Direct injection of fuel into the engine cylinders has been regarded as a way of the reduction in fuel consumption and pollutant emissions. The spray generated from the injector is of paramount importance in Direct Injection Spark Ignition(DISI) engines in that the primary atomization process must meet the requirement of quick and complete evaporation with proper mixing followed by combustion especially to prohibit the excessive unburned hydrocarbon(UHC) emissions.
A high pressure swirl-spray injector and a high pressure fan-spray injector were investigated to be applied in a DISI engine. Swirl-spray injector makes a sac spray at first, velocity of which is very fast compared to main swirl spray. The sac spray has large momentum owing to the large droplet size. The main spray should formulate a vortex motion which mixes fuel and air. Fan spray injector provides fuel spray with a shape of a sheet.
The interaction between air flow and fuel spray was investigated in a steady flow system embodied in a wind tunnel to simulate wide range of flow conditions inside the cylinder of the DISI engine. The direct Mie scattered images presented the macroscopic view of the liquid sprays and vapor fields. The velocity and particle size of fuel droplets were investigated by phase Doppler anemometer(PDA) system. The processes of atomization and evaporation with a DISI injector were observed and consequently utilized to construct the data-base for the spray and fuel-air mixing mechanism as a function of the flow characteristics. Fuel vaporization and coalescence were found to be proportional to crossflow velocity. Air entrainment in the vortices was found still active in strong flow field.
The supplying pressure to injectors varies the fuel spray field. In case of the spray from high pressure swirl injector, the increased supplying pressure makes the sac spray and main spray velocity high. Sauter mean diameter(SMD) value was increased and average diameter of the fuel particles was decreased with increasing fuel supplying pressure. Fan spray injector made the axial and radial penetration longer when the injection pressure increased. The average diameter and SMD were simultaneously decreased with increasing injection pressure.
A purpose-built single-cylinder optically-accessible DISI engine was used to investigate the influence of injection pressure and averaged flow velocity inside the cylinder. Fan spray injector was used to ensure the connection with spray results.
The higher supplying pressure to injector made no difference with the engine performance, represented by indicate mean effective pressure(IMEP). But it reduced coefficient of variation of ${IMEP}(COV_{IMEP})$, the emission level of CO and UHC lower. The fuel droplets were atomized more easily at higher injection pressure so they could be mixed with ambient air effectively. The fuel distribution was investigated by laser induced fluorescence(LIF) technique. It showed the compact and effective location of fuel cloud to the point of spark plug gap at higher injection pressure.
Averaged in-cylinder tumble velocity was estimated by particle image velocimetry(PIV) method. The mean flow velocity was proportional to engine operating speed. Air excess ratio and fuel amount were fixed to ensure consistency of the experiment. The IMEP value was highest and $COV_{IMEP}$ value was lowest at 1200RPM with operating condition 800RPM~1600RPM. Corresponding NOx emission level was highest at 1200RPM. CO and UHC emission levels were lowered at higher engine speed. The air-fuel distribution pictures showed the most compact and accurate feature at 1200RPM near the spark plug at the ignition timing. The faster speed of engine leaded the insufficient mixing duration. The lower speed of engine made the air-fuel mixture too spreaded for the sufficient compact air-fuel field to be produced.
