BACKGROUND TO THE INVENTION AND PRIOR ART

The present invention relates to an arrangement for cooling of compressed air and/or recirculating exhaust gases which are led to a combustion engine according to the preamble of claim 1.

In supercharged combustion engines, the air is compressed before it is led to the combustion engine. A large amount of air can thus be led into the combustion engine. However, the compression results in the air acquiring a raised temperature which leads to the specific volume of the air increasing. For this reason, the compressed air is cooled in at least one charge air cooler before it is led to the combustion engine.

The technique called EGR (exhaust gas recirculation) is a known way of leading back part of the exhaust gases from a combustion process in a combustion engine, via a return line, to a line for supply of air to the combustion engine. A mixture of air and exhaust gases is thus supplied to the engine's cylinders in which the combustion takes place. Adding exhaust gases to the air causes a lower combustion temperature resulting inter alia in the exhaust gases having a reduced content of nitrogen oxides NO _x . This technique is used both for Otto engines and for diesel engines. However, the exhaust gases of a combustion engine will be at a high temperature. The recirculating exhaust gases are therefore subjected to cooling in one or more EGR coolers before they are mixed with the air and led to the combustion engine.

A known practice is to cool the compressed air in charge air coolers and the recirculating exhaust gases in EGR coolers directly or indirectly by means of air which is at the temperature of the surroundings. The recirculating exhaust gases and the compressed air are thus cooled to a temperature which exceeds the temperature of the surroundings by about 10-15°C. The recirculating exhaust gases and the compressed air are therefore cooled to a temperature which depends on the temperature of the surroundings. In situations where the surroundings are at a high temperature, the result is significantly poorer cooling than when the surroundings are at a low temperature. SUMMARY OF THE INVENTION

The object of the present invention is propose an arrangement which cools compressed air and/or recirculating exhaust gases which are led to a combustion engine in a relatively simple and economic way to a desired low temperature even when the surrounding air is at a relatively high temperature.

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. According to the invention, the compressed air and/or the recirculating exhaust gases are subjected to at least one step of cooling in a cooler, which may be by air or by a circulating coolant. The compressed air and/or the recirculating exhaust gases may therefore acquire a relatively low temperature before they are subjected to a final step of cooling in an evaporator of a cooling circuit which contains a phase-transforming cooling medium. Such a cooling circuit therefore need only be used for subjecting the compressed air and/or the recirculating exhaust gases to a final step of cooling. The cooling circuit may thus effect cooling of the medium to a temperature which corresponds to or is below the temperature of the surroundings, without being of overly large dimensions. Thermal energy is used to power the cooling circuit. There is plenty of surplus heat around a combustion engine. Such an existing energy source can be used to power the cooling circuit in a very economical way. The compressed air and the recirculating exhaust gases may be cooled separately in their respective evaporators or in a joint evaporator after they have been mixed with one another.

According to an embodiment of the present invention, said power means is a vapour generator in which the cooling medium is intended to be warmed by said heat source to a temperature at which it vaporises. A vapour generator makes it possible for a relatively large amount of thermal energy to be absorbed from a heat source in a very effective way when the cooling medium vaporises. Said power means comprises with advantage an ejector means adapted to mixing gaseous cooling medium from the evaporator with gaseous cooling medium from the vapour generator. The ejector means may be an ejector pump or a venturi which has a narrowing portion through which the gaseous cooling medium from the vapour generator passes at high velocity. The result is low static pressure within the ejector means such that cooling medium from the evaporator can be drawn in and mixed with cooling medium from the vapour generator. The ejector means thus makes it possible to mix cooling medium from the evaporator which is at a low pressure with the cooling medium from the vapour generator which is at a significantly higher pressure. The fact that the ejector means is able to draw cooling medium from the evaporator allows circulation of cooling medium to and from the evaporator, making it possible to cool the compressed air and/the recirculating exhaust gases in the evaporator.

According to another preferred embodiment of the present invention, the cooling circuit comprises a condenser in which the cooling medium from the ejector means is intended to be cooled to a temperature at which it condenses. In a closed cooling circuit, a circulating cooling medium which vaporises in an evaporator has to be brought back to a liquid state in a condenser. The cooling circuit may comprise, downstream from the condenser, a manifold comprising a first line with a pump which pumps liquid cooling medium from the condenser to the vapour generator and a second line with an expansion valve which leads liquid cooling medium from the condenser to the evaporator. The cooling medium which is led to the vapour generator is used to absorb heat from the heat source to power the cooling circuit, whereas the cooling medium which is led to the evaporator is used to cool the compressed air and/or the recirculating exhaust gases in the evaporator.

According to a preferred embodiment of the present invention, the arrangement comprises a control unit adapted to controlling the operation of the cooling circuit. The control unit may be a computer unit with suitable software. The control unit can assess when the cooling circuit needs to be used. In certain operating situations it may be sufficient to cool the compressed air and/or the recirculating exhaust gases in said coolers. The cooling circuit should at least not be activated in situations where there is risk that the compressed air and/or the recirculating exhaust gases might be cooled to a temperature below 0°C, in which case there would be risk that ice might form within the cooler and that the flow to the combustion engine might be obstructed. The control unit may receive information from, for example, a sensor which detects the

temperature of the compressed air and/or the recirculating exhaust gases after they have been cooled in the cooler or by the temperature of the surroundings, in order to assess whether the cooling circuit has to be activated or not. According to a preferred embodiment of the present invention, the compressed air and/or the recirculating exhaust gases are intended to be cooled in said cooler directly or indirectly by air which is at the temperature of the surroundings. The cooler may therefore be air-cooled and have air at the temperature of the surroundings flowing through it or be liquid-cooled by a circulating coolant which is itself cooled by air at the temperature of the surroundings. With such a cooler, the compressed air and/or the recirculating exhaust gases can be cooled to a relatively low temperature, relative to the temperature of the surroundings, before the final cooling in the evaporator. The cooling circuit may therefore be quite small.

According to an embodiment of the present invention, said vapour generator is situated at such a location that it absorbs heat from the exhaust gases in an exhaust line of the combustion engine. The exhaust gases from a combustion engine are an existing energy source which can with advantage be utilised to power the cooling circuit.

Alternatively, the vapour generator may be situated at such a location that it absorbs heat from the compressed air and/or the recirculating exhaust gases in said line at a location upstream of the cooler. This makes it possible for the compressed air and/or the recirculating exhaust gases to give up thermal energy to the vapour generator and thereby undergo a first step of cooling. The load on the ordinary radiator downstream is thus lightened. According to a further alternative, the vapour generator may be situated at such a location that it absorbs heat from coolant circulating in a cooling system which cools the combustion engine. The coolant in the cooling system will be at a constant high temperature and therefore be appropriate for use as heat source to vaporise the cooling medium in the vapour generator. The load on the ordinary radiator of the cooling system is thereby reduced. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below by way of examples with reference to the attached drawings, in which: Fig. 1 depicts an arrangement for cooling of compressed air and recirculating

exhaust gases which are led to a combustion engine,

2 depicts the ejector pump in Fig. 1 in more detail and

3 depicts an alternative arrangement for cooling of compressed air and

recirculating exhaust gases which are led to a combustion engine. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE

INVENTION

Fig. 1 depicts a vehicle 1 powered by a supercharged combustion engine 2. The vehicle 1 may a heavy vehicle powered by a supercharged diesel engine. The exhaust gases from the cylinders of the combustion engine 2 are led to an exhaust line 4 via an exhaust manifold 3. The exhaust gases in the exhaust line 4, which will be at above atmospheric pressure, are led to a turbine 5 of a turbo unit. The turbine 5 is thus provided with driving power which is transferred, via a connection, to a compressor 6. The compressor 6 compresses air which is led into an air line 8 via an air filter 7. A charge air cooler 9 is provided in the air line 8. The charge air cooler 9 is arranged at a front portion of the vehicle 1. The purpose of the charge air cooler 9 is to cool the compressed air before it is led to the combustion engine 2. The compressed air is cooled in the charge air cooler 9 by air at the temperature of the surroundings which is caused to flow through the charge air cooler 9 by a radiator fan 10. The radiator fan 10 is powered by the combustion engine 2 via a suitable connection.

The combustion engine 2 is provided an EGR (exhaust gas recirculation) system for recirculation of the exhaust gases. Mixing exhaust gases with the compressed air which is led to the engine's cylinders lowers the combustion temperature and hence also the content of nitrogen oxides NO _x formed during the combustion processes. A return line 11 for recirculation of exhaust gases extends from the exhaust line 4 to the air 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 to steplessly control the amount of exhaust gases which is led from the exhaust line 4 to the air line 8 via the return line 11. The return line 11 comprises a first EGR cooler 14 and a second EGR cooler 15 to subject the recirculating exhaust gases to two steps of cooling. 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 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 for example possible to use a venturi 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 the exhaust gases in the exhaust line 4 of an Otto engine in substantially all operating situations will be at a higher pressure than the compressed air in the inlet line 8. After the exhaust gases have been mixed with the compressed air at a location 8a in the inlet line 8, the mixture is led to an evaporator 16 in which the mixture undergoes a final step of cooling before it is led to the respective cylinders of the diesel engine 2 via a manifold 17.

The combustion engine 2 is cooled in a conventional way by a cooling system which contains a circulating coolant. A coolant pump 18 circulates the coolant in the cooling system. The coolant pump 18 circulates a main flow of the coolant through the combustion engine 2. After the coolant has cooled the combustion engine 2, it is led in a line 21 to a thermostat 19 in the cooling system. When the coolant has reached a normal operating temperature, the thermostat 19 is adapted to leading it to a radiator 20 in order to be cooled. However, part of the coolant in the cooling system is led via a line 22 to the first EGR cooler 14 in which it subjects the recirculating exhaust gases to a first step of cooling. After the coolant has cooled the exhaust gases in the first EGR cooler 14, it is led back to the line 21 via a line 23. The warm coolant is cooled in the radiator 20 which is fitted at a forward portion of the vehicle 1 but downstream of the charge air cooler 9 and the air-cooled second EGR cooler 15 with respect to the intended direction of flow of the air. Such positioning of the second EGR cooler 15 and the charge air cooler 9 makes it possible for the recirculating exhaust gases and the compressed air to be cooled by air which is at the temperature of the surroundings. With normal dimensioning of such coolers 9, 15, the compressed air and the recirculating exhaust gases here undergo cooling to a temperature which exceeds the temperature of the surroundings by about 10-15°C. The lower the temperature of the air and the exhaust gases, the greater the amount of air and recirculating exhaust gases which can be led to the cylinders of the combustion engine.

The evaporator 16 is a component of a cooling circuit containing a circulating cooling medium. The cooling circuit comprises a vapour generator 24 situated in the exhaust line 4 at a location downstream of the turbine 5. The cooling medium is warmed in the evaporator 24 by the exhaust gases in the exhaust line 4 to a temperature at which it vaporises. The vaporised cooling medium is led to an ejector pump 25. Gaseous cooling medium from the vapour generator 24 is here mixed with gaseous cooling medium from the evaporator 16. The gaseous cooling medium is led from the ejector 25 to a condenser 26. A fan 27 driven by an electric motor 28 here forces a cooling air flow through the condenser 26 so that the cooling medium is cooled to a temperature at which it condenses. The cooling circuit comprises a manifold situated downstream of the condenser 26. The manifold comprises a first line with a pump 29 which leads liquid cooling medium from the condenser 26 to the vapour generator 24 and a second line with a throttle valve 30 which leads liquid cooling medium from the condenser 26 to the evaporator 16. A control unit 31 is adapted to controlling the operation of said cooling circuit on the basis of information from, for example, a sensor 32 which detects the temperature of the charge air and the recirculating exhaust gases in the inlet line 8.

During operation of the combustion engine 2, when the EGR valve 12 is open, warm exhaust gases are returned via the return line 11. The exhaust gases may be at a temperature of 500-600°C when they reach the first EGR cooler 14. The exhaust gases are subjected to a first step of cooling in the first EGR cooler 14 by the coolant. After the exhaust gases have been cooled in the first EGR cooler 14, they proceed in the return line 11 to the second EGR cooler 15. The exhaust gases are here subjected to a second step of cooling by air which is at the temperature of the surroundings to a temperature which exceeds that of the surroundings by about 10-15°C. The compressed air is cooled in the charge air cooler 9 by air at the temperature of the surroundings. The compressed air may thus likewise be cooled to a temperature which exceeds that of the surroundings by 10-15°C. The cooled exhaust gases are mixed with the cooled air at the location 8a in the inlet line 8.

The sensor 32 detects the temperature of the mixture of air and exhaust gases in the inlet line 8. The control unit 31 uses inter alia this information to assess whether it is possible to subject the mixture to a further step of cooling in the evaporator 16. Air and, in particular, exhaust gases contain relatively large amounts of water vapour.

When the air and the exhaust gases are cooled, water in liquid form precipitates within the charge air cooler 9 and the EGR cooler 15. Water in liquid form will therefore be led to the combustion engine 2. This is no great problem provided that it takes place in controlled forms. However, the air and the exhaust gases should not be cooled to a lower temperature than 0°C because of the risk that the water precipitated might then freeze to ice and obstruct the flow in the inlet line 8. When the control unit 31 deems that there is no risk of ice formation, the cooling circuit is activated. In this case the control unit activates the pump 29 to pump liquid cooling medium to the vapour generator 24. The coolant acquires a pressure p ₃ in the vapour generator 24. The warm exhaust gases in the exhaust line 4 warm the cooling medium so that it vaporises at the prevailing pressure p ₃ . The gaseous cooling medium formed in the vapour generator 24 is led to the ejector pump 25.

Fig. 2 depicts the ejector pump 25 in more detail. When the gaseous cooling medium from the vapour generator 24 reaches the ejector pump 25, it undergoes acceleration in a narrowing portion 25a. The gaseous cooling medium from the vapour generator 24 will thus be at a very high velocity when it flows out from the narrowing portion 25 a into an internal space 25b of the ejector pump 25. The result is a very low static pressure po in the space 25b. The ejector pump 25 comprises an inlet 25c close to the space 25b. The inlet 25c is connected to a line which leads cooling medium from the evaporator 16 to the ejector pump 25. The low static pressure po in the space 25b is lower than the pressure pi which prevails in the evaporator 16. The result is that gaseous cooling medium is drawn from the evaporator 16 to said space 25b in the ejector pump 25. The gaseous cooling medium from the evaporator 16 is here mixed with the gaseous cooling medium from the vapour generator 24. The ejector pump 25 comprises finally an expanding portion 25 d with an increasing cross-sectional area in which the velocity of the gaseous cooling medium decreases. The gaseous cooling medium is at a pressure p ₂ when it is led from the ejector pump 25 to the condenser 26. Pressure p ₂ is higher than pressure pi but lower than pressure p ₃ . The numerical designations of the aforesaid pressures p ₀ , pi, p ₂ , p ₃ are related to their respective magnitudes.

The cooling medium is cooled in the condenser 26 by air which is forced through the condenser 26 by the fan 27. The cooling medium is cooled here to a temperature such that it condenses at the prevailing pressure p ₂ within the condenser 26. The cooling circuit therefore comprises a manifold in the form of two lines downstream of the condenser 26. One of the lines comprises the pump 29 which leads part of the liquid cooling medium from the condenser 26 back to the vapour generator 24. The pump 29 here exerts a pressure upon the cooling medium such that the latter' s pressure increases from p ₂ to p ₃ . The second line, which comprises the expansion valve 30, leads a remaining portion of the liquid cooling medium to the evaporator 16. The expansion valve 30 lowers the pressure of the cooling medium from p ₂ to pi. The liquid cooling medium will at pressure pi be at a lower temperature than the air and the exhaust gases in the inlet line 8. The cooling medium therefore absorbs heat from the air and the exhaust gases in the evaporator 16. The cooling medium has at pressure pi an evaporation temperature which is with advantage lower than the temperature of the surroundings. The mixture of air and exhaust gases can thus be cooled in the evaporator 16 to a temperature equal to or lower than the temperature of the surroundings. The cooling medium which vaporises in the evaporator 16 is drawn to the ejector pump 25, in which it therefore mixes with the cooling medium from the vapour generator 24.

In this case, a cooling circuit is thus used to subject the compressed air and the recirculating exhaust gases to a final step of cooling before they are led into the combustion engine 2. The operation of the cooling circuit is conducted substantially on the basis of the thermal energy which the cooling medium absorbs in the vapour generator 24 from the exhaust gases in the exhaust line 4. This thermal energy is used to vaporise the cooling medium at the highest pressure p ₃ in the cooling circuit. The vaporised cooling medium, which is thus at a relatively high pressure p ₃ , is used to create a high- velocity flow through the ejector pump 25. This may result in a very low static pressure p ₀ , making it possible for the cooling medium from the evaporator 16 at pressure pi to be drawn into the ejector pump 25 and be mixed with the cooling medium from the vapour generator 24. Apart from said thermal energy, energy need only be supplied to run the pump 29. The energy required to run the pump 29 is substantially negligible. In this case existing thermal energy is thus used to power the cooling circuit.

In situations where the control unit 31 receives information from the sensor 32 which indicates risk that the air and the exhaust gases might be cooled to a temperature below 0°C, the operation of the pump 29 and hence of the cooling circuit is deactivated. In this case, the evaporator 16 is therefore not used to cool the air and the exhaust gases before they are led to the combustion engine 2. An alternative vapour generator 35 is represented by broken lines in Fig. 1. When such a vapour generator 35 is used, the coolant in the cooling system which cools the combustion engine 2 is used as heat source. The coolant may be at a temperature of about 100°C when it leaves the combustion engine 2. In this case the coolant is thus provided with extra cooling, thereby reducing the load upon the ordinary radiator 20.

Fig. 3 depicts an alternative cooling circuit for cooling the air and the exhaust gases which are led to the combustion engine 2. This case involves using a vapour generator 33 which absorbs heat from the compressed air at a location upstream of the charge air cooler, and a vapour generator 34 which absorbs thermal energy from the recirculating exhaust gases at a location upstream of the EGR cooler 15. When the compressed air and, in particular, the recirculating exhaust gases may both be at a very high temperature, the cooling medium can be warmed effectively to the temperature at which it vaporises in said vapour generators 33, 34. Here again, the thermal energy absorbed is used to power a cooling circuit similar to that described in the embodiment in Fig. 1. No further description of the cooling circuit's function is therefore given here. The two vapour generators 33, 34 are here arranged in parallel. Since thermal energy is absorbed from the compressed air and the recirculating exhaust gases, they undergo a first step of cooling in the two vapour generators 33, 34. Thereafter, the compressed air is cooled in the charge air cooler 9 and the recirculating exhaust gases are cooled in the EGR cooler 15 before they are mixed. The mixture undergoes a final step of cooling in the evaporator 16. When the two vapour generators 33, 34 provide initial cooling, the cooling of the compressed air and the recirculating exhaust gases becomes more effective.

The invention is in no way limited to the embodiments to which the drawings refer but may be varied freely within the scopes of the claims. It is for example possible to cool only the recirculating exhaust gases or the compressed air in the manner described above, in which case the evaporator 16 will be situated in a line which conveys only recirculating exhaust gases or compressed air.