Drop size measurements were performed in the intake port of a motoring engine using a laser diffraction particle sizing technique. The experimental parameters which were varied include number of injection cycles, start of injection timing, engine speed and manifold pressure. Two injectors having different atomization and dispersion characteristics were used in the study, a production dual jet injector which produced Sauter Mean Diameters (SMDs) in the range of 250 to 400 μm and an air assist injector which had a line-of-sight SMD of 39 μm.In measurements with the dual jet injector, after initial injection, the quantity of fuel present in the intake port was observed to increase with each subsequent injection event, reaching a steady state value after 6 to 10 injection cycles. The SMD produced by the back-flow atomization was independent of the number of injection events and independent of engine speed over a range of 750 to 1500 RPM. In addition, the start of injection timing had no effect on measured SMD for closed valve injection timings, if all the fuel had reached the intake valves before the valves opened. As the injection timing was retarded so that injection took place as the intake valves were opening, the measured SMD increased due to a higher portion of airborne droplets, which originated directly from the injector, contributing to the back-flow spray. Also, the crank angle degree at which drop size measurements were performed had a substantial effect on measured SMD and this resulted in significant variation in SMD at each measurement point.For the dual jet injector, the measured back-flow SMDs were in the range of 20 to 80 μm, hence the back-flow process offers enhanced atomization for typical port fuel injectors which produce SMDs larger than 100 μm. For the air assist injector, the back-flow SMD was found to be insensitive to injection timing, being constant at 40 μm. Hence, the back-flow process does not improve atomization for injectors producing more finely atomized sprays.