Fuel Effect on Particulate Matter Composition and Soot Oxidation in a Direct-Injection Spark Ignition (DISI) Engine

Particulate matter (PM) composition and soot oxidation were investigated in a single-cylinder spray-guided direct-injection spark ignition (DISI) research engine using the thermogravimetric analysis (TGA) technique. Fuels including gasoline, ethanol, 25% volumetric blend of ethanol in gasoline (E25), and a new biofuel candidate (2,5-dimethylfuran, DMF) were studied. The engine was operated at 1500 rpm with a rich fuel/air ratio (lambda = 0.9) and late fuel injection strategy, representing one of the worst scenarios of PM emissions from DISI engines. A TGA method featuring devolatilization and soot oxidization functions was developed and a kinetic model was used to analyze the soot oxidation process. The results show that volatile components are the main contributor to the PM produced from gasoline, E25, and DMF, and elemental soot accounts only up to 35% of PM mass at 8.5 bar IMEP. Ethanol combustion is so clean that only 6.3% of PM mass comes from elemental soot. The reaction rate of the soot oxidation is highly dependent on fuel and is sensitive to engine load. Soot from ethanol combustion is the most easily oxidized, indicated by the lowest temperature and activation energies (83 kJ/mol) required for oxidization. Soot from gasoline combustion is the most difficult to be oxidized, requiring the highest temperature and activation energy. It is found that the activation energy required for the soot from gasoline combustion increases with the engine load; however, the increase for soot from DMF combustion is very small.