Spray/wall interaction in direct-injection spark ignition engines equipped with multi-hole injectors

The spray injected from a six-hole injector into a constant-volume chamber has been characterised, before and after fuel impingement on a hot surface simulating the piston in direct-injection gasoline engines, by means of spray visualisation and phase Doppler anemometry (PDA) measurements. Spray i mages have confirmed that penetration decreases with increasing chamber pressure or decreasing injection pressure, that higher droplet velocities are present at the spray centre and that a multi layer structure is formed along the spray axis. Stronger interaction between the six sprays is observed as the chamber pressure increases. The PDA measurements under both atmospheric and high pressure revealed that the mean droplet velocity prior to fuel impingement on the surface reaches its peak at the spray axis unlike the droplet mean diameter which exhibits an off-centre peak. The results also showed that the temporal variation of the mean droplet velocity at the spray centreline is similar to the predicted injection velocity variation at the nozzle exit while, further downstream and away from the centreline, three stages have been identified in the spray structure: leading edge, steady-state and tail. After the droplets impinge on the hot plate they move downstream forming a jet parallel to the plate surface with increasing or decreasing mean diameter depending on the surface inclination angle. The effect of injection pressure and ambient pressure/temperature on the droplet characteristics has been demonstrated and quantified.