Distribution Of In-cylinder Soot Size And Number Density In A Diesel Engine

In recent years, the haze weather appeared frequently in Chinese multiple cities. Haze weather can lead to reducing visibility, and doing harm to human body and mind. And the most important substances that coursed haze weather are inhalable particles. The emission of inhalable particles is mainly derived from combustion source. One of the main channels of combustion source emissions is from motor vehicle exhaust, and the particles in the exhaust gas of the vehicles are mainly produced by diesel engines. It can be said that the particle emission of diesel engines is one of the main sources of haze weather.Particle emission of diesel engines has been one of the focuses in the research field of internal combustion engine. Many countries have set up regulations on the quality of diesel particles. In addition, due to the small size particles are far more damaging to the human body, and accounted for over 90% of the number of diesel particles. The requirement for limiting the number of particle emission was first put forward in Euro 5 emission regulation.In this paper, a geometric model of the combustion chamber of diesel engine is established, and the mesh generation is performed. The initial conditions(including the fuel injection, the turbulent kinetic energy and the turbulent length scale) and the boundary conditions were set up. The suitable mathematical models were selected including K-epsilon model, KH-RT model, flame coherent model and kinetic model. Thus, a 3D numerical model of the diesel engine was established.Through the acquisition of diesel engine cylinder pressure, soot emissions and fuel consumption rate and other parameters under the rated condition, the calculation model was calibrated and validated. On this basis, the influences of the fuel injection system parameters(nozzle hole diameter, nozzle hole angle and injection advance angle) and blends(B10, B20 and B50) on combustion process and soot emissions were investigated. Much emphasis was put on the distribution regulars of soot mass fraction, soot size and soot number density in cylinder of diesel engine.With the increase of the crank angle, soot mass fraction of the original engine increases rapidly, and then decreases. The overall trend of soot size and soot number density both firstly increase and then decrease. And the distribution range gets wide, then narrowd, and finally tends to be stable.While the diameter of the nozzle hole decreases, the amount of soot formation increase in the early stage of combustion and decrease in the later stage of combustion. From the initial stage of combustion to TDC, with the decrease of the diameter of the nozzle hole, the soot size and the soot number density both increase continuously. But in the middle and later stage of combustion, the soot size and the soot number density have no obvious change trend. The results show that the trade-off relationship of the soot number density and soot size is found in the later stage of combustion.With the decrease of the nozzle hole angle, the mean soot mass fraction increases gradually. There is no fixed trend of the soot size, but the soot number density increases while the nozzle hole angle decreases.With the increase of fuel injection advance angle, soot formation increases slightly in the early stage of combustion, and decreases in the later stage of combustion. The fuel injection advance angle has little effect on the final mean soot size and mean soot number density.Compared with B0, the final mean soot mass fraction of B20 decreases mostly in three of the oxygen blends, while the B50 decreases less than B10. The mean soot size and the mean soot number density of diesel have double peaks with the crank angle, and that of the oxygen blends have single peak. Overall, in the early and later stage of combustion, the mean soot size of the oxygen blends is smaller than that of diesel. The mean soot number density of the oxygen blends is higher than that of diesel in initial and later stage of combustion, but is lower in the medium term of combustion. And the trade-off relationship of the soot number density and soot size also exists.