In the last 30 years, diesel engines have made rapid progress to increased efficiency, environmental protection and comfort for both light- and heavy-duty applications. The technical developments include all issues from fuel to combustion process to exhaust gas aftertreatment. This paper provides a comprehensive summary of the available literature regarding technical developments and their impact on the reduction of pollutant emission. This includes emission legislation, fuel quality, diesel engine- and exhaust gas aftertreatment technologies, as well as particulate composition, with a focus on the mass-related particulate emission of on-road vehicle applications. Diesel engine technologies representative of real-world on-road applications will be highlighted.Internal engine modifications now make it possible to minimize particulate and nitrogen oxide emissions with nearly no reduction in power. Among these modifications are cooled exhaust gas recirculation, optimized injections systems, adapted charging systems and optimized combustion processes with high turbulence. With introduction and optimization of exhaust gas aftertreatment systems, such as the diesel oxidation catalyst and the diesel particulate trap, as well as NOx-reduction systems, pollutant emissions have been significantly decreased. Today, sulfur poisoning of diesel oxidation catalysts is no longer considered a problem due to the low-sulfur fuel used in Europe. In the future, there will be an increased use of bio-fuels, which generally have a positive impact on the particulate emissions and do not increase the particle number emissions.Since the introduction of the EU emissions legislation, all emission limits have been reduced by over 90%. Further steps can be expected in the future. Retrospectively, the particulate emissions of modern diesel engines with respect to quality and quantity cannot be compared with those of older engines. Internal engine modifications lead to a clear reduction of the particulate emissions without a negative impact on the particulate-size distribution towards smaller particles. The residual particles can be trapped in a diesel particulate trap independent of their size or the engine operating mode. The usage of a wall-flow diesel particulate filter leads to an extreme reduction of the emitted particulate mass and number, approaching 100%. A reduced particulate mass emission is always connected to a reduced particle number emission.

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In the present work, an exhaust aftertreatment system combining lean NO X trap (LNT) plus exhaust bypass, passive selective catalytic reduction catalyst (SCR) and engine-independent LNT reductant supply by onboard exhaust fuel reforming was developed. Further, it was experimentally investigated in steady-state operation on an engine test bench and on road in a demonstrator vehicle. The intrinsic NH 3 formation during the LNT regeneration was intensively studied as one key function for passive SCR without active urea dosing. LNT regeneration duration and temperature are the most significant parameters for NH 3 emission. In steadystate operation, the passive SCR could be forced to a contribution to the total NO X conversion of up to 35 %. At 170°C, 50 % steady-state total NO X conversion was achieved, and at 250°C and a fuel consumption penalty of <2 %, complete NO X conversion could be demonstrated. Compared to conventional LNT operation with frequent engine enrichment, the engine-independent LNT regeneration with reformate gas is energetically more efficient and advantageous regarding CO 2 emissions. Additionally, by shifting NO X reduction from LNT to SCR, fuel consumption penalty due to LNT enrichment can be reduced. Finally, the combination system was demonstrated in transient test cycles and by real driving operation on road.

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