Firing TDC identification through wasted spark duration measurements during cranking of small engines

Implementing control techniques through “virtual sensors” is extremely attractive for small size engines, given that cost effectiveness is essential. This work presents a routine for identifying the firing TDC through measurement of spark duration. Previous capability of correctly identifying cycle phasing through this route was confirmed during normal operation of a power unit that featured a wasted spark ignition system. Starting with the hypothesis that this could be implemented during engine cranking, the procedure was adapted for identifying the firing TDC as quickly as possible; it was also developed with the specific task of requiring less time for synchronization, compared to the previous version. The new method was verified on a small size 50 cc single cylinder engine that featured a recoil starter mechanism. Correct identification was confirmed, with the possibility of generating the reference signal as early as the 2nd cycle that featured normal operation of the ignition system; to put things into perspective, a minimum of 5-6 such cycles were recorded during multiple cranking events. Compared to the rpm based evaluation, more consistent identification could be achieved, mostly due to the fact that the spark duration measurements were not influenced by the varying engine speed during cranking.