Opto-thermal analysis of the combustion process in a DISI engine fueled with gasoline and ethanol

In the quest for ever higher efficiency and reduction of exhaust emissions, combustion control is becoming an essential aspect. This is even more important when considering the stochastic nature of turbulence, which determines cycle-by-cycle variability specific for spark ignition (SI) engines. Within this context, combustion characterization was performed through in-cylinder pressure analysis and spectroscopy, in an optically accessible single cylinder direct injection (DI) SI engine. Gasoline, ethanol and their blend containing 20% alcohol were tested at 2000 rpm in WOT conditions; stoichiometric and lean burn cases (λ≅1.2) were considered. The optical signal recorded through a window fitted in the piston crown allowed detailed UV-visible spectroscopic characterization of three distinct phases: ignition (identified through the presence of the spark), flame development and the main combustion stage (related to high rates of heat release). CN (B-X) emission bands were used for identifying the initial stages of ignition and evaluate the gas temperature during spark discharge. Flame development was associated to the UV emission mainly due to OH (A-X) transitions. CO2* and OH represented the markers of the main combustion phase. Moreover, spectral features of soot precursors were detected and associated to the oxidation of fuel deposits. Optical results were also correlated to the particle emissions at the exhaust. Although the overall engine output was practically the same for both fuel types, less stable combustion was noted for the alcohol, given by higher coefficient of variation recorded for the indicated mean effective pressure. Incomplete fuel evaporation was identified as the main reason for the observed differences and the basis was set for evaluating the correlation between local air-fuel variation and optical parameters.