The automotive industry has made great efforts in reducing fuel consumption. The efficiency of modern spark ignition (SI) engines has been increased by improving the combustion process and reducing engine losses such as friction, gas exchange and wall heat losses. Nevertheless, further efficiency improvement is indispensable for the reduction of CO2 emissions and the smart usage of available energy. In the previous years the Atkinson Cycle, realized over the crank train and/or valve train, is attracting considerable interest of several OEMs due to the high theoretical efficiency potential.In this publication a crank train-based Atkinson cycle engine is investigated. The researched engine, a 4-stroke 2 cylinder V-engine, basically consists of a special crank train linkage system and a novel Mono-Shaft valve train concept. The idea of a Mono-Shaft valve train mechanism is to realize the valve actuation without the need for separate cam shafts and gears, but via a cam disk rotating with crankshaft speed, thus enabling the integration of the cam disk in the crankshaft.This publication first gives a brief overview of the specific engine design. The second section includes a comprehensive kinematic analysis of the crank train linkage system with respect to feasible piston motions. In the third section a detailed 1D-CFD (one-dimensional computational fluid dynamics) simulation is presented calculating the engine's indicated efficiency. The simulation is adjusted via the experimental data obtained by a conventional spark ignition engine (burn rate, flow coefficients, etc.) using the valve timing of the Mono-Shaft valve train concept. The conclusions are drawn in the final section.The results of this study show that the Atkinson Cycle-based twin engine concept has high potential concerning efficiency improvement of SI engines. Regarding the characteristic efficiency results this engine concept could be favorably used for range extender applications.