Abstract
This paper is the second investigation on the effects of intake flow control methods on the part load performance in a spark ignition engine. In the previous work, two control methods, port throttling and masking, were compared with respect to lean misfire limit, fuel consumption and emissions. In this work, the effects of these two methods on EGR characteristics were studied and simultaneously the differences between EGR and lean combustion as a dilution method were investigated. The results show that EGR limit is expanded up to 23% and 3 ~ 5% improvement in the fuel consumption are achieved around 8 ~ 13% rates by the flow controls comparing with 10% limit and 1.5% reduction around 3% rate of non-control case. The masking method is more effective on the limit expansion than throttling as like as lean misfire limit; however there is no substantial difference in fuel consumptions improvement regardless the control methods except high load condition. Also it is observed that there exist critical EGR rates around which the combustion performance and NOx formation change remarkably and these rates generally coincide with optimum rates for the fuel consumption. In addition, dilution with fresh air is much more advantageous than that of the exhaust gas from the view point of dilution limit and fuel consumption, while utilization of the exhaust gas is more effective on NOx reduction in spite of considerably small dilution compared with the use of fresh air. Finally, the improvement of fuel consumption by massive EGR is highly dependent on the EGR limit at which the engine runs stably, therefore the stratified combustion technique might be a best solution for this purpose.
