Reduction of Steady State NOx Levels from an Automotive Diesel Engine Using Optimised VGT/EGR Schedules

Currently, 80% of European diesel passenger cars are turbocharged and as emission standards become more stringent exhaust gas recirculation (EGR) will be the primary means of suppressing oxides of nitrogen (NOx). The lighter the load the greater will be the combustion tolerance to increased EGR flow rates and hence increased NOx suppression. Automotive diesel engines using wastegated turbochargers cannot recirculate above 50% EGR without some sort of “added” device or system, which is able to displace the inlet fresh air charge. This has been demonstrated by throttling the diesel intake to reduce the fresh air inlet manifold pressure so allowing more EGR flow by virtue of a higher exhaust-side pressure due the effects of the turbocharger. The method reported here investigates a different approach to increasing the EGR rates by replacing a fixed geometry turbocharger (FGT) with a variable geometry turbocharger, (VGT). Under part-load operation, where emission production is significant in the European drive cycle, independent control of both VGT vane position and EGR valve position was used to optimise emission levels. A reduction in the FGT levels of NOx of up to 45 percent was observed at discrete operating points without compromising fuel consumption or smoke.