Device for diagnosing ignition system for use in internal combustion engine

An ignition system for internal combustion engine, which controls the ignition coil primary current according to the magnitude of the floating capacitance in the secondary side wiring section of the ignition coil, by determining the floating capacitance from the negative slope of rising of the secondary voltage produced in the ignition coil in response to the cutoff of the primary current and the primary cutoff current value, the slope being determined by measuring the period T until the secondary voltage reaches a predetermined voltage value. When the floating capacitance is increased, the primary cutoff current value is increased to increase the coil energy so as to increase the secondary voltage generated in the ignition coil for preventing the generation of a miss-spark.

This invention relates to ignition systems for internal combustion engines 
and, more particularly, to a system in which the floating capacitance 
which has great influence upon the transmission of a high voltage is 
measured. Also, the invention relates to a system, in which when the high 
voltage transmission loss is increased so that miss-sparks are likely to 
be generated the coil energy is increased to prevent the generation of 
miss-sparks. 
In the usual ignition system for an internal combustion engine, a high 
voltage produced from an ignition coil is transmitted through a high 
tension line and a distributor to each ignition plug. Usually, however, 
the output impedance of the ignition coil is comparatively high, and also 
the high tension line code lies in the close proximity of the engine body. 
Therefore, there always exists a distributed electrostatic capacitance or 
so-called floating capacitance in the wiring section of the secondary of 
the ignition coil. This floating capacitance increases when water or 
saline water is attached to the high tension code. In such a case, the 
high voltage to be impressed upon the ignition plug electrode is reduced 
compared to the voltage produced in the ignition coil. FIG. 1 shows this 
relationship. In the Figure, the ordinate is taken for the maximum value E 
of the generated voltage, and the abscissa is taken for the floating 
capacitance C. Plots a and b represent characteristics for respective 
ignition coil primary cutoff current values of 5.7 and 3.8 A. In the 
graph, 0 pF of the floating capacitance is shown in the abscissa for the 
sake of comparison although actually there exists some floating 
capacitance. The voltage generated in the ignition coil is readily reduced 
with the increase of the floating capacitance, while increasingly high 
voltage has been demanded as the ignition coil secondary voltage for such 
purpose as the exhaust gas recirculation (EGR) to cope with exhaust gas 
problems. Thus, there is a trend for increasing probability of the 
miss-spark generation, posing problems in the engine performance. 
To solve these problems, the development of ignition coils and high tension 
line codes, which is highly reliable and less likely to give rise to the 
reduction of the high voltage, is called for. Also, for diagnosing the 
ignition system, ignition system diagnosing means, particularly floating 
capacitance measuring means, are necessary. 
Although the measurement of the floating capacitance can be made with a 
commercially available electrostatic capacitance meter, extreme 
difficulties are involved in the measurement in this case since the 
ignition coil and each ignition plug are normally separated from each 
other by the distributor and also since a high voltage is impressed. Also, 
it is almost impossible to record the condition of the system during 
actual running. 
To overcome the above difficulties, the invention is predicated in the fact 
that the secondary high voltage generated in the ignition coil varies with 
the increase of the floating capacitance, and according to the invention 
the floating capacitance involved in the ignition system is measured by 
measuring the ignition coil voltage. When the floating capacitance as 
shown by a broken curve in FIG. 2 is increased, the ignition coil 
secondary voltage as shown by a dashed curve in FIG. 2 is changed such 
that its peak value and also its period are increased. The floating 
capacitance can be measured by constantly measuring the peak value 
V.sub.max or the period T.sub.0. Usually, however, with a spark discharge 
caused in the ignition plug electrode section the secondary voltage is 
reduced as shown by a solid curve in FIG. 2, so that neither V.sub.max or 
T.sub.0 can be directly measured. 
According to the invention, the floating capacitance is measured by 
determining the slope of a negatively rising portion of the secondary 
voltage waveform. This slope is found to vary with the ignition coil 
energy for the same floating capacitance, so that it is compensated for 
the coil energy. The coil energy is usually given as 
EQU 1/2L.sub.1 .multidot.I.sub.off.sup.2 .times..eta. 
where L.sub.1 is the primary coil inductance of the ignition coil, .eta. is 
the efficiency of energy transfer from the primary to the secondary of the 
ignition coil, the I.sub.off is the primary cutoff current in the ignition 
coil. Assuming L.sub.1 and .eta. to be constant, I.sub.off can be taken as 
the coil energy. FIG. 3 shows a relationship among the rising period T, 
which is required for the secondary voltage to rise from zero to a 
constant voltage V.sub.a, the primary cutoff current I.sub.off and the 
floating capacitance. In the Figure, the ordinate is taken for the rising 
period T required for reaching V.sub.a =-5 kV, and the abscissa is taken 
for the primary cutoff current I.sub.off. Plots a to d represent 
characteristics for respective floating capacitance values of 0, 50, 100 
and 150 pF. It will be seen from FIG. 3 that the floating capacitance can 
be determined by measuring the rising period T and the primary cutoff 
current I.sub.off and finding a point correlating the two measured values. 
An object of the invention is to provide an ignition system for an internal 
combustion engine, which can estimate the reduction of the ignition coil 
secondary voltage by the aforementioned method. 
Another object of the invention is to provide an ignition system for an 
internal combustion engine, which always detects the floating capacitance 
and, when the floating capacitance is increased, makes the energization 
period of the ignition coil primary longer to increase the coil energy so 
as to increase the secondary voltage for preventing the generation of a 
miss-spark. 
A further object of the invention is to provide an ignition system for an 
internal combustion engine, which always detects the floating capacitance 
and, when the floating capacitance is increased, increases the primary 
cut-off current to increase the coil energy so as to increase the 
secondary voltage for preventing the generation of a miss-spark. 
According to the invention, according to which the floating capacitance in 
the ignition system is measured by determining the slope of rising of the 
ignition coil secondary voltage, the reduction of the secondary voltage 
can be estimated from the result of the measurement, so that it is 
possible to effect the diagnosis as to whether or not the layout of the 
ignition system components such as ignition coil, distributor, high 
tension codes and ignition plugs is satisfactory and also as to what 
effects the changes of the environmental conditions have upon the ignition 
coil voltage. 
Further, since the system according to the invention has a simple 
construction, it can be mounted in a vehicle to permit the diagnosis of 
the ignition system during the running of the vehicle. 
Furthermore, since the system according to the invention measures the 
floating capacitance and makes the energization period of the primary coil 
longer or increases the primary cutoff current when the floating 
capacitance is increased, it is possible to reliably prevent the 
generation of a miss-spark with the ignition coil voltage increased by 
increasing the coil energy at the time when the floating capacitance is 
increased.

Now, preferred embodiments of the invention will be described with 
reference to the accompanying drawings. FIG. 4 shows an embodiment of the 
ignition system for an internal combustion engine according to the 
invention. Designated at 1 is an ignition coil, and at 2 an igniter for 
controlling the energization and de-energization of a primary coil 1a of 
the ignition coil. The igniter 2 is connected to an ignition timing 
control means not shown. Designated at 3 is a distributor, and at 4 
ignition plugs. A high voltage produced across a secondary coil 1b of the 
ignition coil 1 is applied through a high tension line 5 to the 
distributor 3 and thence through high tension lines 6 to ignition plugs 4. 
The floating capacitance is the capacitance component present in this high 
voltage transmission system. Designated at 7 is an external resistor 
connected in series with the primary coil 1a of the ignition coil 1, and 
at 8 a battery. Designated at 9 is a voltage divider for detecting the 
secondary high voltage across the ignition coil 1 through voltage 
division, and at 10 an ignition system diagnosing unit according to the 
invention. 
An example of the ignition system diagnosing unit 10 will now be described 
in detail. FIG. 5 is its block diagram, and FIG. 6 is a time chart 
illustrating waveforms appearing at various parts of it. Designated at 100 
is a floating capacitance detecting section. It includes a shaping circuit 
110 with an input terminal thereof connected to the point b in FIG. 4, 
i.e., the juncture between the ignition coil 1 and igniter 2. The waveform 
appearing at the point b is as shown in (b) in FIG. 6. The shaping circuit 
110 shapes this waveform into a pulse signal having a predetermined 
duration as shown in (d) in FIG. 6. The detecting section includes another 
shaping circuit 120 with an input terminal c' thereof connected to the 
point c' in FIG. 4. The point c' is connected through the voltage divider 
9 to the high tension line 5. The voltage divider 9 is of a well-known 
type using a resistor and a capacitor and dividing the input voltage to 
1/1,000. The waveform appearing at the point c' is as shown in (c) in FIG. 
6. The shaping circuit 120 includes a comparator for comparing this 
waveform with a constant voltage V.sub.a as shown by a dashed line in (c) 
in FIG. 6 and producing an output at a level "1" when the value is 
surpassed, and it produces an output as shown in (e) in FIG. 6. A 
flip-flop circuit 130, which consists of a well-known R-S flip-flop, 
receives the outputs of both the shaping circuits 110 and 120 and produces 
a pulse as shown in (f) in FIG. 6. The duration T of this pulse represents 
the slope of rising of the secondary voltage generated in the ignition 
coil 1. A gate 140 passes clock pulses from an oscillator 150 to a counter 
160 for a period corresponding to the duration of the output pulse from 
the flip-flop circuit 130, thus measuring the period T. A counter 180 
produces pulses spaced apart in time (pulses in (g) and (h) in FIG. 6) for 
causing a latch 170 to take out the result of the count from the counter 
160 and subsequently resetting the counter 160. More particularly, the 
result of the count of the counter 160 is temporarily stored in the latch 
170 under the control of the pulse in (g) in FIG. 6, and the counter 160 
is subsequently reset under the control of the pulse in (h) in FIG. 6. The 
measurement value T temporarily stored in the latch 170 is then supplied 
to a memory section 300. Designated at 200 is a primary cutoff current 
measuring circuit. It includes a differential amplifier 210 which detects 
the primary current by detecting the potential difference between the 
opposite ends of the external resistor 7. The detected waveform is as 
shown in (a) in FIG. 6. The peak of this waveform is held by a peak hold 
circuit 220 as shown by a dashed line in (a) in FIG. 6, and is converted 
by an analog-to-digital (A/D) converter 230 into a corresponding digital 
value. This digital signal is taken out by a latch 240 at the timing of 
the afore-mentioned latch signal shown in (g) in FIG. 6 to be supplied to 
the memory section 300. 
The memory section 300 includes a read only memory (ROM) 310 and a 
digital-to-analog (D/A) converter 320. The ROM 310 receives as its input 
the output of the latch 170 in the floating capacitance detecting circuit 
100 and the output of the latch 240 in the primary cutoff current 
detecting circuit 200. These two data respectively represent the rising 
period T and the primary cutoff current I.sub.off, and the ROM 310 
produces a value representing the floating capacitance determined from the 
two input values. In the ROM 310, data as shown in FIG. 3 (representing 
the floating capacitance correlating the rising period T and primary 
cutoff current I.sub.off) are memorized. The D/A converter 320 converts 
the digital value produced from the ROM 310 into an analog voltage, that 
is, it produces a voltage value as shown in (i) in FIG. 6 which represents 
the magnitude of the floating capacitance. 
A second embodiment of the invention will now be described. While in the 
preceding first embodiment the period T from the rising of the primary 
voltage till the reaching of a constant voltage V.sub.2 is measured for 
determining the slope of rising of the secondary voltage, in the second 
embodiment the slope is determined by measuring the secondary voltage a 
predetermined period after the rising of the primary voltage. 
FIG. 7 shows a graph, in which the secondary voltage E.sub.2 50 .mu.sec. 
after the rising of the primary voltage is plotted. Plots a, b and c 
represent characteristics for respective floating capacitance values of 0, 
50 and 100 pF. As is shown, the secondary voltage E.sub.2 increases with 
increase of the primary cutoff current I.sub.off while it decreases with 
increase of the floating capacitance. It will be seen from FIG. 7 that the 
floating capacitance can be determined from the secondary voltage E.sub.2 
and primary cutoff current I.sub.off if these values are obtained. The 
secondary voltage is actually negatively as high as several ten kV, but 
one-thousandth of its value is measured by virtue of the fact the 
afore-mentioned voltage divider 9 dividing a high voltage is used. 
FIG. 8 shows a second example of the ignition system diagnosing unit, which 
is generally designated at 10. Designated at 400 is a rising slope 
measuring circuit. It includes a shaping circuit 410 with the input 
terminal thereof connected to the point b in FIG. 4, i.e., the juncture 
between the ignition coil 1 and igniter 2. At this point b a waveform as 
shown in (b) in FIG. 9 appears. The shaping circuit 410 converts this 
waveform into a pulse as shown in (d) in FIG. 9. A delay circuit 420 
receives the output pulse of the shaping circuit 410 as trigger pulse to 
produce a pulse having a duration T' as shown in (e) in FIG. 9. A counter 
430 receives the output pulse of the delay circuit 420 as reset input and 
starts counting of clock pulses from an oscillator 440 after the falling 
of this pulse. It produces as its outputs Q.sub.1 and Q.sub.2 pulses 
spaced apart in time as shown in (f) and (g) in FIG. 9. The rising slope 
measuring circuit 400 further includes an inverting circuit 450, which 
receives as its input the output of the voltage divider 9 as shown in (c) 
in FIG. 9. This input is obtained by dividing the secondary voltage to 
1/1000. Since the secondary voltage is a negative voltage, the inverting 
circuit 450 inverts the divided voltage input to a positive one. An A/D 
converter 460 converts the output of the inverting circuit 450 into a 
digital value. The output of the A/D converter 460 is temporarily stored 
in a latch 470 at a timing as shown in (f) in FIG. 9 before being supplied 
to a memory section 600. 
Designated at 500 is a primary cutoff current measuring circuit. It 
includes a differential amplifier 510 for detecting the primary current by 
measuring the potential difference between the opposite terminals of the 
external resistor 7 in series with the ignition coil 1. The detected 
waveform is as shown by a solid line in (a) in FIG. 9. A peak hold circuit 
520 holds the peak of the primary current waveform as shown by a dashed 
line in (a) in FIG. 9, and an A/D converter 530 converts this value into a 
digital one. This digital value is taken out by a latch 540 at the timing 
of the latch signal shown in (f) in FIG. 9 to be supplied to the memory 
section 600. 
The memory section 600 includes a ROM 610 and a D/A converter 620. The ROM 
610 receives as its input the output of the latch 470 in the rising slope 
measuring circuit 400 and the output of the latch 540 in the primary 
cutoff current measuring circuit 500. These two data respectively 
represent the secondary voltage E.sub.2 and primary cutoff current 
I.sub.off, and the ROM 610 produces the floating capacitance value 
determined from these two values. In the ROM 610, data regarding the 
one-thousandth of the secondary voltage value are memorized. 
The D/A converter 620 converts the output digital value of the ROM 610 into 
an analog voltage, that is, it produces a voltage value as shown in (h) in 
FIG. 9 corresponding to the magnitude of floating capacitance. 
While in the preceding first and second examples respectively shown in 
FIGS. 5 and 8 the slope has been measured respectively by determining the 
time elapsed until the reaching of a predetermined voltage and the 
secondary voltage after a predetermined period of time, in a third example 
the slope is determined from the time elapsed until the breakdown takes 
place and the breakdown voltage. As a means for determining the floating 
capacitance by this slope determination method, there is a map method, 
which makes use of three parameters, namely the cutoff current, time until 
the break takes place and breakdown voltage. Also, there is another 
method, in which an approximation to the secondary voltage is obtained by 
solving differential equations set up under the assumption of an 
equivalent circuit of the ignition system, and a formula for calculating 
the floating capacitance is derived to determine the floating capacitance 
from this formula. With the calculation system based on this formula, a 
formula for calculating the generated secondary voltage (i.e., the maximum 
value of the open waveform where the breakdown does not take place) can 
also be derived from the approximation formula for the secondary voltage, 
and the generated secondary voltage can be determined. The latter 
calculation system will now be described. 
FIG. 10 shows an equivalent circuit of the ignition system. Labeled E is 
the battery, R.sub.1 the sum of the external resistance and the resistance 
of the coil primary, L.sub.1 the inductance of the coil primary, Tr the 
last stage power transistor in the igniter, R.sub.2 the resistance of the 
coil secondary, L.sub.2 the inductance of the coil secondary, C.sub.2 the 
sum of the capacitance of the coil secondary and the floating capacitance, 
M the mutual inductance of the coil, i.sub.1 the primary current, i.sub.2 
the secondary current, v.sub.1 the primary voltage, and v.sub.2 the 
secondary voltage. From FIG. 10, there are set up differential equations: 
##EQU1## 
There is taken several ten .mu.sec. before the primary current is cut off 
by the last stage power transistor in the igniter. Under the consideration 
of this cutoff time T.sub.s of the transistor, the primary current i.sub.1 
is assumed to be 
##EQU2## 
(It is also possible to linearly approximate i.sub.1 to be 
##EQU3## 
Then, by solving the above differential equations under this assumption we 
have, for 0&lt;t&lt;T.sub.s, 
##EQU4## 
and for T.sub.s &lt;t, 
##EQU5## 
where k is the coefficient of coupling of the coil, i.e., 
##EQU6## 
FIG. 13 compares the experimental true value and calculated value of the 
secondary voltage v.sub.2. These two values coincide well in a region from 
the rising of the secondary voltage till the reaching of the maximum value 
of the secondary voltage, in which the break takes place. Denoting the 
floating capacitance by C* and the generated secondary voltage by V.sub.G, 
we have 
##EQU7## 
where C.sub.L2 is the capacitance of the coil secondary, T is the time 
until the break takes place, and V.sub.B is the breakdown voltage. It is 
possible to compensate V.sub.B in the above equations for the energy loss 
due to the discharge in the distributor, and by so doing the accuracy will 
be further improved. 
FIG. 11 shows the third example of the ignition system diagnosing unit, 
which is generally designated at 10. Designated at 2100 is a time 
measuring circuit for measuring the time from the rising of the secondary 
voltage until the breakdown takes place. It includes a shaping circuit 
2110 with an input terminal b thereof connected to the point b in FIG. 4. 
The waveform appearing at this input terminal is as shown in (b) in FIG. 
12. The shaping circuit 2110 shapes this waveform into a pulse as shown in 
(d) in FIG. 12. The time measuring circuit also includes a differentiating 
circuit 2120 with an input terminal c' thereof connected to the point c in 
FIG. 4. The circuit 2120 differentiates a waveform as shown in (c) in FIG. 
12 to produce a waveform as shown in (e). Its output is coupled to a 
shaping circuit 2130, in which a suitable threshold level is provided so 
that it does not detect the discharge in the distributor but detects only 
the discharge in the plug section to produce a waveform as shown in (f) in 
FIG. 12. A flip-flop circuit 2140 produces from the waveforms (d) and (f) 
in FIG. 12 a waveform representing the period of time T until the break 
takes place as shown in (g). A gate 2160 passes clock pulses from an 
oscillator 2150 to a counter 2170 for a period of time corresponding to 
the duration of the output pulse of the flip-flop circuit 2140, and thus 
it measures the time T. A counter 2180 produces pulses spaced apart in 
time (i.e., pulses as shown in (i) and (h) in FIG. 12) for transferring 
the result of the counter 2170 to a latch 2190 and subsequently resetting 
the counter 2170. More particularly, the result of the counter 2170 is 
transferred to and temporarily memorized in the latch 2190 under the 
control of the pulse (i), and the counter 2170 is subsequently reset under 
the control of the pulse (h). The measurement value T temporarily stored 
in the latch 2190 is supplied to an arithmetic section 2400. 
Designated at 2200 is a breakdown voltage measuring circuit. Here, a peak 
hold circuit 2310 holds the peak of the secondary voltage waveform (c) in 
FIG. 12. It holds the peak of the waveform as shown by a dashed line in 
(c) in FIG. 12, and an A/D converter 2320 converts this value into a 
corresponding digital value, which is taken out by the latch 2330 at the 
timing of the latch signal (h) shown in FIG. 12 to be supplied to the 
arithmetic section 2400. 
Designated at 2300 is a primary cutoff current measuring circuit. Here, a 
differential amplifier 2310 detects the primary current by measuring the 
potential difference between the opposite terminals of the external 
resistor 7 shown in FIG. 4. A peak hold circuit 2320 holds the waveform of 
its input, as shown by a solid line in (a) in FIG. 12, in a manner as 
shown by a dashed line, and an A/D converter 2330 converts this value into 
a digital value. A latch circuit 2340 supplies this digital value to the 
arithmetic section 2400 at the timing as shown in (h) in FIG. 12. 
The arithmetic section 2400 includes a central processing unit (CPU) 2410 
and a D/A converter 2420. In the CPU 2410, the values in the latches 2190, 
2230 and 2340 are taken out, and the floating capacitance and generated 
secondary voltage are calculated with these values substituted into the 
afore-mentioned formulas for obtaining the floating capacitance and 
generated secondary voltage. 
FIG. 14 shows a second embodiment of the ignition system for an internal 
combustion engine according to the invention. In this embodiment, a 
primary current control section 20 is provided in lieu of the ignition 
system diagnosing unit 10 in the previous embodiment of FIG. 4. In other 
words, this embodiment is the same as the embodiment of FIG. 4 except for 
that the primary current control section 20 controls the igniter 2 for 
on-off controlling the primary current in the ignition coil and that the 
ignition coil 1' in this case is of an improved type with the current 
therein increasing linearly with time as shown by a solid line or dashed 
line in FIG. 15. 
The primary current control section 20 is a gist of this embodiment, and it 
determines the energization period of the primary of the coil 1 from the 
magnitude of the floating capacitance and controls the energy supplied to 
the coil without varying the ignition timing but by varying the timing of 
the commencement of the conduction. 
Now, the primary current control section 20 will be described. FIG. 16 
shows its block diagram, and FIG. 17 is a time chart illustrating its 
operation. In FIG. 16, designated at 100 is a floating capacitance 
detecting section. Its input terminals b and c' are connected to the 
respective points b and c' in FIG. 14, and waveforms as shown in (b) and 
(c) in FIG. 17 appear at the respective points b and c'. The floating 
capacitance detecting section 100 shown in FIG. 16 is the same as the 
floating capacitance detecting section 100, so its detailed description is 
omitted. The waveforms of the outputs of the shaping circuits 110 and 120 
in the floating capacitance detecting section 100 in FIG. 16 are 
respectively shown in (d) and (e) in FIG. 17. Also, the output waveform of 
the flip-flop circuit 130 is shown in (f) in FIG. 17, and the output 
waveform of the counter 180 is shown in (g) and (h) in FIG. 17. The 
measurement value T obtained by measuring the period T shown in FIG. 2 is 
latched in the latch 170 and is supplied to an energization period control 
section 700. The value T here represents the period until the secondary 
voltage across the ignition coil 1 reaches a constant voltage V.sub.2, 
i.e., the slope of rising of the secondary voltage. Designated at 800 is a 
primary cutoff current measuring section. It detects the primary current 
from the potential difference between the opposite terminals of the 
external resistor 7 in series with the primary coil. A peak hold circuit 
810 holds the peak of the potential difference between the opposite ends 
of the resistor 7 (of a waveform as shown by a solid line in (a) in FIG. 
17), and an A/D converter 820 converts this value into a digital value. A 
latch 830 takes out this digital value under the control of the 
afore-mentioned latch signal as shown in (g) in FIG. 17 and supplies it to 
a ROM 750 in the control section 700. The content of the program stored in 
the ROM 750 is, for instance, as shown by the plot c for a floating 
capacitance value of 100 pF as shown in the graph of FIG. 3. When the 
primary cutoff current is 3 A and the rising period T is 34 .mu.sec., a 
point on the plot c is taken out, showing that the floating capacitance is 
increased by 100 pF. As the content of the ROM 750, the rising period, for 
instance one corresponding to the plot for the floating capacitance value 
of 100 pF, is memorized as a corresponding count number of clock pulses 
produced from the oscillator 150. The peak hold circuit 810 is reset by 
the afore-mentioned period control signal as shown in (h) in FIG. 17. 
A comparator 710 in the energization period control section 700 compares 
the output of the latch 170, i.e., the measured rising period, and the 
output of the ROM 750, i.e., the rising period corresponding to a 
predetermined primary cutoff current value for the floating capacitance 
value of 100 pF, and it produces an output of a level "1" when the former 
is longer than the latter. At this time, in an adder 720 a basic dwell 
angle (K.sub.1) which is always provided from a basic dwell angle setting 
circuit 730 and a compensating dwell angle (K.sub.2) provided from an 
angle setting circuit 740 are added together to produce a dwell angle 
(K.sub.1 +K.sub.2). Normally, (i.e., when the output of the comparator 710 
is at a level "0"), the sole basic dwell angle (K.sub.1) from the basic 
dwell angle setting circuit 730 is provided from the adder 220. Designated 
at 900 is an ignition timing control section for determining the 
energization commencement timing and ignition timing. In this section, an 
ignition timing calculating section 920 calculates the ignition timing 
from a r.p.m. value N and an intake pressure value P supplied to it, and 
an advancement angle calculating section 940 produces from a top dead 
center signal (TDC) as shown in (i) in FIG. 17 a crank angle signal as 
shown in (j) in FIG. 3. A down-counter 430 down-counts this value for each 
one-degree crank angle signal (1.degree. CA). 
Meanwhile, a dwell angle calculating section 940 produces a dwell angle 
signal as shown in (k) in FIG. 17, and a down-counter 950 down-counts this 
value for each one-degree crank angle signal (1.degree. CA). When the 
outputs of the counters 930 and 940 become zero, a signal is supplied to a 
flip-flop circuit of a well-known construction constituted by NAND 
circuits 960 and 970, and the energization commencement timing and 
ignition timing are controlled by the output signal from this flip-flop as 
shown in (l) in FIG. 17. Thus, when the floating capacitance is increased, 
the energization period can be increased to increase the coil energy 
without changing the ignition timing, as shown by a dashed line in (l) in 
FIG. 17. The normal energization period is indicated by a solid line in 
(l) in FIG. 17. By providing a longer period for energizing the coil 
primary the primary cutoff current I.sub.off can be increased from the 
value shown by the solid line in FIG. 15 to the value of the dashed line 
to increase the coil energy. The one-degree crank angle signal (1.degree. 
CA) and top dead signal (TDC) are provided from a signal generator, which 
comprises a slit disc installed on the engine crankshaft and a 
photo-sensor for detecting the slit. 
A second example of the primary current control section 20 will now be 
described. While in the preceding first example the energization period is 
controlled such that when the floating capacitance exceeds a predetermined 
value the energization period is made longer by an extent corresponding to 
a predetermined crank angle, in the second embodiment the energization 
period is continuously controlled according to the floating capacitance 
value. FIG. 18 shows a portion of the second example that sets this 
example apart from the first example; namely an energization period 
control section 1000 corresponding to the section 700 shown in FIG. 16. In 
FIG. 18, a latch 170 corresponds to the latch 170 in FIG. 16, and when the 
pulse signal shown in (g) in FIG. 17 is produced it supplies the count 
number corresponding to the rising period T until the reaching of the 
constant voltage V.sub.a by the secondary voltage, obtained in the 
preceding stage circuit, to a ROM 1010. A latch circuit 830 corresponds to 
the latch circuit 830 in FIG. 2, and it supplies the primary cutoff 
current derived in the preceding stage circuit to the ROM 1010 under the 
control of the pulse signal shown in (g) in FIG. 6. In the ROM 1010, data 
concerning the compensation angle which is determined as a function of the 
floating capacitance which is in turn determined from the rising period T 
and primary cutoff current I.sub.off and to be added to the basic dwell 
angle are memorized. This compensation angle increases with increasing 
floating capacitance to increase the energization period and hence the 
coil energy. Table below shows an example of the memory content of the ROM 
1010. The compensation angle memorized in this example is, for instance, 
1.0.degree. for 20 .mu.sec. as the value of T, 7.0.degree. for 30 
.mu.sec., 14.0.degree. for 40 .mu.sec. and so forth with 3.0 A as the 
value of I.sub.off. Values within parentheses given below these 
compensation angle values represent the corresponding floating 
capacitance. 
______________________________________ 
I.sub.off 
20 30 40 50 60 
______________________________________ 
2.0 3.5 7.0 9.5 14.0 
(35) (70) (95) (140) 
2.5 0 5.0 10.0 15.0 
(-5) (50) (100) (150) 
3.0 1.0 7.0 14.0 20.0 
(10) (70) (140) (200) 
3.5 1.5 9.0 17.0 
(15) (90) (170) 
4.0 1.5 100 19.0 
(15) (100) (190) 
______________________________________ 
In an interporating section 1020, the compensation dwell angle is 
determined, in an adder 1040 and the compensation dwell angle is added to 
the basic dwell angle from a basic dwell angle setting circuit 1030 to 
produce the dwell angle output supplied to the dwell calculating section 
940. As an example, when the rising period T is 35 .mu.sec. and the 
primary cutoff current I.sub.off is 3 A, the compensation angle is 
obtained from 14.degree. C. for T=40 .mu.sec. with I.sub.off =3A and 
7.degree. for T=30 .mu.sec. with I.sub.off =3A by the interpolation 
method, and is 10.5.degree. (the corresponding floating capacitance being 
105 pF). In this case, the output dwell angle specified by the adder 1040 
is greater than the basic dwell angle by 10.5.degree., and the coil energy 
is increased by the corresponding amount. 
While in the above embodiments the voltage division ratio of the voltage 
divider 9 is set to 1/1000, this is by no means limitative. Also, the 
ignition coil 1 is not limited to the one, in which the current increases 
linearly with time as shown in FIG. 15, and it is possible to use as well 
an ordinary coil in which the current varies in a manner as shown in FIG. 
19. In FIG. 19, a solid curve shows the waveform of the current normally 
caused, and a dashed curve of the current that is caused when the 
energization period is increased. 
FIG. 20 shows a third embodiment of the ignition system for an internal 
combustion engine according to the invention. In the embodiment of FIG. 
20, unlike the embodiment of FIG. 14 in which the igniter 2 is controlled 
by the primary current control section 20, the igniter 2 is on-off 
controlled by an ignition signal from an ignition signal generating means 
2a for controlling the energization of the primary coil 1a of the ignition 
coil 1 to produce a high voltage across the secondary coil 1b therein. 
External resistors 7 and 7a are connected in series with the primary coil 
1a of the ignition coil 1, and as a primary current control circuit a 
relay 30 is connected in parallel with the resistor 7a. The relay 30 is 
controlled by a coil energy control section 40, which is a gist of the 
invention such that the resistor 7a is shunted when an output of a level 
"1" is produced from the control section 40. The ignition coil 1 is an 
ordinary ignition coil, that is, it is not of the improved type with the 
current linearly increasing with time as shown in FIG. 14. In the other 
construction, the embodiment of FIG. 20 is the same as the embodiment of 
FIG. 14. 
An example of the coil energy control section 40 will now be described. 
FIG. 21 is its block diagram, and FIG. 22 is a time charge illustrating 
the operation of it. In FIG. 21, designated at 100 is a floating 
capacitance detecting section with its input terminals b and c' connected 
to the respective points b and c' in FIG. 20. Waveforms as shown in (b) 
and (c) in FIG. 22 appear in the respective points b and c'. The 
construction of the floating capacitance detecting section 100 in FIG. 21 
is the same as that of the section 100 in FIG. 5, so its detailed 
description is omitted here. The waveforms of the outputs of the shaping 
circuits 110 and 120 are respectively shown in (d) and (e) in FIG. 17. 
Also, the waveform of the output of the flip-flop circuit 130 is shown in 
(f) in FIG. 17, and the waveform of the output of the counter 180 is shown 
in (g) and (h) in FIG. 17. The measurement value T obtained by measuring 
the period T in FIG. 2 is latched in the latch 170 and supplied to a 
comparator section 1100. 
Designated at 1200 is a level setting section, in which the primary current 
is detected from the potential difference between the opposite terminals 
of the external resistor 7 in series with the primary coil. A peak hold 
circuit 310 holds the peak of the potential difference between the 
opposite terminals of the resistor 7 (the waveform as shown by a solid 
curve in (a) in FIG. 22) as shown by a dashed line in (a) in FIG. 22. The 
peak hold circuit 1210, an A/D converter 1220, a latch 1230 and a ROM 1240 
in the level setting section 1200 are respectively the same in 
construction, connection and operation as the peak hold circuit 810, A/D 
converter 820 and latch 830 in the primary cutoff current section 800 and 
the ROM 750 in the energization period control section 700 in FIG. 16, so 
their detailed description is omitted here. The comparator section 1100 
includes a digital comparator 1110, which compares the output of the latch 
170, i.e., the period of rising of the secondary voltage, and the output 
of the ROM 1240, and a control circuit 1120 for controlling the relay 30 
according to the output of the digital comparator 1110. When the measured 
rising period T is longer the rising period corresponding to a 
predetermined primary cutoff current for the floating capacitance value of 
100 pF, the comparator 1110 produces an output of a level "1" showing that 
the floating capacitance is increased. The control circuit 1120 amplifies 
this signal up to a level capable of operating the relay 30 so that the 
relay 30 is turned "on". As a result, the total resistance on the primary 
side of the ignition coil 1 is reduced to increase the primary cutoff 
current I.sub.off as shown in FIG. 23 so as to increase the coil energy. 
Thus, the secondary voltage produced in the ignition coil 1 is increased 
to prevent the generation of a miss-spark. 
A second example of the coil energy control section 40 will now be 
described. While in the preceding example the period T until the secondary 
voltage reaches a constant value V.sub.2 has been measured for determining 
the slope of rising of the secondary voltage, in this example the slope is 
determined by obtaining the secondary voltage after the lapse of a 
predetermined period of time. 
FIG. 24 shows, similar to FIG. 7, the secondary voltage E.sub.2 50 .mu.sec. 
after the rising of the primary voltage. Plots a, b and c represent 
characteristics for respective floating capacitance values of 0, 50 and 
100 pF. The floating capacitance can be determined from the secondary 
voltage E.sub.2 and primary cutoff current I.sub.off with reference to 
this Figure. When the measured secondary voltage is found to be lower than 
the value in the graph for, for instance, the floating capacitance value 
of 100 pF, the resistance on the primary side of the ignition coil 1 
(resistance of a circuit including the external resistors 7 and 7a in 
series) is reduced. 
FIG. 25 shows the second example of the coil energy control section 40, and 
FIG. 26 is a time chart illustrating the operation of it. Designated at 
1300 is a floating capacitance detecting section. It includes a shaping 
circuit 1310 with the input terminal thereof connected to the point b in 
FIG. 4, i.e., the juncture between the ignition coil 1 and igniter 2. At 
this point b appears a waveform as shown in (b) in FIG. 26 similar to the 
waveform shown in (b) in FIG. 22. The shaping circuit 1310 converts this 
waveform into a pulse as shown in (d) in FIG. 26. A delay circuit 1320 
produces a pulse as shown in (e) in FIG. 26, having a duration T' from the 
rising of the pulse in (d) in FIG. 26. A counter 1330 counts clock pulses 
from an oscillator 1340 and produces a pulse as shown in (f) in FIG. 26 
immediately after the duration T' of the pulse in (e) in FIG. 26. 
The section 1300 further includes an inverting circuit 1350 with the input 
terminal thereof connected to the output terminal of the voltage divider 9 
and receiving a waveform as shown in (c) in FIG. 26. This waveform is a 
negative voltage, and an inverting circuit 1350 inverts this voltage into 
a positive one. A hold circuit 1360 samples and holds the output of the 
inverting circuit 1350 at the timing of the output of the counter 1330 
(i.e., the pulse shown in (f) in FIG. 26). Designated at 1500 is a level 
setting section. It detects the primary current from the potential 
difference between the opposite terminals of the external resistor 7 in 
series with the primary coil 1. A peak hold circuit 1510 holds the peak of 
the potential difference between the opposite terminals of the resistor 7 
(i.e., a waveform as shown in (a) in FIG. 26), and a hold circuit 1520 
also effects sampling and holding at the timing of the output of the 
counter 1330 as shown in (f) in FIG. 26. The hold circuit 1520 has a 
construction as shown in FIG. 27. Its time constant is suitably set by 
appropriately selecting the resistance of its resistor 1520a and the 
capacitance of its capacitor 1520b so that a change of I.sub.off can be 
detected. It further has an analog switch 1520c which is turned on when 
the signal shown in (f) in FIG. 26 is at level "1". 
The section 1500 further includes an amplifier 1530. It produces an output 
as a function of the sampled value of the primary cutoff current 
I.sub.off, for instance as shown by a dashed plot d in FIG. 24. While the 
scale of the ordinate of the graph of FIG. 24 is in the order of kV, the 
actual scale is one-thousandth of the scale of the graph because of the 
fact that the voltage divider 9 is used. While in the preceding example 
the rising period programmed with I.sub.off for 100 pF is memorized in the 
ROM, in this example an approximation to the divided secondary voltage 
characteristic for 100 pF, i.e., the dashed plot in FIG. 24, is used. The 
program of this characteristic may of course be memorized by using a ROM 
as in the preceding example. 
Designated at 1400 is a comparator section. It includes an analog 
comparator 1410 and a control circuit 1420 for controlling the relay 30 
according to the output of the comparator 1410. The comparator 1410 
compares its two inputs, i.e., the value obtained by sampling the divided 
secondary voltage a predetermined period of time T' after the rising of 
the primary voltage and a predetermined voltage value programmed with the 
primary cutoff current I.sub.off for the floating capacitance value of 
substantially 100 pF, and when the former becomes lower than the latter it 
produces an output at a level "1", whereby the relay 30 is turned "on" by 
the control circuit 1420. 
The peak hold circuit 1510 is reset when a pulse shown in (g) in FIG. 26, 
slightly delayed after the pulse in (f) in FIG. 26, is produced from the 
counter 1330. While the voltage division ratio of the voltage divider 9 is 
set to 1/1000, this is by no means limitative, and any suitable ratio may 
be selected by considering the source voltage for the circuit and the 
amplification degree of the amplifier 1530. 
FIG. 28 shows a third example of the coil energy control section 40. 
Designated at 2000 is a power transistor for controlling the energization 
of the ignition coil 1, and at 2001 a detecting resistor for detecting the 
primary current in the ignition coil 1. Designated at 2004 is a bias 
control circuit for controlling the base current in the transistor 2000. 
Designated at 2002 is a transistor for on-off controlling the power 
transistor 2000 and controlled by a control circuit 2003. The control 
circuit 2003 receives as its input an ignition timing control and 
energization control signal produced from a well-known ignition signal 
generating means 2005. Thus, a signal as shown in (a) in FIG. 29 appears 
at a point X in FIG. 28. Resistors 2006, 2007, 2009 and 2011, a transistor 
2010 and an inverter 2008 constitute a level switching circuit 2012, and 
the potential at a point Y is changed by the signal from the control 
circuit 1120 shown in FIG. 21 or control circuit 1420 shown in FIG. 25. 
When the energization of the primary coil 1a of the ignition coil 1 is 
started with the triggering of the power transistor 2000, the potential at 
a point Z, i.e., one end of the detecting resistor 2001, increases with 
current therethrough as shown in (b) in FIG. 29. 
The bias control circuit 2004 compares the potential at the point Z and a 
predetermined potential at the point Y, and when the potential at the 
point Z is higher than that at the point Y it functions to reduce the 
potential at the point X for reducing the base current in the transistor 
2000. As a result, the operation of the transistor 2000 is controlled 
toward the cutoff, whereby the primary current is reduced to reduce the 
potential at the point Z. Consequently, the potential at the point Y 
becomes higher than the potential at the point Z, whereby the base current 
in the power transistor 2000 is increased to bring the power transistor 
again toward the conduction. In this way, during the energization of the 
primary coil the power transistor 2000 is controlled to make the potential 
at the point Z equal to that at the point Y, and thus the primary current 
in the ignition coil 1 trimmed at a certain value as shown in (b) in FIG. 
29. In this construction, when the floating capacitance is less than a 
predetermined value (for instance 100 pF), at which time the output of the 
control circuit 1120 or 1420 is "0", the transistor 2010 is "on". Thus, at 
this time the potential at the point Y is at a low level, and the primary 
current which is controlled to a constant value is at a low level as shown 
by a solid line in (b) in FIG. 29. 
When the floating capacitance is increased, the output of the control 
circuit 1120 or 1420 is changed to "1". As a result, the transistor 2010 
is cutoff, increasing the potential level at the point Y, whereby the 
primary current is controlled to a high level as shown by a dashed line in 
(b) in FIG. 29 to increase the coil energy so as to increase the generated 
voltage for preventing the generation of a miss-spark. 
While in the above embodiments the primary current is increased in a 
non-continuous way with the increase of the floating capacitance beyond a 
predetermined value, it is also possible to permit the primary current to 
be continuously increased with increasing floating resistance. 
Also, while in the above embodiments the floating capacitance has been 
digitally calculated by using a floating capacitance calculating circuit 
constituted by a memory section using a ROM, it is also possible to 
calculate the floating capacitance analog-wise with a floating capacitance 
calculating circuit using a function generator circuit or the like.