This invention relates to internal combustion engines, and more particularly to an internal combustion engine ignition device having a radio noise preventing device in the low voltage side circuit of an ignition circuit thereof.
It has been well known in the art that the main cause for radio noise generated by the ignition system of an internal combustion engine is a capacity discharge current generated during the spark discharge of a spark plug. It has been considered in the art that this capacity discharge current is attributed to the spark discharge of the spark plug, and therefore it has been considered that the capacity discharge current flows in the spark plug side, or the high voltage side of the circuit. Accordingly, a radio noise preventing device for suppressing the capacity discharge current in the high voltage side is provided for the high voltage side only in order to prevent the radio noise. However, it has been found that in the conventional ignition device sometimes the capacity discharge current I.sub.1 in the low voltage side circuit becomes larger than the capacity discharge current in the high voltage side circuit. This will be described in detail with reference to FIG. 3.
In conventional ignition circuits shown in FIGS. 1 and 2, the capacity discharge current I.sub.1 in a low voltage side circuit comprising a battery 1, the primary side of an ignition coil 2, and an interrupter 3, and the capacity discharge current I.sub.2 in a high voltage side circuit comprising a spark plug 4 connected to the secondary side of the ignition coil 2 were measured by using different noise preventing devices 5 in the high voltage side circuit with the following results: In FIG. 3, reference character (a) indicates the case where an ordinary spark plug was employed without using a noise preventing device as shown in FIG. 1, and reference characters (b) through (e) indicate the cases where a spark plug with a 1 K.OMEGA. resistor, a spark plug with a 5 K.OMEGA. resistor, a spark plug with a 5 K.OMEGA. resistor and a cap with a 5 K.OMEGA. resistor, and a 5 K.OMEGA. winding shield cap and a spark plug with a 5 K.OMEGA. resistor were employed as shown in FIG. 2, respectively. The measurement values of the capacity discharge currents I.sub.1 and I.sub.2 in the various cases described above are as indicated in FIG. 3. As is apparent from the graph shown in FIG. 3, in the case where the resistance value of the high voltage side noise preventing device is changed or a noise preventing device having an inductive component is provided in the circuit, (1) the low voltage side current I.sub.1 decreases in proportion to the value of the resistor series-connected to the high voltage side if the value is within approximately one K.OMEGA.; however, it does not decrease further even if the value of the resistor is increased. Furthermore, (2) in the case where an excellent noise preventing device is connected to the high voltage side, the value of the current I.sub.1 tends to become larger than the value of the high voltage side current I.sub.2. Thus, even if the noise preventing device is provided in the high voltage side as in the conventional method, it is impossible to obtain a satisfactory noise preventing effect when the current I.sub.1 becomes larger than the current I.sub.2.