The Threshold Sooting Index (TSI) of a blend is estimated based on a molar averaging of the pure component values. TSI is a fuel-independent derived quantity, originally established by Calcote and Manos [43], that correlates linearly with the smoke points of fuels:

(4–13)

Where, and are constants derived from statistical fits, is molecular weight for fuel, and is smoke point.

According to Calcote and Manos, TSI can be used to predict sooting tendencies in jet engines and from both diffusion and premixed flames. However, smoke-point measurements [44] use only diffusion flames. Gill and Olson [45] performed TSI measurements using pure, binary, and ternary fuel mixtures, and derived the best fit for Equation 4–13 with =3.32 (mol.mm/g) and =–1.47.