ExxonMobil told airport fuel suppliers in a letter last month that it cannot support the use of jet fuel in the new diesel-powered aircraft that are coming on to the U.S. market.

The letter, signed by U.S. General Aviation Operations Manager Martin Tippl, says that the U.S. currently lacks a system that requires testing every batch of jet fuel for cetane number. "Knowing the minimum cetane value allows the establishing of a restart envelope and the definition of engine start limitations," ExxonMobil points out in the letter.

Without knowing the cetane, that means the fuel supplier "cannot guarantee the ignition performance of the jet fuel it supplies and cannot know if the aircraft will be airworthy after fuelling."

Freeze point is another issue. "Unlike turbine powered aircraft, piston-powered aircraft do not reach speeds that cause heating of the fuel in the wing due to friction caused by airflow," ExxonMobil points out.

"High performance pressurized piston powered aircraft essentially fly no higher than about 25,000 feet versus the 40,000 feet of turbine powered aircraft. At these lower altitudes, the outside air temperature rarely gets colder than about -55oC. This is the origin of the -58oC freezing point requirement for avgas used in spark ignition aircraft engines."

In contrast, jet aircraft have maximum fuel freeze-point temperatures of -40o to -47oC, ExxonMobil points out.