Method for controlling a corona ignition system of a cyclically operating internal combustion engine

Abstract

A method for controlling a corona ignition system in which a corona discharge is produced at an ignition electrode by exciting a resonating circuit with an AC voltage produced by a high-frequency generator. The AC voltage is adjusted to a target value depending on an operating state of the engine. Combustion onset is determined by evaluating an electrical variable of the resonating circuit and the target value of the AC voltage is reduced by a predefined value following a predefined number of engine cycles or a predefined operating period. The determined combustion onset is evaluated in one or more engine cycles. The target value for the momentary engine operating state is then increased if, by evaluation of the combustion onset, it is found that a predefined requirement is no longer met. Otherwise, the reduced target value is stored as a new target value for the momentary engine operating state.

Claims

1. A method for controlling a corona ignition system of a cyclically operating internal combustion engine, the method comprising: producing a corona discharge at an ignition electrode by exciting a resonating circuit with an AC voltage produced by a high-frequency generator, said resonating circuit comprising the ignition electrode; adjusting the AC voltage to a target value that is set as a function of an operating state of the engine; detecting a combustion onset by evaluating an electrical variable of the resonating circuit; and evaluating the combustion onset from one or a number of engine cycles, wherein the target value of the AC voltage is increased if, by evaluation of the combustion onset, it is determined that a predefined requirement is no longer met.

2. The method according to claim 1, wherein: the target value of the AC voltage is reduced by a predefined value whenever a predefined number of engine cycles or a predefined operating period has passed; the combustion onset determined with the reduced target value of the AC voltage is evaluated for one or more engine cycles; and if it is determined from the evaluation of the combustion onset that the predefined requirement is no longer met, the target value of the AC voltage for the engine operating state is increased by more than the predefined value, and, if it is determined by the evaluation of the combustion onset that the predefined requirement is still met, the reduced target value is stored as a new target value for the engine operating state.

3. The method according to claim 1, wherein the combustion onset is evaluated by being compared with a reference value for the operating state of the engine, wherein the predefined requirement is met when the combustion onset does not exceed the reference value.

4. The method according to claim 1, wherein the combustion onset is evaluated by comparing a characteristic variable of a scattering of the combustion onset with a reference value, wherein the predefined requirement is met when the combustion onset does not exceed the reference value.

5. The method according to claim 4, wherein the characteristic variable of the scattering is the variance.

6. The method according to claim 2, wherein, if, by evaluation of the combustion onset, it is determined that the requirement defined by a reference value is no longer met, the target value of the AC voltage is increased by a value that is dependent on the deviation from the reference value.

7. The method according to claim 1, wherein the predefined value is fixed as an absolute value.

8. The method according to claim 1, wherein the target value of the AC voltage is increased at least by twice the predefined value if, by evaluation of the combustion onset in one or more engine cycles, it is determined that the predefined requirement is no longer met.

9. The method according to claim 1, wherein the predefined value is at most 2% of the target value of the AC voltage.

10. The method according to claim 1, wherein, by evaluating an electrical variable of the resonating circuit, it is monitored whether an arc discharge occurs and, if so, the target value of the AC voltage is reduced by a value that is greater than the predefined value by which the target value of the AC voltage is reduced following a predefined number of engine cycles or following a predefined operating period.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 shows a schematic illustration of a corona ignition system.

DETAILED DESCRIPTION

(3) The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of this disclosure.

(4) The corona ignition system illustrated schematically in FIG. 1 comprises a central unit 1 and a plurality of igniters 5, which are connected to the central unit 1 and which each comprise a resonating circuit 5b with an ignition electrode 5a. Each of these igniters 5 is associated with a combustion chamber of a cyclically operating engine and produces a corona discharge at the ignition electrode 5a thereof.

(5) The central unit 1 comprises a step-up converter 3, a plurality of high-frequency generators 4, which are connected to the step-up converter and which are connected to the individual igniters 5, and a control unit 2 which controls the step-up converter 3 and the high-frequency generators 4. The step-up converter 3 produces an input voltage from an on-board supply voltage of the vehicle. This input voltage is provided to the high-frequency generators 4. The high-frequency generators 4 each produce from this input voltage an alternating voltage with which the resonating circuit 5b of the igniter 5 connected to the high-frequency generator 4 in question is excited.

(6) The control unit 2 contains a memory, in which target values of the AC voltage produced by the high-frequency generators 4 are stored for different engine operating states. The target values can be stored in a matrix or a characteristics map. The control unit 2 may explicitly store target values for the output voltage of the high-frequency generators 4 or alternatively may set target values for the input voltage of the high-frequency generators 4, since the output voltage of the high-frequency generators 4 and thus the AC voltage with which the resonating circuits 5b of the igniters 5 are excited is defined by the input voltage of said high-frequency generators.

(7) When the corona ignition system is used for the first time, target values provided by the manufacturer may initially be stored in the memory of the central unit 2.

(8) During operation of the corona ignition system the target value is repeatedly reduced by a predefined value ΔU, for example whenever a predefined operating time has or a defined number of engine cycles have passed. The operating time and the number of engine cycles may be counted in an absolute manner, or may refer only to the current engine operating state.

(9) The control unit 2 determines the combustion onset of the fuel in the combustion chamber of the engine from the electric characteristic variables of the resonating circuit 5b of the igniter 5 in question. The combustion onset can be identified for example on the basis of an extremum in the progression of an electrical variable of the resonating circuit 5b, in particular on the basis of a local maximum of the impedance or a local minimum of the resonance frequency of the resonating circuit 5b. Details concerning the determination of the combustion onset are explained in US 2013/0319095 A1incorporated herein by reference.

(10) The control unit 2 then compares the combustion onset determined with the reduced target value with a reference value. Reference values for the combustion onset in the different engine operating states are stored in the memory of the control unit 2, for example in a matrix or a characteristics map.

(11) If a comparison of the combustion onset with the reference value for the engine operating state in question reveals that a predefined requirement is no longer met, that is to say for example the combustion onset is later than the reference value, the target value is increased by a value that is greater than the value ΔU by which the target value was previously reduced. For example, the target value can be increased by a value 2ΔU or more. If the evaluation of the combustion onset has revealed that the combustion onset has deteriorated by the reduction of the target value, such that it no longer meets the requirements defined by the reference value, this reduction is reversed and the target value is increased to a value higher than before the reduction.

(12) If, otherwise, it is determined by evaluation of the combustion onset that the predefined requirement or the predefined requirements are still met, that is to say for example the combustion onset has not shifted behind the reference value, the reduced target value is maintained until a renewed reduction of the target value has been performed after some time. By way of example, the target value can be reduced at predefined intervals, for example 10 seconds or more, or following a predefined number of engine cycles, for example 20 engine cycles or more.

(13) The evaluation of the combustion onset by comparison with a reference value can be replaced or supplemented by detecting the combustion onset for a number of successive engine cycles and evaluating the scattering of the combustion onset. A small scattering is typical for a well-configured corona ignition system, whereas a large scattering indicates that the resonating circuit 5b is excited with a voltage that is too low. In order to evaluate the scattering, the control unit 2 can calculate a characteristic variable of the scattering of the combustion onset, for example the variance. This characteristic variable of the scattering is then compared with a reference value of the scattering. If this comparison reveals an excessively high scattering, for example if the variance exceeds the reference variable, the target value is increased, for example by a value of 2ΔU or more.

(14) The reduction of the target value by the predefined value ΔU should not be so great that the engine operation is significantly disturbed thereby. If the predefined value ΔU is no more than 2% of the target value, a disruption of the engine operation can generally be avoided, even if the target value following a reduction by 2% is in many cases too low for optimal engine operation. For example, the predefined value ΔU can be selected so as to be so small that it is no more than 1% of the momentary target value of the AC voltage with which the resonating circuit 5b is excited. The value ΔU can be predefined as an absolute value in volts or as a fraction of the momentary target value. The value ΔU can set in a constant manner or can be set in a variable manner by the control unit 2, for example depending on the engine operating state.

(15) If, after a reduction of the target value, it is determined from one or more engine cycles by evaluation of the combustion onset that a predefined requirement is no longer met, the momentary target value can be increased by a fixed value, for example always by 2ΔU or more. Another possibility lies in making the increase dependent on the extent to which a predefined requirement was missed, for example the extent to which a predefined reference value was exceeded. If, for example, the combustion onset is too late, the increase of the target value can be selected so as to be proportional to the value by which the combustion onset lies behind the reference value. It is also possible to determine the increase value using a non-linear function of the deviation of the combustion onset from the reference value.

(16) When the scattering of the combustion onset is evaluated, the increase of the target value can also be determined by a function or characteristic variable of the scattering, for example the variance. If the variance or another characteristic variable of the scattering exceeds a predefined reference value, the target value can be increased by a value that is proportional to the difference between the variance or another characteristic variable of the scattering and the reference value. The increase of the target value can also be chosen to be disproportional to the deviation.

(17) While exemplary embodiments have been disclosed hereinabove, the present invention is not limited to the disclosed embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of this disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.