Studies of start-of-injection (SOI) timing impact on particle size distributions were performed for conventional diesel combustion (CDC) and reactivity-controlled compression ignition (RCCI) combustion using a single injection strategy. The influence of post injection timing on PSDs from a triple injection cold start rapid catalyst heating strategy was also studied. PSD measurements were performed using a scanning mobility particle sizer (SMPS) combined with a condensation particle counter (CPC) and measurements were performed without and with a thermodenuder (TD).
In the single-injection CDC and RCCI injection timing study, CDC particle size distributions were found to be significantly influenced by the SOI timing. An initial increase in the peak of the PSD was observed when retarding SOI from -12 to -3 CAD, and significantly reduced particle concentrations were observed at larger particle sizes for the most retarded SOI times of 0 and +2 CAD. For RCCI combustion, the PSD concentrations were over an order of magnitude lower than for CDC. Only a slight dependence of the PSD on injection timing was seen for the RCCI PSD results.
In the rapid catalyst heating strategy study, particle size distributions were found to be highly dependent on post injection timing, the amount of exhaust gas recirculation, and the type of fuel used. With 30% EGR, it was observed that concentrations of larger particles decrease initially with delayed injection timing (post SOIC = +11, +15, +19 CAD), and then increased significantly for later injection cases (post SOIC = +23 and +25 CAD). The 30% EGR cases were found to have higher particle concentrations than the 0% EGR cases, regardless of the type of fuel used. Results for a primary reference fuel (PRF) blend of 34 vol% isooctane with a balance of n-heptane were found to have significantly lower particle numbers for both 0% and 30% EGR cases relative to results using diesel fuel due to the lower chemical sooting propensity and the higher volatility of the PRF blend.