Cooled exhaust-gas recirculation for fuel economy and emissions improvement in gasoline engines

摘要

Modern gasoline engines face fuel-efficiency challenges due to inherent limitations including knock, pumping losses, and fuel enrichment. The addition of exhaust-gas recirculation (EGR) has been shown to improve the fuel consumption of gasoline engines, either port fuel injected or direct injected, by reducing pumping losses and knock and eliminating the enrichment region. In addition, the use of EGR has been shown to substantially reduce emissions of nitrogen oxides (NO_x) and CO. A 2.4-litre multi-point injection engine and a 1.6-litre gasoline direct injection engine were run with high levels of both cooled and uncooled EGR. Unlike numerous previous publications, these engines included a modified ignition system that allows extension of the cooled EGR limit of the engine to greater than 25 per cent and improves combustion at lower EGR levels. The results showed that an improvement of between 5 and 30 per cent in fuel consumption is possible, with the largest improvement occurring in the typical enrichment region. In addition, the results showed that EGR can reduce knock, resulting in an improvement in combustion phasing. Finally, the high levels of EGR reduced the emissions of CO by 30 per cent and of NO_x by up to 80 per cent. A detailed effort has been made to quantify the sources of improvement throughout the engine cycle and to demonstrate an EGR strategy (cooled EGR at high loads, internal EGR at low loads) that will maximize fuel consumption improvements. The results presented here indicate that the use of EGR in gasoline engines has the potential to reduce fuel consumption and emissions in a very cost-effective manner.