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The effect of SCR on nanoparticle emissions has been a subject for some recent diesel particle emission related studies. In this study, the effect of after-treatment (DOC and SCR) on particle emissions was studied with a heavy-duty off-road diesel engine (emission level stage 3b with an SCR). A special “transient cold test cycle” (TCTC) was designed to describe the SCR system operation at low exhaust gas temperatures. The particle instrumentation made it possible to measure on-line the particle number concentration, particle size distribution and chemical composition of particles. The largest particle number concentrations were measured after the exhaust manifold. The exhaust after-treatment was observed to reduce the total particle number concentration by 82.5% with the DOC and 95.7% with the DOC+SCR.

During the past few decades the exhaust emissions of diesel engines have significantly decreased due to efficient emissions regulation. Compared to the situation in the 1990s, the nitrogen oxide (NOx) and particulate matter (PM) emissions, the main challenges for diesel engines, are now reduced 80-95 % in many industrialized countries. To meet the demanding requirements, engine technologies have been updated and improved step by step. These improvements have also kept Specific Fuel Consumption (SFC) figures at a low level or they have even improved. The latter issue is of great significance for consumers (cost) and also for the environment (CO2). Nowadays many diesel engine fuel injection strategies rely on the use of exhaust after-treatment systems. Efficient and clean combustion is obtained by utilizing high injection pressure and advanced injection timing.

Particle emissions have been generally associated to diesel engines. However, spark-ignition direct injection (SI-DI) engines have been observed to produce notable amounts of particulate matter as well. The upcoming Euro 6 legislation for passenger cars (effective in 2014, stricter limit in 2017) will further limit the particulate emissions from SI engines by introducing a particle number emission (PN) limit, and it is not probable that the SI-DI engines are able to meet this limit without resorting to additional aftertreatment systems. In this study, the solid particle emissions of a SI-DI passenger car with and without an installed Particle Oxidation Catalyst (POC®) were studied over the New European Driving Cycle (NEDC) on a chassis dynamometer and over real transient acceleration situations on road. It was observed that a considerable portion of particle number emissions occurred during the transient acceleration phases of the cycle.