Octane 101: Autogas vs. avgas

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
www.zeltex.com