Gemini Diesel Engine Attracts Industry’s Eye

At first glance the Gemini 100 looks like a compact water-cooled aircraft engine with four opposing cylinders. Narrow and not very tall or deep, it’s roughly the same size as a Continental O-200 and has the same output, 100 hp. That’s where the similarities end, says Tim Archer, president and CEO of Powerplant Developments, the Gemini’s manufacturer. With three cylinders and six pistons, the Gemini burns diesel/Jet A.

Come again?

Like a lot of pilots, I’m not much of a gearhead, but this caught my attention. The Gemini is an opposed piston engine, meaning the combustion chamber is formed between two pistons in the same cylinder, Archer explained. The intake and exhaust ports are orifices in the cylinder wall and the piston’s movement opens and closes them.

Parallel crankshafts transfer the power to the reduction gearing built into the front of the case. Being shorter, these cranks are not subject to the torsional loads imposed on the longer cranks in traditional engines. To ensure reliability, the Gemini cranks are made of EN40 steel, the same steel often used to make cranks for Formula 1 engines, which routinely run at 19,000 rpm, Archer said.

Apparently I’m not the only one attracted to this innovative powerplant. Powerplant Developments has signed a memorandum to supply a pre-certification Gemini 100 and two turbocharged Gemini 125 engines to an Italian aircraft manufacturer, Tecnam. And during EAA AirVenture, seven other airframers expressed interest in the engine, especially the forthcoming 180/200 hp and 300 hp versions. The company has completed design studies for Gemini engines up to 600 hp.

With liquid cooling, weight has always been an issue with aviation diesels. This innovative design seems to have found the solution. The installed weight of the 1,600-cc/98-cubic-inch Gemini 100 is 166 pounds. Installed, the O-200 weighs nearly 200 pounds, and the 112-hp Lycoming O-235 tips the scales at 242 pounds. 

Tecnam will evaluate the Gemini 100 in its P92 Eaglet, an LSA powered by a 100-hp Rotax 912 ULS2, which has an installed weight of roughly 170 pounds. The Gemini 125s will power the  P2006T twin.

Fuel is another consideration of weight. The Gemini 100 has a specific fuel consumption of .38. Archer reluctantly converted that to 3 to 3.5 gph, depending on the airframe and prop, other variables in the equation. It’s interesting that a prolonged Internet search for the specific fuel consumption numbers for avgas engines provided no joy.

Archer said the Gemini 100 will retail for $18,000 and the 125 will be $23,500. “TBO looks like it will be 2,000 hours,” he said, adding that with operational data on the engine it may grow to 3,000 hours. He foresees no pre-TBO inspections, such as on the reduction gears, and the three cylinders won’t need to be replaced until the third overhaul.

All of it sure sounds good, but the skeptic in me says engineers have been trying to develop a workable aviation diesel since World War II. Designs come and disappear, but few fly. Tecnam will give the Gemini some test wings, and it will be interesting to see if the other manufactures interested in the bigger engines follow through with their initial interest. — Scott Spangler