Why lean AFR results in extra heat?
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AJAY
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Hi folks. Been on the Busa sidelines for awhile. The subject of fuel mixture is a very big deal in the piston-engine airplane world, where the pilot has control over fuel mixture as part of basic engine management. Historically, you run a piston engine at "rich of peak", where there is more fuel than oxygen to keep the engine cool enough that it doesn't burn up the rings and valves - or worse! One company, GAMI, has been downright evangelistic about running engines "lean of peak" where there are too few fuel molecules for the oxygen molecules present. This gives the engine a tad less power, but with the advantage of cleaner operation and total fuel burn. GAMI's science and engineering is fundamentally sound, at least for fuel injected engines, and this is no longer a controversial subject like it was 10 years ago.
Some "science-lite" for you readers: a perfect mixture, that is, exactly perfect ratio of oxygen to fuel (stoichiometric mixture) will give you the most power, but also the most heat - and heat is what makes the engine go. Generally we run engines a little rich to keep them cool. You can run them lean of peak, but the mixture becomes very sensitive and it's easy to damage the engine if your fuel management is not perfect - at least for an aircraft engine where the amount of oxygen changes due to changes in altitude. If I'm rich mixture and I climb to thinner air, the mixture gets richer and the engine gets cooler. But if I did the same with an initially lean mixture and climb, then the mixture approaches stoichiometric and the engine gets too hot under full power, burning up. This is the worry. The opposite happens when descending altitude, but in that case you usually use low engine power and there is less risk of accidentally damaging the engine by failing to readjust mixture.
Sorry for the long-winded post. Most of the data about fuel mixture and temperature is very well documented EMPIRICALLY (experimentally) in a 1955 technical report, "The Aircraft Engine and its Operation" by Pratt and Whitney, which you can still buy or find on the web. But here is an article from GAMI that shows the key graphs and explains what happens very well:
http://home.pcisys.net/~aghorash/Mixture_Magic.pdf
This doesn't really explain the physics (chemistry) of why especially well, but it does show exactly what happens with fuel mixture and temperature and power and fuel burn in fairly easy terms for the layman.
Again, sorry for the long-winded post. - AJ
Some "science-lite" for you readers: a perfect mixture, that is, exactly perfect ratio of oxygen to fuel (stoichiometric mixture) will give you the most power, but also the most heat - and heat is what makes the engine go. Generally we run engines a little rich to keep them cool. You can run them lean of peak, but the mixture becomes very sensitive and it's easy to damage the engine if your fuel management is not perfect - at least for an aircraft engine where the amount of oxygen changes due to changes in altitude. If I'm rich mixture and I climb to thinner air, the mixture gets richer and the engine gets cooler. But if I did the same with an initially lean mixture and climb, then the mixture approaches stoichiometric and the engine gets too hot under full power, burning up. This is the worry. The opposite happens when descending altitude, but in that case you usually use low engine power and there is less risk of accidentally damaging the engine by failing to readjust mixture.
Sorry for the long-winded post. Most of the data about fuel mixture and temperature is very well documented EMPIRICALLY (experimentally) in a 1955 technical report, "The Aircraft Engine and its Operation" by Pratt and Whitney, which you can still buy or find on the web. But here is an article from GAMI that shows the key graphs and explains what happens very well:
http://home.pcisys.net/~aghorash/Mixture_Magic.pdf
This doesn't really explain the physics (chemistry) of why especially well, but it does show exactly what happens with fuel mixture and temperature and power and fuel burn in fairly easy terms for the layman.
Again, sorry for the long-winded post. - AJ
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