ABSTRACT A newly-invented "X"-configuration engine utilizing the Scotch yoke mechanism renders potential for the best power/weight ratio of any diesel engine. Given the efficiency advantage of 4-stroke turbo-diesel cycle over turbine engines, a study reveals that X-engines may be a preferable solution to turbines for airplanes, helicopters and UAVs up to 60000 lbs max. takeoff weight. Calculations indicate potential for 35 to 60% lower fuel consumption with no compromise to payload, range, cruise speed, maximum speed, takeoff power or maximum takeoff weight. Outline: I. CAE study of the double-acting Scotch yoke mechanism indicates feasibility for 4-stroke turbo-diesel cycle. a) kinematic analysis b) essential new components: piston, yoke, bearing block c) FEA optimized DASY system for 4-stroke diesel cycle d) apples-to-apples comparison of DASY to connecting rod: bearing pressures, part count, torque delivery, engine balance, package. II. Scotch yoke X-engine design. a) DASY X-engine aero-diesel overview b) sub-systems: cranktrain, cylinder block assembly, valvetrain, cylinder heads, HPCR fuel injection, accessories, manifolds, turbocharger, intercooler. c) assembly process for engine bottom end d) IP coverage: 3 U.S. and 3 foreign patents III. Advantages of DASY X-engine over con-rod-based engines (X, V, L, boxer, horizontal, radial, etc.). a) past X-engines using con-rods b) lightest weight, least volume, smallest cross-sectional area c) X-engine aluminum cylinder block is stronger & stiffer d) less friction - crankcase pumping work e) perfect dynamic balance f) scalable from 8 to 32 cylinders g) smoothest torque delivery IV. DASY X-engines for aircraft / comparison to turbine engines a) 100hp/cylinder series: X-16 (1600hp), X-24 (2400hp), X-32 (3200hp) b) package comparison of X-engine to comparable turbine engine c) fuels: Jet A, Jet A-1, JP-5, JP-8, diesel d) TBO target 7500 hours e) fuel consumption advantage over comparable turbine engines f) advantages for lower noise and less heat signature g) other capabilities for aircraft: horizontal or vertical engine orientation (airplanes/helicopters); potentially both orientations for VTOL airplanes. h) left-hand/right-hand rotation engine pairs using 95% common parts V. Other applications: aircraft APUs, tanks, military ground vehicles, marine propulsion, diesel-electric systems, more.