A new model high-efficiency gas engine has been developed that achieves a generating efficiency of 41.5 %, the highest in the world for a medium-sized gas engine. This represents a 1.1 % further increase over the 40.4% generating efficiency attained in the previous model. This achievement was made possible by improving combustion efficiency through the optimization of the shape of the combustion chamber, reducing various losses through the optimization of valve timing, and enhancing the performance of the turbocharger, all of which were concomitant with the adoption of a long stroke design. At the same time, development efforts were also focused on making the co-generation system more compact, improving the maintainability of the system, and bolstering the competitiveness of the system as a product.