Gasoline Injection and Spray Combustion in a Cell with Conditions Typical of Direct Injection Engines

Abstract

Penetration and combustion of fuel sprays is studied in conditions similar to gasoline direct injection engines. A closed pressurized and heated injection cell is used. It is equipped with quartz windows providing large optical accesses. A homogeneous flammable mixture is introduced in the cell and ignited to raise the internal pressure and temperature. Liquid fuel is injected at the time when the desired thermodynamic conditions are reached. Conditions representative of late injection in a direct-injection engine are selected. Gasoline spray ignition and combustion is provided by a spark plug with long electrodes, locating the electrode gap right in the middle of the spray. The combustion does not reach the wall, which makes this experiment interesting for the validation of combustion in CFD codes. Two pressure swirl injectors with spray angles of 60 and 90 degree are used. The fuel is iso-octane with 5% 3-pentanone as tracer. The spray without combustion is characterized by laser diagnostics: planar Mie scattering for the liquid phase and planar Laser-induced fluorescence for the vapor phase. Natural light emitted by the spray combustion is collected, showing the burnt gas volume. Pressure cell is recorded to calculate heat release. It is found that the spray shape and penetration without (or before) combustion is very sensitive to the ambient density. Spray contraction caused by ambient gas entrainment is very strong and causes a complete transformation of the initial hollow cone spray. A narrow plain spray with high penetration is observed with the 60 degree injector. The spray shape with the 90 degree injector is close to a ring vortex, with low penetration. These features are similar to those observed in previous experiments with lower temperatures. During the spray combustion experiments, an optimum ignition delay was observed. Spray combustion exibits a rapid combustion with a high heat release rate for 3 to 4 ms, followed by a slow pressure rise during typically 20 ms. This latest phase was shown to be generated by mixing of the hot burnt gases pocket with the fresh surrounding gases. The maximum mean burnt fractions are: 60 % with the 60 degree injector and 75 % with the 90 degree injector. This shows that combustion is far from being complete in the selected conditions.