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The interaction between spark discharge and low-temperature oxidation (LTO) was investigated using an optical compression and expansion machine fueled with n-C7H16 or i-C8H18 for an equivalence ratio of 0.33. Charge pressure was adjusted so that the compression stoke could induce LTO for n-C7H16, but could not lead to high-temperature reactions. A spark was discharged in the field before, during, or after the LTO for n-C7H16 or in the field without LTO for i-C8H18. Reaction zones were induced in the field after the LTO, whereas no reaction zones were induced in the fields before the LTO and without LTO. Local ignitions were induced in the areas surrounding the propagating reaction zones. The reaction zone propagation with the low equivalence ratio must be a different phenomenon from conventional flame propagation. The reaction zones can compress or heat the surrounding areas containing H2O2 and CH2O, and accelerate an H2O2 regeneration loop in the pre-reaction zones.

Formaldehyde artificially added into hydrocarbon/air mixtures is efficacious as an ignition control medium for the piston engines. The vapor added into the mixtures gives the premixed compression-ignition (HCCI) engine a stable ignition timing. It allows us to get the MBT controllable by the amount of formaldehyde to be added. The added formaldehyde could be a sole controlling factor to the ignition timing. Depending on the temperature to which mixtures are compressed, the effect of formaldehyde adulteration is either suppressing or promoting. The effect of formaldehyde addition is suppressing on the ignition events belonging originally to the cool-flame dominant ignition. In the preflame induction period of cool-flame dominant ignition the added formaldehyde is superfluous to the cool-flame generating processes.