Ignition circuit with automatic spark advance

[57] ABSTRACT

A solid state capacitor discharge ignition system for use in a single-cylinder engine having a magneto for an electrical energy source wherein the capacitor is discharged through a silicon controlled rectifier in response to trigger signals generated by a trigger assembly mounted on the rotor and stator of the magneto. The trigger assembly generates at least two discrete spaced pulses at running and cranking speeds, and the spacing between the two pulses corresponds to a desired shift in the engine timing between cranking and running speeds. The trigger assembly, the silicon controlled rectifier and the circuit connection therebetween are such that both pulses are applied to the controlled rectifier at all engine speeds; but the retarded ignition pulse is effective at cranking speeds and the advance pulse is effective at running speeds.

Claims, 15 Drawing Figures

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PATENTED*» 9I9?2

3,661,132

SHEET 1 OF 3

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ATTORNEYS

PATENTEDMAY 91972

3,661,132

SHEET 2 OF 3

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PATENTED HAY 9 1972

3,661,132

SHEET 3 OF 3

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ΤΊβ. 15

ATTORNEYS

3,661,132

IGNITION CIRCUIT WITH AUTOMATIC SPARK ADVANCE

This application is a continuation of my copending application (now abandoned) Ser. No. 684,052, filed Nov. 17, 1967, and entitled IGNITION CIRCUIT WITH AUTOMATIC SPARK ADVANCE.

It is well known that the combustible charge in an internal combustion engine is normally ignited at or near top dead center during the compression stroke of the piston. Engine performance can be improved if the ignition timing varies with engine speed. Ignition may be timed at substantially top dead center during starting with the timing being advanced as the speed of the engine increases. With breaker point ignition systems, ignition timing may be advanced using mechanical means that are responsive to engine speed. Such ignition systems have numerous disadvantages associated primarily with wear of the breaker points and other parts that in time cause unwanted variation in the ignition advance.

Relatively high manufacturing costs as well as service and maintenance problems have generally precluded the use of such ignition systems having automatic timing advance on single-cylinder engines that are mass produced at the lowest possible cost. It is essential that the cost of an ignition system for single-cylinder engines be held to a minimum due to vigorous competition in the small engine field. Hence it is common practice in the small engine field to select a fixed ignition timing representing a compromise between the optimum timing at starting and at running speeds. Easy starting has been a problem with small engines, particularly with engines having manual starting. However, as long as engine timing is fixed at a compromise for starting and for running, there is a limit to which the ease in starting may be improved. Hence, it is highly desirable to provide a timing advance between starting and running speeds at a low cost. Automatic timing advance at a low cost would be particularly desirable when incorporated in solid state ignitions such as disclosed in my copending application entitled “Ignition System,” Ser. No. 654,860, filed July 20, 1967, now U.S. Pat. No. 3,490,426.

The objects of the present invention are to provide ignition systems having automatic timing advance that are compatible with solid state ignition circuits; that eliminate moving parts and hence eliminate well-known disadvantages from wear in prior ignition systems having automatic timing advance; that are simple in construction, relatively low in cost and reliable by comparison with prior ignition systems having automatic timing advance; that provide an effective shift in engine timing from an optimum timing for starting to an effective timing at running speeds; and that are particularly suited to ignition systems for single-cylinder engines that are mass produced at the lowest possible cost.

Other objects, features and advantages of the present invention will be apparent in connection with the following description. the appended claims and the accompanying drawings in which:

FIG. 1 is a view diagrammatically illustrating a magneto having a main charging coil and a pair of trigger coils having different turns;

FIG. 2 is a circuit diagram of a solid state, capacitordischarge ignition circuit used with the magneto shown in FIG. 1;

FIG. 3 is a diagram illustrating waveforms of voltages generated in the trigger coils of FIGS. 1 and 2;

FIG. 4 is a fragmentary view of a magneto showing another embodiment of the present invention wherein two trigger coils are arranged with different air gaps;

FIG. 5 is a circuit diagram illustrating the modification in the circuit of FIG. 2 for the dual coil arrangement of FIG. 4;

FIG. 6 illustrates a still further modification for the circuits of FIGS. 2 and 5 to isolate the trigger coils from each other;

FIG. 7 is a fragmentary view of a magneto showing yet another embodiment wherein a single trigger coil is arranged on a narrow U-shaped core to develop consecutive triggering pulses;

FIG. 8 is a circuit diagram illustrating modification of the circuit of FIG. 2 for the single trigger coil and U-shaped core of FIG. 7;

FIG. 9 is a diagram illustrating waveforms for voltages generated in the modification of FIGS. 7 and 8;

FIG. 10 is a fragmentary view of a magneto showing a still further embodiment wherein a single trigger coil is arranged on a wide U-shaped core;

FIG. 11 is a diagram illustrating waveforms of voltages generated in the single coil modification of FIG. 10;

FIG. 12 illustrates a still further modification of the present invention;

FIG. 13 illustrates a modification of the present invention wherein two coils are wound on the same core and connected in push-pull;

FIG. 14 is a diagram illustrating the waveforms of the voltages generated in modification of FIG. 13; and

FIG. 15 is a circuit diagram illustrating one temperature compensation circuit.

Referring more particularly to FIGS. 1-3, there is illustrated a magneto designated generally at 10 and comprising a stator 12 and a rotor 14 which is drivingly connected to the crankshaft of a single-cylinder engine (not shown) so as to rotate in a clockwise direction as viewed in FIG. 1 in synchronism with the engine. A permanent magnet 18 embedded in rotor 14 has a north pole face 20 and a south pole face 22 that extend circumferentially along the inner periphery of rotor 14. Faces 20, 22 have a magnetic gap 23 therebetween. The stator 12 is fastened On the engine by suitable means and is stationary relative to rotor 14. Mounted on stator 12 is a main charging coil assembly 26 which includes a charging coil 28 wound on the center leg 32 of an E-shaped core 34. This arrangement provides a rapid flux reversal through the center leg 32 generating a relatively high voltage in the coil 28. Two trigger coil assemblies 40, 41 are also mounted on stator 12 in spaced relation to each other and to the main coil assembly 26.

The trigger coil assembly 40 generally comprises a coil 42 wound on a core 44 which is mounted at its radially inner end on a plate 46 adjustably fastened on the stator 12. Core 44 projects radially outwardly of stator 12 with the radially outer end spaced from rotor 14 to form an air gap 48 with magnet 18. Trigger coil assembly 41 comprises a trigger coil 43 wound on a core 45 which is mounted at its inner end on a plate 47 also adjustably fastened on stator 12. Core 45 has an air gap 50 with magnet 18. As illustrated in FIGS. 1 and 2, coil 42 has fewer turns than coil 43 and the air gaps 48, 50 are substantially equal. The angular displacement between axis 50 of the charging coil 28 and the axis 52 of the trigger coil 42 is designated Θ , whereas the corresponding angular displacement of the axis 53 of the coil 43 is designated Θ ₂ and the angular displacement between the axes 52, 53 is designated θ ₃. Θ , and Θ ₂ may also be considered as representing crankshaft angles and also time. In general the crankshaft angle, Θ „ of the trigger coil assembly 40 is selected to provide an advanced ignition timing when the engine is running. The crankshaft angle, 0₂, of the trigger coil assembly 41 is selected to provide a retarded ignition timing when the engine is cranked during starting.

Referring more particularly to the circuit in FIG. 2, a Zener diode 58 is connected directly across the charging coil 28 to regulate the maximum positive voltage generated in coil 28 when the upper terminal as viewed in FIG. 2 is positive. Also connected across the charging coil 28 is a series circuit comprising a silicon diode 60, a capacitor 62 and the primary winding 64 of an ignition transformer 66. The secondary winding 68 of transformer 66 is directly connected across a spark plug 72. Connected directly across the serially connected capacitor 62 and winding 64 is a silicon controlled rectifier 74 having an anode 76, a cathode 78 and a gate 80. The trigger coils 42, 43 are connected in parallel with each other and across the gate 80 and cathode 78 to provide triggering signals to the gate to initiate conduction of rectifier 74 as the coils 42,

3,661,132 are swept by a magnet 18. The coils 42, 43 are wound on the cores 44, 45, respectively, and are connected to the gate 80 so as to have the same relative polarity as indicated by the dots in FIG. 2.

The operation of the ignition described hereinabove can best be understood in connection with the waveforms illustrated in FIG. 3 wherein crankshaft angles, β , are plotted along the abscissa axis and voltage is plotted along the ordinate axis. The abscissa axis can also be considered as generally representing time at different scales for different engine speeds. It will be understood that the waveforms in FIG. 3 are for purposes of explanation and are not necessarily intended to be to scale. When the engine is turned at a relatively low cranking speed during starting, as magnet 18 sweeps past the charging coil 28, the alternating voltage generated in coil 28 is rectified by diode 60 to charge capacitor 62 to the polarity indicated in FIG. 2. As magnet 18 continues rotation in a clockwise direction past trigger coil assembly 40, an alternating triggering signal 84 (FIG. 3) is generated in coil 42 and applied to gate 80. The signal 84 comprises three pulses 87, 87', 87 of alternating polarity. Coil 42 is connected to gate 80 so that the first pulse 87 is negative, the second pulse 87' is positive and the third pulse 87 is negative. In the preferred embodiment, only the positive pulse 87' is used. Pulse 87' is generated when gap 23 passes core 44. Rectifier 74 has a critical gate voltage designated at the voltage level 86 in FIG. 3. The number of turns in coil 42 is selected so that at cranking speeds pulse 87' has an amplitude substantially below level 86 and hence does not fire rectifier 74. As magnet 18 continues to rotate past the trigger coil assembly 41, an alternating voltage 88 is generated in the second trigger coil 43. The signal 88 also includes a first negative pulse 89, a second positive pulse 89' and a third negative pulse 89. The number of turns in coil 43 is selected so that at cranking speeds, pulse 89' exceeds level 86 to initiate discharge of capacitor 62 through rectifier 74. The duration of pulse 89' is sufficient to allow capacitor 62 to completely discharge in a damped oscillatory manner through rectifier 74 on one discharge half-cycle and through diodes 58,60 on the opposite discharge half-cycle.

By way of example, coil 43 may have 10 times the number of turns in coil 42 to assure sufficient amplitude difference between pulses 87', 89' at cranking speeds so that the pulse 87' will not fire rectifier 74 but pulse 89' will. A ratio of at least greater than five to one between the peak amplitudes of the pulses 89', 87' is preferred to obtain sufficient amplitude separation. The location, Θ ₂, of trigger coil 43 is correlated to the engine cycle so that the retarded ignition pulse 89' fires rectifier 74 at the desired crankshaft angle to facilitate easy starting, for example, at or near top dead center in the compression stroke.

As soon as the engine starts the voltage generated in coil 42 increases substantially. Hence at running speeds the first positive pulse 91’ in the gate voltage from coil 42, corresponding to pulse 87', exceeds the threshold level 86 at the crankshaft angle Θ , to fire rectifier 74 and initiate discharge of capacitor 62. The location, Θ „ of coil 43 is selected so that the advanced ignition timing pulse 91' exceeds level 86 at the desired crankshaft angle for running speeds, for example, an angle of 22° before top dead center. Although the amplitude of the retarded pulse corresponding to pulse 89' is also increased substantially at running speeds, the retarded pulse is ineffective since capacitor 62 is substantially fully discharged in response to pulse 9Γ. By way of example, on engines in the 2.5-7 horsepower range a typical cranking speed is in the range of 300-400 RPM with minimum cranking speeds of 100-150 RPM and a typical idle speed is above 1,500 RPM. The circuit is designated to provide a timing shift in a speed range of 800-1,000 RPM. The timing shift provides easy starting and acceptable engine performance.

Although pulses 87' and 89' are used in the preferred embodiment, it will be apparent that other pulse pairs may also be used. For example, by reversing the coil leads the first pulses corresponding to pulses 87, 89 will be positive and have the desired time separation for a timing shift of about 20°. Referring to the embodiment of FIGS. 4 and 5, the trigger coil assemblies 40, 41 are replaced by corresponding trigger coil assemblies 100,101. Except for the triggering circuit for rectifier 74, enclosed in dashed lines in FIGS. 2 and 5, the ignition circuit for the dual coil arrangement illustrated in FIGS. 4 and 5 is the same as that shown in FIG. 2. The trigger coil assembly 100 includes a trigger coil 102 wound on a core 104 which is mounted at its inner end on a plate 106 adjustably fastened on stator 12. Core 104 has an air gap 108 with magnet 18 (FIG. 1). The trigger coil assembly 101 comprises a trigger coil 103 wound on a core 105 which is mounted at its radially inner end on a plate 107 adjustably fastened on stator 12. Core 105 has an air gap 111 with magnet 18. Coils 102, 103 have the same number of turns but the air gap 108 is substantially greater than the air gap 111. With the difference in air gaps at any given speed of rotor 14, the signal generated in coil 103 will have a substantially higher amplitude than the signal generated in coil 102.

More particularly the air gaps 108, 111 are selected so that the triggering signal generated in coil 102 has the same relationship to the triggering signal generated in coil 103 as signal 84 (FIG. 3) generated in the coil 42 (FIGS. 1 and 2) has to the signal 88 generated in coil 43. Coils 102,103 are connected in parallel with each other and directly across the gate 80 and the cathode 78. Coils 102, 103 have the same relative polarity as indicated by the dots in FIG. 5 and are connected to gate 80 so that the second pulse in each coil generated when gap 23 passes the respective cores will be positive as with pulses 87', 89' for coils 42, 43 (FIGS. 1-3). Hence the operation of the modification of FIGS. 4 and 5 will be apparent from the description in connection with FIGS. 1-3.

Referring to FIG. 6, a modification for the circuit of FIG. 2 is illustrated wherein a silicon diode 114 is connected in series with coil 42 across coil 43. Diode 114 is poled in the direction shown to decouple coil 42 from coil 43 during the positive pulse 89'. To simplify the description, the various pulses are illustrated in FIG. 3 as relatively sharp pulses occuring at different times. However the pulses may be of longer duration with some of the pulses coincident with, or at least overlapping, other of the pulses depending on the configuration of magnet 18 together with the parameters and the separation of coil assemblies 40, 41. For example, if the negative pulse 87 overlaps the positive pulse 89' the gate voltage may be reduced to such an extent that the retard pulse 89' may not fire rectifier 74 at low cranking speeds. Additionally, with coil 42 connected directly across coil 43, coil 42 will load coil 43 impairing the effect of the retard pulse 89', particularly where, as in FIGS. 1 and 2, coil 42 has fewer turns than coil 43. However diode 114 minimizes these problems where required.

FIGS. 7-9 illustrate still a further modification for providing consecutive timing pulses whose phase and amplitude relationship at low cranking speed and at higher running speed provide automatic timing advance. The coils 40, 41 (FIG. 1-3) are replaced with a single trigger coil assembly 120 which comprises a U-shaped core 122 mounted at its radially inner end on stator 12. Core 122 has legs 124, 126 spaced closely together. A single coil 128 is wound on leg 124 which is considered as a leading leg in that it is the first of the legs 124, 126 encountered by the leading edge 131 of the north pole face 20 during rotation of rotor 14. The angular separation between the core legs 124, 126 and the circumferential length of the leading pole face 20, that is, the angular separation between the leading and trailing edges of the pole face 20 are correlated to the engine timing cycle to obtain the required timing shift. More particularly, the leading edge 131 of the pole face 20 and the center line 130 of the gap 23 (in effect, the trailing edge of face 20) have an angular displacement 9'_a equal to the desired timing shift, corresponding to the angle Θ ₃ (FIGS. 1 and 3). Coil 124 is connected directly across gate 80 and cathode 78 of rectifier 74 as illustrated in FIG. 8. The ignition circuit for the narrow U-shaped core arrangement of FIG. 7 is identical to that disclosed and

3,661,132 described in connection with the circuit of FIG. 2, except for the trigger circuit enclosed in dotted lines (FIGS. 2 and 8), i.e., the connection of the trigger coil 128 to the rectifier 74.

The manner in which the advance and retard pulses are developed using the U-shaped core illustrated in FIGS. 7 and 8 will be more apparent in connection with the waveforms illustrated in FIG. 9. FIG. 9a is the plot of flux, &lt;/&gt;, versus crankshaft angle, Θ, or the equivalent time, t, and similarly FIG. 9b is a plot of the voltage, V, versus crankshaft angles, Θ, or the corresponding time, t. As magnet 18 approaches the trigger coil assembly 120 the first flux change is in a direction which is assumed to be negative going as illustrated by the waveform portion 113 (FIG. 9α). When the leading edge 131 of pole 20 reaches the trailing core leg 126, some of the flux from leg 124 is shunted to the leg 126 causing a flux reversal and providing a positive going flux at coil 124 designated at 135 in FIG. 9α. As magnet 18 sweeps past the trigger coil assembly 120, the flux in leg 124 remains substantially constant until the gap 123 reaches the trigger coil assembly 120 causing a rapid flux change in a positive going direction designated at 137 (FIG. 9b). The remaining portion of the flux waveform is not utilized in the embodiment being described. Referring to the voltage waveform illustrated in FIG. 9b, the first positive going flux change 135 generates a low amplitude positive timing pulse 136 in coil 124 and the more rapid positive going flux change 137 generates a larger amplitude positive timing pulse 138 in the coil 124. The magnitude of the advanced timing pulse 136 generated at low cranking speeds is substantially below the critical threshold level 86 (FIGS. 3 and 9b). The gating pulse 138 is substantially greater than the critical level 86 and passes through level 86 at the desired crankshaft angle 0'., to facilitate easy starting, that is at or just about top dead center. When the engine starts the rate of change of the first positive going flux reversal, corresponding to the flux reversal designated at 135, increases substantially generating an advanced timing pulse 140 which exceeds the threshold level 86 at the desired advanced timing position θ', in FIG. 9b. Thus discharge of capacitor 62 is initiated at the desired crankshaft angle, θ',, in response to pulse 140 when the engine is operating at running speeds.

In the embodiment illustrated in FIG. 10, the trigger coil assembly 144 generally comprises a U-shaped core 146 mounted on stator 12. The core 146 has radially disposed legs 148, 150 whose circumferential displacement to each other is wide by comparison to the spacing between legs 124, 126 in FIG. 7. A single coil 152 is wound on the trailing core leg 150. The angular displacement between legs 148, 150 is selected relative to the circumferential length of the pole shoes 20, 22 to obtain the flux waveform 160 (FIG. 11α) which generates consecutive timing pulses having the required phase and amplitude relationships at cranking speed and at running speed. The connection (not shown) of the trigger coil 152 to the gate 80 is the same as that shown in FIG. 8 for the coil 124.

More particularly, the angular displacement between legs 148, 150 is equal to or less than the angular separation between the leading edge 154 of pole face 20 and the trailing edge 156 of pole face 22 but greater than the length of pole face 20. For the clockwise rotation of magnet 18 illustrated in FIG. 10, as magnet 18 approaches the leg 148 and moves to the position relative to core 146 illustrated in FIG. 10, the flux linking coil 152 increases in a negative direction as illustrated by the first negative going portion 162 of waveform 160. As magnet 18 continues past the position illustrated in FIG. 10 and the trailing edge 156 of the south pole 22 breaks from leg 148, the flux in coil 152 reverses to a positive going flux at 166. As the north pole 20 sweeps past the leg 150 the flux in coil 152 remains relatively constant at 168 until the air gap 23 reaches the leg 150 at which point there is a rapid flux change 170 in a positive going direction. The flux then remains substantially constant and finally drops off to zero as the south pole 20 clears leg 150.

The corresponding voltages generated in coil 152 are shown in FIG. 116. At cranking speeds the first positive going flux change at 166 is relatively slow and generates a corresponding advance ignition pulse 176 whose amplitude is substantially below the critical threshold level 86. However at cranking speeds the relatively rapid flux change 170 generates a retarded ignition timing pulse 180 whose amplitude exceeds the critical threshold value 86 at the desired crankshaft angle, θ”₂, to facilitate easy starting. After the engine starts the rate of change of flux in coil 152 when the leg 148 clears the trailing edge 156 of the south pole 122 increases substantially. This causes a corresponding large increase in the amplitude of the advanced ignition timing pulse as illustrated by the pulse 182 (FIG. 11). Pulse 182 exceeds the level 86 at the crankshaft angle 0”, to provide the desired ignition timing advance at running speeds. Hence during cranking the retarded ignition timing pulse 180 is effective to generate a spark in plug 72. When the engine is running the advanced ignition timing pulse 182 is effective to generate the spark at plug 72.

FIG. 12 illustrates another modification in a triggering coil circuit wherein each of the coils 186, 188 are connected in series with respective isolating diode 190, 192 across the gatecathode of rectifier 74. Coils 186, 188 have a different number of turns as illustrated in FIG. 12. Diode isolation would also be useful with trigger coils having different air gaps as described hereinabove in connection with the embodiment of FIGS. 4 and 5. Diode 190 corresponds to diode 114 (FIG. 6) whereas diode 192 isolates coil 186 from negative pulses generated in coil 188 and from loading by coil 188. The operation of the circuit illustrated in FIG. 12 is similar to the operation of the circuit illustrated in FIG. 6. For example, referring to FIGS. 3 and 12 diode 192 would block the first negative pulse 89 in coil 188 so that pulse 89 does not interfere with the advanced timing pulse 87' generated in coil 186. The isolation provided by the diodes 190, 192 may be required to provide effective separation between the triggering pulses generated in the two coils 186, 188 depending on the particular configuration of magnet 18 and the design of coils 102, 103.

FIG. 13 illustrates still another embodiment for generating two spaced triggering pulses having the desired phase and amplitude relationships at cranking speed and at running speed. Two trigger coils 204, 206 are wound on a common core 208 which is mounted on the stator 12 so as to be swept by magnet 18 during each revolution of rotor 14. Coil 204 has fewer turns than coil 206. Coils 204, 206 are each connected in series with a respective isolating diode 210, 212 across gate 80 and cathode 78 through a common connection 214. A single coil having a tap at the connection 214 is also contemplated. As indicated by the dots in FIG. 13, coils 204, 206 are oppositely poled in push-pull to gate 80.

At cranking speed magnet 18 will generate in coil 204 the waveform illustrated in FIG. 14α. The first positive pulse 220 generated when the leading pole 20 reaches coil 204 has a peak amplitude substantially below the critical voltage level 86. The second pulse 221 which is negative is generated by the rapid flux reversal as gap 23 sweeps coil 204. However, pulse 221 is blocked from gate 80 by diode 210. The second positive pulse 222 is generated as the trailing edge of the trailing pole 22 breaks from coil 204. Pulse 222 is not used in the embodiment being described. However since coil 206 is oppositely phased relative to coil 204, magnet 18 generates in coil 206 a voltage having a waveform shown in FIG. 14b wherein the first pulse 223 and the last pulse 224 are negative. The second pulse 225 is positive and has an amplitude that exceeds the voltage level 86 at the crankshaft angle 0'₂. Hence at cranking speeds the advanced ignition timing pulse 225 fires rectifier 74 at the desired crankshaft angle that facilitates easy starting. After the engine starts the magnitude of the voltages generated in coil 204 increases substantially so that at running speeds the first positive pulse 226 (FIG. 14α) generated in coil 204 exceeds the level 86 at the desired crankshaft angle 0'”,. As with the embodiments described in connection with FIGS. 7 and 10, the amount of the timing shift 0'₃ is determined in part by the configuration of magnet 18. More particularly for the embodiment of FIG. 13, the shift depends on the length of

3,661,132 the leading pole 20. It will also be apparent that although the desired amplitude relationship between pulse 220 and pulse 225 is obtained by having fewer turns in coil 204 than in coil 206, the amplitude difference is in part due to the configuration of magnet 18 and the gap 23.

It has been found particularly desirable to provide temperature compensation in ignition circuits having automatic advance of the type described hereinabove to assure that the ignition timing occurs at the proper crankshaft angle regardless of temperature. FIG. 6 illustrates one circuit that is useful with the various trigger coil arrangements to compensate for variations in the critical threshold level 86 with temperature. In the circuit of FIG. 15 the trigger coils are illustrated in block form at 240, i.e., either coils having a different number of turns (FIGS. 1-3), coils having different air gaps (FIGS. 4 and 5), a single coil on a narrow U-shaped core (FIGS. 7-9), a single coil on a wide U-shaped core (FIGS. 10 and 11), or push-pull coils on a single core (FIGS. 13 and 14). A silicon diode 242 is connected in series with cathode 78 to the common return lead 244. Connected across the trigger circuit 240 is a voltage divider comprising a resistor 246 and a thermistor 248. Thermistor 248 has a negative temperature coefficient so that its resistance decreases with increasing temperature. Gate 80 is connected to the divider between resistor 246 and thermistor 248. With increasing temperatures anode-cathode leakage current through rectifier 74 develops a small voltage drop across diode 242 so that the gate-cathode junction becomes reversed biased. With increasing temperature, the gate voltage developed across the thermistor drops off. Temperature compensation using either thermistor 248 or diode 242, alone, is also contemplated. The circuit also minimizes spurious triggering by ripple voltages generated by stray flux, particularly at high running speeds and at high temperatures.

It will be understood that the ignition system having automatic ignition timing advance has been described hereinabove for purposes of illustration and is not intended to indicate limits of the present invention, the scope of which is defined in the following claims.