Ignition system with idle speed governor apparatus

An alternator driven capacitive discharge ignition system includes a tachometer circuit monitoring the speed-dependent repetition rate of discharge at an internal trigger power supply, the tachometer circuit causing the triggering threshold bias to be reduced below idle speed to electronically advance the timing. The ignition circuit includes a triggering threshold bias capacitor in series the trigger pulse source and a trigger power supply having a pilot capacitor to alternately fire a pair of ignition silicon controlled rectifiers. The pilot capacitor is charged in series with an RC filter network of a "bucket" tachometer circuit to create a speed signal voltage proportional to engine speed with the pilot capacitor functioning as the bucket capacitor. The speed signal is a voltage which is positive relative to a common signal ground while the threshold bias capacitor voltage is negative relative to such signal ground. The speed signal is applied to the gate of a P-channel depletion-mode junction field-effect transistor (JFET). The source-to-drain channel of the transistor is connected in parallel with the threshold bias capacitor. Below a selected idle speed, the source-to-drain channel resistance provides a maximum bleed current to the threshold bias capacitor, thereby reducing the triggering threshold voltage and creating an automatic spark advance. As the engine speeds up, the source-to-drain resistance increases and virtually eliminate the bleed current at speeds slightly above idle.

BACKGROUND OF THE INVENTION 
This invention relates to a solid state ignition system for internal 
combustion engines having an alternator power supply. 
Capacitive discharge ignition systems for internal combustion engines 
provide highly reliable and satisfactory ignition and have been 
particularly satisfactory for two cycle engines used in recreational 
products such as outboard motors, snowmobiles, motor cycles and the like. 
Such ignition systems may employ a battery power supply with an auxiliary 
alternator for maintaining the battery fully charged. In smaller engines, 
such as employed in outboard motors or snowmobiles and the like, the 
alternator may be connected directly to the engine crankshaft and provide 
the sole power supply for the capacitive discharge ignition system. 
Generally, the capacitive discharge ignition system employs one or more 
energy storage ignition capacitors via the ignition transformers to the 
spark plugs of the internal combustion engine for discharge of the 
capacitor or capacitors at appropriate times for proper ignition of the 
combustible charges in the cylinders of the engine. 
In some alternator driven systems, the engine flywheel is constructed with 
two separate sets of magnets; the first set to generate the appropriate 
charging voltages for the energy storage capacitor or capacitors, and the 
second set to generate the appropriate trigger pulses for firing of the 
controlled rectifiers. 
In other alternator driven systems, the engine flywheel is constructed with 
only a single set of magnets to generate both the appropriate charging 
voltages and the appropriate trigger pulses. 
In either type of system the trigger pulses are normally generated in a 
movable trigger pulse winding or coil arranged to allow angular movement 
both in a direction opposite to the flywheel rotation to mechanically 
advance the ignition timing and in a direction the same as the flywheel 
rotation to mechanically retard the ignition timing. 
An example of a practical and highly satisfactory alternator driven system 
of the first type, utilizing two separate sets of magnets for the 
capacitor charging and the triggering, respectively, is disclosed in U.S. 
Pat. No. 3,805,759 which issued Apr. 23, 1974 to Arthur O. Fitzner. The 
triggering circuit of U.S. Pat. No. 3,805,759 includes a bias capacitor 
operationally in series with, and charged by, the trigger pulse winding. 
The output of the trigger pulse winding or generator changes with speed, 
and the bias capacitor is charged therefore in accordance with the speed 
of the engine. The capacitor is connected to effectively create a variable 
triggering threshold matching the varying output characteristic of the 
trigger pulse generator. This provides an essentially constant timing 
characteristic over the normal operating speed range for marine outboard 
motors and has been found to contribute to improved operation. The timing 
is, of course, then separately controlled by mechanically positioning of 
the trigger pulse winding means. 
An example of a practical and highly satisfactory alternator driven 
ignition system of the second type, utilizing only a single set of magnets 
for both the capacitor charging and the trigger pulse generation is set 
forth in U.S. Pat. No. 3,937,300 which issued Feb. 10, 1976 to Richard L. 
Sleder et al. 
Systems having only a single set of magnets as disclosed in U.S. Pat. No. 
3,937,200 are advantageously employed on the smaller outboard motors which 
need no provisions for maintaining the charge of an external battery. 
Although alternator driven capacitor discharge ignition systems provide 
highly satisfactory operation of internal combustion engines for outboard 
motors, snowmobiles and the like, smooth and particularly satisfactory 
engine operation may not be created under idle speed settings. The 
analysis of the inventors indicates that with pulse generator triggered 
capacitor discharge ignition systems, the timing characteristic 
automatically and adversely changes to some degree at the extremely low 
instantaneous speeds sometimes encountered at idle. This is true even with 
the use of the unique stabilizing circuit in U.S. Pat. No. 3,805,759. 
Generally, the timing characteristic, particularly at momentary speeds 
below the idle speed, includes an automatic retard of the spark or timing 
which is detrimental. 
At idle, the flywheel speed varies as it rotates through one full 
revolution, slowing down as the engine compresses the combustible gases 
prior to maximum compression, perhaps speeding up very slightly as the 
peak of compression is passed, and them accelerating rapidly once the 
combustible gases are ignited. 
The triggering threshold is set by the sum of the relatively fixed 
threshold of the controlled rectifier devices plus the variable threshold 
voltage contributed by the bias capacitor. 
When the flywheel speed slows momentarily, as it tends to do just prior to 
ignition, the trigger pulse in the trigger coil, which is proportional to 
flywheel speed, will be weakened, and the timing will be retarded. 
When the flywheel speed increases momentarily, as it tends to do 
immediately after ignition, the trigger pulse in the trigger coil will be 
strengthened and will tend to charge the bias capacitor accordingly. This 
will raise the variable threshold voltage in the bias capacitor and 
contribute to a more retarded ignition timing on the very next firing. 
Overall, the result is to rapidly reduce the power output from the idling 
engine when the speed drops off momentarily, causing a less than desirable 
idle characteristic. 
SUMMARY OF THE PRESENT INVENTION 
The present invention is particularly directed to an alternator driven 
solid state ignition system having a pulse actuated discharge switch means 
and an engine driven pulse generator connected to actuate the solid state 
switch means for firing of the spark plugs, in combination with an 
automatic electronic idle speed governor means to provide smooth idle 
speed operation. The idle speed operation is improved by eliminating the 
undesired automatic electronic spark retard of the timing characteristic 
and, in fact, providing a significant advance in the timing at speeds 
below the desired idle speed. 
Generally, in accordance with the present invention, a speed sensitive or 
monitor means is coupled to the engine and produces a threshold 
modification in the trigger circuit to create a distinct and significant 
advance at the spark at speeds just below idle speed, with a resulting 
programmed stabilization in engine operation at idle speed. 
In a preferred and a unique embodiment of the present invention, a 
tachometer-type speed signal is developed from the charging and 
discharging surges of the tirgger power supply. The tachometer-type speed 
signal varies directly with the speed and is automatically strongly 
coupled to the trigger pulse circuit only below a selected minimum speed, 
preferably selected just below the idle speed. In a system employed a 
triggering threshold bias capacitor operatively in series opposition to 
the trigger pulse source, the inventors have found the timing 
characteristic may be appropriately modified by ahtomatically and rapidly 
reducing the bias voltage of such capacitor as the speed is reduced below 
the desired idle speed. In one embodiment of the present invention, the 
tachometer circuit may employ a bucket capacitor coupled to the trigger 
power supply for charging a resistor-capacitor filter network through 
suitable steering diodes. The resistor-capacitor filter network provides 
the voltage signal proportional to speed. A voltage-controlled "resistor" 
in the form of a field-effect transistor is connected in parallel with the 
threshold bias capacitor and at speeds below a selected speed becomes 
conducting to increase the drain or bleed of the bias capacitor. Above 
such selected speed, the "resistor" means effectively goes to a 
non-conducting state and the bias capacitor functions in the normal manner 
to maintain the essentially constant timing. Temperature stabilizing means 
may be provided, preferably in that portion of the circuit providing the 
voltage signal proportional to speed. 
In a particularly unique embodiment of the present invention in an ignition 
system having a pilot capacitor to fire the selected ignition control 
rectifier, the pilot capacitor functions dually to provide energy storage 
for the firing of the selected ignition control rectifier and also as the 
bucket capacitor for the tachometer circuit, transferring an essentially 
constant bucketful of electrical charge to the resistor-capacitor filter 
network during each recharge following firing of an ignition control 
rectifier. 
The pilot capacitor is charged in series with the RC filter network of the 
tachometer circuit to create a speed signal voltage proportional to the 
frequency of the charging, and thus proportional to speed. The speed 
signal is a voltage which is positive relative to a common signal ground 
while the bias capacitor voltage is negative relative to such signal 
ground. The speed signal is applied to control the conductivity of a 
silicon semiconductor means connected across the bias capacitor. The 
semiconductor means is selected to properly combine and respond to the 
opposite polarity of the speed signal and the bias capacitor voltage. A 
particularly unique and satisfactory semiconductor means is a P-channel 
depletion-mode junction field-effect transistor (JFET) in which a positive 
gate signal is operative to reduce the apparent conductivity of the 
source-to-drain channel of the transistor. The transistor source is 
connected to the common signal ground, which is also the positive terminal 
of the bias capacitor, and the transistor drain is connected to the 
negative side of such capacitor. The source-to-drain channel of the 
transistor is, therefore, connected directly in parallel with the bias 
capacitor. The speed signal will always be either a positive voltage or 
zero and is connected to the transistor gate. When the signal is zero, and 
thus the gate-to-source voltage is zero, the source-to-drain resistance is 
a minimum and the maximum drain or bleed current can flow to reduce the 
bias voltage on the bias capacitor which results in an automatic advance 
of the spark. As the engine speeds up, the speed voltage signal increases 
with a corresponding increased gate voltage which increases the apparent 
source-to-drain resistance and reduces the bleed-off current which can 
flow from the capacitor. By proper selection and design, the effect of the 
JFET is virtually eliminated at an engine speed increased slightly above 
idle speed, and the system continues to operate with the bias capacitor 
maintaining the highly desired and effective constant timing 
characteristic with speed. 
The idle speed governor therefor electronically produces an automatic 
advance of the spark as the engine speed slows down below a selected 
speed. 
The present invention thus provides a highly effective and relatively 
simple means for providing an electronic idle speed governor for 
stabilizing the operation of internal combustion engines particularly 
engines having trigger pulse means of finite trigger pulse slope and where 
controlled variation of the ignition system triggering threshold can 
provide the desired electronic spark advance for idle speed stabilization.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
Referring to the drawings, and particularly to FIG. 1, the illustrated 
embodiment of the invention includes an engine driven alternator unit 1 
coupled to a two-cylinder engine 2 which may be mounted as the power head 
of an outboard propulsion drive unit 3. The outputs of alternator 1, 
comprising the capacitor charging output and the trigger pulse output, are 
connected to an ignition circuit 4 for firing of the engine 2. The 
ignition circuit 4 is a capacitive discharge system including a separage 
pair of ignition capacitors 5 and 6, which are alternately and cyclically 
charged and discharged from alternator unit 1 to an interrelated and 
connected spark plug 7 and 8 through suitable gated discharge circuits 9 
and 10. The illustrated alternator unit 1 and ignition circuit 4 provide a 
highly satisfactory ignition system for normal operation. However, at 
engine idle speed, an uneven running condition is often encountered 
resulting from the creation of a retard of the spark. In accordance with 
this invention, an idle speed governor circuit 11 is connected to the 
discharging circuit means 9 and 10 and automatically and electronically 
changes the retard of the spark to an advance and thereby eliminates the 
adverse idle speed condition. The alternator unit 1 and the ignition 
circuit 4 employing the alternately operative capacitor 5 and 6 are 
similar to that shown in the previously identified U.S. Pat. No. 3,805,759 
and the copending application of JAMES R. HAGER entitled MULTIPLE 
CAITOR MEANS IGNITION SYSTEM, filed on Jan. 19, 1976 with Ser. No. 
650,418 and assigned to the same assignee. The ignition system is briefly 
described herein with sufficient detail to clearly describe and explain 
the illustrated embodiment of the present invention. 
The alternator 1 includes a magnet secured within the engine flywheel and 
polarized radially of the rotor to define two distinct, circumferential 
magnets as at 12 and 13, which are oppositely and radially polarized to 
define a pair of flux reversal junctions 14 and 15 separated by 
180.degree.. A stator section 16 includes a semi-circular laminated core 
17 having end poles 18, on which charging windings or coils 19 are wound. 
The core is semi-circular such that the coils 19 are also spaced by 
180.degree. and coupled twice in each complete revolution of the rotor to 
the flux reversible junctions 14 and 15. 
The charging coils 19 are connected in series aiding between leads 20 and 
21, the latter being connected to engine block ground. Lead 20 is 
connected to the topside of capacitor 5 by a charging diode 22 and the 
opposite side of capacitor 5 is connected by a diode 23 to ground and thus 
to lead 21. The voltage output of the windings 19 is thus directly 
connected to charge capacitor 5 when lead 20 is positive. Lead 21 is 
similarly connected to the topside of capacitor 6 via a diode 24, and lead 
20 is connected to the bottom side at capacitor 6 by diode 25, to provide 
charging current to the ignition capacitor 6, when lead 20 is negative. 
The flux reversals of the junctions 14 and 15 are in opposite directions 
with respect to the coils 19 to generate opposite polarity pulses and 
thereby alternately charge the capacitors 5 and 6. 
A trigger coil 26 is wound on a core 27 which is mounted for angular 
orientation between the stator poles 18 for sequential coupling to the 
flux reversal junctions 14 and 15. A trigger signal of opposite polarity 
is thus generated between the successive charging pulses. The successive 
opposite polarity triggering signals function to alternately fire the 
discharge circuits 9 and 10 for firing of the two spark plugs 7 and 8 of 
engine 2. The capacitors 5 and 6 are connected to the spark plugs 7 and 8 
by similar circuits 9 and 10 and the circuit for capacitor 5 is described 
specifically, with corresponding elements of the circuit for capacitor 6 
identified by corresponding primed numbers. The capacitor 5 is connected 
in series with an ignition control rectifier 28 and the primary of a pulse 
transformer 29. The secondary of the pulse transformer 29 is connected 
across the spark plug 7 and an ignition pulse is created by triggering 
rectifier 28 on. A trigger pulse circuit 30 is connected to the alternator 
unit 1 and particularly trigger winding 26 and to the rectifiers 28 and 
28' to discharge the capacitors 5 and 6 at the appropriate time. In 
accordance with this invention, the idle speed governor circuit 11 is 
connected to the pulse circuit 30. A pllot pulse or trigger capacitor 32 
is connected to be charged from the output of the alternator unit 1 
simultaneously with the charging of either of the ignition capacitors 5 
and 6. Capacitor 32 is charged from capacitors 5 and 6 through charging 
resistors 33 and 34 working into a common bleeder resistor 35. The 
resistance of the resistors 33 and 34 is high to prevent excessive 
discharge of the main firing capacitors 5 and 6 prior to firing. A pilot 
controlled rectifier 36 connects the trigger capacitor 32 to the gate 37 
of the ignition controlled rectifier 28 for providing a powerful turn-on 
current pulse into the gate 37 of rectifier 28, limited mainly by resistor 
35a. The trigger winding 26 is connected to turn on the pilot rectifier 36 
and discharge the trigger capacitor 32. 
The capacitor 6 is similarly discharged to fire spark plug 8, with the 
trigger capacitor 32 again providing an intermediate powerful trigger 
pulse source for controlled rectifier 28'. A capacitive-resistive coupling 
and timing stabilizing network 38 connects the trigger winding 26 to the 
pilot controlled rectifiers 36 and 36' with the trigger coil leads being 
connected via the coupling and stabilizing network 38 one each to each of 
the gates 39 and 39' of the pilot rectifiers 36 and 36'. The network 38 
includes a triggering threshold bias, and timing stabilizing capacitor 40 
to insure an essentially constant firing angle independent of speed. 
Advance and retard setting of the trigger coil 26 is separately 
controlled. This stabilizes the ignition angle generally as taught in U.S. 
Pat. No. 3,805,759, dated Apr. 23, 1974. The trigger coil 26 is movably 
mounted for limited angular orientation, preferably as a predetermined 
function of the throttle setting, for example, as disclosed in U.S. Pat. 
No. 3,937,200. The trigger coil 26 is illustrated in the FIG. 1 in maximum 
retard firing position and is coupled to the throttle for movement to a 
maximum advance firing position. In accordance with conventional practice, 
initial throttle movement only advances the timing to increase the speed 
and then further throttle movement opens the throttle plate in the 
carburetor in addition to advancing the timing. The unique idle speed 
governor of the present invention automatically and electronically 
modifies the timing characteristic during idle speed operation to 
similarly advance the spark only and thereby stabilize the engine 
operation. 
Generally, in the illustrated embodiment of the invention, the idle speed 
governor circuit 11 is a "bucket" type tachometer circuit which is 
connected to the pilot trigger capacitor 32 to receive a pulse rate 
related charging signal. The output of the tachometer circuit is applied 
to a junction field-effect transistor 41 (JFET) which is connected as a 
variable bleed resistance means across the timing stabilizing threshold 
bias capacitor 40. Generally, as the engine speed slows down below the 
desired idle speed, the field-effect transistor 41 acts as a relatively 
heavy drain or bleed on the stabilizing capacitor 40. Any reduction in the 
bias voltage acts to lower the triggering threshold and automatically 
advance the spark. This, in turn, causes the engine speed to increase back 
toward the desired idle speed and effectively produces a much more 
constant engine idle speed. As the engine speed is deliberately increased 
by control of the throttle, the tachometer-type voltage signal 
correspondingly increases. The increased voltage signal on the 
field-effect transistor 41 increases its apparent resistance connected 
across the stabilizing capacitor 40 and at a speed very slightly higher 
than that of idle speed, the effect of JFET 41 is virtually removed from 
the circuit. The ignition system then operates to maintain an essentially 
constant timing over the normal operating speed of the engine, 
disregarding the angular repositioning of the trigger coil of course, as 
disclosed in the previously referred to patent. 
The idle speed governor circuit of the present invention with its 
electronic adjustment of timing significantly improves the idle speed 
operation of the engine while still permitting the normal satisfactory 
stabilized timing operation of the engine at all other speeds. 
More particularly, in accordance with the illustrated embodiment, the 
trigger capacitor 32 is coupled to the common signal ground through the 
idle speed governor unit 11 as follows. A diode 42 connects the negative 
side of the trigger capacitor 32 in series with a tachometer capacitor 44 
to the common signal ground lead 44A. A resistor 45 is connected in 
parallel with capacitor 44, and a voltage proportional to the frequency of 
ignition system firing is developed on the tachometer capacitor. The 
resistor 45 in parallel with the capacitor 44, controls the time rate of 
discharge and thereby provides a means of calibrating the voltage on the 
tachometer capacitor 44 against speed. A trigger capacitor discharge diode 
46 is connected between the junction of the trigger capacitor 32 with the 
diode 42 and the common signal ground lead 44A. When a pilot rectifier 36 
is triggered, the trigger capacitor 32 is discharged through the common 
resistor 35A, the triggered rectifier 36, the gate-to-cathode circuit of 
the main silicon control rectifier 28 to the common signal ground lead 44A 
and back to the trigger capacitor 32 via the return diode 46. The positive 
side of the tachometer capacitor 44 is connected in series with a resistor 
49 to the gate 50 of the JFET 41. A filter capacitor 51 is connected 
between the gate 50 and the signal ground lead 44A, and with the signal 
coupling resistor 49 constitutes a filter to remove much of the ripple of 
the tachometer voltage being supplied by capacitor 44. 
The transistor 41 is a P-channel depletion mode junction field-effect 
transistor. The source 52 is connected directly to the signal ground lead 
44A and consequently to the positive side of the bias capacitor 40 of the 
trigger circuit 11. The drain 53 of the field-effect transistor 41 is 
connected to the opposite or negative side of the bias capacitor 40. The 
cource-to-drain channel of the field-effect transistor 41 is thus 
connected directly in parallel with the capacitor 40 and depending upon 
its effective resistance may increase the discharge rate of the capacitor. 
A maximum drain or bleed-off current flows when the field-effect 
transistor 41 has zero voltage on the gate and a minimum current flows 
when a large positive voltage is applied to the gate 50. A junction 
field-effect transistor employs the characteristics of a reverse-biased 
junction to develop a depletion region between the source and the drain 
and thereby control the source-to-grain current. The P-channel junction 
field-effect transistor is ideally suited to the present application 
because the polarity relationships are uniquely suited to the application 
of employing the positive voltage developed by the tachometer signal 
capacitor 44 for controlling the negative voltage developed by the timing 
stabilizing capacitor 40. 
In the illustrated embodiment of the invention, the tachometer voltage is 
either zero or positive. When the tachometer voltage is zero, the gate 
voltage is zero and the field-effect transistor exhisibts a minimum 
resistance between the positive source and negative drain conditions. As a 
result, the stabilizing capacitor 40 is subjected to a maximum discharge 
or draining current. This reduces the vias voltage of the timing 
stabilizing capacitor to a minimum level. Any reduction in such bias 
voltage automatically advances the triggering and the timing of the spark, 
thereby effectively increasing the engine speed. The tachometer voltage is 
small at idle and lower speeds and is set to a level to establish the 
required transistor conductivity by proper trimming of the calibration 
resistor. 
As the engine speed increases, the tachometer voltage rises, providing a 
corresponding increased positive voltage on the gate. In accordance with 
the well-known characteristics of a JFET, the resistance of the flow path 
or channel between the source and drain increases and the drain current 
from the bias capacitor is correspondingly reduced. At engine speeds only 
slightly above idle operating speed the effective resistance of the JFET 
41 increases such that the effect of the JFET is virtually absent. 
Overall, the effect is to advance the spark as the engine slows down below 
the selected speed but conversely to completely eliminate the idle 
governor action as the engine speed increases above the selected speed. 
The particular speed at which the effect of JFET 41 is effectively removed 
can be readily controlled by adjustment of the trimming resistor 45 
connected in parallel with the tachometer capacitor 44. The trimmable 
resistor 45 is preferably of the type which may be machine trimmed to 
permit accurate and production adjustment of the idle speed governor 
circuit. 
Thus, in the illustrated embodiment of the invention, the pilot or trigger 
capacitor 32 has the dual function of providing a pilot trigger pulse as 
in the prior art and further providing the function of a "bucket" 
capacitor for a tachometer circuit. 
The voltage pulse generated by trigger winding 26 spans a finite number of 
crankshaft degrees and this pulse width is essentially a constant 
independent of engine speed. However, the amplitude of this voltage pulse 
is proportional to engine speed. Therefore, the slope (volts/degree) of 
the trigger voltage pulse is proportional to engine speed. At relatively 
low engine speeds such as around idle, the engine's ignition timing will 
retard due to the decreasing slope of the generated trigger voltage as the 
engine speed decreases. Instantaneous speed changes around idle are 
related to the varying cylinder pressures and the flywheel moment of 
inertia. Also, poor fuel distribution may contribute to relatively large 
instantaneous speed variations. If the timing retards as a function of 
decreasing speed, this characteristic will further contribute to uneven or 
poor idling of the engine. 
Without the idle governor circuit 11, the timing characteristic with the 
trigger coil 26 in a fixed position is typically as shown at 55 in FIG. 2. 
Idle speed is approximately 600 to 620 RPM. Below idle speed, the timing 
characteristic includes a very slight advance and then a relatively sharp 
retard as the speed further decreases. Above idle speed the angle of 
retard increases slowly and generally along a straight line curve. The 
characteristic above idle is highly satisfactory for smooth engine 
operation. The characteristic below idle speed, however, contributes to 
uneven running of the engine. 
The idle speed governor circuit 11 drastically changes the timing 
characteristic immediately below the idle speed, as shown at 56, in FIG. 
2, without changing the characteristic above idle speed, as also shown in 
FIG. 2. The governor, by drastically reducing the charge on the 
stabilizing capacitor 40 causes a very rapid advance in the timing as the 
speed decreases below idle speed. The timing advance tends to increase the 
engine speed and thereby produce a stable idle speed operation. The idle 
timing characteristic shown is typical of a four horsepower engine 
employed in small outboard motors and illustrates a characteristic 
particularly designed to establish one degree of advance at 550 RPM which 
rises rapidly to a peak of seven degrees at 400 RPM. This timing 
characteristic provides a relatively large timing advance with relatively 
small decreases in speed, such that the idle speed governor circuit is 
sensitive and highly responsive at speeds slightly below idle speed. Above 
idle speed the timing characteristic with and without the idle speed 
governor circuit are essentially identical. 
However, a separate tachometer control system can, of course, be employed. 
For example, referring particularly to FIG. 3, a separate "bucket" 
tachometer circuit 57 is connected to the charging circuit and includes a 
separate "bucket" capacitor 58 connected directly to be charged in 
parallel with the trigger capacitor 32 and functions as a small "bucket" 
capacitor. The tachometer capacitor 59 is similarly connected in the 
circuit to the "bucket" capacitor 58 and to the gate of a field-effect 
transistor 60. In the embodiment of the invention shown in FIG. 3, a drain 
resistor 62 in parallel with a thermister 63 is connected in the drain 
lead. The thermister may be employed to provide additional temperature 
stabilization. The inventors have found that such a circuit is not 
necessary because satisfactory temperature stabilization may be provided 
by proper selection of the calibrating resistor and bucket capacitor. 
The present invention thus provides a simple and reliable idle speed 
governor for solid state ignition systems having a retarding timing 
characteristic below idle speeds. 
Various modes of carrying out the invention are contemplated as being 
within the scope of the following claims particularly pointing out and 
distinctly claiming the subject matter which is regarded as the invention.