Measurements of Fuel Film Thickness in the Inlet Port of an S.I. Engine by Laser Induced Fluorescence 952483

Fuel wetting in the inlet port of a gasoline engine was studied using Laser-Induced Fluorescence (LIF). The measurements were done directly on the metal wall surface. Quantitative results were be obtained using a special calibration procedure. The sensitivity of the technique was found to correspond to a fuel layer thickness in the order of 1 μm, and the accuracy was estimated to be approx. 10 %. The engine was run on iso-octane, and in order to obtain fluorescence a dopant (3-pentanone) was added to the fuel. Laser light with a wave length of 266 nm was generated by frequency doubling the light from a Nd-YAG laser in two steps. A laser sheet was directed into the intake port and the fuel layer on the wall could be studied along a line on the bifurcation wall. The fluorescence light was detected with an intensified diode-array camera.

The measurements from the fuel film thickness were compared with measurements of the total fuel film mass using an A/F response method. In general, an increase in the thickness of the fuel film leads to a larger total mass of fuel in the film. However, when the injector type is changed, a reduction of the fuel film thickness at the measurement location does not necessarily imply a reduction of the total fuel film mass.

The results show that open valve injection timing creates a significant reduction in the fuel film thickness compared with closed valve injection. In a warm engine there is no significant fuel film remaining from one cycle to the next at open valve injection. The fuel film is also largely dependant on engine temperature and load.

The fuel film from a twin-spray injector was shown to be thinner than that from a standard single-spray injector. A corresponding reduction of the total fuel film mass could, however, not be observed using the A/F response measurement technique.