On the Effects of Urea and Water Injection on Particles across the SCR Catalyst in a Heavy - Duty Euro VI Diesel Engine

Particle emissions from heavy-duty engines are regulated both by mass and number by Euro VI regulation. Understanding the evolution of particle size and number from the exhaust valve to the tail pipe is of vital importance to expand the possibilities of particle reduction. In this study, experiments were carried out on a heavy-duty Euro VI engine after-treatment system consisting of diesel oxidation catalyst, diesel particulate filter and selective catalytic reduction (SCR) unit with AdBlue injection followed by ammonia slip catalyst. The present work focusses on the SCR unit with regard to total particle number with and without nucleation particles both. Experiments were conducted by varying the AdBlue injection quantity, SCR inlet temperature [to vary the reaction temperature], exhaust mass flow rate [to vary the residence time in SCR], and fuel injection pressures [to vary inlet particle number and inlet NO_x]. Sampling for particle measurements was performed at the inlet, upstream of the urea injector and the outlet of the SCR. Particle measurements were made using two different two-stage dilution systems, for measuring non-volatile and volatile particles, respectively. The total particle number (PN) was measured using two different condensation particle counters (CPC), one with the cut-off size at 23nm and another with a cut-off size at 7 nm to capture nuclei mode particles. An increase in the total number of particles was observed at 400°C and 1200 bar of fuel injection pressure with the higher AdBlue injection quantities compared to the dry-run SCR. At 400°C, a surge in nucleation occurs irrespective of the fuel injection pressures. An experiment was also made to isolate the water droplets effect on PN from nitrate formation. The SCR was injected with de-ionized water and a PN reduction was observed across the SCR and the reduction was higher with higher inlet PN to the SCR.