Strategies for emissions control in heavy-duty diesel engines to achieve low-emissions combustion with a high efficiency

Abstract

In this paper, the strategies for emission controls to achieve clean and high-efficiency combustion were studied in detail in a common-rail heavy-duty diesel engine coupled with exhaust gas recirculation. The results show that, in comparison with a single-stage turbocharger, a two-stage turbocharger dramatically improves the exhaust gas recirculation recyclability and enhances the air-to-fuel mass flow ratio. Therefore, nitrogen oxide emissions can be reduced to a much lower level, and the nitrogen oxide–brake specific fuel consumption trade-off relationship as well as the nitrogen oxide–soot trade-off relationship can be greatly improved simultaneously at low-speed high-load conditions. However, the peak cylinder pressure increases evidently at high-load conditions for a two-stage turbocharger. The brake specific fuel consumption, the soot emissions and the peak cylinder pressure can be lowered simultaneously by appropriately reducing the compression ratio and optimizing the geometry of the combustion chamber (such as the re-entrant diameter and the bowl depth) as well as using an injector with a tapered nozzle hole. At low- speed high-load conditions, the soot emissions can be reduced obviously without serious penalties in the brake specific fuel consumption when the nitrogen oxide emissions were kept constant by delaying the main-injection timing and reducing the injection pressure. However, it is necessary to employ a higher injection pressure together with a post-injection to reduce the brake specific fuel consumption and the soot emissions at high speeds. The combustion characteristics are mainly dominated by the cetane number of the fuel. With the addition of lower-cetane-number fuel to pure diesel, the ignition delay is prolonged as the exhaust gas recirculation rate increases, thereby also increasing the premixed combustion and reducing the soot emissions sharply. Meanwhile, the crank angle at which there is 50% completion of the accumulated heat release has less influence on the brake thermal efficiency with the addition of lower-cetane-number fuel. 2,5-dimethylfuran, as a novel oxygenated biofuel with a lower cetane number, is superior to gasoline for reducing the soot emissions, which may be a promising and clean alternative in compression ignition engines coupled with exhaust gas recirculation.