| With the emission regulations increasingly stringent and the environmental pollution more serious,Clean and efficient combustion technology theory and technology has become the focus of domestic and foreign scholars.The premixed low temperature combustion mode,relying on its potential and advantages in emissions,is one of the best choices to achieve efficient and clean combustion for diesel engine.Compared with the traditional combustion mode,premixed low temperature combustion based on early injection strategy shows a different air flow characteristics and fuel distribution characteristics in cylinder.So it is a new challenge to the enhancement of air-fuel mixing.In this paper,from the point of view of improving the fuel distribution and enhancing air-fuel mixing,the effect of combustion system parameters on the mixing of fuel and air was conducted based on a light vehicle diesel engine focusing on the control of soot emission,and the co-optimization of combustion system parameters and combustion control parameters was carried out.Based on the premixed low-temperature combustion mode,the influence of spray cone angle and bowl location on the air flow and the air-fuel mixing process was studied,and the mechanism to enhance the air-fuel mixing process based on early injection strategy was compared.The result shows that the fuel falls into the clearance due to the advance of injection timing in the premixed low-temperature combustion mode.Reducing the spray cone angle and expanding combustion chamber bowl towards cylinder liner make the distribution of fuel in the cylinder more uniform,which promotes the formation of mixture and reduces the soot emissions.However,reducing spray cone angle may lead to the increasing of NOx emissions due to high temperature of concentrated mixture.While expanding combustion chamber bowl towards cylinder liner can make full use of spray kinetic energy,which helps to enhance the mixing process and avoid a significant increase in NOx emissions.In order to further improve the distribution of fuel in the cylinder and enhance the airflow in the circumferential and axial directions,the effects of different factors on the mixing and combustion process was studied by changing the nozzle hole number,swirl ratio and throat contract ratio.The results show that,when the nozzle hole number increase from 5 to 8,the fuel distribution in the ring cavity is more uniform for the entrainment area increasing of fuel beam,and soot emissions is decreased due to the uniform mixing of fuel and air.NOx emissions increase first and then decrease When swirl ratio increases from 1.8 to 2.6,the circumferential velocity in combustion chamber increases,which promote the diffusion of fuel to circumferential direction,reduce the local equivalent ratio,and improve the uniformity of the mixture.The soot emissions decrease,but NOx emissions increase.When throat contract ratio of combustion chamber is reduced from 0.9 to 0.8,it is observed that the mixing process is enhanced for improvement of squeeze flow and turbulence intensity.But the too small throat contract ratio increases the fuel quality into the clearance and makes the mixture focus on the slope of the combustion chamber boss because of strong squeeze flow,resulting in uneven distribution of mixture.The soot emissions decrease first and then increase,but NOx emissions increase first and then decrease.Finally,the co-optimization of combustion system parameters and combustion control parameters was carried out by orthogonal design method.And the influence degree of different parameters on soot emission is judged by range analysis.The results show that the influence degree of various factors on the soot emission is in the order of injection pressure,nozzle hole number,throat contract ratio,swirl ratio,EGR rate,injection timing and spray cone angle.The optimum scheme shows that soot emissions were reduced by 96% and NOx emissions were reduced by 39% compared to the original scheme. |
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