In situ measurement of hydrocarbon fuel concentration near a spark plug in an engine cylinder using the 3.392 μm infrared laser absorption method: Discussion of applicability with a homogeneous methane-air mixture

A fibre optic system was developed to determine the fuel concentration near a spark plug using an infrared absorption method. The system was linked to an optical sensor installed in the spark plug, from which light could pass through the combustion chamber. By using this modified spark plug, successive measurements of the fuel concentration near the spark plug before ignition were performed in a spark-ignition engine burning homogeneously mixed methane-air. The fuel concentration was determined from the Lambert-Beer law by considering the dependence of the methane molar absorption coefficient on pressure and temperature. Three main conclusions were drawn from this study. First, the methane molar absorption coefficient was greater for lower pressures and decreased with increasing temperature and pressure above atmospheric pressure. The temperature and pressure effects were offset by each other, since the temperature effects were positive and the pressure effects were negative. Second, precise time-series data for the local fuel concentration were obtained by considering the in-cylinder pressure and temperature from an estimate of the methane molar absorption coefficient. And third, the measured air/fuel ratio near the spark plug before ignition agreed with the preset value when the developed optical sensor was used under motoring and firing conditions.