CAPS (Cirrus Aircraft Parachute System)

SR22T – Fuel Flexibility

Cirrus introduced a new model aircraft – the SR22T in June 2010. One of the significant changes of the SR22T is the TCM TSIO-550-K turbocharged engine. The "K" model engine installation was a result of cooperative designs between Cirrus and TCM to make the necessary optimizations to both the engine and airframe to ensure desirable performance, engine operations and airframe integration. TCM has chosen the TSIO-550-K engine to aid in the preparation for the worst case (octane) fuel scenario (94UL). Many would say that the turbocharged engines are the most vulnerable to a significant drop in octane, therefore making them a perfect test subject. A 94UL fuel speciation has gone through the ASTM fuels committee so a full range of testing of 94UL, including certification, with the TSIO-550-K engine could be performed. Testing is currently underway.

During testing, one thing became clear. There are many ways to accommodate the lower octane fuels but the most viable, near term solution for high performance airplanes is a reduced compression ratio. While more sophisticated solutions are in the works, longer lead times and economic reasons may not make them an appropriate solution.

The SR22T works with a 7.5:1 compression ratio which is sufficient to adapt to lower octane fuels. The lower compression ratio, however, does result in less efficiency (approximately 2-3%). The compromise results in a more fuel tolerant engine which reduces the future fuel risk, with the sacrifice of 20-40 nm in range. Fuel Flexibility, just another choice offered from Cirrus.

Discontinuous leading edge of a Cirrus wing

Aviation Fuel Future

Background:

In 1976 under the Clean Air Act, lead was identified by the Environmental Protection Agency (EPA) as a major pollutant and subsequent action was taken to get the "lead out". Remarkable progress has been made and according to the EPA: Average concentrations of lead in the air have dropped 91 percent between 1980 and 2008. Much of the credit for the dramatic drop of lead concentrations is given to the permanent phase-out of lead in motor vehicle gasoline. From 1976-1996 lead was gradually reduced to completely non-existent in highway use vehicles. In the 1996 ban on leaded gasoline for highway use vehicles, the EPA also singled out "ok" usage of lead in fuels for: aircraft, racing cars, marine engines, etc.

The natural progression of further eliminating lead concentrations leads to the next largest source, leaded avgas. The EPA, as required by the Clean Air Act, has formally begun the regulatory process that will ultimately result in the removal of lead from aviation gasoline. The industry expects a solution before the current 100LL becomes unavailable.

CIRRUS participates in finding a solution:

Cirrus is actively working with all industry partners to try and find the best possible transition to a future unleaded avgas. Cirrus currently:

  • Chairs the General Aviation Manufactures Association (GAMA) fuels committee which leads the various industry groups working with the EPA and the FAA to find a viable transition plan to an unleaded avgas future.
  • Is a member of the FAA's Unleaded Avgas Transition Aviation Rulemaking Committee. The membership consists of the FAA, EPA, GA associations, Manufacturers, Petroleum companies and possible future fuel providers. This is the first time a committee was formed with all affected stakeholders.
  • Is a member of the ASTM aviation fuels committee which is establishing the specifications and methods to validating new aviation fuels.
  • Participates in the Coordination Research Counsel (CRC) – the aviation fuels committee which works with fuel suppliers, engine manufactures and the FAA on researching and testing high octane fuels.
  • Is working directly with fuel suppliers to test promising new fuels.

The Search for Unleaded Avgas

The industry collectively agrees that every effort should be made to find a transparent to near transparent 100 octane fuel replacement. Keeping the 100 octane rating is actually easy with many different additives such as MTBE, Ethanol, Manganese, Xylene just to name a few. The tricky part is finding the solution that not only produces 100 octane but also address the many issues such as:

  • operation at temperature extremes
  • fuel stability over time
  • health and safety aspects of the fuel and its combustion products (the current problem with 100LL)
  • material compatibility
  • the ability to produce the fuel
  • cost

Generally speaking there are three possible avgas replacement scenarios (in order of highest octane):

  1. Find a drop-in replacement fuel that's equivalent to 100LL. No fuel has been identified, but the search is extremely active.
  2. Find the closest to 100 octane fuel replacement that address all issues listed above.
  3. Adopt 94 octane unleaded fuel (94UL). While this is the lowest octane rating, it does comply with all the issues listed above.

To date many would say no fuel has established itself as a clear front runner for a 100 octane or near 100 octane solution, but there are many hopefuls such as Swift fuel and GAMI's G100UL.

The "worst case scenario", in most people's opinion, is the 94UL fuel solution. To produce it, the lead in the 100LL refinement process is removed. This solves all the issues listed above, but the drop in octane will mean changes (or power reductions) to many higher performance engines.

What will those changes to higher performance engines be?

Each engine will have its own unique case and until the future fuel is identified an ultimate solution to each unique engine may not be clearly identified, but the solutions could be as simple as operating limitations at the "corner" of the performance envelope to a more complicated solution of replacing the pistons.

There are also some promising new technologies on such as FADEC, anti-knock sensors and GAMI's Prism system that could ease any fuel transition by the use of electronics.

Cirrus Approach

Due to the uncertainty involved in the fuel issue, Cirrus is developing solutions for both existing and new aircraft that address a wide range of possible fuel specification outcomes. This doesn't change the Cirrus commitment to finding a transparent solution. An example of such a solution was the recent unveiling of the SR22T model aircraft that offers flexibility to a wide range of fuels on what is believed to be the most affected engine by any octane decrease – a high horsepower, turbo-charged engine. Preparations are already being made for engine solutions on the current fleet of almost 5,000 aircraft. Efforts will be more focused, however, when the future fuel is ultimately identified.