Particulate Emissions from a Direct-Injection Spark-Ignition Engine

Particulate mass (PM) emission rate and size distribution measurements were performed in a direct-injection two-stroke engine under a wide range of conditions using a venturi-type mini-dilution tunnel. Air-assisted and nitrogen-assisted liquid fuel injection were both tested to investigate subtle changes in local equivalence ratio; gaseous propane injection using the same injection system was investigated to isolate the effects of liquid fuel impingement.

Under overall lean operating conditions the PM emissions were found to decrease when the air-assisted injection was changed to N2-assisted injection with all other parameters equal. The suggested cause for this behavior was a reduction in the PM formation and oxidation rates due to lower local temperatures. A similar effect (lower particulate matter emissions with a locally richer air-fuel ratio) was observed for a light load condition where the local oxygen concentration was varied by changing the exhaust gas recirculation rate. The use of propane injection resulted in lower PM emissions for all conditions except a medium load 2000 RPM condition. The reduction cannot, however, be solely related to the elimination of liquid fuel impingement since differences in the size distributions suggest that fuel composition played a significant role. Under some conditions there were significant differences between the air- and N2-assisted injected case where there are not expected to be significant differences in the amount of liquid impingement. Thus, while impingement is likely an important mechanism for particulate matter formation, it is not considered to be controlling under all cases.