Fuel Film Measurement in a SI Gasoline Engine Using a Newly Developed MEMS Sensor

The previously developed capacitance sensor for detecting a liquid fuel film was modified to apply to the in-cylinder measurement. On the developed sensor surface, comb-shaped electrodes were circularly aligned. The capacitance between the electrodes varies with the liquid fuel film adhering. The capacitance variation between the electrodes on the sensor surface was converted to the frequency variation of the oscillation circuit. In the previous study, it was revealed that the frequency of the oscillation circuit varies with the variation of the liquid fuel coverage area on the sensor surface. The developed sensor was installed in the combustion chamber of the rapid compression and expansion machine, and the performance of the developed sensor was examined. Iso-octane was used as a test fuel to explore the sensor that had been developed. As a result, the adherence of the liquid fuel directly injected into the cylinder was successfully detected under the quiescent and motoring conditions without a combustion event. The frequency decreases with the increase in the duration and amount of fuel injection. The frequency variation was saturated for more than 4 ms injection duration. With the combustion event, the frequency varied during combustion duration even without fuel injection and fuel adhering—the cause of the variation during combustion. The liquid fuel adhering before the combustion event can be detected. The sensor can be applied to the fuel film measurement during cold start and warm-up conditions.