Experimental Measurement of the Isothermal Bulk Modulus of Compressibility and Speed of Sound of Conventional and Alternative Jet Fuels

The use of jet fuels in compression-ignition engines with pump-line-nozzle (PLN) fuel injection systems requires consideration of differences in bulk modulus and speed of sound versus diesel fuels, thus requiring thorough characterization of jet fuels before they are applied. In this study, we measured the isothermal bulk modulus of compressibility of three different petroleum-based jet fuels (i.e., POSF 4658, POSF 6169, and POSF 10325) at 313 K (0-4000 psig) and of three different alternative jet fuels (i.e., Farnesane, hydrotreated renewable jet-derived from Camelina (HRJC), and Alcohol-to-Jet (ATJ)) at 308 K (0-4000 psig). Measured isothermal bulk modulus is used to calculate the specific heat ratio of each test fuel by comparing it to the isentropic bulk modulus referenced from previous literature. Measurement results show that the bulk modulus of petroleum-based jet fuels is similar to 20%-25% lower than that of the petroleum-based diesel fuel: 984, 932, and 953 MPa for POSF 4658, POSF 6169, and POSF 10325, respectively, at 0.1 MPa. The bulk modulus of Farnesane (982 MPa at 0.1 MPa) was similar to that of petroleum-based jet fuels while those of the other alternative jet fuels were similar to 4% (HRJC) and 14% (ATJ) lower than that of the petroleum jet fuels. We also measured the pseudoisothermal speed of sound of these jet fuels using a pair of fuel line pressure sensors installed on the fuel supply line of a diesel engine in a temperature range between 318 K and 338 K. Results showed a lower pseudoisothermal speed of sound for both petroleum-based (1114-1122 m/s at 328 K) and alternative jet fuels (1038-1116 m/s at 328 K) relative to petroleum-based diesel fuel. Isentropic speed of sound estimated by applying the conversion factor to the pseudoisothermal speed of sound measurement showed a similar trend: a lower isentropic speed of sound for jet fuels relative to petroleum-based diesel fuel. This suggested an unintended retarding in the actual fuel injection timing when these jet fuels are used in a CI engine in lieu of regular diesel fuel without the adjustment in nominal fuel injection timing.

Automated summary

This study measured and compared the bulk modulus and speed of sound of different jet fuels, revealing that petroleum-based jet fuels have lower bulk modulus compared to petroleum-based diesel fuel, while alternative jet fuels have even lower bulk modulus. This difference may result in unintended retardation of actual fuel injection timing when using jet fuels in compression-ignition engines without adjusting the nominal fuel injection timing.