There have been strong demands recently for reductions in the fuel consumption and exhaust emissions of diesel engines from the standpoints of conserving energy and curbing global warming. A great deal of research is being done on new emission control technologies using direct-injection (DI) diesel engines that provide high thermal efficiency. This work includes dramatic improvements in the combustion process. The authors have developed a new combustion concept called Modulated Kinetics (MK), which reduces smoke and NOx levels simultaneously by reconciling low-temperature combustion with pre-mixed combustion [1, 2]. At present, research is under way on the second generation of MK combustion with the aim of improving emission performance further and achieving higher thermal efficiency . Reducing heat rejection in the combustion chamber is effective in improving the thermal efficiency of DI diesel engines as well as that of MK combustion. In this study, experimental investigations were conducted to examine transient heat transfer in a single-cylinder DI diesel engine. Transient surface temperature data obtained from the piston cavity and piston head were used as the basis for determining the transient heat flux rates. The results showed the heat rejection level of MK combustion and the effects of a combustion control parameter incorporated in this combustion concept. The results also made clear the effects of the fuel injection parameters of a common rail injection system, including their respective influence during MK combustion. Based on these results, this paper discusses the effects of combustion control parameters on transient heat transfer in the combustion chamber as well as on combustion characteristics, including their influences during MK combustion. It also describes the reasons why fuel consumption does not deteriorate during MK combustion under a retarded injection timing and presents a general approach to improving the thermal efficiency of DI diesel engines further.