Wasted spark duration measurement as a method for firing TDC identification in small engines

Distributed generation represents a valid option for reducing transmission losses and overall power system complexity. Back-up power is another option that can ensure valuable safety margins in the scenario of extensive penetration of renewables, and can also act as balancing sources. Spark ignition (SI) engines are often the prime choice for such applications with sizes ranging from single cylinder configurations to large bore units. Augmenting control margins without increasing the number of sensors is an essential aspect for small size engines. Common practice for such power units is to employ fixed ignition timing with so called wasted spark systems, i.e. two events per cycle one during compression and one during the exhaust stroke. Nonetheless, application of more complex control systems such as fuel injection are becoming more and more widespread even for this engine category. Within this context, a method was developed for identifying TDC phasing based on current measurements in the secondary ignition circuit for engines that feature wasted spark systems. The method was applied on a 50 cc SI unit connected to a 1 kW power generator. The only modification to the ignition system was to substitute the coil with an automotive type piece; this was required because the original component featured common ground wiring for the primary and secondary side, thus rendering current measurements much more difficult to implement. Duration of the actual spark event was found to be a good parameter for distinguishing between ignition occurring during the compression stroke or during exhaust. Statistical distribution during sequences of 200 cycles recorded at three different levels of load was used for testing the identification algorithm. The developed method was verified with pressure measurements to distinguish between the two events, i.e. compression and exhaust. Limitations of the method are discussed and the possible ways of improving its application were identified.