But somehow there is no discussion of the PM (soot) emissions from GDI engines??? Because the combustion process is a hybrid of the gasoline spark ignition and the diesel compression engine, there is a natural increase in soot emissions from a GDi engine (see here and here, for example). This similarity is more easily identified when particule number concentration is measured (instead of particulate mass). The increased PM emissions have been documented well in several studies in europe, and in fact, the Euro 6 emission regulations are expected to require GDI to meet the same PM/NOx criteria as the diesel engines in europe. While in the short term this may be possible using engine control technologies, and operating in certain regions of the engine map, this may not be possible for a long time. Under transient conditions GDi engines produce about as much soot as a modern diesel engine, and hence to reach PM regulatory limits, a particulate filter may be required.