Measurement of hydrocarbon fuel concentration by means of infrared absorption technique with 3.39 μm He-Ne laser

The infrared absorption technique with a 3.39 μm He-Ne laser is useful for measuring hydrocarbon concentrations. First, molar absorption coefficients of propane or methane for the 3.39 μm wavelength were investigated in the temperature range of 285-420 K and in the pressure range of 100-800 kPa. It was found that the molar absorption coefficient, ∈, is independent of temperature, and that pressure has little effect on ∈ in propane, but a strong effect in methane. Second, by applying this technique to a spark ignition engine, the fuel concentration in the vicinity of a spark plug and the hydrocarbon concentration in the exhaust pipe were measured under the conditions of no residual gas and homogeneous mixture. For the lean mixture, although cycle-to-cycle fluctuation of the fuel concentration was very small, the fluctuation of pressure in the cylinder was large.