Self-Ignition Delay Prediction in PCCI Direct Injection Diesel Engines Using Multi-Zone Spray Combustion Model and Detailed Chemistry

A previously developed multi-zone direct-injection (DI) diesel combustion model has been refined for more accurate simulation of a full cycle of a turbocharged diesel engine. The combustion model takes into account the following features of the spray dynamics:

• Detailed evolution process of fuel sprays;
• Interaction of sprays with the in-cylinder swirl and the walls of the combustion chamber;
• Evolution of a Near-Wall Flow (NWF) formed as a result of a spray-wall impingement as a function of the impingement angle and the local swirl velocity;
• Interaction of Near-Wall Flows formed by adjacent sprays;
• Effect of gas and wall temperatures on the evaporation rate in the spray and NWF zones.

In the model each fuel spray is split into a number of specific zones with different evaporation conditions. Zones, formed on the cylinder liner surface and on the cylinder head, are also taken into account. The piston bowl in the modelling process is assumed to have an arbitrary axi-symmetric shape. The combustion model considers all known types of injectors including non-central and side injection systems. A NOx calculation submodel uses detailed chemistry analysis which considers 199 reactions of 33 species. A soot formation calculation sub-model used is the phenomenological one and takes into account the distribution of the Sauter Mean Diameter (SMD) in the injection process. The ignition delay sub-model implements two concepts. The first concept is based on calculations using the conventional empirical equations. In the second approach the ignition delay period is estimated using relevant data in the pre-calculated comprehensive 4-D map of ignition delays. This 4-D map is developed using CHEMKIN detailed chemistry simulations and takes into account effects of the temperature, the pressure, the Fuel/Air ratio and the Exhaust Gas Recirculation (EGR) on the ignition delay period. The above approach is also planned to be used in future for calculations of ignition delays in diesel engines fuelled by bio-fuels. The model has been validated using published experimental data obtained on high- and medium-speed engines. Comparison of results demonstrates a good agreement between theoretical and experimental sets of data.

The above sub-models were integrated into special software, which is a full-cycle engine simulation tool, allowing more advanced analysis of Premixed Charge Compression Ignition (PCCI) and Homogeneous Charge Compression Ignition (HCCI) diesels.