MECHANISTIC STUDIES ON THE COMBUSTION OF ISOTOPICALLY LABELED CYCLOHEXANES WITHIN A SINGLE CYLINDER INTERNAL-COMBUSTION ENGINE

Hydrocarbon emissions from a single cylinder Ricardo Hydra research en gine have been analysed by gas chromatography-mass spectrometry for tw o isotopically labelled cyclohexane fuels: (1) an equimolar mixture of cyclohexane and cyclohexane-d(12), and (2) cyclohexane-1,1,3,3-d(4). Isotopic distributions of cyclohexane, cyclohexene, benzene, 1,3-butad iene and propylene within the exhaust gases have been used to investig ate possible mechanistic pathways in the formation of these emission s pecies. Results from the first fuel show that the exhaust cyclohexane consists entirely of unburnt fuel with a [C6D12]/[C6H12] ratio of 1.4, which indicates the presence of a kinetic isotope effect in the consu mption of cyclohexane. The isotopic distributions within the other spe cies are indicative of intramolecular decomposition pathways from the cyclohexyl radical. The exhaust benzene appears to be formed by succes sive dehydrogenations from cyclohexyl rather than by build up from sma ller molecular weight species. 1,3-Butadiene is formed by beta-scissio n of hexenyl species, whilst propylene is formed through an intramolec ular rearrangement, probably involving a methylcyclopentyl intermediat e. The second fuel shows that intramolecular hydrogen migration in the cyclohexyl radical is of minimal significance prior to its decomposit ion to cyclohexene. The isotopic distribution within the observed benz ene, however, indicates partial scrambling prior to formation. A preli minary experiment using toluene-d(8) fuel indicates that the condition ing fuel or its breakdown products may pray a role in the formation of exhaust hydrocarbons, particularly in samples taken shortly after the fuel changeover, but that the lubricating oil plays no significant ro le in the formation of these products.