Influence of Inlet Air Temperature on Gasoline HCCI Particulate Emissions

Particulate emissions from homogeneous charge compression ignition (HCCI) engines have been usually considered negligible, and the measurement of particulates (PM) with HCCI combustion systems has been extremely rare. A report from Ford and the authors' own recently published work suggest that PM emissions from gasoline HCCI engines should not be neglected. It has been shown that PM emissions for SI and HCCI engines can be very similar, especially in the accumulation mode. In the present work, the influence of inlet air temperature was investigated. A purpose-built inlet air thermal management system was in use for this purpose. An electric heater in line with an air/engine coolant heat exchanger was used to heat inlet air to 30-40C above standard operation conditions. A water-cooled intercooler was in use during the second set of tests. Inlet air was cooled approximately 20C below the case where no heating or cooling was implemented. It has been found that increasing the air temperature will lead to a higher concentration of nucleation mode particles (having diameters 10-20nm) and lower concentration of accumulation mode particles (with diameters 80-90nm). When inlet air was cooled, a reverse trend has been observed, especially in the accumulation mode. It was also observed that increasing the load will reduce the differences caused by changes in the inlet air temperature. Finally, trends similar to those observed for direct injection spark ignition (DISI) engines have been observed as well. Increasing either engine speed or engine load leads to increased PM emissions.