DE19753842C2 - Method for operating an exhaust gas catalytic converter for an internal combustion engine - Google Patents

Description

The invention relates to methods for operating an exhaust gas Talysators for an internal combustion engine according to the upper term fen of independent claims 1, 9 and 13.

The pollutant emission of an internal combustion engine can be through catalytic aftertreatment using a three-way Catalytic converter in connection with a lambda control device reduce effectiveness effectively. An important prerequisite for this is, however, that in addition to the lambda probe, the control device tion, even the catalytic converter's light-off temperature (light Off temperature). Below that temperature, in the typical automotive catalytic converters is approximately 300 ° C the catalyst is ineffective to ineffective and the reaction finds only with insufficiently low conversion rates («10%) instead of. To quickly reach the light-off temperature ensure and thus the emission of pollutants during the Cold start phase of the internal combustion engine, in which approx. 70 to 80% the total pollutants of HC and CO are emitted, Nevertheless, various warm-up strategies are to be reduced known.

Rapid heating of the catalyst can be increased the exhaust gas energy u. a. by retarding the ignition angle, Raising the idle speed, mixture enrichment, mixture dim or secondary air injection (e.g. DE 40 29 811 A, DE 41 32 814 A1, DE 41 33 117 A1). Another possibility possibility is to remove the catalytic converter with one electrical heating system powered by the motor vehicle equipment.  

Since the above-mentioned processes for heating the catalyst only may be active until the light-off temperature is reached, be it for reasons of increased emissions, driving comfort or the desired power of the internal combustion engine they are deactivated again after a certain time the. This can be done via time integration, for example the air mass and comparison with a constant threshold or via a fixed time. These well-known Measures do not lead to the ge in all operating areas wished success.

DE 43 23 243 A1 describes a needs-based Heizver drive for a catalyst in the exhaust system of an internal combustion engine known machine, in which a convertibility of the Ka Talysators is determined, and the catalyst depends on Convertibility is heatable. Depending on the operating conditions the internal combustion engine will be different measures men used to heat the catalyst. Lead the Heating measures within a predetermined first time interval valls or load integrals fail to achieve the desired success, so a diagnostic function is started. As part of the diagnosis particularly effective heating measures come into play Use if it occurs within a predeterminable second time tervalls or load integrals is not possible with these Measures a sufficient convertibility of the kata to reach lysators.

In a device according to DE 196 43 674 A1 for diagnosis damage a catalytic converter for cleaning Exhaust gases from a machine will affect the feasibility of the Diagnosis based on the catalyst temperature. A Value which, in the event of condensation within the cat tors is to be determined and a value without this condensation are used as reference heat quantities for cold start and warm start set. A catalyst temperature on normal machines run is calculated based on the running conditions of Ma seem. The amount of heat generated by the exhaust gases from the Kataly  sator is fed from the catalyst temperature and the amount of air absorbed. If the entire heat memenge that the catalyst after starting the machine too has been conducted equal to or less than a reference heat quantity, the estimated catalyst temperature will be on egg set a fixed value. In all other cases, the ge estimated temperature value of the catalyst by an equation calculated in which the catalyst temperature at normal Running conditions as parameters. That way the catalyst temperature is estimated with high accuracy become.

DE 43 30 997 A1 describes a method for monitoring the Starting behavior of a catalyst system in one force vehicle described. A temperature of an An jumping range of the catalyst system is detected, the An represent the area of the catalyst system tiert, which significantly influences the warm-up emissions. The An air-fuel mixture is supplied to the catalyst system and the impact that this supply of air Fuel mixture to the temperature of the light-off area has assessed the light-off behavior of the catalyst system. To assess the light-off behavior of the catalyst system the deviation of the temperature of the light-off area from a reference value and / or the first and / or the second time derivative of the temperature of the light-off area with Threshold values compared, the reference value being the tempera represents the light-off area in the event that Light-off area no conversion of air-fuel Mixture takes place.

The invention is therefore based on the object of methods specify with the help of criteria for ending the on heating measures for the catalyst can be determined.

The object of the invention is through the features of the unab dependent claims 1, 7 and 11 solved.  

The period of time within which the heating measures are active are, according to the invention, of the operating state of the internal combustion engine machine selected. For this, either the thermi cal energy used, the exhaust gas to the catalyst is supplied, or there are temperatures of the exhaust gas upstream and / or downstream of the catalyst or the Mo measured temperature itself, read from a map sen or by means of a physical temperature model calculates.

The temperature model is based on the energy balance of the Catalyst, the catalyst above the exhaust gas masses electricity supplied with a certain temperature, the energy transfer between the exhaust gas and the catalytic converter, the energy flow generated by the pollutant conversion, the exhaust gas discharged downstream of the catalyst Energy and that emitted by radiation and convection Energy is taken into account.  

The methods have the advantage of being reliable criteria to deliver the end of the heating measures and thus unnecessary warming of the Catalyst is avoided.

Advantageous developments of the invention are in the depend given claims.

The invention is explained in more detail below with reference to the figures purifies; show it:

Fig. 1 shows schematically the structure of an exhaust gas cleaning system and

Fig. 2 shows a physical model for calculating the monolith temperature and the outlet temperature after the pre-catalyst of the exhaust gas cleaning system.

Fig. 1 shows an internal combustion engine 2 with an injection system 7 , which is connected to an intake tract 1 and an exhaust tract 3 . In the exhaust tract 3 , a precatalyst 4 and a main catalyst 5 are provided separately therefrom in the flow direction. Seen in the flow direction of the exhaust gas (arrow symbol) in front of the pre-catalytic converter 4 is a first temperature sensor 11 and between the pre-catalytic converter 4 and the main catalytic converter 5 a second temperature sensor 12 is introduced into the exhaust gas tract 3 . A first lambda probe 19 is arranged in front of the pre-catalyst 4 in the gas tract 3 . In a known manner, it serves as a control probe for a lambda control, with the aid of which the fuel / air mixture is adjusted. After the main catalytic converter 5 , a second lambda probe 13 , also referred to as a monitor probe or trim probe, is arranged in the exhaust tract 3 .

A secondary air supply line 16 opens into the exhaust tract 3 between the internal combustion engine 2 and the pre-catalyst 4 . In the secondary air supply line 16 , a valve 8 , a secondary air pump 9 and a second air mass meter 10 are arranged.

In the intake tract 1 , a first air mass meter 6 is introduced, which is connected via a signal line to a control unit 15 which has a data memory 14 . The control unit 15 is connected via a data bus 17 to the internal combustion engine 2 and the injection system 7 . In addition, the control unit 15 is connected via signal lines to the second air mass meter 10 , the first temperature sensor 11 , the second temperature sensor 12 and the lambda sensor 13 . The secondary air pump 9 and the valve 8 are connected to the control unit 15 via control lines.

The control unit 15 controls the injection of the internal combustion engine 2 and the supply of the secondary air into the exhaust tract 3 as a function of the engine air mass supplied and the exhaust gas composition after the main catalyst 5 .

In addition, the control unit 15 takes over, among other things, the processing of the temperature signals from the sensors 11 and 12 , or it calculates the temperature values in the catalytic converter (monolith temperature) and downstream of the precatalyst from the value for the temperature in front of the precatalyst via a physical model. Various criteria for reaching the light-off temperature of the catalyst are derived from this. In particular, the temperature values obtained in this way are compared with predetermined limit values and, if these limit values are exceeded, the warm-up measures for catalyst heating are deactivated.

The following are three options, each as a criterion for ending the warm-up measures determines who that can.

Reaching the catalytic converter light-off temperature depends on the thermal energy supplied to the catalytic converter via the exhaust gas. After supplying a certain amount of energy, a corresponding catalyst temperature is established. Since part of the amount of energy supplied, in particular when the vehicle is driving, is released again via convection processes, the amount of energy supplied is corrected by a convection term which depends on the driving speed of the vehicle equipped with the internal combustion engine. The heating measures remain active until the amount of energy supplied exceeds a predetermined threshold:

With
_{Exhaust gas} as a mass flow from the intake air mass and the introduced secondary air mass of a certain size,
c _p as the specific heat capacity of the exhaust gas,
T _VVK as the exhaust gas temperature upstream of the pre-catalyst,
T as the time during which the catalyst heating measures are active
_Conv (v) as the energy flow (convection power) emitted by the catalyst by convection, which depends on the driving speed v and is determined to

_Kon (v) = A _O. k ₂ (v). (T _K - T _U )

being with
A _{O is} the outer _{surface of} the catalyst, with which constant k ₂ (v) denotes the heat transfer coefficient of the catalyst surface to the ambient air as a function of the vehicle speed v. The constant k ₂ is applied as a function of the vehicle speed.

The values for _Konv (v) are stored in a corresponding map KF1 of the data memory 14 .

SW1 as a threshold value, which is stored in the data memory 14 . It is determined by driving tests on the test bench.

With this energy balance, an energy release by heat measurement radiation should be disregarded, as this higher temperatures (approx. 500 ° C) has a noticeable influence.

Another option is temperature differences to use the catalyst as a criterion whether the light-off temperature of the catalyst is reached. For this, the Temperatures upstream and downstream of the pre-catalyst determined.

The temperature T _VVK upstream of the pre-catalyst 4 is either measured again by means of the temperature sensor 11 ( FIG. 1) or determined via a map KF2, which is stored in the data memory 14 as a function of at least one of the variables air ratio λ, air mass flow _LM in the intake tract, ignition angle IGA is laid down. These options for determining the temperature T _VVK also apply to the first-mentioned method.

The temperature T _NVK downstream of the pre-catalyst 4 is either measured by means of the temperature sensor 12 or determined via a physical temperature model described in more detail later.

The difference is formed from the two temperature values and this is compared with a threshold value SW2. If the threshold value SW2 is exceeded, the heating measures (warm-up strategies) are ended:

T _NVK - T _VVK <SW2

The threshold value SW2 is taken from a, depending on the exhaust gas mass flow, _{exhaust gas} and speed n of the internal combustion engine on a tensioned map KF3, which is stored in the data memory 14 . The exhaust _gas mass flow exhaust _gas is composed of the mass flow for the intake air mass _LM and the mass flow of the introduced secondary air _SLM :

_{Exhaust gas} = (1 + 1 / c _s ) _LM + _SLM

with c _s as the coefficient for the amount of fuel supplied.

Fig. 2 shows schematically the model with which different temperatures in or on the pre-catalyst can be calculated. The energy balance of the catalyst is derived from the heat energy supplied _in current, the current through the exhaust gas _exhaust masses having the inlet temperature T _presale, the Ka is supplied talysator. Energy is transferred from the exhaust gas to the catalytic converter in the catalytic converter. This is symbolically represented in FIG. 2 as a change over time in the energy transfer (power transfer) _{gas → cat} . The energy flow generated by the pollutant _conversion is called _Exoth . These two energy flows heat the catalyst to the catalyst temperature T _K. In addition, the catalyst performance are in the form of convection and radiation _{Conv wheel} to the environment. At the same time, an output (energy flow) of _{exhaust gas is} discharged from the catalyst with the _{exhaust gas} .

This temperature model is used to calculate both the temperature of the monolith T _k itself and the starting temperature of the exhaust gas T _NVK downstream of the pre-catalyst.

Starting from the energy balance equation, the starting temperature T _{NVK is} calculated from the temperature upstream of the pre-catalyst T _VVK and the exhaust _gas mass flow exhaust _gas , which the exhaust gas stream has after the pre-catalyst. The temperature T _VVK is measured with the first temperature sensor 11 in front of the pre-catalyst 4 or determined via the map KF2. The _{exhaust gas} mass flow from _{exhaust gas} is determined on the basis of the intake engine air and on the basis of the secondary air supplied, which is measured with the first air mass meter 6 and the second air mass meter 10 .

The pre-catalyst 4 the energy supplied current _in calculation net as follows:

_in = _{exhaust gas} . T _VVK . c _p , (1)

whereby with _{exhaust gas} the exhaust gas mass flow, with T _VVK the measured inlet temperature of the exhaust gas upstream of the pre-catalyst 4 and with c _p the specific heat capacity of the exhaust gas at constant pressure are designated.

The exhaust gas mass flow ( _{exhaust gas} ) is calculated using the following formula:

_{Exhaust gas} = (1 + 1 / C _S ). _LM + _SLM , (2)

where _{SLM is} the secondary air mass flow, _{LM is} the motor air mass flow and C _{S is} a coefficient for the supplied fuel quantity which has a value of 14.7 at λ = 1.

The power transfer (change over time of the energy transfer) _{gas → cat} from the exhaust gas to the catalyst monolith is described with the following equation:

_{Gas → Kat} = _{exhaust gas} . k ₁ . A _M (T _VVK - T _K ) (3)

where k _{1 is} the heat transfer coefficient from the exhaust gas to the catalyst monolith, A _M denotes the surface of the catalyst monolith around which exhaust gas flows and T _K denotes the temperature of the catalyst monolith.

According to the Boltzmann radiation law gives the radiated power from the pre-catalyst _wheel to:

_Rad = A _O. k _B. (T _K ⁴ - T _U ⁴ ) (4)

with A _O the outer catalyst surface, with K _B the Bolzmann constant, with T _U the ambient temperature and with T _K the catalyst temperature (monolith temperature).

By convection, a convection current is given off by the catalyst, which is described by the following equation:

_Conv = A _O. k ₂ (v). (T _K - T _U ) (5)

where with _Konv the convection power, with A _O the outer catalyst surface, with the constant k ₂ (v) the heat transfer coefficient of the catalyst surface to the ambient air depending on the vehicle speed v. The constant k ₂ is applied depending on the vehicle speed.

A first order differential equation for the temperature T _{K of} the catalyst results from the energy balance equation:

where m _{K denotes} the mass of the catalyst monolith and c _{K denotes} the specific heat capacity of the catalyst monolith.

It is assumed that the exothermic energy thermal energy of the monolith increased.

The energy flow ( _Exoth ) generated by the exothermic conversion of the pollutant _components in the catalytic converter is _stored in a map (KF4) of the data memory ( 14 ) depending on the monolith _temperature (T _K ).

Finally, the temperature downstream of the pre-catalyst is calculated from equations (3) and (6):

For an optimal temperature model for calculating the initial temperature T _NVK for the catalyst, temperature profiles of the catalyst are measured on the engine test stand and used to adapt the model parameters k ₁ , k ₂ and c _K accordingly, so that the error occurring between the temperature model and the actual heating of the catalyst is minimized.

A further criterion as to whether the heating measures initiated for the catalyst can be switched off again is the monolith temperature T _{K of} the catalyst. This can be measured either directly by means of a suitable temperature sensor 18 (shown in dashed lines in FIG. 1) in the pre-catalyst 4 , or be calculated using the physical model described above (equation (6)).

The value for T _K is compared to a threshold value SW3 and the heating measures are deactivated when the threshold value is exceeded:

T _K <SW3

The threshold value SW3 is again taken from a map KF5 spanned depending on the _{exhaust gas} mass flow _{exhaust gas} and speed n of the internal combustion engine and stored in the data memory 14 .

Claims (14)

1. A method for operating an exhaust gas catalytic converter for an internal combustion engine, in which at least one heating measure is initiated for accelerated catalytic converter heating and when the light-off temperature of the catalytic converter is reached it is deactivated again, characterized in that

• 1. the criterion for reaching the light-off temperature is the thermal energy (∫ _in (dt)) supplied to the catalyst ( 4 ) via the exhaust gas,
• 2. the supplied thermal energy (∫ _in (dt)) from the exhaust gas mass flow ( _{exhaust gas} ), the specific heat capacity of the exhaust gas (c _p ) and the temperature (T _VVK ) of the exhaust gas upstream of the catalyst ( 4 ) is determined,
  • 1. this value is corrected by means of a term ( _conv ) _dependent on the driving speed (v) of the vehicle equipped with the internal combustion engine, which takes into account the power output by convection, and
• 3. the heating measure is deactivated when the amount of energy supplied exceeds a predetermined threshold value (SW1).

2. The method according to claim 1, characterized in that the exhaust gas mass flow ( _{exhaust gas} ) from the internal combustion engine air mass flow ( _LM ) and by means of a Se kundärlufeinrichtung ( 8 , 9 , 16 ) introduced secondary air mass flow ( _SLM ) is determined. 3. The method according to claim 2, characterized in that the exhaust gas mass flow ( _{exhaust gas} ) is calculated according to the following equation:
_{Exhaust gas} = (1 + 1 / C _S ). _LM + _SLM ,
where C _S denotes a coefficient for the quantity of fuel supplied, which has a value of 14.7 at λ = 1. 4. The method according to claim 1, characterized in that the speed-dependent values for the convection power ( _Konv ) are _stored in a map (KF1) of a data memory ( 14 ) egg nes control unit ( 15 ) for the internal combustion engine. 5. The method according to claim 1, characterized in that the temperature (T _VVK ) by means of a temperature sensor ( 11 ) is measured. 6. The method according to claim 1, characterized in that the temperature (T _VVK ) via a map (KF2) depending on at least one of the sizes air ratio (λ), air mass flow ( _LM ) in the intake tract, ignition angle (IGA) is determined. 7. A method of operating an exhaust gas catalytic converter for an internal combustion engine, in which at least one heating measure is initiated for accelerated catalytic converter heating and deactivated when the light-off temperature of the catalytic converter is reached, characterized in that the temperature difference from the as a criterion for reaching the light-off temperature Temperature values at the input (T _VVK ) and at the output (T _NVK ) of the catalyst ( 4 ) is used, and the heating measure is deactivated when the temperature difference exceeds a predetermined threshold value (SW2). 8. The method according to claim 7, characterized in that the temperatures at the input (T _VVK ) and at the output (T _NVK ) of the Kata lysators ( 4 ) by means of temperature sensors ( 11 , 12 ) who measured the. 9. The method according to claim 7, characterized in that the temperature at the outlet (T _NVK ) of the catalyst ( 4 ) is calculated by means of egg nes based on the energy balance of the catalyst ( 4 ) temperature model, the catalyst ( 4 ) via the exhaust gas mass flow ( _{Exhaust gas} ) with the temperature (T _VVK ) supplied energy flow ( _in ) and the power transfer ( _{gas → cat} ) between the exhaust gas and the catalytic converter ( 4 ) is taken into account. 10. The method according to claim 9, characterized in that the temperature (T _NVK ) at the outlet of the catalyst ( 4 ) is calculated using the differential equation
with c _p as the specific heat capacity of the exhaust gas. 11. A method of operating an exhaust gas catalytic converter for an internal combustion engine, in which at least one heating measure is initiated for accelerated catalytic converter heating and when the light-off temperature of the catalytic converter is reached it is deactivated again, characterized in that

• 1. the temperature (T _K ) of the catalyst monolith is used as a criterion for reaching the light-off temperature,
• 2. the temperature (T _K ) of the catalyst monolith is calculated by means of a temperature model based on the energy balance of the catalyst ( 4 ), the catalyst ( 4 ) via the exhaust gas mass flow ( _{exhaust gas} ) with the temperature (T _VVK ) supplied energy flow ( _in ), the power transfer ( _{gas → cat} ) between the exhaust gas and the catalytic converter ( 4 ), the energy flow ( _Exoth ) generated by the pollutant _conversion , the energy flow ( _{exhaust gas} ) discharged downstream of the catalytic converter ( 4 ) and the radiation ( _conv ) and convection output ( _conv ) is taken into account, and
• 3. the heating measure is deactivated when this temperature (T _K ) exceeds a predetermined threshold value (SW3).

12. The method according to claim 11, characterized in that the temperature (T _K ) of the catalyst is calculated from the energy balance equation with the aid of the first order differential equation
where m _{K denotes} the mass of the catalyst monolith and c _{K denotes} the specific heat capacity of the catalyst monolith. 13. The method according to claim 12, characterized in that the value for the energy flow generated by the pollutant _conversion ( _Exoth ) depending on the monolith _temperature (T _K ) in a map (KF4) of a data memory ( 14 ) of a control unit ( 15 ) for the Internal combustion engine is stored. 14. The method according to claim 11, characterized in that the threshold value (SW3) in a, depending on the exhaust gas mass flow ( _{exhaust gas} ) and speed (s) of the internal combustion engine expanded map (KF5) is taken, which in a data memory ( 14 ) is filed.