Fuel/Air Stoichiometry vs Peak EGT?
 

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?

Fuel/Air Stoichiometry vs Peak EGT?
 

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

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.

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.

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.

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.