Other Future Fuels
Mogas and Jet Fuel:
While the future of avgas is being debated, other thinking has been towards other fuels already available across the world. Two often discussed are motor fuel (mogas, petrol, gasoline) and jet fuel (or road diesel, a similar fuel).
Various companies over the past few years have developed engines for motor fuel. Some are widely used, Rotax being the best example.
All motor-fuel engines that have entered production are low horsepower, typically no more than 100hp, of little value for larger, more capable traveling aircraft requiring 200hp or more.
There is no short-term expectation of higher-powered engines running normal motor fuel.
In the same period others developed "diesel" engines burning jet fuel. Jet fuel, of course, is available throughout the world. The term "diesel", sometimes known as "compression ignition", applies to the way the engine operates: these are piston engines that compress fuel/air to very high pressures initiating spontaneous ignition instead of using a spark plug. No electric/ignition/spark plug system is used.
Not all these companies have survived; orphaning customers. Always a risk with a new technology.
These engines are becoming more and more refined (don't think of a truck engine) but there is a fundamental issue which dramatically limits diesel engine use in general aviation: the higher internal pressures require stronger, heavier structures; contibuting to much higher engine weights.
Some diesel engines are derivatives of auto engines that run at high rpm, meaning a gearbox is needed for typical propeller speeds. This introduces yet more weight, higher complexity and new maintenance needs.
These engines have also, like mogas engines, been fairly low power engines, rarely exceeding 150hp. The exception to this is SMA, whose 227hp engine, is to appear in the latest incarnation of Cessna's C182 Skylane (see sidebar).
There are no diesel engines available for a Cirrus SR22 300hp-class traveling aircraft.
An exhaust driven turbo-charger (air compressor) allows normal gasoline engines to maintain sea-level power to high altitudes. You normally expect turbo-charged aircraft to go faster as you go higher. Diesel engines are different.
Turbo-chargers are used on virtually all diesel engines and are used to "force feed" outside air into the very high internal pressures of the engine - not just to boost performance.
So diesel aero-engines have very low critical altitudes (above which the turbo-boost cannot maintain sea level power). Sometimes this is as low as sea level itself but more often in the 6,000 - 10,000 ft. range.