Autogas vs. avgas
By John Szymanski
More and more pilots are obtaining STCs for their aircraft to use autogas. Why? Because autogas is less expensive and the FAA has found no statistically significant safety-related issues with using autogas as long as the gasoline meets the octane requirements for your engine. You have probably heard the term, "caveat emptor." It means, "buyer beware." The wrong fuel octane used in your engine can mean the early demise of your engine and you, if you are not careful.
The octane rating of gasoline refers to the fuel's anti-knock quality, Autogas octane is rated differently than that of avgas. Two CFR (Cooperative Fuel Research) knock-test engines, adopted by ASTM, are used to test automotive gasoline according to ASTM D 2699 and D2700 standards. A modified version of the CFR engine is used to test avgas. The ASTM D 2699 test refers to the Research Octane Method (RON) and serves as the essential index of acceleration knock. ASTM D 2700 refers to the Motor Octane Number (MON) and provides an index of knock at high engine speeds. The (MON) method engine test differs from the (RON) method by using preheated fuel mixtures, higher engine speeds and variable ignition timing, placing more stringent thermal demands on the fuel under test. The (MON) number is typically 8 to 10 octane numbers lower than the (RON) number.
The autogas (MON) octane number is similar to the aviation rating of octane up to 100 octane, according to ASTM D 2700. When you purchase autogas, the octane rating is the average of the RON and MON, (R+M)/2 and the formula is posted on the gasoline dispenser. However, you have no way of knowing if the (MON) octane number meets the requirements of your aircraft engine unless you have documentation or a means to test gasoline octane on-site.
Another potential problem is the mixing of turbine fuel with avgas, When turbine fuel is added to avgas, the octane level drops significantly. A 10-percent mixture of turbine fuel and 90-percent 82-octane avgas can lower the octane of the gasoline over two-octane numbers.
Many people think high-octane gasoline is more powerful than low octane gasoline. This is not true. The energy produced from a gallon of high and low octane gasoline is almost the same. Any minor variation depends on what additives are used by refiners and blenders. The key features of high-octane gasoline are a higher ignition temperature and a slower burning rate.
The higher ignition temperature of high octane gasoline reduces the chance of detonation from "hot spots" within the engine's cylinders and minimizes pre-ignition. A slower burn rate allows for more efficient use of the ignited fuel's pressure buildup to be converted to mechanical energy instead of heat. That is why a high performance engine will run smoother and will feel more powerful when high-octane gasoline is used.
Using a low-octane gasoline whose ignition temperature is too low causes pre-ignition. Low-octane automotive gasoline (87-octane) has a typical ignition temperature of 300 degrees Celsius; high-octane (93-octane) automotive gasoline has a typical ignition temperature of 400 degrees Celsius. Aviation gasoline is blended to ignite at 500 degrees Celsius. High compression and high cylinder temperature will cause the fuel to ignite before the sparkplug fires.
Opposition to Autogas The autogas debate is nearly 20 years old and there is still plenty of fuel for the fire. With over 50,000 STCs on the market autogas is definitely in for the long haul...
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