The droplet velocity, size and distributions of iso-octane fuel from single hole and twin jet air-assist injectors have been measured by phase Doppler velocimetry in the pent-roof for two cylinder head designs of firing four-valve engines running at 1500 rpm, together with the airflow during induction and compression. The use of the twin jet air-assist injector together with the introduction of a transfer-passage between the two intake ports of a Honda VTEC-E valve train arrangement resulted in reduction in ISNOx and COV-1mep of the order of half of those with the single hole injector design without a transfer passage. Droplets, for both heads and injectors, having passed the inlet valves, impinged directly onto the sleeve opposite to their entry without striking the exhaust valves and had velocities up to 30 m/s and Sauter mean diameters which varied from 20 to 50pm. During the intake stroke, some air passed through the transfer-passage from the secondary intake valve VTEC-E port to the primary valve port and probably convected some evaporated fuel vapor thereby redistributing the proportion of fuel vapor entering the combustion chamber from each port relative to the design without the transfer passage. The use of the twin jet air-assist injector resulted in the impingement of droplets on the sleeve 15° CA earlier than for the single hole injector and to displacement of the location of maximum droplet flux by about 21/2 cm along the cylinder periphery, away from one exhaust valve and towards the primary inlet valve. In this region and over the duration of vapor release, approximately between 450° and 500° CA, the vapor cloud was convected by the air motion towards the cylinder head, in contrast to the use of the single hole injector which was generated at a location which resulted in convection of the corresponding cloud towards the piston crown. The pathlines during induction and compression of the rich vapor clouds are therefore different, providing evidence of a different mechanism for mixture stratification for the engine with the twin jet air-assist injector accounting for the improvement in ISNOx and COV-imep observed in the dynamometer tests.