The Effect of EGR on the Performance and Pollutant Emissions of Heavy Duty Diesel Engines Using Constant and Variable AFR

Pollutant emissions and specifically NO and soot are one of the most important problems that engineers have to face when developing heavy duty DI diesel engines. Two main strategies exist as options for their control, reduction inside the engine cylinder using advanced combustion and fuel injection technologies and use of after-treatment systems. In the present work it is examined the use of EGR to control the formation of NO inside the cylinder of an engine with extremely high peak pressure. The work is applied on a single cylinder truck test engine developed under a project funded by the European Community focusing on the improvement of heavy duty DI diesel engine efficiency using increased injection timing. Use is made of a simulation model to predict the effect of more advanced injection timing on engine performance and emissions. The model has been modified to include the effect of EGR used to c ontrol the formation of NO which is considerably increased at high injection timings. Experimental results from the engine without EGR at various engine operating conditions and injection timings have been used to determine the baseline for evaluating the effect of EGR on engine performance and emissions and to validate the model used. Various engine conditions are examined covering the operating range of the engine. Using the simulation model results are produced concerning engine performance, i.e. cylinder pressure, heat release, BSFC and tailpipe values for NO and soot. For each engine operating condition various percentages of EGR are examined and their impact on engine BSFC and pollutant emissions is determined. As observed the use of EGR results to a slight increase of BSFC, a reduction of NO and an increase of soot emissions. Due to the last it was decided to examine using the simulation code the possibility for increasing boost pressure to maintain the overall AFR the same as in the case without EGR. The findings of the investigation are interesting since it is revealed that using an increased boost pressure soot is not increased seriously with EGR, BSFC is maintained fairly the same while the reduction of NO is slightly lower but in any case very significant compared to operation without EGR. Results are also provided concerning the distribution of thermodynamic parameters and pollutants inside the fuel jet revealing the actual effect of EGR on their formation. Especially important is the information obtained when comparing values under EGR operation at constant AFR with the ones obtained with a constant boost pressure since a better explanation for the observed results is provided helping us understand the actual effect of EGR on the pollutant formation mechanism.