We continue to receive a number of questions regarding the use of automobile gas in aircraft engines. This information may likely be more than you need to know, but should answer most questions.  

When most current piston aircraft engines were designed and certified in the 1950’s, lead was still a normal gas additive, mostly because it was a cheap way to increase octane number (by about 20%). The higher the octane number, the higher the compression and temperatures in the cylinder can be before fuel begins to self-ignite prior to the piston reaching the top of the cylinder and before spark plug ignition. We know this as detonation, or pre-detonation. Aircraft piston engine have rudimentary mechanical ignition timing systems and need the higher octane to prevent detonation, and lead was a cheaper way to increase the octane number.

Octane number required by an engine is based mostly on the compression ratio of the cylinder; the higher the compression (which creates greater temperatures and pressures within the cylinder as the cylinder squeezes the air/fuel mixture to greater extents) the higher the octane number must be to resist the mixture from self-detonating. This is why lower compression engines can burn lower octane gas. The FAA has issued STC’s that allow use of automotive unleaded, non-ethanol gas for some lower compression aircraft engines. These STC’s simply requires a new placard be installed on the fuel tank filler.

Because minimum octane is mostly based on the temperatures and pressure within the cylinders as the piston compresses the air/fuel mixture, the temperature and pressure of the outside air being sucked into the cylinder from the air filter can considerably lower or raise cylinder temperatures and pressures. The colder and/or lower pressure of the intake air (such as on a cold day or at higher altitudes), the lower the cylinder temperatures will be during the compression stroke, decreasing the likelihood of detonation.

So, octane ratings can be reduced with lower ambient temperatures and pressures on normally aspirated engines. This is one reason why gas station fuels available during winter months, or at higher altitude locations, may have lower octane ratings compared to stations in warmer or at lower altitudes.

For example, a car -with a minimum octane requirement of 85 and recommended octane of 92- may run great with 85 octane fuel in Denver (elevation 5280 ft) during winter months; but experience “pinging”, detonation, or preignition with the same fuel when driven to Arizona (elev. 1000 ft) during summer where temperatures normally exceed 100F.

Octane ratings normally are calculated at a temperature of 77F at sea level pressure. A good rule of thumb is the octane number can be reduced by 1 number per 1000 feet elevation, and by 1/2 number per each 10 degrees F below 77F. The ALPHA engine recommended octane number is 87, which should cover all normal ambient conditions.

Operating with too low an octane number may cause a pinging noise (although the knock sensors should cause the ECM to adjust timing before pinging occurs), indicating a higher-octane rating fuel should be used under current conditions. Engine controllers/ ECMs can only retard timing so much to combat detonation though, and such extreme adjustment greatly decreases the efficiency and performance of the engine, and should not be relied upon to save money at the pump.

Because lighter fuel elements evaporate at higher engine compartment temperatures, fuel line vapor lock in carburetors and aviation mechanical fuel injection systems becomes a real common problem. Anyone that has tried to start an aircraft engine on a hot day, especially when the engine is still hot, can attest to the frustration. Because modern EFI engines constantly bypass a portion of fuel from the engine back to the fuel tank, pressurized fuel is always cool which eliminates the vapor lock.

Lead as an additional lubricant. Lead provided another vital requirement with early engines; lead acts to lubricate/cushion engine components such as valve seating surfaces which extended engine life (metals used in modern engine valves are far harder and don’t need this lubrication effect). Unfortunately, lead is a poison and after illness and deaths of many early engineers and workers producing leaded fuels, the government was pressured to outlaw it soon after its introduction. But by then, engines required it and there was no cheap alternative……similar to today’s piston aircraft fuel dilemma. 

Lead pollution
Lead doesn’t disappear during combustion, it circulates through the engine and flows out the exhaust pipe. The lead was mostly outlawed in the US as a fuel additive in the 1970s under the Clean Air Act with considerable positive health effects. In the US, 78% of children had elevated lead blood levels, wherein by 1996 only 7% had elevated levels; contemporary studies also found higher lead blood levels in persons living near airports with high piston engine traffic. Aviation is one of the few industries still allowed to use leaded fuels simply because the majority of aviation engines, originally designed in the 1940s and ’50s, require leaded fuels and most of the general aviation fleet would be grounded otherwise.

Enter ethanol to replace lead
Necessity is the mother of invention, as leaded fuels were being outlawed, automotive manufacturers developed new engine designs with harder metals, adjustable ignition timing components that mostly prevented detonation, and helped develop other fuel additives to replace lead; ethanol became the primary choice as it was cheap, non-poisonous and it increased the octane level while decreasing harmful pipe emissions…..a win-win, especially for corn farmers whose crops were used to produce the ethanol (ethanol is essentially alcohol derived from corn)-but ethanol brought new problems.

Ethanol absorbs water from surrounding air far more than earlier additives, which contaminates the fuel. Eventually, water absorbed in the fuel will separate and collect on the bottom of the tank/plumbing/carburetor, which disrupts fuel flow to the engine and can degrade and corrode fuel system components. The greater the humidity of the surrounding air, the greater the rate of contamination and shorter the safe useful life of the fuel.

Ethanol also acts as a solvent that may break down other system materials such as rubber seals and hoses, fiberglass and other composite materials used in older aircraft fuel systems. So, modifying older fuel systems to modern ethanol fuels often requires replacing components such as lines, seals, and possibly tanks. This is why part of the Corsair engine conversion requires the installation of new fuel system seals and hoses, as well as additional redundant filter elements.

Ethanol also evaporates quicker than other additives, which lowers the fuel’s octane rating over a shorter time period compared to non-ethanol fuel types. Considering ethanol fuel properties of increased water absorption and its quicker loss of octane rating due to evaporation, ethanol fuel is best suited for aircraft that are used on a regular basis, stored in dryer climates, and do not maintain fuel in its tanks for extended periods. Corsairs POH list recommends when using ethanol fuels to curtail contamination and max period of time ethanol fuels should be in the aircraft’s tanks.

Ethanol fuels have proven to be safe and effective in gasoline engines for decades under all climates, conditions, and multiple engine types. Automobile owners rarely maintain their fuel system per manufacturer recommendations with little trouble as a result. Experimental aircraft have used automotive engines and MOGAS for decades without issue.

Normal automotive fuel systems and filters, as well as the engine itself, were designed for these fuels and no reason these fuels would act differently simply because of the aircraft application, especially if a few extra steps are followed.

As with all modern engines, the Alpha is controlled by an electronic engine controller that monitors several sensors to deliver efficient fuel delivery and spark ignition. Oxygen sensors allow the EEC to monitor exhaust emissions for air/fuel ratios and immediately adjust timing and fuel injector pulse rates to maintain ideal ratios. Pre-ignition and detonation is detected by the Corsair ECM, which will adjust timing and annunciate the condition on the SED.

Oxygen sensors and leaded gas
Because lead in aviation fuel doesn’t combust and exits the exhaust mostly intact, lead eventually builds up on the O2 sensors. The lead soon isolates the sensor surface from the exhaust gases passing by it, and valid signals to the ECM stop. When this occurs, the ECM ignores the O2 sensor inputs and annunciates a fault code indicating the sensor needs to be replaced. There have been few long-term success of cleaning O2 sensors of the lead build-up, but Corsair recommends replacing the sensor when needed. However, O2 sensors are not required to be used on Corsair engines as the ECM will compensate with no noticeable change in engine performance or reliability.

Fuel freezing temperatures
MOGAS and AVGAS have similar freezing levels- freezing is not a practical concern under normal piston engine operations. Jet fuel has additional additives to prevent freezing issues due to the longer cruise periods and altitudes where air temperatures remain well below freezing.

Summary
There is no fundamental reason MOGAS cannot be safely used to power automotive engines in aircraft applications. The same concerns of octane ratings, effects of ethanol, and useful life of gas remain the same. However, because aircraft cannot simply pull over when engine troubles occur, these same concerns must be considered with greater scrutiny. Routine maintenance must include additional checks to ensure the quality of fuel, shorter storage periods of fuel, and develop an effective schedule of checking filter elements and components. The results will be a far cheaper aircraft to operate, cleaner tailpipe emissions into our air supply, and greater fuel options/availability throughout the world.