An experimental and numerical study of the spray from an air-assisted fuel injector in a direct-injection spark ignition (DISI) engine is presented. The experiments are performed using ultraviolet laser diagnostics in a motoring optical engine that is a DISI variant of a production port-fuel-injected engine. Non-optical single-cylinder firing engine results of experiments and simulations are also presented. Recent related work by the present authors has shown that, under design conditions, the air-assisted injector exhibits strong coupling between the liquid and gas phases. It results in a filled-in conical spray of relatively fine atomization and low penetration, with liquid and vapour fuel concentrations close to the centrally located injector and spark plug, as required for lean stratified operation. This paper shows the strong effect of increasing the chamber temperature on spray evaporation and penetration. This observed temperature effect shows that spray evaporation should vary significantly during cold start. Exhaust gas recirculation (EGR) may also have a beneficial effect on DISI engine performance through enhanced spray evaporation, although this is difficult to verify experimentally. If true, this effect of EGR is a separate mechanism to the known beneficial effects of EGR in air-assisted DISI engines.

The transient behaviour of the fuel spray from an air assisted fuel injector has been investigated both numerically and experimentally in a Constant Volume Chamber (CVC) and an optical engine. This two phase injector is difficult to analyse numerically and experimentally because of the strong coupling between the gas and liquid phases. The gas driven atomization of liquid fuel involves liquid film formation, separation and break up and also liquid droplet coalescence, break up, splashing, bouncing, evaporation and collision. Furthermore, the liquid phase is the dominant phase in many regions within the injector. Experimental results are obtained by using Mie scattering, Laser Induced Fluorescence (LIF) and Laser Sheet Drop sizing (LSD) techniques. Computational results are obtained by using a mixed Lagrangian/Eulerian approach in a commercial Computational Fluid Dynamic (CFD) code. Injector rig results show a good atomization of the spray with low spray width and penetration and Sauter Mean Diameters SMD) of droplets of order 10 μm. Engine results show a spray with a relatively low penetration producing liquid fuel and vapour fuel concentrations close to the central location of the injector and spark plug for stratified, lean mode of operation. Imaging of this injector operating inside an engine confirms the overall design approach for stratified operation.

The transient behaviour of the fuel spray from an air-assisted fuel injector has been investigated experimentally in a constant volume chamber (CVC), whilst varying the chamber pressure. Ensemble averaged, planar images of the laser induced fluorescence (LIF) and Mie scattering from the spray have been obtained, as well as back-illuminated images. The effect of the ambient pressure on the spray penetration of the air-assisted fuel injector is determined. Unlike conventional single-phase injectors, large vortex structures are observed, indicating that the air and liquid motion are strongly coupled. These results also show that the spray penetration varies significantly with chamber pressure.

The transient behaviour of the fuel spray from an air-assisted fuel injector in a constant volume chamber has been investigated experimentally. The relative Sauter mean diameter (SMD) of the spray droplets was determined using planar laser induced fluorescence (PLIF) and planar Mie scattering. Planar images of the ensemble averaged relative SMD with various injection conditions were obtained by calculating the ratio between the two laser light intensities at a given point. The penetration length and the spray shape factor were also obtained. The ensemble averaged results suggest the existence of vortices that are shed from the injector tip, and which entrain the smaller droplets. Results also show that the characteristics of the injector vary weakly with several particular injection parameters, notably the fuel injection pressure and the delay between fuel and air injection.