| As the crisis of energy and environment becomes more and more severe,and the emission regulations becomes more stricter,the realization of efficient and clean combustion of internal combustion engine has become the priority.The cost of diesel engine after-treatment device is expensive,and there are still many shortcomings in reliability and adaptability.Therefore,reducing the original emissions through combustion optimization and fuel improvement technology is the direction of future research.The internal EGR strategy can capture the required amount of combustion gas by changing the valve phase.Compared with the external EGR system,the internal EGR system has the advantage of low cost and good durability.And it has a great potential in reducing the NOx emission of diesel engine.The Negative Valve Overlap(NVO)strategy and Secondary Exhaust Valve opening(SEVO)strategy are the most important ways to realize the internal EGR.The oxygen-carrying characteristic of oxygenated fuel can supply the oxygen element in combustion area,and greatly reduce the Soot emissions under different operating conditions.Therefore,this paper combines the internal EGR strategies with oxygenated fuel,and tries to find the best way to achieve the compromise between emissions and economy by coordinating the intake boundary condition.The Late Intake Valve Closing(LIVC)strategy can effectively control the ignition time and realize variable compression ratio,which shows great advantages in the technology route of efficient and clean combustion.This paper analyzes the potential of the LIVC strategy to improve the combustion and emission characteristics by combining the optimization of the intake pressure and the intake components.The "trade-off" relationship between NOx and Soot has always been a key problem in the emission control of diesel engine.In this paper,the oxygen concentration stratification technology is applied to create a reasonable exhaust gas distribution and oxygen distribution in the cylinder to achieve the effect of inhibiting the generation of NOx and Soot at the same time.The internal EGR can also regulate the oxygen distribution,and the closing time of the intake valve will also affect the oxygen concentration distribution in the cylinder during the compression process.Therefore,this paper combines the oxygen concentration stratification technology with the internal EGR strategy and LIVC strategy respectively,the aim is to explore the influence of the variable valve phase on the oxygen concentration distribution characteristics in cylinder,as well as the promotion and restriction to the effect of oxygen concentration stratification combustion.In this paper,one-dimensional and three-dimensional simulation model of diesel engine for test is established by GT power and CONVEREG respectively.GT power is used to calculate the initial temperature,pressure and gas composition in cylinder / exhaust port under different valve strategies for CONVERGE.The main research contents and conclusions are as follows:1.Under the internal EGR strategy,the exhaust gas not only acts as a diluent to inhibit the reaction rate,but also as a heating agent to promote combustion process.The change of ignition delay depends on the combined action of the two aspects.Under the condition of medium load and speed,a large proportion of internal EGR can significantly reduce NOx emissions,but lead to a significant deterioration of soot emissions and a decrease in the indicated efficiency.In order to improve the emission of Soot and the indicated efficiency,this paper adopts measures to improve the internal EGR tolerance,such as increasing the intake pressure and using oxygenated fuel.Finally,a compromise between emissions and economy is achieved.2.Under the LIVC strategy,as the closing time of intake valve delays,the effective compression ratio decreases and the ignition delay period prolongs,which lead to the decrease of soot emission,but the indicated efficiency decreases due to the excessive mixing of fuel and air as well as the increase of heat transfer loss.The increase of intake pressure is helpful to improve the indicated thermal efficiency,and the combination with homogeneous EGR can further inhibit NOx generation.3.By introducing exhaust gas into the inlet near the center of the cylinder in the helical inport,the "lean in the middle and rich all around" oxygen concentration stratification pattern is formed in the cylinder at the time of injection,which is in good correspondence with the pollutant generation position.After the combination of stratified EGR and the NVO strategy,when the angle of negative valve overlap is small,the distribution of oxygen concentration in cylinder still presents the ideal stratified state of " lean in the middle and rich all around ".When the angle of negative valve overlap is large,the position of piston is low when the intake valve opens,resulting in the reduction of air which enters the bottom area of piston,and finally the oxygen-rich zone is formed above the combustion chamber.Without the use of external EGR,the backflow movement of the SEVO strategy just causes the oxygen concentration stratification pattern of " lean in the middle and rich all around".When combined with stratified EGR,the oxygen-lean zone is more concentrated in the center of the combustion chamber.After the combination of stratified EGR and the LIVC strategy,the stratification form of oxygen concentration is almost the same under different closing time of intake valve,which is close to the ideal stratification pattern on the whole,but the axial symmetry of oxygen concentration distribution is insufficient.4.Under the effect of stratified distribution of oxygen concentration,the NOx emission is reduced by 1.5% and the soot emission is reduced by 3.1% compared with the homogeneous distribution of oxygen concentration at the original valve phase.After the combination of oxygen concentration stratification technology and different valve strategies,the most important factors that affect the effect of oxygen concentration stratification on the improvements of pollutant include: the relative change of ignition delay period and combustion temperature between oxygen concentration stratification distribution and homogeneous distribution combustion,and the change of activation stratification state in cylinder.In all cases,the ignition delay period of oxygen concentration stratified combustion is longer than that of homogeneous combustion,which helps to improve the quality of oil-gas mixture and increase the proportion of premixed combustion,which lead to the improvement of the Soot emission,except for when IVC=270°CA ATDC,the highest stratification degree of oxygen concentration in the cylinder cause the oxygen concentration in the central area of the combustion chamber is too lean,resulting in higher Soot emission by oxygen concentration stratified combustion.In other cases,the Soot emission of oxygen concentration stratified combustion of is not higher than that of homogeneous combustion.The difference of the maximum combustion temperature and the duration of high temperature between oxygen concentration stratified and homogeneous combustion is the decisive factor affecting the improvement effect of NOx,and this difference is caused by the change of oxygen concentration distribution form,premixed combustion amount and the combustion duration under different valve strategies. |
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