Patent Application
20110253114
The invention is directed to a method having the features indicated in the preamble of claim 1. Such a method and such a system are known from WO 2010/011838 A1.
Document WO 2004/063560 A1 discloses how a fuel/air mixture can be ignited in a combustion chamber of an internal combustion engine by a corona discharge created in the combustion chamber. For this purpose an ignition electrode extends in an electrically insulated manner through one of the walls of the combustion chamber, that are at ground potential into the combustion chamber, preferably opposite of a reciprocating piston provided in the combustion chamber. The ignition electrode constitutes a capacitance together with the walls of the combustion chamber that are at ground potential and function as counterelectrode. The combustion chamber and the contents thereof act as a dielectric. Air or a fuel/air mixture or exhaust gas is located therein, depending on which stroke the piston is engaged in.
The capacitance is a component of an electric oscillating circuit which is excited by a high-frequency voltage created using a transformer having a center tap. The transformer interacts with a switching device which applies alternately a specifiable DC voltage to the two primary windings of the transformer separated by the center tap. The secondary winding of the transformer supplies a series oscillating circuit comprising the capacitance formed by the ignition electrode and the walls of the combustion chamber. The frequency of the alternating voltage which excites the oscillating circuit and is delivered by the transformer is controlled such that it is as close as possible to the resonance frequency of the oscillating circuit. The result is a voltage step-up between the ignition electrode and the walls of the combustion chamber in which the ignition electrode is disposed. The resonance frequency is typically between 30 kilohertz and 3 megahertz, and the alternating voltage reaches values at the ignition electrode of 50 kV to 500 kV, for example.
Thus, a corona discharge can be created in the combustion chamber. The corona discharge should not break down into an arc discharge or a spark discharge. Measures are therefore implemented to ensure that the voltage between the ignition electrode and ground remains below the voltage required for a complete breakdown. For this purpose, it is known from WO 2004/063560 A1 to measure the voltage and the current intensity at the input of the transformer and, on the basis thereof, to calculate the impedance as the quotient of the voltage and the current intensity. The impedance calculated in this manner is compared to a fixed setpoint value for the impedance, which is selected such that the corona discharge can be maintained without the occurrence of a complete voltage breakdown.
This method has the disadvantage that the formation of the corona is not optimal and, in particular, an optimal size of the corona is not always attained. Specifically, the corona increases in size the closer the oscillating circuit is operated to the breakdown voltage. To ensure that the breakdown voltage is never reached, the setpoint value of the impedance that must not be exceeded must be so low that a voltage breakdown and, therefore, an arc discharge, is always prevented. A point that must be considered when specifying the setpoint value of the impedance is that the current-voltage characteristic curve of the circuit driving the transformer, which is also referred in the following as the igniter, is subject to production-related fluctuations. If structural or production-related changes are made to igniters that cause the voltage-current characteristic curve to change, it may be necessary to redetermine the setpoint value of the impedance using trials, to prevent the situation in which a corona of inadequate size is formed or, in the worst case, a corona is not formed at all.
On the basis of document WO 2010/011838 A1 it is known to control the transformer on the primary side thereof by specifying, initially at a low voltage, a setpoint impedance by determining a so-called baseline impedance at the input of the transformer. Starting at a low voltage, the voltage-current characteristic curve at the input of the transformer initially has a linear shape, which indicates that impedance remains the same: The current intensity initially increases in proportion to voltage. The baseline impedance is characteristic for the particular igniter. If a certain voltage is exceeded, the impedance increases, which is indicated by the fact that the intensity of the current measured on the primary side of the transformer is no longer proportional to the voltage, but rather increases at an increasingly slower rate as the voltage continues to increase, until a voltage breakdown occurs. In the method known from document WO 2010/011838 A1, the setpoint impedance is determined as the sum of the baseline impedance and an additional impedance. The additional impedance is increased in small increments by increasing the voltage until a spark discharge occurs. As soon as a spark discharge is detected, the additional impedance is reduced by an amount that is slightly greater than the preceding increment, in order to prevent further spark discharges and keep the oscillating circuit in resonance. In this way it is possible to hold the current intensity and voltage at the input of the transformer below the amount at which a spark discharge can occur, i.e. to limit them to an amount at which the corona reaches a maximum size.
The disadvantage of the method known from document WO 2010/011838 A1 is that the fuel/air mixture cannot be ignited by a corona discharge without spark discharges occurring from time to time, because observing the occurrence of spark discharges is the precondition for specifying the setpoint value of the impedance. However, even if a spark discharge occurs only sporadically, the result can be unideal combustion or misfirings, or even erosion of the ignition electrodes.
The object of the present invention is a method for igniting a fuel/air mixture in one or more combustion chambers using corona discharge, which allows for optimal formation of the corona and avoids the initially described disadvantages to the greatest extent possible.
This object is attained by way of a method having the features indicated in claim 1. Advantageous refinements of the invention are the subject matter of the dependent claims.
In the method according to the invention for igniting a fuel/air mixture in a cyclically operating internal combustion engine having one or more combustion chambers delimited by walls that are at ground potential, an igniter having an electric transformer, which has on the primary side thereof a baseline impedance ZBaseline that is characteristic for the ignition system, is used to excite an electric oscillating circuit which is connected to a secondary winding of the transformer. In the oscillating circuit an ignition electrode, which is guided through one of the walls delimiting the combustion chamber in an electrically insulated manner and extends into the combustion chamber, forms a capacitance together with the walls of the combustion chamber that are at ground potential. The excitation of the oscillating circuit is controlled such that a corona discharge igniting the fuel/air mixture is created in the combustion chamber at the ignition electrode. For this purpose, in each cycle of the internal combustion engine, the electric voltage applied at a primary winding of the transformer—referred to hereinbelow as primary voltage—is increased incrementally, wherein the increments by which the primary voltage is increased are selected such that the intensity of the electric current flowing in the primary winding—referred to hereinbelow as primary current—increases incrementally due to the stepwise increase in the primary voltage by amounts that become smaller as the impedance on the primary side of the transformer increases, and move toward a specifiable minimum upon approaching the breakdown voltage. In this case, the breakdown voltage is understood to be the primary voltage that, when exceeded, results in the corona discharge transitioning into a spark discharge or arc discharge. The ignition system comprises the components that are necessary for the ignition by a corona discharge and which are used in the method of the present invention.
The invention has substantial advantages:
There are several practical implementations of the method according to the invention. The specified minimum, toward which the incremental increases in intensity of the primary current move, can be zero, although it can also be a limiting value that differs from zero. The latter can be advantageous in order to meaningfully limit the increments before halting the increase of the primary voltage. Advantageously, the increase of the primary voltage is halted at the latest when the specifiable minimum of the amount by which the intensity of the primary current increases as the primary voltage is increased incrementally is reached or fallen below for the first time. Another advantageous possibility is not to halt the incremental increase in the primary voltage once a specifiable minimum is reached or fallen below, but rather as soon as a limiting value of the increase in primary current intensity is reached, wherein this limiting value is located above the stated minimum by a specifiable amount. This is recommended, in particular, if the value zero was selected as the specifiable minimum.
To excite the oscillating circuit with a high-frequency alternating voltage, a transformer is advantageously used which comprises on the primary side thereof a center tap at which two primary windings meet. They can be connected to a DC voltage source in opposite directions in alternation, and therefore the two primary windings are excited inversely in alternation, thereby inducing an alternating voltage in the secondary winding of the transformer, the frequency of which is determined by the frequency at which the two primary windings are connected in alternation to the DC voltage source. Advantageously, this frequency can be changed, thereby ensuring that the oscillating circuit on the secondary side of the transformer can be excited with the resonance frequency thereof. It is known to provide a high-frequency switch on the primary side of the transformer for this purpose, which connects the two primary windings in opposite directions in alternation to the DC voltage source provided. Documents WO 2004/063560 A1 and WO 2010/011838 A disclose additional details thereto. The excitation of the oscillating circuit is advisably discontinuous with a present frequency that is set by a control device that is adapted to the method of the present invention.
In a combustion engine, the fuel/air mixture has to be ignited in each cylinder in every motor cycle. It is also possible to cause more than one ignition process by corona discharge in each cylinder during each motor cycle. An advantage of this is that fuel can be more completely burned by a post-combustion or exhaust gas with fewer harmful contents can be achieved, for example.
There are advantageous embodiments of the method according to the invention which require knowledge of the magnitude of the baseline impedance on the primary side of the transformer. As mentioned above, this can be determined in the case of voltages having a linear U/I characteristic curve by measuring the primary voltage and the primary current, and calculating the quotient thereof. The baseline impedance is preferably determined prior to every ignition. This ensures that its value is always current.
According to a particularly advantageous embodiment of the method, the primary voltage U is increased incrementally by applying an iteration method for calculating the primary voltage Un for the nth step on the basis of the primary current having intensity In−1, which was induced by the primary voltage Un−1 applied in the (n−1)th step, according to the formula
U
n
=Z
Baseline
*I
n−1
*k (1)
wherein k is greater than 1. In this manner a series of primary voltages Un which converge toward the breakdown voltage UD or a final value UB of the primary voltage that is slightly below the breakdown voltage UD is obtained, and a series of primary current intensities In is obtained, the increases of which converge toward zero.
The factor k influences the final value of the convergence, i.e. the final value UB of the primary voltage that is below the breakdown voltage UD and toward which the primary voltage converges. The factor k can be suitably determined in preliminary trials. The factor k must be less than or equal to the quotient of the breakdown voltage UD and the product of the breakdown current ID and the baseline impedance ZBaseiine:
k<=U
D/(ZBaseline−ID). (3)
It can be determined for a certain engine and a corona ignition device provided therefor using preliminary trials, and can then be applied in the method according to the invention for the entire series of identical engines. The breakdown current ID is intended to mean the maximum of the primary current intensity that occurs before voltage breakdown takes place.
In a piston engine, the breakdown voltage UD depends on the distance between the ignition electrode and the piston or—in other words—on the position of the crankshaft or—in other words—on the ignition angle. Since the ignition of the fuel/air mixture should take place at a certain piston position or a certain ignition angle, and the latter can be changed by the engine control unit, it is advantageous to define the factor k differently for different piston positions and ignition angles. Defined values of k can be stored in a control unit as a function of one of the three parameters “piston position”, “position of the crankshaft”, and “ignition angle”, e.g. in an engine control unit that is present anyway, or in an ignition control unit which is provided separately for controlling the ignition method according to the invention. The value of k that depends on the parameter that was selected can then be applied in the formula provided for the iteration
U
n
=Z
Baseline
*I
n−1
*k (1)
Provided the value of the selected parameter remains the same, the factor k should also remain the same.
Theoretically, the iteration method could be carried out using an infinitely large number of steps. However, the iteration method is advantageously halted when the increase In−In−1 in primary current intensity attained in a step n, or the increase Un+1−Un in primary voltage calculated on the basis thereof reaches or falls below a specified limiting value. This limiting value can be selected such that it defines how closely one approaches the final value UB of the primary voltage in the iteration, wherein the final value UB of the primary voltage is defined by suitably selecting the factor k, and is less than the breakdown voltage UD.
According to another advantageous embodiment of the method, the primary voltage U is increased incrementally by applying an iteration method for calculating the primary voltage Un for the nth step on the basis of the primary current having intensity In−1, which was induced by the primary voltage Un−1 applied in the (n−1)th step, according to the formula
U
n
=Z
Baseline
*I
n−1
+U
ADD (2)
wherein UADD is an additional voltage that is slightly lower than the difference between the breakdown voltage UD and the voltage determined by the product of the baseline impedance ZBaseline and the breakdown current ID. The quantity UADD is preferably determined in advance in preliminary trials conducted on an engine, and is then applied for a series of identical engines comprising the same ignition electrode located in the same position.
For a piston engine, the additional voltage UADD is determined in a manner similar for factor k in the above-described embodiment of the method according to the invention as a function of the distance of the piston from the tip of the ignition electrode, or the position of the crankshaft driven by the piston, or the ignition angle. UADD can be stored and applied according to the invention for identical engines comprising the same ignition electrode located in the same position, as a function of one of these three parameters, in the equation
U
n
=Z
Baseline
*I
n−1
+U
ADD (2)
wherein UADD should change only if the value of one of these parameters changes, but should otherwise remain the same.
Preferably, the additional voltage UADD is determined in preliminary trials such that it is slightly lower than the difference between the breakdown voltage UD and the voltage determined by the product of the baseline impedance ZBaseline and the breakdown current ID. The primary voltage then converges toward a value that is only slightly lower than the breakdown voltage UD. An advantage of the method is that changes in the baseline impedance are automatically compensated for, and so the method may be applied using the previously determined quantity UADD even in cases where the ignition device was structurally redesigned for engines that are otherwise identical, provided the design and placement of the ignition electrode within the combustion chamber remain unchanged.
Theoretically, the iteration method could be carried out using an infinitely large number of steps. In this embodiment of the invention, the iteration method is advantageously halted when the increase In−In−1 in primary current intensity attained in a step n, or the increase Un+1−Un in primary voltage calculated on the basis thereof, reaches or falls below a specified limiting value.
In a third advantageous embodiment of the invention, the primary voltage is increased by applying an iteration method incrementally from a value Un to a value Un+1, and the intensity of the resulting primary current In+1 is measured and compared to the current intensity In measured in the preceding step n. On the basis thereof, the mean slope of the U/I characteristic curve is determined for the dependence of the primary current on the primary voltage in the region between the nth step and the (n+1)th step, and the iteration method is halted when the mean slope determined in the last step reaches or exceeds a specified limiting value. Advantageously, the mean slope of the U/I characteristic curve is determined as
Z
av=(Un+1−Un)/(In+1−In) (4)
This iteration method converges as well. The limiting value that is selected determines how closely the breakdown voltage UD is approached. In the simplest case, the primary voltage is increased in uniform increments. It is also possible, however, to increase the primary voltage in the non-linear part of the U/I characteristic curve by increments Un+1−Un, the size of which decreases linearly. As a result, the steps used to approach the primary voltage of the breakdown voltage become smaller as they get closer, thereby making it easier to more closely approach the breakdown voltage UD.
The invention is explained in greater detail below with reference to the attached schematic drawings.
A high-frequency generator 10 is provided for excitation of oscillating circuit 7, and comprises a dc voltage source 11 and a transformer 12 having a center tap 13 on the primary side thereof, thereby enabling two primary windings 14 and 15 to meet at center tap 13. Using a high-frequency switch 16, the ends of primary windings 14 and 15 opposite center tap 13 are connected to ground in alternation. The switching rate of high-frequency switch 16 determines the frequency with which series oscillating circuit 7 is excited, and can be changed. Secondary winding 17 of transformer 12 supplies series oscillating circuit 7 at point A. High-frequency switch 16 is controlled using a not-shown control loop such that the oscillating circuit is excited with the resonant frequency thereof. The voltage between the tip of ignition electrode 5 and walls 2 to 4 that are at ground potential is therefore at a maximum.
Cylinder head 2 comprises a passage 20 through which ignition electrode 5 is guided in an electrically insulated and sealed manner. Ignition electrode 5 is enclosed along a portion of the length thereof by an insulator 6 which can be composed of a sintered ceramic, e.g. an aluminium oxide ceramic. Ignition electrode 5 extends via the tip thereof into combustion chamber 1 and extends slightly past insulator 6, although it could be flush therewith.
A few sharp-edged projections 21 can be provided on the top side of piston 18 in the environment of the tip of ignition electrode 5, which are used to locally increase the electric field strength between ignition electrode 5 and piston 18 situated opposite thereto. When oscillating circuit 7 is excited, a corona discharge forms primarily in the region between ignition electrode 5 and optionally provided projections 21 of piston 18, and can be accompanied by a more or less intensive charge carrier cloud 22.
A housing 23 is placed onto the outer side of cylinder head 2. Primary windings 14 and 15 of transformer 12, and high-frequency switch 16 interacting therewith, are located in a first compartment 24 of housing 23. A second compartment 25 of housing 23 contains secondary winding 17 of transformer 12 and the remaining components of series oscillating circuit 7, and, optionally, means for observing the behavior of oscillating circuit 7. An interface 26 can be used to establish a connection, for example, to a diagnostic unit 29 and/or an engine control unit 30.
U
n
=Z
Baseline
*I
n−1
*k (1)
In the formula, k is a factor that is greater than 1 and should be less than or equal to the quotient of the breakdown voltage UD and the product of the breakdown current ID and the baseline impedance ZBaseline:
k≦U
D/(ZBaseline*ID).
The factor k can be determined in advance in a suitable manner for an engine of a specified version, and, in fact, as a function of the distance between ignition electrode 5 and piston 18 or—expressed another way—the position of the engine crankshaft driven by piston 18 or—expressed yet another way—the ignition angle. The factor k is determined in preliminary trials such that the iteration method implemented using the stated formula (1) converges toward a final value UB of the voltage which is slightly less than the breakdown voltage UD. The factor k, which is determined in preliminary trials, can be used for a series of identical engines. The convergence toward a suitable final value of the primary voltage UB is insensitive to production tolerances and changes in baseline impedance caused, for example, by ageing, production tolerances of the ignition device, contamination of the ignition electrode, temperature differences due to different exhaust gas recirculation rates or different leanings of the fuel/air mixture, or due to the use of different control devices; it is also insensitive to changes in the design of the igniter, provided the geometry of the igniter and the ignition electrode inside the combustion chamber remain unchanged.
The iteration method is carried out for each cylinder of the engine in every cycle of the engine, i.e. once for every two revolutions of the crankshaft in the case of a 4-stroke engine, before the particular moment of ignition. To do this, first the baseline impedance ZBaseline is determined in a linear part of the characteristic curve, below point A, preferably in the vicinity of point A, and, in fact, as the quotient of the primary voltage U and the associated primary current intensity I, e.g. as Z=UA/IA. The voltage Un of the next iteration step in each case is determined using the formula (1). The primary voltage then converges toward the value UB, and the intensity of the primary current converges toward the value IB. Three iteration steps are sketched in
U
n
=Z
Baseline
*I
n−1
+U
ADD (2)
wherein UADD is an additional voltage that is slightly less than the difference between the breakdown voltage UD and the primary voltage which is determined by the product of the baseline impedance ZBaseline and the breakdown current ID. The quantity UADD can be determined suitably using an engine having a specified configuration, and can then be applied for a series of identical engines. The convergence toward a suitable final value of the primary voltage UB is insensitive to production tolerances and changes in the baseline impedance caused, for example, by ageing, production tolerances of the ignition device, contamination of the ignition electrode, temperature differences due to different exhaust gas recirculation rates or different leanings of the fuel/air mixture, or due to the use of different control devices; it is also insensitive to changes in the design of the igniter, provided the design and placement of the ignition electrode inside the combustion chamber remain unchanged.
The iteration method is carried out for each cylinder of the engine in every cycle of the engine, i.e. once for every two revolutions of the crankshaft in the case of a 4-stroke engine, before the particular moment of ignition. To do this, first the baseline impedance ZBaseline is determined in the linear part of the U/I characteristic curve, as in the embodiment depicted in
In the example shown in
The limiting value of the slope can be determined suitably in preliminary trials, in particular such that it is the slope of the characteristic curve at point B, at which the primary voltage UB is slightly less than the breakdown voltage UD.
As shown in
The final value UB can be suitably selected in preliminary trials as a function of the distance of the tip of ignition electrode 5 from piston 18 or—expressed another way—as a function of the position of the crankshaft or—expressed yet another way—as a function of the ignition angle, to obtain a corona of optimal size. An engine control unit can specify when ignition should take place in terms of the distance between the tip of ignition electrode 5 and piston 18, the position of the crankshaft, or the ignition angle, and, based thereon, one of the following is selected accordingly: the limiting value of the slope, per the example shown in
The limiting value of the slope of the U/I characteristic curve shown in
The factor by which the baseline impedance is multiplied to determine the limiting value, or the additional impedance that is added to the baseline impedance to determine the limiting value of the slope can be determined in preliminary trials as a function of the distance of the tip of the ignition electrode from the piston, or the position of the crankshaft, or the ignition angle. They are also stored in a control unit, and can be specified by same. The values can be stored in an engine control unit which is present anyway, although they are preferably stored in a separate ignition control unit.
In all variants of the method according to the invention described above, the situation is prevented in which the breakdown voltage UD is reached or exceeded during operation of the engine, in contrast to the methods in the prior art: In the method known from WO 2004/063560 A1, the increase of the primary voltage is halted as soon as the measured impedance exceeds a fixed threshold value Zfix, to reliably prevent a spark discharge from occurring. The impedance threshold value Zfix must ensure that a corona occurs without an arc of a spark for various igniters. Since this must also apply for igniters having U/I characteristic curves with different shapes due to changes related to design or production, Zfix must be selected relatively low. Therefore, the straight line representing the value Zfix intersects the U/I characteristic curve at a point D far below the breakdown voltage UD, see
According to the method described in WO 2010/011838 A1, the baseline impedance is first determined in the linear part of the characteristic curve. Next, the impedance is increased incrementally until a spark discharge is detected. A spark discharge is detected when the measured impedance exceeds a threshold value ZArc, see
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 015 344.3 | Apr 2010 | DE | national |
