Particulate emission reduction has long been a challenge for diesel engines as the diesel diffusion combustion process can generate high levels of soot which is one of the main constituents of particulate matter. Gasoline engines use a pre-mixed combustion process which produces negligible levels of soot, so particulate emissions have not been an issue for gasoline engines, particularly with modern port fuel injected (PFI) engines which provide excellent mixture quality.
Future European and US emissions standards will include more stringent particulate limits for gasoline engines to protect against increases in airborne particulate levels due to the more widespread use of gasoline direct injection (GDI). While GDI engines are typically more efficient than PFI engines, they emit higher particulate levels, but still meet the current particulate standards. In Europe there is a proposal for the Euro 6 standard to require GDI engines to meet the particulate number (PN) standard introduced at Euro 5 for diesel engines. In the US, more stringent particulate mass (PM) limits have been proposed by CARB and EPA. Testing has shown that current Euro 5 GDI engines would exceed the proposed Euro 6 PN standard and that additional effort may be required to meet the proposed US standards.
While a particulate filter could be used to reduce both particulate mass and number, the preferred approach would be to reduce engine-out particulates which may be possible through combustion system, controls and calibration improvements.
This paper describes an investigation where combustion system improvements and control strategies were developed then applied to provide a significant particulate number reduction from a gasoline turbocharged direct injection engine (GTDI). The mechanisms of particulate formation are described and countermeasures for reduction are discussed. In particular, particulate formation resulting from interactions between the fuel and the combustion chamber walls (liner, piston) are investigated, and it is shown how multiple injection strategies can be used to minimize these interactions and reduce particulates. The effects of component temperature on particulates are discussed and the particulate reduction benefits of a control strategy to provide rapid piston heating are shown. The development of injector technology for particulate emission reduction is also discussed.
The potential for a turbocharged GTDI engine to meet the proposed Euro 6 PN standard without the use of a particulate filter is demonstrated.
