AV8ORJWC
Joined: 13 Jul 2006
Posts: 1149
Location: Aurora, Oregon "Home of VANS"
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 Posted: Sat Sep 23, 2006 8:49 am Post subject: Detonation vs. PreIgnition |
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My apologies to the guys getting this a second time after the Aero-electric list, but many believe blending of Mogas and Avgas helps the pocketbook and has no detrimental side effects. Hopefully Walter will address this in a future Kitplanes article.
Good comments, Ron. Yes, the octane rating is a measure of the resistance of the fuel/air mixture to auto-ignite, or ignite without the presence of an open flame. It's a complex phenomenon and autoignition is a function of time, pressure and temperature. The rating methodology, developed by Ethyl Corporation way back when, compares the resistance to auto-ignition of the test fuel to mixtures of normal heptane to iso-octane. The octane number is the percentage of iso-octane in the mix that behaves like the test fuel (80-octane fuel behaves like a a mixture of 20% n-heptane and 80% iso-octane). Since the actual test is really done by measuring the BMEP of a test engine, numbers over 100 are possible - 110-octane fuel will allow the test engine to operate at 110% of the BMEP allowed by iso-octane(not exactly correct, but close enough). The method isn't exact and if the test engine is operated at different temperatures and loads the observed octane number comes out differently. Running at light load creates an octane rating called the "research octane" number and running at high temperature/high load produces a number called "motor octane". The law says the average octane rating of an automotive fuel must be displayed on the pump and that's why it says (R+M)/2 on the pump. Aviation fuel, because engines are usually operated under conditions more like the the motor method, are typically rated by the motor method - I believe 100LL is more correctly called 100/120 (100 motor octane and 120 research octane). That's why some of the past-history fuels were called 110/130 etc.
When the fuel is ignited by an open flame front there is little difference in flame speed between high octane and low octane fuel: Flame speed is NOT the primary difference between the fuels. Detonation is auto-ignition of the "end gases" late in the combustion process. Because of combustion in the cylinder the end gases(the portion of the charge furthest from the ignition source) are compressed to very high pressures, which because of the gas law PV=NRT, raises their temperature. They will only remain unburned for a limited time and if the flame front doesn't arrive soon enough, igniting them progressively, they will auto-ignite all at once, "exploding." This will cause all sorts of problems, but will not usually result in immediate engine failure.
As Ron correctly described below, "pre-ignition" is simply ignition of the fuel charge before the spark occurs. Because the charge burns and raises the pressure before the piston fully compresses the charge the temperature and pressure can go to extremely high levels, causing major damage very quickly. For example (George Braly would have the real numbers) normal compression (no ignition) pressure might be 350psi, combustion results in pressures of 800 to 1,000 psi and pre-ignition could result in pressures up to 2,000 psi. Pre-ignition can (likely) result from pre-ignition because of the extremely high pressure and temperature of the end gases. One thing that isn't usually thought about is the rate of heat transfer from a gas to the cylinder, which is proportional to the velocity, temperature difference AND PRESSURE of the gas. Double the pressure and double the temperature and the heat transfer will go up by a factor of 4. Detonation results in sonic shock waves traveling through the combustion chamber, which means locally the hot gases are moving at sonic velocity. Much of the damage from both detonation and pre-ignition is caused by the extremely high heat transfer rates, not just the pressure. Detonation under high loads can increase the local heat transfer rate to the point that after a short time a component (edge of the exhaust valve, carbon deposit, etc) can be heated to the point it causes pre-ignition. This has sometimes been referred to as "runaway detonation."
I hope from the above description you get the idea that detonation and pre-ignition can be a real horror story for the engine. The best thing is to maintain a healthy detonation margin. In general, detonation is suppressed by increasing the octane rating, lowering the compression ratio, retarding the ignition timing, lowering the inlet temperature and pressure, lowering the cylinder head temperature, increasing turbulence and reducing the distance from the spark plug to the end gases(by running on both plugs). And you can dilute the charge by adding water, excess fuel or excess air (run LOP). As you can see, most of these things will reduce the power output of the engine and therein lies the conundrum. For a standard unmodified aircraft engine the worst operating case would be a takeoff from Death Valley on a dry, 115-degree day with the carb heat left on, mixture leaned and one of the mags inoperative. As I recall it is just those conditions that the FAA requires to be demonstrated for a certified engine.
Sorry about the excessively long non-electric dissertation.
Gary Casey
Know what actions or inactions you can take to keep your Detonation Margins in a safe range. When you increase the piston C/R you are moving closer to No Margin. The big boys at Reno lean into the wind with a runway/desert always immediately below them.
John – 40600
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