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Formation of nitrogen oxides (NOx) in direct-injection premixed charge compression ignition (DI-PCCI) combustion simulated in a constant volume vessel was investigated using an ignition-combustion model that combines a stochastic mixing model with a reduced chemical reaction scheme. Several improvements were made to the model in order to predict the combustion processes in DI-PCCI. Calculations were carried out for the injection and ambient conditions equivalent to the measurements using the constant volume vessel. Analysis of the calculated results clarified the effects of mixture heterogeneity on NO concentrations and the mechanisms are discussed. The results show that the model successfully represents the experimental tendency for NO concentration when the injection conditions and ambient oxygen mole fraction are varied.

Multi-stage injection with pilot injection and post injection has been widely used for the noise and emissions reduction of diesel engines. Considering many parameters to be decided for optimal combustion, computer simulations such as three dimensional computational fluid dynamics (3D-CFD) and lower dimensional codes should play a role for optimal selection of intervals and quantity ratios. However, the data for the sprays are insufficient for reproducing the actual fuel-air mixture formation process related to pilot and post injection. Hence, there is a need for experimental data with a small-quantity injection. The small-quantity injection is characterized with an injection rate shape similar to a triangle rather than a rectangle. This study is mainly focused on the spray characteristics of diesel sprays in which the entire process is dominated by unsteady injection processes.

To clarify the relationship between the local heat release and the velocity distribution inside the diesel spray flame, simultaneous optical diagnostics of OH* chemiluminescence and particle image velocimetry (PIV) have been applied to the diesel spray flame under the elevated in-cylinder pressure and temperature conditions formed in a rapid compression expansion machine (RCEM). The cranking speed of the RCEM was 900 rpm, and the in-cylinder pressure and temperature were 8 MPa and 800 K at the start of injection, respectively. The amount of fuel was 10.2 mg. The injection pressure was 120, 90, and 60 MPa. To minimize the disturbance of luminous flame on optical diagnostics, a solvent, with comparable combustion characteristics to diesel fuel was used as fuel. The oxygen concentration was set to 15%. Results clearly show that PIV can successfully analyze the velocity distribution in diesel spray flames.

Hydrogen–diesel dual-fuel combustion processes were visualized using an optically accessible rapid compression and expansion machine (RCEM). A hydrogen-air mixture was introduced into the combustion chamber, and a pilot injection of diesel fuel was used as the ignition source. A small amount of diesel fuel was injected as the pilot fuel at injection pressures of 40, 80, and 120 MPa using a common rail injection system. The injection amounts of diesel fuel were varied as 3, 6, and 13 mm3. The amount of hydrogen was manipulated by varying the total excess air ratio (λtotal) at 3 and 4. The RCEM was operated at a constant speed of 900 rpm, and the in-cylinder pressure and temperature at the top dead center (TDC) were set as 5 MPa and 700 K, respectively. The combustion processes were visualized via direct photography and hydroxyl (OH*) chemiluminescence photography using a high-speed camera and an image intensifier.