The Compact Compression Ignition (CCI) engine concept was first described  in 1999. The design has since evolved with greater emphasis on high efficiency and a demonstration model is currently the subject of preliminary tests. This paper compares results of mechanical and thermodynamic simulations of two 450 kW engines. One is a conventional turbocharged six cylinder diesel engine and the other is a three module naturally aspirated CCI engine.The ubiquitous slider-crank mechanism that forms the basis of all current automotive engines has geometric limitations which are discussed as an introduction to the radically different CCI.According to the analysis, the CCI engine offers both higher efficiency and smaller size than the conventional engine. The reasons for the differences are described and quantified in the paper.