A method to improve fuel consumption in diesel engines is to enhance their theoretical thermal efficiency by increasing their compression ratio. However, this results in an increase in heat loss due to the elevation of the concomitant in-cylinder temperature and the expansion of the impingement area between fuel spray and chamber wall. Therefore, reducing heat loss to the chamber wall is important to effectively benefit from a high compression ratio.To meet this challenge, in this study, we optimized the combustion chamber shape using the three-dimensional computational fluid dynamics (CFD) simulation software, CONVERGE. A rationale proposed by the University of Wisconsin-Madison was selected to outline the shape and combined with a multiobjective optimization software, modeFRONTIER. The calculations produced a shallow dishlike combustion chamber comprising a plateau at its center that may reduce heat loss. In this system, a portion of fuel spray remained at the center of the combustion chamber because of wall impingement. The lower half of this spray developed along the chamber wall, partially blocking air entrainment. These phenomena resulted in weak premixed combustion, lowering the combustion temperature and thus reducing heat loss.Furthermore, experiments using a single-cylinder engine were performed to evaluate the effect of the optimized combustion chamber shape on heat loss. The optimized combustion chamber improved fuel consumption under high load and advanced injection timing conditions. A weak premixed combustion was observed, as predicted by the CFD calculation. The thermal balance analysis also revealed that heat loss from the cylinder decreased while exhaust loss increased.