The purpose of this paper is to investigate combustion and performance characteristics for an advanced class of diesel engines which support future Army ground propulsion requirements of improved thermal efficiency, reduced system size and weight, and enhanced mobility. Advanced ground vehicle engine research represents a critical building block for future Army vehicles. Unique technology driven engines are essential to the development of compact, high-power density ground propulsion systems.Through an in-house analysis of technical opportunities in the vehicle ground propulsion area, a number of dramatic payoffs have been identified as being achievable. These payoffs require significant advances in various areas such as: optimized combustion, heat release phasing, and fluid flow/fuel spray interaction. These areas have been analyzed in a fundamental manner relative to conventional and low heat rejection “adiabatic” engines. Of particular note, in the research being reported here complex interacting mechanisms within the low heat rejection (LHR) class of engine were seen to be extremely sensitive to heat release phasing and its breakdown between premixed and diffusion combustion. Small parameter changes can significantly enhance or degrade engine performance depending upon characteristics such as engine geometry, heat transfer, and fluid flow/chemical kinetic phasing interactions. These sensitivities are not nearly as great in conventional water cooled engines.Studies to date reveal that while chemical kinetics form a critical dominance in a conventionally cooled engine, an LHR engine is more critically affected by mixing controlled phenomena. The LHR engine combination of 1) shortened ignition delay, and 2) diffusion mixing limitations, is responsible for producing longer heat release periods in non-optimized LHR engines. Experimental results to fundamentally understand and improve this characteristic will be presented.