A two-stroke diesel engine was outfitted for operation with an electronic solenoid-controlled unit injector and an additional solenoid-controlled air-assisted injector at the inlet ports. Factorial experiments were designed in order to quantify, in a statistically representative manner, the effects of pilot (or ‘split’) and port auxiliary injection on main fuel combustion.Results indicated that interactions between experimental parameters (such as between pilot fuel quantity and pilot-to-main spacing), as well as main effects are important in analyzing auxiliary fuel injection. The bulk gas temperature at main injection was determined primarily by the experimental parameters acting independently of one another, which is a case where main effects only are important. Conversely, analysis of indicated specific fuel consumption and peak cylinder pressure involved interactions of the experimental parameters in both cases.Other results indicated that addition of a pilot injection could effectively separate the premixed and diffusion portions of the combustion process. The port fuel was observed to undergo a two-stage ignition process and suppress the premixed burn of the main fuel. The early premixed burn spike of the pilot fuel could increase the bulk gas temperature at main injection by over 350 K. The two-stage ignition process of the port fuel could cause an increase in this quantity of up to 200 K.