Diesel Engines May Make a Comeback as Planemakers Seek Alternative Fuels

At least four firms are developing diesel engines for non-turbine, high-performance light aircraft that will run on Jet A1, thereby freeing operators from dependency upon leaded high-octane avgas that is headed toward extinction early in the 21st century. The new diesels promise one-third better specific fuel consumption than current leaded gas piston engines.

Morane Renault Engines' 200-hp, five-liter, four-stroke, four-cylinder MR 200 turbo-diesel made its U.S. debut at EAA AirVenture '98 (the Oshkosh show). It made its first flight on an Aerospatiale Socata TB20 in March.

Meanwhile Lycoming, together with partner Detroit Diesel, is developing a turbo-diesel engine. And Continental is pressing ahead with a 4.7-liter, two-stroke, 4-cylinder CSD-283 engine as part of the NASA-GAP (General Aviation Propulsion) program. The Morane Renault MR 200, including accessories, weighs 13 pounds less than a 200-hp, four-cylinder, Lycoming piston engine, according to Luc Pelon, the company's program manager. Turbocharging and the large intercooler enable the engine to develop 70 percent power at 25,000 feet. The engine has a digital electronic control and power is set with a single lever.

The MR 200 features an 18:1 compression ratio, plus head-to-head through bolts that allow use of very high boost pressures. Similar to slow turning diesel engines used in trucks, the MR 200 generates its 200 hp at a comparatively slow, constant 2,000 rpm. It uses high boost, instead of high rpm to generate its rated power and make possible a 3,000-hour TBO, according to Pelon. External noise levels should drop five to six dB because of low prop rpm plus the muffling effect of the turbocharger, said Pelon.

JAA Part 23 certification is slated for third quarter 1999. FAA certification is expected shortly thereafter. The turbo-diesels will make their production debut aboard Socata general aviation singles.

Morane Renault also is developing 250- and 300-hp versions of the four-cylinder turbo-diesel. Both engines have 3:2 reduction gearing, thereby allowing the prop to turn 2,000 rpm while the engine turns at 3,000 rpm. The MR 250 and MR 300 also are scheduled for certification in 1999 and they weigh considerably less than gasoline-fueled, six-cylinder engines that produce equivalent horsepower. Installed cost is projected to be about 10% more compared to engines that burn avgas. For example, retrofitting an MR 300 might cost $60,000 to $90,000. Life cycle costs, however, should be lower because of increased maintenance intervals.

In contrast to Morane Renault's MR 200, Lycoming-Detroit Diesel's four-stroke, four-cylinder engine weighs 20% more than its current 200-hp, four-cylinder gasoline powered IO-360. The TDIO-360 has a 16:1 compression ratio, it runs at a maximum 2,400 rpm and maximum boost is limited to 60 inches MAP. The engine has a conventional hydro-mechanical fuel control. Preliminary tests of the TDIO-360 proof-of-concept engine are complete.

The next step is a 500-hour endurance test in preparation for fourth quarter 1999 certification. Lycoming hopes to pare 10 to 15% of the excess weight from the engine by certification. Continental is pressing ahead with a 4.7-liter, two-stroke, four-cylinder CSD-283 engine as part of the NASA-GAP (General Aviation Propulsion) program.

By Fred George