Gas VS Diesel Engines for Airplanes
By: Peter P Field
In looking around for a power plant for my experimental airplane I discovered several different kinds of engines which were suitable in weight, horsepower, and size. Mine is a single engine airplane so naturally I gravitated towards engines that had a long history of reliability. My search turned up several diesel or compression ignition engines and they had some really interesting features, most notably fairly light weight. Light weight diesels – what an innovative idea!
What my search uncovered was that the Blue Collar diesel engine is on the verge of a renaissance with regard to aircraft. With aviation fuel prices increasing, averaging $4.50 to $5.50 a gallon for 100 Low Lead gasoline, at the time of this writing, any improvement in fuel consumption for General Aviation piston powered aircraft will be welcome indeed. As any trucker knows, diesel fuel is just as expensive as gasoline, but kerosene in its many forms has a higher specific gravity and is a more energetic fuel and is thus more economical.
Diesel engines have long been relegated to the heavy lifting jobs in trucking, farming, locomotives, and heavy equipment because the added weight penalty of these very high compression engines doesn’t affect their use in these applications. Airplane powerplants, however, demand high power and very light weight; requirements difficult to blend in a diesel. Most aircraft engines are of the direct drive type, in that that the propeller rotates at the same RPM as the engine. Direct drive aircraft engines are designed to cope with the necessity to keep propeller tip speeds below Mach 1 where propeller efficiency deteriorates dramatically. This requirement means that for light planes, maximum RPM’s generally do not exceed 2700. Thus higher horsepower levels cannot be developed by running the engine at higher RPM. In some gasoline aircraft engines a propeller speed reduction unit (a gear box) is used to allow the engine to run at higher RPM but keep prop tip speeds below subsonic levels. The disadvantage of a geared aircraft engine is the added weight of the gear reduction unit. Unlike automobile engines where there is continual stop and go driving, shifting gears, speeding up and slowing down, then re-accelerating; in aircraft, the engine is operated at 100 % power during take off and for part of the climb. At cruise altitude, power is reduced to a constant output in the 65% - 75% horsepower range where the engine will be operated for many hours during the remainder of the flight. Finally, power is reduced during the descent and landing to some lower output range. This operational profile is ideally suited for the diesel if the engine weight can be controlled to acceptable levels. Operating at 2700 RPM or lower results in lower piston travel per mile covered and translates to engine longevity, a highly desirable characteristic in any aircraft engine.
The advent of Computer Aided Design (CAD) technology has allowed the diesel to re-emerge as a viable aircraft power plant. Engineers in several innovative companies have been able to remove weight from the engine block by careful analysis of the stress paths insuring strength is only where it needs to be. Other differences between diesels and gasoline engines have to be taken into account as well. Diesels don’t need an ignition system so the weight of magnetos, wiring harnesses, and spark plugs can be eliminated. The addition of supercharging and turbo charging works particularly well in a diesel in that it can allow slightly lower compression ratios (thus lower weight) and keep the engine horsepower out put at high levels as the airplane is flown to higher altitudes. The high temperatures generated by diesels generally require liquid cooling so radiators must be added to the overall weight of the engine installation although one aircraft diesel is air cooled. Almost all present day gasoline aircraft engines are air cooled. Some of the more recent diesel aircraft engines have done away with valves and operate like model airplane engines on a two stroke cycle. We’re not talking about the “mixed gas” sort of 2 cycle operation here; these engines are fully lubricated with independent oil delivery systems. One big advantage of the 2 cycle operation is increased power per crank shaft revolution.
Diesels are reasonably easy to adapt to existing airframes. A more robust engine mount is required to handle the adverse torque caused by the loss of a cylinder. Kerosene based fuels weigh more (6.8 pounds per gallon versus 6.0 for gasoline) so attention must be paid to airframe loads and gross weights must be reduced in existing designs if full fuel capacity is to be used. In new designs diesel adaptation is straight forward as airframe loads standards and fuel capacities can be easily engineered into the design.
Aviation is the only place where leaded gasoline fuel is still used in the US and some experts contend that it is only a matter of time before these leaded fuels will disappear. Improved fuel consumption, robust construction, reliability, and a good match for aviation make the diesel engine an ideal selection for small and medium sized airplanes. You can expect to see more diesels in general aviation aircraft in the future.