Investigation of Intake Port Fuel Films in a Small Utility Air-Cooled Engine

Four techniques were investigated for the characterization and quantification of fuel films in the intake port in a small four-stroke, air-cooled utility engine modified to use a fuel injector mounted in the place of the carburetor float bowl: step-fueling with constant air flow, step-throttle with constant fuel flow, skip-injection, and stop-injection tests. In the first two tests the exhaust air-fuel (A/F) ratio was measured with a fast-response universal exhaust gas oxygen (UEGO) sensor, while a fast flame ionization detector (FFID) was used in the latter two test methods. The engine was fueled with indolene, iso-octane, and propane to investigate and separate combustion and oil absorption / desorption from fuel film effects. The results indicate that the air flow through the intake port had the largest impact on the fuel film dynamics. Step-fueling tests showed only a short (less than 5 engine cycles) period during which the inducted A/F differed from the delivered A/F, whereas step-throttle tests showed a more pronounced A/F excursion that persisted for close to 20 engine cycles. The skip- and stop-injection tests indicated that vaporization from the fuel film contributed approximately 30% of the fuel inducted per cycle, regardless of load or the liquid fuel type. The overall film mass was found to be directly proportional to engine load (throttle position).