Investigations on the pollutant emissions of gasoline direct injection engines during cold-start

Abstract

As the CO2 emission standards around the world become more stringent, the turbocharged downsized gasoline direct injection (GDI) engine provides a mature platform to achieve better fuel economy. For this reason, it is expected that the GDI engine will capture increasing shares of the market during the coming years. The in-cylinder liquid injection, though advantageous in most engine operation regimes, creates emissions challenges during the cold crank-start and cold fast-idle phases. The engine cold-start is responsible for a disproportionate share of the hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter (PM) emitted over the certification cycle. Understanding the sources of the pollutants during this stage is necessary for the further market penetration of GDI under the constraint of tighter emission standards. This work aims to examine the formation processes of the HC, NOx and PM emissions during the cold-start phase in a GDI engine, and the sensitivity of the pollutant emissions to different operation strategies. To this end, a detailed analysis of the crank-start was carried out, in which the first three engine cycles were individually examined. For the steady-state phase, the trade-off between low fast-idle emissions and high exhaust thermal enthalpy flow, necessary for fast catalyst warm-up, is investigated under several operation strategies. The pollutant formation processes are strongly dependent on the mixture formation and on the temperature and pressure history of the combustion process. The results show that unconventional valve timing strategies with large, symmetric, negative valve overlap and delayed combustion phasing are the most effective ways to reduce engine-out emissions during both crank-start and fast-idle phases.