A Thermo-Resistive Flow Sensor for Injection Rate Measurements 2002-01-0212
In this study, a micromachined hot film anemometer, placed on a pressure stable LTCC substrate, is presented to measure the different injection quantities, needed in modern direct injection (DI) system for optimum performance. A bi-layer of Titanium (Ti) and Platinum (Pt) as advanced metallization system for the thin film resistors is used due to a lower resistivity and a higher temperature coefficient of resistance (TCR) compared to Molybdenum (Mo), which was taken for the first prototypes. Especially the increase in TCR is recommended for thermal mass flow sensors, as the resulting signal height is linearly related to this material parameter. Therefore, the new technology steps, to fabricate the sensor elements, are given in the paper. FEM simulations on the temperature distribution around the hot film anemometers in the injection nozzle reveal a minimum distance, where the temperature is increased compared to the ambient, of about 30 μm at a injection pressure of 20 MPa. This demonstrates, that the two thin film sensors, which would be needed for bi-directional mass flow measurements, have to placed within this distance to each other. The latest injection rate measurements, however, performed for the first time up to 90 MPa with a nozzle integrated mass flow sensor, yield a strong decrease in sensor amplitude due to the closing of the nozzle. Subsequent pressure waves, caused by the strong deceleration of the fuel column, are clearly detectable. Finally, the relationship between the height of the sensor signals and the injection pressure, ranging from 40 MPa to 90 MPa, is presented and discussed.