DE10036942B4 - Method for determining the temperature distribution of a NOx catalyst - Google Patents

Abstract

Method for determining the temperature distribution of a catalytic converter 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 in 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.

Description

The The invention relates to a method for determining the temperature distribution a catalyst, in particular a NOx storage catalyst and the use of the method for controlling the internal combustion engine. On the basis of such a temperature distribution can in a corresponding Method of operating mode in lean-burn internal combustion engines be determined.

today lean-burn internal combustion engines, for example direct injection Internal combustion engines, whether direct injection petrol or diesel engines, generate during of lean operation polluting Nitrogen oxides NOx. These nitrogen oxides are during lean operation of the Internal combustion engine in one of the internal combustion engine downstream NOx storage catalyst stored. Due to the finite absorption capacity of the NOx storage catalyst becomes the stored NOx with simultaneous reduction to nitrogen N2 released by a brief rich operation of the internal combustion engine. To help reduce fuel consumption and emissions To be able to control the operating mode of the internal combustion engine as precisely as possible, are preferably exact knowledge of the conditions necessary in the exhaust gas purification system, for example at the time the beginning and the end of the regeneration of a NOx storage catalyst to be able to determine.

• – Anreicherung eines Ottomotors im oberen Teillast- und Vollastbereich zur Begrenzung der Katalysatortemperatur;
• – Änderung des Betriebsmodus bei magerlauffähigen Ottomotoren (Saugrohreinspritzer mit homogenem Magerbetrieb, Direkteinspritzer mit homogenem und geschichtetem Magerbetrieb) im Bereich der NOx-Thermo-Desorption von NOxSpeicherkatalysatoren (um 550°C, je nach NOx-Speicherkatalysatorsystem);
• – Ergreifen von abgas- und/oder katalysatortemperatursteigernden Maßnahmen bei Auskühlung des Katalysators nahe an die Anspringtemperatur;
• – Aussetzen von abgas- und/oder katalysatortemperatursteigernden Maßnahmen nach Auskühlung und nachfolgendem Erwärmen eines hinreichend großen Bereichs des Katalysators, und
• – Sicherstellen einer hinreichenden Erwärmung eines ausreichend großen Bereichs des Katalysators bei der Entschwefelung.

Therefore, knowledge of the course of the temperature in the catalytic converter can contribute to tailoring the operation of an internal combustion engine more precisely to the boundary conditions and limits of the exhaust gas purification system, in particular mentioning:

• - Enrichment of a gasoline engine in the upper part-load and full load range to limit the catalyst temperature;
• - Modification of the operating mode for lean-burn gasoline engines (homogeneous lean-burn intake manifold injectors, direct lean and homogeneous lean-burn engines) in the field of NOx thermal desorption of NOx storage catalytic converters (around 550 ° C, depending on the NOx storage catalyst system);
• - Taking measures of exhaust gas and / or catalyst temperature-increasing measures when cooling the catalyst close to the light-off temperature;
• - Exposure of exhaust gas and / or catalyst temperature-increasing measures after cooling and subsequent heating of a sufficiently large range of the catalyst, and
• - Ensure sufficient heating of a sufficiently large area of the catalyst during desulfurization.

At the current For example, in the art, the catalyst temperature may be by measuring the exhaust gas temperature both before and after the Catalyst can be determined. It can also be done by measuring the exhaust gas temperature before the catalyst and calculating a homogeneous catalyst temperature, predominantly by analysis of the heat transfer between the catalyst and the exhaust gas. With the so determined mean catalyst temperature, however, are effects that in the dynamic Operation by inhomogeneous temperature distribution in the catalyst occur can, not to be detected. Thus, for example, by a mean, determined NOx storage catalytic converter temperature near the NOx thermo-desorption limit can not be detected, whether at very large actual Temperature differences between the foremost and the rearmost Catalyst zone already from the foremost catalyst zone NOx thermo-desorbed and thus a NOx breakthrough takes place. To exclude such effects, points the calculated catalyst temperature threshold, in the case of lean-running internal combustion engines to avoid NOx breakthroughs to lambda = 1 operation is switched, a big one Safety distance to the actual Thermo-desorption threshold on. As a result, higher time shares driven as required in a more fuel-efficient operating mode. Meaningfully, through a determination of the temperature profile in the NOx storage catalytic converter even after a full load drive with very high catalyst temperatures a sufficiently prolonged cooling below the thermo-desorption threshold be determined so that too one possible early Time switched from operation with lambda = 1 to lean operation can be.

Thus, it would be advantageous to know the local temperature distribution in the catalyst determined by means of a calculation, in order to be able to assign a temperature to each infinitesimal small catalyst element and, if appropriate, to be able to assign specific conversion and storage properties in combination with other input variables. From the sum of the properties of each individual catalyst element results in a behavioral prediction of the total catalyst, which compared to the consideration of the catalyst as an element much more precisely reflects the real behavior of the catalyst. The timing and extent of interventions in the operating mode of the internal combustion engine then depend better on the real behavior of the exhaust gas cleaning system, since the intervention thresholds closer to the critical sizes of Catalyst can be introduced.

by The vehicle and catalyst manufacturers are in the past several programs for calculating the catalyst temperature and the Conversion behavior has been developed. The ones used, However, very accurate mapping algorithms are because of their structure and scope are not suitable for integration in engine control units, because the software scope, the hardware requirements, type and scope the required input quantities and the computation times significantly over the possibilities current and in the near future expected ECUs are.

DE 197 52 271 A1 discloses a method and an apparatus for deriving and controlling the temperature of a _NOx trap arranged in the exhaust passage of an internal combustion engine downstream of a three-way catalyst and in which a temperature sensor is used to correct a temperature estimation model under relatively static conditions, so that an estimate of the NO _x is made possible -Fallentemperatur and the temperature of the three-way catalyst. In a second embodiment, the three-way catalyst for estimating the instantaneous catalyst temperature also has a temperature sensor.

Of the The invention is therefore based on the object, a method for calculation the temperature distribution to create, at much lower Extent and at the expense of some inaccuracy statements about the local Temperature distribution in the catalyst allows. Further, a method for controlling a storage catalyst using this Calculation method specified.

The The object is achieved by the method according to claim 1 and the use the method of claim 15 solved. Preferred embodiments The invention are the subject of the dependent claims.

According to the method of the invention for the calculation of the temperature distribution is to simplify the Systems assumed the radial temperature distribution to be constant, the catalyst is divided into n consecutively arranged segments and the temperature of each segment is determined. Where n is a natural number greater than one. The segments assigned to each n can differ be long to achieve a better fit. By subdivision of the catalyst in n successively arranged segments is the Number of catalyst elements to be calculated on a practice-oriented Limited measure.

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

One Heat transfer between the discs takes place essentially only by the gas flowing through instead of. The axial heat conduction is therefore preferably set to zero; However, it can be used for special applications in the included in the calculation algorithms listed below become.

First, will the adiabatic heat transfer between the exhaust and the catalyst plate and thus the through thermal energy transfer achieved temperature change calculated of the exhaust gas and the catalyst disk. Subsequently, will dependent the exhaust gas mass flow and the temperature of each individual catalyst plate a local variable implementation of the inflowing Pollutants accepted. The heat losses over the Catalyst become approximate dependent on taken into account by the temperature of the catalyst disk.

to Calculation of the thermal heat transfer between the exhaust gas and the catalyst on each individual catalyst plate, of the energy transfer by pollutant conversion and the heat losses in the front pipe can be particularly advantageously uses the algorithms described in Appendix 1 and 2 become.

The Exhaust gas temperature in the flow of the catalyst is provided by a temperature sensor the catalyst or by a temperature sensor behind a possibly ascertained upstream precatalyst. In the latter case, the temperature loss over the Exhaust pipe between the pilot and main catalyst can be modeled.

To calculate the pollutant conversion and the resulting increase in temperature, a profile is first created depending on the space velocity, which indicates the conversion proportion of the residual pollutants at the individual catalyst disks. While at low space velocities, the reaction predominantly takes place in the foremost catalyst disk (s), it shifts with increasing space speed the reaction into the catalyst disks located further back. At very high space velocities, even the catalyst can be completely or partially overflowed. With the temperature of the currently calculated catalyst disk, the conversion rate applicable for this calculation loop is determined separately for each individual pollutant from a characteristic diagram.

The HC, CO, NOx, O2 and H2 raw emissions usually can not be measured become. It is, however, an approximate Determination from a map possible, that from the stationary, warm-burning emissions with additive or multiplicative correction for dynamic operations and cold start effects calculates the real emissions. The map is determined in a known manner on an engine test bench.

There modern emission control systems increasingly from a close-coupled small-volume Pre-catalyst and a downstream main catalyst consist, becomes the main catalyst predominantly no longer charged with the total amount of Rogehissionsmenge, but only with a subset. Depending on the space velocity of the precatalyst, a raw emission content is determined, which is not is reacted by the precatalyst and thus the main catalyst applied.

The temperature-dependent Conversion behavior of the precatalyst after a cold start must be there not considered because it can be assumed that the precatalyst due to its arrangement, coating and mass starts faster than the main catalyst and that at Onset of catalytic reactions on the main catalyst of Pre-catalyst its working temperature and thus its maximum temperature-dependent Conversion rate has already reached.

With the temperature and the space velocity becomes - as described above - the conversion rate calculated the catalyst disk. The inflowing calculated residual pollutants be fully or partially observed on the catalyst plate implemented the residual oxygen supply. The energy balance at the Catalyst disk is then to recalculate, taking into account the converted chemical energy, around the outflowing Adiabatic exhaust gas temperature to determine. The exhaust gas and catalyst disk temperature be final still about the heat loss corrected.

A preferred embodiment The invention is explained below with reference to the single figure.

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

to Calculation of the temperature distribution of the catalyst will be different Input variables needed, the are determined in two sub-bills, whereby the input variables for Calculation of a slice i from the results of the calculation of Temperature of the disc i - 1 depend can.

From the size of raw emissions 1 , which are taken from a map determined on a test bench, as well as a motor dynamics correction factor 2 and a coolant correction factor 3 be in 4 the emissions calculated before the pre-catalyst. The mentioned correction factors 2 and 3 are determined in a known manner. In 5 will be out of the in 4 calculated emissions, the catalytic conversion of emissions in the pre-catalyst is calculated, for which calculation a further input variable is necessary, namely in the step 7 certain conversion rate of the precatalyst for each pollutant, which is dependent on the space velocity of the exhaust gas. The space velocity of the exhaust gas is from the block 6 provided. From the block 5 done calculation of the catalytic conversion of in 4 corrected raw emissions 1 results in the upcoming emissions before the catalyst 8th more precisely the emissions 8th , which are in front of the first catalyst disk. These emissions are the HC, CO, NOx, H2 and O2 mass flows.

By means of a measurement of the air mass in step 9 and measuring the temperature in step 10 will be in step 11 the mass flow and the temperature of the exhaust behind the precatalyst determined. By way of example in the appendix 2 , shown algorithm is in step 12 a calculation of the influences of the front pipe made and it results in the step 13 the temperature and the mass flow of the exhaust gas in front of the first catalyst plate. In step 14 will be from the data of the previous step 13 the provisional temperature t ' _{Kat, 1 determines} the first catalyst plate. The determination of this preliminary temperature can be made according to Equation (3) given in Annex 1.1. In step 15 is calculated from the preliminary temperature of the first catalyst plate, the temperature increase by exothermic and it results in 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} . One way of calculating is given by the algorithms in Appendix 1.2. In order to be able to calculate the increase in temperature due to exothermicity, the in step 8th determined emissions before the first catalyst plate needed. Further, the conversion rate of the first catalyst plate, namely the conversion rate of each pollutant present in the exhaust gas. These conversion rates are in the step 16 taking into account the space velocity-dependent conversion rate of the precatalyst (step 7 ) and the preliminary temperature t ' _{Kat, 1 of} the first catalyst disk (step 14 ) determined from a determined on an engine test bench map. The determination of the temperature of the first catalyst disk is thus completed.

In step 17 is determined, for example, according to the algorithm underlying the equation 7, the cooling of the exhaust gas and it results in the temperature of the exhaust gas in front of the second catalyst disk t ' _{Abg, 2} . Furthermore, the in step 15 calculated temperature increase caused by exothermic reactions in the first catalyst plate. These reactions have an effect on the emissions upstream of the second catalyst plate, so that in step 18 the emissions before the second catalyst disc are determined, in other words, before the second catalyst disc pending HC, CO, NOx, H2 and O2 mass flows. The method is carried out for the second and each further disc analogous to the calculation of the first disc. The following therefore only briefly summarizes the meaning of the following method steps, the calculation for three slices, namely 1, 2 and 3 or n, being shown in the figure.

In step 19 the determination of the provisional temperature t ' _{Kat, 2 of} the second catalyst disk is carried out; in step the increase of the temperature due to the exotherm is calculated (t _{Kat, 2} , t _{exhaust, 2} ); step 21 involves determining the conversion rates in the second catalyst plate for each pollutant; the cooling of the exhaust gas and thus the temperature t ' _{Abg, 3 of} the exhaust gas before the third or nth disc is in step 22 set; in step 23 the emissions are determined in front of the nth catalyst disk; step 24 includes the determination of the preliminary temperature t ' _{Kat, n} ; step 25 includes the calculation of the temperature increase by exotherm and thus the determination of t _{Kat, n} and t _{Abg, n} ; in step 26 the conversion rates in the nth catalyst plate are determined for each pollutant; and in step 27 the exhaust gas cooling of the effluent from the n-th disc exhaust gas t 'exhaust _{, n + 1 is} determined.

attachment

A_Kat

Katalysatoroberfläche

A_KRohr

Oberfläche Katalysatorcanning (Hüllrohr)

A_Rohr

Rohroberfläche

C_p,Abg

spezifische Wärmekapazität des Abgases

c_p,Kat

spezifische Wärmekapazität des Katalysators,

c_p,Rohr

spezifische Wärmekapazität des Rohres

H_U,Schadstoff

Heizwert der Schadstoffe

m_Kat

Katalysatormasse,

m_Rohr

Rohrmasse

ṁ_Schadstoff

Schadstoffmassenstrom

ṁ_Abg

Abgasmassenstrom

R_K

Konvertierungsrate

t_Abg

Abgastemperatur

t_Kat

Katalysatortemperatur

t'_Kat

Katalysatortemperatur am Eintritt in die Katalysatorscheibe

t_U

Umgebungstemperatur

t_Rohr

Rohrtemperatur

α

Wärmeübergangskoeffizient

α_AK

Wärmeübergangskoeffizient Abgas-Katalysator

α_AR

Wärmeübergangskoeffizient Abgas-Rohr

α_KRU

Wärmeübergangskoeffizient Katalysatorcanning-Umgebung

Δτ

Zeitintervall

τ

Zeit

For the following considerations, the following definitions are made:

A _cat

catalyst surface

A _KRohr

Surface catalyst scanning (cladding)

A _pipe

pipe surface

C _{p, exhaust}

specific heat capacity of the exhaust gas

_{cp, cat}

specific heat capacity of the catalyst,

c _{p, tube}

specific heat capacity of the pipe

H _{U, pollutant}

Calorific value of pollutants

_cat

Catalyst mass,

m _tube

pipe mass

ṁ _pollutant

Pollutant mass flow

ṁ _exhaust

Exhaust gas mass flow

R _K

conversion rate

t _outg

exhaust gas temperature

_cat

catalyst temperature

_cat

Catalyst temperature at the inlet of the catalyst plate

t _U

ambient temperature

t _pipe

pipe temperature

α

Heat transfer coefficient

α _AK

Heat transfer coefficient exhaust catalyst

α _AR

Heat transfer coefficient exhaust pipe

α _KRU

Heat transfer coefficient Catalyst scanning environment

Δτ

time interval

τ

Time

'

Eintritt in die Katalysatorscheibe

i

örtlicher Laufindex

τ

zeitlicher Laufindex

Furthermore, the indices used have the following meaning:

'

Entry into the catalyst disc

i

local running index

τ

temporal index

1. calculation loop for a catalyst disk

1.1 Calculation of the thermally adiabatic power transmission between the exhaust and the considered catalyst disk for detection the conversion rate of the catalyst disk

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) = t U (2)

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

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

1.2 Calculation of the energy transfer through the pollutant conversion

With the aid of a preliminary catalyst temperature, a conversion rate R _{K of} the current catalytic converter disk can be determined from a characteristic diagram with the input data t ' _{Kat, i} and the local space velocity. When calculating the energy transfer through the pollutant conversion in the catalyst, both the residual oxygen content present and the conversion in the precatalyst are taken into account. From the energy balance can be calculated in a known manner, the temperature of the exhaust gas:

From this exhaust gas temperature, which takes into account the chemical conversions, the real temperature of the considered catalyst disk can be determined as follows:

Due to the heat emission of the exhaust gas to the catalyst plate, the exhaust gas cools down and out of the energy balance m ' Abg · c p, Abg · t Abg, i = ṁ Abg · c p, Abg · T ' Abg, i + 1 + α AK · A Kat · (T Abg i - t Kat, i ) (6) If the exhaust gas temperature at the next catalyst disk is:

1.3 Consideration of losses

wird dann die sich ergebende Temperatur der Katalysatorscheibe ermittelt:

At the catalyst surface heat loss occur, for example by radiation or convection, so that forms a radial temperature gradient. This heat loss is determined by a rough estimate. Again about an energy balance

is then determined the resulting temperature of the catalyst plate:

2. Calculation of the pre-pipe

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

The calculation of the cooling of the exhaust gas can be made by a corresponding application of the equation (7), 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 emission to the environment, whereby these losses occur by convection or radiation. Neglecting the radiation losses and excluding the convection losses, the final result is a temperature of the pre-pipe of:

1

raw emissions from map

2

correction factor engine dynamics

3

correction factor Coolant temperature

4

emissions in front of the precatalyst

5

catalytic Reaction in the precatalyst

6

space velocity the exhaust gas

7

conversion rate of the precatalyst at working temperature

8th

emissions in front of the first catalyst disk

9

Air mass measurement

10

temperature measurement Exhaust after precatalyst

11

exhaust after precatalyst

12

calculation An exhaust pipe

13

exhaust in front of the first catalyst disk

14

preliminary temperature the first catalyst disk

15

Temperature increase by Exothermicity of the first catalyst disk

16

conversion rates the first catalyst disk

17

exhaust gas cooling

18

emissions in front of the second catalyst disc

19

preliminary temperature the second catalyst disc

20

Temperature increase by Exothermicity of the second catalyst plate

21

conversion rates the second catalyst disc

22

exhaust gas cooling

23

emissions in front of the nth catalyst disk

24

preliminary temperature the nth catalyst disc

25

Temperature increase by Exothermicity of the nth catalyst disc

26

conversion rates the nth catalyst disc

27

exhaust gas cooling

Claims (15)

Method for determining the temperature distribution of a catalytic converter 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 in 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. Method according to claim 1, characterized in that that each Slice a length of 1 / n of the total length of the catalyst. Method according to claim 1, characterized in that that the Slices of different lengths exhibit Method according to one of the preceding claims, characterized characterized in that for n: 2 ≤ n ≤ 50. Method according to claim 4, characterized in that that for n: 3 ≤ n ≤ 12. Method according to claim 4, characterized in that that for n: 5 ≤ n ≤ 7. Method according to one of the preceding claims, characterized characterized in that axial heat conduction is set equal to zero. Method according to one of the preceding claims, characterized characterized in that temperature change a catalyst plate by chemical reaction of a catalyst plate n oncoming Pollutants as local variable function of the last calculated catalyst disk temperature and the exhaust gas mass flow is calculated from the energy balance. Method according to claim 8, characterized in that that the temperature change a catalyst disk n by heat losses over the Catalyst as a function of the last calculated temperature of the Catalyst disk n is determined. Method according to claim 9, characterized that the Reaction rate of pollutants for each catalyst disk as Function of the space velocity is determined. Method according to claim 10, characterized in that that for everyone Pollutant separately, the conversion rate is determined. Method according to claim 11, characterized in that that the Implementation rates of pollutants are taken from a map. Method according to one of the preceding claims, characterized characterized in that Calculation frequency for calculating a temperature profile between 0.1 to 10 Hz. Method according to one of the preceding claims, characterized characterized in that Exhaust gas temperature in the flow of the catalyst determined by a temperature sensor is, wherein the temperature loss of the exhaust gas through the exhaust pipe between the place of the feeler and the entry of the exhaust gas into the catalyst in the calculation considered becomes. Use of the method according to one of the preceding claims for controlling 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.