Liquid fuel attached to the wall surface of the intake port, the piston and the combustion chamber is one of the main causes of the unburned hydrocarbon emissions from a port fueled SI engine, especially during transient operations. To investigate the liquid fuel film formation process and fuel film behavior during transient operation is essential to reduce exhaust emissions in real driving operations, including cold start operations. Optical techniques have been often applied to measure the fuel film in conventional reports, however, it is difficult to apply those previous techniques to actual engines during transient operations. In this study, using MEMS technique, a novel capacitance sensor has been developed to detect liquid fuel film formation and evaporation processes in actual engines. A resistance temperature detector (RTD) was also constructed on the MEMS sensor with the capacitance sensor to measure the sensor surface temperature. The response and the sensitivity of the developed sensor were examined at the atmospheric conditions at first. As a result, it was found that though the sensor shows less sensitivity to pure commercial gasoline, it has enough sensitivity to gasoline fuel containing 20% ethanol (E20 gasoline). After the sensitivity test, the sensor was installed into the intake pipe of the single cylinder engine and examined to detect the liquid fuel film on the wall of the intake pipe. The engine was operated at a constant speed of 2000 rpm with E20 gasoline fuel. The sensor performed well during the engine operation, and the liquid fuel impingement and evaporation process could be monitored.
