Automotive fuel from the pump is NOT the same everywhere you go. There are summer blends, winter blends, geographical blends within the seasonal changes, varying levels and types of oxygenates (not just ethanol) and when tanks are switched over a mixture of "in between". The pump labeling is accurate for octane, and in some parts of the world, the maximum percentage of ethanol. It doesn't tell you everything you need to know to make "pump gas" fit-for-purpose for aviation and it's not just about octane and ethanol. We need to control a wider range of fuel properties for aviation. Your equipment, your life and the lives of your passengers depend on it. Let's walk through exactly what Lycoming did in authorizing automotive gasoline - "mogas" - for our engines. It's also suggested that you read Section B of the latest revision to Lycoming Service Instruction 1070 - and read it carefully.
First, notice that we specified 93 AKI as the octane level. This octane level provided the required detonation margin without modification of our engine FAA Type Design allowances, including cylinder head temperature limits. 93 AKI is "Super Premium" fuel. That AKI rated octane is what is needed to achieve the exact same power performance on the approved engines as 100LL avgas. 93AKI is produced worldwide, but not necessarily distributed worldwide.
Next, Lycoming limited vapor pressure to Class A-4. This was done to prevent vapor lock and A-4 was chosen as it overlaps with avgas properties. The Class A-4 convention arrives from the U.S. Environmental Protection Agency's influence on the automotive gasoline specification and the need to control emissions on ground vehicles. The automotive fuel spec has a method that allows us to control when vapor lock would occur and lends predictability to fuel evaporation through all phases of the aircraft operating envelope. Ground transportation gasoline - heavily influenced by EPA - controls vapor pressure for emissions, startability and driveability on a seasonal and local geographic basis. Vapor pressure is not labeled for retail "pump gas" but is controlled via the wholesale distribution chain. To make fit-for-purpose aviation "mogas", you need to control vapor pressure and you need to know that it matches what your equipment needs.
No ethanol. Most assume that the restrictions on ethanol are related to corrosion. That is just one factor. The other equally important parts are (a) that the automotive gasoline specifications allow vapor pressure to rise outside of the base control limits as you add ethanol and (b) ethanol reduces the energy density of the fuel, increasing fuel consumption. Lycoming restricted ethanol more for vapor pressure control and fuel burn rates than corrosion. When you add ethanol, at certain percentages the automotive fuel spec vapor pressure is allowed to increase. Fuel consumption rates increase from what the POH may indicate as you add ethanol because the energy Page 2 of 2 density drops. Retail "pump gas" is not labeled for exactly how much ethanol or oxygenates are included - maybe a maximum in some of the United States - thus you do not know if it is matching what your equipment needs and consistent with POH performance curves. To make fit-for-purpose aviation "mogas", you need to control the exact amount of ethanol and you cannot allow ethanol inclusion to grant a "bye" on vapor pressure.
Lastly, in approving "mogas" Lycoming locked in the revision of ASTM D4814 to 09b and EN228 to 2008:E. Automotive specifications change frequently, almost yearly, in response to changing environmental regulations and political mandates such as ethanol inclusion. Freezing our mogas specification in time gives refiners a fixed standard and one that matches against the stated performance of the engine. Predictability.
Lycoming is well aware that other aviation equipment manufacturers have allowed "pump gas," in some cases with ethanol. Notice that in the details they provide warnings and cautions addressing the issues we have raised, leaving it to the operator to assess risk in using a fuel where they do not know its core properties other than octane. We do not agree with that approach.
Summarizing Part 2 of this series, the Lycoming approval provides details on how to specify a "fit-for-purpose" aviation engine fuel from fuel that is designed to start and run ground transport engines. The controls we have placed on the fuel are necessary for engines in the currently existing fleet that were designed and certified for operation with avgas. Yes, you can maintain performance ratings on some engines. Yes, the Lycoming "mogas" might be in the tanks at your corner market or local gas station. No, you cannot determine if the retail "pump gas" is controlled as it needs to be for aviation because filling station pumps do not provide the information needed to match it to "mogas" requirements. Yes, it is produced under existing ASTM and EN automotive fuel specifications and can be ordered wholesale, but we do not believe that it can supplant the 100LL avgas that is widely distributed already.
What our mogas approval really says is that using automotive fuel blends in aviation is possible for certain engine models if you put the correct controls in place and those controls are not in place for "pump gas." It's an option to consider that would eliminate lead emissions and bring the potential for more fuels flexibility to customers, who would still need an airframe TC or STC to operate.
Airworthiness by Design. In the next article we'll go deeper into Lycoming's motivation to approve "mogas" - a low grade aviation fuel from automotive blend stocks.
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