Fuel combustion additives: A study of their thermal stabilities and decomposition pathways

Cetane improvers are relatively unstable molecules, and their decomposition at low temperatures produce free radicals. It has been suggested that these radicals speed up the rate of fuel autoxidation and help to initiate combustion. The relationship between thermal stabilities of selected fuel additives and their effectiveness as diesel fuel cetane improvers was studied. The additives were 2-ethylhexyl nitrate, isopropyl nitrate, tetraethylene glycol dinitrate, di-tert-butyl peroxide, and methylcyclopentadienyl manganese tricarbonyl. The thermal decomposition rates of the additives were measured, and their decomposition mechanisms were elucidated by identifying their thermolysis products. The thermal stability studies of each compound were performed in a solvent mixture designed to approximate a typical diesel fuel, in two full-distillate diesel fuels, and in pure hydrocarbons. In all cases, decomposition kinetics were first-order. Neither relative thermal stability nor heat release measured in the solution phase can be directly correlated with in-cylinder cetane improver effectiveness. Unlike cetane effectiveness, additive thermal stability was independent of fuel. It appeared that cetane effectiveness correlated with fragmentation of the additive molecule, a set of reactions that occurred after the rate-limiting initial bond cleavage