LP EGR loop is one of the method to further reduce NOx while maintaining PM Emissions at low level. When variable geometry turbine (VGT) get closed there are chances of higher boost pressure allowing a high rate of supplemental EGR. The study discusses about the influence of boost pressure and injection pressure on combustion which gives optimal pressures for each operating point in terms of NOx and PM Emissions. It also includes the study of increased boost pressure by closing VGT veins in LPL EGR. NOx PM trade off gets worst when compared to constant boost pressure for EGR at constant A/F ratio. Low pressure EGR is used by varying the EGR rate the gas flow through the turbine is unchanged. Higher boost pressure can be achieved when VGT veins are closed which increases the dilution ratio while maintaining a suitable in-cylinder oxygen quantity for PM emissions. This is the main reason why LP EGR is mostly studied by manufactures for future automotive diesel engines. The study mainly focuses on NOx and PM Emission reduction by LP EGR loop.
The study is conducted on 2.0-liter water cooled HSDI diesel engine. The test setup and engine specification is shown in Figure 1 .
OPERATING POINTS AND RESULTS
The engine is equipped with the combination of HP EGR loop and LP EGR loop. The two operating conditions studied in this experiment are load and speed. The EGR results increases with the decrease in O2 concentration in inlet and decrease of boost pressure O2 of in-cylinder decreases. During the middle load operation, the load for above conditions, critical value of air rises. This increases the PM, CO, HC Emissions and specific fuel consumptions. This is the reason for these operating conditions NOx emission reduction is difficult.
Effect of Boost Pressure on NOx Emissions
Boost pressure can be increased by LP EGR loop by varying LP EGR rate. In this study for A and B operating points four level of boost pressure are studied which gave the optimal pressure in terms of NOx and PM Emissions. The boost pressure is increased by closing VGT veins.
A. Operating Point, A
Effect of boost pressure on varying EGR on NOx emissions for operating point A is shown in Figure 6. For a given boost pressure the increase of EGR rate results in the decrease of EGR rate. There are two condition for which the NOx emissions are affected because of boost pressure. The first one is, the boost pressure increases results in higher in-cylinder oxygen quality that increases NO formation rate. Second is increase of in-cylinder trapped which results in decrease of local temperatures at particular flame temperature that control NOx formation.
B. Operating Point, B
Figure 7 shows the effect of boost pressure with varying dilution ratio on NOx emissions. Contrary to operating point A boost pressure increases at a given dilution ratio increases NOx emissions which shows there is no unique trend of NOx emission with varying boost pressure.
Effect of Injection Pressure on NOX Emission.
As the injection pressure increases there is much negative impact on NOx emissions for low dilution ratio and little impact at higher dilution ratio which is shown in Figure 12. Often due to the increase in speed there is a rush of fuel entering the combustion chamber leading to increase in NOx emission. Also due to the rush of fuel entering the combustion chamber at higher rate the local combustion temperature increases leading to higher NOx emissions.
DEVELOPMENT OF LOW PRESSURE LOOP EGR SYSTEM
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