Autoignition and Sooting Characteristics of Iso -Octane and Ethanol in an Optical Rapid Compression Machine 2022-01-0419

With the introduction of EV technology into the light-duty vehicle market, the demand for gasoline in conventional spark ignition engines is projected to decline in the coming decades. Therefore, researchers have been investigating the use of gasoline and other light fuels in heavy-duty engine applications. In heavy-duty engines, the combustion mode will likely be non-premixed, mixing-controlled combustion, where the rate of combustion is determined by the fuel-air mixing process. This creates a range of mixture conditions inside the engine cylinder at every instance in time. The goal of this research is to experimentally quantify the sooting behaviors of light fuels under a range of compression ignition engine mixture conditions (i.e., a range of equivalence ratios). Accordingly, an optical rapid compression machine (RCM) was used to compress charges of iso-octane and ethanol to autoignition conditions, which varied between a fuel-air equivalence ratio of 1.6 to 2.3 and 1.9 to 2.5, respectively. This equivalence ratio span showed a transition from non-sooting conditions to a limit where the burned mixture after combustion became optically thick, as measured by a line-of-sight laser extinction diagnostic. Data from the laser extinction diagnostic indicated that pure iso-octane begins to form measurable soot at an equivalence ratio of ~1.85, whereas pure ethanol begins forming soot at an equivalence ratio of ~2.2. If the data are compared on the basis of an oxygenated equivalence ratio, which accounts for fuel-bound oxygen, both fuels begin to form soot at a similar oxygenated equivalence ratio of ~1.85.