Effects of exhaust gas recycle in a downsized gasoline engine

Lately, the exhaust gas recycle usage, widely diffused in Diesel engines, has been adopted in SI engines as well. It is a cheap technique which allows a sound control of NO formation meanwhile it can improve engine thermodynamics.In this paper, the influence of EGR on the operation of a turbocharged spark ignition engine has been evaluated by using both experimental and numerical techniques. In particular, since knock occurrence is a crucial point in the optimization of a turbocharged SI engine, the improvement in knock resistance, at high load operation, has been assessed. First, a method for knock detection and quantification has been illustrated. Then, the influence of EGR on engine performance, octane requirement and exhaust gas temperature was measured at two different rotational speed values and WOT operation.Since EGR has produced a drop in engine performance (between 10% and 13%) and an increase in knock resistance, a new set of main control variables has been determined in order to restore the original torque level while achieving a significant decrease in specific fuel consumption (between 6% and 11%).At the end, numerical analyses of engine combustion, aimed to explain the results of experimental investigations, have been carried out and a summary is reported in the paper.

► EGR reduces both knock intensity and exhaust gas temperature with respect to pure air–gasoline fueling.
► Both air–fuel ratio and spark advance can be improved by adopting new boost pressure levels.
► Optimizing the main control parameters the power output can be recovered.
► Experimental analyses show that the new control strategy allows increasing fuel economy at the same power output.