WO2007011300A1

WO2007011300A1 - Arrangement and method for recirculation of exhaust gases in a combustion engine

Info

Publication number: WO2007011300A1
Application number: PCT/SE2006/050227
Authority: WO
Inventors: Kim KYLSTRÖM; Vesa Hokkanen
Original assignee: Scania Cv Ab (Publ)
Priority date: 2005-07-18
Filing date: 2006-06-29
Publication date: 2007-01-25
Also published as: SE0501702L; JP2009501876A; DE112006001857T5; SE528878C2

Abstract

The present invention relates to an arrangement and a method for recirculation of exhaust gases of a combustion engine (1). The arrangement comprises a valve device (V1, V2) arranged in the return line (9) upstream from the EGR cooler (11) with respect to the intended direction of exhaust gas flow in the return line (9), and a control unit (10) which in certain situations is adapted to placing the valve device (V1, V2) in a cleaning position in which a portion of the return line (9b) which comprises the EGR cooler (11) is connected to surrounding air (13) so that compressed air from the inlet line (6) flows through said portion of the return line (9b) and the EGR cooler (11) before it is led out to surrounding air (13) by means of the valve device (V1, V2).

Description

Arrangement and method for recirculation of exhaust gases of a supercharged combustion engine

BACKGROUND TO THE INVENTION, AND STATE OF THE ART

The present invention relates to an arrangement and a method for recirculation of exhaust gases of a supercharged combustion engine according to the preambles of claims 1 and.

The technique known as EGR (exhaust gas recirculation) is a known way of leading part of the exhaust gases from a combustion process in a combustion engine back, via a return line, to an inlet for supply of air to the combustion engine. A mixture of air and exhaust gases is thus supplied via the inlet line to the engine's cylinders in which the combustion takes place. Adding exhaust gases to the air causes a lower combustion temperature which results inter alia in a reduced content of nitrogen oxides NO_x in the exhaust gases. This technique is used for both Otto engines and diesel engines.

The return line for exhaust gases comprises inter alia an EGR valve which is settable to provide a desired amount of EGR. An electrical control unit is adapted to controlling the EGR valve on the basis, inter alia, of information concerning the load of the combustion engine. In the case of supercharged combustion engines, the exhaust gases are mixed with air which is at a relatively high pressure. When a rapid increase in engine load is required, the EGR valve closes to increase the flow of fresh air to the engine so that the required engine load can be reached quickly without any increase in soot emissions. A rapid reduction in engine load also requires closure of the EGR valve. The EGR valve closes to maintain the charge pressure of the air as long as possible so that it can be used for any possible subsequent rapid increase in engine load. The return line also comprises an EGR cooler adapted to cooling the exhaust gases in the return line before they mix with the air in an inlet line to the engine. In course of time, soot deposits from the exhaust gases inevitably build up on the inside surfaces of the EGR cooler, thereby impairing the heat transfer capacity of the EGR cooler and at the same time increasing the resistance to the flow of exhaust gases through the EGR cooler. Insufficient cooling of the exhaust gases leads inter alia to impaired engine performance. WO 2004/067945 refers to an arrangement for recirculation of exhaust gases of a supercharged combustion engine, which arrangement can in certain situations provide a backflow of air through an EGR cooler to clean the latter from soot deposits. Such a backflow of air can only be effected, however, in situations where the pressure of the exhaust gases is lower than the pressure of the compressed air in an inlet line. This pressure difference is also often relatively limited, resulting in a relatively small air flow which is not sufficient to clean the EGR cooler effectively.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an arrangement and a method of the kind mentioned in the introduction whereby the inside surfaces of the cooler are kept substantially free from soot deposits from the exhaust gases in an effective and simple manner.

This object is achieved with the arrangement of the kind mentioned in the introduction which is characterised by the features indicated in the characterising part of claim 1. Since the compressed air in the inlet line is at a relatively large overpressure compared with the pressure of the surroundings, it is possible to create a powerful flow of air from the inlet line to the surroundings via the EGR cooler when the valve device is placed in the cleaning position. Since the passages of the EGR cooler are dimensioned to accommodate and cool a relatively small amount of exhaust gases, such a powerful air flow exerts a strong action on the inside surfaces of the EGR cooler so that they are effectively cleaned from soot deposits.

According to a preferred embodiment of the present invention, the valve device comprises a first valve means adapted to controlling the flow of exhaust gases from the exhaust line to the return line. Such a first valve means may be a two-way valve of any desired kind. With advantage, the first valve means is a damper which can be placed in an open position whereby it allows exhaust gases from the exhaust line to pass through the return line, and in a closed position whereby it prevents exhaust gases from passing through the return line. Such a damper may also be placed in partly open positions so that the amount of exhaust gases returned through the return line can be controlled. The first valve means may be a conventional EGR valve. According to another preferred embodiment of the present invention, the valve device comprises a second valve means situated downstream from the first valve means with respect to the intended direction of exhaust gas flow in the return line, whereby the control unit is adapted to placing the second valve means in an open position so that it connects said portion of the return line which comprises the EGR cooler to the surrounding air when the valve device is in a cleaning position. Such a second valve means maybe arranged directly adjacent to the first valve means so that the two valve means are comprised within a combined valve device. Alternatively, a separate second valve means may be arranged at a suitable point in the return line between the first valve means and the EGR cooler. The second valve means may be a two-way valve of any desired kind.

According to another preferred embodiment of the present invention, the control unit is adapted to placing the valve device in the cleaning position in at least certain situations during operation of the combustion engine where no exhaust gases are returned through the return line and where the combustion engine requires substantially no supply of air. Since the cleaning of the EGR cooler need not normally be carried out immediately, it is advantageous to effect it in situations where the cleaning has substantially no effect on the combustion engine's normal operation and discharge of emissions. Such a situation is when the return line during operation is not used for returning exhaust gases, i.e. when the first valve means is in a closed position. The first valve means is normally closed at times when the load of the combustion engine is increasing rapidly or decreasing rapidly. During normal operation of a vehicle, rapid increases and decreases in the load of the combustion engine occur relatively frequently. There are therefore plenty of opportunities for cleaning the EGR cooler without affecting the recirculation of exhaust gases. To avoid disturbing the operation of the combustion engine, it is advantageous to carry out cleaning of the EGR cooler in situations where the combustion engine requires substantially no air supply. Such situations occur in connection with the vehicle's gear changes when the driver releases the accelerator pedal. Although such opportunities are relatively brief, they last long enough for it to be possible to bring about an air flow which blows the EGR cooler clean.

According to a preferred embodiment of the present invention, the arrangement comprises a sensor adapted to detecting a parameter related to the degree of deposits in the EGR cooler and to informing the control unit about the value of said parameter. Such a sensor may be a pressure sensor by which it is possible to determine the pressure drop of the exhaust gases in the EGR cooler. The pressure drop of the exhaust gases in the EGR cooler is a parameter which increases with the degree of soot deposits in the EGR cooler. An alternative sensor may be a flowmeter which measures the exhaust flow through the EGR cooler. The exhaust flow is a parameter which decreases with the degree of soot deposits in the EGR cooler. With advantage, the control unit is adapted to placing the valve device in a cleaning position after it has received values pertaining to said parameter which indicate that the degree of deposits in the EGR cooler exceeds a maximum acceptable value. Alternatively, the control unit may initiate a cleaning process of the EGR cooler at substantially specified intervals of time during the operation of the combustion engine.

The object indicated above is also achieved with the method of the kind mentioned in the introduction which is characterised by the features indicated in the characterising part of claim 8.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below by way of example with respect to the attached drawings, in which:

Fig. 1 depicts an arrangement for recirculation of exhaust gases of a supercharged combustion engine according to a first embodiment,

Fig. 2 depicts an arrangement for recirculation of exhaust gases of a supercharged combustion engine according to a second embodiment and

Fig. 3 depicts a flowchart of a method according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE

INVENTION

Fig. 1 depicts an arrangement for recirculating part of the exhaust gases of a supercharged combustion engine 1, which may be a diesel engine or an Otto engine.

Such recirculation is usually called EGR (exhaust gas recirculation). The combustion engine 1 is with advantage intended to power a heavy vehicle. Exhaust gases from the cylinders of the combustion engine 1 are led via an exhaust manifold 2 to an exhaust line 3. The exhaust gases in the exhaust line 3, which are at above atmospheric pressure, are led through a turbine 4. The turbine 4 is thus provided with driving power which is transmitted via a connection to a compressor 5. The compressor 5 thereupon compresses air which is led to the combustion engine 1 via an inlet line 6. The inlet line 6 comprises a charge air cooler 7 for cooling the compressed air before it is led, via a manifold 8, to the respective cylinders of the combustion engine 1. A return line 9 has the function of recirculating part of the exhaust gases from the exhaust line 3. The return line 9 comprises a valve device which comprises a first valve means V₁ and a second valve means v₂. An electrical control unit 10 is adapted to placing the valve means V₁, v₂ in desired positions during operation of the combustion engine 1. The control unit 10 may be a computer unit which is provided with software stored on a data carrier 10a. The return line 9 comprises an EGR cooler 11 for cooling the recirculating exhaust gases and an EGR mixer 12 which mixes the recirculating exhaust gases with the compressed air in the inlet line 6. The control unit

10 is intended to receive operation-related information 14 concerning the combustion engine 1 in order to control the valve means V₁, v₂ during operation.

The first valve means V₁ is here exemplified as a damper which can be placed in an open position when exhaust gases are to be returned through the return line 9 from the exhaust line 3 to the inlet line 6, and in a closed position when no exhaust gases are to be returned. The first valve means V₁ divides the return line 9 into a first portion 9a situated upstream from the valve means V₁ with respect to the intended direction of exhaust gas flow in the return line 9, and a second portion 9b situated downstream from the valve means V₁. The construction of the first valve means V₁ is such that it is possible to control its degree of opening steplessly so that a desired amount of exhaust gases can be returned through the return line 9. The first valve means vi may take the form of a conventional EGR valve. The second valve means v₂ is arranged in the second portion of the return line 9b at a position upstream from the charge air cooler

11 with respect to the intended direction of exhaust gas flow in the return line 9. The second valve means v₂ is normally held in a closed position during operation of the combustion engine 1. When the second valve means v₂ is placed in an open position, it connects the second portion of the return line 9b to surrounding air 13.

During operation of the vehicle, the control unit 10 substantially continuously receives operation-related information 14 concerning the combustion engine 1. On the basis of information about, for example, the fuel supply to the combustion engine 1, the control unit 10 can determine the load of the combustion engine. The information also comprises information about the engine's running speed n and other parameters which is required for regulating the amount of exhaust gases which is to be recirculated. When the combustion engine 1 is running at substantially constant load, the control unit 10 more or less holds the first valve means V₁ in an open position so that a suitable amount of exhaust gases from the exhaust line 3 is led through the return line 9 and mixes with the compressed air in the inlet line 6. During, for example, rapid changes in the load of the combustion engine, the control 10 unit closes the first valve means V₁ and thereby stops the recirculation of exhaust gases to the combustion engine 1. The control unit 10 closes the first valve means V₁ during operating situations where a rapid load rise is required by the combustion engine in order to increase the proportion of fresh air which is led, via the inlet line 6, to the combustion engine 1. The load of the combustion engine 1 is thus increased rapidly without increasing the amount of emissions. The control unit 10 also closes the first valve means V₁ during operating situations where the load of the combustion engine is reduced rapidly in order to maintain as long as possible the charge pressure of the air, which may be used during a subsequent rapid increase in the engine's load. Rapid load reduction occurs, for example, during a gearchange process in the vehicle when a driver of the vehicle releases the accelerator pedal.

The exhaust gases led through the return line 9, however, contain soot particles which while, passing through the EGR cooler 11, may well settle on the heat transfer surfaces of the EGR cooler 11, resulting in the formation of soot deposits. Soot deposits on the heat transfer surfaces of the EGR cooler 11 impair the cooler's ability to cool the exhaust gases. Insufficient cooling of the exhaust gases leads inter alia to impairment of the engine's performance. Soot deposits also obstruct the exhaust gas flow passages through the EGR cooler 11, thereby reducing the exhaust gas flow through the return line 9, which may lead to increased discharge of emissions from the combustion engine 1.

When the control unit 10 receives operation-related information 14 which indicates a rapid load decrease or a rapid load increase, the control unit 10 places the first valve means V₁ in a closed position so that no exhaust gases are returned through the return line 9. There is thus a possibility of placing the second valve means v₂ in an open position. In the open position, the second valve means v₂ connects the second portion 9b of the return line to surrounding air 13. When the first valve means V₁ is in a closed position and the second valve means v₂ is in an open position, the valve device is placed in a cleaning position. The cleaning position results in a powerful flow of air from the inlet line 6 through the second portion of the return line 9b and the EGR cooler 11 and out to the surroundings 13 via the second valve means v₂. The rapid air flow is initiated by the great difference in pressure between the compressed air in the inlet line 6 and surrounding air 13. The return line 9 is dimensioned to accommodate a considerably smaller amount of exhaust gases than the amount of compressed air which in this situation flows through the second portion of the return line 9b and the EGR cooler 11. This flow of fresh air passing at high velocity through the EGR cooler 11 in an opposite direction to that which occurs when exhaust gases are recirculated cleans its internal surfaces from soot deposits. The control unit 10 has the possibility of placing the second valve means v₂ in an open position whenever the first valve means V₁ is in a closed position. To avoid affecting the operation of the combustion engine 1, however, it is advantageous only to place the second valve means v₂ in an open position during situations where the combustion engine 1 requires substantially no air supply. Such a situation occurs inter alia during a gearchange process in the vehicle immediately before a gear is disengaged. During this relatively short time, the second valve means v₂ can be opened to provide a cleaning rapid air flow through the EGR cooler 11. The control unit 10 can place the second valve means v₂ in an open position whenever the first valve means V₁ is in the closed position and the combustion engine 1 requires no air supply. In many cases, however, it is not permissible to clean the EGR cooler 11 in each such operating situation and the control unit 10 may be preprogrammed to start cleaning after a specified number of opportunities, e.g. every tenth time such an operating situation occurs, or when a predetermined amount of time has passed since the latest cleaning.

Fig. 2 depicts an arrangement which corresponds to the arrangement described above but also comprises a pressure sensor 15 arranged in the second portion of the return line 9b downstream from the EGR cooler 11. The function of the pressure sensor 15 is to detect the pressure of the exhaust gases after they have left the EGR cooler 11. The pressure sensor 15 is adapted to sending to the control unit 10 a signal concerning measured pressure values. The control unit 10 is supposed to have knowledge of the exhaust gas pressure in the exhaust line 3 or the like so that the pressure drop of the exhaust gases through the EGR cooler 11 can be determined. The pressure drop of the exhaust gases is related to the degree of soot deposits in the flow ducts of the EGR cooler 11. To this end, the control unit 10 may compare estimated values for the pressure drop of the exhaust gases, when they have passed through the EGR cooler 11, with a reference value. When the estimated pressure drop exceeds the reference value, it is time to clean the EGR cooler 11. In this situation, the control unit 10 may place the second valve means v₂ in the open position as soon as an operating situation occurs where the first valve means V₁ is in the closed position and the combustion engine 1 requires no air supply. The result is a brief powerful air flow through the return line 9 and the EGR cooler 11 which effectively cleans their inside surfaces from soot deposits. The soot deposits are blown out to surrounding air 13 or are gathered in a filter or the like fitted close to the outlet of the second valve means v₂.

Fig. 3 depicts a flowchart of a method for controlling recirculation of exhaust gases. The method starts at 16. At 17, the control unit 10 receives operation-related information 14 and decides whether exhaust gases have to be returned through the return line 9 or not. If the combustion engine 1 receives information 14 which indicates substantially constant load of the combustion engine, the control unit 10 will find that there is nothing to prevent recirculation of exhaust gases through the return line 9. In this case, at 18, the control unit 10 places the first valve means vi in an open position and the valve means v₂ in a normal position. If the control unit 10 at the same time receives information from, for example, the pressure sensor 15 which indicates that the EGR cooler 11 needs cleaning, this may be left until a subsequent more convenient opportunity, since obstruction of an EGR cooler 11 is usually not so acute as to need remedying immediately. Thereafter the process starts again at 16.

If instead the control unit 10 receives information 14 which indicates a rapid increase or decrease in the load of the combustion engine 1, it will find that the recirculation of exhaust gases through the return line 9 has to be stopped. The control unit 10 will then, at 19, place the first valve means V₁ in a closed position. Thereafter the control unit 10 will decide, at 20, whether the EGR cooler 11 needs cleaning. The control unit 10 may make such a decision on the basis of information from the pressure sensor 15 or knowledge of when the latest cleaning was carried out. If no cleaning is needed, the control unit 10 will, at 21, continue to hold the second valve means v₂ in a closed position. Thereafter the process starts again at 16. If on the contrary the control unit 10 finds that cleaning of the EGR cooler 11 is needed, the valve means v₂ will be placed in an open position, at 22, thereby connecting the compressed air in the inlet line 6 to air 13 which is at ambient pressure. The pressure difference between the compressed air in the inlet line 6 and the surrounding air 13 results in a relatively large air flow at high velocity passing through the second portion of the return line 9b and the EGR cooler 11 so that the latter' s inside surfaces are cleaned from soot deposits. Such regular cleaning processes during operation make it possible for the combustion engine 1 to maintain stable performance over a long operating period without needing to carry out service operations in order to clean the EGR cooler 11.

All the process steps, and any desired subsequences of steps, described above can of course be controlled by a computer programme which is directly loadable to the internal memory of a computer and comprises suitable software for controlling the necessary steps when the programme is run on the computer. In addition, even if the embodiment of the invention described with reference to the drawings is software- controlled by means of a computer and processes performed by a computer, the invention also extends to a computer programme, particularly such a computer programme which is stored on a data carrier adapted to implementing the invention. The programme may be in the form of source code, object code, a code at a level between source and object code, e.g. in partly compiled form, or in whatever other form may be advantageous for use in implementing the method according to the invention. The data carrier may be any desired entity or device capable of storing the programme. For example, the data carrier may comprise a storage medium such as ROM (Read Only Memory), PROM (Programmable read-only memory), EPROM (Erasable PROM), Flash or EEPROM (Electrically EPROM). Moreover, the data carrier may take the form of a transferable carrier, such as an electrical or optical signal which can be transferred via an electrical or optical cable or by radio or in some other way. Where the programme is contained in a signal which can be carried directly via cable or other device or means, the data carrier may take the form of such a cable, device or equipment. Alternatively, the data carrier may be an integrated circuit in which the programme is stored, whereby the integrated circuit is adapted to performing, or being used in the performance of, relevant processes.

The invention is in no way limited to the embodiment referred to in the drawings but may be varied freely within the scopes of the claims. The pressure sensor depicted in Fig. 2 may be replaced by another sensor which detects a parameter related to the degree of soot deposits in the EGR cooler. The valve device need not comprise two separate valve means V₁, v₂ but may comprise a unitary valve of any desired kind which has a corresponding function. The invention is applicable to substantially all types of combustion engines where air is supplied at above atmospheric pressure to the combustion engine.

Claims

1. An arrangement for recirculation of exhaust gases of a supercharged combustion engine (1), whereby the arrangement comprises an inlet line (6) adapted to leading compressed air to the combustion engine (1), a return line (9) adapted to recirculating exhaust gases from an exhaust line (3) of the combustion engine (1) to a connection point (12) to the inlet line (6), and an EGR cooler (11) arranged in the return line (9) in order to cool the exhaust gases, characterised in that the arrangement comprises a valve device (V₁, v₂) arranged in the return line (9) upstream from the EGR cooler (11) with respect to the intended direction of exhaust gas flow in the return line (9), and a control unit (10) which in specified situations is adapted to placing the valve device (V₁, v₂) in a cleaning position in which the valve device (V₁, v₂) connects a portion of the return line (9b) which comprises the EGR cooler (11) to surrounding air (13) so that compressed air from the inlet line (6) flows through said portion of the return line (9b) and the EGR cooler (11) before it is led out to surrounding air (13).

2. An arrangement according to claim 1, characterised in that the valve device comprises a first valve means (V₁) adapted to controlling the flow of exhaust gases from the exhaust line (3) to the return line (9).

3. An arrangement according to claim 2, characterised in that the first valve means (V₁) comprises a damper which can be placed in an open position whereby it allows exhaust gases from the exhaust line (3) to pass through the return line (9), and in a closed position whereby it prevents exhaust gases from passing through the return line (9).

4. An arrangement according to claim 2 or 3, characterised in that the valve device comprises a second valve means (v₂) situated downstream from the first valve means (V₁) with respect to the intended direction of exhaust gas glow in the return line (9), whereby the control unit (10) is adapted to placing the second valve means (v₂) in an open position so that it connects said portion of the return line (9b) which comprises the EGR cooler (11) to surrounding air (13) when the valve device (V₁, v₂) is in a cleaning position.

5. An arrangement according to any one of the foregoing claims, characterised in that the control unit (10) is adapted to placing the valve device (V₁, v₂) in the cleaning position in at least certain situations during operation of the combustion engine (1) where no exhaust gases are returned through the return line (9) and where the combustion engine (1) requires substantially no supply of air.

6. An arrangement according to any one of the foregoing claims, characterised in that the arrangement comprises a sensor (15) adapted to detecting a parameter which is related to the degree of deposits in the EGR cooler (11) and to informing the control unit (10) about the value of said parameter.

7. An arrangement according to claim 6, characterised in that the control unit (10) is adapted to placing the valve means (V₁, v₂) in a cleaning position after it has received values concerning said parameter which indicate that the degree of deposits in the EGR cooler (11) exceeds a maximum acceptable value.

8. A method for recirculation of exhaust gases of a supercharged combustion engine (1), whereby the method comprises steps of leading compressed air to the combustion engine via an inlet line (6), of recirculating exhaust gases from an exhaust line (3) of the combustion engine (1) via a return line (9) to a connection point (12) to the inlet line, and of cooling the exhaust gases in the return line (9) in an EGR cooler (11), characterised by steps of arranging a valve device (vι , v₂) in the return line (9) upstream from the EGR cooler (11) with respect to the intended direction of exhaust gas flow in the return line (9) in the inlet line (6), and of placing the valve device (V₁, v₂), in specified situations, in a cleaning position in which the valve device (V₁ , v₂) connects a portion of the return line (9b) which comprises the EGR cooler (11) to surrounding air (13) so that compressed air from the inlet line (6) flows through said portion of the return line (9b) and the EGR cooler (11) before it is led out to surrounding air (13).

9. A method according to claim 8, characterised by the step of controlling the flow of exhaust gases from the exhaust line (3) to the return line (9) by means of a first valve means (V₁).

10. A method according to claim 9, characterised by steps of controlling the flow of exhaust gases from the exhaust line (3) to the return line (9) by means of a damper (V₁) which can be placed in an open position whereby it allows exhaust gases from the exhaust line (3) to pass through the return line (9), and in a closed position whereby it prevents exhaust gases from passing through the return line (9).

11. A method according to claim 9 or 10, characterised by the step of placing a second valve means (v₂), which is situated downstream from the first valve means (V₁) with respect to the intended direction of exhaust gas flow in the return line (9), in an open position so that it connects said portion of the return line (9b) which comprises the EGR cooler (11) to surrounding air (13) when the valve device (V₁, v₂) is in a cleaning position.

12. A method according to any one of claims 8-11 above, characterised by the step of placing the valve device (V₁, v₂) in the cleaning position in at least certain situations during operation of the combustion engine (1) where no exhaust gases are returned through the return line (9) and where the combustion engine (1) requires substantially no supply of air.

13. A method according to any one of claims 8-12 above, characterised by the steps of detecting a parameter which is related to the degree of deposits in the EGR cooler (11), and of informing the control unit (10) about the value of said parameter.

14. A method according to claiml3, characterised by the step of placing the valve device (V₁, v₂) in the cleaning position after values of said parameter have been detected which indicate that the degree of deposits in the EGR cooler (11) exceeds a maximum acceptable value.

15. A computer programme, directly loadable to the internal memory of the computer, comprising software for controlling the method according to any one of claims 8-14 when the programme is run on a computer.

16. A computer-readable medium (10a) on which a programme is stored which is suitable for enabling a computer to control the method according to any one of claims 8-14.