.2 (0.066).

Download here: http://www.tc.faa.gov/its/worldpac/techrpt/ar0853.pdf

As a practical matter, I just consider peak EGT to be stoich and carry on.
 
Lycoming, peak at 0.065:



C.F. Taylor, from previous studies, peak about 0.066



The recent (relatively speaking) Swift fuel study done at the FAA's Hughes Technical Center is one of the few windows for folks like us, given that manufacturers like to label everything Top Secret.

It lists the calculated stoichiometric ratio for 100LL at 1/14.9, or 0.067. The graphs for baseline runs at various power settings (IO-540K) have peak EGT right around 1/15.2 (0.066).

Download here: http://www.tc.faa.gov/its/worldpac/techrpt/ar0853.pdf

As a practical matter, I just consider peak EGT to be stoich and carry on.


Ah, excellent, thank you! So with 15.2 being the empirical stoich for 100LL6, the average molecular weight (carbons in the chain) must be higher than straight octane, which makes sense since there are bound to be some other constituents in the blend. Anyone have a chemical analysis of 100LL? Taking into account that there are probably some esters and alcohols in there contributing oxygens of their own, the amount of carbon is probably even a bit higher.
 
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I also now see that my ratio of 0.08:1 could be seen as the "best power" mixture as it represents the most fuel it is possible to burn under normally aspirated conditions. However, I would imagine there are not insignificant effects associated with the nitrogen content of the combustion air and other things like water vapor.
 
With straight Mogas (no ethanol) we figure 14.7 AFR is stoich and peak EGT which is .068 in the units on the charts.
 
My assumptions are that at a given MAP, the combustion chamber fills with ambient air and that the fuel metering device, be it a carburetor or fuel injection system, is in turn metering Fuel in based on the air flow. It seems that if I use the MAP as the basis for calculating the molar composition of the precombustion mixture, I get lower fuel flows than expected at a given engine speed.

The volumetric efficiency of an engine is rarely 100%. Generally less, but sometimes more. If you'e assuming you get 90.25 cubic inches of air into your 360's cylinder every time it goes down, that will throw off your oxygen molecule count calculations significantly. You need a table of volumetric efficiency or a direct measurement of air like a mass air flow meter to do this correctly.
 
The volumetric efficiency of an engine is rarely 100%. Generally less, but sometimes more. If you'e assuming you get 90.25 cubic inches of air into your 360's cylinder every time it goes down, that will throw off your oxygen molecule count calculations significantly. You need a table of volumetric efficiency or a direct measurement of air like a mass air flow meter to do this correctly.

Even more complicated: that volumetric efficiency usually depends, to some extent, on the intake air temperature. Just take a look at any manufacturer's power data as a function of temperature: it's more than just the air density changing.
 
The volumetric efficiency of an engine is rarely 100%. Generally less, but sometimes more. If you'e assuming you get 90.25 cubic inches of air into your 360's cylinder every time it goes down, that will throw off your oxygen molecule count calculations significantly. You need a table of volumetric efficiency or a direct measurement of air like a mass air flow meter to do this correctly.


Yes, my calcs right now are a first approximation to validate my method. I will tighten things up later to include this sort of concern. I have a factor in there but it is just set at one right now.

Things like the effects of valve overlap and incomplete exhaustion are in the future.
 
Interesting Approach

It looks like you have things well in hand. IIRC, the offset of equivalency ratio to peak EGT is due to latent heat of the fuel, on a natural gas engine it is right on. Again IIRC. For your calculations I would not worry about humidity and certainly not nitrogen. Temperature and In-Man-P yes.
 
Anyone have a chemical analysis of 100LL?

Much like automotive gasoline, there is not a single formula that defines 100LL. ASTM D910-14A defines the performance that 100LL must meet, but that document leaves it up to each refiner as to how to achieve the required performance. This means that the chemical composition can vary from refiner to refiner. They will be similar, but just realize that there will be some variation.

This is why isooctane is used in ICE research - to eliminate variability of the fuel composition.
 
Dave,

Just installed an Air/Fuel Ratio monitor, and have had a few longish legs to do some data collection during leaning (painfully slow leaning to evaluate injectors and learn what AFR is telling me). Lot's of raw data to wade through, as its collected by hand from my VM-1000 :eek::
IMG_1599_2.jpg


My first peek at this data seems to correlate well with the charts and numbers Dan shared here. Best power, roughly based on cruise speed during leaning, occurs about 12.5:1, Peak occurs in the mid- to hi- 14:1 range (14.5 to 14.9) and my motor starts to grumble a bit and speeds drop off markedly just before reaching 16:1 (the top of the scale on my AFR monitor). I'm going to try to hand-jam this into a few spreadsheets, and will be happy to share what I come up with. The sensor is on one side of the engine versus each cylinder, the data is eyeball collected, and the AFR reading jumps around a bit, so it may not be at the super-fine data level you may be generating...but if its of value, happy to share.

Its giving me some good data on injector management too!

Interesting thread! :)

Cheers,

Bob
 
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