Electronic system for controlling the ignition of an internal combustion engine, particularly for motor vehicles

A control circuit which includes a circuit for regulating the time taken to desaturate the switching transistor. This regulating circuit is arranged to control the time for which the transistor is conductive, so that the desaturation time of the transistor is substantially nil when the rate of rotation of the engine is greater than a predetermined value.

DESCRIPTION 
The present invention relates to an electronic system for controlling the 
ignition of an internal combustion engine, specifically a system 
comprising: 
a magnetic pulse generator for outputting a signal having a frequency and 
amplitude proportional to the rate of rotation of the engine, 
an ignition coil the secondary winding of which can be coupled selectively 
and cyclically to the plugs of the engine to generate the spark, 
a switching transistor connected to the primary winding of the coil, and 
an control circuit which is connected to the pulse generator and to the 
switching transistor and which, in order to cause a spark to be generated, 
is arranged to cause 
a. the transistor to become saturated at a first moment determined by the 
characteristics of the signal from the pulse generator to allow current to 
pass through the primary winding of the ignition coil, 
b. the transistor to be desaturated immediately the current has reached a 
predetermined value in order to limit the intensity of the current, and 
c. the transistor to be cut off at a second moment determined by the 
characteristics of the signal from the pulse generator to interrupt the 
current. 
In electronic ignition control systems of this type made up till now, the 
switching transistor is generally of the Darlington type. The primary 
winding of the ignition coil typically also has a very low resistance to 
allow the current circulating in it to rise rapidly when the switching 
transistor is saturated. This is due to the fact that it is necessary to 
allow the current in the primary winding of the ignition coil to reach a 
sufficiently high value even when the battery is running down. 
At low rates of rotation of the engine, the current in the primary winding 
of the coil can thus even reach very high values which could damage it. 
For this reason, the switching transistor is desaturated, that is, 
returned to linear operating conditions, immediately the current in the 
ignition coil has reached a predetermined limit, for example about 6A. 
When the switching transistor is saturated, the current in the ignition 
coil increases exponentially with an almost linear initial trace. When the 
current reaches the predetermined final value, the transistor is 
desaturated and remains in this condition until it is cut off, 
interrupting the current sharply, in order to cause sparking. The time 
interval for which the transistor is kept desaturated is such as to enable 
the current in the ignition coil to reach a sufficient magnitude in 
various dynamic operating conditions, for example, even when the rate of 
rotation of the engine is increased sharply. In fact, the increase in the 
rate of rotation of the engine causes a reduction in the time between two 
successive sparks and hence a reduction in the time available to bring the 
current in the primary winding of the ignition coil to a magnitude 
sufficient to cause to the discharge. 
When the switching transistor is saturated, it dissipates a small power 
since the value V.sub.ce is very small. However, the energy dissipated by 
the transistor is considerable when it operates in desaturated conditions, 
since the collector current and V.sub.ce both assume high values. 
The object of the present invention is to provide an electronic ignition 
control system of the type mentioned above, which enables the dissipation 
of energy by the switching transistor to be reduced noticeably. 
This object is achieved according to the invention by means of an 
electronic ignition control system of the type specified above, the main 
characteristic of which lies in the fact that the control circuit 
includes: 
electrical sensor means for providing a signal indicative of the rate of 
rotation of the engine, and 
a circuit for regulating the desaturation time of the transistor, coupled 
to the electrical sensor means and arranged to control the time for which 
the switching transistor is conductive so that the length of the 
desaturation time of the transistor is substantially nil when the rate of 
rotation of the engine is greater than the predetermined value.

With reference to FIG. 1, an electronic ignition control system according 
to the invention, for internal combustion engines, includes a magnetic 
pulse generator 1 including, in known manner, a toothed rotor 2 rotated by 
the engine, and a stationary inductive receiver (pick-up) 3. The pulse 
generator 1 is mounted in known manner, for example, within the body of 
the ignition distributor (not illustrated). 
The pick-up 3 may, for example, include a coil of conductive wire located 
in a magnetic circuit with a variable gap. During rotation, each time a 
tooth of the rotor 2 passes in front of the coil 3 the flux in the 
magnetic circuit varies and, by induction, generates an electrical signal 
at the ends of the coil 3. The waveform of the signal generated by the 
pulse generator is, for example, that indicated A in FIG. 4. The form of 
this signal depends on the geometry of the rotor but, in general, the 
signal includes a portion with a positive amplitude in the phase in which 
a tooth approaches a pick-up 3, a nil amplitude (passage through zero) 
when a tooth faces the pick-up, and a negative portion corresponding to 
the phase in which the tooth moves away from the pick-up. 
In other words, the signal output by the pick-up is proportional to the 
variation in the magnetic flux with time and its amplitude is therefore 
proportional to the rate of rotation (number of revolutions) of the 
engine, and the negative passages through zero are proportional to the 
frequency and precisely identify a particular position of the engine 
shaft. 
With reference again to FIG. 1, the ignition control system includes, in 
known manner, an ignition coil or transformer 4 with a primary winding 5 
and a secondary winding 6. The winding 5 is connected between a source of 
d.c. voltage V (a motor vehicle battery) and the collector of a power 
switching transistor 7 of Darlington type. A feedback resistor 8 for 
controlling the current in the winding 4 is connected between the emitter 
of this transistor and earth. 
The secondary winding 6 of the transformer 4 can be connected to the plugs 
SP of the engine selectively and cyclically, for example, through a rotary 
distributor of known type. 
The base of the transistor 7 is connected to the output of a pilot (driver) 
circuit 9 having its input connected to the output of a logic control 
circuit 10. 
The pulse generator 1 is connected to the input of a passage-through-zero 
monitor 11 of known type. 
In ignition control systems of known type, the current in the primary 
winding 5 of the ignition coil is controlled through a switching 
transistor 7 so that, for any rotational speed of the engine, it has a 
trace of the type shown by the waveform I in FIG. 1a. In order to achieve 
this current trace, in known systems, the signal A output by the pulse 
generator is compared with the value of a reference signal processed by an 
angle advance control circuit. When the signal A passes through the level 
of the reference signal, the switching transistor 7 is changed from the 
cut-off condition to the saturated condition, so that the current starts 
to increase practically linearly (instant T.sub.0 in FIG. 1a) in the 
primary winding 5 of the ignition coil. The instantaneous value of the 
current in the primary winding of the ignition coil is controlled by means 
of the feedback resistor connected in the collector-emitter circuit of the 
switching transistor. In present ignition control systems, the current in 
the primary winding of the ignition coil 4 is limited to a maximum value, 
indicated I.sub.M in FIG. 1a, for example equal to 6A, to avoid damage to 
the primary winding of the ignition coil and to the Darlington transistor 
itself. This current limitation is, for the most part, obtained by a 
reduction in the base current of the Darlington transistor 7, so as to 
bring the latter from the saturated condition to a condition of operation 
in the linear zone (desaturation). 
In known systems (and also in the system of the invention) immediately the 
pulse generator signal A presents a negative passage through zero (that 
is, a passage through zero in which a previously positive signal becomes 
negative), the switching Darlington transistor is cut off and the current 
in the primary winding of the ignition coil is interrupted suddenly 
(instant T.sub.2 in FIG. 1a). Consequently, a high voltage sufficient to 
cause sparking is applied to the plug or plugs connected to the ignition 
coil at that moment. 
The time interval between the instants t.sub.1 and t.sub.2 in FIG. 1a 
represents the so-called desaturation time of the transistor 7. The power 
dissipated by the transistor is very considerable in this period of time. 
The electronic ignition control systems made up to now are arranged to 
control the switching transistor so that it has a specific desaturation 
time whatever the rate of rotation of the engine. This desaturtion time of 
the transistor is provided, among other things, to allow the current to 
reach a sufficient magnitude in the primary winding of the ignition coil 
even with sharp variations in critical parameters for the ignition, for 
example, when the rate of rotation of the engine increases sharply and 
hence the time available to bring the current in the ignition coil to the 
desired level is reduced. 
With reference again to FIG. 1, in the control system of the present 
invention, a signal processing circuit, indicated 12, is connected to the 
output of the magnetic pulse generator 1. This circuit is arranged to 
output a signal, indicated V.sub.i in FIGS. 1 and 4, which corresponds 
substantially to the sum of a first signal proportional to the signal A of 
the pulse generator 1 and a second signal substantially proportional to 
the integral of the signal from the pulse generator. FIG. 1 illustrates a 
particularly simple and convenient embodiment of the signal processing 
circuit 12. In this embodiment, the circuit includes a resistor R.sub.1 
connected to the pick-up 3 of the magnetic pulse generator, and a 
capacitor C.sub.1 and a resistor R.sub.2 connected in series between the 
other terminal of R.sub.1 and a terminal a maintained at a constant 
reference potential. The common terminal of the capacitor C.sub.1 and the 
resistor R.sub.1 is also the output of the processing circuit 12. 
The signal V.sub.i output by the processing circuit 12 is applied to an 
input of a dwell angle control circuit 13, to the input of a peak monitor 
14 and to a threshold comparator circuit 15. 
A possible structure of the dwell angle control circuit 13 is illustrated 
in greater detail in FIG. 2. 
The peak monitor 14 outputs a signal V.sub.p (FIGS. 1 and 4) the amplitude 
of which is indicative of the frequency of the signal V.sub.i and hence 
substantially of the frequency of the signal A, and therefore of the rate 
of rotation of the engine. This signal is used as a threshold reference 
signal for the comparator circuit 15. 
A possible structure of the dwell angle control circuit 13 is illustrated 
in greater detail in FIG. 2. 
The peak monitor 14 outputs a signal V.sub.p (FIGS. 1 and 4), the amplitude 
of which is indicative of the frequency of the signal V.sub.i and hence 
substantially of the frequency of the signal A, and therefore of the rate 
of rotation of the engine. This signal is used as a threshold reference 
signal for the comparator circuit 15. 
The passage-through-zero monitor 11 is connected to the first input 16a of 
an AND gate 16 having another input 16b connected to the output of the 
threshold comparator 15. The output of the AND gate 16 is connected to a 
first input 10a of the logic control circuit 10. The other input 10b of 
this circuit is connected to the output of the dwell angle control circuit 
13. This latter circuit is also connected to the output of the peak 
monitor 14, the output of the AND gate 16, and a first output 17a of a 
comparator circuit 17 having its input connected to the unearthed terminal 
of the feedback resistor 8. 
The logic circuit 10 has its output connected to the input of the driver 
circuit 9 and to the dwell angle control circuit 13. The circuit 9 is also 
connected to a second output 17b of the comparator circuit 17. 
This comparator circuit outputs a signal at its output 17a when the voltage 
across the ends of the resistor 8 indicates that the current I in the 
primary winding of the ignition coil has exceeded a predetermined value 
less than the value which is sufficient to trigger sparking. This value 
may be 3A, for example. The circuit 17 outputs a second signal at the 
output 17b when, on the other hand, the current I reaches the maximum 
predetermined value, for example 6A. For this purpose, the circuit 17 may 
simply include two threshold comparators. 
In the ignition control system of FIG. 1, the signal V.sub.i output by the 
processing circuit 12 is used to determine the instants at which the 
primary winding of the ignition coil starts to conduct current. As already 
stated, this signal is proportional to the magnetic flux linkage in the 
pulse generator and is "cleaner" than the signal A from the pulse 
generator, in that the integration eliminates noise with an average zero 
value, and also has a decidely smaller dynamic as the frequency, that is, 
the rate of rotation of the engine, varies. 
The instants at which current conduction is initiated are determined in the 
dwell angle control circuit 13 which is illustrated in greater detail in 
FIG. 2. This circuit includes a comparator 18 having one input connected 
to the output of the processing circuit 12 and a second input connected to 
a reference voltage generator circuit, generally indicated 19. The latter 
circuit includes a capacitor C.sub.2 connected between an input of the 
comparator 18 and a terminal a kept at a reference voltage. Two current 
generators 20 and 21 are connected to this capacitor. When activated, the 
first generates a current I.sub.20 which tends to charge the capacitor 
C.sub.2. When activated, the generator 21 allows the capacitor to 
discharge. 
A logic control circuit, indicated 22, controls the activation of the 
current generators 20 and 21. Reference 23 indicates a circuit for 
controlling the intensity of the current generator by the generators 20 
and 21. 
The logic circuit 22 has two inputs connected to the output 17a of the 
comparator circuit 17 and to the output of the logic circuit 10. The 
circuit 23, however, is connected to the output of the peak monitor 14. 
As apparent from FIG. 3, the logic circuits 10 may include two mono-stable 
circuits 30 and 31 connected to the inputs 10a and 10b and arranged to 
generate a pulse of predetermined duration when they detect a descending 
trace and a rising trace respectively in the respective signals supplied 
to them. The outputs of the mono-stable circuits 30 and 31 are connected 
respectively to the reset and set inputs respectively of a bi-stable 
circuit (flip-flop) 33. 
As is immediately understood, the logic circuit 10 outputs a signal 
V.sub.in (FIGS. 1 and 4) which has a rising trace for each rising trace of 
the signal V.sub.dw and a descending trace for each descending trace of 
the signal V.sub.ps. 
The circuit 22 of the dwell circuit 13 is arranged to activate the current 
generator 21 when the signal output by the comparator 18 indicates that 
the signal V.sub.i has exceeded the voltage V.sub.c localised at the 
capacitor C.sub.2 (as occurs, for example, at the instants t.sub.10, 
t.sub.20, t.sub.30 in FIG. 4 and at the instant indicated t.sub.40 in FIG. 
5). When this occurs, the current generator 21 causes partial discharging 
of the capacitor C.sub.2 at constant current. The voltage V.sub.c across 
the capacitor thus decreases linearly, as illustrated in FIGS. 4 and 5. 
As will become more apparent below, when the signal V.sub.i exceeds the 
voltage V.sub.c, the switching transistor 7 is made to change from the cut 
off condition to the saturated condition and the current I in the primary 
winding of the ignition coil starts to increase almost linearly. 
Immediately this current reaches the lower threshold of the comparator 
circuit 17 (equal to 3A in the embodiment given previously), the logic 
control circuit 22 de-activates the current generator 21 and activates the 
current generator 20. This occurs, for example, at the instant indicated 
t.sub.41 in FIG. 5. 
Starting from this instant, the current generator 20 causes the capacitor 
C.sub.2 to be recharged with a constant current and the voltage V.sub.c 
starts to increase linearly, as indicated in FIGS. 4 and 5. 
The recharging phase of the capacitor C.sub.2 is interrupted when the 
signal A from the pulse generator 1 passes through zero in the negative 
sense (instant of sparking), as occurs, for example, at the instants 
t.sub.11, t.sub.21, t.sub.31 in FIG. 4 and at the instant t.sub.42 in FIG. 
5. 
As stated previously, the instants at which current starts to flow in the 
primary winding of the ignition transformer correspond to the instants at 
which the trace of the signal V.sub.i intersects and rises through the 
voltage V.sub.c. If the engine is running at a constant rate of rotation 
and the currents I.sub.20 and I.sub.21 are equal to each other, this 
voltage V.sub.c, after a discharge-charging phase of the capacitor 
C.sub.2, has the same level that it had before the discharge-charging 
phase, as indicated by the right-hand branch of V.sub.c shown in full 
outline in FIG. 5. When the rate of rotation of the engine increases 
suddenly, however, the signal A from the pulse generator increases in 
frequency and the negative passage through zero of the signal A is 
advanced. Consequently, the recharging phase of the capacitor C.sub.2 is 
interrupted before time, for example, at the instant indicated t.sub.43 in 
FIG. 5, and the voltage across the capacitor V.sub.2 at the end of the 
charging pulse assumes a level, indicated V'.sub.c in FIG. 5, which is 
less than the level before the discharge-charging phase. Consequently, the 
subsequent intersection of the signal V.sub.i and the voltage across the 
capacitor is also advanced. Thus, the instant at which the primary winding 
of the ignition coil starts to conduct is also advanced correspondingly. 
This advance enables the desired value of the current in the coil to be 
achieved at the moment of discharge. 
On the contrary, when the engine slows, the final voltage at the end of a 
discharge-charging phase of the capacitor C.sub.2 will be higher than the 
initial value, as indicated, for example, by the level V".sub.c in FIG. 5. 
Consequently, the intersection of the trace of the signal V.sub.i and the 
voltage across the capacitor is delayed and the instant at which the 
primary winding of the ignition coil starts to conduct current is 
therefore delayed. 
The dwell angle control circuit thus enables a self-adapting type of 
control of the initiation of the conduction of current by the ignition 
coil to be achieved: if the rate of rotation of the engine increases this 
instant of initiation is gradually advanced, while if the engine slows it 
is retarded. 
The signal output by the comparator 18 is indicated V.sub.dw in FIGS. 1 and 
4. In the same Figures, V.sub.A and V.sub.B indicate the signals output by 
the passage-through-zero monitor 11 and by the comparator 15. Reference 
V.sub.ps indicates the signal output by the AND circuit 16. This signal 
has a descending trace (FIG. 4) in correspondence with each negative 
passage through zero of the signal A of the magnetic pulse generator 1. 
The comparator circuit 15 and the AND circuit 16 have the function of 
eliminating pulsed interference which may possibly "pollute" the signal A 
supplied by the pulse generator 1, essentially in accordance with the 
technique described in Italian patent application No. 67560-A/84 in the 
name of the same Applicants. 
The inputs 10b and 10a of the logic circuit 10 are thus supplied with the 
signals V.sub.dw and V.sub.ps which define respectively the instants of 
initiation and the instants of cut off of the conduction of current by the 
primary winding 5 of the ignition coil 4. 
The driver circuit 9 causes the Darlington switching transistor 7 to be 
conductive for the duration of each pulse of the signal V.sub.in. 
Consequently, the current I in the ignition coil takes the form indicated 
I in FIG. 4. 
The control system according to the invention is arranged to drive the 
switching transistor 7 so that the desaturation time of this transistor 
obeys, in dependence on the rate of rotation n of the engine, a law of 
variation which is essentially of the type illustrated in FIG. 7. As shown 
in this Figure, the desaturation time t.sub.desat decreases when the rate 
of rotation is less than the predetermined value n.sub.0 (for example 3000 
rpm) and is substantially zero when n is n.sub.0. 
In order to achieve this law of variation of the desaturation time, the 
dwell angle control circuit 13 includes the regulating circuit 23 which 
can change the intensity of the current generated by the generator 20 (and 
possibly also the generator 21) in dependence on the amplitude of the 
signal V.sub.ip. The amplitude of the signal is, as stated above, 
indicative of the rate of rotation of the engine. More particularly, the 
regulating circuit 23 is arranged to reduce the current I.sub.20 from the 
generator 20 when the signal V.sub.ip indicates that the rate of rotation 
of the engine is less than n.sub.0, so that, in the phase in which the 
capacitor C.sub.2 is recharged, the voltage V.sub.c increases more slowly, 
with a rate of increase which depends, in the final analysis, on the rate 
of rotation of the engine. The overall saturation and de-saturation 
conduction time of the switching transistor is therefore extended. Thus, 
the current in the coil may easily reach the prefixed limit (for example, 
6A) and the switching transistor is maintained in the desaturated 
condition for a certain further period of time. 
When the signal V.sub.ip indicates that the rate of rotation of the engine 
is greater than n.sub.0, the current I.sub.20 no longer varies as the rate 
of rotation varies and the desaturation time of the switching transistor 7 
is always practically zero. 
The system according to the invention thus ensures that the transistor has 
a desaturation time which is not zero at low rates of rotation of the 
engine and a desaturation time which is practically nil at high rates of 
rotation. At low rates, that is, at rates for which the relative 
variations from one period between two ignitions and the subsequent period 
may be very considerable, a sufficiently intense current in the primary 
winding of the ignition coil is thus ensured even during sharp 
accelerations. At high rotational rates, when the relative variations from 
one period between two ignitions and the subsequent period are modest, 
however, the dissipation of power by the transistor is drastically reduced 
even during acceleration. 
Conveniently, the regulating circuit 23 of the dwell angle control circuit 
13 may be arranged also to modify the intensity of the current from the 
generator 21 in a predetermined manner. This enables the decrement of the 
voltage V.sub.c during the discharge phases of the capacitor C.sub.2 to be 
modified. This possibility enables the overall time for which the 
transistor is conductive to be varied and hence the final value achieved 
by the current in the primary winding of the ignition coil to be varied. 
Naturally, the principle of the invention remaining the same, the forms of 
embodiment and details of construction may be varied widely with respect 
to that described and illustrated purely by way of non-limiting example, 
without thereby departing from the scope of the present invention.