Effect of implementing large-scale charge motion, reducing hydraulic flow of the injector and increasing injection pressure on particle emissions of a GDI engine at WOT and boosted operation

Abstract

Gasoline direct injection (GDI) shows advantages compared with port fuel injection (PFI) regarding efficiency and specific power. Due to stricter regulations for fuel consumption (via the regulation of carbon dioxide), GDI engines are becoming increasingly favourable compared with PFI engines. Therefore the share of GDI engines, especially in combination with turbocharging, is increasing in most of the markets with CO2 regulations. Challenging for GDI engines is the mixture formation process due to the short time between fuel injection and the start of combustion. Thus, the injector needs to provide a fine fuel atomization in a considerably short time. The generated spray pattern thereby interacts with the in-cylinder charge motion to generate an appropriate air–fuel mixture. Because of this challenging mixture formation process, the formation of soot in local fuel-rich areas is possible. Thus GDI engines emit more particles compared with PFI engines and need special attention on the mixture formation process. To understand the reasons for the increased particle number (PN) emissions, a project concerning the cause of particle emissions was started at Institute of Internal Combustion Engines (IFKM) in Karlsruhe in 2011. During the project, different causes for PN emissions were identified. This article discusses the possible reasons for particle emissions under high engine load and low engine speed and shows some possible solutions to reduce the emission of particles. Discussed possible solutions to enhance the mixture formation process are the generation of a large-scale charge motion (tumble and swirl), the reduction of the hydraulic flow of the multi-hole, solenoid-activated injector and an increase of the rail pressure up to 50 MPa. The reduction of the hydraulic flow and the increase of the injection pressure lead to smaller average droplets and thus to a faster evaporation. An implementation of a large-scale charge motion enhances the mixture formation process and leads to a reduction of the emitted PN concentration at high engine load. This is shown for wide open throttle (WOT) as well as for boosted operation. The reduction of the hydraulic flow of the injector by reducing the bore hole diameter at constant number of holes and spray targeting of the injector leads to smaller droplets. By increasing the injection pressure, the injection duration as well as the average droplet size can be reduced and leads to a better homogenization and a further reduced PN concentration.