DETECTION OF ABNORMAL FLAME FRONTS IN ROAD TESTS WITH AN ENGINE USING INDEPENDENT IONIZATION GAPS

Recently, a number of investigators (1), (2), and (3) have reported detection by means of ionization gaps of flame fronts other than those initiated by the spark discharge. Since some of the findings in these studies appeared to be contrary to road-test experience at the General Motors Research Laboratories, the significance of ionization-gap data was questioned. For example, Reference (1) shows abnormal flame fronts occurring in an engine in the absence of combustion chamber deposits, whereas in road testing, it is usually necessary to accumulate deposits before abnormal flame fronts become noticeable. Reference (2) shows data wherein abnormal combustion was unaffected by fuel octane number, while in road testing, objectionable abnormal combustion is usually eliminated by increasing fuel octane number.

In order to investigate these apparent anomalies, engine tests were made using special independent ionization gaps. In addition to studying the effects of deposits and fuel octane number on abnormal combustion, a comparison was made between ionization-gap and “ignition-off” detection of abnormal combustion, since the latter has been used extensively in road testing (4).