wailunwu

HI

Assuming even distribution of fuel and air in the engine, why is peak/best engine power not at peak EGT (with stoichiometric fuel to air ratio 14.7 to 1)? I was told that best power is at a slightly rich fuel to rich ratio.

If 14.7:1 gives the most efficient combustion (with no excess fuel/gas) why doesn't the engine power peak at this point.

Please see link below:
http://www.mummbrothers.com/SRF_Stuff/images/Stoich.gif

Thanks
Al

kme9418

Stoichiometric ratio is where the chemical equation is balanced for the reaction. This is theoretical. It does not exactly coincide with the max power output. Also, 14.7 pertains to gasoline and is not the same for every fuel. (14.7:1 is air/fuel not fuel/air)

Here's a more detailed version of the graphic related to piston engines:

http://images.thesamba.com/vw/gallery/pix/435515.jpg

BSFC: brake specific fuel consumption...a measure of efficiency

Note that peak BSFC is on the lean side. Max EGT is at stoic which indicates max heat generated. Efficiency could mean 3 or more things depending on your goal: 1. max power (rich of stoic) 2. max heat generated (stoic) 3. min fuel burn (lean of stoic).

Disclaimer: I am not a chemist (but I was the teacher's favorite in high school Chemistry class)

Twin Wasp

From "Aerodynamics for Naval Aviators" -

The chemically correct proportions of air and hydrocarbon fuel would be 15 lbs. of air for each lb. of fuel, or a fuel/air ratio of 0.067. This chemically correct, or “stoichiometric,” fuel-air ratio would provide the proportions of fuel and air to produce maximum release of heat during combustion of a given weight of mixture.

Because of the effects of flame propagation speed, fuel distribution, temperature variation, etc., the maximum power obtained with a fixed airflow occurs at fuel-air ratios of approximately 0.07 to 0.08.

Combustion can be supported by fuel-air ratios just greater than .0.04 but the energy released is insufficient to overcome pumping losses and engine mechanical friction. Essentially, the same result is obtained for the rich fuel-air ratios just below 0.20. Fuel-air ratios between these limits produce varying amounts of output power and the maximum power output generally occurs at fuel-air ratios of approximately 0.07 to 0.08. Thus, this range of fuel/air ratios which produces maximum power for a given airflow is termed the “best power” range. At some lower range of fuel/air ratios, a maximum of power per fuel-air ratio is obtained and this the “best economy” range.

The best economy range generally occurs between fuel-air ratios of 0.05 and 0.07. When maximum engine power is required for takeoff, fuel-air ratios greater than 0.08 are necessary to suppress detonation. Hence, fuel-air ratios of 0.09 to 0.11 are typical during this operation.

There are some nice graphs explaining this in the book but it's too much cut and paste. AFNA is available free for down load from the FAA web site.