Mass density (ρ) is a fundamental physical property of a fuel that determines its energy content. It is frequently used as a target parameter when formulating fuel surrogates . It may also impact soot formation in advanced engine concepts . Mixing rules are more easily formulated for specific volume (ν), which is the inverse of ρ, since ν is a property that increases with more substance. For an ideal solution, the specific volume of the mixture νmix is given by