Apparatus for suppression of individual ignition events in an ignition system

An apparatus for suppression of individual ignition events in an ignition system includes a short-circuit switch, connected in parallel to the primary winding of the ignition coil. The short-circuit switch is controlled by a control switch. In order to avoid having to provide a separate power supply to furnish triggering energy to the control switch, this energy is derived from the energy stored in the ignition coil during a closing phase of the ignition switch. This requires fewer components than prior art circuits designed for the same purpose.

BACKGROUND OF THE INVENTION 
The invention relates generally to apparatus for suppression of individual 
ignition events in an ignition system. An apparatus of this kind is known 
for instance from German Patent 24 43 403, Knoedler & Blum, assigned to 
Robert Bosch GmbH. The known circuit arrangement is used in ignition 
systems that have an ignition coil and an interruptor switch disposed in 
the primary circuit of the ignition coil. To suppress individual ignition 
events, a controllable switch is provided, which is connected parallel to 
the primary winding of the ignition coil. For triggering the switch, a 
trip switch device is provided, which draws its requisite triggering 
energy from a separate circuit, for instance in the form of a blocking 
oscillator. With the aid of the blocking oscillator, the necessary control 
voltage for the switch, which is above the operating low voltage of the 
ignition system, is simultaneously furnished. 
The object of the invention is to simplify the known circuit arrangement 
for suppression of individual ignition events in an ignition system. 
ADVANTAGES OF THE INVENTION 
The apparatus according to the invention has the advantage that only a few 
components are needed. Production involves only low cost. Moreover, 
because of the low number of components, the operational reliability of 
the circuit rises. The ease of connection to the existing ignition system, 
since only two connections have to be made on the primary side of the 
ignition system, is especially advantageous. The simplification of the 
circuit arrangement is attained by providing that the control energy for 
actuating a short-circuit switch disposed on the primary side is furnished 
after the opening of an ignition switch, from the energy stored in the 
ignition coil during the closing phase of the ignition switch. 
The connection of the electrically controllable short-circuit switch 
parallel to the primary winding of the ignition coil is advantageous. This 
provision keeps the load on the ignition coil low. 
The actuation of the short-circuit switch is especially simple if a further 
electrically triggerable control switch, which is preferably in the form 
of a MOSFET, is provided between the primary-side connection of the 
ignition coil, to which the ignition switch is connected, and the control 
input of the short-circuit switch. 
A triac, which is preferably located in an optical coupler, is especially 
suitable as the control switch. The optical coupler assures an electrical 
separation between the control circuit of the short-circuit switch and the 
external trigger circuit of the short-circuit switch. 
When the control switch is closed, a diode located between the primary-side 
connection of the ignition coil, to which the ignition switch is 
connected, and the short-circuit switch keeps blocking-state voltages of 
predetermined polarity away from the control input of the short-circuit 
switch. 
A thyristor is especially suitable as the short-circuit switch.

DETAILED DESCRIPTION 
FIG. 1 shows an ignition coil 10, which includes at least one primary and 
at least one secondary winding 11, 12. The generated ignition voltage 
appears at a secondary winding connection 13. A second secondary winding 
connection 14 is connected to a first primary winding connection 15. A 
diode and an ignition switch 17 are connected to the first primary winding 
connection 15. The ignition switch 17 is connected to a ground 18. A 
second primary winding connection 19 of the ignition coil 10 leads to a 
power supply connection 20. A short-circuit switch 21 is connected between 
the diode 16 and the power supply connection 20. A control switch 23 is 
connected between the diode 16 and one control input 22 of the 
short-circuit switch 21. The control switch 23 has a control input 24 that 
leads to a trigger circuit, not shown in further detail in FIG. 1. 
FIG. 2 shows another exemplary embodiment of the apparatus according to the 
invention. Elements shown in FIG. 2 and matching those of FIG. 1 have the 
same reference numerals in both drawings figures. The triggering of the 
short-circuit switch 21 differs from that in the apparatus of FIG. 1. The 
control switch 23 is located in an optical coupler 30, whose control input 
31 leads to an optical element 32 that emits visible radiation. 
The apparatus according to the invention will now be described in further 
detail, referring to FIGS. 1 and 2: 
The ignition coil 10 is for instance provided in order to generate ignition 
pulses for an internal combustion engine. The secondary ignition voltage 
appears at the first secondary connection 13. From there, it travels 
either to a distributor or directly to one or more spark plugs. Neither a 
distributor nor spark plugs are shown in the drawings. An ignition pulse 
is tripped if the ignition switch 17 is opened after a closing phase. 
During the closing phase, magnetic energy is stored by means of the 
current flowing in the primary winding 11 of the ignition coil 10. After 
the opening of the ignition switch 17, the abrupt break in the flow of 
current causes a steep voltage increase in the primary winding 11 at the 
first primary winding connection 15, which is transmitted to the secondary 
side. If the ignition switch 17 is in the form of a mechanical interruptor 
or an end stage transistor of a transistor ignition system, then a 
capacitor must be connected parallel to the ignition switch. On the one 
hand, this capacitor limits the voltage amplitude; on the other, it leads 
to the development of an oscillation, which is damped in cooperation with 
the lost resistances of the circuit. 
It may be necessary to suppress individual ignition events, for the sake of 
diagnosis in an engine. This is true, for instance, if the contributions 
of the various cylinders to the engine output are to be determined 
separately. To suppress individual ignition events, the short-circuit 21 
is therefore provided, which prevents secondary ignition voltage pulses 
from appearing. For instance, at a predetermined instant, it 
short-circuits the primary winding 11 of the ignition coil 10. 
The instant is defined by the trigger circuit, not shown, which is 
connected to the control input 24 of the control switch 23. This instant 
is preferably at the end of the closing phase of the ignition switch 17. 
At that time, the energy stored in the ignition coil 10 is at a maximum. 
This assures the furnishing of adequate control energy for actuating the 
short-circuit switch 21. 
In principle, the control switch can be closed at any arbitrary instant 
before the ignition event that is to be suppressed. As long as the voltage 
at the primary winding connection 15 is less than the supply voltage, it 
remains out of operation. 
After the opening of the ignition switch 17, a high primary-side voltage 
would arise at the first primary winding connection 15; transformed to the 
secondary side, it produces the ignition pulse. When the control switch is 
closed, the short-circuit switch 21 prevents the appearance of the 
primary-side high voltage and thus inhibits the ignition pulse. 
The control switch can be opened again at any arbitrary time between the 
opening of the ignition switch 17 and the next ignition that is not to be 
suppressed. A high-voltage-proof transistor is for instance provided as 
the short-circuit switch. A thyristor is especially suitable; it is 
relatively economical under the prevailing conditions, in terms of 
electrical strength and peak current capacity. The imposition of a high 
voltage at a predeterminable polarity upon the control input of the 
short-circuit switch 21 is avoided with a diode 16 connected in series 
with the short-circuit switch 21 and located between the first primary 
winding connection 15 of the ignition coil 10 and the arrangement 21. In 
the exemplary embodiments shown in the drawings, the voltage components at 
the primary winding connection 15 that are negative relative to the power 
supply connection 20 are kept away from the short-circuit switch 21. A 
series-connected resistor that limits the maximum current can also be 
provided. 
For triggering the short-circuit switch 21, a control switch 23 is 
provided, for instance in the form of a MOSFET of the self-inhibiting 
type. MOSFETs of this kind, which can handle the voltage rises that occur 
in the ignition system, can currently be obtained inexpensively. A control 
circuit, not shown in FIG. 1, outputs suitable trigger pulses, which lead 
to the suppression of the ignition voltage, to the control input 24 of the 
MOSFET. A complete electrical separation between the triggering and the 
control switch 23 is possible, for instance with an optical coupler 30 
shown in FIG. 2. An optically triggerable thyristor can for instance be 
used as the short-circuit switch 21. A triac is also highly suitable for 
that purpose and is inexpensive; it can be used both in the exemplary 
embodiment of FIG. 1 and in the exemplary embodiment of FIG. 2. Optical 
couplers 30 having a triac 23, which withstand the high voltage rises that 
occur, are currently available. The triggering is effected via the control 
input 31, which leads to an optical element 32, preferably a 
light-emitting diode, located in the optical coupler 30.