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#72
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What/how does compression ratio affect an engine?
No extra heat in high Octane. Same hydrocarbon structure is both. Same
BTUs. Only difference is the speed of combustion is controlled so it is slower in the higher octane to prevent pre-ignition. -- Cy Galley - Chair, AirVenture Emergency Aircraft Repair A 46 Year Service Project of Chapter 75 EAA Safety Programs Editor - TC New address - EAA Sport Pilot "Scott" wrote in message ... One thing I think "might" be a concern is that burning 100LL (can't get 80 octane avgas these days) in an engine built for 80 octane is the extra heat. I think valves are most likely to be affected by burning the 100LL instead of 80. Might be all wet on this, but that's what I've heard and I'm NOT an engine mechanic...just a user You might try digging in the Lycoming site and see what they say about 80 vs 100LL... Scott wrote: OK, not cool to reply to my own post, I know. But I just found this great resource which basically answers all my questions: http://www.lycoming.textron.com/supp.../key-reprints/ "Lycoming provides helpful information in various publications, including Lycoming Flyer Key Reprints. Lycoming's Key Reprints is our effort to continually share our best practices, key lessons and engines systems knowledge to empower our customers." Lower compression = lower chance of preignition = lower octane required (ie. auto fuel)! Yes, that would make a difference to the pocket book! The issue of using auto fuel is addressed in this series also: "Auto fuel is now being used as a substitute for Grade 80 aviation gasoline under STCs issued by the FAA. Most major oil companies and engine manufacturers continue to recommend that aircraft piston engines be operated only on aviation gasoline. Deterioration of engine and fuel system parts have been reported in aircraft using auto fuel. Operators should consider the added risk of using auto fuel in aircraft. Remember -- a pilot can't pull over to the side of the road when fuel creates a problem with the engine." -- Scott http://corbenflyer.tripod.com/ Gotta Fly or Gonna Die Building RV-4 (Super Slow Build Version) |
#73
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What/how does compression ratio affect an engine?
On Thu, 13 Dec 2007 04:00:58 GMT, "Cy Galley"
wrote: No extra heat in high Octane. Same hydrocarbon structure is both. Same BTUs. Only difference is the speed of combustion is controlled so it is slower in the higher octane to prevent pre-ignition. WAY off base, Cy. On ALL counts, except the no extra heat and same BTU's. -- Posted via a free Usenet account from http://www.teranews.com |
#74
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What/how does compression ratio affect an engine?
On Dec 12, 9:08 pm, clare at snyder.on.ca wrote:
On Thu, 13 Dec 2007 04:00:58 GMT, "Cy Galley" wrote: No extra heat in high Octane. Same hydrocarbon structure is both. Same BTUs. Only difference is the speed of combustion is controlled so it is slower in the higher octane to prevent pre-ignition. WAY off base, Cy. On ALL counts, except the no extra heat and same BTU's. -- Posted via a free Usenet account fromhttp://www.teranews.com Higher-octane rated fuels have about the same flame front speed as lower-octane fuels, at around 100 feet per second. It's their resistance to detonation, which involves flame fronts speeds of 5000 feet per second or more, that makes them necessary in high-compression engines. As the combustion process begins in the cylinder, the burning raises the pressure through the entire air/fuel mix, and low-octane rated fuels will break down under that increasing pressure and spontaneously combine with the oxygen, igniting almost all at once instead of *waiting* (key word) for the flame to set them off in a controlled chain reaction. Detonation therefore requires time to develop. Low RPM with high MP is pro-detonation. Lean mixtures burn more slowly, so they're pro-detonation. Big cylinders take more time for the flame front to cross, so they suffer more from detonation. Preignition is something else. It's ignition of the mix during compression by some hot spot in the cylinder, perhaps a glowing bit of carbon in the head or on the piston. The burn begins early, before the sparkplug was scheduled to fire, and gets way ahead of itself due to the still-increasing compression as well as the combustion, and detonation might happen. The damage is about the same. Engines that aren't leaned properly will develop more carbon to cause preignition. Too much lead fouling can do it. An oil-burner will carbon up, too. Octane ratings are different than octane content. In the old way, fuel under test was fed to a variable-compression test engine and the compression was raised until detonation began to occur. Then that fuel was shut off and a mix of octane and heptane was introduced, engine still running, and the octane/heptane ratio was varied until the detonation point was established. An 80-octane fuel had the same detonation characteristics as a mix of 80% octane and 20% heptane, hence the rating. 80/87 reflected the detonation resistance at lean and rich settings. Fuels with ratings above 100 obviously have to be rated using some other method, since we can't have a fuel consisting of, say, 115% octane. Dan |
#75
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What/how does compression ratio affect an engine?
On Thu, 13 Dec 2007 04:00:58 GMT, "Cy Galley"
wrote: No extra heat in high Octane. Same hydrocarbon structure is both. Same BTUs. Only difference is the speed of combustion is controlled so it is slower in the higher octane to prevent pre-ignition. Cy I'm on your side. See my prior post. Lot of people are using different words to describe the same thing which may be part of the problem. Big John |
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