DE19753842A1 - Determining when to discontinue auxiliary heating of catalyst from cold start - Google Patents

Abstract

The criterion indicating achievement of initiation- or light-off temperature for catalytic action, is the time integral of thermal energy supplied by exhaust gases to the catalyst (4). Additional heating is shut off, once energy supplied exceeds a given threshold (SW1). Preferred Features: Thermal energy supplied is determined from: exhaust gas mass flowrate, its specific heat at constant pressure and the exhaust temperature upstream of the catalyst. Exhaust mass flowrate is determined from the inlet air mass flowrate induced directly by the engine, and from the secondary air mass flowrate. The latter is introduced by the secondary air injection system (8, 9, 16). A correction included, takes into account the quantity of fuel introduced. The calculated total energy supplied to the catalyst is corrected for convective heat losses, taking into account vehicle speed. For this purpose, corrections in terms of speed are stored in e.g. a look-up table in computer memory. A sensor (11) measures exhaust temperature. Alternatively its temperature is deduced from stored data, in terms of measured stoichiometry, air inlet flowrate, and ignition timing. In a similar method, the temperature reached by the catalyst is deduced from its measured inlet and outlet temperatures, to indicate when additional heating can be discontinued. A further related method is based on direct measurement of the catalyst substrate temperature. For each of the foregoing methods, the mathematical relationships are set out.

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.

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, 9 and 13 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 .

Between the internal combustion engine 2 and the pre-catalyst 4 , a secondary air supply line 16 opens into the exhaust tract 3. 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
_Konv (v) = A _O .k ₂ (v). (T _k -T _U ) where 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 depending on 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 NEN 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:

_Konv = 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 starting temperature T _NVK for the catalytic converter, temperature profiles of the catalytic converter were 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 (18)

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 is deactivated again when the catalytic converter light-up temperature is reached, characterized in that the criterion for reaching the light-off temperature is that of the exhaust gas Thermal energy (∫ _in (dt)) supplied to the catalytic converter ( 4 ) is used and 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 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. 3. The method according to claim 2, 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. 4. The method according to claim 3, 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 ,
C _s denotes a coefficient for the quantity of fuel supplied, which has a value of 14.7 at λ = 1. 5. The method according to any one of claims 1 to 4, characterized in that before the comparison with the threshold value (SW1), the energy supplied (∫ _in (dt)) by means of one of the driving speed (v) of the equipped with the internal combustion engine Corrected vehicle-dependent _terms ( _conv ) that takes into account the power output by convection. 6. The method according to claim 5, 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. 7. The method according to claim 2, characterized in that the temperature (T _VVK ) by means of a temperature sensor ( 11 ) is measured. 8. The method according to claim 2, characterized in that the temperature (T _VVK ) via a map (KF2) depending on at least one of the quantities air ratio (λ), air mass flow ( _LM ) in the intake tract, ignition angle (IGA) is determined. 9. 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 is deactivated again when the catalytic converter's light-off temperature is reached, characterized in that the temperature difference from the temperature values is used as a criterion for reaching the light-off temperature is used at the input (T _VVK ) and at the output (T _NVK ) of the catalyst ( 4 ) and the heating measure is deactivated when the temperature difference exceeds a predetermined threshold value (SW2). 10. The method according to claim 9, characterized in that the temperatures at the input (T _VVK ) and at the output (T _NVK ) of the catalyst ( 4 ) by means of temperature sensors ( 11 , 12 ) are measured. 11. The method according to claim 9, characterized in that the temperature at the outlet (T _NVK ) of the catalyst ( 4 ) 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 ) and the power transfer ( _{gas → Kat} ) between the exhaust gas and the catalytic converter ( 4 ) is taken into account. 12. The method according to claim 11, 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. 13. 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 is deactivated again when the catalytic converter's light-off temperature is reached, characterized in that the temperature (T _K ) of the catalyst monolith is used and the heating measure is deactivated when this temperature (T _K ) exceeds a predetermined threshold value (SW3). 14. The method according to claim 13, characterized in that the temperature (T _K ) of the catalyst monolith is measured by means of a temperature sensor ( 18 ). 15. The method according to claim 13, characterized in that 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 _discharged downstream of the catalytic converter ( 4 ) from the exhaust gas ( _{Exhaust gas} ) and the power emitted by radiation ( _conv ) and convection ( _conv ) is taken into account. 16. The method according to claim 15, characterized in that from the energy balance equation, the temperature (T _K ) of the catalyst is calculated using 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. 17. The method according to claim 12 or 16, characterized in that the value for the energy flow _generated by the pollutant _conversion ( _Exoth ) is _stored depending on the monolith _{temperature of} a control unit ( 15 ) for the internal combustion engine. 18. The method according to claim 13, 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 spanned map (KF5) is taken, which in a data memory ( 14 ) is filed.