Ignition System for an Internal Combustion Engine

The invention relates to an ignition system.

The use of alternating current for separate ignition in internal combustion engines is known. The use of alternating current for ignition has the advantage that the spark discharge at the spark plug can be maintained for any length of time so that the engine requirement at any point in time can easily be matched, increasing the efficiency of the internal combustion engine through more complete utilization of the fuel mixture and reducing the pollutants in the exhaust. DE-OS 1 539 183 shows an ignition system with a primary and secondary circuit of a step-up transformer, whose primary circuit is designed as a parallel and series resonance circuit. This resonance circuit, following a rapid discharge in the secondary circuit, generates an alternating current at the spark plug cathodes In addition, DE-OS 25 17 940 teaches a capacitor ignition system for internal combustion engines with ferromagnetic resonance in which a second control circuit generates an oscillating current in the primary and secondary windings only after each discharge of the primary capacitor, allowing an alternating current to flow for a predetermined period of time at the spark plug.

Another alternating current ignition system is described in DE-OS 29 34 573. In this ignition system, an oscillator circuit controls a transistor feedback circuit connected to the primary winding of an ignition coil. This oscillator circuit is controlled by the switch positions of the breaker contacts of an ignition distributor and generates an alternating current signal with constant frequency at the spark plugs.

One disadvantage of this known ignition system is that generating the alternating current signal assumes an input signal triggered by a starting pulse, necessitating additional circuit elements to generate the starting pulse.

Another disadvantage of these known ignition systems is that the energy supply is provided over a constant ignition interval, thus generating an alternating current signal with constant power at the ignition contacts. If the secondary circuit is not satisfactorily closed, for example if the mixture fails to ignite, if the ignition contacts are short-circuited, or if the spark plug connector become disconnected, this can lead to excessive power being supplied, which can result in damage or even destruction of the electrical components of the ignition system.

The goal of the present invention is to provide an ignition system of the species recited above which does not suffer from the listed disadvantages, and exhibits or permits the following characteristics:

1. The fuel mixture is ignited a short time after triggering, in a few microseconds if possible.

2. Combustion time adjustable at will.

3. Reliable static and dynamic triggering.

4. Combustion voltage tolerances which occur as a result of fluctuations in ionization as the ignition current passes through zero, vorticization, and pressure changes in the fuel mixture present in the cylinder, should have little or no effect on the combustion current.

5. High operating safety both in normal operation and in the case of malfunctions, for example if the spark plug output is open or shorted, over a wide range of temperatures.

6. Minor high-frequency problems.

This goal is achieved according to the invention.

According to the latter, the basic idea of the invention consists in connecting an ignition final stage (primary and secondary circuit) in such a way that it operates in a current-control blocking and conducting mode.

The blocking and conducting times of a switching transistor in the primary circuit of the final ignition stage is controlled as a function of the ignition energy consumed in the secondary circuit in such a way that when the energy consumption in the secondary circuit increases, the ignition current frequency increases as well and decreases when energy consumption falls. The control parameter which the on cycles of the transistor determine is the energy which is not completely drawn from the primary circuit by the secondary circuit with the constant energy supply to the output circuit being ensured by the current control at a resistor in the primary circuit. As a result, only as much energy is supplied to the primary and hence to the secondary circuit as is needed to generate an ignition spark and for control.

By using an energy recovery diode, the unused energy is recycled into the energy storage means (battery) resulting in lower consumption of electrical power.

Possible circuits are described hereinafter. In particular, two spark plugs may be ignited with one ignition final stage. In order to reduce the transient recovery time of the ignition final stage, the adjusting element that controls the energy supply (ohmic resistance) may be switched by additional circuit means.

The free-running ignition final stage (output circuit) consists of a switch (transistor), an energy recovery diode, a load coil, a primary resonant circuit capacitor, and a secondary circuit coil connected in series with a spark plug capacitor. The function of the output circuit is comparable to that of a band filter. Two electrical states are possible:

1. Ignition has not occurred:

In this case the secondary circuit, because of its approximately 50% coupling, is coupled supercritically with the primary circuit by the counter inductance. As a result the high voltage in the secondary circuit is available in full very rapidly within a few periods.

2. Ignition has occurred:

In this case the secondary circuit is loosely coupled by the strong damping with the primary circuit. This guarantees a nearly constant current supply which is almost independent of the ignition voltage.

The circuit according to the invention which, in the nonignited state, takes the spark plug capacitance into account, offers the following protections and possibilities:

a) The required ignition voltage amplitude is reached quickly, so there is no need for a distributor to connect the spark plug to the ignition voltage when or after it is reached.

b) For example if a spark plug connector has been pulled off, and there is no capacitance in the secondary circuit, the complete high voltage is not applied to the secondary output to protect the ignition coil, so that higher operating reliability of the ignition system is achieved.

c) With a spark plug short circuited or in the case of normal operation with different ignition voltages, the ignition current is always limited to a value that is harmless to the ignition system.

This technology using the previously described self-oscillating ignition stage permits considerable reduction of the volume of an ignition coil since the total ignition spark energy is fed to the spark plug over a longer space of time and because the transmission frequency is high and the circuit operates both in the blocked and conducting states.

Another advantage of this ignition final stage is that coupling of only about 50% is required to build up the ignition coil. This feature means that such a miniature ignition coil is economical and simple to manufacture.

Since every spark plug is provided with a miniature coil and since the circuit operates in the conducting and isolation transformer modes, high voltage is available practically immediately after triggering, so there is no difficulty in eliminating the distributor. Especially suitable, small, and efficiently, manufacturable ignition coils are also the subject of the invention.

The invention also proposes means for controlling, especially triggering, the ignition paths.

To trigger the individual ignition paths, various methods are known of themselves. Thus for example DE-OS 36 30 272 Al teaches a device for controlling an internal combustion engine in which the position of a transmitter disk connected with a shaft of the internal combustion engine, said disk having a hole designed as a mark, is recorded by a fixed recording segment. Using an inductively operating sensor, which operates for example by the eddy current principle, pulses are obtained which are evaluated electronically. A control and regulating circuit then uses these pulses to generate the on and off signals for the individual ignition branches. This known method is also suitable for triggering the high-frequency alternating current ignition. One disadvantage of the dynamic detection of the ignition time described above is that, to determine the position, the transmitter disk must move in order to determine clearly the position of the camshaft or crankshaft.

According to the proposal of the invention, a wheel is mounted on the camshaft to detect the correct triggering time for ignition, said wheel having a clearly identifiable code on its surface, said code being scanned by a sensor. The scanning by the sensor is inductive or optical for example. Thus for example a ten-bit Gray code can be placed on the peripheral surface of a camshaft wheel, said code being scanned by an inductive multifunction sensor with integral electronics and the position of the camshaft wheel supplies corresponding electrical signals. Improved resolution is achieved by making the code nonlinear, in other words by providing high resolution only in the vicinity of top dead center. This sensor and transmitter arrangement permits static and/or dynamic detection for example of the crankshaft angle, so that the position of the pistons and the ignition sequence for the individual cylinders of the internal combustion engine can be determined. Advantageously this permits self-starting without using a starting device, for example an electrical starter motor.

The components required for the ignition system according to the invention, especially for the control and regulating circuit and the sensor for static and/or dynamic determination of the crankshaft angle, can be powered in the conventional manner directly by means of a known low-voltage source, for example a 12 volt DC battery. The disadvantage of a low-voltage power supply like this one is that supplying electrical consumers that require a high operating voltage, such as headlights with high-pressure gas discharge bulbs or the ignition system described above, is possible only with an unsatisfactory degree of efficiency. This disadvantage can be overcome in a motor vehicle according to the invention in an advantageous manner by using a chopper-type power supply, in other words an inverter with a transformer. When a battery with a terminal voltage of 6 to 18 volts is used, output voltages of 150 volts for example can be obtained using the chopper-type power supply with better efficiency than when using a low-voltage power supply for the electrical consumers and their supply network in the motor vehicle.

The invention will now be described in greater detail with reference to the embodiments in the drawing.

FIG. 1 is a schematic overall view of the ignition system according to the invention;

FIG. 2a is a circuit for an ignition final stage according to a first embodiment;

FIG. 2b is a circuit for an ignition final stage according to a second embodiment;

FIG. 2c is a substitute circuit diagram of the ignition final stages shown in FIGS. 2a and 2b;

FIG. 3a is a schematic of an ignition final stage according to a third embodiment;

FIG. 3b is a schematic of the circuit shown in FIG. 3a;

FIG. 4a is a graph showing the pattern of the drain voltage U.sub.D of switching transistor TR1 or TR2 in the circuits in FIGS. 2 and 3 as a function of time;

FIG. 4b is a graph of the secondary circuit voltage UH corresponding to the drain voltage according to FIG. 4a as a function of time;

FIG. 4c is a graph of drain current I.sub.D of the switching transistor corresponding to the drain voltage shown in FIG. 4a as a function of time;

FIG. 5a is a graph of drain current I.sub.D of the switching transistor during ignition as a function of time;

FIG. 5b is a graph of arc voltage U.sub.B at the spark plug during ignition as a function of time;

FIG. 5c is a graph of drain voltage U.sub.D of the switching transistor during ignition as a function of time;

FIG. 6 is a schematic according to another embodiment for three ignition pathways for each two spark plugs;

FIG. 7 is a schematic of an ignition final stage module according to one embodiment;

FIG. 8 is a schematic of an ignition final stage module according to another embodiment;

FIG. 9 is a schematic diagram of a complete ignition final stage according to one embodiment;

FIG. 10 is a partially cut-away side view of an assembled miniature ignition coil according to one embodiment;

FIGS. 11a-11c show the individual parts of the ignition coil according to FIG. 10 in an exploded view, namely

FIG. 11a the coil housing,

FIG. 11b the coil core, and

FIG. 11c the coil body;

FIG. 12 is a schematic diagram of a trigger device for static and/or dynamic determination of the crankshaft angle according to one embodiment;

FIG. 13 is a schematic diagram of a trigger device for static and/or dynamic determination of the crankshaft angle according to another embodiment; and

FIG. 14 is a schematic diagram of a chopper-type power supply.

The ignition system according to the invention consists of the components shown schematically in FIG. 1 namely

a low-voltage power supply (BAT)

a central chopper-type supply (UF1)

high-voltage generating ignition final stages (ZST) corresponding to the number of cylinders

and miniature ignition coils (ZSP) for each spark plug.

The ignition final stage according to the invention shown in FIG. 2a consists of a primary resonant circuit and a secondary resonant circuit. The primary resonant circuit has a control and regulating circuit 2 with a trigger input 4, a trigger output 6 and a supply lead 8, as well as the primary winding Pl of an ignition coil. In series with primary circuit coil Pl is an resonant circuit capacitor Cl and parallel with it, an energy recovery diode Dl. A transistor TR1 is connected on the drain side with capacitor Cl and power recovery diode Dl. On the source side, transistor TR1 is connected to ground by a current-limiting resistor R1. A lead 10 connects the transistor on the source side with current-limiting resistor R1 and control and regulating circuit 2. On the secondary side, secondary coil (S1) is connected in series with the winding and ignition capacitor CW, as shown in the schematic in FIG. 2c. In the otherwise corresponding, embodiment shown in FIG. 2b, one output stage with electrically separated inductive decoupling is provided.

A complete circuit for an ignition final stage with three ignition parts for each two spark plugs, in other words for a six-cylinder engine for example, is shown in FIG. 6.

Supplying two spark plugs Z1 and Z2 with a common ignition final stage is shown in FIG. 3a. With a secondary circuit wired in this fashion, the effective winding and spark plug capacitance CW is preferably reduced by a factor of 2, as shown in the schematic diagram in FIG. 3b.

The theoretical function of the ignition final stage according to the invention is explained with reference to the graphs in FIGS. 4a to 4c and in FIGS. 5a to 5c for the above embodiments of the ignition final stages.

The function of the self-oscillating ignition final stage will be explained first for the non-ignited case (graph in FIGS. 4a to 4c).

Here we assume the steady state with sufficient battery voltage. The voltage at point A in the circuit according to FIG. 6 permits operation at the low level as soon as amplifier OP1 conducts. A trigger input, e.g. trigger input 3, is grounded in response to control. Since the reference voltage at point B is more positive than the voltage at the inverting input of amplifier OP4, transistor T30 conducts. Then a drain current I.sub.D begins to flow (FIG. 4c, time interval T1). The voltage drop at resistor R37 increases until the voltage at the inverting input (minus) of amplifier OP4 is more positive than the reference voltage at point B.

At this point transistor T30 is blocked. The energy contained in storage coil SP30 causes the entire output circuit to oscillate. A portion of the energy is transferred to capacitor C33 in the primary area (C1 and/or C2 in schematic diagram 2c or 3b) and the other part is transferred to capacitance CW in the secondary circuit (time interval t2, FIGS. 4a and 4b).

The voltage U.sub.D on capacitor C33 increases sinusoidally until no energy is left in the storage coil. During time interval T3 the capacitively stored energy is returned to inductance L1 until the voltage in capacitor C33 is equal to 0. At this point in time (start of time interval t4), storage coil SP30 delivers its available energy on the secondary side into circuit capacitor CW. On the primary side, this is not possible by analogy for C33 since the voltage U.sub.D at the drain of transistor T30 cannot go negative because the internal diode (energy recovery diode D1 or D2 in FIG. 2a, 2b, or 3a) conducts. The energy available in primary inductance L1 is returned through diode D30 to the on-board electrical system (time interval t4, see FIG. 4c).

The secondary circuit can continue resonating during this time interval t4 (see U.sub.H in FIG. 4b). Its frequency is somewhat higher than before because the scattered inductance L (FIG. 2c, FIG. 3b) is now parallel to counter inductance M (see FIGS. 2c and 3b). During this time interval t4 transistor T30 conducts again because the same voltage conditions prevail as at the beginning of time interval t1. When the energy of inductance L1 has been discharged completely into the voltage source (on-board network), a new cycle begins.

In order to understand the circuit, it should be mentioned that transistor T30 is only blocked when the voltage at the inverting input (minus) of amplifier OP4 is more positive than the reference voltage at point B. This case always occurs when the charging current I.sub.D reaches a limiting value determined by resistor R37. This current control guarantees a constant energy supply to primary inductance L1, with the energy, apart from minor losses, being recycled completely into the on-board network in the event of failure to ignite. The blocked state of transistor T30 is maintained by the voltage drop across resistor R36 as long as voltage U.sub.D on the drain of transistor T30 is more positive than the battery voltage.

The described function of self-excitation does not change in the event of ignition, because the inductive coupling between the primary and secondary inductances amounting to about 50% prevents total damping of the primary circuit by the heavily damped secondary circuit. In the event of ignition, the following operation takes place:

Because of the ignition current now flowing through the spark plug, much less energy is returned from the voltage source, in other words into the on-board network (FIG. 5a). Time interval t4 decreases considerably. One advantage of this circuit design is that only as much energy is recycled as was present after the ignition phase.

This behavior makes it possible for the desired current supply to be largely independent of ignition voltage U.sub.B over a wide range. When ignition voltage U.sub.B is high, a large share of the energy in the light arc of the spark plug is converted into heat. In this case less residual energy is returned to the voltage source. Consequently, time interval t4 becomes shorter, the ignition frequency rises, and the current draw increases.

For the opposite case, in other words for low ignition voltage U.sub.B, the opposite behavior applies, namely time interval t4 increases, the ignition frequency decreases, and the current draw declines.

In the embodiment described above, there are different primary and secondary circuit frequencies.

With suitable circuit design, for example the primary free running circuit frequency is about 18 kHz and the secondary circuit frequency is 43.5 kHz with an open primary circuit and 60 kHz with a shorted primary circuit.

The basic frequency with spark plug termination is about 20 kHz at an ignition voltage of 900 Vss.

So that the high voltage be fully at the will available spark plug immediately after the on signal comes from the control and regulating circuit, it is advantageous if, for a specified time interval, drain current I.sub.D through the drain-source lead of transistor T30 is greater than in the completely steady state. To achieve this, in the circuit according to FIG. 7, using a bistable flip-flop FF1, which controls the gate of transistor T40, the actual measured value of the drain-current-proportional voltage at point C is reduced. The current amplitude is adjusted by resistor R40 so that the stored energy in primary inductance L1 is sufficiently high to replace the residual energy in the output circuit which is not present when the circuit is switched on. In this way the maximum high voltage U.sub.H is achieved even during the first oscillation period.

Flip-flop FF1 can be reset by the negative flank (reset flank) of the first current pulse. Resetting of flip-flop FF1 however can be made dependent on whether ignition has taken place or not. The information on this can be derived for example from the changing frequencies.

According to another preferred embodiment which is shown in FIG. 8, by means of an additional monostable flip-flop FF2, bistable flip-flop FF1 can be made to reset only during the time interval during which transistor current I.sub.D would flow, provided that ignition had occurred. This arrangement has the advantage that with very seriously contaminated spark plugs, ignition voltage U.sub.H increases further, providing a voltage reserve for heavily worn and contaminated spark plugs.

The overall design of an ignition final stage (see FIG. 9) with an ignition module IZM with integrated circuit and an ignition coil ZSP is shown in FIG. 9. The complete circuit of the ignition module with a high degree of integration thus permits economical manufacture and high operational reliability.

The miniature ignition coil used in cooperation with the ignition final stages mentioned above in an advantageous manner is shown in detail in FIGS. 10 and 11a-11c. The miniature ignition coil consists of three individual components, namely coil body 20, coil core 22, and coil housing 24. Coil body 20 has a cylindrical shape on one of whose end surfaces a plug-in socket 26 is integrally connected. This socket 26 is surrounded by a cylindrical wall 28 that acts a protective cap providing a positive and closely fitting seat on the spark plug.

Individual chamber segments 30a to 30g and 32 are formed on jacket surface 29 of coil body 20 by a plurality of circumferential segment ribs. Preferably, chamber segment 32 with the largest chamber rib interval 1 receives the coil winding of the low-impedance primary circuit coil, since the primary circuit is made with greater tolerances when forming the winding and can be made chamberless to improve space utilization. Preferably, the coil winding of the high-ohmage secondary coil is located in chamber segments 30a to 30g which are spaced closer together. One advantage of this chamber winding technique for the secondary circuit is that higher voltage strength is achieved and smaller winding tolerances are easier to achieve. Terminals 34 for the primary circuit are brought out at the end of coil body 20.

To receive coil core 22, coil body 20 has a concentric bore 33 (see FIG. 11c).

Coil core 22 is made mushroom- or T-shaped. This shape permits both simple installation and also provides magnetic shielding while increasing the Q factor of the primary circuit. Coil core 22 preferably is made of ferrite, which advantageously shows no saturation phenomena up to 200 C.

To hold coil core 22 in the coil body and to protect the coil windings, coil housing 24 for coil body 20 is made cap- or pot-shaped with coil core 22 in place (see FIGS. 11a). To protect the electrical leads against mechanical stress, a tubular stub 36 is mounted on coil housing 24 on its top lid.

In the end-mounted form (see FIG. 10), the coil body is potted in watertight fashion with coil housing 24, thereby advantageously increasing corrosion resistance. The potting compound 38 preferably extends over chamber segments 30a to 30g that receive the secondary windings. The potting material used is preferably composed of silicone. "Plastoferrit" is suitable for coil housing 24, enriched for example with conductive carbon black, whereby magnetic and electrostatic shielding is provided against external electromagnetic fields. Overall, the simple design of ignition coil 22 permits economical manufacture and the small volume of ignition coil 22 makes it possible to mount it directly on the spark plugs, increasing the operational reliability of the ignition system and resulting in low HF noise.

To trigger the individual ignition paths, the angular position of a crankshaft or camshaft is determined by means of a coding disk 40, 42 firmly connected therewith, as shown in FIGS. 12 and 13. FIG. 12 shows a code which can be used to trigger three ignition paths. The binary code of the radially disposed code tracks 44a, b, and c is read by an inductive sensor 46 and evaluated in electronics 48. This electronics provides at its output 50 the trigger signals required for the individual ignition paths. The code is advantageously designed in its phase position for the highest engine rpm, so that electronics 48 connected downstream, depending on the rpm, supplies the trigger signal to the ignition final stages on a delayed basis.

A fully digital circuit in which the ignition phase is directly evaluated by means of an on-board computer 52 is shown in FIG. 13. Code pattern 53 is located on jacket surface 42 of the code wheel nonrotatably connected for example with the camshaft. A ten-bit Gray code is used preferably as the code, and is read for example by an inductive multifunction sensor 54 or by an optical scanner. These signals are evaluated in a downstream integrated electronic circuit 52, for example an on-board computer, for determination for example of individual piston positions. This information is used to trigger the individual ignition final stages and also to meter and provide controlled direct injection of the fuel mixture into the cylinders.

Using a code wheel of this kind, the absolute position of the crankshaft or camshaft can be determined even statically, in other words in the resting position, which makes it possible to start the internal combustion engine from a resting position without using an electrical starting device (starter).

The voltage and current supply for electrical devices can be provided by a chopper-type power supply (DC-DC converter). The schematic diagram of a preferred embodiment is shown in FIG. 14. It shows a known schematic diagram of a secondarily regulated single-ended isolated transformer.