The initiation of the pre-flame reaction of n-heptane in a motored engine

A motored, unfired CFR engine was fed with a rich heptane/air mixture. The jacket temperature was maintained at 100 degreesC with a few experiments at 0 degreesC. The exhaust vapours were totally condensed by a novel exhaust recovery system. After completely removing cylinder deposits, no reaction products were found at compression ratios up to 14:1. At a compression ratio of 16:1 the charge autoignited/detonated. Subsequently, reaction products were found at a compression ratio of 6:1 and thereafter in increasing quantities up to 14.6:1, the reproducible threshold of autoignition. This entire sequence of experiments was reproduced four times. Samples of the condensate were concentrated by vacuum distillation and subjected to qualitative and quantitative analyses. A very limited range of peroxides and carbonyls were formed in very low concentrations in all experiments where autoignition did not occur. These were identified and their rates of formation calculated. Some thermodynamic inferences were drawn as to their heats of formation. The maximum compression temperatures were calculated. At compression ratios below 16: 1, it was concluded that the preflame reaction was initiated on the surface of the combustion chamber, after it had been subjected to autoignitions/detonations.