Hydrocarbon emissions remain an important concern for the automotive industry due to increasingly strict regulations. In an investigation of possible emission sources within the engine, the concentration of fuel absorbed in the oil film on the cylinder wall of a small internal combustion engine has been measured with laserinduced fluorescence (LIF) spectroscopy. A laser pulse from a nitrogen laser (337.1 nm) provided the excitation, and the fluorescence was monitored with an intensified-charge-coupled device (ICCD) array mounted to a spectrometer. The excitation laser pulse was launched through a window in the engine head onto a fiber-optic probe mounted flush with the cylinder wall. The laser-excited oil film on the fiber-optic probe produced an LIF signal that was collected by the fiber-optic probe and analyzed for fuel content. The timing of the laser pulse and ICCD gate were controlled in order to synchronize the collection of data with a particular point in the engine cycle. Measurements made in situ, while the engine was running, yield information on the amount of unburned fuel stored in the oil film for various engine conditions. Fuel-in-oil concentrations were determined for various engine temperatures during cold starts, for different fuel enrichment levels, and as a function of the crank angle of the engine cycle. Fuel concentrations as high as 50% were detected during cold starts, and fuel concentrations reached levels greater than 25% for warm engine operation. Changes in the fuel content were also found to be related to enrichment and crank angle.

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