Particle Number Emissions From Compression Ignition Engines: A Critical Review of Current Understandings

Particulate matter (PM) emitted from compression ignition (CI) engines is an important source of pollution in the urban atmosphere and has adverse effects on the environment and human health. For CI engines, the development of engine technologies puts controlling ultrafine particle emissions, especially the number of particle emissions (PN), as the ambitious vector, making breakthroughs necessary to further reduce the particle emissions of CI engines. This review is aimed at giving the most recent updates on the effects of control strategies on PN emissions. The present paper starts with the chemical composition and formation mechanism of particle emissions from CI engines and discusses the contribution of each component and evolution process to the final PN emissions. With the objective to explore the fundamental causes of PN emissions, in-cylinder technologies such as injection parameters, operating conditions, and fuel formulation were also addressed in terms of their impacts on particle size distribution and PN emissions, by discussing the fuel-air mixing, combustion characteristics, and fuel kinetics during in-cylinder combustion processes. The effects of the after-treatment systems and their combined effects are also presented. This section highlighted the surprising finding that the interaction of catalyst and diesel particle filter (DPF) regeneration elevated the PN emissions. Finally, toward coming up with the optimized technologies for PN reduction, the efficacies of all the technologies on PN emissions were compared. It was found that DPF exhibited the highest reduction degree in PN emissions among the technologies, whereas the alternative fuels that were generally believed to be beneficial for PM reduction showed inconsistent efficacy on PN emissions. This review can serve as a reference for the further reduction of PN emissions and updating of emission regulations.