Numerical investigation is carried out in order to explore diesel combustion using advanced turbulence and combustion models. Turbulence is modeled by one-equation non-viscosity dynamic structure Large Eddy Simulation (LES) model. Sub-grid fuel-air mixing is calculated using a dynamic scale similarity sub-grid scalar dissipation model to represent the local state of combustion. Fuel-air mixing time scale is used in order to determine the local in-homogeneity and rate of mixing of fuel and air.Diesel combustion is studied and compared with experimental results for high power diesel engine setup at different conditions representing both low temperature combustion and traditional high temperature combustion regimes. Further studies are carried out in diesel engine to investigate in-cylinder fuel air mixing and the onset of ignition. Engine experiments were conducted to investigate Low Temperature Combustion (LTC) and conventional diesel combustion modes by varying fuel injection event(s) and intake charge conditions such as Exhaust Gas Recirculation (EGR). Additionally a preliminary numerical investigation is carried out in order to study the effects of mixed mode combustion in a high powered diesel engine. A single cycle mode switch analysis is performed from high temperature combustion to partially premixed compression ignition (PCCI) mode. In this preliminary investigation several aspects of combustion control are investigated. The effects of fast response controls such as fuel injection timing on in-cylinder combustion are studied. Slow response controls, such as EGR and intake boost are not realized in single cycle mode switching. The effects of such transients on CA50 and emission characteristics are presented. It is found that in order to control combustion during mode transition, fuel-air mixing stratification is required, which can be achieved by fuel injection timing and spray targeting techniques.