In the world of military aviation, the piston engine was left for dead sometime in the 1950s and it would still be thus if drones hadnt come along. But piston engines swinging old-school props give unmanned aircraft something that jet engines cant: unrivaled mid-altitude efficiency and endurance. So as unlikely as it seems, drone technology is pushing piston engine and prop design and thats why Lycoming has a diesel engine that we know absolutely nothing about. Its designated the DEL 120 and has 205 HP, meaning its in the IO-360 class. Its the prime powerplant in the General Atomics MQ-1C Improved Gray Eagle, an Army drone. The Gray Eagle is a follow-on to the MQ-1 Predator, which has similar mission capabilities.
This is a companion discussion topic for the original entry at https://www.avweb.com/ownership/lycoming-has-a-diesel