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Originally Posted by v8capri
Um All this octane talk for avgas has me confused.
Are we talking RON or MON ?
and my changing the mixture ( A/F ratio), you can change the octane rating ?? now you have me confused.
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Ok, the octane rating for avgas is RON, and is rated by the American Petroleum Institute. Piston aircraft engines have compression ratios higher than 13:1, and need a much higher octane fuel to prevent pre-detonation within the various conditions, atmospheric pressures that they operate at.
Essentially, the octane rating of all petroleums changes due to altitude and the effect of a lower density air on the perfect chemical condition.
At a lean cruise (lean being set by the actual mixture control lever in the ИИИИpit) setting ie, 10000 ft with a mixture of say 1 part fuel to 110 parts air, the nominal octane rating of the fuel is 100 RON, this takes into account the much lower density of the air at that altitude. As a result, less fuel is used to counteract the fact that less oxygen is available due to density. The engine is still trying to draw in the same amount of air in the cylinders but is not able to (like when you stand atop a 10000 ft mountain and notice its harder to breathe) so less fuel/air mix results in a lower compression ratio and a lower specific energy/octane value for the fuel at that condition. The fuel is engineered to specific values to take into account that an engine operating at 10000 ft might only have a compression ratio of 6:1 at that altitude.
At sea level, when you have the mixture set to the full rich setting say 1 part fuel to 75 parts air, the nominal octane value of the fuel is 130 RON. This is because there is more air (higher compression ratio for same volume compared to altitude) and the air being more dense lowers the flash point of the fuel due to increased volatility of that particular richer mix. This is a chemically perfect condition of the fuel under these circumstances and more power is attainable.
To overcome these problems aircraft engine manufacturers have put on more expensive planes turbocharged engines.
If you recall the Whyalla aircraft crash in that Piper chieftain the guy was running twin Lycoming TSIO-JT2B turbo charged 540 cubic inch 6 cylinder engines. The problem was, the turbo's run about 40 pounds of boost at sea level and around 8 pounds at altitude, so there is alot of scope for setting the correct mix (due to the atmospheric density being overcome by pressurised air fed to the engines). Combined with this and a lack of that particular engines characteristics for lean cruise setup the pilot (who had minimal hours and was only about 20) ran too lean a mixture (not enough fuel, high temperatures), ran too high a temperature and ended up hole-ing a couple of pistons, melting bearings conrods and damaging both crankshafts. Unfortunately that plane and all PAX perished due to a double engine failure where one failed and he put full power on the other failing engine causing it to prematurely fail.