Factors influencing the effect of pilot-injection on main-injection combustion were investigated using heat release analysis in a heavy-duty diesel engine fuelled with standard diesel fuel, and included the effect of those factors on engine performance and emissions.Combinations of pilot injection parameters i.e. pilot start of injection, pilot mass, pilot-main injection separation, and rail pressure were studied for various operating conditions and combustion phases.It was concluded that the effect of pilot-injection combustion on main injection can be studied based on the phase of pilot combustion at the start of main injection. Four cases were identified: a) main injection during the mixing phase of pilot injection; b) main injection during the premixed phase of pilot combustion; c) main injection during the diffusive phase of pilot combustion and d) main injection after pilot combustion was completed.The effect of pilot injection was illustrated by the conceptual model of interaction modes. This approach has the advantage that the effect of each mode on the main injection is independent of the pilot injection parameters. For combustion control stability and convergence reasons, it is important to consider the transition between the four different interaction modes in regard to variances in the chamber conditions, pilot injected mass and pilot injection combustion.