Factors influencing the pilot-injection combustion, the effect of pilot-injection on main-injection combustion, performance and emissions were investigated by heat release analysis in a heavy duty diesel engine fueled with standard diesel fuel. Combinations of pilot injection parameters i.e. pilot start of injection, pilot mass, pilot-main injection separation and rail pressure were studied for the different operating conditions and combustion phases. The factors influencing the pilot combustion and its impact on the main injection were investigated. It was concluded that the effect of pilot-injection combustion on main injection depends on during which phase of pilot combustion main injection starts. Four cases of how the pilot influences the main injection were identified: a) main injection during mixing phase of pilot injection; b) main injection during premixed phase of pilot combustion; c) main injection during diffusive phase of pilot combustion d) main injection after pilot combustion is completed. The effect of pilot injection was explained by the interaction mode conceptual model. The model has the advantage of being independent of pilot injection parameters. For combustion control stability and convergence reasons, it is important to consider the transition between the four different interaction modes due to variances in the chamber conditions, pilot injected mas and pilot injection combustion.