Results are presented for basic combustion optimisation work with an EUI system on a single-cylinder pressure-charged diesel engine with a near quiescent combustion system. A substantial improvement in the particulate/NOx trade-off was achieved by reduction of spray hole diameter, increase of injection pressure and a longer injection period. Analysis of the injection and heat release diagrams shows how the reduction of soot particulate was achieved by an increase of injection pressure with reduction of spray hole diameter. The tendency for NOx to increase was minimised by the longer injection period. The lowest air swirl gave the best results and further reductions in emissions were achieved by improvements to the nozzle and the EUI system.Steady-state simulation-cycle emission results show that with an injection pressure of 1800 bar, a low soot particulate of 0.024 g/bhp h is achieved and the total calculated particulate and NOx is within the US 94 limits for heavy-duty trucks.