Octane 101: Autogas vs. avgas

Octane 101
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.

Octane 101

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.

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Detonation occurs after the spark plug fires. Ideally, when the spark plug fires and ignites the fuel, a progressive flame path develops producing a smooth increase in temperature and pressure within the engine cylinder to drive the piston. Detonation occurs when other ignition sources ignite the fuel charge. Red-hot bits of carbon and hot spots in the cylinder also ignite the fuel causing a second or third flame path to develop. This phenomenon causes the gasoline charge to burn instantaneously, producing a violent increase in cylinder pressure and temperature.

If you use a fuel with lower-octane than is recommended by the engine's manufacturer, this can cause serious damage to the pistons, valves, cylinders, heads, and bearings in a short period of time. Low-octane gasoline ignites quickly and produces a pressure-temperature peak that can exceed the design limit of the engine. This condition is exacerbated the more you run the engine. The heat buildup can not be dissipated fast enough from within the cylinder, causing more pre-ignition and uncontrolled detonation.

If your engine is designed to use low-octane fuel, the use of a higher-octane gasoline will not improve performance. Engine dynamics, timing and compression ratio are what determine which gasoline octane the engine manufacturer recommends. It is possible to burn the exhaust valves if you use a high-octane gasoline in an engine that is designed to use 82-octane gasoline. Retarded ignition timing and a slower fuel burn rate can increase exhaust temperature because the fuel charge is still burning at high pressure when the exhaust valve opens. Excessive heat and pressure will quickly erode the valves and seats.

Octane testing

New technology is now available to test the gasoline used in light aircraft. Gasoline now may be tested when it is delivered to the FBO, at the refueling truck, or directly from the aircraft with a portable octane analyzer.

Flying is very expensive. The costs of aircraft, service, fuel, insurance and engine repairs are constantly rising. Bad fuel or the wrong fuel will ruin your engine and your day. What better insurance policy could you possibly have now that there is an easy and fast way to test the lifeblood of your aircraft, your fuel.

The Source
Zeltex, Inc.
130 Western Maryland Parkway
Haggerstown, Md 21740
(800) 732-1950