Two-stage ignitions during rapid compression: spontaneous combustion in lean fuel-air mixtures

Abstract

The two-stage ignition of acetaldehyde + oxygen-nitrogen (‘air’) mixtureis investigated under rapid compression. The conditions are parallel to those brought about during the compression stroke in spark ignition and diesel engines. An approximately tenfold compression of pre-mixed gases is brought about mechanically. The time for rapid motion of the piston is 22 ms, and gases are ultimately compressed into a cylindrical volume, length 2.10 cm and diameter 4.50 cm. When air is used the temperature of the gases at the end of compression but before reaction is about 580 K. Heat losses are important in the stages following rapid compression. In a novel development, a miniature turbine is fitted to the combustion chamber. It is used to en­hance cooling rates. Two-stage combustion occurs in the post-compression period. There is a delay before the first stage is perceptible. The first stage itself is only mildly exothermic ( ca . 100 W cm -3 ) and the temperature rises associated with it are not large (∆ T < 150 K). These properties are not greatly affected by initial composition or by the temperature reached during compression. The second stage, however, is very strongly affected in intensity by such changes. Increases of mixture strength and of compressed gas temperature enhance both the extent and the rate of temperature change. For example, the second stage of combustion is weak in very lean mixtures ( ϕ < 0.25) and the approach to the maximum tem­perature is sufficiently slow for reaction to be markedly non-adiabatic; however, with mixtures for which ϕ > 0.5, very vigorous reaction occurs, and flame temperatures achieve adiabatic values. The total delay time before second-stage ignition is least for a mixture with ϕ = 0.6. This minimum reflects differing responses to mixture strength of its two constituent times. The duration of the first phase (ז 1 ) in which there is no measurable heat output, does not vary much in very lean mixtures, but it lengthens in the range 0.5 < ϕ < 1.0. The effect may be attributed to a small diminution in compressed gas temperature due to a decrease in γ . The duration of the second phase (ז 2 ) in which two-stage heat release is observed falls very sharply as the mixture strength is increased, and for ϕ > 0.5 it is negligible. However, the second phase is lengthened substantially when heat-transfer rates are artificially enhanced; the second stage of two-stage ignition may even be quenched in consequence. This is a most important and novel conclusion of the present study.