In situ measurement of hydrocarbon fuel concentration near a sparkplug in an engine cylinder using the 3.392 µminfrared laser absorption method: application to an actual engine

Abstract

An infrared absorption method with a 3.392 µmHe–Ne laser was used to determine the hydrocarbon fuel concentration near thespark plug in a spark-ignition engine. Iso-octane was used for the fuel. Thepressure and temperature dependence of the molar absorption coefficient wasclarified. The molar absorption coefficients of a multi-component fuel such asgasoline were estimated by using the coefficient of each component andconsidering the mass balance. A sensor was developed and installed in a sparkplug, which was substituted in place of an ordinary spark plug in a spark-ignitionengine. Light can pass from the sensor through the engine cylinder tomeasure the fuel concentration. The effects of liquid droplets inside theengine cylinder, mechanical vibrations and other gases such as H2O andCO2on the measurement accuracy were considered. Four main conclusions were drawnfrom this study. First, the pressure and temperature effects on the molarabsorption coefficient of liquid fuel vapour were determined independently inadvance using a constant-volume vessel. The pressure and temperaturedependence of the molar absorption coefficient was determined underengine firing conditions. Second, the molar absorption coefficients of amulti-component hydrocarbon fuel such as gasoline were estimated by consideringthe molar fraction of each component. Third, in situ measurements of thehydrocarbon fuel concentration in an actual engine were obtained usingthe spark plug sensor and the molar absorption coefficient of iso-octane.The concentration near the spark plug just before ignition was almost inagreement with the mean value that was obtained from the measurement ofthe flow rate made with a burette, which represented the mean valueaveraged over many cycles. And fourth, no liquid droplets were observed atnear-idling conditions. The effects of other gases, such as CO, CO2 andH2O,can be neglected.