Patent Grant
8973561
Publications WO 2010/011838 A1 and WO 2004/063560 A1 disclose how a fuel-air mixture in a combustion chamber of an internal combustion engine can be ignited by an HF corona discharge generated in the combustion chamber. To this end, an ignition electrode of an igniter is passed through one of the walls of the combustion chamber in an electrically insulating manner, the walls being applied to ground potential, and projects into the combustion chamber, preferably opposite a piston provided in the combustion chamber. Along with the walls of the combustion chamber, which are applied to ground potential, the ignition electrode forms a capacitance as a counter electrode. With its content, the combustion chamber acts like a dielectric medium. Depending on the cycle of the piston, air or a fuel-air mixture or an exhaust gas is present in said combustion chamber.
The capacitance is a component of an electric resonant circuit which is energized by means of a high-frequency voltage which, according to the prior art, is generated by means of a transformer with a center tap. The transformer cooperates with a switching device which alternately applies a specifiable direct current voltage to the two primary windings of the transformer, which are connected by the center tap. The secondary winding of the transformer feeds a series resonant circuit which primarily consists of ohmic resistances and the inductance of the secondary winding as well as the capacitance which is formed by the ignition electrode, the isolator, the outer conductor of the igniter and the walls of the combustion chamber. The frequency of the alternating current voltage that is supplied by the transformer and energizes the resonant circuit is controlled such that it is as close to the resonant frequency of the resonant circuit as possible. This results in a voltage overshoot between the ignition electrode and the walls of the combustion chamber in which the igniter is arranged.
Typically, the resonant frequency is between 30 kilohertz and 3 megahertz, and the alternating voltage reaches values of, e.g., 50 kV to 500 kV at the ignition electrode.
This allows generating a high-frequency corona discharge in the combustion chamber. The corona discharge is not to change into an arc discharge or spark discharge. For this reason, it is made sure that the voltage across the ignition electrode and the ground remains below the voltage for a complete breakthrough.
WO 2010/011838 A1 discloses that the frequency of the resonant circuit is controlled by measuring the phase shift between current and voltage at the feed points of the resonant circuit and is controlled to a value of zero by means of a phase control loop because, in case there is resonance, current and voltage are in phase in a series resonant circuit (phase shift=zero). The phase control loop controls the switching frequency of a switching device which is used to alternately apply a specified voltage to the one primary winding and to the other primary winding of the transformer, with the result that, on the secondary side of the transformer, current and voltage are in phase with each other at the feed points of the series resonant circuit.
The shift of the resonant frequency of the HF resonant circuit which contains the HF igniter is a major problem in the prior art. There are various causes for this. One cause of the shift of the resonant frequency lies in load changes in the combustion chamber of the internal combustion engine, for example, by changes in temperature, in pressure, in moisture, in soiling of the tip or tips of the ignition electrode of the HF igniter and by changes in other parameters which are associated with the operation of the internal combustion engine. The conditions of the corona formation can also shift the resonant frequency. The problem is solved only partially by feeding in the resonant frequency through a phase control loop, such as it is disclosed in WO 2010/011838 A1. With the phase control loop, the deviation of the frequency from the resonant frequency of the HF resonant circuit is readjusted, there will be control deviations, and there may also be overshoots. Smaller control deviations and shorter control times would be desirable. The prior art is to further disadvantage in that the phase control is susceptible to a temperature drift of the components of the phase control loop and to voltage noise. If frequencies are high, there may additionally be great switching losses of the circuit breakers used in the switching device on the primary side of the transformer when the phase shift between current and voltage increases.
The object of the invention is to create a method of the aforementioned type, in the embodiment of which the disadvantages mentioned above are less grave than in the prior art.
This object is reached by means of a method having the features presented in claim 1. Advantageous refinements are the subject of the subordinate claims.
The method according to the invention deals with the energization of an HF resonant circuit which contains an igniter as a component for igniting a fuel-air mixture in the combustion chamber of an internal combustion engine by means of a high-frequency corona discharge, wherein the igniter comprises an ignition electrode and an insulator surrounding the ignition electrode, with the result that the ignition electrode can be inserted into the combustion chamber such that it is insulated against the walls of the combustion chamber. The insulator can, additionally, be surrounded by a metal outer conductor. According to claim 1, such an HF resonant circuit is energized by means of a DC-AC inverter which, on its direct current side, is activated by a control circuit and energized by means of an electric direct current pulse, said control circuit preferably being operated in a digital manner. The alternating current that occurs in the HF resonant circuit as a response to the current pulse is observed.
To generate and terminate a current pulse, a switch is actuated as soon as an instantaneous value of the alternating current caused in the 1-IF resonant circuit falls below a first switching threshold, and the switch is actuated again as soon as the instantaneous value of the alternating current energized in the HF resonant circuit exceeds a second switching threshold.
In the simplest case, the two switching thresholds may have the same value. For example, a value of zero can be used for both switching thresholds, with the result that the switch is actuated during each zero passage of the alternating current. Preferably, however, the two switching thresholds are different, for example, by the two switching thresholds having different signs. The switching operation of the switch can be carried out during the time period that will then elapse between the point when a switching threshold is reached and a subsequent zero passage, with the result that the start and the end of a current pulse coincide with a zero passage with high precision. Preferably, the switching thresholds are selected such that the switching time deviates from the time that elapses between the point when a switching operation is triggered and a subsequent zero passage of the alternating current by less than a factor of 2, more preferably, corresponds to that time.
With a method according to the invention, the polarity of the current pulse on the direct current side of the DC-AC inverter can be inverted by actuating the switch whenever the zero passage of the current intensity of the alternating current occurs in the HF resonant circuit. The HF resonant circuit is then excited during each half-wave of the alternating current. It is, however, sufficient that the HF resonant circuit is only energized during every second half-wave, i.e., that there is a time lag between two successive current pulses, said time lag corresponding to the time lag between two zero passages. With a variant of the method according to the invention, all current pulses can, therefore, also have the same polarity.
The current pulses for exciting the DC-AC inverter can be generated by means of direct current pulses, preferably square-wave pulses, which are applied to the direct current side of said DC-AC inverter. The instantaneous value of the alternating current or the instantaneous value of the alternating current voltage can be directly observed on the alternating current side of the DC-AC inverter.
The following description of the invention illustrates the advantages and refinements thereof only by means of a monitoring of the alternating current excited in the HF resonant circuit. However, the advantages and refinements also apply to the monitoring of the alternating current voltage in corresponding manner.
The invention has important advantages:
Appropriately, the current pulses which are supplied to the DC-AC inverter on its primary side are formed as square-wave pulses or approximately as square-wave pulses. As a result, the HF resonant circuit which is, preferably, configured as a series resonant circuit can be easily excited and adjusted to its resonant frequency.
The method according to the invention can have different embodiments. It utilizes the behavior of an HF resonant circuit, particularly that of an HF series resonant circuit, during the switch-on operation. The transient behavior of the HF resonant circuit is characterized by the roots or zeros points of its transfer function. A transfer function describes the dependency of the output signal of the HF resonant circuit on the input signal thereof, i.e., current pulses or voltage pulses that generate current pulses. An HF resonant circuit which contains an igniter as a component for igniting a fuel-air mixture in the combustion chamber of an internal combustion engine by means of an HF corona discharge usually reacts with a periodic output signal during switch-on, due to the conjugate complex zero points of its transfer function. The zero points of the current or voltage signal which is generated in the HF resonant circuit due to the energization of the latter are the closer to the resonant frequency of the HF resonant circuit the higher the quality of the HF resonant circuit.
The HF resonant circuit can be energized by planning the first pulse with which to feed the DC-AC inverter for a duration which exceeds half the period at an assumed resonant frequency of the HF resonant circuit. The frequency which is assumed as the resonant frequency of the HF resonant circuit can be obtained as an empirical value. Therein, the resonant frequency of the HF resonant circuit that is the smallest possible one under the basic conditions specified can, initially, be estimated while planning the first pulse for energizing the DC-AC inverter and, therefore, the HF resonant circuit for a duration that exceeds half the period at this assumed resonant frequency that is small as possible. The pulse will then excite the HF resonant circuit to oscillate via the DC-AC inverter. According to the invention, the behavior of the alternating current flowing in the HF resonant circuit is observed and the exciting pulse is terminated when the first zero passage of the alternating current occurs. Depending on the variant of the invention, the pulse can be followed by a pause until the next zero passage or the DC-AC inverter can be fed with a pulse of inverted polarity immediately thereafter, said pulse in turn lasting until the next zero passage of the current in the HF resonant circuit, this being followed by another generation of a direct current voltage pulse for feeding the DC-AC inverter, said direct current voltage pulse again having inverted polarity. In either variant, it is ensured that the pulses which are formed to feed the DC-AC inverter and, therefore, to energize the HF resonant circuit are directly generated with a frequency with which the current signal occurs in the HF resonant circuit. This process which is used to energize the HF resonant circuit is continued automatically. An empirical value of the frequency that can be assumed as resonant frequency during subsequent excitation processes of the HF resonant circuit can be derived from observing the frequency of the current signal in the HF resonant circuit. If there is no detection of the zero passage, this empirical value can be assumed as the desired frequency until a zero passage is detected again.
It is, however, also possible to generate the first direct current voltage pulse for a duration that is shorter than half the period at an assumed resonant frequency of the HF resonant circuit whenever an ignition is to take place. In this case, the first zero passage of the current signal in the HF resonant circuit will occur earlier than in the case discussed above, because the shorter selected direct current voltage pulse which is used to feed the DC-AC inverter shortens the transient process of the HF resonant circuit. If, however, the pulse is terminated when the first zero passage occurs in the current signal of the HF resonant circuit in this case as well, the HF resonant circuit is excited with the correct frequency during and after the second zero passage of the current signal, i.e., with the resonant frequency or with a frequency that is close to the resonant frequency of the HF resonant circuit. Here as well, the first pulse can be followed by a pause until the next zero passage or a pulse with inverted polarity can be generated for the DC-AC inverter, wherein said pulse is left present until the next zero passage of the current signal occurs in the HF resonant circuit.
Alternatively, the energizing pulses can be consistently planned for an indefinite duration and their polarity at the input of the DC-AC inverter can be inverted whenever a zero passage of the current intensity occurs in the HF resonant circuit. In this manner, it is also possible to achieve that the HF resonant circuit is always excited with a frequency which is its resonant frequency or is close to its resonant frequency.
Cases are conceivable wherein the zero passage of the current intensity of the current signal in the HF resonant circuit cannot be measured because the measuring equipment that is provided to that end and is used to observe the behavior of the current intensity in the HF resonant circuit has failed or is malfunctioning or because the signal transmitted by the current measuring equipment does not arrive at the control circuit which controls the feed of the DC-AC inverter with direct current voltage pulses. To ensure that an oscillation can be generated in the HF resonant circuit even in such a case, it can be provided that, in any case, a switching operation takes place, i.e., the pulse is terminated after a defined duration which can be stored in the control circuit and which is longer than half the period of the oscillations occurring in the HF resonant circuit. The terminated pulse can be followed by a pulse with inverted polarity or by a pause. The sum of the two pulses with inverted polarity or the sum of the first pulse and the subsequent currentless pause is a period the duration of which is, preferably, a defined specified value and which, preferably, corresponds to the minimum of the resonant frequency that is possible under the given basic conditions of the HF resonant circuit. In this manner, it is possible to ensure that, even if there is a failure of the detection of zero passages of the current signal in the HF resonant circuit, the HF igniter can still ignite, although no longer under optimal conditions.
The zero passages of the current intensity of the current signal in the HF resonant circuit can be detected in various ways. One possibility is to precisely determine the zero passage by means of a detector which detects the change in the sign of the polarity. Another possibility is to define a positive threshold of the current intensity and a negative threshold of the current intensity in the vicinity of the zero passage and to observe the point when the current intensity in the HF resonant circuit crosses the two thresholds of the current intensity. When the zero passage of the current intensity is approached, the pulse fed into the DC-AC inverter is terminated and the next pulse to be fed into the DC-AC inverter is started during the zero passage or when the other threshold is crossed. Particularly in the case first mentioned, there is therefore only a small switching gap between two successive pulses which are fed to the DC-AC inverter with different polarity.
Another possibility is to utilize the defined thresholds of the current intensity in the HF resonant circuit such that every pulse that is fed into the DC-AC inverter either is started when a threshold immediately following a zero passage is crossed and is terminated with the next zero passage or is started with a zero passage and is terminated when the threshold immediately prior to the next zero passage is crossed.
The first switching threshold can be a switch-on threshold and the second threshold can be a switch-off threshold. With a method according to the invention, it is, however, also possible that the first switching threshold is a switch-off threshold and the second switching threshold is a switch-on threshold.
For example, a current pulse applied to the direct current side of the DC-AC inverter can be terminated when an instantaneous value of the alternating current excited in the HF resonant circuit falls below a specified switch-off threshold value and, thereafter, a further current pulse is applied to the direct current side of the DC-AC inverter when an instantaneous value of the alternating current exceeds a specified switch-on threshold value. It is, just as well, possible to terminate a voltage pulse applied to the direct current side of the DC-AC inverter when an instantaneous value of the alternating current excited in the HF resonant circuit exceeds a specified switch-off threshold value and, thereafter, a further voltage pulse is applied to the direct current side of the DC-AC inverter when an instantaneous value of the alternating current falls below a specified switch-on threshold value.
In the method according to the invention, a transformer is preferably used as DC-AC inverter and the pulses are fed into a primary winding of the transformer. If the transformer has only a single primary winding, the pulses can be fed to said primary winding either alternately with varying polarity or—in a different variant of the method—always with the same polarity. The transformer can also have two primary windings which are separated by a center tap and are alternately supplied with the pulses. Therein, the center tap can be applied to unchanging potential, for example, to ground potential. In this case, direct current pulses alternately flow in different directions through the two primary windings, this corresponding to an alternating polarity of the direct current voltage pulses.
The desired high-frequency high voltage does not have to be generated with a transformer. It can also be generated with a DC-AC inverter which is fed on its input side—herein also referred to as primary side—with a direct current voltage from which a high-frequency high voltage that can be directly tapped on the output side—herein also referred to as secondary side—of the DC-AC inverter by means of semiconductor integrated circuits that are known to persons skilled in the art, e.g., by means of an H-bridge circuit, with a high-frequency switch on semiconductor base being arranged in each of the four branches of said H-bridge circuit.
Below, the invention is illustrated in more detail by means of the enclosed schematic diagrams. Identical or equivalent elements are designated with equal reference symbols in the figures.
a is a detail view of the principal components of an HF igniter which is, at the same time, the essential component of an HF resonant circuit;
A high-frequency generator which has a direct current voltage source and, as DC-AC inverter 6, a transformer with a tap center 6d on its primary side is provided for energizing the HF resonant circuit. Two primary windings 6a and 6b meet at the center tap 6d. The ends of the primary windings 6a and 6b that are remote from the center tap 6d are alternately connected to ground by means of a high-frequency switchover device which comprises two circuit breakers 7 and 8. The switching frequency of the high-frequency switchover device determines the frequency which is used to energize the series resonant circuit (
A detector circuit 5 which serves to determine the zero passage of the current intensity of the current signal in the HF resonant circuit is provided between the HF resonant circuit and the secondary winding 6c of the transformer 6.
In the exemplary embodiment, the center tap 6d of the transformer 6 is connected to a voltage source which supplies the direct current voltage Vcc. The other two connections of the primary windings 6a and 6b of the transformer 6 are switched against ground via the circuit breakers 7 and 8. It is, however, also possible to connect the center tap 6d to ground and to connect the other two connections of the primary windings 6a and 6b to the voltage source which supplies the direct current voltage Vcc via the circuit breakers 7 and 8.
The control circuit 11 controls when and how long the circuit breakers 7 and 8 are closed. To achieve this, the detector circuit 5 signals each zero passage of the current intensity of the current signal flowing in the HF resonant circuit via a line 12 running to the control circuit 11, with the result that, thereafter, the control circuit 11 alternately generates pulse-shaped control signals for closing the circuit breaker 7 and opening the circuit breaker 8 or for closing the circuit breaker 8 and opening the circuit breaker 7, wherein these control signals can still be amplified by means of amplifiers 9 and 10.
The control circuit 11 can have a varying structure. For example, it can be a microcontroller, but it can also be a field programmable gate array (in short: FPGA), i.e., an integrated circuit in digital technology, in which a logic circuit can be programmed. The control device 11 can also be a complex programmable logic device (CPLD) or an ASIC, i.e., an application-specific integrated circuit, or any other logic circuit.
The exemplary embodiment shown in
In either case, i.e., both in the example shown in
in diagram (a) the start and the end of a possible excitation pulse for closing and opening the circuit breaker 7;
in diagram (b) the start and the end of a possible excitation pulse for closing and opening the circuit breaker 8;
in diagram (c) the behavior of the current intensity of a current signal energized in the HF resonant circuit;
in diagram (d) the start and the end of a control pulse which actually closes and re-opens the circuit breaker 7; and
in diagram (e) the start and the end of a control pulse which actually closes and subsequently re-opens the circuit breaker 8.
After the initial excitation pulse 13a, a further excitation pulse is not required. The control pulses 16 and 17 for the circuit breakers 7 and 8 are generated by the occurrence of the further zero passages A+ and A−, so that the activation process for the HF resonant circuit is automatically continued until it is terminated by switching off the voltage supply or the control circuit.
The method shown in
The method shown in
The method shown in
An excitation pulse 14 for the circuit breaker 8 that starts with the zero passage A− is terminated by the occurrence of the next zero passage A+. If, however, the latter fails, the excitation pulse 14 is also prolonged in analogy to the excitation pulse 13c but no longer than to the specified point in time where the falling edge 14d occurs at the latest.
During normal trouble-free operation, the actual activation pulses 16 and 17 are generated by the current zero passages 1A−, A+ and A−, as described in
The excitation pulses 13c and 14 can start at a zero passage of the current intensity 15 in the HF resonant circuit.
The zero passages of the current intensity signal 15 in the HF resonant circuit do not have to be determined exactly. It is also sufficient to provide a positive current threshold above the zero passages and a negative current threshold below the zero passages; see
It is, however, also possible to combine the current passages through the positive and negative current thresholds with the zero passages of the current intensity in order to obtain control signals for the circuit breakers 7 and 8. One possibility is to deactivate the control pulse 16 at the time of the current passage B− and to reactivate it at the time of the current zero passage A+ and to deactivate it again at the time of the current passage C+. Another possibility is to deactivate the control pulse 16 at the time of the current zero passage A− and to reactivate it at the time of the current passage B+, whereas the control pulse 17 is activated at the time of the current passage C− and deactivated again at the time of the current zero passage A+.
The control circuit 11 monitors the excited alternating current i(t) by means of two switching thresholds B− and B+. When the instantaneous value of the alternating current i(t) excited in the HF resonant circuit falls below the first switching threshold B−, the switch 8 is actuated and, therefore, a pulse fed into the DC-AC inverter 6 is terminated. When the instantaneous value of the alternating current i(t) exceeds the second switching threshold B+, the switch 7 is again actuated. This resets the switch 7 to its conducting state, with the result that a voltage or current pulse starts.
A switching time Δ t which is indicated in
In order to keep switching losses as low as possible, it is desirable that the switch 7, which usually is a field effect transistor, changes its switching state in each zero passage or as close to the zero passage as possible.
In order to achieve this, a second switching threshold C+ which is different from the first switching threshold B− can be used according to the exemplary embodiment shown in
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| 10 2010 042 167 | Sep 2010 | DE | national |
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