WO2008069743A1

WO2008069743A1 - Arrangement for a supercharged combustion engine

Info

Publication number: WO2008069743A1
Application number: PCT/SE2007/050915
Authority: WO
Inventors: Håkan SARBY
Original assignee: Scania Cv Ab (Publ)
Priority date: 2006-12-05
Filing date: 2007-11-29
Publication date: 2008-06-12
Also published as: DE112007002957T5; SE0602604L; DE112007002957B4; SE531102C2

Abstract

The present invention relates to an arrangement for a supercharged combustion engine (1). The arrangement comprises an exhaust line (4) intended to lead exhaust gases out from the combustion engine (2), and an inlet line (8) intended to lead air at above atmospheric pressure to the combustion engine (2). The arrangement also comprises a first compressor (6a) for compressing the air in the inlet line (8) in a first stage, a second compressor (6b) for compressing the air in the inlet line (8) in a second stage, and a first charge air cooler (9a) adapted to cooling the air after it has been compressed by the first stage and before it is compressed in the second stage.

Description

Arrangement for a supercharged combustion engine

BACKGROUND TO THE INVENTION, AND STATE OF THE ART

The present invention relates to an arrangement for a supercharged combustion engine according to the preamble of claim 1.

The technique called 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 line 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.

Optimum use of this technique entails recirculation of a relatively large amount of exhaust gases. The recirculating exhaust gases are therefore cooled in at least one EGR cooler to reduce the specific volume of the exhaust gases before they are mixed with air and led to the combustion engine. Conventional EGR coolers use the coolant of the vehicle's ordinary cooling system for cooling the combustion engine. Another known practice is to use an air-cooled EGR cooler in which the exhaust gases are cooled by air which is at the temperature of the surroundings, thereby allowing the recirculating exhaust gases to be cooled to a temperature substantially corresponding to the temperature of the surroundings. The recirculating exhaust gases can thus be subjected to a substantially optimum reduction in specific volume so that a large amount of exhaust gases can be recirculated into the combustion engine.

The amount of air which can be supplied to a supercharged combustion engine depends on the pressure of the air but also on the temperature of the air. Supplying the largest possible amount of air to the combustion engine entails the air being first compressed by a compressor before being cooled in a charge air cooler and thereafter being led to the combustion engine. The compressed air is usually cooled in the charge air cooler by surrounding air. The compressed air can thus be cooled to a temperature which exceeds the temperature of the surroundings by only a few degrees. Despite the air being compressed and cooled as described above, this is not usually sufficient for providing the necessary amount of air which together with the recirculating exhaust gases will enable combustion with optimum use of the EGR technique.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an arrangement which makes it possible to supply a large amount of compressed air to a supercharged combustion engine.

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. The arrangement thus comprises two compressors which compress the air in two stages. The air also undergoes cooling in a charge air cooler between said compression stages. The air will be at higher pressure and temperature after the compression in the first stage. The cooling of the air after it has been compressed in the first stage leads to its being at a lower temperature but maintaining the same pressure. The air thus becomes more compact and thereby assumes a lower specific volume, i.e. it occupies a smaller volume per unit weight. Since the compressor usually has a space with a constant volume for receiving and compressing air, said cooling allows a larger amount of air to be drawn into the second compressor and compressed in the second stage, [t also means that a larger amount of air can be compressed to a very high pressure and led to the combustion engine. The cooling between the two compression stages also means that the work of compressing the air to said high pressure will be less than if the air was compressed to that pressure in only one stage or in two stages without intermediate cooling. Moreover, less warming is thus supplied, thereby reducing the need to cool the air. A charge air cooler with a lower capacity can therefore be used for cooling the air to the same temperature as in the conventional case.

According to a preferred embodiment of the present invention, the first charge air cooler is part of a cooling circuit with a circulating coolant which is intended to cool the air in the first charge air cooler. The amount of air which can be drawn into the second compressor is thus related to the specific volume of the air, which varies with the temperature of the air. It is therefore important to cool the air to as low a temperature as possible in the first charge air cooler. Using the coolant of the cooling system which cools the combustion engine is relatively simple and functional but not particularly effective, since this coolant is usually at a temperature of 70-100⁰C during normal operation of the combustion engine, which would only allow the air to be cooled to a temperature in line with the coolant's operating temperature. It is therefore advantageous for said cooling circuit to take the form of a separate unit relative to the cooling system which is intended to cool the combustion engine. The separate cooling circuit may comprise a cooling element in which the circulating coolant is adapted to being cooled by air. Air is readily available and can easily be caused to flow through the cooling element in order to increase the cooling effect of the circulating coolant. A radiator fan and the air flow generated by movement of the vehicle can be used to cause a considerable amount of air to flow through the cooling element. The cooling element is preferably situated in a region in which it is adapted to being cooled by air which is at the temperature of the surroundings, thereby allowing cooling of the coolant in the cooling element to a temperature close to the temperature of the surroundings. With advantage, the separate cooling circuit comprises a line adapted to leading the coolant from the cooling element to the first charge air cooler without the coolant undergoing substantially any warming on its way between the cooling element and the first charge air cooler. The coolant will therefore be able to cool the compressed air in the first charge air cooler to a temperature close to the temperature of the surroundings.

According to another preferred embodiment of the invention, the arrangement comprises a second charge air cooler fitted in a region in which it is adapted to having air which is at the temperature of the surroundings flow through it and being thereby cooled. The compressed air thus undergoes a second stage of compression after the cooling in the first charge air cooler and thereby acquires not only a further increase in pressure but also an increase in temperature. To enable as large an amount of air as possible to be led into the combustion engine, the air has to be cooled again before it is led into the combustion engine. This can with advantage be done in a charge air cooler which has air at the temperature of the surroundings flowing through it. This makes it possible for the air to be cooled to a temperature close to the temperature of the surroundings and hence for a substantially optimum amount of air to be supplied to the combustion engine.

According to another embodiment of the invention, the arrangement comprises a return line which connects the exhaust line to the inlet line so that it is possible, via the return line, to recirculate exhaust gases from the exhaust line to the inlet line. In this case, the combustion engine is therefore provided with an EGR system intended to reduce the content of nitrogen oxides in the exhaust gases. With such a system in particular, it is extremely important to provide a large air flow to the combustion engine so that the EGR system can be used in such a way as to achieve a substantially optimum reduction in the content of nitrogen oxides. Accordingly, the separate cooling circuit may comprise a line adapted to leading the coolant from the first charge air cooler to a first EGR cooler in which it is adapted to cooling the recirculating exhaust gases in the return line. This may entail the separate cooling system also being used for cooling the recirculating exhaust gases. The arrangement comprises with advantage a second EGR cooler for cooling the recirculating exhaust gases in a second stage, which second EGR cooler is fitted in a region in which it is adapted to having air at the temperature of the surroundings flow through it. It is also important that the recirculating exhaust gases are cooled in such a way that they assume as low a specific volume as possible so that a large amount of exhaust gases can be led into the combustion engine. With such a second EGR cooler, the exhaust gases can be cooled to a temperature close to the temperature of the surroundings. Thus a substantially optimum amount of recirculating exhaust gases can be led into the combustion engine. A low temperature of the compressed air and a low temperature of the recirculating exhaust gases when they are led into the combustion engine also result in a lower combustion temperature and hence a lower content of nitrogen oxides in the exhaust gases.

According to another embodiment of the invention, at least one of said compressors forms part of a turbo unit which comprises a turbine adapted to being driven by the exhaust gases in the exhaust line. With advantage, the arrangement comprises two turbines which extract from the exhaust gases energy which is used for driving said compressors.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below by way of examples with reference to the attached drawings, which:

Fig. 1 depicts an arrangement for a supercharged combustion engine according to a first embodiment of the invention and

Fig. 2 depicts an arrangement for a supercharged combustion engine according to a second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 depicts an arrangement for recirculation of the exhaust gases of a supercharged combustion engine which is adapted to powering a schematically depicted vehicle 1. The combustion engine is here exemplified as a diesel engine 2. Such recirculation is usually called EGR (Exhaust Gas Recirculation). Adding exhaust gases to the compressed air which is led to the cylinders of the engine lowers the combustion temperature and hence also the content of nitrogen oxides (NO_x) which are formed during the combustion processes. The diesel engine 2 may be intended to power a heavy vehicle 1. The exhaust gases from the cylinders of the diesel engine 2 are led via an exhaust manifold 3 to an exhaust line 4. The diesel engine 2 is provided with a first turbo unit comprising a turbine 5a and a compressor 6a, and a second turbo unit comprising a turbine 5b and a compressor 6b. The exhaust gases in the exhaust line 4, which are at above atmospheric pressure, are led initially to the turbine 5b of the second turbo unit. The turbine 5b is thus provided with driving force which is transmitted via a connection to the compressor 6b of the second turbo unit. The exhaust gases are led thereafter via the exhaust line 4 to the turbine 5 a of the first turbo unit. The turbine 5a is thus provided with driving force which is transmitted via a connection to the compressor 6a of the first turbo unit.

The compressor 6a of the first turbo unit thus compresses air which is drawn into an inlet line 8 via an air filter 7. The air is cooled thereafter in a first charge air cooler 9a. The first charge air cooler 9a is a component of a separate cooling system with a coolant which is circulated by a coolant pump 18. The separate cooling system comprises also a cooling element 19 fitted in a peripheral region A' of the vehicle 1. In this case the peripheral region A' is situated at a front portion of the vehicle 1. A radiator fan 20 is adapted to providing a flow of surrounding air through the cooling element 19. The cooling fan 20 is driven by an electric motor 21. The coolant is cooled in the cooling element 19 by air which is at the temperature of the surroundings. The coolant is led, substantially without being warmed, from the cooling element 19 to the first charge air cooler 9a via a line 22. The coolant will thus be at substantially the same temperature when it reaches the first charge air cooler 9a as when it leaves the cooling element 19. It is therefore possible to cool the coolant in the first charge air cooler 9a to a temperature close to the temperature of the surroundings. A line 23 leads the coolant from the first charge air cooler 9a back to the cooling element 19. The control unit 13 is adapted to controlling the coolant pump 18 of the separate cooling system, thereby making it possible to vary the coolant flow in the separate cooling circuit. The control unit 13 is also adapted to controlling the radiator fan 20 in the separate cooling system, thereby making it possible to vary the cooling of the coolant in the cooling element 19. The compressed and cooled air leaving the first charge air cooler 9a is led to the compressor 6b of the second turbo unit, in which it is compressed in a second stage. The air is cooled thereafter in a second charge air cooler 9b. The second charge air cooler 9b is arranged in a peripheral region A of the vehicle 1 , which in this case is at a front portion of the vehicle 1. The compressed air is cooled in the second charge air cooler 9b by surrounding air which is caused to flow through the second charge air cooler 9b by a radiator fan 10 and by the air flow generated by movement of the vehicle. The radiator fan 10 is driven by the diesel engine 2 via a suitable connection.

An arrangement for providing recirculation of part of the exhaust gases in the exhaust line 4 comprises a return line 11 which extends between the exhaust line 4 and the inlet line 8. The return line 11 comprises an EGR valve 12 by which the exhaust flow in the return line 11 can be shut off. The EGR valve 12 may also be used for steplessly controlling the amount of exhaust gases which is led from the exhaust line 4 to the inlet line 8 via the return line 11. A control unit 13 is adapted to controlling the EGR valve 12 on the basis of information about the current operating state of the diesel engine 2. The control unit 13 may be a computer unit provided with suitable software. The return line 11 comprises a first EGR cooler 14a for cooling the exhaust gases in a first stage and a second EGR cooler 14b for cooling the exhaust gases in a second stage. In certain operating states of supercharged diesel engines 2, the pressure of the exhaust gases in the exhaust line 4 will be lower than the pressure of the compressed air in the inlet line 8. In such operating situations it is not possible to mix the exhaust gases in the return line 11 directly with the compressed air in the inlet line 8 without special auxiliary means. To this end it is possible to use, for example, a venturi 16 or a turbo unit with variable geometry. If the combustion engine 2 is instead a supercharged Otto engine, the exhaust gases in the return line 11 can be led directly into the inlet line 8, since in substantially all operating states of an Otto engine the exhaust gases in the exhaust line 4 will be at a higher pressure than the compressed air in the inlet line 8. When the exhaust gases have mixed with the compressed air in the inlet line 8, the mixture is led via a manifold 17 to the respective cylinders of the diesel engine 2.

During operation of the diesel engine 2, exhaust gases flow through the exhaust line 4 and drive the turbines 5a, b of the turbo units. The turbines 5a, b are thus provided with driving power which powers the compressors 6a, b of the turbo units. The compressor 6a of the first turbo unit draws surrounding air in via the air filter 7 and compresses the air in the inlet line 8 in a first stage. The air thus acquires a higher pressure and a higher temperature. The compressed air is cooled in the first charge air cooler 9a by the coolant of the separate cooling system. If the cooling of the air in the first charge air cooler 9a needs boosting, the control unit 7 can control the coolant pump 18 so that it increases the coolant flow through the first charge air cooler 9a, and/or control the radiator fan 20 so that it boosts the cooling which the circulating coolant undergoes in the cooling element 19. The coolant may thus be at a temperature substantially corresponding to the temperature of the surroundings when it reaches the first charge air cooler 9a, thereby allowing the compressed air to be cooled to a temperature close to the temperature of the surroundings. The air maintains its pressure during the cooling in the first charge air cooler 9a. Air which is cooled acquires a lower specific volume, i.e. occupies a smaller volume per unit weight. The air thus becomes more compact. A compressor normally has a space with a constant volume for receiving and compressing air. The cooling of the air in the first charge air cooler 9a thus makes it possible for a larger amount of air to be compressed in the compressor 6b of the second turbo unit. The air is here compressed as a second stage so that it assumes a still higher pressure. The compressed air is led thereafter through the second charge air cooler 9b, in which it is cooled by surrounding air. The compressed air can here be cooled back to a temperature close to the temperature of the surroundings.

In most operating states of the diesel engine 2, the control unit 13 will keep the EGR valve 12 open so that part of exhaust gases in the exhaust line 4 is led into the return line 11. The exhaust gases in the exhaust line 4 may be at a temperature of about 500- 600⁰C when they reach the first EGR cooler 14a. The recirculating exhaust gases are cooled in the first EGR cooler 14a in a first stage. The coolant of the cooling system which cools the diesel engine may here be used as cooling medium. During normal operation of the vehicle, this coolant will be at a temperature within the range 70- 100⁰C. The recirculating exhaust gases can therefore be cooled in a first stage to a temperature close to the temperature of the coolant. Thereafter the exhaust gases are led to the second EGR cooler 14b, which is situated in a peripheral region A of the vehicle 1. The second EGR cooler 14b has air at the temperature of the surroundings flowing through it. With a suitably dimensioned second EGR cooler 14b, the recirculating exhaust gases can be cooled to a temperature substantially corresponding to the temperature of the surroundings. Exhaust gases in the return line 11 can thus undergo cooling to substantially the same temperature as the compressed air in the second charge air cooler 9b.

The recirculating exhaust gases are thus cooled to a temperature substantially corresponding to the temperature of the surroundings before they are mixed with the air and led into the combustion engine 2. The compressed air is thus likewise cooled in two stages. Its cooling between the compressions in the compressors 6a, b results in the air assuming a relatively small specific volume when it is compressed by the compressor 6b in the second stage. Thus a relatively large amount of air can be compressed by the compressor 6b in the second stage. The compressed air is cooled thereafter in the second charge air cooler to a temperature substantially corresponding to the temperature of the surroundings. Both the exhaust gases and the compressed air will thus be at a temperature substantially corresponding to the temperature of the surroundings when they mix, thereby making it possible for a substantially optimum amount of recirculating exhaust gases and a substantially optimum amount of air to be led into the combustion engine at a high pressure and consequently affording the possibility of combustion in the combustion engine with a substantially optimum reduction of the nitrogen oxides in the exhaust gases. A low temperature of the compressed air and a low temperature of the recirculating exhaust gases when they are led into the combustion engine 2 also results in a lower combustion temperature and hence a lower content of nitrogen oxides in the exhaust gases.

Fig. 2 depicts an alternative arrangement for a supercharged diesel engine 2. In this case the separate cooling system is used for cooling also the recirculating exhaust gases in the first EGR cooler 14a. The coolant is cooled in the cooling element 19 by means of surrounding air. The coolant is led from the cooling element 19 to the first charge air cooler 9a via a line 22. When the coolant has cooled the air in the first charge air cooler 9a, it is led via a line 24 to the first EGR cooler 14a. The coolant will here be at a higher temperature than the surroundings, since it will have already been used for cooling the air in the first charge air cooler 9a, but it will be at a considerably lower temperature than the recirculating exhaust gases, which may here be at a temperature of about 500-600⁰C. The recirculating exhaust gases can thus be cooled in a first stage to a temperature close to the temperature of the coolant. The coolant is thereafter led via a line 25 to the cooling element 19, in which it is cooled again by air which is at the temperature of the surroundings.

The invention is in no way limited to the embodiments described with reference to the drawings but may be varied freely within the scopes of the claims.

Claims

1. An arrangement for a supercharged combustion engine (2), which arrangement comprises an exhaust line (4) intended to lead exhaust gases out from the combustion engine (2), an inlet line (8) intended to lead air at above atmospheric pressure to the combustion engine (2), a first compressor (6a) for compressing the air in the inlet line (8) in a first stage, a second compressor (6b) for compressing the air in the inlet line (8) in a second stage, a first charge air cooler (9a) adapted to cooling the air after it has been compressed by the first stage and before it is compressed in the second stage, a return line (11) connecting the exhaust line (4) to the inlet line (8) so that it is possible, via the return line (11), to recirculate exhaust gases from the exhaust line (4) to the inlet line (8), and a cooling circuit which constitutes a separate unit relative to a cooling system which is intended to cool the combustion engine (2), which cooling circuit comprises a circulating coolant, which is intended to cool the air in the first charge air cooler (9a), and a cooling element (19) in which the circulating coolant is adapted to being cooled by air, characterised in that the cooling element (19) is situated in a region (A') in which it is adapted to being cooled by air which is at the temperature of the surroundings, and that the separate cooling circuit comprises a line (24) adapted to leading the coolant from the first charge air cooler (9a) to a first EGR cooler (14a) in which it is adapted to cooling the recirculating exhaust gases in the return line (11).

2. An arrangement according to claim 1, characterised in that the separate cooling circuit comprises a line (22) adapted to leading the coolant from the cooling element (19) to the first charge air cooler (9a) without the coolant undergoing substantially any warming on its way between the cooling element (19) and the first charge air cooler (9a).

3. An arrangement according to any one of the foregoing claims, characterised in that the arrangement comprises a second charge air cooler (9b) adapted to cooling the compressed air in a second stage and fitted in a region (A) in which it is adapted to having air which is at the temperature of the surroundings flow through it and to being thereby cooled.

4. An arrangement according to any one of the foregoing claims, characterised in that the arrangement comprises a second EGR cooler (14b) for cooling the recirculating exhaust gases in a second stage, which second EGR cooler is fitted in a region (A) in which it is adapted to having air which is at the temperature of the surroundings flow through it.

5. An arrangement according to any one of the foregoing claims, characterised in that at least one of said compressors (6a, b) is part of a turbo unit which comprises a turbine (5a, b) adapted to being driven by the exhaust gases in the exhaust line (4).