Abstract
Formaldehyde artificially added into the mixtures is efficacious as an ignition control medium for hydrocarbon fuels in engine cylinders. The vapor added into the mixtures has given the premixed compression-ignition (HCCI) engine a stable ignition timing, which is controllable to get the MET by the amount of formaldehyde to be added. The effect of formaldehyde addition is either suppressing or promoting. A universal criterion of the formaldehyde effect resulting whether in the advanced or in the retarded hot-flame appearance from the original ignition events has been elucidated through experiments over a wide range of parameters on the fuel octane or butane rating, the equivalence ratio of the mixture, and the mixture temperature to be raised. The formaldehyde would be a suppressing additive under the cool-flame generating constituents, temperature and pressure conditions, and a promoting additive under the poor cool-flame generating conditions in the cylinder charge during the preflame reactions leading to the ignition. The added formaldehyde is superfluous to the ignition event originally belonging to the cool-flame dominant regime, which would give a suppressing effect. On the other hand, the artificially added formaldehyde could be a starting point of preflame reactions leading to the final hot ignition belonging to the blue-flame-dominant regime, where a few fuel transition and low intermediates appear, and where the high temperature chemistry dominates. It allows a short cut of the preflame induction period, in consequence, a promoting effect for the ignition. The low-temperature-ignition classification; three typical regime, "cool-flame dominant regime", "negative-temperature-coefficient regime" and "blue-flame dominant regime" would be a universal advanced/retarded criterion on ignition of fuel/air mixtures with formaldehyde doping for the ignition timing control in the piston compression engines.
