DE102017211850A1 - Method for heating at least one catalytic converter of an internal combustion engine with an odd number of cylinders - Google Patents

Abstract

Method for heating at least one catalytic converter of an internal combustion engine (10) with an odd number of cylinders, wherein two successive cylinders form a pair in a predeterminable ignition sequence, one cylinder of the pair having an air-fuel ratio (λ) greater than one and the other cylinder of the pair is operated at an air / fuel ratio (λ) less than one, wherein an overall air / fuel ratio (λ _ges ) of the pair gives a predetermined value.

Description

State of the art

The invention relates to a method for heating at least one catalytic converter of an internal combustion engine with an odd number of cylinders according to the independent claims. Furthermore, the invention relates to a computer program that is configured to perform one of the methods.

Currently, a catalyst heating is carried out with various engine measures to bring the catalyst as quickly as possible in its exhaust optimal temperature window to minimize emissions and to ensure high efficiency of the catalyst. To z. B. even after the engine start to achieve an optimal operating point within the conversion window of the catalyst, further motor measures are taken to keep the catalyst warm. One of these measures is z. B. the lambda split. The cylinders are alternately operated by a certain factor lean and rich.

The DE 13 20 890 B4 discloses a method for heating at least one of its catalysts ( 36 . 38 ) in the exhaust gas of a spark-ignition and direct injection of fuel in air fillings of at least one combustion chamber ( 11 ) working internal combustion engine ( 12 by alternately generating lean combustion chamber fillings and rich combustion chamber fillings in the at least one combustion chamber ( 11 ), wherein the internal combustion engine ( 12 ) is operated in a first operating mode with stratified combustion chamber fillings and is operated in a second operating mode with homogeneous combustion chamber fillings, wherein lean combustion chamber fillings are generated in the first operating mode and rich combustion chamber fillings are generated in the second operating mode and where spark ignition ( 65 ) of the rich combustion chamber fillings compared to spark ignition ( 63 ) of the lean combustion chamber fillings are delayed, characterized in that the delay of the spark ignition ( 65 ) is dimensioned so that the same moments result from burns of the lean combustion chamber fillings and the rich combustion chamber fillings.

The EP 0 902 172 A2 discloses a method of operating a multi-cylinder internal combustion engine with gasoline direct injection into the cylinders, wherein at least one cylinder is operated cycle to cycle alternately at a lambda value greater than or less than or equal to 1.0 for afterburning the gasoline within the exhaust tract and at least one other cylinder of Cycle to cycle alternately with a lambda value smaller or larger or if necessary equal to 1.0 is operated. As a result, an afterburning can take place within the exhaust tract of the internal combustion engine, which requires no additional injection and ignition device, without secondary air pump and without expensive temperature-dependent control.

For engines with an odd number of cylinders, a rigid assignment of the air / fuel ratio λ changing per cycle inevitably leads to an even distribution due to the odd number of cylinders is not possible, so that it comes to unintentionally unequal torques.

Due to the different air / fuel ratios λ and above all by the unequal number of cylinders, the cylinder torques provided by the cylinders change during the transition from normal operation without lambda split for operation with lambda split. Such a change can be perceived by the driver.

Against this background, the object of the invention is to provide optimum heating of at least one catalytic converter of an internal combustion engine with an odd number of cylinders, in which all the cylinders are used for heating the catalytic converter.

Disclosure of the invention

The invention relates to a method and a device for heating at least one catalytic converter of an internal combustion engine with an odd number of cylinders and a computer program on a storage medium for carrying out the method. Advantageous developments are the subject of the independent claims.

In a first aspect, the invention relates to a method for heating at least one catalytic converter of an internal combustion engine having an odd number of cylinders, wherein two successive cylinders form a pair in a predeterminable firing order, one cylinder of the pair being greater than one at an air / fuel ratio other cylinder of the pair is operated with an air / fuel ratio less than one, wherein an overall air / fuel ratio of the pair gives a predetermined value.

It is particularly advantageous that the alternating operation of the cylinder with an air / fuel ratio greater and smaller one and by pairing together two consecutive cylinders, as well as by specifying a common air / fuel ratio for the couple, a good smoothness of Internal combustion engine can be ensured because torque inequalities evenly over the heating process be distributed. Thus, a cathexis for internal combustion engines with an odd number of cylinders, in particular for internal combustion engines with three cylinders is possible without a leader of the vehicle perceives unwanted torque drops or a disturbing running noise of the internal combustion engine. That is, all cylinders take part in the heating process, in particular in firing order, so that a faster heating of the catalyst can be achieved.

Furthermore, it can be provided that the predefinable value for the total air / fuel ratio of the pair is one.

This is particularly advantageous when using a three-way catalyst, since in this case a high conversion of the pollutants generated by the internal combustion engine can be ensured.

By alternately lean and rich operation of the cylinder is achieved that the exhaust gas of two cylinders in total results in the desired air / fuel ratio and thus the best possible exhaust gas conversion is achieved.

In another aspect, a remaining cylinder in a combustion cycle forms a pair with a cylinder from a subsequent combustion cycle, particularly in the predetermined firing order of the cylinders. By forming this pair, a smooth running smoothness of the internal combustion engine with an odd number of cylinders can be ensured.

Further, one cylinder of the pair is operated in response to a first factor having an air-fuel ratio less than one and another cylinder of the pair is operated in response to a second factor having an air-fuel ratio greater than one.

Furthermore, the first factor is limited as a function of a comparison with a predefinable maximum threshold value, in particular by an air / fuel ratio.

This has the particular advantage that the efficiency and the burn limits can be met by limiting the first factor. By taking into account a limiting air / fuel ratio is prevented in variable lambda lambda the lambda of the rich cylinder within the prescribed by combustion limits, Zündwinkelverstellbereich and possibly for driveability reasons range.

Furthermore, the second factor is limited as a function of a comparison with a predefinable minimum threshold, in particular by an air / fuel ratio.

By limiting the second factor, safe operation of the internal combustion engine and compliance with combustion limits can be maintained. By taking into account a limiting air / fuel ratio is prevented in variable lambda lambda the lean cylinder is within the range specified by combustion limits, Zündwinkelverstellbereich and possibly for driveability reasons range.

In a further development, a value for an oxygen level of the at least one catalyst is determined and when a predefinable threshold value is exceeded by the value of the oxygen level of the at least one catalyst, the process of heating the at least one catalyst with an air / fuel ratio smaller than one is started.

If the at least one catalyst has already stored sufficient oxygen, it is particularly advantageous to begin the process of cat heating with an air / fuel ratio of less than one, since oxygen is already present as a reaction partner for the heating process. This prevents the oxygen storage level of the catalytic converter from reaching its limits and its exhaust gas conversion decreasing.

In a flexible development, a value for an oxygen level of the at least one catalyst is determined and falls below a predefinable threshold by the value of the oxygen level of the at least one catalyst, the process of heating the at least one catalyst is started with an air / fuel ratio greater than one.

This is advantageous, since thus a heating of the catalyst can be efficiently implemented. This prevents the oxygen storage level of the catalytic converter from reaching its limits and its exhaust gas conversion decreasing.

Advantageously, the internal combustion engine is operated in a homogeneous operating state.

In a further aspect, the internal combustion engine may be a gasoline direct injection and / or a gas-fueled internal combustion engine, in particular a liquefied petroleum gas (LPG) or an internal combustion engine (CNG) compressed natural gas (CNG).

In other aspects, the invention relates to a device, in particular a control device and a computer program, which are set up for executing one of the methods, in particular programmed. In yet another aspect, the invention relates to a machine-readable storage medium on which the computer program is stored.

list of figures

• 1 eine schematische Darstellung einer Brennkraftmaschine mit Direkteinspritzung und einem Abgastrakt mit einem Katalysator;
• 2 den beispielhaften Ablauf des Verfahrens zum Aufheizen wenigstens eines Katalysators einer Brennkraftmaschine mit einer ungeraden Anzahl von Zylindern.

The invention is described in more detail below with reference to the accompanying drawings and to exemplary embodiments. Showing:

• 1 a schematic representation of an internal combustion engine with direct injection and an exhaust tract with a catalyst;
• 2 the exemplary sequence of the method for heating at least one catalytic converter of an internal combustion engine with an odd number of cylinders.

Description of the embodiments

In 1 is a schematic representation of an internal combustion engine 10 shown with a direct injection and an exhaust tract with a catalyst. The internal combustion engine 10 has at least one combustion chamber 11 on top of a piston 12 is sealed movable. The change of the filling of the combustion chamber 11 is via at least one inlet valve 6 and at least one exhaust valve 7 controlled. intake valve 6 is from an intake valve actuator 4 operated and exhaust valve 7 is from an exhaust valve actuator 5 actuated. Both intake valve actuator 4 as well as exhaust valve actuator 5 can be realized both by camshafts as a mechanical actuator and by electric or pneumatic actuator.

When the inlet valve is open 6 the piston sucks 12 Air from a suction pipe 13 at. During the intake process and / or during the subsequent compression process, fuel is injected via an injection valve 8th directly into the combustion chamber 11 meted out. The resulting combustible mixture in the combustion chamber 11 comes with a spark plug 9 ignited (spark ignition).

With the exhaust valve open 7 the burnt residual gases are released from the combustion chamber 11 ejected into the exhaust tract. Downstream of the exhaust tract, ie downstream of the exhaust pipe 17 , there is a catalyst 18 , The catalyst 18 can z. B. of three carrier structures 20 . 21 and 22 exist that differ in their catalytic coating. Preferably, this catalyst 18 is designed as a three-way catalyst and is controlled at an air / fuel ratio λ equal to one. Alternatively and additionally, a plurality of catalysts can also be arranged one behind the other in series or in parallel. At a lambda equal to one control, the internal combustion engine 10 operated alternately with oxygen excess and lack of oxygen, wherein the period of the fat-lean change is in the second range. In this case, the catalyst stores 18 In oxygen excess phases oxygen and nitrogen oxides and can react in low oxygen phases with then contained in the exhaust gas hydrocarbons and carbon monoxide to carbon dioxide, water and nitrogen catalytically. One speaks in this case of a storage catalyst 18 ,

Will the internal combustion engine 10 on the other hand, for a long time with a lean mixture, that is to say operated with an excess of oxygen, the nitrogen oxides which are then emitted more intensively become the catalytic coating of the carrier structures 20 . 21 and 22 in the catalyst 18 recorded and stored in the form of nitrogen nitrates.

These nitrogen nitrates are caused by a short-term operation of the internal combustion engine 10 degraded again with a rich mixture. In contrast to the lambda = 1 control, the lean and rich phases with which the catalyst 18 is operated, not approximately symmetrical. The catalyst 18 can store oxygen and nitrogen oxides for periods of the order of minutes and release them in converted form in a time interval of the order of seconds. This allows the internal combustion engine 10 be operated lean in the sum, without the emitted nitrogen oxides emitted in lean operation in larger quantities into the environment.

Alternatively, the catalyst 18 also be a particulate catalyst or a 3-way catalyst.

The control of the internal combustion engine 10 done by a control unit 100 that at least signals an air mass meter 1 , one with a sender wheel 14 interacting speed sensor 15 and a driver's desire 24 processed. In addition, the controller can 100 Signals of a first exhaust gas sensor 23 , Signals of a second exhaust gas sensor 19 and the signals of other sensors, not shown, on pressures and / or temperatures in the range of the internal combustion engine 10 or the exhaust tract be supplied. From these and optionally further input signals forms the controller 100 Control signals with which the internal combustion engine 10 can be operated according to the driver's request and / or according to pre-programmed requirements.

For example, the filling of a combustion chamber 11 in homogeneous operation of Internal combustion engine 10 about the position of a throttle valve 2 that of a throttle actuator 3 is pressed, be set. In homogeneous operation, that of the internal combustion engine 10 generated torque substantially determined by the mass of the combustion chamber filling and the selected ignition timing. In stratified operation, the internal combustion engine 10 however, largely unthrottled with open throttle 2 and maximum combustion chamber filling of the combustion chamber 11 with air. In this case, that of the internal combustion engine 10 generated torque substantially determined by the injected fuel mass and the ignition timing.

In the 2 FIG. 10 is a flowchart for the method of heating at least one catalyst 18 an internal combustion engine 10 shown with an odd number of cylinders. The method may also be for an internal combustion engine 10 with an odd number of cylinders with more than three cylinders ( 5 . 7 . 9 ... cylinder) are used. The internal combustion engine 10 can continue a gas-powered internal combustion engine 10 , such as As an internal combustion engine, which is operated with LPG (Liquefied Petroleum Gas - LPG) or compressed natural gas (CNG - compressed natural gas) or as a hybrid with gasoline and gas, are used.

In the following, the embodiment for an internal combustion engine 10 described with three cylinders.

In a first step 500 is the temperature of the catalyst by means of a temperature sensor or a temperature model 18 determined. Is the determined temperature value of the catalyst 18 below a first predetermined threshold, e.g. B. 500 ° C, and above a second predetermined threshold, z. B. between 200 and 250 ° C, so the catalyst 18 be heated to reach a minimum temperature in which the catalyst can ensure its catalytic activity. Preferably, the operating temperature for the catalyst is about 500 ° C.

As an alternative or additional criterion for deciding whether the catalyst 18 can be heated, z. B. an evaluation of an operating condition of the internal combustion engine 10 be performed, such. B. the detection of a cold start of the internal combustion engine 10 , This operating state of the internal combustion engine 10 , z. B. via one in the control unit 100 deposited information happen.

Heating the catalyst 18 is achieved by post combustion of fuel within the exhaust tract.

For this purpose, a lambda split for the cylinder of the internal combustion engine 10 carried out. That is, the cylinders of the internal combustion engine 10 are operated alternately by a certain factor lean or rich, ie with an air / fuel ratio λ greater or smaller one.

Due to the odd number of cylinders, this distribution of the different air / fuel ratios λ within a combustion cycle inevitably leads to an imbalance in the distribution to the existing cylinders. A combustion cycle is preferably understood to mean that each cylinder has been operated once in a predeterminable firing order, in particular for a 4-stroke or 2-stroke operation.

For an internal combustion engine 10 with three cylinders, two cylinders with an air / fuel ratio λ greater than one and a cylinder with an air / fuel ratio λ less than one or vice versa are operated in a combustion cycle.

Is the temperature of the catalyst 18 above the first predefinable threshold and below the predetermined second threshold, so in step 500 continued from the front.

Will in step 500 realized that a heating process of the catalyst 18 is requested, so will in one step 510 a different division of the air / fuel ratio λ for the three cylinders requested. This process is also called lambda split.

The division of the air / fuel ratio λ depends mainly on the current operating variables of the internal combustion engine 10 from. Operating variables with great influence on the air / fuel ratio λ and the distribution of the air / fuel ratio λ are, for example, the speed and the torque of the internal combustion engine 10 and the difference of a predetermined target catalyst temperature with the current temperature of the catalyst 18 ,

In step 510 the division for an odd number of cylinders is now made as follows. Starting with a predeterminable firing order, in particular a recurrent firing order, for the cylinders, two cylinders each form a pair, one cylinder of the pair at an air / fuel ratio λ greater than one and the other cylinder of the pair with an air / fuel ratio λ smaller one is operated, so that an entire air / fuel ratio λ _ges , in particular on average over the cylinder pair, a predetermined value, in particular one, results.

In 3 a total of two combustion cycles (A; B) are shown. A possible Ignition order for the first combustion cycle (A) starts with cylinder 1 and then follow cylinders 2 and cylinders 3 , The second combustion cycle B then starts again with cylinder 1 followed by cylinder 2 and 3 etc..

Depending on the set in the first combustion cycle A air / fuel ratio λ for the cylinder 1 Thus, either with an air / fuel ratio λ greater or smaller one, the air / fuel ratio λ is set for the following cylinders in firing order, so that the cylinders are always operated with an alternating air / fuel ratio λ greater or lesser one.

From combustion cycle to the following combustion cycle, the air / fuel ratio λ alternates for each cylinder.

That Cylinders operated at an air-fuel ratio λ greater than one are operated at an air-fuel ratio λ less than one in the following combustion cycle, and vice versa.

In the example of 3 becomes the first cylinder 1 the ignition order with an air / fuel ratio λ greater than one operated. It follows that the cylinder 2 is operated with an air / fuel ratio λ less than one. Subsequently, the cylinder 3 operated again with an air / fuel ratio λ greater than one, etc ..

For a single combustion cycle, it is easy to see that such operation is the cylinder 1 . 2 and 3 would lead to an imbalance of the air / fuel ratio λ.

This imbalance is now eliminated by two consecutive cylinders forming a pair in a predeterminable firing order, so that an overall air / fuel ratio λ _ges , in particular as an average over the pair of cylinders, results in a predefinable value, in particular one. In the combustion cycle A form cylinders 1 and cylinders 2 the first couple 70 , The cylinder 3 in the first combustion cycle A then forms the second pair 80 with the cylinder 1 from the combustion cycle B, followed by the third pair 90 consisting of cylinder 2 and cylinders 3 of the second combustion cycle B. It is easily understood that each cylinder of the internal combustion engine 10 is operated from combustion cycle to the following combustion cycle alternately with an air / fuel ratio λ greater or smaller one.

By this recurring change of the air / fuel ratio λ of the cylinder with an air / fuel ratio λ greater and smaller one and the pairwise division of the cylinder can be a good running smoothness for an internal combustion engine 10 are guaranteed with an odd number of cylinders, so that unequal torques are distributed evenly and barely noticeable. Furthermore, the exhaust gas conversion is improved.

For the application of this method is especially suitable for a homogeneous operation of the internal combustion engine 10 so that the air-fuel ratio λ is preferably varied between 0.8 and 1.2, that is, in particular, operated at an air-fuel ratio λ close to one.

Preferably, the first cylinder becomes after the activation of the heating process of the catalyst 18 with a rich air / fuel ratio λ, ie with an air / fuel ratio λ less than one, started.

The lambda split for the cylinders is controlled via a characteristic map, which depends on, for example, speed and / or the moment of the internal combustion engine 10 and / or the current temperature control for the catalyst 18 determines a split factor.

$$fakto{r}_{mager}=2-fakto{r}_{fett}$$

$$lambd{a}_{mager}=\frac{lambd{a}_{gesamt}}{fakto{r}_{mager}}$$

, wobei lambda_gesamt das Luft-/Kraftstoffverhältnis für das Zylinderpaar, faktor_fett den Faktor für mit einem Luft-/Kraftstoffverhältnis kleiner eins zu betreibenden Zylinder und faktor_mager den Faktor mit einem Luft-/Kraftstoffverhältnis größer eins für den zu betreibenden Zylinder beschreibt. Vorzugsweise werden die Faktoren immer ausgehend vom fett zu betreibenden Zylinder durch das Steuergerät 100 ermittelt.The split factors are calculated for a pair of cylinders: $$faktO{r}_{fett}=\frac{lambd{a}_{Gesamt}}{lambd{a}_{fett}}$$

$$faktO{r}_{maGer}=2-faktO{r}_{fett}$$

$$lambd{a}_{maGer}=\frac{lambd{a}_{Gesamt}}{faktO{r}_{maGer}}$$

, where lambda _totals the air / fuel ratio for the cylinder pair, factor _fat the factor for cylinders with an air / fuel ratio less than one to operate and factor _lean the factor with an air / fuel ratio greater than one for the cylinder to be operated. Preferably, the factors are always based on the cylinder to be operated by the control unit 100 determined.

The factor _lean describes a factor with an air / fuel ratio λ greater than one and the factor _fat an air / fuel ratio λ less than one. Alternatively or additionally, in one step 520 the two factors factor _fat and factor _lean additionally be limited by a comparison with each one threshold.

Exceeds the factor _fat a predetermined threshold, so z. B. a predefinable maximum substitute value for factor _bold , such as a value, which gives a maximum air / fuel ratio λ of 0.8, can be specified. With active restriction of the factor _fat must factor _lean according to equation ( 2 ) are also adjusted.

If the factor falls _below a predeterminable threshold value, then z. B. a predeterminable minimum replacement value for factor _lean , such. For example, a value which gives a minimum air / fuel ratio λ of 1.1 or 1.2 (for CNG) can be given.

With active limitation of the factor _lean , factor must be _bold according to equation ( 2 ) are also adjusted.

The limitation of the factors for the air / fuel ratio λ to be set provide additional safety, since combustion limits of the cylinders can be monitored and too short injection times for the combustion can be prevented, so that the drivability of the internal combustion engine 10 can be guaranteed.

Alternatively or additionally, in one step 530 by means of one on the control unit 100 determined model for the oxygen loading of the catalyst 18 be decided whether the starting cylinder of the heating process of the catalyst 18 to be operated with an air / power ratio greater or smaller one.

In the control unit 100 becomes an actual value for the oxygen loading of the catalyst 18 determined.

Exceeds the actual value for the oxygen loading of the catalyst 18 a predefinable threshold, so does the catalyst 18 already stored a sufficient amount of oxygen for the heating, so that the heating process with a rich combustion, ie with an air / fuel ratio λ less than one, can be started.

If the actual value for the oxygen loading of the catalyst falls below 18 a predefinable threshold, so does the catalyst 18 currently one or no amount of oxygen stored, so that the heating is started with a lean combustion, ie with an air / fuel ratio λ greater than one, so that the catalyst 18 oxygen is first supplied to allow for the fastest possible exothermic chemical reaction. In one step 540 Then, a target amount of fuel for the current cylinder by the controller 100 and then multiplied by the split factor specified for the cylinder. If the cylinder is to be operated with an air / fuel ratio greater than one, the factor _{lean is} used. If the current cylinder is to be operated with an air / fuel ratio smaller than one, then the factor factor is used in _bold .

Subsequently, the other cylinder of the pair is operated with the determined factor for him, so that an overall air / fuel ratio λ _ges , in particular on average over the cylinder pair, a predetermined value, in particular one, results.

Subsequently, the currently determined temperature of the catalyst 18 be compared with the target catalyst temperature to be reached. Is the current temperature of the catalyst 18 below the target catalyst temperature, the heating of the catalyst 18 continued.

Is the current temperature of the catalyst 18 greater than or equal to the target catalyst temperature so the catalyst heating is stopped and it can in step 500 to be continued.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 1320890 B4 [0003]
• EP 0902172 A2 [0004]

Claims (13)

Method for heating at least one catalytic converter of an internal combustion engine (10) with an odd number of cylinders, characterized in that two successive cylinders form a pair in a predeterminable ignition sequence, one cylinder of the pair being greater than one and at an air / fuel ratio (λ) the other cylinder of the pair is operated at an air / fuel ratio (λ) less than one, wherein an overall air / fuel ratio (λ _ges ) of the pair gives a predetermined value. Method according to Claim 1 , characterized in that the predeterminable value for the total air / fuel ratio (λ _ges ) of the pair is one. Method according to Claim 1 or 2 , characterized in that a remaining cylinder in a combustion cycle forms a pair with a cylinder from a subsequent combustion cycle, in particular in the predeterminable firing order of the cylinders. Method according to one of the preceding claims, characterized in that the one cylinder of the pair in response to a first factor (factor _fat ) with an air / fuel ratio (λ) less than one and another cylinder of the pair in dependence on a second factor (factor _lean ) are operated with an air / fuel ratio (λ) greater than one. Method according to Claim 4 , characterized in that the first factor (factor _fat ) is limited as a function of a comparison with a predefinable maximum threshold value, in particular by an air / fuel ratio (λ). Method according to one of Claims 4 or 5 , characterized in that the second factor (factor _lean ) is limited in dependence on a comparison with a predeterminable minimum threshold, in particular by an air / fuel ratio (λ). Method according to one of the preceding claims, characterized in that a value for an oxygen level of the at least one catalyst is determined and at a predeterminable threshold value is exceeded by the value of the oxygen level of the at least one catalyst (18) the process of heating the at least one catalyst ( 18) is started with an air / fuel ratio (λ) less than one. Method according to one of the preceding claims, that a value for an oxygen level of the at least one catalyst is determined and falls below a predetermined threshold by the value of the oxygen level of at least one catalyst (18) the process of heating the at least one catalyst (18) an air-fuel ratio (λ) greater than one is started. Method according to one of the preceding claims, characterized in that the internal combustion engine (10) is operated in a homogeneous operating state. Method according to one of the preceding claims, characterized in that the internal combustion engine (10) a gasoline direct injection and / or a gas-powered internal combustion engine (10), in particular one with liquefied petroleum gas (LPG) or compressed natural gas (CNG - compressed natural gas ) operated internal combustion engine (10). Computer program, which is adapted to a method according to one of Claims 1 to 10 perform. Electronic storage medium with a computer program after Claim 11 , Device, in particular control device (100), which is adapted to a method according to one of Claims 1 to 10 perform.

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