No. 251

1970

On the Principles of Designing Ignition Systems

Nanin

(Presented by the scientific seminar of the Research Institute of High Voltages at the Tomsk Polytechnic Institute)

One of the important aspects in the technology of igniting combustible mixtures in fuel-combustion systems, in techniques for investigating the explosion hazard of vapor-gas systems, and in a number of other electrophysical studies and tests is the problem of creating the most expedient ignition devices. The importance of the subject is illustrated, in particular, by the recently published work [1], in which the current state of the theory and practice of existing systems is systematically set forth.

Both in fuel-combustion systems and in studies of the explosion hazard of gas systems, the most widespread are electrical ignition systems, specifically spark-discharge sources. (The term spark-discharge ignition sources should be understood to mean devices in which the ignition pulse is a spark or an arc—regardless of the method by which they are produced: magneto, battery, high-frequency, spark-capacitor circuits, circuits with a discharge along a surface, etc. Unlike those listed, hot-wire systems—with heating of igniting bodies by Joule heat—are considerably less widespread.)

For fuel-combustion systems, spark-discharge sources are convenient because of their compactness and reliability [1]; for devices for testing vapor-gas mixtures for explosion hazard, they, in addition to the above advantages, imply increased accuracy in measuring the parameters of the spark-discharge pulse that determine its energy content (current and voltage) and, importantly, in many cases reproduce real (potential) ignition pulses characteristic of modern electrical and electronic systems in various fields of technology: short-circuit arcs, lu-

… arcs and sparks of normal and abnormal commutations, discharges of static electricity, etc.

An analysis of the present state of the theory and practice of ignition systems and apparatus, based on the literature, shows that at the present time neither in the field of systems for fuel combustion nor in the field of devices for testing vapor–gas systems for explosion hazard have principles been developed for the construction of ignition apparatus. In the work already mentioned [1], in particular, it is not stated what the form and duration of the most effective ignition pulse should be—that is, a pulse that reliably ignites a mixture at some minimum energy content. Knowledge of the parameters of the most effective spark-arc pulse can place work on the construction of ignition systems on a principled basis and will undoubtedly promote progress in this field.

A similar situation is also observed in work on the study of the combustible properties of vapor–gas systems and, in particular, their explosion hazard. It is known that one of the main characteristics of the explosion hazard of a mixture is the minimum energy of its ignition, which in its significance approximates a physical characteristic of the given mixture. Knowledge of the minimum energies makes it possible to carry out the most reliable detuning of explosion-hazardous systems from ignitions by spark-arc (and not only by such) pulses, for example in creating intrinsically safe circuits. However, in practically none of the experimental works on estimating minimum ignition energies have conditions been realized under which ignition was achieved at the smallest energies (of course, what is meant here are the conditions determined by the ignition source).

Meanwhile, from the works of Ya. B. Zel’dovich [2], V. Lewis and G. Elbe [3,4], Jost [5], and others, it follows, in essence, that the ignition process, like any other inertial process occurring in space and in time, is naturally predisposed to optimization: a certain amount of energy introduced into a mixture at a low rate and, consequently, over a long time may fail to ignite the mixture because of the large heat transfer to the surrounding medium; the same amount of energy introduced into the mixture at a very high rate may also fail to ignite the mixture because of…

the inertia of the flame-formation process. Accordingly, there exists a certain intermediate rate of energy input that “resonates,” in a certain sense, with the process of growth of the initial flame kernel and optimizes ignition—at this rate the probability of ignition is maximal.

Moreover, in [6, 7] a measure of the inertia of formation of a stable flame was essentially determined: it was established that a flame is formed if its initial formation—the flame kernel—reaches certain critical dimensions; the time required to reach these dimensions is \(10^{-4}\) sec.

Thus, by the present time a stable opinion has developed that the greatest ignition efficiency should occur when the time of energy input into the mixture is of the order of \(10^{-4}\) sec, [8] et al.

In light of the foregoing, it seems obvious that apparatus and devices should be constructed in such a way that the ignition pulses act during the same indicated time. In this case they will approach the most effective ones.

It is another matter that, up to the present time, the optimal form of the energy pulse has not been established: should the instantaneous power of the pulse increase continuously—right up to the end of initiation—or, conversely, is increased power advisable at the beginning of the discharge with the possibility of decreasing it by the moment when a flame kernel of critical dimensions is formed; or, perhaps, should the instantaneous power of the pulse be constant from the beginning to the end of ignition? Both the first, the second, and the third variants are problematic—special theoretical and experimental developments are necessary.

It may nevertheless be assumed that a number of questions concerning effective ignition of gas-vapor systems can be solved by using a rectangular (or nearly rectangular) form of the ignition pulse, i.e., with constancy of the instantaneous power of the ignition pulse throughout the entire initiation process, or—what is the same thing—with a constant rate of energy input into the mixture.

In any case, using this model it is possible to obtain a completely unambiguous solution to the question of the optimal rate of energy input and, correspondingly, of the minimum ignition energy of the given mixture.

What is the possible practical payoff from implementing the rectangular model?

If, in the course of investigations of mixture ignition using the rectangular model, a substantial reduction in the ignition energy is established for pulses of duration \(10^{-4}\) sec in comparison with the minimum ignition energy established for pulses of microsecond and millisecond duration, the question will arise of the practical implementation of the optimal ignition pulse in working fuel-ignition systems.

The implementation of the rectangular model of the ignition pulse and its use in investigations of the explosion safety of vapor-gas systems will make it possible, first, to establish the threshold of the smallest energies still capable of igniting the system, and, second, to investigate—practically under ideal conditions—the stability of the concentration ignition limits of mixtures under the action of spark pulses with a large range of energies and durations.

The proposed range of experimental work on the search for the optimal form of the ignition pulse on the basis of the rectangular model does not, of course, exclude experimental and theoretical work to seek it using other models. The importance of these works for formulating the principles of designing ignition systems appears unquestionable.

REFERENCES

1. V. A. Balagurov, Ignition Apparatus, Mashinostroenie Publishing House, Moscow, 1968.

2. Ya. B. Zel’dovich, N. N. Semenov, ZhFKh, 23, No. 11, 1949.

1. B. Lewis, G. Elbe, Combustion, Flames and Explosions of Gases, No. 4, 1951.

2. B. Lewis, G. Elbe, “Basic Principles of Ignition and Flame Propagation,” in Problems of Combustion of Reactive Fuels, Moscow, 1959.

3. W. Jost, Zeit. Phys. Chem., 196, 4, 1950.

4. D. R. Lintin, E. R. Wooding, “Investigation of the Ignition of a Gas by an Electric Spark,” British Journal of Applied Physics, 10, Apr., 1959.

5. D. Arnold, R. Sherburne, “Investigation of Ignition in the Initial Stage of Flame-Kernel Development in Hydrocarbon–Air Mixtures,” Problems of Combustion, IV International Symposium on Combustion and Detonation Waves, 1958.

6. E. S. Shchetinkov, Physics of Gas Combustion, Nauka Publishing House, Moscow, 1965.