The Spark Energy And Duration

With a homogeneous mixture in the in the cylinder, spark energy of the order of 1 mJ and a duration of a few micro-seconds would suffice to initiate the combustion process. However, in practice, circumstances are less than the ideal. The pressure, temperature and density of the mixture between the spark plug electrodes have a considerable influence on the voltage required to produce a spark. Therefore, the spark energy and duration are to be of sufficient order to initiate combustion under the most unfavourable conditions expected in the vicinity of the spark plug over the complete range of engine operation. Usually, if the spark energy exceeds 40 mJ and the duration is longer than 0.5 ms, reliable ignition is obtained. If the resistance of the deposits on the spark plug electrodes is sufficiently high, the loss of electrical energy through these deposits may prevent the spark discharge.

Energy Requirements For Ignition

The total enthalpy required to cause the flame to be self sustaining and promote ignition, is given by the product of the surface area of the spherical flame and the enthalpy per unit area. It is reasonable to assume that the basic requirement of the ignition system is that it should supply this energy within a small volume. Further, ignition should occur in a time interval sufficiently short to ensure that only a negligible amount of energy is lost other than to establish the flame. In view of this last mentioned condition, it is apparent that the rate of supply of energy is as important factor as the total energy supplied.
A small electric spark of short duration would appear to meet most of the requirements for ignition. A spark can be caused by applying a sufficiently high voltage between two electrodes separated by a gap, and there is a critical voltage below which no sparking occurs. this critical voltage is a function of the dimension of the gap between the electrodes, the fuel air ratio and the pressure of the gas. Additionally, the manner in which the voltage is raised to the critical value and the configuration and the condition of the electrodes are important in respect of the energy required.
An ignition process obeys the law of conservation of energy. Hence, it can be treated as a balance of energy between:

1. that provided by an external source,
2. that released by chemical reaction and,
3. that dissipated to the surroundings by means of thermal conduction, convection and radiation.