Abstract:
An igniter for an internal combustion engine has a plurality of drive circuits for generating an ignition voltage in the secondary winding of an ignition coil. A high-voltage diode is connected in series between the secondary winding of the ignition coil and a spark plug. The diode prevents destructive interference between the ignition voltages generated in the secondary winding for successive firings of the spark plug.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an igniter for an internal combustion engine. More particularly, it relates to an igniter for a multi-channel ignition system. 
     A multi-channel ignition system has a plurality of drive circuits connected to a single ignition coil. By individually controlling the drive circuits, the interval between successive firings of a spark plug can be adjusted. A drive signal from each drive circuit is applied to the same ignition coil. In a conventional multi-channel ignition system, when the interval between two successive firings becomes small, the drive signal from one drive circuit may overlap the drive signal from the other drive circuit, causing destructive interference in the secondary winding of the ignition coil. As a result, an ignition voltage having a desired peak value can not be generated in the secondary winding for the second of two successive firings, so the spark plug can not generate a good spark for the second firing. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an igniter for an internal combustion engine which can prevent destructive interference between the drive signals for successive firings of a spark plug. 
     An engine igniter in accordance with the present invention has an ignition coil and a plurality of drive circuits for generating ignition voltages in the secondary winding of the ignition coil. The secondary winding of the ignition coil is connected to a spark plug by a high-voltage diode which prevents a positive voltage from being generated in the secondary winding. When the interval between successive firings of the engine becomes small and ignition voltages in the secondary winding of the ignition coil overlap one another, since both voltages are of the same polarity, there can be no destructive interference between the two ignition voltages. Therefore, a high peak voltage can be generated in the secondary winding for each ignition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of an embodiment of an engine igniter according to the present invention. 
     FIGS. 2(a-h) are a wave form diagram showing the output signals of various portions of the embodiment of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of an engine igniter according to the present invention will now be described while referring to the accompanying drawings. FIG. 1 is a circuit diagram of this embodiment, which is a two-channel igniter for double ignition. The igniter has a drive circuit for a first channel and an identical drive circuit for a second channel. The reference numerals for the elements of the first channel drive circuit are affixed with the letter &#34;a&#34;, and the reference numerals for the elements of the second channel drive circuit are affixed with the letter &#34;b&#34;. As the two drive circuits have the same structure, only that of the first drive circuit will be described. As shown in this figure, a timer 1a for the first channel is connected to an unillustrated engine controller which provides it with ignition timing signals. The timer 1a generates an output pulse having a prescribed pulse width which is input to the base of a drive transistor 2a. The collector of the drive transistor 2a is connected to one end of the primary winding of a step-up transformer 3a. A battery voltage B+is applied to the other end of the primary winding of the step-up transformer 3a. The emitter of transistor 2a is grounded. The above-described structure constitutes a voltage booster circuit (a DC--DC converter). One end of the secondary winding of transformer 3a is connected to one end of an energy storage capacitor 4a. The other end of the secondary winding of transformer 3a is grounded. A high-resistance resistor 6a is connected across capacitor 4a. One end of resistor 6a is connected to the anode of a diode 5a and the cathode of an interference-prevention diode 7a for preventing interference between the first and second channels, and the other end of resistor 6a is connected to the anode of another diode 10a. The anode of interference-prevention diode 7a is connected to the primary winding of an ignition coil 8. The secondary winding of the ignition coil 8 is connected to the cathode of a high-voltage diode 16, the anode of which is connected to a spark plug 9. 
     An ignition timing signal train is transmitted from the unillustrated engine controller by a signal bus 11 and is provided to a timer circuit 12a for the first channel. The timer circuit 12a generates an output pulse of a prescribed width which is applied to the base of transistor 13a and which controls the discharge of capacitor 4a. The collector of transistor 13a is connected to the cathode of diode 10a while its emitter is grounded. 
     The operation of this embodiment will be explained while referring to FIG. 2, which is a wave form diagram of the output signals of various portions of this embodiment. Lines (a) and (b) of FIG. 2 illustrate ignition timing signals which are input to the first timer 1a and the second timer 1b by the unillustrated controller. The controller varies the interval between successive timing signals in accordance with engine operating conditions. The first and second timers 1a and 1b are triggered by the ignition timing signals and generate output pulses of a prescribed width as shown in lines (c) and (d). These pulses are used to gate the drive transistors 2a and 2b, which conduct for the duration of the output pulses. When the levels of the output pulses of the timers 1a and 1b fall and drive transistors 2a and 2b stop conducting, a voltage is generated in the secondary windings of step-up transformers 3a and 3b, and capacitors 4a and 4b are charged with the illustrated polarity by charging currents which flow in the directions shown by lines 14a and 14b, respectively. When the capacitors 4a and 4b have been charged, in response to signals which are provided by the input bus 11, the first timer circuit 12a and the second timer circuit 12b each generate an output pulse of a prescribed width. These pulses cause transistors 13a and 13b to conduct, upon which capacitors 4a  and 4b discharge their stored energy by the paths illustrated by lines 15a and 15b, respectively. Lines (e) and (f) illustrate the voltages across capacitors 4a and 4b, respectively, as they charge and discharge. When the capacitors 4a and 4b discharge, a high voltage is generated in the secondary winding of the ignition coil 8, and this voltage causes the spark plug 9 to fire. 
     Line (g) of FIG. 2 shows the voltage which would be generated in the secondary winding of the ignition coil 8 of a conventional engine igniter, in which the secondary winding of the ignition coil 8 is connected directly to the spark plug 9. In the absence of the high-voltage diode 16, current could flow through the secondary winding in both directions, so the voltage generated in the secondary winding of the ignition coil 8 would have both a positive and a negative component. When the intervals between two successive ignitions grow smaller, i.e., when the intervals between the timing signals for the first and second channels shown in lines (a) and (b) decrease, the ignition voltage in the secondary winding due to the discharge of capacitor 4b would begin to overlap the ignition voltage in the secondary winding due to the discharge of capacitor 4a, and the positive component of the voltage due to capacitor 4a would partly cancel out the negative component of the voltage due to capacitor 4b. As a result, the peak voltage for the second ignition would be greatly reduced, and a good spark could not be produced for the second ignition. 
     In contrast, in the present invention, the high-voltage diode 16 which is connected between the secondary winding of the ignition coil 8 and the spark plug 9 prevents a positive voltage from being generated in the secondary winding. Line (h) of FIG. 2 shows the ignition voltage in the secondary winding. It can be seen that because of the diode 16, the ignition voltage has only a negative component. Therefore, even when the interval between two successive ignitions grows small, the ignition voltages for the first ignition does not destructively interfere with the ignition voltage for the second ignition, and a high peak voltage can be obtained for both ignitions. Accordingly, a good spark can be generated by the spark plug for both ignitions. 
     The illustrated embodiment has only two different channels, but an engine igniter according to the present invention can have a larger number of channels.