Discharge ignition apparatus for internal combustion engine having automatic spark advance

An improved capacitive discharge ignition apparatus provides spark advance with engine speed. The ignition apparatus includes at least one trigger circuit including a trigger coil preferably contained in a common housing with other circuit elements. In this case, an auxiliary coil may be provided to cancel certain pulses in the trigger coil output that could otherwise interfere with desired operation. In some embodiments, a second trigger circuit is provided to derive a triggering signal from the primary coil of the ignition's step-up transformer. A triggering signal of appropriate magnitude received from either trigger circuit will cause discharge of an energy storage element through the primary coil.

BACKGROUND OF THE INVENTION 
The present invention generally relates to an improved ignition system for 
use in an internal combustion engine. More particularly, the invention 
relates to a discharge ignition apparatus that provides automatic spark 
advance at predetermined operating speeds. 
In the case of large displacement engines, ignition circuits of relatively 
elaborate design have often been provided to advance the ignition spark as 
engine speed is increased. For example, the spark may be delayed at 
starting speeds until approximately peak compression of the engine's 
piston. At higher engine speeds, the spark is preferably advanced to occur 
before peak compression. In many cases, an advance of twenty (20) 
mechanical degrees or more would not be uncommon. 
The present invention is directed to various novel ignition arrangements 
providing automatic spark advance that are particularly well-suited for 
smaller displacement engines. 
SUMMARY OF THE INVENTION 
In one aspect, the present invention provides an ignition apparatus for use 
with an internal combustion engine to produce an electrical spark at a 
spark ignition device. The apparatus comprises a magnet assembly, 
including a pair of pole faces, operatively revolved along a circular 
path. A magnetically permeable core having at least two leg portions each 
including a respective end face is mounted adjacent the circular path. The 
leg portions are situated such that the pole faces pass proximate to the 
end faces during revolution of the magnet assembly. As a result, a 
time-varying magnetic flux is produced in the magnetically permeable core. 
The ignition apparatus further includes a transformer having a primary coil 
and a secondary coil related by a predetermined step-up ratio. The 
secondary coil is electrically connected during operation to the spark 
ignition device. A spark generation circuit is operative to apply a 
primary voltage pulse to the primary coil responsive to a triggering 
signal of a predetermined polarity. A spark generating pulse is 
responsively produced in the secondary coil. 
In addition, the ignition apparatus includes triggering circuitry having a 
trigger coil and an auxiliary coil connected in circuit. For example, the 
auxiliary coil may be connected in series with the trigger coil. The 
triggering circuitry applies a trigger coil triggering signal of the 
predetermined polarity to the spark generation circuit to produce a 
predetermined spark advance at predetermined operating speeds. In some 
exemplary embodiments, the trigger coil operatively produces first and 
second excursions of the predetermined polarity with the auxiliary coil 
substantially cancelling the second excursion from the trigger coil 
triggering signal. 
Often, the spark generation circuit may include a charge coil situated 
about one of the leg portions of the magnetically permeable core. In such 
cases, the auxiliary coil is preferably wound coaxial with the charge coil 
with the trigger coil being spaced apart from the transformer by a 
predetermined physical angle. For example, the auxiliary coil may comprise 
a tap from the charge coil at a predetermined number of turns. The trigger 
coil may be wound about a magnetically permeable pole piece separate from 
the magnetically permeable core. Preferably, the transformer and the 
trigger coil are mounted inside of a common housing. 
In some exemplary embodiments, the triggering circuitry further comprises a 
primary coil trigger circuit adapted to apply a primary coil triggering 
signal to the spark generation circuit. For example, the primary coil 
trigger circuit may comprise a voltage divider network connected across 
the primary coil for producing the primary coil triggering signal at a 
divider node thereof. The spark generation circuitry will often include an 
electronic switch having a common node to which the trigger coil 
triggering signal and the primary coil triggering signal are applied. 
In other aspects, the present invention provides an ignition apparatus for 
use with an internal combustion engine to produce an electrical spark at a 
spark ignition device. The apparatus comprises a magnet assembly, 
including a pair of pole faces, operatively revolved along a circular 
path. A magnetically permeable core having at least two leg portions each 
including a respective end face is mounted adjacent to the circular path. 
The leg portions are situated such that the pole faces pass proximate to 
the end faces during revolution of the magnet assembly. As a result, a 
time-varying magnetic flux is produced in the magnetically permeable core. 
The ignition apparatus further includes a transformer having a primary coil 
and a secondary coil related by a predetermined step-up ratio. The 
secondary coil is electrically connected during operation to the spark 
ignition device. A spark generation circuit is operative to apply a 
primary voltage pulse to the primary coil responsive to a triggering 
signal of a predetermined polarity. A spark generating pulse is 
responsively produced in the secondary coil. 
In addition, the ignition apparatus includes triggering circuitry having a 
trigger coil spaced apart from the transformer by a predetermined physical 
angle. The trigger coil applies a trigger coil triggering signal to the 
spark generation circuit to produce a predetermined spark advance at 
predetermined operating speeds. The trigger coil is wound about a 
magnetically permeable pole piece separate from the magnetically permeable 
core, but is mounted inside a common housing with the transformer. 
In some exemplary embodiments, the triggering circuitry further comprises a 
primary coil trigger circuit adapted to apply a primary coil triggering 
signal to the spark generation circuit. For example, a voltage divider 
network may be connected across the primary coil for producing the primary 
coil triggering signal at a divider node thereof. The spark generation 
circuitry will often include an electronic switch having a common node to 
which the trigger coil triggering signal and the primary coil triggering 
signal are applied. 
The triggering circuitry of the ignition apparatus may further comprise an 
auxiliary coil electrically connected in series with the trigger coil. The 
spark generation circuit may include a charge coil situated about a leg 
portion of the magnetically permeable core, with the auxiliary coil being 
wound coaxial with the charge coil. For example, the auxiliary coil may 
comprises a tap from the charge coil at a predetermined number of turns. 
Still further aspects of the present invention provide a discharge circuit 
for use in a discharge ignition system of the type operative to produce an 
electrical spark at a spark ignition device. The discharge circuit 
comprises a storage capacitor, a charge coil, and a rectifier electrically 
connected therebetween. A transformer is also provided, including a 
primary coil and a secondary coil. The secondary coil is electrically 
connected during operation to the spark ignition device to produce the 
electrical spark. 
The discharge circuit further includes an electronic switch electrically 
connected in circuit with the storage capacitor and the primary coil. The 
electronic switch is rendered conductive by a triggering signal applied to 
a triggering node thereof. Toward this end, a triggering circuit is 
electrically connected to the triggering node. The triggering circuitry 
includes a trigger coil and an auxiliary coil arranged in series to apply 
a modified trigger coil signal to the triggering node. In some exemplary 
embodiments, a diode is electrically connected between the trigger coil 
and the triggering node. Preferably, a resistor is also electrically 
connected between the trigger coil and the triggering node. 
Additional aspects of the present invention are provided by a discharge 
ignition apparatus for use with an internal combustion engine to produce 
an electrical spark at a spark ignition device. The apparatus comprises a 
movable magnet assembly including a pair of pole faces. A magnetically 
permeable core is provided, having at least two leg portions each 
including a respective end face. The magnetically permeable core is 
mounted such that the pole faces pass proximate to the end faces as the 
magnet assembly is operatively moved in a cyclical manner to produce a 
time-varying magnetic flux in the magnetically permeable core. A housing 
is mounted to at least one of the leg portions of the magnetically 
permeable core. A transformer having a primary coil and a secondary coil 
is located in the housing and situated about the magnetically permeable 
core. The secondary coil of the transformer is electrically connected 
during operation to the spark ignition device. 
The discharge ignition apparatus further comprises a discharge circuit 
located in the housing. The discharge circuit includes a charge coil 
situated about the magnetically permeable core to have a charging voltage 
induced thereon by the magnetic flux. As a result, a charging energy is 
supplied to an energy storage element. 
An electronic switch is electrically connected in circuit with the energy 
storage element and the primary coil. Activation of the electronic switch 
during operation produces a voltage on the primary coil. Toward this end, 
triggering circuitry is provided, including a trigger coil spaced apart 
from the transformer by a predetermined physical angle and having a pole 
piece separate from the magnetically permeable core. The triggering 
circuitry operates to apply a triggering signal to the electronic switch 
yielding a predetermined spark advance at predetermined operating speeds. 
In some presently preferred embodiments, the triggering circuitry further 
comprises a primary coil trigger circuit such as a voltage divider network 
connected across the primary coil. In many such embodiments, the 
electronic switch will have a single triggering node. 
The triggering circuitry may also comprise an auxiliary coil electrically 
connected in series with the trigger coil. Preferably, the auxiliary coil 
may be wound coaxial with the charge coil. For example, the auxiliary coil 
may comprise a tap from the charge coil at a predetermined number of 
turns. 
Other objects, features and aspects of the present invention are discussed 
in greater detail below.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
It is to be understood by one of ordinary skill in the art that the present 
discussion is a description of exemplary embodiments only and is not 
intended as limiting the broader aspects of the present invention, which 
broader aspects are embodied in the exemplary constructions. 
FIG. 1 illustrates a discharge ignition apparatus constructed in accordance 
with the present invention. The apparatus is configured to produce the 
requisite spark at spark plug 10 to ignite the air-fuel mixture within the 
piston cylinder of the engine. The apparatus may be used with various 
devices powered by gasoline engines, particularly hand-held two-cycle and 
four-cycle gasoline engines. 
Generally, the apparatus includes a stator unit 12 and a rotatable flywheel 
14. Flywheel 14 typically includes a central bore for mounting to a 
rotatable spindle mechanically interconnected with the engine's drive 
shaft. As a result, rotation of the spindle will produce a concomitant 
rotation of flywheel 14 (such as in the direction indicated by the arrow). 
Stator unit 12, which typically remains fixed with respect to the engine 
during use, includes a magnetically permeable core 16. In this case, core 
16 includes two depending leg portions, respectively indicated at 18 and 
20. In many embodiments, however, the magnetically permeable core may be 
constructed having three such leg portions. 
A sealed housing 22 maintains the various coils and other components 
utilized to produce a spark at spark plug 10. In particular, housing 22 
includes a transformer having a primary coil 24 and a secondary coil 26. 
In the illustrated embodiment, coils 24 and 26 may be mounted coaxially 
about leg portion 20. A charge coil 28, which may also be mounted about 
leg portion 20, provides a source of energy for the ignition spark as will 
be explained more fully below. 
A trigger coil 30 is also contained within housing 22, wound about a 
separate, magnetically permeable pole piece 32. An auxiliary coil 34 is 
wound coaxial with charge coil 28 to provide a "tight" electromagnetic 
coupling thereto. Preferably, charge coil 28 and auxiliary coil 34 are 
configured as a single coil from which the ground node is tapped at a 
predetermined number of turns. 
The various components within housing 22 may be protected and maintained 
securely in position via a suitable potting compound. Electrical 
connection with spark plug 10 is achieved by a typical interconnecting 
wire 36. 
A magnet assembly is mounted adjacent the periphery of flywheel 14 to 
revolve about a circular path in synchronism with operation of the engine. 
The magnet assembly includes a permanent magnet 38 having pole pieces 40 
and 42 mounted at respective ends thereof. It will be appreciated that the 
circumferential faces of pole pieces 40 and 42 will pass proximate to the 
end faces of leg portions 18 and 20 as flywheel 14 is rotated. Rotation of 
flywheel 14 thus produces a time-varying magnetic flux within core 16, as 
desired. 
It can be seen that pole piece 32 is separated from leg portion 20 by a 
predetermined physical angle. The separation between pole piece 32 and leg 
portion 20 causes a time-varying flux to be induced in pole piece 32 
before the flux induced in core 16. This leading flux can be used to 
provide spark advance in the manner described in more detail below. 
Preferably, the air gap between core 16 and pole pieces 40 and 42 will be 
much smaller than the air gap between pole piece 32 and pole pieces 40 and 
42. For example, the air gap with core 16 may fall within a range of 
0.010" to 0.015" in some preferred embodiments. In such cases, the air gap 
with pole piece 32 may be on the order of 0.25". The larger air gap 
produces a more rounded pulse shape used for spark advance as will be 
described below. 
With smaller engines, the flywheel will often have a radius of between 
about 1.75" to 2.0". In such cases, the physical separation between leg 
portion 20 and pole piece 32 will often be about 0.5", or about 14 to 16 
mechanical degrees. 
FIG. 2 illustrates the various electronic components contained within 
sealed housing 22 in one preferred embodiment of the present invention. As 
can be seen, secondary coil 26 is connected across the gap 44 of spark 
plug 10. Charge coil 28 is electrically connected to a storage capacitor 
46 through a rectifier diode 48. A resistor 50 is provided to attenuate 
transient voltages produced as diode 48 changes from forward conducting to 
reverse blocking. In addition, diode 48 may be a breakdown diode of 
predetermined characteristics to prevent overvoltage on charge coil 28 
which could occur by transformer action during the main discharge. 
Capacitor 46 is, in turn, electrically connected in circuit with primary 
coil 24 through a silicon-controlled rectifier (SCR) 52. Diode 54 
functions as a ringback diode for reversal of the polarity of capacitor 46 
during discharge. A relative ground, as indicated at 56, typically 
provides electrical communication with the engine block. Although not 
shown, a stop switch may be provided to selectively ground charge coil 28 
and thereby disable operation of the ignition system. 
SCR 52 is rendered conductive by application of a triggering signal to 
triggering node 58, which is the SCR gate. In this case, the triggering 
signal is produced by a triggering circuit including trigger coil 30 and 
auxiliary coil 34 connected in series as shown. It can thus be seen that 
the triggering signal is the output of trigger coil 30, as modified by the 
various elements of the trigger circuit. 
In the illustrated embodiment, the trigger circuit includes a loading 
resistor 60 provided across auxiliary coil 34. A further loading resistor 
62 is provided between ground and the output of trigger coil 30. As one 
skilled in the art will recognize, resistors 60 and 62 appropriately scale 
the induced voltage on their respective coil. Diode 64 and resistor 66 act 
together to protect SCR 52 from harmful voltages and suppress any unwanted 
leakage pulses. 
The operation of the circuit shown in FIG. 2 will now be explained with 
reference to the waveforms illustrated in FIGS. 3A through 3D. The 
illustrated waveforms are merely diagrammatic in nature for which scale is 
not implied. In addition, one skilled in the art will recognize that 
references to "positive" or "negative" are merely a matter of convention. 
It will also be appreciated that the illustrated sequence is repeated for 
every revolution of the magnet assembly. 
FIG. 3A illustrates a waveform V.sub.c of the voltage produced across 
charge coil 28 during one passage of the magnet assembly carried by 
flywheel 14. As can be seen, waveform V.sub.c includes a first positive 
excursion 68 followed by a relatively large negative excursion 70. A 
further positive excursion 72 follows negative excursion 70. One skilled 
in the art will recognize excursion 72 as a typical series RCL response to 
the exponential voltage generated by the charge coil. Capacitor 46 is 
charged to its peak value at this time. 
FIG. 3B illustrates a waveform V.sub.a such as may be induced across 
auxiliary coil 34 at corresponding points in time. Because auxiliary coil 
34 is tightly coupled to charge coil 28 as explained above, waveform 
V.sub.a will have a shape similar to V.sub.c, but opposite polarity and 
different magnitude. Thus, waveform V.sub.a exhibits a negative excursion 
74 followed by a positive excursion 76. Positive excursion 76 is then 
followed by a negative excursion 78. 
FIG. 3C illustrates waveform V.sub.tr produced across trigger coil 30, 
which has four excursions 80, 82, 84 and 86. It can be seen that 
excursions 80 and 84 are negative, with excursions 82 and 86 being 
positive. Excursions 80, 82 and 84 are produced by generator action as the 
magnet assembly carried by flywheel 14 passes pole piece 32. This part of 
waveform V.sub.tr is time shifted forward with respect to waveform 
V.sub.c. As noted above, the relatively "loose" coupling between pole 
piece 32 and magnet 38 provides a more rounded waveform shape, which 
allows the time-shifted signal to provide a relatively large spark 
advance. 
Excursion 86 of waveform V.sub.tr is produced by transformer interaction 
between charge coil 28 and trigger coil 30 during the capacitor charging 
cycle. This results from the close proximity of these two coils in compact 
module packages such as that shown in the illustrated embodiment. It will 
be appreciated that excursion 86 should not reach node 58, or capacitor 46 
will be shorted at the onset of when it would otherwise be charged. 
While a number of electronic techniques may suitably suppress excursion 86, 
the illustrated embodiment utilizes coils to provide an equal and opposite 
pulse. In particular, auxiliary coil 34, strongly coupled to the source of 
unwanted pulse, produces excursion 78 in opposition to excursion 86. 
Resistor 60 loads excursion 78 to the appropriate level for addition to 
the output of trigger coil 30. 
FIG. 3D shows the resultant waveform V.sub.tm, which may be applied to 
triggering node 58 through diode 64. As can be seen, the trigger coil 
output has been modified to eliminate the undesired positive excursion 
during charging of capacitor 46. The resulting negative excursions 88 and 
90 are blocked by diode 64. The relatively large positive excursion 92 
passes through diode 64 and gates SCR 52. As a result, energy stored in 
capacitor 46 during the previous charging cycle is released to primary 
coil 24. 
It will be appreciated that excursion 92 will not gate SCR 52 until a 
certain gating threshold is reached. At low engine speed, the gating 
threshold will not be reached until a time t.sub.2 near the peak of 
excursion 92. As speed is increased, the point on the waveform where 
gating occurs will move down the rounded curve (or back in time), yielding 
spark advance with speed. The maximum spark advance occurs at time 
t.sub.1. In many embodiments, the advance angle e represented by the 
region between times t.sub.1 and t.sub.2 may be at least fifteen 
mechanical degrees. 
FIG. 4 illustrates an alternative circuit arrangement that may achieve an 
even greater degree of spark advance. Many aspects of this circuit are 
similar to the circuit of FIG. 2 and, for the sake of brevity, will not be 
discussed in detail. Common or analogous elements have been given a 
reference number augmented by one-hundred in relation to their 
counterparts in the previous embodiment. 
In this case, it can be seen that a voltage divider network has been 
provided across primary coil 124. This voltage divider network includes a 
pair of serially connected resistors 192 and 194 defining a divider node 
196 therebetween. Divider node 196 is, in turn, connected to triggering 
node 158 through diode 198. 
FIGS. 5A through 5E illustrate various waveforms produced in the circuit of 
FIG. 4. The first four waveforms, i.e., FIGS. 5A through 5D, are similar 
to waveforms produced at corresponding locations in the circuit of FIG. 2. 
For purposes of simplicity, FIG. 5B also indicates the primary coil 
waveform V.sub.p ', since this waveform will have a polarity and shape 
similar to the auxiliary coil waveform V.sub.a '. 
The primary coil waveform V.sub.p ' is scaled by the voltage divider 
network and applied to node 158 through diode 198. This is in addition to 
the waveform V.sub.tm ' which is also applied to node 158 through diode 
164. The cumulative waveform V.sub.g applied to node 158 through the 
combined action of these two triggering circuits is shown in FIG. 5E. As 
can be seen, waveform V.sub.g has a first positive excursion 200 
contributed by the waveform V.sub.tm, followed by a second positive 
excursion 202 of greater amplitude contributed by waveform V.sub.p '. 
The manner in which the cumulative gating waveform produced in the circuit 
of FIG. 4 provides the desired spark advance can be most easily understood 
with reference to FIGS. 6A and 6B. Specifically, FIG. 6A shows a gating 
waveform V.sub.g ' such as may be produced at relatively low engine 
speeds. In this case, excursion 200' remains below the gating threshold 
and will thus pass without causing SCR 152 to gate. At the lowest speeds, 
SCR 152 is gated near the peak of excursion 202'. As shown in the region 
A, gating will occur earlier along the leading edge of excursion 202' as 
engine speed is increased. 
As shown in FIG. 6B, further increases in engine speed will cause excursion 
200" to eventually reach the gating threshold. As this point, indicated by 
the region B, the gating point will advance in a stepwise manner to the 
peak of excursion 200". As speed further increases, the gating point will 
advance down the leading edge of excursion 200", as shown in region C. 
FIG. 7 illustrates a typical timing curve that may be produced in the 
circuit of FIG. 4. As described, the spark will advance gradually in 
region A before making a significant step advance when region B is 
reached. After region B, the spark again advances gradually in region C 
until the maximum advance has been achieved. In many embodiments, a spark 
advance of at least twenty-five mechanical degrees can be achieved in this 
manner. 
It will be appreciated that various circuit arrangements can be utilized to 
achieve functional results as described herein. Thus, FIGS. 8A through 8C 
illustrate further exemplary embodiments of an ignition discharge circuit 
constructed in accordance with the present invention. Elements common or 
analogous to previous embodiments are indicated by references numbers 
augmented by one-hundred in each case. One skilled in the art will 
understand operation of these circuits without a detailed explanation. 
While preferred embodiments of the invention have been shown and described, 
modifications and variations be made thereto by those of ordinary skill in 
the art without departing from the spirit and scope of the present 
invention. For example, it may be desirable in some circuit arrangements 
to substitute an inductor or other circuit component as the energy storage 
element. In addition, it should be understood that aspects of various 
embodiments of the invention may be interchanged both in whole or in part. 
Furthermore, those of ordinary skill in the art will appreciate that the 
foregoing description is by way of example only, and is not intended to be 
limitative of the invention so further described in the appended claims.