Abstract: To obtain higher current yield from ion current measurable across the electrodes of a spark plug upon application of a biasing potential thereto, occurring immediately after an ignition event due to ionization of burning hydrocarbons within the combustion chamber of an internal combustion engine (ICE), the polarity of the circuit including a spark plug is so selected that the smaller one of the electrodes, typically the center electrode of the spark plug, has a positive measuring voltage and positive ignition voltage applied thereto so that the positively charged ions, which have a substantially greater mass than the electrons, will be caused to flow to the negatively charged substantially greater electrode, thus resulting in an enhanced ion current. A standard automotive ignition circuit can be used, with the battery connected, as customary, with negative-ground (FIG.

Abstract: The output of a combustion chamber sensor is analyzed in two succeeding time intervals defined in term of crankshaft position by use of a reference mark. The sensor output is compared with prescribed threshold values in the respective intervals, and a logic circuit recognizes whether normal combustion, slow combustion or a combustion failure is present and an engine control, such as fuel injection or ignition timing, or a combination of both is modified accordingly.

Abstract: A light guide element, secured in a spark plug-like housing and positioned, for example, within a metallic tube forming a spark plug connector, is coated at the end facing the combustion chamber with a light-impervious material viz: graphite, the graphite emitting radiation upon influence of heat arising within the combustion chamber, which radiation is transmitted to the optical pick-up for subsequent photo-electric evaluation thereof. The optical pick-up is, for example, a quartz glass rod coupled to a light guide cable, or directly coupled to an opto-electrical transducer.

Abstract: In order to eliminate from an ion current sensor signals which are not derived from knocking of an engine, the ion current sensor signal is evaluated by a high-pass - low-pass filter to filter out signals of frequencies extraneous to knocking frequencies, using, preferably, a digital filter including two timing stages (11, 13) connected to a counter; to inhibit response of the circuit, an interrupter switch (8) is included in the evaluation circuit, which is opened upon sensing of disturbance signals due to ignition events, for example upon occurrence of ignition, by setting a timing stage (20) which maintains the switch open, and thus inhibits evaluation of oscillatory signals from the ion current sensor (1) for the timing duration thereof.