EP0640761A2 - Controllable ignition system - Google Patents

Abstract

The invention relates to a method for controlling an ignition system for internal-combustion engines, in which invention the spark firing current and its firing period can be adjusted. The described method permits each cylinder of the engine to be supplied with that ignition energy which corresponds to the instantaneous requirement of the engine, as a result of which a spark plug changing interval of more than 100,000 km can be achieved. According to the invention, both the value of the spark firing current and its firing duration can be controlled as a function of engine parameters, in particular of load, speed and temperature. Preferably, the method according to the invention can be used in an alternating current or high voltage capacitor ignition system. <IMAGE>

Description

The invention relates to a method for controlling an ignition system for internal combustion engines according to the preamble of claim 1.

A generic ignition system is known from DE-OS 39 28 726, which has the advantage over conventional ignition systems, for example so-called transistor ignitions with static high voltage distribution, that small and thus inexpensive ignition coils can be used. Furthermore, according to the above. The optimal ignition is ensured by the fact that it remains switched on for the entire burning time, regardless of the speed. Such an ignition system is referred to as an AC ignition system because it generates a bipolar spark current.

In the ignition concepts known to date, the following requirements were in the foreground: to ensure a safe cold start and to reliably ignite the fuel / air mixture in the cylinder even with sooty spark plugs. A correspondingly large ignition energy was provided to meet this requirement. This ignition energy, designed for the maximum demand of the engine, is not required for normal operation (warm engine). This leads to an unnecessarily high electrode erosion of the spark plugs, which in turn increases the service life the spark plugs will lower and the candles will have to be changed frequently.

The object of the present invention is to provide a method for controlling an ignition system according to the type mentioned at the outset, so that the spark plug change intervals are at least 100,000 km.

This object is achieved by the characterizing features of patent claim 1. After this, the value of the spark combustion current and its burning time are controlled depending on engine parameters. Such an ignition with controlled parameters causes a much lower burn on the spark plugs than a conventional series ignition. This significantly extends the spark plug replacement intervals.

In an advantageous development of the method according to the invention, the engine load, speed and engine parameters are used to control the ignition current and also its burning time. For this purpose, characteristic maps stored in the control unit are preferably used. A base value for the ignition current value or for the combustion duration are preferably taken for the engine load and the speed from an ignition current map or a combustion duration map.

According to a further preferred embodiment, these basic values for the ignition current value and the burning duration are corrected in accordance with the current operating state of the internal combustion engine. Temperature compensation is carried out if the motor temperature has not yet reached a certain threshold. This improves the cold start property of the engine. Furthermore, the base value for the ignition current value in the event of a dynamic change in the state of the Motor is loaded with a dynamic factor that is proportional to the change in load value and decreases with time. After a certain delay, the dynamic factor has reached zero, the corrected base value assuming the base value for the new load condition.

The method according to the invention can advantageously be used to control AC or high-voltage capacitor ignitions.

Figur 1

ein Blockschaltbild einer Wechselstrom-Zündanlage zur Durchführung des erfindungsgemäßen Verfahrens,

Figur 2

ein detailliertes Schaltbild einer Zündendstufe einer Wechselstrom-Zündanlage gemäß Figur 1,

Figur 3

Strom- und Spannungszeitdiagramme zur Erläuterung der Funktionsweise der Wechselstromzündung,

Figur 4

ein Brennstrom-Kennfeld gemäß des erfindungsgemäßen Verfahrens,

Figur 5

ein Zünddauer-Kennfeld gemäß des erfindungsgemäßen Verfahrens und

Figur 6

ein Diagramm zur Darstellung des Elektrodenabbrandes als Funktion der zurückgelegten Fahrstrecke.

In the following, the method according to the invention is to be illustrated and explained using an AC ignition system as an example. Show it:

Figure 1

2 shows a block diagram of an AC ignition system for carrying out the method according to the invention,

Figure 2

2 shows a detailed circuit diagram of an ignition output stage of an AC ignition system according to FIG. 1,

Figure 3

Current and voltage time diagrams to explain how the AC ignition works,

Figure 4

a combustion current map according to the inventive method,

Figure 5

an ignition duration map according to the inventive method and

Figure 6

a diagram showing the electrode erosion as a function of the distance traveled.

FIG. 1 shows a block diagram of an alternating current ignition for carrying out the method according to the invention for a 4-cylinder machine. In this case, an ignition output stage Z1-Z4 is provided for each spark plug ZK1. These ignition output stages are connected via a circuit 9 for cylinder selection to a control unit 1, which generates an ignition signal 1 to 4 for each ignition output stage and simultaneously outputs a modulation voltage U _Mod for all ignition output stages, which is processed by a current control circuit 10. This modulation voltage sets a target value of the ignition current I _{is intended to} represent and is by means of a comparator with an on a shunt resistor R (see FIG. 2) of the primary circuit of the ignition actual value I generated _is compared. The result of the comparison is fed to the cylinder selection circuit 9. Furthermore, the control unit 1 is equipped with sensors 4, 5 and 6 for detecting the speed n, the load L and the engine temperature T and with a device 7 for cylinder 1 detection and via lines 1a for controlling the electronic injection with an injection system 11, which contains the corresponding actuators. Finally, a switching power supply 3 generates the supply voltages (18 V / 180 V) for the ignition output stages Z1-Z4, which is fed by an on-board battery 2.

An embodiment of an ignition output stage for controlling a single ignition coil according to FIG. 1 is shown in FIG. 2 and essentially consists of a transistor T, in the form of an IGBT transistor (isolated gate bipolar transistor), an energy recovery diode D, a primary resonant circuit capacitor C. , an ignition coil Tr constructed from a primary and secondary winding with a coupling of approximately 50%, a spark plug ZK and a simple control circuit 10 which corresponds to the current control circuit 10 according to FIG. 1, however additionally contains a gate of the cylinder selection circuit 9. This control circuit 10 is therefore supplied with the control signals prepared by the control unit 1, namely the ignition signal 1 and the modulation voltage U _Mod . The first-mentioned control signal sets the ignition point and the burning time t _B , while the second-mentioned control signal U _{Mod determines} the value of the primary current I _p and subsequently the ignition voltage U _k , that is to say the value of the spark current i _B. The generation of these two control signals ignition signal 1 and U _Mod according to the invention is explained further below.

The ignition output stage according to FIG. 2 operates in the current-controlled flyback and forward converter mode. For the duration of the switching on of the transistor T, a collector current I _{k flows} , which corresponds to the primary coil current I _p according to FIG. 3. This collector current I _k is _intended by the control circuit 10 to a particular modulation of the voltage U _Mod value I limited. In order to obtain a short charging time, the ignition output stage is supplied with a voltage of 180 V using a switching power supply unit which has already been explained in connection with FIG. If the collector current I _{k has reached} the value specified by I _soll , the transistor T is switched off. The energy contained in the storage coil stimulates the output circuit (secondary inductance, spark plug capacity) to vibrate. Part of the energy transfers into the capacitor C and the other part into the spark plug capacitance. The voltages U _c on the capacitor C and the ignition voltage U _B on the spark plug ZK rise - as shown in FIG. 3 - in a sinusoidal fashion until there is no more energy in the storage coil, that is to say the primary coil.

In the subsequent period, the capacitively stored energy is fed back to the primary coil inductance until the voltage U _c at the capacitor C reaches the value zero (see FIG. 3). The primary side voltage U _c cannot become negative through the diode D. On the secondary side, the oscillation continues due to the only approx. 50% strong coupling between primary and secondary inductance. During this time period, the transistor T is switched on again, because the same voltage conditions are now present as before the transistor was first switched on. The current control always guarantees the same supply of energy to the primary coil. The portion of the energy that was not used in the spark channel is completely fed back into the vehicle electrical system. The coupling of approx. 50% prevents total damping of the primary resonant circuit (primary coil, capacitor C) due to the strongly damped secondary resonant circuit in the event of a spark breakdown.

As can be seen from FIG. 3, the duration of the complete cycle (charging the primary coil, decay process until the voltage U _c at capacitor C crosses zero) is approximately 80 μs. Thus the loading time of the coil can be neglected. In contrast to transistor coil ignition, a closing angle control is therefore not necessary. On the other hand, the burning time t _B per ignition process can be changed as desired by varying the number of switching cycles. The spark combustion current i _B is modulated by changing the energy fed in on the primary side. Parallel to the spark current, the secondary-side high-voltage supply U _k on the spark plug ZK also changes in certain areas due to the non-ideal power source character of the output stage. When reducing the spark current i _B must therefore in each case the decrease in the maximum high voltage must also be observed.

This technology of the self-oscillating ignition output stage allows a considerable reduction in the volume of the ignition coil because, in contrast to transistor coil ignition, not all of the energy for an ignition process has to be stored in the coil, but is supplied in several small units. Therefore, only a reduced coil volume is required to store the smaller amount of energy. Another advantage for the construction of the ignition coil is the coupling required of only approx. 50%, since this can be achieved with a simple rod core.

The control unit 1 represents a μ-controller system, for example based on a Motorola module MC68HC811E2, which is an 8-bit controller with an internal EEPROM program memory. The voltage supply of this control unit 1 comes from the on-board electrical system fed by the battery 2. In order to control the AC ignition system correctly, the control device 1 needs a signal about the cylinder sequence (cylinder 1 detection 7 according to FIG. 1). For this purpose, a magnet can be attached to the toothed disk of the camshaft, for example, which is queried by a Hall sensor. This delivers a signal every 360 ° of the camshaft or every 720 ° of the crankshaft: the cylinder 1 mark.

With the method according to the invention, the AC ignition system according to FIG. 1 becomes an ignition system which makes it possible to control the ignition energy with the aid of two parameters. The first parameter is the modulation voltage U _Mod , with the aid of which the primary current I _p (cf. FIG. 2) of the ignition coil is regulated. With this Current I _{p influences} the high voltage U _{k of} the secondary coil or the spark current i _B with which the spark burns. This is a higher-frequency PWM signal which is smoothed by an RC filter in the ignition output stage and which is output together for all 4 cylinders, as shown in FIG. 1. For this purpose, control unit 1 has a PWM output. According to Figure 1, the individual cylinders are ignited with the ignition signals 1 to 4. The burning time t _{B of} the ignition process represents the second parameter and is also determined by the control unit 1 and implemented via the pulse width of the respective ignition signal.

The control program stored in the control unit 1 for the ignition output stages ensures, on the one hand, the correct ignition distribution and, on the other hand, the calculation of the optimal ignition parameters, namely in the form of the modulation voltage U _Mod and the burning time t _B and their output. Before the control of the ignition output stages can begin, the control device 1 must be synchronized, ie it waits for the first signal of the cylinder 1 detection of the device 7 (cf. FIG. 1). This is followed by an endless loop in which all calculations are carried out and which is repeated with each ignition process. An analog-to-digital conversion is carried out in this loop in order to detect the motor parameters, such as load and temperature, generated by the sensors 5 and 6. The speed is determined by evaluating the time interval between successive pulses of the speed sensor.

The new ignition parameters are calculated with the aid of the engine load L (which is determined either by the position of the throttle valve potentiometer or by recording the amount of air in the intake manifold), For this purpose, the associated basic values U _Basis and t _{Basis of} the modulation voltage U _Mod and the burning time t _{B are} taken from two characteristic maps stored in the memory of the control unit 1. These two maps are shown in FIGS. 4 and 5, namely the combustion current map and the ignition duration map. The design of these maps is based on the ignition energy requirement. The characteristic diagram for the spark combustion current i _B according to FIG. 4 takes into account the current offered with a safety factor of 1.2. The highest current at idle speed is required regardless of the load. In full-load operation, the required spark current decreases successively with the speed, whereas in part-load and zero-load operation, the value drops more quickly and reaches the minimum of 40 mA at medium speeds. The minimum burning time was determined on a test bench in the map for the burning time. In the entire partial and full load range, an ignition duration of 120 µs (corresponds to an ignition pulse) was found to be sufficient. In contrast, in the no-load range, especially at medium speeds, the burning time must be extended considerably. All of the operating points shown with the two maps according to FIGS. 4 and 5 correspond to a stationary engine. The temperature and the dynamic behavior of the engine are also taken into account by control unit 1, as shown below.

The basic values U _Basis and t _Basis described above for the modulation voltage U _Mod and the burning time t _B are corrected in accordance with the current operating state of the engine in the following way:



U _Mod = U _Basis + U _Temp + U _Dyn ,



where U _{Basis is} the _base value determined from the load-speed map, U _{Temp is} the temperature correction value and U _{Dyn is} the dynamic correction value.

The temperature correction value results from the following formula:



U _Temp = (T _{70 ° C} - T _ist ) · k _T ,



where T _{70 ° C is} a certain threshold temperature, for example 70 ° C, T _is the current engine temperature and k _{T is} a proportional factor. Thus, the temperature correction is a proportional correction, ie if the engine temperature falls below a certain threshold value, e.g. B. 70 ° C, a factor U _{Temp is} calculated by which the modulation voltage U _{Mod is} increased. This factor U _Temp is proportional to the difference between the engine temperature and the temperature threshold. This correction is not carried out when the engine is warm.

In the event of a dynamic change in the operating state of the engine, an increased high voltage is offered for a short time, namely by the factor of the dynamic correction U _Dyn . This factor U _Dyn results from the following formula:



U _Dyn = (L _ist - L _alt ) · k _B + U _{Dyn, alt} · k _B-1 ,



where L _is or L _{old is} the current load value or the load value before the change in the operating state. k _B and k _B-1 are proportional factors that are determined by practical driving tests. After a change in load, the modulation voltage U _Mod increases by this dynamic factor U _Dyn , which is proportional to the change in the load signal and decreases over time. After a Delay time of 2 s, for example, this factor U _Dyn has dropped to zero, whereby the modulation voltage U _{Mod reaches} the new static basic value for the new load state.

A similar procedure is used to calculate the burning time t _B. Starting from the base value t _base already described above, only a temperature correction is carried out according to the following formula:



t _B = t _base + t _temp ,



where t _{basis is} the basic burn duration determined from the load-speed map and the temperature _correction value t _{temp is} calculated using the following formula:



t _temp = (T _{70 ° C} - T _ist ) · k _Tt ,



where T _{70 ° C represents} a certain threshold value, for example 70 ° C and T _is the current engine temperature, while k _{Tt is} a proportionality factor as in the corresponding temperature correction of the modulation voltage U _Temp . Even when calculating the burning time t _B , the temperature is only taken into account when the engine temperature T _is below the threshold temperature, for example 70 ° C.

In a test run of the above-described alternating current ignition in a test vehicle, electrode wear on the spark plugs of 0.03 mm was found after driving 15,000 km, compared to 0.09 mm for spark plugs with a conventional series ignition. Accordingly, the response voltages of the spark plugs in a pressure chamber only increased by 3.7 kV or 2.7 kV compared to 5.5 kV or 4.5 kV for spark plugs with a series ignition. The more than three times the life of the spark plugs can be expected.

Finally, a long-term test also showed corresponding good results, which is shown in FIG. 6, according to which, at the end of the endurance test, the mileage reached the value of 120,000 km for the spark plugs operated with the AC ignition described above (dashed line). Over the same period of time, the spark plugs operated with a conventional series ignition (shown with a solid line) had to be replaced 4x, since they each have the wear limit, i. H. Individual misfires could be recognized when the load changed. The spark plugs with AC ignition could have been used if the experiment was continued.

The electrode erosion of these spark plugs was 3.9 times smaller than that of the spark plugs operated with the series ignition.

By controlling the ignition according to the invention via a map, the AC ignition also meets the increased requirements placed on future ignition systems. Optimized combustion, in particular, is expected to improve the exhaust gas values. It is also conceivable to use the method according to the invention in future lean burn engines over an extended combustion time.

With the AC ignition according to the invention, an ignition system is available which is optimally adapted to the different ignition energy requirements of the engine, without having to forego operational reliability.

Claims (12)

Method for controlling an ignition system for internal combustion engines, consisting of at least one ignition output stage (Z₁ ... Z₄) for controlling at least one ignition coil (Tr) which generates an ignition current (i _B ), the value of the spark combustion current (i _B ) as well whose burning time (t _B ) is adjustable, characterized in that the value of the spark combustion current (i _B ) and its burning time (t _B ) are controlled as a function of engine parameters. Method according to Claim 1, characterized in that the engine parameters correspond to the engine load (L), the speed (n) and the engine temperature (T). Method according to claim 1 or 2, characterized in that the value of the spark combustion current (i _B ) and its burning duration (t _B ) is determined as a function of the engine parameters (L, n, T) by means of characteristic maps stored in a control unit (1). Method according to claim 3, characterized in that a base value (U _basis ) for the value of the spark combustion current (i _B ) is taken from the ignition current characteristic diagram for the engine load (L) and speed (n). Method according to Claim 3 or 4, characterized in that a base value (t _base ) for the combustion duration (t _B ) is taken from the combustion duration map for the engine parameter load (L) and speed (n). Method according to Claim 4 or 5, characterized in that the basic values (U _basis , t _basis ) are corrected in accordance with the current operating state of the internal combustion engine. Method according to Claim 6, characterized in that a temperature _correction (U _Temp , t _Temp ) is carried out as a function of the instantaneous engine temperature (T _ist ). Method according to Claim 6 or 7, characterized in that the basic value (U _basis ) for the value of the spark combustion current (i _B ) is subjected to a dynamic correction in the event of a dynamic change in the operating state of the internal combustion engine. Method according to Claim 8, characterized in that after a change in the load, the base value (U _basis ) increases by a dynamic factor (U _Dyn ) which is proportional to the change in the load value (L _ist - L _alt ) and which decreases with time. Method according to Claim 9, characterized in that after a certain delay time the dynamic factor (U _Dyn ) reaches the value zero, the corrected base value adopting the base value for the new load condition. Method according to one of the preceding claims for controlling an AC ignition system. Method according to one of claims 1 to 10 for controlling a high-voltage capacitor ignition system.

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