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Low temperature, highly premixed compression ignition strategies have proven to produce high efficiency and low soot emissions, but struggle to reach high loads within normal operating constraints. Recent research has suggested that a mixed mode combustion strategy using a premixed main heat release followed by a mixing controlled load extension injection can retain the part-load thermal efficiency and emissions reduction potential of premixed compression ignition strategies, while enabling high load operation. However, soot emissions under this type of mixed mode combustion strategy have been shown to be problematic. This work investigates soot formation and mitigation methods using a combination of detailed engine experiments and computational fluid dynamics modeling. A premixed compression ignition combustion event was achieved using a premixed charge of gasoline and n-heptane to control combustion phasing, and a load extension injection of gasoline was added near top dead center. The experiments showed negligible engine out soot under the premixed compression ignition operating conditions (i.e. without the load extension injection). When the load extension injection was added, soot increased by several orders of magnitude. Detailed experiments were used to isolate the effects of injection timing, injection pressure, charge conditions (e.g. air-fuel ratio), and fuel type. Computational fluid dynamics modeling considering polycyclic aromatic hydrocarbon chemistry up to pyrene was then used to explain the experimentally observed soot trends. As expected, the soot emission results showed a strong impact of oxygen concentration and injection pressure for injection timings near top dead center; however, as the load extension injection event was delayed beyond the end of the premixed compression ignition heat release, the soot formation decreased and became independent of oxygen concentration. At these conditions, the computational fluid dynamics modeling showed that soot formation is dependent solely on temperature. The results identify a pathway to enable premixed compression ignition load extension, while minimizing soot emissions.

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