Today automotive gasoline combustion engine’s are relatively inefficient. Diesel engines are more efficient, but are large and heavy, and are typically not used for hybrid electric applications. This paper presents an optimized thermodynamic cycle dubbed the High Efficiency Hybrid Cycle, with 75% thermodynamic efficiency potential, as well as a new rotary ‘X’type engine architecture that embodies this cycle efficiently and compactly, while addressing the challenges of prior Wankel-type rotary engines, including sealing, lubrication, durability, and emissions. Preliminary results of development of a Compression Ignited 30 kW X engine targeting 45%(peak) brake thermal efficiency are presented. This engine aims to fit in a 10” box, with a weight of less than 40 lb, and could efficiently charge a battery to extend the range of an electric vehicle.