The fuel-independent particulate emissions of a direct injection gasoline engine were investigated. This was done by running the engine with reference gasoline at four different loads and then switching to hydrogen or methane port fuel operation and comparing the resulting particulate emissions and their size distribution. Differences in the combustion characteristics of hydrogen and gasoline were accounted for by diluting the inlet air with nitrogen and matching the pressure or heat release traces to those of gasoline operation.
Methane operation is expected to generate particulate emissions lower by several orders of magnitude compared to gasoline and hydrogen does not contribute to carbon soot formation because of the lack of carbon atoms in the molecule. Thus, any remaining particulate emissions at hydrogen gas operation must arise from non fuel related sources, e.g. from lubrication oil, metal abrasion or inlet air. With methane and hydrogen operation, only a very small amount of particles larger than 30 nm could be measured. The comparison between CEC reference fuel and hydrogen operation showed similar size distributions for particles smaller than 10 nm at low loads, which was also consistent with the comparison between methane and gasoline operation.
The tentative conclusion is that particles smaller than 10 nm do not, for the most part, originate from the combustion of the fuel. However, at higher loads, the differences between gasoline and hydrogen operation were more pronounced at all particle sizes. Thus, while there seems to be a fuel-independent baseline of particulate emissions, the contribution of the fuel combustion itself is also important and is the dominant contribution at higher engine loads.
