WO2007011299A1

WO2007011299A1 - Arrangement and method for recirculation of exhaust gases in a combustion engine

Info

Publication number: WO2007011299A1
Application number: PCT/SE2006/050225
Authority: WO
Inventors: Kim KYLSTRÖM
Original assignee: Scania Cv Ab (Publ)
Priority date: 2005-07-18
Filing date: 2006-06-28
Publication date: 2007-01-25
Also published as: JP2009501875A; DE112006001858T5; SE0501701L; SE528881C2

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 means (S2) arranged in the inlet line (6), a line (13) extending from the valve means (S2) to a connection point (9a) to the return line, which connection point (9a) is situated 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 means (S2) in a cleaning position so that at least part of the compressed air in the inlet line (6) is led via the valve means (S2) and said line (13) to the return line (9), whereby the air is led in such a direction that it flows through the EGR cooler (11).

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, hi 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. With such a valve means arranged in the inlet line it is possible to lead the compressed air from the inlet line to the return line and on through the EGR cooler. As the air flow in the inlet line is considerably greater than the normal exhaust flow through the return line, the result is an air flow of such magnitude and with such force through the EGR cooler that the latter' s inside surfaces are effectively cleaned from soot deposits. The valve means is preferably placed in such a position as to lead the whole air flow in the inlet line to the return line, resulting in maximum air flow and optimum cleaning of the return line and the EGR cooler.

According to a preferred embodiment of the present invention, the return line comprises an EGR valve adapted to controlling the flow of exhaust gases from the exhaust line to the return line, and the connection point of said line to the return line is situated downstream from the EGR valve but upstream from the EGR cooler with respect to the intended direction of exhaust gas flow in the return line. It is therefore possible with the EGR valve in a closed position to guide the air flow in the return line in such a direction that the whole air flow passes through the EGR cooler. With advantage, the control unit is adapted to placing the valve means in the cleaning position during operating states of the combustion engine where the EGR valve is closed. Since no exhaust gases are led into the return line when the EGR valve is closed, it is advantageous to switch to cleaning the EGR cooler in such situations. Thus cleaning the EGR cooler does not result in any increase in emissions from the combustion engine as compared with those during conventional operation. The EGR valve 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 exhaust gas recirculation process.

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 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 means in a cleaning position in situations where it receives 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 specified intervals of time during the operation of the combustion engine.

According to another preferred embodiment of the present invention, the control unit is adapted to receiving information concerning the speed of the combustion engine and the control unit is adapted to placing the valve means in said cleaning position and the EGR valve in an open position if the speed of the combustion engine exceeds a maximum acceptable speed value. Placing the valve means and the EGR valve in the aforesaid positions results in insufficient air being led to the combustion engine's cylinders for the fuel to ignite. The combustion engine is subjected to an emergency stop and the risk of damage which might be caused by overspeed of the engine is eliminated. A further function of the valve means is thus effected.

According to another preferred embodiment of the present invention, the inlet line comprises a compressor which compresses the air in the inlet line, and the valve means is situated downstream from the compressor with respect to the direction of air flow in the inlet line. For it to be possible for compressed air to be led through the EGR cooler, it is necessary that the valve means be situated downstream from the compressor. With advantage, the inlet line comprises a charge air cooler and the valve means is situated upstream from the charge air cooler with respect to the direction of air flow in the inlet line. Compressed air is thus led at high velocity through the EGR cooler. Warm compressed air cleans the EGR cooler more effectively than compressed air which has been cooled in the charge air cooler.

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 9.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is 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 the arrangement in Fig. 1 during cleaning of an EGR cooler,

Fig. 3 depicts the arrangement in Fig. 1 during an emergency stop of the combustion engine,

Fig. 4 depicts an arrangement for recirculation of exhaust gases of a supercharged combustion engine according to a second embodiment and

Fig. 5 depicts a flowchart of a method according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figs. 1-3 depict 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 is intended to recirculate part of the exhaust gases from the exhaust line 3. The return line 9 comprises an EGR valve in the form of a first settable damper S₁ by which the exhaust flow in the return line 9 can, when necessary, be shut off. To some extent the first damper S₁ can also be used for controlling the amount of exhaust gases which is led through the return line 9. An electrical control unit 10 is adapted to placing the first damper S₁ in a desired position 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 exhaust gases led through the return line 9 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 an increased discharge of emissions from the combustion engine 1. A valve means in the form of a second damper S₂ is arranged in the inlet line 6 at a position between the compressor 5 and the charge air cooler 7. A line 13 extends between the second damper S₂ and a connection point to the return line 9. The connection point to the return line 9 is situated between the first damper S₁ and the EGR cooler 11. The electrical control unit 10 is adapted to receiving operation-related information 14 concerning the combustion engine 1 to make it possible to control the first damper S₁ and the second damper s₂.

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. When the combustion engine 1 is running at substantially constant load, the control unit 10 holds the first damper S₁ in an open position, resulting in a suitable amount of exhaust gases from the exhaust line 3 being led through the return line 9 and mixing with the compressed air in the inlet line 6. m certain operating situations, however, the control unit 10 closes the first damper S₁, e.g. during rapid changes in the load of the combustion engine. The control unit 10 closes the first damper S₁ during rapid load increase in order to increase the proportion of fresh air led via the inlet line 6 to the combustion engine 1. Load increase is thereby achieved quickly without increasing the amount of emissions. The control unit 10 closes the first damper S₁ during a rapid load decrease in order to maintain as long as possible the charge pressure of the air, which may be used for a subsequent rapid increase in the engine's load. Rapid load reduction occurs inter alia during a gearchange process in the vehicle when a driver of the vehicle releases the accelerator pedal.

Fig. 1 depicts the arrangement at a time when the control unit 10 receives operation- related information 14 which indicates that the combustion engine is running at substantially constant load. The control unit 10 has placed the first damper si in an open position so that exhaust gases are returned from the exhaust line 3 to the inlet line 6 via the return line 9. The control unit 10 has placed the second damper s₂ in a normal position so that all the compressed air compressed by the compressor 5 in the inlet line 6 is guided by the second damper S₂ to the charge air cooler 7 before the air mixes with the returned exhaust gases in the EGR mixer 12.

Fig. 2 depicts the arrangement 1 at a time when the control unit 10 has received operation-related information 14 which indicates a rapid load reduction. A driver of the vehicle may here have released the accelerator pedal in order to engage a different gear in the combustion engine's gearbox. In this situation, the control unit 10 stops the recirculation of exhaust gases through the return line 9 by placing the first damper S₁ in a closed position. The control unit 10 will now have the possibility of placing the second damper S₂ in a cleaning position so that compressed air from the compressor 5 is led to the return line 9. In the cleaning position, the compressed air is led through the line 13 before it reaches the return line 9 at a position between the first damper Si and the EGR cooler 7. As the first damper S₁ is closed, the air can only continue to flow in one direction through the return line 9. The air thus passes through the EGR cooler 7. The return line 9 is dimensioned to allow only a considerably smaller amount of exhaust gases to pass through it than the amount of compressed air which in this situation flows through the return line 9 and the EGR cooler 7. This large flow of fresh air flowing through the EGR cooler 11 at high velocity effectively cleans the EGR cooler's inside surfaces from soot deposits. The control unit 10 has the possibility of placing the second damper s₂ in a cleaning position each time the first damper S₁ is placed in a closed position. The control unit 10 may be adapted to placing the second damper s₂ in a cleaning position at predetermined intervals, e.g. the control unit 10 may place the second damper s₂ in a cleaning position every tenth time the first damper S₁ is placed in the closed position. The control unit 10 may also be adapted to placing the second damper S₂ in a cleaning position at specified intervals of time in order to clean the EGR cooler 11. When such an interval of time has passed, the control unit 10 may place the second damper S₂ in the cleaning position on the next occasion when the first damper S₁ is closed. Alternatively, the control unit 10 may close the first damper S₁ directly and place the second damper s₂ in a cleaning position when such an interval of time has passed, irrespective of the combustion engine's load.

Fig. 3 depicts the arrangement at a time when the control unit 10 has received information 14 concerning the speed n of the combustion engine 1 which indicates that the combustion engine 1 is racing. Engine racing may be regarded as occurring when the engine's speed n exceeds a maximum acceptable engine speed n_max. When the control unit 10 receives information which indicates that the engine's speed n exceeds the maximum acceptable engine speed n_max, the control unit 10 places the first damper Si in an open position and the second damper S₂ in a cleaning position, thereby guiding the compressed air in the inlet line 6 to the return line 9 via the second damper s₂. Since the first damper S₁ is in an open position, the air is led mainly towards the exhaust line 3, resulting in so little air being led to the combustion engine 1 that combustion in the combustion engine's cylinders becomes impossible. The combustion engine 1 is thus subjected to an emergency stop and damage due to overspeed can be avoided.

Fig. 4 depicts an arrangement equipped with a pressure sensor 15 arranged in the return line 9 downstream from the EGR cooler 11. The pressure sensor 15 detects 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 damper s₂ in the cleaning position during a subsequent operating situation where the first damper S₁ is closed, resulting in an abundant air flow at high velocity through the return line 9 and the EGR cooler 11, thereby effectively cleaning the inside surfaces of the EGR cooler 11 from soot deposits.

Fig. 5 depicts a flowchart of the function of the arrangement as above. The process starts at 16. At 17, the control unit 10 receives information 14 concerning the combustion engine's speed n. The control unit 10 compares whether the speed n is below or equal to a maximum acceptable speed n_max. At 18, if the speed n is greater than the maximum acceptable speed n_max, the control unit opens the first damper S₁ if it is not already open and places the second damper s₂ in the cleaning position, resulting in an emergency stop of the combustion engine 1 so that overspeed of the combustion engine 1 is avoided. Thereafter the process starts again at 16.

If the control unit 10 finds that the speed n is acceptable, the process continues at 19, where the control unit 10 receives 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 situation, at 20, the control unit 10 places the first damper Si in an open position and the second damper s₂ 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 load increase or load decrease 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 21, place the first damper S₁ in a closed position. Thereafter the control unit 10 will decide, at 22, 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 23, continue to hold the second damper s₂ in a normal 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 second damper s₂ will be placed in a cleaning position, at 24, resulting in a large air flow at high velocity through the EGR cooler 11 so that the latter' s inside surfaces are cleaned from soot deposits. Such regular cleaning of the EGR cooler makes it possible to maintain good performance over a long operating period without needing to carry out service operations in order to clean or replace it.

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, hi 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 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 means (s₂) arranged in the inlet line (6), a line (13) extending from the valve means (s₂) to a connection point (9a) to the return line, which connection point (9a) is situated 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 means (s₂) in a cleaning position so that at least part of the compressed air in the inlet line (6) is led via the valve means (s₂) and said line (13) to the return line (9) when the air is led in such a direction that it flows through the EGR cooler (11).

2. An arrangement according to claim 1, characterised in that the return line (9) comprises an EGR valve (S₁) adapted to controlling the flow of exhaust gases from the exhaust line (3) to the return line (9) and that the connection point (9a) of said line (13) to the return line is situated downstream from the EGR valve (si) but upstream from the EGR cooler (11) with respect to the intended direction of exhaust gas flow in the return line (9).

3. An arrangement according to claim 2, characterised in that the control unit (10) is adapted to placing the valve means (s₂) in the cleaning position during operating states of the combustion engine (1) where the EGR valve (S₁) is closed.

4. 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.

5. An arrangement according to claim 4, characterised in that the control unit (10) is adapted to placing the valve means (s₂) in a cleaning position in situations where it has received values concerning said parameter which indicate that the degree of deposits in the EGR cooler (11) exceeds a maximum acceptable value.

6. An arrangement according to any one of claims 2 to 5 above, characterised in that the control unit (10) is adapted to receiving information concerning the combustion engine's speed (n) and that the control unit (10) is adapted to placing the valve means (s₂) in said cleaning position and the EGR valve (S₁) in an open position if the combustion engine's speed (n) exceeds a maximum acceptable speed value (n_max).

7. An arrangement according to any one of the foregoing claims, characterised in that the inlet line (6) comprises a compressor (5) which compresses the air in the inlet line and that the valve means (s₂) is situated downstream from the compressor (5) with respect to the direction of air flow in the inlet line (6).

8. An arrangement according to claim 7, characterised in that the inlet line (6) comprises a charge air cooler (7) and that the valve means (s₂) is situated upstream from the charge air cooler (7) with respect to the direction of air flow in the inlet line (6).

9. 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 means (s₂) in the inlet line (6), of arranging a line (13) which extends from the valve means (s₂) to a connection point (9a) to the return line, which connection point (9a) is situated upstream from the EGR cooler (11) with respect to the intended direction of exhaust gas flow in the return line (9), and of placing the valve means (s₂), in specified situations, in a cleaning position in which at least part of the compressed air in the inlet line (6) is led via the valve means (s₂) and said line (13) to the return line (9), whereby the air is led in such a direction that it flows through the EGR cooler (11).

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 an EGR valve (S₁) and of arranging the connection point (9a) of said line (13) to the return line downstream from the EGR valve (S₁) but upstream from the EGR cooler (11) with respect to the intended direction of exhaust gas flow in the return line (9).

11. A method according to claim 10, characterised by the step of placing the valve means (s₂) in the cleaning position during operating states of the combustion engine

(1) when the EGR valve (S₁) is closed.

12. A method according to any one of claims 9-11 above, characterised by the step of detecting, by means of a sensor (15), a parameter which is related to the degree of deposition of pollutants in the EGR cooler (11).

13. A method according to claim 12, characterised by the step of placing the valve means (s₂) in a cleaning position in situations where the value of said parameter indicates that the degree of deposits in the EGR cooler (11) exceeds a maximum acceptable value.

14. A method according to any one of claims 10 to 13 above, characterised by steps of receiving information concerning the combustion engine's speed (n) and of placing the valve means (s₂) in said cleaning position and the EGR valve (S₁) in an open position if the combustion engine's speed (n) exceeds a maximum speed value (n_max).

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 9-14 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 9-14.