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Scientific news from Division 1 |
The hotter the engine oil, the slower the ultrasound
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As commissoned by Hella Fahrzeugkomponenten GmbH, Bremen, measurements of the ultrasound propagation speed in engine oils as a function of temperature in the range from 8°C to 153°C were performed. The data obtained are important for the estimation of the uncertainty of the ultrasound based oil-filling level measurement in car engines recently developed.
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Recently, sensors for the automatic monitoring of the oil-filling level in car engines have been developed which are based on ultrasound measurements. To determine the accuracy of this technique, fundamental investigations had to be carried out to examine the propagation speed of ultrasound in car engine oils as a function of the temperature.
A measurement set-up for the determination of the sound speed over a large temperature range, from 8°C to 153°C, was designed and installed. In particular, the fact had to be considered that only small volumes of approximately 200 ml were available for specifically treated engine oil samples. To exclude drift effects - caused by material expansion due to the large range of temperatures investigated - from the measurements and uncertainty considerations, a differential measurement technique was chosen. At each temperature of interest, ultrasound time-of-flight measurements were carried out in such a way that two echoes from an acoustic reflector were recorded. Between these recordings the reflector was displaced by a certain, precisely specified distance. The time-of-flight was then determined by a cross-correlation analysis of these two echo signals. The sound speed was determined for 10 engine oil samples having different viscosity specifications and usage times, including unused samples, and stemming from different oil producers.
The results show a qualitative similar behaviour of all oil samples tested. The sound speed decreases monotonously from about 1485 m/s at 8°C to about 1025 m/s at 150°C. Only slight differences in the absolute height of the curves were observed. The maximum deviations from the mean value of all oil samples were 0.52% and +0.66% at 8°C, and -0.94% and +1.33% at 150°C. The results will be used to improve the temperature dependence compensation integrated in the filling-level meter sensor and for an improved uncertainty estimation for the ultrasound-based oil level measurements in car engines. The investigations were commissioned by Hella Fahrzeugkomponenten GmbH, Bremen, Germany.
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Contact person: |
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V. Wilkens, AG 1.62, Email: Volker.Wilkens@ptb.de
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© Physikalisch-Technische Bundesanstalt
Page created: 27.06.2007, last update: 27.06.2007,
Dirk Röske
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