U.S. patent number 3,619,634 [Application Number 05/056,182] was granted by the patent office on 1971-11-09 for alternator and combined breakerless ignition system.
This patent grant is currently assigned to R. E. Phelan Company Inc.. Invention is credited to Bob O. Burson.
United States Patent |
3,619,634 |
Burson |
November 9, 1971 |
ALTERNATOR AND COMBINED BREAKERLESS IGNITION SYSTEM
Abstract
An alternator for use with an internal combustion engine
includes a rotor having a plurality of generating magnets and a
stator carrying a number of coils in which voltages are induced by
the movement of the generating magnets therepast. One of these
coils is used to charge the capacitor of an associated capacitor
discharge ignition system for the engine, and triggering of the
ignition system to control the discharge of the capacitor and
firing of the associated spark plug is achieved by a trigger signal
producing means including a magnet separate from the generating
magnets, a trigger coil, and irregularities such as pins on the
rotor for varying the flux of the trigger magnet through the
trigger coil in response to rotation of the rotor.
Inventors: |
Burson; Bob O. (East
Longmeadow, MA) |
Assignee: |
R. E. Phelan Company Inc. (East
Longmeadow, MA)
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Family
ID: |
22002715 |
Appl.
No.: |
05/056,182 |
Filed: |
July 10, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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834847 |
Jun 19, 1969 |
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675429 |
Oct 16, 1967 |
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Current U.S.
Class: |
307/10.6;
310/153; 123/406.57; 123/600 |
Current CPC
Class: |
F02P
1/086 (20130101) |
Current International
Class: |
F02P
1/08 (20060101); F02P 1/00 (20060101); F02p
003/06 () |
Field of
Search: |
;307/1R
;123/148AC,148E,149 ;310/153 ;320/14 ;322/91 ;323/58
;315/209,218 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schaefer; Robert K.
Assistant Examiner: Smith; William J.
Parent Case Text
This application is a continuation of copending application Ser.
No. 834,847, filed June 19, 1969 and entitled "Alternator and
Combined Breakerless Ignition System," which application in turn is
a division of previous application Ser. No. 675,429, filed Oct. 16,
1967 and entitled "Inductively Triggered Breakerless Ignition
System With Automatic Spark Advance."
Claims
I claim:
1. In an electrical system for a spark ignition engine the
combination comprising: an alternator having a stator fixed
relative to the stationary structure of said engine and a rotor
rotated in synchronism with the operation of said engine, said
stator including a plurality of poles consisting of a first group
of poles containing more than one pole and a second group of poles
containing at least one additional pole, a first group of windings
each of which is received on a respective one of said first group
of poles, a second group of windings each of which is received
solely on a respective one of said second group of poles, said
rotor including a plurality of generating magnets which cooperate
with said stator poles to induce voltage waveforms in said windings
as said rotor rotates, a load circuit, means connecting said first
group of windings to said load circuit so as to provide electrical
power thereto, a spark gap ignition device, an electrical storage
device connected with said second group of windings for storing
electrical power provided by said second group of windings, means
including an electronic switching device connected with said
storage device for controlling the transmission of power from said
storage device to said spark gap device and for causing the
occurrence of a spark at said spark gap device as said switching
device is switched from a first state to a second state, a
triggering coil separate from said stator located adjacent said
rotor and coupled with said switching device and operable to switch
said switching device from said first state to said second state
when the voltage across said triggering coil reaches a
predetermined level, a triggering magnet separate from said
generating magnets, and means providing a circuit for the magnetic
flux of said triggering magnet which circuit passes through said
triggering coil and has a reluctance dependent on the angular
position of said rotor so that different magnitudes of voltage are
induced in said triggering coil at different angular positions of
said rotor.
2. The combination defined in claim 1 further characterized by said
storage device being a capacitor.
3. The combination defined in claim 1 further characterized by said
rotor including an outer circumferentially extending rim of
magnetic material surrounding said stator and within which said
generating magnets are received, said triggering coil being located
adjacent the outer surface of said rim, and said magnetic flux
circuit providing means for said triggering coil including an
irregularity on said outer surface of said rim.
4. The combination defined in claim 3 further characterized by said
irregularity comprising a protrusion extending radially outwardly
from said outer rim surface.
5. The combination defined in claim 3 further characterized by said
irregularity comprising a recess formed in said outer rim
surface.
6. The combination defined in claim 1 further characterized by said
rotor including an outer circumferentially extending rim of
magnetic material surrounding said stator and within which said
generating magnets are received said triggering coil being located
adjacent the outer surface of said rim, and said magnetic flux
circuit providing means for said triggering coil including at least
two angularly spaced irregularities which are of substantially
different configurations so as to induce voltage waveforms of
substantially different peak magnitudes in said triggering coil as
they move therepast.
7. The combination defined in claim 1 further characterized by each
of said windings of said first group of windings having a low
number of turns in comparison to the number of turns of each of
said windings of said second group so that each of said windings of
said second group has induced therein a substantially higher peak
voltage than that induced in each of said windings of said first
group.
8. The combination defined in claim 1 further characterized by said
second group of poles being comprised of only one pole and said
second group of windings being comprised of only one winding
received solely on said only one pole.
9. The combination defined in claim 8 further characterized by said
only one winding having a substantially larger number of turns than
each of said windings of said first group of windings.
Description
BACKGROUND OF THE INVENTION
This invention relates to ignition systems for spark ignited
engines, and deals more particularly with a breakerless ignition
system for use in combination with an alternator driven by the
associated engine.
Several different breakerless ignition systems have been proposed
in the past wherein the conventional mechanical breaker is replaced
by a transistor, silicon controlled rectifier, thyratron or other
electronic switch element controlled by a triggering signal. These
systems are generally desirable in that they can usually be made
more reliable than equivalent mechanical breaker systems and, being
less subject to mechanical wear and deterioration, have a longer
service life. Nevertheless, up to this time, the providing of an
automatic spark advance in breakerless systems has presented
problems, and the absence of a good means for obtaining an
automatic advance has in many cases been a drawback to the
acceptance of breakerless systems. Also, the providing of high
voltage power for effecting the spark at the spark plug and the
provision of suitable trigger signals has caused other
problems.
The general object of this invention is, therefore, to provide a
breakerless ignition system having a simple, inexpensive and
reliable means for producing the high voltage power required for
the firing of a spark plug and also for providing a suitable
trigger signal for controlling the timing of the firing. A further
object is to provide such a system which lends itself well to the
further provision of a simple, inexpensive and reliable means for
automatically producing an advance in the timing of the spark as
the speed of the associated engine increases. In the description
which follows, the invention is described in conjunction with an
ignition system utilizing a condenser as a storage device for
storing electrical power used to obtain the spark at the spark
plug, and this is the presently preferred application. This,
however, should not be taken as limiting the scope of the
invention, as the invention, in its broader aspects, may be applied
as well to ignition systems utilizing other types of storage
devices.
SUMMARY OF THE INVENTION
The invention resides in the combination of an alternator and
breakerless ignition system for use with an internal combustion
engine. The alternator includes a rotor carrying a plurality of
generating magnets and a stator having a plurality of load coils
and one or more high-voltage coils for use with the ignition
system. The voltage produced by the high-voltage coil or coils is
used to charge a capacitor or other storage device which is
discharged through a step-up transformer to produce a still higher
voltage used to produce a spark at the associated spark plug.
Discharge of the capacitor is controlled by a triggered electronic
switch device such as a silicon controlled rectifier triggered by
triggering signals appearing in synchronism with the rotation of
the rotor. These triggering signals are in turn produced by a
triggering magnet separate from the generating magnets, a
triggering coil, and irregularities, such as pins on the rotor, for
varying the flux of the triggering magnet through the triggering
coil in response to the rotor rotation. Preferably the triggering
magnet is located externally of the rotor with the triggering coil
being wound upon it.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view taken through an alternator comprising
part of an ignition system embodying this invention.
FIG. 2 is a schematic wiring diagram of an ignition system of which
the parts shown in FIG. 1 are a part.
FIG. 3 is a diagram illustrating the voltage waveform induced in
the triggering coil of FIG. 1 during each revolution of the
rotor.
FIG. 4 is a fragmentary view illustrating another form of
irregularity which may be used on the rotating part of an ignition
system.
FIG. 5 is a development of the surface of the rotating part of FIG.
4 taken along the line 5--5 of that figure.
FIG. 6 is a diagram illustrating the voltage waveform produced in
the triggering coil during each revolution of the rotor of FIG.
4.
FIG. 7 is a fragmentary view showing another form of irregularity
which may be used on the rotating part of an ignition system
embodying this invention.
FIG. 8 is a development of the surface of the rotating part of FIG.
7 taken along the line 8--8 of that figure.
FIG. 9 is a diagram illustrating the signal induced in the
triggering coil during each revolution of the rotor of FIG. 7.
FIG. 10 is a fragmentary view showing another form of irregularity
which may be used on the rotating part of an ignition embodying
this invention.
FIG. 11 is a development of the surface of the rotating part shown
in FIG. 10 and taken along the line 11--11 of that figure.
FIG. 12 is a diagram showing the signal produced in the triggering
coil during each revolution of the rotating part of FIG. 10.
FIG. 13 is a fragmentary sectional view showing another form of
irregularity which may be used on the rotating part of an ignition
system embodying this invention.
FIG. 14 is a development of the surface of the rotating part of
FIG. 13 taken along the line 14--14 of that figure.
FIG. 15 is a diagram showing the signal produced in the triggering
coil during each revolution of the rotating part of FIG. 13.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning to the drawings and first considering FIGS. 1 and 2, the
ignition system 16 there shown includes an alternator 14 which is
attached to the associated engine and which is used to provide both
power for the ignition system and power for a load 18 separate from
the ignition system. The load 18 may, for example, constitute a
charging circuit for a battery used for starting the engine and/or
a circuit including lights or other auxiliary electrically powered
equipment. The alternator 14 includes a stator 20 fixed to the
stationary structure of the engine and a rotor 22 which is fixed to
a shaft 24. The shaft 24 is one which is rotated in synchronism
with the operation of the engine and may be the crankshaft or
camshaft of the engine. In any event, the shaft 24 in the
illustrated case is one which rotates at the same speed as the
engine crankshaft so that a given angular displacement of the
crankshaft results in an equivalent angular displacement of the
rotor. The rotor has a central hub 21 which is received on the
shaft 24 and an axially extending rim or flange 23 connected to the
hub by a radial web (not shown).
The stator 20 is located inside of the rotor 22 in the space
between the hub 21 and rim 23 and includes a core 25 of laminated
magnetic material having twelve equally angularly spaced radially
outwardly extending poles 26, 26. Eleven of the poles 26, 26
receive associated windings 28, 28 having a relatively low number
of turns, for example, 15 turns, of relatively heavy wire, and
these windings are connected in series with one another as shown to
provide a source of alternating power for the load 18. The twelfth
pole 26, however, receives a winding 30 made up of a large number
of turns, for example 4,000 turns, of relatively fine wire and this
winding is used to provide a source of higher voltage power for the
ignition system 16.
As shown in FIG. 1, the rotor rim or flange 23 is comprised of an
outer band or shell 27, which is made of iron or steel or other
magnetic material, and an annular insert 29. The insert 29 is made
of a nonmagnetic base material, such as aluminum, and has six
magnet assemblies embedded therein with each such assembly
consisting of two pole pieces 32, 32 and a tangentially charged
generating magnet 34. The pole pieces 32, 32 provide a series of
twelve alternately magnetically charged pole faces on the inner
surface of the rotor rim. It will, therefore, be obvious that as
the rotor 22 is rotated, the magnet assemblies induce alternating
voltages in the various coils 28, 28 and 30 with the voltage
induced in the coil 30 being relatively high in comparison with
that induced in each of the coils 28, 28. During normal operation,
the rotor 22 is rotated by the engine in the direction of the arrow
36.
Considering the ignition system 16 of FIG. 2 in more detail, this
system in addition to the energizing coil 30 includes a condenser
38 connected across the coil 30 and a step-up transformer 40. A
diode 42 connected in parallel with the coil 30 rectifies the power
supplied to the condenser 38 so that only positive pulses are
transmitted thereto, and another rectifier 44 connected in series
with the coil 30 prevents the charge on the condenser 38 from
returning to the coil 30 during the negative portion of each cycle.
The condenser 38 is also connected across the primary winding 46 of
the transformer 40 through a silicon controlled rectifier 48.
Therefore, when the condenser 38 is charged and the silicon
controlled rectifier 48 triggered from a nonconducting to a
conducting state, the condenser is discharged through the primary
winding 46 to produce a surge of current through the primary
winding, which induces a high voltage in the secondary winding 49
and accordingly produces a spark at the spark plug 50 or other
spark gap ignition device connected to the secondary winding
49.
In accordance with the invention the triggering of the silicon
controlled rectifier 48 is controlled by a triggering coil 52
connected between its gate and cathode terminals. The triggering
coil 52 is in turn, as shown in FIG. 1, located adjacent the outer
surface of the alternator rotor 22 and is part of an assembly
including a core of magnetic material around which the coil 52 is
wound and having a magnetic pole face 54 facing inwardly toward the
rotor. Associated with the coil core is a source of magnetic flux
or magnetomotive force for establishing a flux circuit of which the
coil core is a part. The coil core may in some instances be part of
a laminated iron piece and the source of magnet flux a separate
permanent magnet fixed to such iron piece. In the illustrated case,
however, the coil 52 is wound on a radially charged permanent
triggering magnet 56 which therefore serves both as a magnetic core
for the coil and as a source of magnetic flux. The triggering
magnet 56 has a north pole at its inner end and a south pole at its
outer end. Lines of magnetic flux therefore tend to flow from one
pole of the magnet to the other and, as shown by the flux lines 58,
58 of FIG. 1, part of the path for this flow of flux tends to be
through the outer magnetic shell 27 of the rotor flange. The amount
of flux flowing through the magnet 56 accordingly depends, among
other things, on the reluctance of the airgap between the pole face
54 and the adjacent surface of the rotor rim and, as part of the
invention, irregularities are provided on the rim so that the
airgap reluctance is substantially different at different rotor
positions.
In the embodiment shown in FIG. 1 these irregularities are provided
by two pins 60 and 62 fixed to the magnetic shell 27 of the rotor
and extending outwardly therefrom. The outer portions of the pins
60 and 62 are cylindrical and of substantially the same diameter
but the leading pin 62 is of a substantially shorter height than
the trailing pin 60. The pins are spaced circumferentially of the
rotor about 20.degree. from one another and are axially aligned
with the pole face 54 of the magnet 56. During each revolution of
the rotor 22, the short pin 62 first passes beneath the magnet 56
and then 20.degree. later the tall pin 60 passes beneath the
magnet. When the short pin 62 passes beneath the magnet it reduces
to some extent the reluctance of the flux path through the coil 52
and a small voltage wave is induced in the coil. When the taller
pin 60 later passes the magnet 56 the reluctance of the flux path
is further reduced and a larger voltage wave is induced in the
coil. The peak values of the voltage waves induced in the coil 52
as the pins pass the magnet is dependent on the rate of change of
the reluctance and therefore on the speed of the rotor, and the
height of the pin 60 and other parameters are chosen such that even
at low cranking speeds the voltage induced in the coil 52 by the
passage of the tall pin 60 is sufficient to trigger the silicon
controlled rectifier 48.
The nature of the voltages induced in the triggering coil 52 and
the way in which advance of the spark is achieved is better
understood by reference to FIG. 3. In this figure, the solid lines
indicate the waveforms obtained at a low rotor speed and the broken
lines indicate the waveforms obtained at a higher rotor speed. The
horizontal line 66 indicates the voltage level required to trigger
the silicon controlled rectifier 48. The pin 62 passes the magnet
56 at the low rotor speed and the wave 70 is that induced in the
coil as the tall pin 60 passes the magnet. The voltage wave 68 does
not rise to the triggering level but the wave 70 does reach and
exceed such level. Triggering of the silicon controlled rectifier
to produce a spark at the spark plug 50, therefore, occurs upon the
appearance of the wave 70, and the tall pin 60 which causes the
production of this wave is so located on the rotor 22 that the wave
70 occurs at approximately the rotor position equivalent to the top
dead center position of the engine crankshaft. The wave 72
indicates the wave produced by the passage of the short pin 62 at
the high rotor speed and the wave 74 indicates that produced by the
passage of the tall pin 60 at the same high speed. Both of these
waves have values which exceed the triggering level 66 and,
therefore, the silicon controlled rectifier will be triggered by
the initial wave 72 to cause a spark at the spark plug 50. In the
illustrated case, the short pin 62, as mentioned, is located
approximately 20.degree. in advance of the tall pin 60 and
therefore at the high rotor speed the firing of the spark plug
occurs approximately 20.degree. in advance of the position at which
firing occurs at low speed.
From the foregoing, it should be obvious that many different
changes may be made from the illustrated system of FIGS. 1 and 2
without departing from the invention. For example, the triggering
coil and core assembly may be associated with any part rotated in
synchronism with the engine and need not necessarily be associated
with the rotor of an alternator. Also, the advance as provided by
the coil and magnet assembly may be used with other electrically
triggered switching devices such as transistors and thyratrons, and
the ignition system itself need not necessarily be a capacitor
discharge system. The pins 60 and 62 may also be moved to different
angular spacings to vary the degree of advance provided between
high and low speed operation and, if desired, a larger number of
pins may be used to provide a number of different degrees of
advance at different speeds. In fact, the nature of the
irregularities on the rotating part may take many different forms
some of which are shown by way of example in the other figures of
the drawings and described below.
In FIGS. 4 and 5, the rotating part corresponding to the rotor 22
of FIG. 1 is shown at 76 and has an irregularity on its outer
surface formed by a circumferentially elongated protrusion 78
having such a shape that the voltage induced in the triggering coil
52, as the protrusion passes the magnet 56, has a ramp-shaped
waveform starting at a low value when the leading end of the
protrusion reaches the magnet and increasing steadily to a high
value when the trailing end of the protrusion reaches the magnet.
As shown by the solid lines of FIGS. 4 and 5, the protrusion 78 has
a constant height, but has an axial width which increases in a
nonlinear fashion in going from its leading end 80 to its trailing
end 82. The length of the air gap between the protrusion 78 and the
pole face 54 therefore remains constant but its area decreases as
the protrusion 78 is moved past the magnet 56. As is well known the
voltage induced in a coil, such as the triggering coil 52, is
directly related to the rate of charge of the magnetic flux passing
therethrough, and the amount of flux is directly related to the
reluctance of the flux path. Therefore, in order to induce a
steadily increasing voltage in the coil 52 the reluctance of the
flux path should change in such a manner that it increases, as the
magnet 56 goes from the leading end 80 to the trailing end 82 of
the protrusion 78, in accordance with the square of the
displacement of the magnet from the leading end 80. Such a changing
reluctance may be obtained, as shown in FIG. 5, by shaping the
protrusion with curved sides 79, 79 the curvature of which is such
that the area of the protrusion covered by the magnet 56 at any
point along the protrusion is approximately directly related to the
square of the displacement of the magnet from the leading end
80.
Due to fringing from the sides 79, 79 of the protrusion 78 a
curvature of the sides 79, 79 which yields an exact second order
relation between the airgap area and its displacement from the
leading end 80 may not in all cases produce a sufficiently straight
ramp-shaped waveform in the triggering coil, and therefore the
curvature may have to depart from such ideal curvature, and be
determined best by trial and error, in order to obtain the desired
waveform. Also, it is not necessary that the protrusion have a
constant height and, as shown by the broken line 81 of FIG. 4, the
desired changing reluctance may be obtained by varying both the
height and the width of the protrusion 78.
It will be obvious that the steepness of the ramp-shaped waveform
induced in the triggering coil 52 by the passage of the protrusion
78 is dependent on the speed of the rotating part 76, the ramp
increasing in steepness as the speed increases. In FIG. 6 the line
84 indicates the waveform produced at a low speed corresponding to
the cranking speed of the engine. The line 86 represents the
waveform produced at a slightly higher speed, the line 88
represents the waveform produced at a still higher speed, and the
line 90 the waveform produced at close to maximum speed. The points
A, B, C and D indicate the points at which triggering of the
associated silicon controlled rectifier 48 occurs for each of the
represented speeds and, therefore, it will be noted from FIG. 6
that as the speed increases the point at which firing occurs
advances, the maximum advance being determined by the angular
extent of the protrusion 78 which in the illustrated case is shown
to be approximately 20.degree..
One possible disadvantage of the single circumferentially elongated
protrusion 78 of FIGS. 4 and 5 is that the point of firing at low
cranking speed is not definitely fixed and may vary to an
undesirable extent. To overcome this, an arrangement such as shown
in FIGS. 7 and 8, and embodying this invention, may be employed
wherein the irregularity on the rotating part 76 includes a
circumferentially elongated protrusion 92 and a second separate
protrusion 94 spaced from the trailing end 98 of the protrusion 92.
Similarly to the protrusion 78 of FIGS. 4 and 5 the protrusion 92
is so shaped as to induce a generally ramp-shaped voltage waveform
in the coil 52 as it passes the magnet 56 and as shown in FIGS. 7
and 8 has a constant height and has a narrow leading end 96 and
gradually increases in width in going to its trailing end 98. The
protrusion 94 is of the same height and width as the trailing end
98 of the protrusion 92. The voltage waveform induced in the coil
52 of FIGS. 7 and 8 is shown for different rotor speeds in FIG. 9,
the line 100 representing the waveform produced at low cranking
speed, the line 102 the waveform produced at a higher speed, the
line 104 the waveform produced at still higher speed, and the line
106 the waveform produced at or near a maximum speed. From FIG. 9
it will be observed that for each speed of the rotor the waveform
produced consists of an initial generally ramp-shaped portion
produced by the elongated protrusion 92 and a subsequent
spike-shaped portion produced by the protrusion 94. At low speeds
the ramp-shaped portion does not rise to the triggering level, but
the spike-shaped portion does rise beyond the triggering level to
cause triggering and firing. Furthermore, the spike-shaped portion
occurs at a definite rotor position and therefore it definitely
fixes the timing of the firing at low speeds.
FIGS. 10 and 11 show an arrangement including a number of
protrusions on the rotating part 76 for causing the firing to occur
at any one of a number of definite rotor positions according to the
rotor speed. In the illustrated system, these protrusions consist
of seven lugs or ribs, 108 to 114, equally spaced from one another
along approximately a 20.degree. extent of the circumference of the
rotating part 76. The lugs 108 to 114 are of substantially the same
height so as to have concentric end faces but, as shown in FIG. 11,
are of varying axial extent and are so arranged as to be of an
increasing axial length in going from the leading lug 108 to the
trailing lug 114. As shown in phantom in FIG. 9, the magnet 56 is
of a rectangular cross section and of such a size that its pole
face 54 is at least as large as the outer face of the largest or
trailing lug 114. Therefore, as each lug is brought in succession
into alignment with the magnet 56, the airgap thickness remains
unchanged but the area over which the airgap extends is varied.
The waveform produced by the arrangement of FIGS. 10 and 11 is
shown in FIG. 12 with the solid lines showing the waveform at low
speed and the broken lines showing the waveform at a higher speed.
At the illustrated low speed the first small voltage wave 122 is
produced by the leading lug 108 and the subsequent waves 123, 124,
125, 126, 127 and 128 are produced respectively by the lugs 109,
110, 111, 112, 113 and 114. Similarly, at the higher illustrated
speed the smallest wave 136 is again produced by the lug 108 and
the subsequent waves 137, 138, 139, 140, 141 and 142 are produced
respectively by the lugs 109, 110, 111, 112, 113 and 114. At the
illustrated low speed firing occurs at point A where the wave 128
produced by the lug 120 crosses the triggering level 66, all other
waves failing to rise to such level. At the higher illustrated
speed, however, the waveform 139 produced by the lug 111 rises to
the triggering level and therefore firing occurs at the point B,
the previous pulses 136, 137 and 138 failing to rise to the
triggering level. At still other speeds of the rotating part, the
magnitude of the waveforms may be greater or less than that shown
by the broken lines of FIG. 12 so that as the speed of the rotating
part increases from its lowest speed to its highest speed, the
spark will occur in succession at the positions determined by the
various lugs. That is, at the lowest speed, firing occurs when the
lug 114 passes the magnet 56. At a slightly higher speed firing
will occur when the lug 115 passes the magnet 56 and at still
higher speeds firing will occur when the other lugs pass the
magnet.
In addition to the irregularities being formed by protrusions on
the surface of the rotating part they may also be formed by
recesses in such a part, and such an arrangement is shown in FIGS.
13 and 14 wherein irregularities are formed by a number of holes
150, 151, 152, 153 and 154 drilled in the part, the holes being
angularly spaced from one another and of a gradually increasing
diameter in going from the leading hole 150 to the trailing hole
154. The action of the holes on the coil 52 is substantially the
same as that produced by the protrusions in FIGS. 10 and 11 except
that the polarity of the waveform is reversed. FIG. 15 shows the
waveform produced by the device of FIGS. 13 and 14, the solid lines
representing the waveform obtained at a low speed and the broken
lines showing the waveform obtained at a higher speed. At the
illustrated low speed the small leading opening 150 first passes
the magnet 56 and slightly reduces the reluctance of the flux path
through the magnet 56 to induce a small voltage wave 160 in the
coil 52. As the subsequent openings 151, 152, 153 and 154 pass the
magnet they reduce the reluctance of the flux path by increasing
amounts and produce increasingly larger voltage waves 161, 162, 163
and 164, respectively. The largest wave 164 is the only one which
crosses the triggering level 66 and, therefore, firing occurs at
the point A. At the higher speed illustrated, the waves 170, 171,
172, 173 and 174 correspond respectively to the openings 150, 151,
152, 153 and 154 and firing occurs at the point B on the wave 171,
this wave being the first one to cross the triggering level 66.
Again, it should be understood that the drawings and description
are not to be construed as defining or limiting the scope of the
invention, the claims which follow being relied upon for that
purpose.
* * * * *