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U. Schmid, R. Hoffmann, G. Krötz
A Micromachined High Pressure Stable Mass Flow Sensor for Common-Rail Injection Systems
We report for the first time about a micromachined mass flow sensor directly integrated between the double guidance and the needle seat into the body of a Common Rail (CR) injection nozzle. The thermal measurement principle was chosen because gaseous as well as liquid mass flows can be determined fast and precisely. Additionally, with glass-ceramic materials fabricated in a low cost batch process, a high temperature and especially pressure stable substrate can be used. The flow sensitive thin film is realized by an e-beam evaporated 110 nm thick molybdenum (Mo) layer. The latter is electrically characterized by the resistivity of ρ=8.2·10–7Ω·m(±2%) at room temperature as well as by the first α=4.5·10–4K–1(±5%) and second β=3.0·10–6K–2 (±12%) temperature coefficients of resistance up to 300°C. The static temperature field at and around the thin film sensor on the glass-ceramic substrate is investigated with an IR imaging system at different electrical heating powers Pelec. For optimized sensor designs a constant temperature profile is measured, being in good agreement with the prediction of an analytical model. The first mass flow measurements at 500 bar exhibit excellent sensor characteristics, measuring even pressure waves occurring during and after injection. Therefore, injection rates can precisely be measured “on board”. Hence we believe that with such micromachined mass flow sensors the injection system of combustion engines can further be optimized by a closed-loop control of the magnetic or piezoactuatoric driven valve.
tm – Technisches Messen, Oldenbourg Wissenschaftsverlag
Print ISSN: 0171-8096
Volume: 68, 05/2001
Pages: 208