The performance of a four-valve engine operating with combustion in all cylinders has been determined in terms of indicated mean-effective pressure, drivability and concentrations of unburned hydrocarbon in the exhaust gases with a stoichiometric mixture of gasoline and air and four injectors including two with air assist. In addition, size and velocity characteristics of the fuel sprays were measured with a phase-Doppler velocimeter outside and inside the engine.With operation at a steady rotational speed of 1200 rpm, the indicated mean- effective cylinder pressure and its covariance were found to be nearly constant with the initiation of injection from 150 to 600 degrees of crank angle after top-dead-centre of intake. The reduction in mean effective pressure with injection with the valves open between 30 and 90 degrees, previously observed with a production double-jet injector, was also found with air assist, with a smaller reduction when the air-assisted spray corresponded to a single and narrow cone so that more fuel impinged on the walls in the region of the port divider. The averaged Sauter mean diameter of droplets from the air-assisted injectors was found to decrease with manifold depression and at low engine load it was of the order of 50 μm and in contrast to the values in excess of 100 μm with a production type.Unsteady operation of the engine involved adjustment of throttle and injection period over ten cycles so that the load was changed at constant speed. With the injection schedule of a production management system, the instantaneous equivalence ratios were up to 12 % below stoichiometric and the deficiency was smaller with injection with the valves open and mono-jet sprays. Experiments with control of the fuel injected during the transient showed that it was possible to reduce the duration and magnitude of the lean excursions as can be required in order to maintain the efficiency of the catalytic converter.