Spray and Self-Ignition Visualization in a DI Diesel Engine

In a DI Diesel engine with optical access, spray and self-ignition were visualized. The engine was designed to provide a large optical access, no spray impingement and thermodynamic conditions close to those of commercial engines. The spray from a single-hole injector and the combustion zone can be fully observed. Laser induced exciplex fluorescence is used to provide information on the spray structure as well as penetration. Luminous emission from the burnt gases served as a marker of the self-ignition sites. The flow field was characterized with laser anemometry.

Data from these three techniques were collected for a set of operating conditions involving changes of air temperature, pressure in the cylinder, injected quantity and engine speed. The effect of varying these parameters is discussed. At the beginning of spray penetration, the liquid and vapor advance at the same speed. Quite soon, the liquid phase stops its progression while the gas phase continues to progress. Self-ignition occurs in a region of high mean shear quite close to the nozzle, often close to the liquid phase tip. This was explained by the presence of coherent strucures in the liquid spray creating favorable self-ignition sites. This mechanism was confirmed by simultaneous self-ignition and liquid Mie scattering visualizations.