Characterization of GDI PM during vehicle start-stop operation 2019-01-0050
As the 2025 fuel economy regulations approach, many manufacturers are implementing start-stop operation to enhance vehicle fuel economy. During start-stop operation, the engine shuts off when the vehicle is stationary for more than a few seconds. When the brake is released by the driver, the engine restarts. Depending on traffic conditions, start-stop operation can result in fuel savings from a few percent to close to 10%. Gasoline direct injection (GDI) engines are also increasingly available on light-duty vehicles. While GDI engines offer fuel economy advantages over port fuel injected (PFI) engines, they also tend to have higher PM emissions, particularly during start-up transients. Thus, there is interest in evaluating the effect of start-stop operation on PM emissions. In this study, a 2.5L GDI vehicle was operated over the first 1372 s of the FTP75 drive cycle, a cycle commonly known as the LA4 cycle. Cold starts (FTP 75 phases 1 and 2) and multiple hot starts (FTP 75 phases 3 and 4) were performed each day, and three fuels were evaluated: an 87 AKI gasoline (E0); a 20% blend of ethanol and the 87 AKI gasoline (E20) and a 12% blend of iso-butanol and the 87 AKI gasoline (iB12). Note that the FTP 75 phases 3 and 4 are identical to phases 1 and 2 except that the engine is warmed up. PM mass, transient particle number concentration and size distribution, soot concentration, and PM chemistry were evaluated for both start-stop operation and no start-stop operation. Three cold cycles and as many as 27 hot cycles were performed for each fuel-mode combination. Composite FTP emissions for E0 and iB12 show increased total PM emissions with start-stop operation, but E20 shows no difference. Statistical analysis of the effects of start-stop on PM number and soot emissions showed different trends for different fuels. For example, when E0 is used the soot tends to increase with start-stop operation, but the particle number decreases. The results of the study have implications for hybrid vehicle operation as well.
John M. Storey, Melanie Moses-DeBusk, Shean Huff, John Thomas, Mary Eibl, Faustine Li
Oak Ridge National Laboratory
International Powertrains, Fuels & Lubricants Meeting