A newly-invented "X"-configuration engine utilizing the Scotch yoke mechanism renders potential for the best power/weight ratio of any piston engine. Due to its inherent space and weight efficiency, low stress levels on critical components and low bearing pressures, this new configuration can be designed for aircraft applications using high-pressure 4-stroke diesel cycle with large numbers of cylinders - as many as 24 or 32 cylinders - to minimize engine weight and cross-sectional area.Given the efficiency advantage of 4-stroke turbo-diesel cycle over turbine engines, a study reveals that diesel X-engines may be a preferable solution to turbine engines for airplanes, helicopters and UAVs up to approximately 60000 lbs max. weight @takeoff. Calculations using existing turbine-powered aircraft as a baseline indicate potential for 35 to 50% lower fuel consumption with no compromise to maximum takeoff weight, payload, range, cruise speed, maximum speed or takeoff power. While the X-engine diesel weighs more than the equal-power turbine engine, the reduction in fuel load @takeoff makes up for the heavier engine weight.The direct competition for X-engine aero-diesels are turboprop and turboshaft engines which justifies a substantial budget to produce X-engine aero-diesels. Hence, the best diesel engine components and systems available can be used while implementing lightweight aerospace materials where appropriate. The result is liable to be an aero-diesel engine that can dramatically improve fuel efficiency and operating costs while still having a lower price than the turbine engine.Besides lower fuel consumption and lower carbon emissions, other potential benefits are for lower noise and lower heat rejection compared to turbine engines.