WO2014137269A1

WO2014137269A1 - Method and arrangement for exhaust aftertreatment at a combustion engine

Info

Publication number: WO2014137269A1
Application number: PCT/SE2014/050227
Authority: WO
Inventors: Magnus Mackaldener; Fredrik Swartling; Robert NORDENHÖK; Ola SANDSTRÖM; Marcus Larsson; Fredrik Brandt; Raymond Reinmann
Original assignee: Scania Cv Ab
Priority date: 2013-03-05
Filing date: 2014-02-25
Publication date: 2014-09-12
Also published as: DE112014000860T5; SE1350261A1; SE536981C2

Abstract

In a method for exhaust cleaning at a combustion engine, exhaust gases are caused to pass through a particulate filter which is part of a filter configuration and in which particles from the exhaust gases are oxidised. The method comprises the step of continuously assessing (S1 ) amounts of particles accumulated in the filter as a basis for preventing their uncontrollable exothermic oxidation in it, and the filter is provided with catalytic material for varying the rate of said oxidation. The step of continuously assessing amounts of particles accumulated in the filter comprises using a particulate prefilter which is part of the filter configuration and is provided with catalytic material for a lower oxidation rate in said prefilter. The method further comprises the step, as a basis for said assessment, of determining (S2) the occurrence of said uncontrollable exothermic oxidation by determination of temperatures of the prefilter, which oxidation will take place in the prefilter before it takes place in the filter. A device for exhaust cleaning of a combustion engine comprises similar parts to those mentioned above.

Description

METHOD AND ARRANGEMENT FOR EXHAUST AFTERTREATMENT AT A COMBUSTION ENGINE

TECHNICAL FIELD The invention relates to a method for exhaust cleaning of a combustion engine and a device for exhaust cleaning of a combustion engine according to the preambles of the independent claims. It relates also to a combustion engine and a motor vehicle.

BACKGROUND The purpose of particulate filters in exhaust systems of combustion engines is to capture and store soot so that it can be oxidised. It is desirable that the soot oxidation rate be such that there is a balance between soot inflow and the amount of soot being oxidised, in which case a state of equilibrium has been reached. Certain forms of engine operation do not meet the

requirements for achieving satisfactory oxidation of soot, mainly because sufficient exhaust temperatures and NO2 concentrations are not reached. In such situations the filter will store more soot than is oxidised away, so no state of equilibrium will be reached. If this situation is allowed to continue, the soot in the filter may well ignite and burn in an uncontrolled way, resulting in a soot exotherm and in the filter consequently being destroyed by the high temperature. A limit value for how much soot may be stored in the filter is therefore set to avoid risk of soot exotherm.

When there is risk of soot exotherm, i.e. when the amount of soot

approaches the limit value, a function of the engine is activated to

automatically push the exhaust temperature up so that the soot particles are oxidised, reducing the amount of soot.

One way of monitoring the amount of soot in a particulate filter is to measure the pressure difference across the filter while making an estimate of the pressure difference at which the limit for the amount of soot is reached. One problem is that the pressure difference across the filter depends on how the soot is stored, which varies inter alia with the rise and fall of the exhaust temperature. The result is a large uncertainty factor which means that an ample margin is required to avoid risk of soot exotherm, entailing activation of the engine's heating function at a relatively smaller amount of soot, with consequent increase in fuel consumption. An alternative variant is to estimate the amount of soot by a model which calculates how much of it is caught in the filter. This method is likewise subject to a large uncertainty factor and the consequent need for an ample margin, with similar

disadvantages to the variant of monitoring the pressure difference across the filter.

OBJECT OF THE INVENTION

One object of the invention is to propose a method for exhaust cleaning of a combustion engine whereby exhaust gases are caused to pass through a particulate filter which is part of a filter configuration and particles in the exhaust gases are oxidised, making it possible to assess amounts of particles accumulated in the filter more accurately.

SUMMARY OF THE INVENTION These and other objects indicated by the description set out below are achieved by a method and a device for exhaust cleaning and by a

combustion engine and a motor vehicle of the kinds indicated in the introduction which further present the features indicated in the characterising parts of the independent claims. Preferred embodiments of the method and and the device are defined in the dependent claims.

The invention achieves the objects with a method for exhaust cleaning of a combustion engine whereby exhaust gases are caused to pass through a particulate filter which is part of a filter configuration and in which particles from the exhaust gases are oxidised, comprising the step of continuously assessing amounts of particles accumulated in the filter as a basis for preventing uncontrollable exothermic oxidation of particles in it, and the filter is provided with catalytic material for varying the rate of said oxidation, which step of continuously assessing amounts of particles accumulated in the filter involves using a particulate prefilter which is part of the filter configuration and is provided with catalytic material for a lower oxidation rate in said prefilter, and the step, as a basis for said assessment, of determining the occurrence of said uncontrollable exothermic oxidation by determination of temperatures of the prefilter, which oxidation will take place in the prefilter before it takes place in the filter. More accurate assessment of amounts of particles accumulated in the filter is thus made possible as compared with estimation of their amounts by measurement of pressure difference across the filter or modelling of amounts of particles, since such a prefilter is better suited to the physics which govern the accumulation of particles in the filter and their oxidation. The resulting possibility of more accurate assessment of amounts of particles accumulated in the filter and the consequent reduction of the uncertainty factor allow larger amounts of particles to be present in it before the engine's heating function is activated, with consequent reduction in fuel consumption. In one embodiment of the method the step of determining said occurrence comprises determination of temperatures downstream of the prefilter. This makes it easy to determine the temperature at which uncontrollable exothermic oxidation takes place in the prefilter.

In one embodiment of the method the prefilter is configured to tolerate uncontrollable exothermic oxidation, making it possible to use the prefilter again after such oxidation has taken place in it.

In one embodiment of the method the particulate filter configuration is situated downstream of an oxidising catalyst which is configured for exhaust pretreatment. In one embodiment of the method said prefilter is situated upstream of said filter, making it possible to use existing means for determination of temperatures in the form of sensors for determining them downstream of the oxidising catalyst. In one embodiment of the method the prefilter is integrated with the filter, making it possible to accommodate the filter configuration in an efficient way with regard to space.

The invention achieves the objects with a device for exhaust cleaning of a combustion engine whereby exhaust gases are intended to pass through a particulate filter which is part of a filter configuration and in which particles from the exhaust gases are intended to be oxidised, comprising means for continuously assessing amounts of particles accumulated in the filter as a basis for preventing uncontrollable exothermic oxidation of particles in the filter, and the filter is provided with catalytic material for varying the rate of said oxidation, which means for continuously assessing amounts of particles accumulated in the filter comprise a particulate prefilter which is part of the filter configuration and is provided with catalytic material for a lower oxidation rate in said prefilter, and means, as a basis for said assessment, for determining the occurrence of said uncontrollable exothermic oxidation by determination of temperatures of the prefilter. More accurate assessment of amounts of particles accumulated in the filter is thus made possible as compared with estimation of their amounts by measurement of pressure difference across the filter or modelling of amounts of particles, since such a prefilter is better suited to the physics which govern the accumulation of particles in the filter and their oxidation. The resulting possibility of more accurate assessment of amounts of particles accumulated in the filter and the consequent reduction in the uncertainty factor allow larger amounts of particles to be present in it before the engine's warming function is activated, with consequent reduction in fuel consumption. In one embodiment of the device said means for determining said occurrence comprise means for determination of temperatures downstream of the prefilter. This makes it easy to determine the temperature at which uncontrollable exothermic oxidation takes place in the prefilter. In one embodiment of the device the prefilter is configured to tolerate uncontrollable exothermic oxidation.

In one embodiment of the device the filter configuration is situated

downstream of an oxidising catalyst which is configured for exhaust pretreatment. In one embodiment of the device the prefilter is situated upstream of said filter, making it possible to use existing means for determination of

temperatures in the form of sensors for determining them downstream of the oxidising catalyst.

In one embodiment of the device the prefilter is integrated with the filter, making it possible to accommodate the filter configuration in an efficient way with regard to space.

In one embodiment of the device the prefilter is situated in a space in said filter, making it possible to accommodate the filter configuration in an efficient way with regard to space. In one embodiment of the device said space is arranged to run right through the particulate filter, resulting in a pressure drop across the prefilter which is similar to that across the filter, and in particle accumulation in the prefilter which is likewise similar to that in the filter. The risk of the filter burning when there is uncontrollable exothermic oxidation in the prefilter is thus further reduced.

DESCRIPTION OF DRAWINGS The invention will be better understood by reading the detailed description set out below in conjunction with the attached drawings exemplifying the invention, in which the same reference notations are used for similar items throughout the various views and

Fig. 1 illustrates schematically a motor vehicle provided with a combustion engine and with devices for exhaust cleaning,

Figs. 2 - 5 illustrate schematically various devices for exhaust cleaning of a combustion engine, Fig. 6 illustrates schematically the particle oxidation rate as a function of the temperature of a particulate prefilter and a particulate filter which are parts of a filter configuration of a device for exhaust cleaning, and

Fig. 7 is a schematic block diagram illustrating a method for exhaust cleaning of a combustion engine. DESCRIPTION OF EMBODIMENTS

Fig. 1 illustrates schematically a motor vehicle 1 in the form of a heavy vehicle, more specifically a truck. The vehicle is provided with a device I; II; III; IV for exhaust cleaning which may be configured in various ways described in more detail below. Fig. 2 illustrates schematically a device I for exhaust cleaning of a

combustion engine 10 according to a first embodiment.

The device I for exhaust cleaning comprises a filter configuration 20 situated in an exhaust pipe 30 for exhaust gases F formed by combustion in an engine 10 which is connected to the exhaust pipe. The exhaust gases F are intended to pass through the filter configuration 20. The engine may for example be a diesel engine. The filter configuration 20 comprises a particulate filter 22 in which particles from the exhaust gases F are intended to be oxidised. The filter is provided with catalytic material for varying the rate of said oxidation. The filter is so configured, dimensioned and situated in the exhaust pipe 30 that particles in the exhaust gases F are captured by the filter.

The filter 22 may take the form of any suitable filter which can filter particles from exhaust gases, e.g. soot particles.

The filter configuration 20 further comprises a particulate prefilter 24 provided with catalytic material for a lower oxidation rate in the prefilter than the oxidation rate in the particulate filter 22. The catalytic material in the filter and the prefilter takes the form in one variant of noble metal, in one variant the form of so-called platinum group metals (PGM), and the amount of noble metal per unit volume in the filter is greater than in the prefilter. When the exhaust temperature rises, the oxidation rate will therefore increase more slowly in the prefilter than in the filter, as illustrated in Fig. 5.

The particulate prefilter 24 is configured to tolerate uncontrollable exothermic oxidation, making it possible for it to be used again after such oxidation has taken place in it. In one variant the prefilter is composed of material which comprises silicon carbide configured to tolerate uncontrollable exothermic oxidation. The particulate filter 22 in one variant is composed of material comprising cordierite, which is cost-effective and allows manufacture in relatively larger dimensions than with, for example silicon carbide.

The exhaust gases F in the exhaust pipe are arranged to pass through an oxidising catalyst 40 for exhaust pretreatment which is situated upstream of the filter configuration 20, comprising inter alia oxidation of hydrocarbons and oxidation of nitrogen monoxide to nitrogen dioxide. In one variant the catalyst is a hydrocarbon oxidation catalyst. In the embodiment illustrated in Fig. 2, the prefilter 24 of the filter

configuration 20 is situated upstream of the filter 22 and therefore

downstream of the oxidising catalyst 40.

The device comprises means for continuously assessing amounts of particles accumulated in the filter 22 as a basis for preventing uncontrollable exothermic oxidation of particles in the filter.

The means for continuously assessing amounts of particles accumulated in the filter 22 comprise the prefilter 24.

The means for continuously assessing amounts of particles accumulated in the filter further comprise means for determining the occurrence of said uncontrollable exothermic oxidation by determination of temperatures of the prefilter 24, comprising means 50 for determination of temperatures downstream of the prefilter. In one variant said means 50 comprise a temperature sensor. The fact that the prefilter 24 is provided with catalytic material for a lower oxidation rate in the prefilter than the oxidation rate in the filter 22 means that uncontrollable exothermic oxidation will take place in the prefilter before it takes place in the filter.

The means 50 for determination of temperatures downstream of the prefilter, here using sensors to monitor them, make it possible to determine the development of uncontrollable exothermic oxidation in the prefilter in order to prevent such oxidation in the filter.

The device I is so configured that when uncontrollable exothermic oxidation is detected by monitoring the temperature downstream of the prefilter 24, a heating function of the engine 10 is activated to warm the exhaust gases in the exhaust pipe 30 and thereby prevent such oxidation in the filter 22.

In a preferred variant the prefilter 24 is dimensioned substantially smaller than the filter 22. In one variant the volume of the prefilter is of the order of 1 per mil to 5 percent of that of the filter. In one variant the surface of the prefilter is adapted to receiving particles of the order of 1 to 10 percent of the surface of the filter. In one embodiment the prefilter is so dimensioned that the thermal energy arising in it does not substantially affect the filter. The fact that the prefilter is substantially smaller than the filter means that uncontrollable exothermic oxidation in the prefilter will result in a relatively high thermal peak but the resulting thermal energy transferred to the filter will be negligible in the context.

The prefilter 24 may be oriented in any suitable way relative to the filter 22. In one variant it is adapted to being brought into line with a central region of the filter in the extent of the exhaust pipe 30.

More accurate assessment of amounts of particles accumulated in the filter is thus made possible as compared with estimation of their amounts by measurement of pressure difference across the filter or modelling of amounts of particles, since such a prefilter is better suited to the physics which govern the accumulation of particles in the filter and their oxidation. The resulting possibility of more accurate assessment of amounts of particles accumulated in the filter and the consequent reduction of the uncertainty factor allow larger amounts of particles to be present in the filter before the engine's heating function is activated, with consequent reduction in fuel consumption.

The prefilter in the embodiment in Fig. 2 being situated upstream of the particulate filter 22 but downstream of the oxidising catalyst 40 makes it possible to use existing means 50 for determination of temperatures in the form of sensors for determining them downstream of the oxidation catalyst. Fig. 3 illustrates schematically a device II for exhaust cleaning of a

combustion engine 10 according to a second embodiment.

The device II in the embodiment illustrated in Fig. 3 differs essentially from the device I in the embodiment in Fig. 2 in the form of the filter configuration 120 and the position of the particulate prefilter 124 relative to the particulate filter 122.

In this embodiment the prefilter 124 is integrated with the filter 122. It is situated in a space 160 in the filter. In this variant the space 160 is in a central region of the filter.

In this embodiment the space 160 takes the form of a blind hole with its aperture facing towards the oxidising catalyst 40. The integration of the prefilter 124 with the filter 122 makes it possible to accommodate the filter configuration 120 in an efficient way with regard to space. The device in this embodiment comprises means 150 for determination of temperatures downstream of the prefilter 124, here comprising a temperature sensor 150 situated downstream of the prefilter. In this variant the sensor 150 is integrated with the filter. It is thus used beyond the temperature sensor 50 which is situated between the filter configuration 120 and the oxidising catalyst 40.

Fig. 4 illustrates schematically a device III for exhaust cleaning of a combustion engine 10 according to a third embodiment.

The device III in the embodiment illustrated in Fig. 4 differs essentially from the device II in the embodiment in Fig. 3 in the form of the space 260 in which the particulate prefilter 224 of the filter configuration 220 is situated.

In this embodiment the space 260 is arranged to run right through the particulate filter 222, and the prefilter 224 is situated in the part of the space 260 in the filter which is nearest to the oxidising catalyst 40. The risk of the filter burning when there is uncontrollable exothermic oxidation in the prefilter is thus reduced.

The device in this embodiment comprises means 250 for determination of temperatures downstream of the prefilter 224, here in the form of a sensor 250 situated downstream of the prefilter. In this variant the sensor 250 is situated downstream of the filter configuration 222, close to said continuous aperture 260. It is thus used beyond the temperature sensor 50 which is situated between the filter configuration 220 and the oxidising catalyst 40.

Fig. 5 illustrates schematically a device IV for exhaust cleaning of a combustion engine 10 according to a fourth embodiment.

The device IV in the embodiment illustrated in Fig. 5 differs essentially from the device III in the embodiment in Fig. 4 in the particulate prefilter 324 which is part of the filter configuration 320.

In this embodiment the space 360 is arranged to run right through the particulate filter 322, and the prefilter 324 is situated in the continuous space in the filter. The result is a similar pressure drop across the prefilter 324 to that across the particulate filter 322, and a similar particle accumulation in the prefilter to that in the filter. The risk of the filter burning when there is uncontrollable exothermic oxidation in the prefilter is thus further reduced. The device IV in this embodiment comprises means 350 for determination of temperatures downstream of the prefilter 324, here in the form of a sensor 350 situated downstream of the prefilter. In this variant the sensor 350 is situated downstream of the filter configuration 322, close to said continuous aperture 360. It is thus used beyond the temperature sensor 50 which is situated between the filter configuration 320 and the oxidising catalyst 40.

Fig. 6 illustrates schematically the particle oxidation rate v as a function of the temperature of particulate prefilters and particulate filters which form part of filter configurations of the device according to the invention. The curve P1 represents the oxidation rate as a function of the exhaust temperature T in the particulate filter, and the curve P2 the oxidation rate as a function of the exhaust temperature T in the particulate prefilter. When there is a rising exhaust temperature T, the oxidation rate v therefore increases more slowly in the prefilter than in the filter because of the prefilter being provided with catalytic material for a lower oxidation rate. Uncontrollable exothermic oxidation will thus take place in the prefilter before it takes place in the filter and it is possible, by monitoring the temperature downstream of the prefilter, to determine the development of uncontrollable exothermic oxidation in the prefilter with a view to preventing it in the filter. Fig. 7 is a schematic block diagram of a method for exhaust cleaning of a combustion engine according to one embodiment of the invention.

In one embodiment the method for exhaust cleaning of a combustion engine comprises a first step S1 in which amounts of particles accumulated in the filter are continuously assessed as a basis for preventing their uncontrollable exothermic oxidation in the filter, the latter being provided with catalytic material for varying the rate of said oxidation, comprising the use of a prefilter which is part of the filter configuration and is provided with catalytic material for a lower oxidation rate in said prefilter.

In one embodiment the method for exhaust cleaning of a combustion engine comprises a second step S2 in which the occurrence of said uncontrollable exothermic oxidation is determined, as a basis for said assessment, by determination of temperatures of the prefilter, which oxidation will take place in the prefilter before it takes place in the filter.

Claims

1 . A method for exhaust cleaning at a combustion engine whereby exhaust gases are caused to pass through a particulate filter (22; 122; 222; 322) which is part of a filter configuration (20; 120; 220; 320), particles from the exhaust gases are oxidised in said particulate filter (22; 122; 222; 322), comprising the step of continuously assessing (S1 ) amounts of particles accumulated in the filter as a basis for preventing their uncontrollable exothermic oxidation in it, and the filter (22; 122; 222; 322) is provided with catalytic material for varying the rate of said oxidation, characterised in that the step of continuously assessing amounts of particles accumulated in the filter (22; 122; 222; 322) comprises using a particulate prefilter (24; 124; 224; 324) which is part of the filter configuration (20; 120; 220; 320) and is provided with catalytic material for a lower oxidation rate in said prefilter (24; 124; 224; 324), and the step, as a basis for said assessment, of determining (S2) the occurrence of said uncontrollable exothermic oxidation by

determination of temperatures of the particulate prefilter (24; 124; 224; 324), which oxidation will take place in the prefilter (24; 124; 224; 324) before it takes place in the filter (22; 122; 222; 322).

2. A method according to claim 1 , in which the step of determining said occurrence comprises determination of temperatures downstream of the particulate prefilter (24; 124; 224; 324).

3. A method according to claim 1 or 2, in which the particulate prefilter (24; 124; 224; 324) is configured to tolerate uncontrollable exothermic oxidation.

4. A method according to any one of claims 1 -3, in which the particulate filter configuration (20; 120; 220; 320) is situated downstream of an oxidising catalyst (40) which is configured for exhaust pretreatment.

5. A method according to claim 4, in which the particulate prefilter (24) is situated upstream of the particulate filter (22).

6. A method according to claim 4, in which the particulate prefilter (124; 224; 324) is integrated with the particulate filter (122; 222; 322).

7. A device (I; II; III; IV) for exhaust cleaning of a combustion engine (10) whereby exhaust gases (F) are intended to pass through a particulate filter (22; 122; 222; 322) which is part of a filter configuration (20; 120; 220; 320), particles from the exhaust gases are intended to be oxidised in said filter (22; 122; 222; 322), comprising means for continuously assessing amounts of particles accumulated in the filter (22; 122; 222; 322) as a basis for preventing their uncontrollable exothermic oxidation in the filter (22; 122; 222; 322), and the filter is provided with catalytic material for varying the rate of said oxidation, characterised in that said means for continuously assessing amounts of particles accumulated in the filter (22; 122; 222; 322) comprise a particulate prefilter (24; 124; 224; 324) which is part of the filter configuration (20; 120; 220; 320) and is provided with catalytic material for a lower oxidation rate in said prefilter (24; 124; 224; 324), and means (50; 150; 250; 350) for determining, as a basis for said assessment, the occurrence of said uncontrollable exothermic oxidation by determination of temperatures of the prefilter (24; 124; 224; 324).

8. A device according to claim 7, in which said means for determining said occurrence comprises means (50; 150; 250; 350) for determination of temperatures downstream of the particulate prefilter (24; 124; 224; 324).

9. A device according to claim 7 or 8, in which the particulate prefilter (24; 124; 224; 324) is configured to tolerate uncontrollable exothermic oxidation.

10. A device according to any one of claims 7-9, in which the particulate filter configuration (20; 120; 220; 320) is situated downstream of an oxidising catalyst (40) which is configured for exhaust pretreatment.

1 1 . A device according to claim 10, in which the particulate prefilter (24) is situated upstream of the particulate filter (22).

12. A device according to claim 10, in which the particulate prefilter (124; 224; 324) is integrated with the particulate filter (122; 222).

13. A device according to claim 12, in which the particulate prefilter (124; 224; 324) is situated in a space (160; 260; 360) in the particulate filter (122; 222; 322).

14. A device according to claim 13, in which said space (260; 360) is arranged to run right through said particulate filter (222; 322).

15. A combustion engine (10) provided with a device (I; II; III; IV) according to any one of claims 7-14.

16. A motor vehicle (1 ) provided with a device (I; II; III; IV) according to any one of claims 7-14.