DE10036942A1 - Modeling of axial distribution of exhaust catalyst temperature, slices catalyst into discs, calculating temperature of each as function of approaching exhaust gas temperature - Google Patents

Abstract

The catalyst is divided into n discs in its axial direction. The temperature of each is determined as a function of the approaching exhaust gas temperature. The radial temperature distribution is taken as constant. The adiabatic thermal transfer between the exhaust gas and the nth disc is calculated.

Description

The invention relates to a method for determining the temperature distribution of a Catalyst, in particular a NOx storage catalyst. Based on such Temperature distribution can be carried out in a corresponding process lean-burn engines are determined.

Today's lean-burn internal combustion engines, for example direct injection Generate internal combustion engines, whether direct-injection gasoline or diesel engines Environmentally harmful nitrogen oxides NOx during lean operation. These nitrogen oxides are during lean operation of the internal combustion engine in one of the Internal combustion engine downstream NOx storage catalyst stored. by virtue of The finite absorption capacity of the NOx storage catalytic converter becomes the stored one NOx with simultaneous reduction to nitrogen N2 through a short-term grease operation the internal combustion engine is released. To reduce fuel consumption and the emissions make the operating mode of the internal combustion engine as precise as possible to be able to control are as precise as possible knowledge of the conditions in the Emission control system necessary, for example at the time of the start and to determine the end of regeneration of a NOx storage catalytic converter.

Knowledge of the temperature profile in the catalytic converter can therefore help to fine-tune the operation of an internal combustion engine to the boundary conditions and limits of the exhaust gas purification system, in particular:

• - Enrichment of a gasoline engine in the upper part-load and full-load range for limitation the catalyst temperature;
• - Changing the operating mode for lean-burn gasoline engines (intake manifold injection with homogeneous lean operation, direct injection with homogeneous and stratified Lean operation) in the area of NOx thermal desorption of NOx storage catalysts (around 550 ° C, depending on the NOx storage catalyst system);
• - Take measures to increase exhaust gas and / or catalyst temperature Cooling of the catalyst close to the light-off temperature;  
• - Suspension of exhaust gas and / or catalyst temperature increasing measures Cooling down and subsequent heating of a sufficiently large area of the Catalyst, and
• - Ensure sufficient heating of a sufficiently large area of the Desulfurization catalyst.

For example, in the current state of the art, the catalyst temperature determined by measuring the exhaust gas temperature both upstream and downstream of the catalytic converter become. It can also be done by measuring the exhaust gas temperature upstream of the catalytic converter and Calculate a homogeneous catalyst temperature, mainly by analyzing the Heat transfer between the catalytic converter and exhaust gas can be determined. With that certain, mean catalyst temperature, however, are effects that occur in the dynamic Operation cannot occur due to inhomogeneous temperature distribution in the catalyst capture. For example, a medium, determined NOx Storage catalyst temperature near the NOx thermal desorption limit not detected whether very large actual temperature differences between the foremost and the rearmost catalyst zone already from the foremost Catalyst zone NOx is thermally desorbed and thus a NOx breakthrough takes place. Around Excluding such effects is demonstrated by the calculated catalyst temperature threshold that of lean-burn engines to avoid NOx breakthroughs is switched to lambda = 1 operation, a large safety distance from actual thermal desorption threshold. The result is higher time shares run in a less fuel-efficient operating mode than required. Analogous can also by determining the temperature profile in the NOx storage catalytic converter sufficient after a full load run with very high catalyst temperatures extensive cooling below the thermal desorption threshold can be determined, see above that at the earliest possible point in time from operation with lambda = 1 to lean operation can be switched.

It would therefore be advantageous to know the local location determined via a calculation Temperature distribution in the catalyst, ideally every infinitesimally small Catalyst element a temperature and resulting - if necessary in combination with additional input variables - specific conversion and storage properties to be able to assign. From the sum of the properties of each one Catalyst element results in a behavioral forecast of the overall catalyst, the  compared to considering the catalytic converter as an element, this is much more precise reflects real behavior of the catalyst. Time and extent of interventions in the operating mode of the internal combustion engine is then better based on the real one Behavior of the emission control system, since the intervention thresholds are closer to the critical sizes of the catalyst can be introduced.

The vehicle and catalytic converter manufacturers have had several in the past Programs for calculating the catalyst temperature and the Conversion behavior has been developed. The used very precisely imaging calculation algorithms are because of their structure and Scope not suitable for integration in engine control units because of Software scope, the hardware requirements, type and scope of the required Input variables and the computing times clearly above the possibilities of current ones and control units to be expected in the near future.

The invention is therefore based on the object of a method for calculating the To create temperature distribution that is much smaller and below Accepting a certain inaccuracy statements about the local Temperature distribution in the catalyst enables. Furthermore, a method for Control of a storage catalytic converter using this Calculation method specified.

The object is achieved by the method according to claim 1 and the use of the Method according to claim 14 solved. Preferred embodiments of the invention are Subject of the subclaims.

According to the inventive method for calculating the temperature distribution the radial temperature distribution is considered constant to simplify the system Assume that the catalyst is divided into n segments arranged one behind the other and determines the temperature of each segment. Here n is a natural number larger One. The segments assigned to each n can be of different lengths in order to to achieve better customization. By dividing the catalyst into n successive segments will be the number of segments to be calculated Catalyst elements limited to a practical level.  

The catalyst is preferably in n disks of 1 / n each of its total length divided. The number n can be between 1 and infinity; under real Calculation conditions will be between 2 and 50, advantageously between 3 and 12 in particular between 5 and 7.

A heat transfer between the panes takes place essentially only through the flowing gas instead. The axial heat conduction is therefore preferably zero set; however, it can be used for special applications in the following listed calculation algorithms are included.

First, the adiabatic heat transfer between the exhaust gas and the Catalyst disc and thus the one achieved by thermal energy transfer Temperature change of the exhaust gas and the catalyst disc calculated. Subsequently becomes dependent on the exhaust gas mass flow and the temperature of each individual Catalyst disc a locally variable implementation of the incoming pollutants accepted. The heat losses over the catalyst are approximately in Dependence on the temperature of the catalytic converter disc is taken into account.

To calculate the thermal heat transfer between exhaust gas and catalytic converter every single catalytic converter disc, the energy transfer due to pollutant conversion and the heat losses in the front pipe can be particularly advantageous in Appendix 1 and 2 described algorithms can be used.

The exhaust gas temperature in the flow of the catalytic converter is set by a temperature sensor the catalyst or through a temperature sensor behind one upstream pre-catalyst determined. In the latter case, the temperature loss must be above the exhaust pipe between the primary and the main catalytic converter are modeled.

To calculate the pollutant conversion and the resulting temperature increase Depending on the space velocity, a profile is first created that corresponds to the Implementation proportion of the residual pollutants on the individual catalyst disks indicates. While at low space velocities the reaction predominantly in the or the foremost catalytic converter disks, shifts with increasing Space velocity the reaction in the catalyst disks further back. At very high space velocities, even the catalyst can be wholly or partially  be overrun. With the temperature of the currently calculated catalytic converter disc from a map the conversion rate applicable for this calculation loop determined separately for each individual pollutant.

The HC, CO, NOx, O2 and H2 raw emissions can usually not be measured become. However, an approximate determination from a map is possible that from the stationary, warm emissions with additive or multiplicative Correction for dynamic processes and cold start effects the real emissions calculated. The map is determined in a known manner on an engine test bench.

As modern exhaust gas purification systems increasingly come from an engine small volume pre-catalyst and a downstream main catalyst exist, the main catalytic converter will mostly no longer work with the entire Raw emissions amount applied, but only with a partial amount. Depending on A raw emission share is determined for the space velocity of the pre-catalyst is not implemented by the pre-catalyst and thus acts on the main catalyst.

The temperature-dependent conversion behavior of the pre-catalyst after a Cold start does not have to be taken into account, since it can be assumed that the Pre-catalytic converter starts up faster due to its arrangement, coating and mass as the main catalyst and that when catalytic reactions begin on the Main catalytic converter the pre-catalytic converter its working temperature and thus its maximum temperature-dependent conversion rate has already been reached.

With the temperature and the space velocity - as described above - the Conversion rate of the catalytic converter disc calculated. The inflowing calculated Residual pollutants are wholly or partly under on the catalytic converter disc Consideration of the residual oxygen supply implemented. The energy balance at the The catalytic converter is then implemented again, including the chemical energy to calculate the outgoing adiabatic exhaust gas temperature to investigate. The exhaust gas and catalytic converter disc temperatures are still final corrected for heat loss.

A preferred embodiment of the invention is described below with reference to the only one Figure explained.

The figure shows schematically the flow chart of the calculation of the temperature profile a catalyst for a time step, the calculation frequency being 0.1 to 10 Hz, advantageously around 0.5 to 2 Hz and in particular 1 Hz is recommended.

Various are used to calculate the temperature distribution of the catalyst Input variables required, which are determined in two secondary calculations, the Input variables for calculating a slice i from the results of the calculation can depend on the temperature of the disc i-1.

The emissions are calculated prior to the primary catalytic converter in 4 out of the raw emissions size 1, that are taken from a characteristic field determined on a test bench, as well as a motor dynamic correction factor 2 and a coolant Korrekturtaktor. 3 The correction factors 2 and 3 mentioned are determined in a known manner. 5, the catalytic conversion of emissions from the calculated in 4 emissions calculated in the pre-catalyst, wherein this calculation, a further input variable is necessary, namely the determined at step 7 Conversion Rate of pre-catalyst for each pollutant, which is dependent on the space velocity of the exhaust gas , The space velocity of the exhaust gas is provided by block 6 . From made in block 5 Calculation of the catalytic conversion of the corrected in 4 1 raw emissions, emissions 8, the queue before the first catalyst disk results in the pressure upstream of the catalytic converter 8 emissions, specifically. These emissions are the HC, CO, NOx, H2 and O2 mass flows.

By measuring the air mass in step 9 and measuring the temperature in step 10 , the mass flow and the temperature of the exhaust gas downstream of the pre-catalyst are determined in step 11 . Using, for example, the one in Appendix 2 . The algorithm shown is calculated in step 12 of the influences of the front pipe and results in step 13 the temperature and the mass flow of the exhaust gas upstream of the first catalytic converter disk. In step 14 , the preliminary temperature t ′ _{Kat, 1 of} the first catalytic converter disk is determined from the data from the previous step 13 . This provisional temperature can be determined according to equation (3) given in Appendix 1.1. In step 15 , the temperature increase due to exothermic energy is calculated from the preliminary temperature of the first catalyst disk and the real temperature t _{Kat, 1 of} the catalyst disk and the temperature of the exhaust gas in the first catalyst _disk t _{Abg, 1 result} . One possibility of the calculation is given by the algorithms in Appendix 1.2. In order to be able to calculate the temperature increase due to exothermic energy, the emissions determined in step 8 are required before the first catalytic converter disk. Furthermore, the conversion rate of the first catalytic converter disc, namely the conversion rate of each pollutant present in the exhaust gas. These conversion rates are determined in step 16 , taking into account the space velocity-dependent conversion rate of the pre-catalytic converter (step 7 ) and the provisional temperature t ' _{Kat, 1 of} the first catalytic converter disk (step 14 ) from a characteristic diagram determined on an engine test bench. This concludes the determination of the temperature of the first catalytic converter disk.

In step 17 , the cooling of the exhaust gas is determined, for example in accordance with the algorithm on which equation 7 is based, and the temperature of the exhaust gas in front of the second catalytic converter disk t ' _{Abg, 2 is obtained} . Furthermore, the temperature increase calculated in step 15 is caused by exothermic reactions in the first catalyst disk. These reactions have an effect on the emissions in front of the second catalytic converter disk, so that in step 18 the emissions in front of the second catalytic converter disk are determined, in other words, the HC, CO, NOx, H2 and O2 mass flows. The procedure for the second and each additional slice is analogous to the calculation of the first slice. The meaning of the following method steps is therefore only summarized below, the calculation for three disks, namely 1, 2 and 3 or n, being shown in the figure.

In step 19 , the provisional temperature t ' _{Kat, 2 of} the second catalyst disk is determined; in step 20 the increase in temperature based on the exothermic energy is calculated (t _{Kat, 2} , t _{Abg, 2} ); Step 21 includes determining the conversion rates in the second catalyst disk for each pollutant; the cooling of the exhaust gas and thus the temperature t ′ exhaust _{, 3 of} the exhaust gas upstream of the third or nth disk is determined in step 22 ; in step 23 , the emissions in front of the nth catalytic converter disk are determined; Step 24 includes determining the preliminary temperature t ' _{Kat, n} ; Step 25 comprises the calculation of the temperature increase due to exothermic energy and thus the determination of t _{Kat, n} , and t _{Abg, n} ; in step 26 the conversion rates in the nth catalytic converter disk are determined for each pollutant; and in step 27 the exhaust gas cooling of the exhaust gas t ' _{Abg, n + 1} flowing out of the nth disk is determined.

attachment

The following definitions are made for the following considerations:
A _Kat catalyst surface
A _CRray surface catalyst _scanning (cladding tube)
A _{pipe pipe} surface
C _{p, Abg} specific heat capacity of the exhaust gas
C _{p, cat} specific heat capacity of the catalyst,
C _{p, pipe} specific heat capacity of the pipe
H _{U, pollutant} calorific value of the pollutants
m _Kat catalyst mass,
m _{pipe pipe} mass
_Pollutant mass flow
_Abg exhaust gas mass flow
R _K conversion rate
t _Exhaust gas temperature
_Kat catalyst temperature t
t ' _Kat catalyst temperature at the entry into the catalyst disc
t _U ambient temperature
t _pipe pipe temperature
α heat transfer coefficient
α _AK exhaust gas catalytic converter heat transfer coefficient
α _AR exhaust gas pipe heat transfer coefficient
α _KRU heat transfer coefficient catalyst _scanning environment
Δτ time interval
τ time

The indices used also have the following meaning:
'' Entry into the catalytic converter disc
i local running index
τ temporal running index

Calculation loop for a catalyst disk Calculation of the thermal adiabatic energy transfer between the Exhaust gas and the considered catalytic converter disc to determine the conversion rate the catalyst disc

In general, the heating of a catalyst can be described by the following differential equation (1):

The above differential equation (1) fulfills the following boundary condition:

t ' _Kat (τ = 0) = τ _U (2)

The temperature of the catalyst can be calculated in local (running index i) and temporal loops (running index τ) from the formal solution of the above differential equation taking into account the boundary condition. The heating results at the entrance to the catalyst disk i:

If the exhaust gas temperature changes over time, the running index τ is restarted for calculation and the previous catalyst temperature t _{Kat, i τ -1 is} used as a boundary condition. However, it is also possible to use the time interval Δτ and the boundary condition for the previous catalyst temperature. The heat transfer coefficient α is approximately determined using the known Nusselt number for the current flow state.

Calculation of energy transfer through pollutant conversion

With the aid of a preliminary catalyst temperature, a conversion rate R _{K of} the current catalyst disk can be determined from a map with the input data t ' _{Kat, i} and the local space velocity. When calculating the energy transfer due to the conversion of pollutants in the catalytic converter, both the existing residual oxygen content and the conversion in the pre-catalytic converter are taken into account. The temperature of the exhaust gas can be calculated in a known manner from the energy balance:

From this exhaust gas temperature, which takes into account the chemical conversions, the real temperature of the catalytic converter disc under consideration can be determined as follows:

The exhaust gas cools down and out of the energy balance due to the heat emission of the exhaust gas to the catalytic converter disk

the exhaust gas temperature at the next catalytic converter disc results:

Taking losses into account

Heat loss occurs on the catalyst surface, for example due to radiation or convection, so that a radial temperature gradient is formed. These heat losses are determined roughly. Again about an energy balance

the resulting temperature of the catalyst disc is then determined:

Calculation of the down tube

The heating of the front pipe, which is assumed to be isothermal, can be calculated as follows:

The calculation of the cooling of the exhaust gas can be carried out by applying equation (7) accordingly, in which equation the sizes of the catalyst are replaced by those of the front pipe. The temperature of the front pipe is reduced by heat being released to the surroundings, these losses being caused by convection or radiation. Neglecting the radiation losses and only taking the convection losses into account, the temperature of the front pipe is:

LIST OF REFERENCE NUMBERS

1

Raw emissions from map

2

Correction factor motor dynamics

3

Correction factor coolant temperature

4

Emissions before the pre-catalyst

5

Catalytic conversion in the pre-catalyst

6

Exhaust gas space velocity

7

Conversion rate of the pre-catalyst at working temperature

8th

Emissions in front of the first catalytic converter disc

9

Air mass measurement

10

Exhaust gas temperature measurement after pre-catalyst

11

Exhaust gas after the pre-catalyst

12

Calculation downpipe

13

Exhaust gas in front of the first catalytic converter disc

14

preliminary temperature of the first catalyst disk

15

Exothermic temperature of the first catalytic converter increases the temperature

16

Conversion rates of the first catalytic converter disk

17

exhaust gas cooling

18

Emissions in front of the second catalytic converter disc

19

preliminary temperature of the second catalyst disk

20

Exothermic temperature of the second catalytic converter disc increases the temperature

21

Conversion rates of the second catalyst disk

22

exhaust gas cooling

23

Emissions in front of the nth catalytic converter disc

24

provisional temperature of the nth catalyst disk

25

Exothermic temperature increase due to the nth catalytic converter disc

26

Conversion rates of the nth catalytic converter disk

27

exhaust gas cooling

Claims (15)

1. A method for determining the temperature of a catalyst of an internal combustion engine, characterized in that the catalyst is divided into n disks, n <1, in its axial direction and the temperature of each disk is determined as a function of the temperature of the exhaust gas flowing into the disk, the radial temperature distribution is assumed to be constant and an adiabatic heat transfer between the exhaust gas and the catalyst disk n is calculated. 2. The method according to claim 1, characterized in that each disc has one Has length of 1 / n of the total length of the catalyst. 3. The method according to claim 1, characterized in that the disks have different lengths 4. The method according to any one of the preceding claims, characterized characterized in that for n: 2 ≦ n ≦ 50. 5. The method according to claim 4, characterized in that for n applies: 3 ≦ n ≦ 12th 6. The method according to claim 4, characterized in that for n applies: 5 ≦ n ≦ 7th 7. The method according to any one of the preceding claims, characterized characterized in that the axial heat conduction is set to zero. 8. The method according to any one of the preceding claims, characterized characterized in that the temperature change of a catalyst disk by chemical conversion of the pollutants flowing onto a catalytic converter disc as locally variable function of the last calculated catalyst disc temperature and of the exhaust gas mass flow is calculated from the energy balance.   9. The method according to claim 8, characterized in that the Temperature change of a catalytic converter disc due to heat loss via the Catalyst as a function of the last calculated temperature of the catalyst disc n is determined. 10. The method according to claim 9, characterized in that the implementation share the pollutants for each catalytic converter disk as a function of space velocity is determined. 11. The method according to claim 10, characterized in that for each pollutant the conversion rate is determined separately. 12. The method according to claim 11, characterized in that the conversion rates the pollutants are taken from a map. 13. The method according to any one of the preceding claims, characterized characterized in that the calculation frequency for calculating a Temperature profile is between 0.1 to 10 Hz. 14. The method according to any one of the preceding claims, characterized characterized in that the exhaust gas temperature in the flow of the catalyst by a Temperature sensor is determined, the temperature loss of the exhaust gas over the Exhaust pipe between the location of the sensor and the entry of the exhaust gas into the Catalyst is taken into account in the calculation. 15. Use of the method according to one of the preceding claims Control of the operating mode of a lean-burn internal combustion engine in an engine control unit for determining the operating mode of the Internal combustion engine.