The process of incorporating the spark plug as a combustion probe, to perform misfire and knock detection, air to fuel ratio and spark timing control has been the subject of research for some time now. , . The feasibility of the approach however depends on being able to correlate some characteristic of the ion current signal to the in cylinder combustion process. Shimaski et al.  and Miyata et al.  suggest such a relationship.The objective of this research has been to extract combustion information from the measured ion current flowing between spark plug electrodes by using various advanced signal processing methods, and to develop a methodology that will permit combustion diagnostics and possibly control based on these measurements. Tests were carried out on a single-cylinder, methane-fueled CFR engine. Results indicate that a nonlinear correlation exists between ion current and combustion pressure and that the developed signal processing methods permit extraction of useful combustion information from the measured ion current signals.