Characteristics of evolution of in-cylinder soot particle size and number density in a diesel engine

Particulate matter (PM) emission from diesel engines has been a critical issue due to its environmental impact. The soot size and number density inside the cylinder can severely affect the size and mass of PM emission emitted from the diesel exhaust pipe. In the present study, the temporal and spatial variations of the mean and instantaneous in-cylinder soot size and number density in a single cylinder direct injection diesel engine have been simulated numerically at different operating conditions, injection system parameters, fuel types and piston bowl geometry using AVL-FIRE code. The results show that the engine load has great effects on the mean soot particle size and number density with crank angle while the engine speed and the fuel injection system parameters have no obvious effects on them. It is observed that there is an obvious shift to larger particles at higher load. The mean soot number densities firstly increase with the increase of loads, but decrease at later crank angle. The in-cylinder soot size distribution shows a unimodal shape at different crank angle under all operating conditions. The soot particle numbers are concentrated from the start of combustion to 40 °CA after top dead center (ATDC). The spatial soot particle size distribution in the combustion chamber is closely related to engine load, impinging angle and airflow motion. The piston bowl diameter has large effect on the mean soot size and number density during the latter phase of the expansion stroke. The results also indicate that both in-cylinder mean soot size and number from biodiesel-diesel blends are affected not only by the oxygen content, but also engine operating conditions.