Note: Descriptions are shown in the official language in which they were submitted.
960~
The present invention relates to an igni~ion system
for internal combustion engines~ and more particularly to
the timing of the switching on and off of the spark.
A known type of ignition system for an internal
combustion engine employs contact breaker points operated
by a cam driven via suitable gearing from the engine
crank-shaft. In this system the contacts are closed
and thus the battery is connected across the ignition
; coil for a fixed number of degrees of crank-shaft rotation
irrespective of engine speed. The duration of the spark~
which commences when the contact breaker points are
openedg is a function of the electrical parameters of
the system, and is substantially independent of engine
speed.
Another known type of ignition system for internal
combustion engines employs a magnetic trigger to switch
- off the current to the ignition coil, so initiating a
spark, the current being switched on again a suitable
time before another spark is required. The period of
time between switch off of the ignition coil, and switch
on of the ignition coil is related by suitable control
means to the speed of therengine such that the coil has
sufficient 'ton'' time for its magnetic field to accumulate
sufficient energy to pr~duce the spark.
A further known type of ignition system is triggered
by opto-electronic means at appropriate crank shaft
positions~ the triggering switching off the current to
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1~49~09
the ignition coil, the coil being reconnected to the
battery a fixed period after switch off. In such a
system the crank-shaft position at which a spark is `
initiated is precisely defined. The spark duration
depends on the electrical parameters of the system, -
switch on of the coil being a fixed time after switch
off rather than a particular number of crank-shaft
degrees later. This system is effectively monostable,
since the spark is produced by an electronic circuit
which has a stable state in which the coil is "on"
and is triggered to the unstable state to interrupt
the current in the primary winding, and thus initiate
the necessary secondary voltage to produce the spark,
the circuit returning to the stable state a fixed time ~
thereafter. :
A known improvement upon the above monosta~le
opto-electronically controlled system is a bistable sys-
tem as disclosed in our Canadian Patents Nos. 879,285 ~
and 932,382, whereby the crank-shaft angles between the `
switch on and off of the coil are fixed.
In our copending Canadian Application No.
195,002, we have disclosed an ignition system for an in-
ternal combustion engine in which not only the timing of
the spark is controlled in accordance with the speed ;
and load on the engine, but the duration of the spark is
controlled relative to the angular position of the crank-
shaft such that the spark at the spark plug is extinguished
.
at a predetermined crank-shaft angle irrespective of
tl~e crank-shaft angle at which the spark is initiated.
In the preferred embodiment disclosed, this position
is in the range of from 0 to 5 after top dead centre
(A.T.D.C.)
The system disclosed in Canadian Application
~O. 195JOO2 utilizes the basic principle disclosed in
our Canadian Patent Application No. 160,849 concerning
the automatic computerized advance and retard of spark
la~ter
~ ignition. In this/Application, the advance and retard
of the spark ignition of an internal combustion engine
is achieved electronically by generating two series of
pulses in synchronism with the engineJ using one series
as a reference for maximum advance and "coil on", and
the other series to operate a counter to count down the
requisite number of pulses beyond the maximum advance
point before the spark is initiated, the count of the
counter being varied from a computer in accordance with
speed and/or load on the engine.
In addition to the speed and/or load of the
engine there are a numlber of other factors which to a
; greater or less extent affect the spark timing of an in-
ternal combustion engine.
The principal factors are: fuel octane rating,
air temperature, humidity and air pressure.
It has been general practice to establish ad-
vance curve requirements for both speed and load changes on
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11~)4~609
the particular engine concerned by dynometer testing using
simulated conditions in the laboratory. ~laving obtained
the advance curves -Eor speed and load, mechanical mcans are
constructed in order to reproduce these curves representing
the required timing of the ignition by means of physical
movcment of the ignition system components relative to the
engine's crank position, i.e. top dead centre ~T.D.C.)
It has further been proposed to use mechanical
means to achieve advance or retard of the ignition timing,
which takes into account the other factors noted above, such
as fuel octane rating, air temperature, humidity and air ;
pressure. Such systems are, of course, mechanically complic-
ated and are liable to error and failure. -
Studies of the combustion process within an internal
combustion engine have shown that the combustion process has
two distinct stages. Firstly, after the initiation of the ;
.
spark, there is a low pressure stage wherein the mixture is
igniting and a flame is starting to propogate within the `~
combustion chamber. At some subsequent point in time a sudden
rise in both the pressure and temperature within the cylinder
is observed, which marks the commencement of the second
explosive stage of combustion. During the initial stage of
combustion temperatures and pressures within the combustion -chamber are low compared with the temperatures, and pressures
within the combustion chamber during the second stage of
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combustion.
~ y the commencement of the second stage of combus-
tion the mixture in the cylinder is well alight, and the
presence of any spark at the spark plug is irrelevant to
the following process of combustion within the cylinder. It
is the timing of the commencement of this second stage which
is extremely important. Tests have shown that for best
engine efficiency this sudden pressure and temperature rise
should always occur at one fixed crank-shaft angle) regardless
of all the variables which influence the spark timing require-
ment.
It is therefore an object of the present invention
to provide a closed loop ignition system in which a predeter-
mined pressure on the pressure rise curve occurring during
the second stage of combustion can be accurately sensed in
,
relation to a given fixed angular position, and maintained
at this position under all engine conditions.
: ::
`~ According to the present invention there is provided
an electronic ignition system for an internal combustion
engineJ including: means for sensing a pre-determined
pressure on the pressure rise curve occurring during the second
:::
stage of combustion in a cylinder of an engine at each firing
cycle; means for comparing the crank-shaft position at said
pre-determined pressure with a fixed pre-determined crank-
: ;
shaft position; and digital means for advancing or retarding
.
~- the ignition by one step at a time so as to maintain this
pre-determined crank-shaft position at said pre-determined
pressure on the pressure rise curve occurring during the
second stage of combustion, irrespective of engine requirements.
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9609
Preferably, the predetermined crank-shaft angle
at which the commencement of the second stage of combustion ~ :
occurs is ideally 10 A.T.D.C., but this may vary according
to certain engine characteristics, more particularly those
relating to the design of the cylinder heads.
Preferably, the electronic ignition system for
an internal combustion engine includes means for generating
a first series of square wave voltage pulses in synchronism
with the engine revolutions to provide a series of alternate
highs and lows; means for generating a second series of
square wave voltage pulses at a frequency greatly in excess ~ `
of the first series; means for counting a given number o~
~'' .
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the second series of voltage pulses from a given point
in relation to the first series of voltage pulses; cmd
means for producing an output of a give~ level from said
counting means after said count has been completed, means
for detecting the presence of both a signal at said given
level from the first pulse generating means and the counting
means, in order to bring about the initiation of the spark,
the extinguishing-of the spark being effected when the
signal from the first generating means changes to the
opposite level; and means for varying the count of the
counting means in accordance with the position of the
commencement of the second stage in the combustion process
if this deviates from the predetermined crank-shaft
position.
Preferably, in one form the electronic device controls
the advance and retard of the ignition, the counting means
starting to count near the position of maximum advance.
Alternatively, the count may be started from the fixed
coil turn on signal. Thus a signal at said given level
from the first trigger initiates the count of the counting
means, which then counts down the number it has been set
to before giving a signal at said given level to cause the
initiation of the spark, and then to permit the spark to
continue until the pressure rise point is reached, and the
coil is turned on~
The counting means is preferably a frequency divider
whose count can be increased or reduced by a single step
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1~)49~ 9
:
at each cycle of ignition.
The present invention will now be described in
greater detai]. by way of examples, with reference to
the accompanying drawings, wherein:-
Figure 1 is a diagram (partly in block form) ofone form of spark control device for use with a spark
ignition system of an internal combustion engine;
Figure 2 is a front view of the disc shown in
Figure l;
Figure 3 is a set of waveforms which assist in
explaining the operatisn of the circuit shown in Figure 1;
Figure 4 is a diagram (partly in block form) of an
alternative form of spa~k control device to that shown
in Figure 1.
Figure 5 is a set of waveforms which assist in ~
explaining the operation of the circuit shown in Figure 4; :
Figure 6 is a cross sectional view through a cylinder : ~
... .
showing a second method of detecting the commencement
of the second stage of the combustion process in the
cylinder using a second spark plug;
Figure 7 is a circuit diagram illustrating this
second method; ~-
Figure 8 is a cross sectional view through a cylinder ;
showing a third method of detecting the commencement of
the second stage of the combustion process in the cylinder
by means of a fibre-optic cable and a translucent window
in the upper part of the cylinder;
_9_
.
60g
Figure 9 is a circuit diagram illustrating this
third method;
Figure 10 is a circuit diagram showing an alter-
native form of combining the logical signals from the first
and third triggers;
Figure 11 is a part cross sectional view through .,.
the inlet manifold of the engine, showing a second embodi-
ment of the invention, utilizing the vacuum in the inlet
manifold;
Figure 12 is a view of the chopper disc for this
second embodiment; and :~
Figure 13 is a circuit diagram of the circuit .
which energizes the solenoid shown in Fi.gure 11.
The ignition control system according to the
present invention will now be described with reference to
a four cylinder internal combustion engine.
Referring to Figures 1 and 2, the device achieves
electronic control of the advance and retard of the initia-.
: tion of the spark through the detection of the crank-shaft
: 20 position of the commencement of the second stage of the
combustion within the cylinders. The device includes a
radiation chopper device generally designated l; a first
fast inverse switching trigger circuit 11; a second fast ~.
inverse switching trigger circuit 12; a counter 1~; a
pressure detector stage 16; an AND gate 19; and a power tran--
sistor stage 18.
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6C39
Circuit details of the two inverse fast switching triggers 11 and
12 and the power transistors stage 18 have been disclosed in our Canadian
Patent No. 98~,935 which issued on March 2, 1976. Each trigger 11 and 12
comprises two transistors arranged to switch in inverse relation to each
other, the first transistor also switching in inverse relation to its
respective associated photo-transistor 7 and 8 to which its base electrode
is connected. The power transistor stage 18 comprises a Darlington pair.
The radiation chopper device 1 consists of a housing 2; a disc 3;
a shaft ~ carrying the disc 3, infra-red radaition sources 5 and 6; and
radiation detectors 7 and 8. The infra-red radiation sources 5 and 6 are
preferably gallium arsenide lamps, and the radiation detectors are prefer-
ably photo-transistors, all these elements being fixed to the housing 2. The
shaft 4 is journalled in bearings ~not shown) in the housing 2, and is
driven at cam shaft speed o~ the engine.
The chopper disc 3 comprises two series of concentric apertures 9
and 10. There are four large apertures 9 in equi-spaced relation, and a
large number of small apertures of slits 10 ~e.g. sixty eight). The aper-
tures 9 permit infra-red radiation from the lamp ~ to reach the photo-tran-
sistor 7, and the slits 10 permit infra-red radiation from the lamp 6 to
reach the photo-transistor 8. The lamps 5 and 6 are energized through a
common stabilized voltage source 20.
The output from the respective phototransistors 7 and 8 is fed to
the inputs of respective fast inverse switching triggers 11 and 12. The
output from the first trigger 11 is applied firstly to the power transistor
stage 18, and seeondly to a first input of the AND gate 19. The output of
the second trigger 12 is fed to the counter 1~, which normally gives a
-11 - ,
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3La~'~.9~ 9
"0" output, but which on completion of the count down set
therein through the operatlon of the AND gate 17 gives a "1"
output. The pressure detector stage 16 is preferably a piezo-
electric detector, and is designed to give an output when
the pressure exceeds a predetermined value. The piezo-electric
detector is housed either in the wall of the cylinder above
the piston when located at T.D.C., or in the upper cylinder
head on the opposite side from the spark plug. The output
from the piczo-electric detector is applied to a third trigger
13, which squares the output pulse, and applies it to a second
input of the AND gate 19. The AND gate 19 detects whether
or not there is a simultaneous coincidence of "1" on both
its inputs and in the event there is coincidence, it provides
an output to adjust the count of the counter 14 by one step
at a time to advance the ignition by a few degrees, in order
to ensure that the pressure peak occurs at a predetermined
crank-shaft angle, which is preferably 10 A.T.D.C. ~e power
transistor stage 18 controls the current flow through the
primary winding of the ignition coil 26. When the outputs
from the stages 11 and 14 are either "0" and "1" or "1' and
"0" or "0" and "0" current flows through the primary winding
of the ignition coil 26, but when both outputs are at the
high level "1", then the current through the coil is inter-
rupted, producing the collapse of the magnetic field and the
resultant high secondary voltage necessary for the spark.
~`~i, '... ~
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6~
The operation of the electronic spark control device
will now be described in greater detail with the aid of the
waveforms shown in Figure 3. As the disc 3 is rotated at
crank-shaf~ speed of the engine, the infra-red radiation from
the lamps 5 and 6 impinges on the respective photo-transistors
7 and 8 through the apertures 9 and slits 10. Accordingly,
the photo-transistor 7 produces four current pulses per
revolution of the disc 3, whilst -the photo-transistor 8
produces a large number (e.g. sixty eight) of pulses per
revolution. The two triggers 11 and 12 fast switch and
amplify these pulses to produce the waveforms (a) and (b) ~-~
respectively. ~uring the interval between crank-shaft
positions tO and tl the photo-transistor 7 is energized by
infra-red radiation, and is therefore conductive. The out-
. .
put from the first trigger is at the low level representing
a "0". At the position tl, the infra-red radiation is cut
off and the output of the first trigger becomes high repre-
senting a "1". This output is applied to both the counter
14 and the first transistor of the power transistor stage 18.
The counter 14 now counts the pulses from the second trigger
12 according to the number set therein. The output of the
counter 14 is at the low level "0" from crank-shaft position
tO up to and beyond the crank-shaft position tl. Therefore,
when the trigger 11 produces a high level output, the power
transistor stage is not switched because of the continued
presence of a low level output from the counter 14. In the
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example illustrated, the counter 14 is set to count down a
total of six pulscs before its output switches to the high
level. Therefore, at crank-shaft position t2, when the count
of six has been completed, the output becom0s high at the
seventh pulse, and the power transistor stage switches off
the flow of current in the primary winding of the ignition
coil 26, and thus initiates the spark through the high induced
secondary voltage on the collapse of the field in the primary
winding of the coil. At crank-shaft position t3 which is the
idealized crank-shaft position at which the commencement of
the second stage of the combustion should occur, the output
of the first trigger reverts to the low level, thus extinguish-
ing the spark, and resetting the frequency divider, which
also reverts to the low level, as shown by waveform ~c),
these events both happening when the photo-transistor 7 is
again energized by infra-red radiation.
Waveform ~d) shows the "1" output from the third
trigger 13 when the combustion within the cyli.nder has reached
its second stage, and has been detected by the piezo-electric
detector 16.
The circuit is designed to operate about the ideal
position when the leading edge of the waveform (d) coincides
with the crank-shaft position t3, this leading edge represent-
ing the commencement of the second stage of combustion. If,
as shown in waveform ~e), the leading
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:
~9~(39
edge of the pulse from the trigger 13 occurs after
the position t3, the AN~ gate 19 does not detect any
coincidence between the "1" outputs from the first
trigger 11 and the third trigger 13, as shown in
waveform tf). Under these conditions the count of
the counter 14 is reduced by one step at a time until
coincidence is detected. I9 on the other hand, as
shown in waveform (g), the leading edge of the pulse
from the trigger 13 occurs before the crank-shaft
position t3~ the AN~ gate detects coincidence as shown
in its output waveform (h). Under these conditions the
count of the counter 14 is increased by one step at a
time until coincidence is no longer detected. Thus,
under normal running conditions, the count of the counter
is adjusted by increments so as to maintain the leading ~-
edge of the waveform (d) at the crank-shaft position of
t3-
An alternative embodiment also using a piezo-electric
detector is shown in Figurès 4 and 5. In this form, the
AND gate 19 is replaced by a comparator 17. A differentiating
circuit 21, a limiter 22, an inverter 24 and a frequency
divider circuit 25 are connec~ed in series between the
output of the first trigger 11 and one input of the
comparator 17. The frequency divider circuit in this
example performs a division by four and is synchronized
from the third trigger 13.
: -
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~0496~
The output of the first trigger 11 which is applied
to the differentiator 21 is the square waveform (a). The
differentiator 21 provides an alternate series of positive
and negative going spikes as shown in waveform (_), the
positive spikes being clipped off by means of the limiter
circuit 22 to provide waveform (_). The negative going
spikes of waveform (k) are then inverted and shaped in the
circuit 24 as shown in waveform (1). The frequency divider
25 allows only one pulse in four to be applied to the compar-
ator 17, such a pulse being shown by waveform (_). In order
to ensure that this selected pulse passed by the frequency
divider 24 corresponds to the firing of the cylinder
associated with the piezo-electric detector 16, a synchroniz-
ation link is provided between the third trigger 13, and the
frequency divider 25 to ensure that the latter remains syn-
chronized at all times.
The comparator 17 thus compares the position of the
pulse output from the third trigger 13 (waveform d) with that
of the pulse output from the frequency divider 25 (waveform m).
If the two pulses are coincident then there is no output from
the comparator 17 to the counter 14, and its count remains
unaltered. On the other hand, if the pulse of waveform (d)
occurs before the pulse of waveform ~_), then there is a
negative output from the comparator, which effects a one step
increase in the count of the counter 14, in order to re~ard
the point at which the spark is induced. Likewise, if the
pulse of waveform ~d) occurs after the pulse of waveform ~m), ~`
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then there is a positive output from the comparator 17 which
effects a one step decrease in the count of the counter 14
in order to advance the point at which the spark is induced.
The increase or decrease oE the count of the frequency divider
14 will continue until the pulse of waveform (d) is coincident
with the pulse of waveform (m).
In the above described alternative embodiment of
Figures 4 and 5, the detection of the second stage of the
combustion process is confined to one cylinder only, but a
separate piezo-electric detector can be provided for each
cylinder, in which case the frequency divider 25 is removed
from the circuit. Such a multi-detector system has the dis- ~ .
advantage that if there are slightly different firing character- ~ ;
istics between cylinders, the co~mt of the frequency 14 will
tend to hunt instead of remaining fixed under constant speed
and/or load conditions.
~` Instead of using an electro-mechanical transducer
device to detect the commencement of the second stage of com-
bustion, in a second preferred form it is possible to utilize
-` 20 a second spark plug as shown in Figure 6, which is a cross
sectional view through one cylinder of the four cylinder
-~ engine. As conventional, a piston 30 reciprocates within the
cylinder wall 32, a connecting rod 34 being connected to the
piston 30 by the little end 36. At the upper end of the
cylinder head 38, there is provided
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as conven-tional~ a spark plug 40 and inlet valve 42 ~the
outlet valve not being shown in the drawing as it is
positioned behind the inlet valve.) A second spark
plug 44 is provided on the other side of the cylinder
head from the ~ain spark! plug 40. This second spark
plug is utilized to detect the second stage of combustion
within the cylinder when the main spark plug has ignited
the compressed combustible mixture within the cylinder.
In order not to interfere with the flame front, which
propagates through the cylinder when the discharge across
the points of the main spark plug 40 occurs, it is desirable ;
that the second spark plug 44 is located as far away as
possible from the main spark plug ~0. In this particular
design of cylinder, having overhead valves, the second
spark plug 44 is conveniently located diametrically
opposite to the main spark plug 40. In order to fit the
second spark plug 44, it is desirable to slightly modify
the position ~f the inlet and/or outlet manifolds from
the cylinder head.
Referring now to Figure 7, one simple circuit for
detecting the second stage in the combustion process by
means of the second spark plug, includes a resistor 46,
a voltage detector 48, a 30 volt battery 50 and a pulse ~-
shaper 52. The resist~r 46 is connected in series with
the points of the second spark plug 44 across the battery
50. ~hen there is no combustion in the cylinder no
current flows in the resistor 46 because the spark plug
,
~gO 496(~
44 presents an open circuit. The voltage detector thus
produces zero output. When the combustible mixture
within the cylinder becomes compressed, and is ignited
by the main spark plug 40, as soon as the gap across
the seconcl spark plug has become fully ionized upon the pressure
wave front reaching the second spark plug at the commence-
ment of the second stage of combustion, the spark plug
presents a short circuit and current flows through the
resistor 46. The voltage across this resistor rises,
and is detected by theevoltage detector 48. The resultant
output pulse produced by the voltage detector during the
brief instant that the spark plug 44 is short-circuited
by the ionization of the gas within the cylinder, is
shaped by the pulse shaper 52 to produce a square wave
output. This positive going pulse is applied to either
the AND gate 19 of the first embodiment, or the comparator
; 17 of the alternative -~mbodiment.
In the third way of detecting the commencement of
the second stage of combustion, use is made of a fibre-
2~ optic cable and translucent window as shown in Figure 8,
which is a similar cross sectional view through one
cylinder of the engine. In this embodiment the second
spark plug is replaced by a quartz glass window 54. A
fibre-optic cable 56 has one end clamped to the window
54 by suitable clamping means 58.
Referring to Figure 9, the other end of the fibre-
optic cable 56 is coupled to a photo-transistor 60,
--19--
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which is mounted in the distributor of the engine along with
the rest of the double-trigger integrated circuitry except
Eor the power transistor stage 18. The photo-transistor 60
is connected in series with a resistor 62 across a battery 64.
A voltage detector 66 is connected across the resistor 62.
The radiation (whether visible and/or infra-red) from within
the cylinder is transmitted through the quartz glass window
; 54, along the fibre-optic cable 56, to the photo-transistor 60.
When the level of radiation exceeds a given value as a result
of the sudden temperature rise at the commencement of the
second stage of combustion, the photo-transistor 60 is
energized. The photo-transistor 60 conducts and current flows ;
from the battery 64 through the resistor 62. The rise in
voltage across the resistor 62 is detected by the voltage
detector 66. The resultant output pulse produced by the
.
voltage detector during combustion is shaped by a pulse shaper :
68 to produce a square wave output. This positive going pulse ~ :
is applied to either the AND gate 19 of the first embodiment,
or the comparator 17 of the alternative embodiment.
With any one of the described embodiments, the commence-
; ment of the second stage of combustion is always maintained -
at the same crank-shaft angle (e.g. 10 A.T.D.C.) irrespective
of the speed of the engine, the load on the engine, air pressure,
air temperature, humidity and other factors which can affect
~ the performance of an internal combustion engine.
; Instead of using an AND gate 19 in the embodiment
shown in Figure 1, the output stages of the first and
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third triggers could be connected as shown in Figure
10. An output transistor 70 of the first trigger 11
is effectively connected in parallel with an output
transistor 72 of the third trigger 13, each transistor
being in series with a resistor 74 across a 12 volt
supply. The logical output at 76 is fed to the counter
14. When both transistors are conductive and when one
or the other is conductive the output is a logical "0~'.
A logical 1l1" is produced when both are non~conductive
simultaneously.
Whilst in the above described embodiments, the
counting means starts to count near the positi0n of
maximum advance, it would be well within the scope of
the invention to start the count from the fixed c0il
"on" position.
In all the above embodiments, use has~been made of
the second trigger 12 and counter 14 in conjunction with
the first trigger 11 and electro-mechanical transducer
to determine the coil off position, i.e. the production
of the spark such that the commencement of the second
stage of combustion occurs at the precise crank-shaft
position at which the~coil switches on to extinguish
the spark. Instead of determining the requirediadvance
and retard electronically~ it may be achieved by electro~
mechanical means. In a second preferred fo~ shQwn in
Figures 11 to 13, the second trigger and frequency divider
are omitted. This second distinct way of carrying out
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the invention utilizes the vacuum manifold of the engine.
As shown in Figure It, a pipe 80 is connected to the
engine inlet manifold of the carburetDrl 82 upstream of
a throttle valve 84. The pipe 80 has a branch 86 connected
to a chamber 88. The chamber 88 is provided with a
diaphragm 90, to which is connected a rod 92 carrying
a forked portion 94. The ends of the forked portion
94 carry the infra- red radiation source 5 and the detector
7 associated with the first trigger 11. A spring 96 is
provided within the chamber 88 to urge the diaphragm 90
to its neutral position against the partial vacuum
produced in the chamber 88. A solenoid valve 98 is
provided to bleed air from the atmosphere into the chamber
88 from a pipe 99.
An apertured M isc 100 is rotated in synchronism with ~`
the crank-shaft of the engine, and is positioned as shown
in Figures 11 and 12 between the infra-red source 5 and
~` detector 7. The disc 100 has four identical apertures 102
equi-spaced around the circumference of the disc. Each
aperture 102 has arcua~ccouter and inner peripheries 104
and 106 respectively, a radial aligned edge 108 and a
~j straight radially inclined edge 110. The radial edge 108 -
provides the constant coil "on~' position to extinguish
the spark and the radially inclined edge 110 provides the ~ -
necessary advance and retard of the production of the
spark ~the coil "off" position) according to the distance
of the infra-red source 5 and detector 7 from the centre
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of ~he disc 100. The output square waveform generated
by the first trigger thus has a variable mark space ratio
which is a function of the dis-tance of the elements 5 and 7
from the centre of the disc.
The logical outputs from the first trigger 11 and
the third trigger 13 are combined as shown in Figure 10,
the combined signal being available at the output 76.
As shown in Figure 13, the signal available at the output
76 is passed through three inverse switching stages 112,
114 and 116~ the last stage 116 being in series with the
energizing coil 118 of the solenoid valve 980
~ his second embodiment operates as follows. The
system is designed so that~the vacuum of the inlet manifold
of the engine pulls theediaphragm 90 against the action of
the spring, so as to move the line of slght of the elements
5 and 7 radially outwards with respect to the disc 100~ thus
tending to increase the mark-space ratio of the output of
the first trigger in order to over advance the ignition
timing. When the timing becomes over-advanced, i.e. the
leading edge of the wave~form (d) is occur~ing before the
crank-shaft position of t3, coincidence is detected at the
output 76. This causes the transistor 112 to conduct, the
transistor ll~ to become non-conductive~ and the transistor
116 to be conductive. When the transistor 116 becomes
conductive, the solenoid 98 is energized to bleed in air
,~ into the chamber 88. This causes the diaphragm to relax
slightly, and to move the line of sight of the elements
,
- .
,,. ~,.
~i ' . .
66~9
5 and 7 Fadially inwards, towards the centre of the
disc 100~ so as to effect a retard of the ignition
timing. As soon as coincidence at the terminal 76
ceases, the circuit of~-` the transistors 112 to 116
reverts to its other stable state to de-energi~e the
solenoid ~8. The system thus hunts about the point
where the leading edge of the waveform (d) is maintained
at the crank-shaft position t3.
'
"-,, .
:: :
~, :
24
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