Ignition System

The present invention relates generally to ignition systems for internal combustion engines and in particular to improvements in ignition systems of the capacitor discharge type.

In order to reduce the complexity, the number of component parts, the package size, and the cost of capacitor discharge ignition circuits, various circuit arrangements have been tried in an attempt to improve such circuits without sacrificing desired operating performance and reliability. In accordance with such objectives, the prior art includes various circuit arrangements for charging the capacitor during a portion of the engine operating cycle and thereafter discharging the capacitor, through an SCR or the like, at a desired time in the engine cycle.

The present invention is directed to improvments in capacitor discharge ignition systems, and particularly, to a capacitor discharge ignition system having novel features which result in important advantages over prior art capacitor discharge ignition systems. One of these features is the utilization of a single winding for both charging the capacitor and for providing a trigger signal for the SCR whereby the cost and complexity of the system are reduced. Another of these is the provision of an ignition coil on the stator structure having the charge/trigger winding so that the primary winding of the ignition coil is energized, not only by the discharge of the capacitor, but also simultaneously by the magnetic field used to induce a charging current in the charge/trigger winding so as to increase the power supplied to the spark plug. Another feature is a provision in the circuit for resonant charging of the capacitor by which substantially maximum potential on the capacitor is attained at the time at which the capacitor is discharged so as to eliminate the necessity of the usual charging diode for the capacitor. And yet another feature is the connection of the charging coil in series with the discharge path for the capacitor so that the charge winding current augments the current from the discharging capacitor to further improve the potential supplied to the spark plug. Further novel features involve the construction and arrangement of the magnetic circuit structure via which the system is energized and various triggering circuits for the SCR through which the capacitor is discharged into the primary of the ignition coil. By virtue of the several embodiments of the invention disclosed herein, a capacitor discharge ignition system can provide excellent performance over a wide range of engine speeds without the complexity generally associated with prior capacitor discharge ignition systems. Thus, a system according to the present invention may be economically constructed and compactly packaged for use with many types of engines, for example, single cylinder gasoline engines such as are used in power lawn motors, etc. Further advantages and features of the invention will be seen in the ensuing description and claims which are to be taken in conjunction with the accompanying drawing.

The drawings illustrate preferred embodiments of capacitor discharge ignition systems according to the present invention and in accordance with the best mode presently contemplated for carrying out the invention.

FIG. 1 is an electrical schematic diagram of a first embodiment of a capacitor discharge ignition system according to the present invention.

FIG. 2 is an electrical schematic diagram of a second embodiment of the invention.

FIG. 3 is an electrical schematic diagram of a third embodiment of the invention.

In FIG. 1, a capacitor discharge ignition circuit 10 according to the present invention is employed with an internal combustion engine 11. Circuit 10 includes a stator assembly 12 mounted on the housing of engine 11 adjacent the periphery of a rotating element such as the engine flywheel 16. A north magnetic pole 18 and a south magnetic pole 20 are arranged on flywheel 16 for rotation therewith past stator 12 as indicated by the arrow 21. As flywheel 16 rotates past the stator 12, the magnetic flux existing between poles 18 and 20 passes through stator 12 to produce a time varying magnetic flux waveform in stator 12 in accordance with principles well known in the art. However, as will be seen in greater detail hereinafter, the particular way in which poles 18 and 20 are arranged and constructed relative to stator assembly 12 and the specific construction of the stator constitute inventive features of the present disclosure resulting in improved performance of a capacitor discharge ignition system.

Stator assembly 12 comprises a U-shaped core 22 having legs 22a and 22b. An ignition coil 24 comprising a primary winding 24p and a secondary winding 24s is wound on leg 22a while a charging and triggering coil 26 is wound on leg 22b. Winding 24p comprises terminals 24p' and 24p" and winding 24s, terminals 24s' and 24s". Coil 26 comprises terminals 26' and 26". Terminals 24p", 24s" and 26" are connected to ground G. A spark plug 28 is connected between ground and terminal 24s' and hence is across secondary winding 24s. (Although a single plug is illustrated, it will be appreciated that the invention may be practiced with multiple cylinder engines.) Preferably, coils 24 and 26 are wound on stator core 22 such that when a voltage is induced in coil 26 which makes terminal 26' positive, a voltage is induced in coil 24 which makes terminal 24p' negative.

Circuit 10 further includes the following elements: an SCR 30, a capacitor 32, a resistor 34, a capacitor 36 and a capacitor 38. Capacitor 32 comprises a terminal 32a which is connected via a lead 40 to terminal 26' and a terminal 32b which is connected by a lead 42 to ground G. Hence capacitor 32 is connected directly across coil 26. SCR 30 comprises an anode terminal 30a, a gate terminal 30g and a cathode terminal 30c. Lead 40 connects terminal 30a with terminals 32a and 26'. In accordance with an important aspect of the present invention, the remaining circuit elements 34, 36 and 38 constitute a triggering circuit for triggering SCR 30 from coil 26. Resistor 34 and capacitor 36 are connected in parallel with each other. Terminal 34a of resistor 34 and terminal 36a of capacitor 36 are connected via lead 40 with terminals 30a, 32a and 26'. Terminal 34b of resistor 34 and terminal 36a of capacitor 36 are connected via a lead 44 with terminal 38a of capacitor 38 and gate terminal 30g of SCR 30. Terminal 30c of SCR 30 and terminal 38b of capacitor 38 are connected via a lead 46 to terminal 24p'.

Circuit 10 operates as follows. As flywheel 16 rotates, magnets 18 and 20 create a time varying magnetic flux in stator core 22. This time varying flux induces a voltage in coil 26 whose waveform includes positive polarity output which appear at terminal 26'. Thus with one magnetic pole pair on flywheel 16 (as illustrated in the drawing) one such positive pulse is generated for each revolution of the flywheel and in this way, the output waveform at terminal 26' includes reptetitive pulses of positive polarity. Since capacitor 32 is coupled directly across coil 26, current flows from coil 26 in response to an initial portion of each positive pulse to charge capacitor 32. The aforementioned triggering circuit causes a triggering signal to be applied to gate terminal 30g in response to each positive pulse from coil 26. Capacitors 36 and 38 constitute a voltage dividing network for dividing the voltage output of coil 26 (i.e. the voltage across capacitor 32) and supplying a selected fraction of this voltage to the gate 30g of SCR 30. Capacitors 36 and 38 are selected such that when the desired voltage has been developed across capacitor 32, a sufficient voltage is applied by capacitor 38 across the gate-cathode of SCR 30 which causes the SCR to be triggered into conduction. Thus, capacitor 38 discharges across the gate-cathode junction of SCR 30 to trigger discharge of capacitor 32 into the ignition coil. Since the voltage applied to the gate of the SCR will be of similar waveform to the voltage waveform across capacitor 32, the requisite gate triggering voltage for turning on SCR 30 can be reached earlier in the waveform across capacitor 32 by increasing the capacitance of capacitor 36 relative to the capacitance of capacitor 38. Similarly, SCR 30 will be triggered later relative to the output waveform across capacitor 32 by increasing the capacitance of capacitor 38 relative to that of capacitor 36. In this way, the spark at plug 28 can be advanced or retarded relative to the operating position of the engine. The advantage of the capacitive voltage dividing network is that hard triggering of SCR 30 is afforded and response is improved over triggering circuits which have softer triggering characteristics. Although resistor 34 is preferred to lend a certain stability to the system, the basic triggering circuit is essentially determined by the characteristics of capacitors 36 and 38. It is contemplated, however, that slight modifications may be made such as adding additional resistance in the triggering circuit to tune the system in for optimum performance to match the characteristics of the specific charging and triggering coil of the ignition system.

When the triggering signal is applied to gate terminal 30g for rendering SCR 30 conductive, capacitor 32 and coil 26 are both immediately used to energize primary winding 24p. Thus, it is desirable for the gating signal for turning on SCR 30 to be generated while the voltage pulse at terminal 26' still has an appreciable magnitude for supplying current from coil 26. With SCR 30 having been triggered into conduction, capacitor 32 discharges into primary winding 24 and current also flows from coil 26 into primary winding 24p. Also the voltage being presently induced in winding 24p by the changing magnetic flux in core 22 creates an additional component of current flow in primary winding 24p. Hence, it will be appreciated that a rapid change in magnetic flux in core 22 is created and this in turn generates a voltage in secondary winding 24s which causes spark plug 28 to fire.

The above enumerated advantages of the invention can now be fully understood. Since the same polarity pulse is used both to charge capacitor 21 and to trigger discharge of the capacitor by firing SCR through the triggering circuit, coil 26 can be wound on stator 22 as a single continuous winding, being wound in the same sense throughout its length and without the need of intermediate tapping. Moreover, circuit 10 requires a relatively small number of individual circuit components and this renders the invention suitable for compact packaging and economical manufacture. It will be noted in this regard also that the only solid state semiconductor which is required in the single SCr 30. It has also been found, contrary to expectations, that omssion of the usual charging diode provides improved circuit performance while at the same time reducing system cost. Heretofore, typical embodiments of capacitor discharge ignition systems had a charging diode interconnected between the charging coil and the capacitor. The charging diode prevented the accumulated charge on the capacitor from discharging back into the charging coil before the capacitor was discharged into the ignition coil. By omitting this diode, it has been found that a greater charge can be developed on the capacitor through what is believed to be a resonant effect. Although specific system construction will necessarily depend upon the particular system requirements, this resonant effect is developed primarily by the charging coil and capacitor constructions. Since specific construction values are difficult to mathematically derive in view of the harmonic content which typically characterizes the magneto output waveform, it is suggested that specific construction values for the charging coil and capacitor may be determining empirically. Desirably, the circuit is constructed so that the capacitor discharges into the ignition coil when the maximum charge is developed on the capacitor. A further benefit can be obtained by using a relatively high impedance primary winding 24p which tends to reduce the primary current. With the reduced primary current, a smaller, and hence less expensive, SCR can be used in the system. Moreover, the relatively high impedance of primary winding 24p benefits low speed operating performance by enhancing ringing in the circuit during firing of the plug.

In accordance with a modified form of the invention, it is possible to eliminate the triggering circuit for SCR 30 and to fire the SCR by means of breaking down the SCR with the potential applied to the anode terminal 30a. In this form, resistor 34, capacitor 36 and capacitor 38 are eliminated. Preferably, SCR 30 is selected to break down slightly before or at the maximum value of each positive output pulse from coil 26. With this type of firing for SCR 30, the circuit complexity is even further reduced and the circuit becomes even more economical. It has also been discovered that another type of breakdown device such as a spark gap will function satisfactorily, hence eliminating the need for any solid state device.

A further inventive feature of circuit 10 involves the way in which poles 18 and 20 are arranged and constructed relative to stator assembly 12. It will be observed in FIG. 1 that poles 18 and 20 are arranged and constructed in relation to stator 12 such that a first imaginary radial line circumferentially bisecting the radially outer face of pole 18 and a second imaginary radial line circumferentially bisecting the radially outer face of pole 20 have a smaller angular separation than do a third imaginary radial line circumferentially bisecting the radially inner face of leg 22b and a fourth imaginary radial line bisecting the radially inner face of leg 22a. In other words, the angular separation between the midpoints of legs 22a and 22b as measured circumferentially across the stator gap is greater than the angular separation between the midpoints of the two poles 18 and 20. It has been found that by increasing the circumferential separation of legs 22a and 22b relative to the circumferential separation of poles 18 and 20, as illustrated by way of example in FIG. 1, spark retard at higher engine rpms is avoided. Hence the particular construction of stator assembly 12 relative to poles 18 and 20 has the advantage of stabilizing to some extent the timing of the ignition spark.

A further benefit is attained by constructing the rotor magnet of a relatively high flux level magnetic material such as smarium cobalt. The use of a smarium cobalt magnet has been found to provide very desirable operating performance. It is preferable to provide a magnetic circuit with a minimum of stray flux paths when smarium cobalt is used for the magnetic material.

FIG. 2 discloses another embodiment of the invention wherein like components from FIG. 1 are identified by like numerals. (Details of the structure of stator assembly 12 are not repeated in FIG. 2; hence coils 26 and 24 are only schematically illustrated.) In the circuit of FIG. 2, it will be observed that resistor 34 and capacitor 36 are omitted, and hence, that energy for triggering of SCR 30 is derived from primary winding 24p. In order to provide suitable triggering for SCR 30, a resistor 48 connects in parallel with capacitor 38, and a resistor 50 connects from gate 30g to ground G. Thus resistor 50, capacitor 38 and resistor 48 are across primary winding 24p and develop a suitable triggering signal for SCR 30 from the voltage induced in winding 24p by the revolving rotor. The effect of the various resistive and capacitive elements 38, 48 and 50 on the triggering of SCR 30 can best be summarized as follows. Assuming for the moment that capacitor 38, resistor 48 and resistor 50 are not electrically connected in circuit, SCR 30 will fire via a breakdown 350-for engine speeds as low as about 35014 400 rpm. When resistor 50 is added, SCR 30 can fire at engine speeds as low as about 100 rpm. By further adding capacitor 38, the lowest engine speed at which SCR 30 will fire is even further reduced to approximately 50 rpm. The provision of resistor 48 keeps the spark from retarding at very high engine rpm. Furthermore, it has been observed that the kilovolt output of spark plug 28 is substantially increased at these relatively low engine rpm, and it is believed that this may be due to a ringing effect provided by the capacitor. Thus it can be seen that with the appropriate selection of specific circuit component values, desired operational circuit performance for small gasoline engine ignition system applicable can be attained with a minimum of circuit complexity. As mentioned earlier, it is preferred to empirically determine specific component values for a given application.

In certain instances, it may be desirable to include a charging diode 52 and a capacitor 54 (shown in phantom in FIG. 2) either singly or in combination in the circuit. As illustrated in the drawing, charging diode 52 would be connected between charge coil 26 and capacitor 32, and capacitor 54 would be connected in shunt with resistor 50. While diode 52 would now necessarily prevent capacitor 32 from discharging back through charge coil 26 were SCR 30 not triggered in accordance with the resonant charging aspect of the invention described above, it is not the purpose of diode 52 to prevent such discharge. Rather, diode 52 is included to alleviate the effects of spikes which may occur in the output waveform developed across coil 26. The reason for including diode 52 is because with certain stator and rotor constructions, it has been observed that the output waveform of the charging coil can contain multiple spikes which may not be desirable. It has been found that the number of excess spikes (i.e., spikes in excess of one for single cylinder engines) can be minimized or eliminated by incorporating diode 52 and/or by changing the resistance of resistor 50. It has been found that increasing the resistance of resistor 50 helps to minimize the number of excess spikes. However, the resistance of resistor 50 necessary to provide proper operation of the circuit at high engine rpm was found to be too small to eliminate these spikes. By including capacitor 54, the number of extra spikes is minimized since the value of resistor 50 can now be increased to reduce the number of excess spikes while providing properly timed triggering action. At lower engine rpm, capacitor 54 provides enhanced voltage to the gate of SCR 30. Thus both diode 52 and capacitor 54 can improve performance in some instances.

In FIG. 3, a further embodiment of the invention is disclosed. In this embodiment charging coil 26 is omitted and primary winding 24p alone is used both for charging capacitor 32 and for triggering SCR 30. In this circuit, capacitor 32 connects directly across primary winding 24p, capacitor 38 connects as above between the gate-cathode of SCR 30 and resistor 50 connects from gate 30g to ground G. The anode of SCR 30 connects to ground. With this circuit, winding 24p energizes both capacitor 32 and capacitor 38. When capacitor 38 is sufficiently charged to trigger SCR 30, primary winding 24p, capacitor 32 and the series combination of resistor 50 and capacitor 38 are effectively short-circuited through the anode-cathode circuit of SCR 30. This creates a sudden large flux change in ignition coil 24 which fires spark plug 28. SCR 30 is turned off on the opposite polarity voltage of primary winding 24p. With this particular circuit construction the values of the various circuit elements are selectable to provide desired circuit operation for use with a small internal combustion engine. Alternatively the anode of SCR 30 could be connected to the ungrounded side of winding 24p, and the cathode of SCR 30 connected to the ground. In this instance the capacitor 38 will remain connected in the gate-cathode circuit of SCR 30 while resistor 50 would remain connected in the anode-gate circuit of SCR 30.

By way of example the following circuit values have been successfully employed in the circuit of FIGS. 2 and 3: capacitor 38, 0.05 microfarads; resistor 50, 15 kilohms when not using capacitor 54 and 22 kilohms when using capacitor 54; capacitor 32, 0.30 microfarads in FIG. 2 and 2.3 microfarads in FIG. 3; capacitor 54, 0.0002 microfarads.

It is to be understood that the foregoing description is that of preferred embodiments of the invention. Various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.