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.