Correlation between Spark Ignition Characteristics and Flame Development in a Constant-Volume Combustion Chamber 920413

The electrical characteristics of transistorized coil ignition (TCI) and capacitor discharge ignition (CDI) systems were investigated in spark-ignited quiescent and flowing propane/air mixtures within an optically-accessible, cylindrical constant-volume combustion chamber. Under quiescent flow conditions, the initial pressure, temperature and equivalence ratio of the mixture as well as the spark gap width and geometry were varied systematically in order to examine the relationship between ignition characteristics and flame initiation and development. The effect of the flow in the spark gap on the electrical characteristics of the ignition system, mixture ignitability and flame development was also examined by varying the pre-ignition mean flow and turbulence as well as the spark plug orientation relative to the mean flow.

Under quiescent flow conditions and despite differences in the electrical characteristics of the two ignition systems examined, TCI and CDI gave rise to similar flame development which implies the absence of a correlation between breakdown/total energy and early flame development; the TCI system, however, with its longer spark duration and higher breakdown energy, allowed extension of the lean ignition limit especially at large spark gaps. For a given ignition system, lower initial mixture pressure, higher initial temperature and wider spark gaps resulted in faster flame propagation.

Under flowing mixture conditions, combustion duration was shortened and the lean limit was extended when the mean flow and turbulence in the spark gap were high provided the orientation of the ground electrode was not in the upwind side of the mean flow direction. As the flow velocities were reduced, the effect of spark plug orientation on ignitability became smaller.

IN SPARK-IGNITION ENGINES, the mechanism of transferring electrical energy from a given ignition system into the mixture in the spark gap is controlled by the thermodynamics of the mixture, the local flow characteristics and the spark plug geometry, e.g.[1,2].* The net total energy deposited into the mixture during the ignition process, which is a fraction of the energy supplied by the spark, and the rate of deposition affect the early flame development which, in turn, is correlated with the burn duration in both quiescent and flowing gasoline/air mixtures.

The ignition criteria vary between quiescent and flowing mixtures. In quiescent mixtures, the energy supplied by the ignition system per unit mixture volume per unit time and the energy deposition duration are the determining factors while in flowing mixtures the above criteria become a function of the mixture velocity in the vicinity of the spark plug. This uncertainty concerning the amount of energy actually deposited into the mixture is creating difficulties in interpreting data in the literature which are mainly based on the energy available in the ignition system.

Understanding the mechanism of energy transfer into the mixture during the three discharge modes of the ignition process [2,3] and the associated energy losses from the early flame kernel to the electrodes [4,5], requires simultaneous measurement on a cycle-to-cycle basis of the time-resolved voltage and current during discharge, of the local flow and mixture strength in the spark gap and of the kernel/flame development in the combustion chamber over a wide range of engine operating conditions. Due to the difficulties involved in performing such simultaneous multi-parameter experiments, engine researchers have attempted to isolate the effects of mixture and flow variations on flame initiation and propagation by examining quiescent or well-premixed mixtures in constant-volume chambers [6,7] and performing experiments in well-characterised engine geometries [8,9].

Along these lines, the present study examines the relationship between the electrical characteristics of spark ignition and flame development in quiescent and flowing propane/air mixtures based on single-shot measurements of electrical characteristics, flame propagation speed and pressure in a constant-volume chamber using transistorized coil ignition (TCI) and capacitor discharge ignition (CDI) systems. The varying energies, discharge periods and associated discharge efficiencies of these two systems allow investigation of the cause and effect relationship between electrical characteristics and flame initiation and propagation.

The present investigation was considered necessary prior to attempting correlation studies in a high-turbulence, four-valve spark-ignition engine in order to allow a reduction of the parameters of interest and better focussing on the lean-burn limit where the problems of misfire and increased cyclic variations and unburned hydrocarbon emissions are still unresolved.

The constant-volume chamber and the experimental techniques employed to characterise the ignition system, flame initiation and development are described in the next section followed by discussion of the results and conclusions.