Impact of Selective Catalytic Reduction Process on Nonvolatile Particle Emissions

Particulate matter (PM) and NO_X are two major pollutants generated by diesel engines. Modern diesel aftertreatment systems include selective catalytic reduction (SCR) technology that helps reduce tailpipe NO_X emissions when coupled with diesel exhaust fluid (DEF/urea) injection. However, this process also results in the formation of urea derived byproducts that can influence non-volatile particle number (PN) measurement conducted in accordance with the European Union (EU) Particle Measurement Program (PMP) protocol. In this program, an experimental investigation of the impact of DEF injection on tailpipe PN and its implications for PMP compliant measurements was conducted using a 2015 model year 6.7 L diesel engine equipped with a diesel oxidation catalyst, diesel particulate filter and SCR system. Open access to the engine controller was available to manually override select parameters. Ammonia-to-NO_X (ANR) ratio was varied to understand the impact of urea injection rate on measured PN emissions. In addition, experiments were conducted with and without exhaust gas recirculation. Limited experiments were also conducted to understand the impact of the PN instrument evaporation tube (ET) temperature setpoint on measured PN. Test cycles included the world harmonized transient cycle (WHTC) and world harmonized stationary cycle (WHSC). Tailpipe PN was simultaneously sampled with two PN instruments that were under ISO 17025 calibration in accordance with PMP requirements, but at two different evaporation tube (ET) temperatures of 300 °C and 350 °C. The instrument using the lower ET temperature reported PN emissions on the order of 50% higher or more, although both ET temperatures were in compliance with the standard. The increase was most pronounced when soot concentration was at its lowest level, suggesting that the presence and increase of soot suppressed the formation of urea derived particles. The work also showed urea derived species contributed to PN emissions, even if the ET temperature was set to 350 °C, when compared with no DEF injection. These results highlight the critical impact DEF-derived species could have on tailpipe PN, and how this may or may not be captured under the framework of the existing PMP regulations.