On the Effects of EGR on Spark-Ignited Gasoline Combustion at High Load

EGR dilution is a promising way to improve fuel economy of Spark-Ignited (SI) gasoline engines. In particular, at high load, it is very efficient in mitigating knock at low speed and to decrease exhaust temperature at high speed so that fuel enrichment can be avoided. The objective of this paper is to better understand the governing mechanisms implied in EGR-diluted SI combustion at high load. For this purpose, measurements were performed on a modern, single-cylinder GDI engine (high tumble value, multi-hole injector, central position). In addition 0-D and 1-D Chemkin simulations (reactors and flames) were used to complete the engine tests so as to gain a better understanding of the physical mechanisms. EGR benefits were confirmed and characterized at 19 bar IMEP: net ISFC could be reduced by 17% at 1200rpm and by 6% at 5000rpm. At low speed, knock mitigation was the main effect, improving the cycle efficiency by a better combustion phasing. At high speed, stoichiometric operation could be achieved, avoiding fuel-costly enrichment. It was also observed that the EGR tolerance decreases with the engine speed: maximum EGR rate was over 30% at low speed, vs. 22% at high speed. EGR composition was varied using synthetic EGR (N2, CO2 or a mixture of N-2 & CO2) in order to control the diluent heat capacity (Cp). These results, coupled with Chemkin simulations showed that the Cp value of the diluent is the main driving mechanism that explains the better auto-ignition resistance of the diluted air-gasoline mixture. The Cp of the diluent is also the 1st order parameter that controls the laminar flame-speed (S-L), chemical effects being of 2nd order and mainly attributed to CO2, which is implied in many elementary reactions of the fuel oxidation process. Then equilibrium constants can be modified when CO2 concentration varies. However the relative importance between thermodynamic (Cp) and chemical effects of the diluent greatly depends on the ambient temperature and pressure conditions. A split-of-losses analysis was also performed. It helped to show that a diluent with a high molar Cp has a greater potential to re-phase the combustion, mitigating knock. A side-effect is a significant increase of thermal losses that almost completely balance the combustion phasing gains that can be obtained with a high-Cp diluent. Engine tests and simulations also showed that a minor species like NO, which is present in the EGR gases, can significantly affect the auto-ignition delay of the fresh gases. A maximum reactivity of the fresh gases was observed for approximately 100ppm NO in the intake air/EGR mixture.