Study On Formation Laws And Size Distribution Of Soot In The Diesel Engines

Soot emission is the main components of diesel particle emission, often account for more than 50%-80% in the total particles. Soot particle is formed because fuel combust under hypoxic condition. Soot particle's damage to human is demonstrated in two aspects: firstly, soot particles suspended in the atmosphere for long time because their diameter is very small; second, soluble organic matter, HC and sulfate et al which are adsorbed on the surface of particles are harmful to human, especially HC included a lot of PAHs that have carcinogenesis. PAHs often are found on the surface of aerosol in the air. So it's very necessary to look into the formation process of soot particle in detail.In this paper, the soot model coupled with complex chemistry mechanism in diesel directly spray combustion, and a multi-phenomenological soot model is used to simulate the formation process of soot particle, that include five global stages:particle nucleation, surface growth, surface oxidation, particle coagulation and PAHs direct deposition. To make clear the relationship between PAHs and soot surface growth, surface growth model that is caused by PAHs directly deposited on the soot surface is added. Multi-steps soot model is completed and developed. In this model, n-tetradecane which is close to diesel components was considered as calculated fuel, and multi-steps soot model couple with n-tetradecane complex chemistry mechanism and other calculation models (shell auto-ignition model, the modified RNG k-εmodel, RT-KH fuel atomization model). Simulation results show soot particles size distribution and formation, development process.The exhaust flow dilution channel was built on the single cylinder diesel engine experimental bench, and LPI was used to measure particle size and quality, and exhaust gas analyzer was used to measure the mass of gaseous pollutants (CO2, NO, CO, HC) in the exhaust. At the same time, the real-time detection of change tendency of pressure and heat release rate with crank angle in the cylinder combustion. We research in detail that engine work conditions effect on soot particles size distribute and formation process. Through experiment results validate the simulation models used in the calculation.The simulation results show that formation laws of soot have a direct relationship with soot precursor (naphthalene, diacetylene). Direct deposition of PAHs (styrene, phenylacetylene) on the surface of soot particles can increase soot quality. With the developing of combustion, the formation of a large number of small diameter soot particles is because of soot precursors (naphthalene, diacetylene) constantly cracking into particles nucleus, the formation of a small number of large diameter particles is result of carbon of condensation, surface growth and PAH direct deposition. The experimental results have confirmed the accuracy of the model.Experimental study results show that:ignition delay period is shorter, the burst pressure increase and occurrence time delay, and quality of pollutants (CO2, CO, NO, HC) and soot emission increase, soot mass obviously move to larger particle size range (soot size distribute transform from carbon nuclear mode to accumulation mode) with torque increase on the same speed condition; according to comparison between soot mass of the different torque on the same speed conditions and that of the different speed on the same torque conditions, it's found that torque has great influence on soot mass.