A novel approach for exhaust gas recirculation stratification in a spark-ignition engine

Over the years, various stratified exhaust gas recirculation (EGR) concepts have been proposed and studied to increase the total amount of EGR that could be trapped within the cylinder so as to decrease NO emissions while lessening the negative impact of high dilution. The different concepts normally involved modifying the intake manifold so as to sequentially feed the cylinder with air and EGR or to create an asymmetrical EGR supply in the intake ports. The present work proposes a novel proof of concept aimed at achieving a stratified EGR in a spark-ignition engine. For the first time, stratified EGR is done by directly injecting exhaust gas through the cylinder head using solenoid valves controlled by an ECU. By this way, the proposed approach allows EGR injection at any time during the intake and compression stroke. The proposed concept was experimentally implemented by modifying a single-cylinder spark-ignition engine. Such an approach allows evaluating EGR stratification with respect to combustion, fuel consumption and emissions, at higher speed and load than previously reported with other stratified concepts. Three different EGR injection timings were evaluated and compared to a homogeneous EGR strategy. It has been possible to simultaneously lower fuel consumption by 6 % at 2100 RPM and high EGR rate while producing 2.6 % less NO emission when compared to a homogeneous EGR. Moreover, by injecting EGR directly in the cylinder allows enhancing the combustion process, as measured by a shorter combustion duration, by 17 % at 2500 RPM with a 20 % EGR-rate. The proof of concept has shown that stratified EGR is possible at high load and that EGR injection timing needs to be retarded with increasing engine speed for better performance.