Previous research has shown that elevating fuel injection pressure results in better air-fuel mixture formation, allowing for a further increase in maximum exhaust gas recirculation (EGR) rate while consequently reducing NOx emissions. The aim of this paper is to find out whether there is an optimum injection pressure for lowest soot-NOx emissions at a given boost pressure in high-speed diesel engines.
Experiments are carried out on a single-cylinder research engine with a prototype common-rail system, capable of more than 200 MPa injection pressure. The effect of injection pressure on soot-NOx formation is investigated for a variety of boost conditions, representing the conditions of single to multi-stage turbocharger systems. Analysis of the data is performed at the application relevant soot to NOx ratio of approximately 1:10.
It is observed that above a critical injection pressure, soot-NOx emissions are not reduced any further. The value of this critical injection pressure increases with load or boost pressure. When increasing the injection pressure above the critical value at a low boost pressure, soot-NOx emissions may rise. The relative air-fuel ratio and EGR rate at the soot to NOx ratio of 1:10 also show effects of saturation when increasing the fuel injection pressure beyond the critical point.
