Thesis Abstract: Various techniques of measuring fuel films in the intake pot of a small air-cooled, homogeneous charged, four-stroke, air-cooled engine were investigated. The techniques used were: step transient fueling tests, step transient throttle tests, capacitance sensor fuel film thikness measurements, skip-injection tests, and stop-injection tests. The engine was fueled with iso-octane, indolene, and propane to investigate and separate combustion from fuel film effects. The engine was operated at 3060 rpm. A heated universal gas oxygen (UEGO) sensor and fast flame ionization detector (FFID) were the diagnostic tools used to measure and characterize fuel film behavior.

Indolene-fueled step transient tests were found to have an increased UEGO sensor response time compared to propane. No overt transient air-fuel (A/F) excursions were noted. This delay in sensor response indicated the presence of fuel films in the intake port, although the film was not overly disturbed during the transient. Indolene-fueled step throttle transient tests indicated notable transient A/F excursions during throttle opening and closing compared to propane-fueled tests. This validated presence of intake port fuel films.

Skip-injection tests determined that approximately 30% of the fuel inducted per cycle comes from the fuel film, regardless of engine load, or the pressure of fuel injection from the siphon-tube fuel injection system. These results were validated during stop-injection tests. From the stop-injection tests it was also found that overall intake port fuel film mass decreases with decreasing engine load. Overall intake port film masses were substantial, often containing numerous engine cycles of fuel. Also, oil consumption was found to contribute to the measured hydrocarbons well after the fuel film was depleted. No technique was developed to determine the point at which the fuel film was depleted.

Capacitance fuel film sensors and calibration techniques were developed. It was found that the sensors were capable of measuring thin films within 50mm. two sensors were used to measure films in the intake port of the engine. The two sensors located in the intake port did not detect the presence of a fuel film. Location of the sensors dictated by engine spatial constraints was thought to be the culprit.