In diesel engines with a straight intake port and a lipless cavity to restrict in-cylinder flow, an injector with numerous small-diameter orifices with a narrow angle can be used to create a highly homogeneous air-fuel mixture that, during PCCI combustion, dramatically reduces the NOX and soot without the addition of expensive new devices.To further improve this new combustion concept, this research focused on cooling losses, which are generally thought to account for 16 to 35% of the total energy of the fuel, and approaches to reducing fuel consumption were explored. First, to clarify the proportions of convective heat transfer and radiation in the cooling losses, a Rapid Compression Machine (RCM) was used to measure the local heat flux and radiation to the combustion chamber wall. The results showed that though larger amounts of injected fuel increased the proportion of heat losses from radiation, the primary factor in cooling losses is convective heat transfer. Next, 3D simulations were used to predict the cooling loss behavior over the entire combustion chamber, and in conjunction with local heat flux measurements on an actual engine, an analysis was performed to determine where cooling losses are significant and when these cooling losses occur. The results showed that because of the convective heat transfer from the reversed squish flow while the piston is descending, the cooling losses were greatest along the side wall of the cavity to the squish region.Based on the findings above, a piston cavity was designed that suppresses reversed squish flow. The shape of this shallow-dish open-chamber cavity suppressed reversed squish flow including local flow, resulting in reduced fuel consumption.