Characterisation of exhaust gaseous and particle emissions from a DI diesel engine with retrofitting of a diesel oxidation catalyst

Abstract

The combined effects of diesel oxidation catalyst (DOC) and ultra low sulphur diesel (ULSD) fuel on the characteristics of diesel exhaust gaseous and particle emissions under a diesel engine dynamometer test bed using a steady-state mode cycle of the standard Economic Commission for Europe Regulation 49 test modes for different engine loads from 10% to 100% of full engine load at a maximum torque of constant speed and idle to fast engine acceleration conditions have been investigated by using the developed gaseous and particle emission measurement systems. In the absence of a standard diesel exhaust particle sampling measurement system, a mini-dilution tunnel sampling (MDTS) and an ejector diluter sampling (EDS) measurement systems have been evaluated to ascertain the most reliable measurement system that can minimise the particle transformations (i.e. nucleation, condensation and coagulation) which affect the exhaust particle number and size distributions during the dilution process. Comparing the particle number and volume concentrations, the MDTS measurement system measures a lower level in the nuclei mode but a higher level in the accumulation mode than EDS measurement system. The measured results also show that the MDTS measurement system shifts the particle count median diameter (CMD) to larger particle number and volume concentration for all engine load conditions. It is mainly because the mini-tunnel dilution leads to particle transformations of nucleation and condensation simultaneously when the exhaust particle emission is cooled and diluted. However, the effect of coagulation on the total number concentration has shown to be negligible. On the other hand, the EDS measurement system can minimise the particle transformations that affect the exhaust particle number and size distributions during the heated dilution process. A fresh DOC can substantially reduce the nuclei mode particle number concentration from 17% to 50%, the total particle number concentration from 15% to 49% and the total particle mass concentration from 24% to 75%, and slightly increase the accumulation mode particle concentration from 1% to 7% when the diesel engine load increases from 10% to 100% of full engine load at a maximum torque of constant speed. Amongst this significant reduction of nuclei mode particle number concentration, 14% to 35% reduction is mainly attributed to the oxidation process that taken place, and the other 3% to 15% reduction comes from the dominant particle thermophoresis and electrostatic deposition mechanism inside the catalyst. However, the alteration level of particle CMD is not pronounced for different engine load conditions. Between the nucleation and accumulation particle modes, it is shown that the used catalyst increases 60% of the particle number concentration in accumulation particle mode (i.e. 650 nm) after 30 operation hours. It is also found that the installation of used DOC can reduce the gaseous emissions from 10% to 97% for CO emission and 4% to 14% for HC, but there is no effect on the NOx when the diesel engine load increases from 10% to 100% of full engine load at a maximum torque of constant speed. In the fast engine acceleration test, it is shown that there is no significant reduction of gaseous emissions in CO and HC at the initial stage until the oxidation process of catalyst had taken place for about 200 seconds.