A Ti/Pt Hot Film Anemometer for CR-Injection Systems

By e-beam evaporating a thin film resistor, consisting of a Titanium (≈10nm)/Platinum (≈100nm) bi-layer, on a LTCC (Low Temperature Cofired Ceramic) substrate, a robust and fast responding thermal mass flow sensor is developed. Due to the integration of the latter into the nozzle of a Common Rail (CR) injection system, important parameters, as the injection rate as well as the begin/end of the injection pulse, can be determined with high accuracy. By a closed-loop control of the magnetic or piezo-electric driven valve, a smooth combustion process with lower NO_x emission values could be achieved. This present paper mainly focuses on analytical calculations to determine the velocity and temperature sensitivity of the thermal mass flow sensors in the constant-current (CC) mode. Because of the cross sensitivity of the thermo-resistive measurement principle, a second thin film resistor, fabricated with the same technology and operated in the low temperature regime, is desired, to detect any temperature changes of the fuel for a proper interpretation of the sensor signals. Further on, the first injection rate measurements up to 130 MPa are presented and the sensor characteristics, which yield high amplitudes during injection, are discussed. For a long-term reliable operation of the hot film anemometers under these harsh environmental conditions, a proper passivation technology is indispensable. Amorphous silicon carbide (a-SiC:H) exhibits an excellent coverage of the surface and a good adhesion to the glass ceramic substrate, enabling a clear reduction of drift effects on sensor constants in oil atmosphere. atmosphere. atmosphere.