Ion formation in compression ignition low-temperature flames.

Abstract

Ion forrmation is detected for the first time in cool and blue flames which appear during the preflame period for compression ignition of a heptane / air mixture. A rapid-compression machine is used for mixture compression and an ionization gap is installed in the combustion chamber. Ion current and emission of active species are observed simultaneously. Ions arise at the time the cool flame degenerates, and the ion current itself will degenerate very late in the blue-flame induction time τ2 just before the blue-flame onset. The behavior of ion-current appearance in a cool flame shows the possibility that the ion is mutually related to the emission of excited oxygen molecules and the generation of olefin. During the blue-flame period the ion current is always detected more definitely. Degeneration of the ion current is often observed late in the blue-flame period. The occurrence of ion-current degeneration after the blue flame comes about can be recognized as a prognostication for the development up to the final real ignition.