Abstract: A spark plug for internal combustion engine, which produces a spark discharge that ignites air-fuel mixture in the combustion chamber and causes combustion such that an ionic current flow during combustion can be utilized for detecting misfire of the engine. The spark plug the core connected to an ignition coil, a center electrode made of at least one of iridium and an iridium alloy and connected to the core through a seat, and a ground electrode separated from the center electrode with a gap. A sum of surface areas of the seat and the center electrode is not less than a prescribed value, specifically, is not less than 11.0 mm2, and more specifically, is not less than 11.47 mm2. Even when the diameter of the center electrode is reduced in the interest of improving ignition performance, improved ignition performance and improved ionic current detection accuracy can be achieved simultaneously.

Abstract: A firing or ignition state discrimination system for an internal combustion engines having a spark plug which produces spark discharge when supplied with discharge current from an ignition coil to ignite air-fuel mixture in the combustion chamber. In the system, ionic-current that flows during combustion of the air-fuel mixture is detected at a predetermined first timing and leakage current that flows across electrodes of the spark plug is detected at a predetermined second timing which is later than the predetermined first timing. The firing or ignition state of the engine is discriminated to one from among normal, misfiring, spark plug fouling, and failure in ignition system or in fuel supply system based on the detected ionic-current and leakage current and presence or absence of fuel cutoff to the engine. With this, it becomes possible to accurately discriminates the firing or ignition state of the spark plug and to inform the fact to the vehicle operator if desired.

Abstract: An ignition coil unit to be installed in a hole opened through a cylinder head of an engine having a case, an ignition coil installed in the case and comprising a primary coil connected to a power source and a secondary coil connected to a spark plug which is installed in a combustion chamber of a cylinder of the engine for producing spark discharge to ignite air-fuel mixture to generate combustion thereof in the combustion chamber, and a circuit board on which an ionic-current detection circuit, connected to the secondary coil, which detects ionic-current that flows during the combustion of the air-fuel mixture; and a processing circuit, connected to the ionic-current detection circuit, which generates an output indicative of the detected ionic-current, are formed; In the unit, an igniter is also installed in the case, thereby improving detection accuracy of ionic-current and misfiring.

Abstract: A spark plug for internal combustion engine which produces a spark discharge that ignites air-fuel mixture in the combustion chamber and causes combustion such that an ionic current flowing during combustion can be utilized for detecting misfire of the engine. The spark plug comprises the core connected to an ignition coil, the center electrode made of iridium or an iridium alloy and connected to the core through a seat and the ground electrode grounded separated from the center electrode with a gap. The characteristic feature is that a sum of surface ares of the seat the center electrode is defined as not less than a prescribed value, specifically, is not less than 11.0 mm2, more specifically is not less than 11.47 mm2. With this, even when the diameter of the center electrode is reduced in the interest of improving ignition performance, the ionic current detection accuracy can nevertheless be enhanced.

Abstract: A firing or ignition state discrimination system for an internal combustion engines having a spark plug which produces spark discharge when supplied with discharge current from an ignition coil to ignite air-fuel mixture in the combustion chamber. In the system, ionic-current that flows during combustion of the air-fuel mixture is detected at a predetermined first timing and leakage current that flows across electrodes of the spark plug is detected at a predetermined second timing which is later than the predetermined first timing. The firing or ignition state of the engine is discriminated to one from among normal, misfiring, spark plug fouling, and failure in ignition system or in fuel supply system based on the detected ionic-current and leakage current and presence or absence of fuel cutoff to the engine. With this, it becomes possible to accurately discriminates the firing or ignition state of the spark plug and to inform the fact to the vehicle operator if desired.

Abstract: An ignition coil unit to be installed in a hole opened through a cylinder head of an engine having a case, an ignition coil installed in the case and comprising a primary coil connected to a power source and a secondary coil connected to a spark plug which is installed in a combustion chamber of a cylinder of the engine for producing spark discharge to ignite air-fuel mixture to generate combustion thereof in the combustion chamber, and a circuit board on which an ionic-current detection circuit, connected to the secondary coil, which detects ionic-current that flows during the combustion of the air-fuel mixture; and a processing circuit, connected to the ionic-current detection circuit, which generates an output indicative of the detected ionic-current, are formed; In the unit, an igniter is also installed in the case, thereby improving detection accuracy of ionic-current and misfiring.