An In-cylinder Laser Absorption Sensor for Crank-angle-resolved Measurements of Gasoline Concentration and Temperature

Simultaneous crank-angle-resolved measurements of gasoline concentration and gas temperature were made with two-color mid-infrared (mid-IR) laser absorption in a production spark-ignition engine (Nissan MR20DE, 2.0L, 4 cyl, MPI with premium gasoline). The mid-IR light was coupled into and out of the cylinder using fiber optics incorporated into a modified spark plug. The absorption line-of-sight was a 5.3 mm optical path located closely adjacent to the ignition spark providing spatially resolved absorption. Two sensor wavelengths were selected in the strong bands associated with the carbon-hydrogen (C-H) stretching vibration near 3.4 μm, which have an absorption ratio that is strongly temperature dependent. Fuel concentration and temperature were determined simultaneously from the absorption at these two wavelengths. The two mid-IR laser wavelengths were simultaneously produced by difference-frequency-generation in a periodically poled lithium niobate (PPLN) crystal using one signal and two pump lasers. The pump lasers were modulated at different frequencies allowing frequency demultiplexing of the laser light transmitted though the spark plug probe, providing simultaneous absorption signals at the two mid-IR wavelengths. A model of the absorption cross section of gasoline was used to account for variation in the absorption signals as a function of fuel blend using the known fractions of olefins, aromatics, and paraffins. Validation experiments of this fuel and temperature sensor included comparison of in-cylinder data with more traditional exhaust gas diagnostics. For an engine operating at steady-state with a nominally homogeneous charge, measurements of air-fuel ratio (A/F) as a function of load agreed well with the air-fuel ratios inferred from analysis of exhaust emissions; in-cylinder measurements performed with quite different fuel blends also agreed well with the exhaust diagnostics.