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With the introduction of more and more stringent exhaust emission limitations for diesel engines by the legislation, engine and vehicle manufactures looking for solutions to meet this legislation limits with minimal hardware changes. In the present study experiments were performed to find possible solutions to meet TIER-4a emission limits without using a Diesel Particulate Filter (DPF). Two similar medium duty diesel engines were chosen to demonstrate this only by changing the fuel injection hardware and calibration. The study was divided into an investigation of the benefit of higher injection pressures while the second part concentrated on the investigation of multiple injection effects. It was found that with the use of high injection pressures from 1800 bar upwards together with high exhaust gas recirculation (EGR) rates the targets can be fulfilled with a single injection. This test was performed at the rated speed 100% load point. The multiple injection experiments demonstrated that the use of an early large pilot injection together with an close post injection can reduce the NOx and smoke emissions efficiently. It was found that with this injection strategy the TIER-4a emission targets can be achieved with a railpressure of only 1300 bar and lower EGR rate of 15% at a mid speed 75% load point (mode-6). The set of experiments also included a investigation into another efficient possibility to achieve TIER-4a emission standards with the use of a DPF but without the use of an EGR system. It was found that with the use of an early pilot injection and a retarded main injection timing it is possible to reduce the NOx formation to achieve the 3 g/kWh NOx limit.