Dynamic measurement of speed of sound in n-Heptane by ultrasonics during fuel injections

•Method for measurement of speed of sound in n-Heptane has been developed.•Time resolved estimate of speed of sound during injection has been provided.•Ultrasound measurement system provides reliable signals in dynamic conditions.•Linear dependence of speed of sound vs pressure during injection is observed.•Characterization of speed of sound with temperature has been accomplished. The paper presents a technique to measure the speed of sound in fuels based on pulse-echo ultrasound. The method is applied inside the test chamber of a Zeuch-type instrument used for indirect measurement of the injection rate (Mexus). The paper outlines the pulse-echo method, considering probe installation, ultrasound beam propagation inside the test chamber, typical signals obtained, as well as different processing algorithms. The method is validated in static conditions by comparing the experimental results to the NIST database both for water and n-Heptane. The ultrasonic system is synchronized to the injector so that time resolved samples of speed of sound can be successfully acquired during a series of injections. Results at different operating conditions in n-Heptane are shown. An uncertainty analysis supports the analysis of results and allows to validate the method. Experimental results show that the speed of sound variation during an injection event is less than 1%, so the Mexus model assumption to consider it constant during the injection is valid.