Method for controlling an electrically heatable catalytic converter and exhaust-gas after-treatment system

Abstract

The invention relates to a method for controlling an electrically heatable catalytic converter for after-treating exhaust gases of an internal combustion engine. The catalytic converter is deactivated or not activated during the start of the engine if the temperature (TKAT(tabst)) of the catalytic converter exceeds a critical temperature TKRIT when starting the engine. The temperature (TKAT(tabst)) of the catalytic converter is estimated via a temperature model. The invention also relates to an exhaust-gas after-treatment system of an internal combustion engine for carrying out the method of the invention.

Description

FIELD OF THE INVENTION

[0001] The invention relates to a method for controlling an electrically heatable catalytic converter for after-treating exhaust gases of an internal combustion engine as well as an exhaust-gas after-treating system for carrying out the method of the invention.

BACKGROUND OF THE INVENTION

[0002] To satisfy strict exhaust-gas requirements, it is necessary that the catalytic converter reaches its operating temperature in a short time after start (that is, that the light-off temperature threshold is exceeded) and therefore the conversion of the exhaust gases begins. To accelerate this warm-up operation, electrically heatable catalytic converters are used which are hereinafter referred to as E-catalytic converters. These catalytic converters usually have a heating power >5 kW and reach a carrier temperature in the region of 500° C. For determining the temperature of the E-catalytic converter, a temperature sensor is provided in known E-catalytic converters which supplies the operating temperature thereof to a control apparatus. Driving the E-catalytic converter then takes place in dependence upon the measured temperature of the E-catalytic converter.

SUMMARY OF THE INVENTION

[0003] It is an object of the invention to provide a method via which a known E-catalytic converter can be simplified with respect to its configuration. Furthermore, the problem of a thermal overload and therefore damage of the E-catalytic converter is, on the one hand, avoided and, on the other hand, the least possible energy is to be taken from an electric energy store for warm-up. It is another object of the invention to provide an exhaust-gas after-treatment system for carrying out the method of the invention.

[0004] The method of the invention is for controlling an electrically heatable catalytic converter for after treating exhaust gases of an internal combustion engine. The method includes the steps of: deactivating or nonactivating the catalytic converter during the start of the engine when the temperature (TKAT(tabst)) of the catalytic converter exceeds a critical temperature TKRIT; and, utilizing a temperature model to estimate the temperature (TKAT(tabst)) of the catalytic converter.

[0005] The method according to the invention for controlling (open loop and/or closed loop) an E-catalytic converter affords the advantage that a temperature, which is measured directly on the E-catalytic converter, is not included in the drive of the E-catalytic converter. Rather, the temperature of the E-catalytic converter is determined by physical condition variables without providing a temperature sensor on or in the E-catalytic converter. This notwithstanding, a thermal overload of the E-catalytic converter is avoided in accordance with the invention and/or only so much energy is drawn from an electric energy store when warming up the E-catalytic converter as is necessary. The invention therefore affords the advantage that by making a temperature sensor in the E-catalytic converter unnecessary, the disadvantages of an E-catalytic converter control can be ameliorated by including additional physical condition variables.

[0006] For the above, three different intervention measures are utilized in the controlled sequence. The most significant intervention measure is the deactivation or non-activation of the E-catalytic converter in the condition wherein it is still hot whereby damage to the E-catalytic converter is prevented.

[0007] With the possibility of the targeted adaptation of the heating duration to the existing requirements, electric energy can be saved and a deterioration of the components of the E-catalytic converter can be countered.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention will now be described with reference to the drawings wherein:

[0009] FIG. 1 is a schematic block diagram showing the functionality of a first intervention possibility;

[0010] FIG. 2 is a block circuit diagram showing a thermal E-catalytic converter model; and,

[0011] FIG. 3 is a schematic showing an exhaust-gas after-treatment system of an internal combustion engine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0012] The method of the invention provides for different intervention possibilities on the controlled sequence of the drive of the E-catalytic converter.

[0013] (a) Condition-Dependent Deactivation of the E-Catalytic Converter during the Start of the Internal Combustion Engine

[0014] A deactivation of the E-catalytic converter takes place in order to protect components when the temperature TKAT(tabst) of the E-catalytic converter already exceeds a critical temperature TKRIT during the start. The temperature TKAT(tabst) can be estimated via the following model the description of which is provided hereinafter.

TKAT(tabst)=[TKAT(0)−TU]·exp(−r·c·tabst)+TU   (see A8 with qu=0).

[0015] For the estimation, the following must be known: the instantaneous switchoff duration tabst from the time point of the last switchoff up to the restart of the engine, the ambient temperature TU and the temperature TKAT(0) of the E-catalytic converter at the time point of switchoff.

[0016] The switchoff duration Tabst can either be read in via a combination instrument or can be determined, in the manner of a substitute, from a model of a cool-down characteristic of the engine.

[0017] The ambient temperature TU can be estimated from the intake-air temperature. In the sense of a worst-case estimate, one can also use a constant substitute value for the ambient temperature TU.

[0018] The temperature of the E-catalytic converter TKAT(0) at the time point of switchoff can be derived from an exhaust-gas temperature model. A typical maximum operating temperature for TKAT(0) can be applied in the sense of a worst-case estimate.

[0019] In total, the functionality shown in FIG. 1 results for the intervention possibility (a). When the E-catalytic converter temperature TKAT(tabst) already exceeds a critical temperature TKRIT during the start, the E-catalytic converter is not activated. If the temperature TKAT(tabst) of the E-catalytic converter lies during the start below a critical temperature TKRIT, then the E-catalytic converter is activated. The values TKAT(0) and/or TU can also be replaced by constant values. These values are then dropped as variable quantities.

[0020] (b) Optimization or Shortening of the Heating Duration of the E-Catalytic Converter

[0021] Although the E-catalytic converter is always operated at a constant heating power, the heat characteristic can run very differently in dependence upon the start temperature and the ambient temperature. The heating duration should be adapted to the start individually for reasons of protecting of components and for saving energy.

[0022] If no temperature information is available, then the heating characteristic and therefore the necessary heating duration can be derived from a temperature model. For the temperature trace when heating up, the following relationship results: 1 T KAT ⁡ ( t ) = [ T KAT ⁡ ( 0 ) - ( q zu r + T U ) ] · exp ⁡ ( - rct ) + ( q zu r + T U ) (A8)

[0023] In addition to the physical condition variables of ambient temperature TU and the temperature TKAT(0) of the catalytic converter at start already mentioned in method (a), the heat entry qzu is needed.

[0024] The heat entry qzu comes essentially from the electrical heating power. This can either be measured by measuring the heating voltage and the heating current or, by the way of substitution, can also be estimated as a constant value.

[0025] The necessary heating duration TEKAT up to reaching the necessary switchoff temperature TKATAB can be determined in the control apparatus either via explicit solution of the above-mentioned mathematical relationship or via continued computation of TKAT with the instantaneously passed heating duration as argument t and subsequent comparison of the result to TKATAB.

[0026] The explicit solution yields the following heating duration: 2 t ekat = 1 r · c · ln ⁡ ( T KAT ⁡ ( 0 ) - q zu r - T U T KATAB - q zu r - T U ) (A9)

[0027] (c) Rapid Switchoff of the E-Catalytic Converter for a Defective Operation

[0028] An activation of the E-catalytic converter drive is to be suppressed or an existing E-catalytic converter drive is to be ended prematurely under specific conditions for reasons of safety. The specific conditions include:

[0029] Diagnosis fault. If the diagnosis of the feed lines to the E-catalytic converter shows a fault, then an activation of the E-catalytic converter is suppressed during start.

[0030] Ambient temperature. At low temperatures, the danger of the E-catalytic converter freezing is present. Under these conditions, an activation of the E-catalytic converter during start is likewise to be suppressed.

[0031] Engine operation. If the engine stalls during the heating phase of the E-catalytic converter, then the E-catalytic converter is advantageously switched off prematurely.

[0032] The model used is described in greater detail in the following wherein the following overview presents the variables used in the description of the model: 1 Abbreviation Explanation Unit TKAT (t) catalytic converter temperature [K] QKAT (t) thermal heat quantity in the E-catalytic [J] converter TU (t) ambient temperature [K] qzu (t) inflowing thermal heat flow [J/s] qab (t) outflowing thermal heat flow [J/s] c thermal proportionality constant [K/J] r thermal transition resistance [J/K*s] tabSt time elapsed since switchoff of the engine [s]

[0033] Relationship of the catalytic converter temperature and heat quantity:

[0034] The temperature TKAT(t) of the E-catalytic converter is, in a first approximation, proportional to the inner thermal heat quantity QKAT(t). The following relationship results when using the specific thermal proportionality factor c:

TKAT(t)=c·QKAT(t)   (A1)

[0035] Supplied Heat Quantity:

[0036] The heat quantity qzu (t) , which flows to the E-catalytic converter during the start, originates from the electric heating power of the E-catalytic converter as well as the heat quantity of the exhaust-gas mass flow. The E-catalytic converter is always driven with the same power and the heat quantity of the exhaust-gas mass flow is approximately reproducible after start in idle. For this reason, qzu(t) is viewed as being constant in the following:

qzu(t)=qzu=const   (A2)

[0037] Out-flowing Heat Quantity:

[0038] A portion of the heat quantity, which is stored in the E-catalytic converter, flows off from the E-catalytic converter in the form of a heat flow qab(t) because of the thermal drop between the temperature TKAT(t) in the E-catalytic converter and the ambient temperature TU.

qab(t)=r·(TKAT(t)−TU)   (A3)

[0039] Presentation of the Characterizing Differential Equation:

[0040] A change of the inner heat quantity QKAT(t) in the E-catalytic converter results exclusively from the in-flow and out-flow of thermal quantities: 3 ∂ Q KAT ⁡ ( t ) ∂ t = q zu - q ab ⁡ ( t ) (A4)

[0041] wherein: qab(t) from equation A3 is inserted into equation A4: 4 ∂ Q KAT ⁡ ( t ) ∂ t = ( q zu + rT U ) - rT KAT ⁡ ( t ) (A5)

[0042] wherein: TKAT(t) from equation A1 is inserted into equation A5: 5 ∂ Q KAT ⁡ ( t ) ∂ t + r · c · Q KAT ⁡ ( t ) = ( q zu + r ⁢   ⁢ T U ) (A6)

[0043] As a solution, one obtains: 6 Q KAT ⁡ ( t ) = [ T KAT ⁡ ( 0 ) c - ( q zu rc + T U c ) ] · exp ⁡ ( - rct ) + ( q zu rc + T U c ) ⁢ ⁢ and (A7) T KAT ⁡ ( t ) = [ T KAT ⁡ ( 0 ) - ( q zu r + T U ) ] · exp ⁡ ( - rct ) + ( q zu r + T U ) (A8)

[0044] wherein: TKAT(0) is the temperature in the E-catalytic converter at the start of the viewing time frame, that is, at t=0.

[0045] The block circuit diagram of FIG. 2 shows the described thermal E-catalytic converter model.

[0046] In FIG. 3, an exhaust-gas after-treatment system 10 of an internal combustion engine in accordance with the invention is shown having an exhaust-gas system 12 for conducting the exhaust gases A. An E-catalytic converter 14 is mounted in the exhaust-gas system 12. The E-catalytic converter 14 is driven by a control apparatus 16 for carrying out the method of the invention.

[0047] It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method for controlling an electrically heatable catalytic converter for after treating exhaust gases of an internal combustion engine, the method comprising the steps of:

deactivating or nonactivating said catalytic converter during the start of said engine when the temperature (TKAT(tabst)) of said catalytic converter exceeds a critical temperature TKRIT; and,

utilizing a temperature model to estimate said temperature (TKAT(tabst)) of said catalytic converter.

2. The method claim 1, wherein said temperature model determines said temperature (TKAT(tabst)) in dependence upon the duration of switchoff when starting said engine.

3. The method of claim 2, wherein said temperature model determines said temperature (TKAT(tabst)) when starting said engine in accordance with the following relationship:

TKAT(tabst)=[TKAT(o)−TU]·exp(−r·c·tabst)+TU

wherein:

TKAT(tabst) is the temperature of said catalytic converter after a switchoff duration (tabst) computed from a switchoff time point of said engine (t=0);

TKAT(0) is the temperature of the catalytic converter at the time point of the switchoff of said engine;

TU is the ambient temperature;

r is a thermal transition resistance; and,

c is a thermal proportionality constant.

4. The method of claim 3, wherein said switchoff duration (tabst) is read in via a combination instrument and/or is determined from a model of the cool-down characteristic of said engine.

5. The method of claim 4, wherein said ambient temperature (TU) is determined from the intake air temperature of said engine.

6. The method of claim 4, wherein a constant substitute value is used for said ambient temperature (TU).

7. The method of claim 3, wherein said temperature of said catalytic converter is derived from an exhaust-gas temperature model at the time point of switching off said engine.

8. The method of claim 3, wherein a constant substitute value is used for said temperature of said catalytic converter at the time point of switching off said engine.

9. A method for controlling an electrically heatable catalytic converter for after treating exhaust gases of an internal combustion engine, the method comprising the steps of:

deactivating said catalytic converter after a required heating duration (Tekat);

utilizing a temperature model to estimate said temperature (TKAT(tabst)) of said catalytic converter; and,

determining a required heating characteristic and/or said required heating duration (Tekat) from said temperature (TKAT(tabst)) during start of the engine and from the ambient temperature (TU).

10. The method of claim 9, wherein said heating characteristic and/or said heating duration (tekat) is derived from the following relationship:

7 T KAT ⁡ ( t ) = [ T KAT ⁡ ( 0 ) - ( q zu r + T U ) ] · exp ⁡ ( - rct ) + ( q zu r + T U )

wherein:

TKATAB(tekat) is the switchoff temperature of said catalytic converter after the required heating duration (tekat);

TKAT (0) is the temperature of the catalytic converter at the time point of the switchoff of said engine;

qzu is the entered heat in the catalytic converter because of the electric heating power thereof;

TU is the ambient temperature;

r is a thermal transition resistance; and,

c is a thermal proportionality constant.

12. The method of claim 11, wherein the entered heat (qzu) is determined by measuring the heating voltage and/or the heating current of the catalytic converter.

13. The method of claim 12, wherein a constant substitute value is used for said entered heat (qzu).

14. The method of claim 10, wherein said required heating duration (tekat) up to reaching said required switchoff temperature (TKATAB) is determined by the following relationship:

8 t ekat = 1 r · c · ln ⁡ ( T KAT ⁡ ( 0 ) - q zu r - T U T KATAB - q zu r - T U ).

15. The method of claim 10, wherein the required heating duration (tEKAT) up to reaching the needed switchoff temperature (TKATAB) is determined from continued computation of the catalytic converter temperature (TKAT) with the currently elapsed heating duration (t) and the subsequent comparison of the result to the switchoff temperature (TKATAB).

16. The method of claim 15, wherein the catalytic converter is deactivated or not activated when a diagnostic method diagnoses a fault.

17. The method of claim 16, wherein said fault is in the feed lines to said catalytic converter.

18. The method of claim 16, wherein said catalytic converter is deactivated or not activated when there is a danger that the catalytic converter can freeze up at low ambient temperature.

19. The method of claim 16, wherein said catalytic converter is deactivated or not activated when said engine stalls during the heating phase of said catalytic converter.

20. An exhaust-gas after treatment arrangement of an internal combustion engine having an exhaust-gas system, the arrangement comprising:

an E-catalytic converter mounted in said exhaust-gas system; and,

a control apparatus for carrying out the method including the steps of:

deactivating or nonactivating said catalytic converter when starting said engine when the temperature (TKAT(tabst)) of said catalytic converter exceeds a critical temperature TKRIT; and,

utilizing a temperature model to estimate said temperature (TKAT(tabst)) of said catalytic converter.