1. Field of the Invention
This invention relates to a spark plug voltage detector probe which is provided to detect a spark plug voltage waveform induced from a secondary circuit of an ignition circuit to be applied to a spark plug in an internal combustion engine.
2. Description of Prior Art
With the demand of purifying emission gas and enhancing fuel efficiency of internal combustion engines, it has been necessary to detect firing conditions in each cylinder of the internal combustion engine so as to protect the internal combustion engine against any type of misfire. In order to detect the firing condition in each of the cylinders, the applicant of the present patent application discloses a misfire detector device in Japanese Patent Appln. No. 3-314783. This device is based on the finding that the misfire in each cylinder is distinguished by detecting a voltage waveform across the spark plug, and analyzing its attenuation characteristics. On the basis of the finding, an electrode plate sensor is embedded in an insulator body which has a plurality of Grooves which have the same diameter as a spark plug cable has. The spark plug cable is placed in the Grooves to form static capacity between the electrode plate sensor and the cable. A lid plate is secured to the insulator body to fix the cable by means of a screw.
In another Japanese Patent Appln. No. 4-303859, an electrode plate is placed in an accommodation space filled with silicone rubber. A spark plug cable is sandwiched by a pair of rubber plates, and placed in Grooves which are diametrically equivalent to the spark plug cable. These rubber plates are interfit into a cavity provided in the accommodation space.
In these types of spark plug voltage detector probes, it is quinteessential to detect the change of the static capacity formed between the electrode plate and the spark plug cable. Upon inadvertently permitting an entrance of foreign matter such as oil, water, dust, soil and the like between the electrode plate and the spark plug cable, it is apparently difficult to precisely detect the true static capacity therebetween. In order to prevent the entry of the foreign matter, the diameter of each of the grooves is the same as the spark plug cable, and the spark plug cable is placed in the Grooves by means of adhesive. Otherwise, the rubber plates are used to ensure a certain tightness between the Grooves and the spark plug cable by their elasticity.
In the case where the rubber plates tightly sandwich the spark plug cable due to their elasticity, it is found that the tightness between the Grooves and the spark plug cable becomes insufficient particularly when placing a plurality of spark plug cables extended from the internal combustion engine.
In the case where the spark plug cable is placed in the Grooves of the insulator body by means of adhesive, it is necessary to disadvantageously replace the detector probe itself upon exchanging the spark plug cable.
Therefore, it is an object of the invention to provide a spark plug voltage detector probe for use in internal combustion engines and in which the component parts are readily replaceable, and for precisely detecting the characteristics of the spark plug voltage waveform applied to each spark plug mounted on the respective cylinders with a relatively simple structure.