Factors influencing the pilot-injection combustion were investigated through heat release analysis in a heavy duty diesel engine fuelled 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. An experiment was designed to investigate the factors influencing the combustion of the pilot. For better injected fuel-mass accuracy, reference data was measured for the injectors in a spray-rig prior to the engine experiments. Results show that cycle-to-cycle variations and cylinder-to-cylinder variations influence the pilot auto-ignition and the amount of heat release. Rail pressure and injected pilot mass affect the obtained variance depending on the chamber conditions. The obtained combustion modes (premixed, diffusive) of pilot combustion depend on the injected mass and rail pressure.