Experimental Study of an LP EGR System on an Automotive Diesel Engine, Compared to HP EGR with Respect to PM and NO_x Emissions and Specific Fuel Consumption

摘要

Previous experimental studies on diesel engines have demonstrated the potential of high-pressure exhaust gas recirculation (HP EGR) as an in-cylinder NO_x control method. With ever more stringent emissions standards, the use of a low pressure EGR loop (LP EGR) seems to be an interesting method to further reduce NO_x emissions while maintaining PM emissions at a low level. Actually, contrary to HP EGR, the gas flow through the turbine is unchanged while varying the EGR rate. Thus, by closing the variable geometry turbine (VGT) vanes, higher boost pressure can be reached, allowing the use of high rates of supplemental EGR. Some experiments are conducted on a 2.0 l HSDI common-rail DI diesel engine equipped with HP and LP EGR loops on a test bench under low and part load conditions, as those encountered in the European emissions test cycle for light-duty vehicles. The dilution ratio, boost pressure, as well as injection pressure are modified using the LP EGR loop to study the influence of boost pressure and injection pressure on combustion (instantaneous rate of heat release) and the NO_x-PM trade-off (while varying EGR rate at a given boost pressure level), thus giving optimal boost pressure and injection pressure for each operating point in terms of NO_x and PM emissions. In particular, higher boost pressures and EGR rates are studied, as compared with the original HP EGR loop. Finally, the impact on LP and HP IMEP, as well as ISFC, when increasing boost pressure by closing VGT vanes with the LP EGR loop is studied.