I was looking around at engine types and thought this one was interesting.
One Crank Opposed Piston Engine
Hybrid Opposite Piston Engine - HOPE & Portable Range Extender / State of the art
Having a single cylinder with two opposed cylinders eliminates cylinder head and valve train, this reduces area for heat rejection and offers opportunity for high efficiency, reduced emissions as well as reliability and wear reduction because of reduction of components.
On the downside it does require presurised air intake to improve scavanging and clearance of exhaust.