In this study, experimental and simulation investigations on the roles of charge density (ρtdc), temperature (Τtdc) at the top dead center and oxygen concentration (φO2) on the combustion paths, emissions and thermal efficiency of a high load operation diesel engine were conducted. Experimental engine was a modified single-cylinder engine equipped with variable mechanisms of boost, exhaust gas recirculation (EGR) and intake valve closing timing (IVCT) to regulate the Ptdc, φO2 and Τtdc. Simulations of engine combustion processes were performed with an ECFM-3Z combustion model.The results revealed that higher Ptdc, leading to lower overall fuel/oxygen equivalence ratio (Φm), enhanced the rate of mixing and chemical reaction and benefited improvement of the thermal efficiency. It was found that increasing charge density played two opposite roles in NOx formation: one was inhibiting combustion temperature rise due to increased total heat capacity of the charge and another was increasing the air entrainment rate resulting mainly in raising mixture temperature. The role of reduced φO₂ by using EGR was essentially to decrease the chemical reactivity of the fuel/gas mixture and to retard the phase of heat release rather than to increase heat capacity and to lower mixing rate. A phenomenon of formation and maintenance of a large amount of incomplete combustion products, i.e., CO, was found in the high load operation engine during the combustion process and was named as "cold storage of carbon-monoxide," which retarded the heat release phase and decreased the burning gas temperature, which led to decreased NOx emissions. It was also found that the engine exhaust soot correlated with the amount of the "cold storage of CO."