A known practice according to a technique called EGR (Exhaust Gas Recirculation) is to lead part of the exhaust gases from a combustion process in a combustion engine back to an inlet duct for supply of air to the combustion engine. A mixture of air and exhaust gases is supplied, via the inlet duct, to the engine's cylinders in which the combustion takes place. Adding exhaust gases to the air causes a lower combustion temperature which results in a reduced content of nitrogen oxides NOX in the exhaust gases.

In supercharged combustion engines, air is supplied to the combustion chamber at a pressure which is higher than the pressure of the exhaust gases. One way of mixing exhaust gases in the inlet duct with the pressurised air is to use a so-called venturi. In the venturi, the pressurised air flows initially through a first portion which has a decreasing cross-sectional area. The velocity of the air therein increases while at the same time the static pressure of the air decreases. In a middle portion of the venturi which has a minimum cross-sectional area, the pressure of the air falls to a level which is lower than the pressure of the exhaust gases. This makes it easy to mix the exhaust gases with the air flow in this middle portion. The resulting composite medium in the form of a mixture of air and exhaust gases flows thereafter through a third portion of the venturi which has an increasing cross-sectional area. At this stage the static pressure of the medium increases again before it is led into the respective cylinders of the combustion engine.

The third portion of a venturi thus takes the form of an expanding flow channel. The flow of a medium through an expanding flow channel leads inevitably to growth of a relatively thick boundary layer of the medium along the wall of the flow channel. A boundary layer of a flowing medium is defined as the layer of the medium which exhibits a velocity of at most 80% of the free-flow velocity in a central portion of the flow channel. An increasingly thick boundary layer along the wall of the flow channel entails risk of the

flow of the medium becoming unstable and of the third portion of the venturi thereby losing its pressure-raising function. The functioning of a venturi depends on good flow quality downstream unaffected by flow-disturbing elements. Ensuring the functioning of the venturi in conventional EGR systems usually involves the inlet duct incorporating a long straight section after the third portion of the venturi. Such a straight section provides uniform and stable flow of the medium after the venturi. A long straight section of inlet duct does mean, however, that EGR systems are space-consuming. For engines intended to power vehicles, this is an obvious problem in that available space is increasingly limited, which leads to being compelled to apply compromise solutions which are not optimum from the flow point of view.

SUMMARY OF THE INVENTION The object of the present invention is to provide an apparatus of the kind mentioned in the introduction which has a venturi whose functioning can be assured even with a relatively short inlet duct situated downstream. With such an inlet duct, the apparatus may be of compact design so that it is possible to install it in a relatively limited space.

This object is achieved with the apparatus of the kind mentioned in the introduction which is characterised by the features indicated in the characterising part of claim 1. To counteract growth of the boundary layer of the composite medium it is advantageous to create in the inlet duct a pressure distribution which tends to equalise the flow velocity between the central and peripheral portions of the inlet duct. To this end, it is appropriate to make the shape and positioning of said flow guide means such that they create a lower static pressure in the flowing medium in the peripheral portion of the inlet duct than in the latter's central portion. With such a static pressure distribution, the flowing medium will exhibit a higher dynamic pressure in the peripheral portion of the inlet duct than in the latter's central portion, thereby leading to equalisation of the medium's velocities in said portions. Equalisation of the medium's velocities in said portions will counteract growth of the boundary layer and the risk of instability in the third portion of the venturi. With appropriate such flow guide means, a uniform and stable flow of the medium can be achieved without having to arrange a long straight section after the venturi. The apparatus may thus be of compact design. With such flow guide means, the inlet duct

may also be provided with a curved shape substantially immediately after the venturi in the direction of flow of the medium without jeopardising the functioning of the venturi.

According to a preferred embodiment of the present invention, said flow guide means are designed to divide the inlet duct into at least two parallel part-ducts. Suitable shape and positioning of the part-ducts makes it possible to create a pressure distribution in the inlet duct after the third portion of the venturi such as to counteract growth of the boundary layer of the composite medium along the third portion. To this end, the respective cross- sectional areas of the inlet and outlet of at least one of said part-ducts may differ in magnitude. Such a part-duct may incorporate a decreasing cross-sectional area in the direction of flow of the medium. This will provide the medium with a greater flow velocity in the part-duct and hence a higher dynamic pressure. This will lead to the static pressure rising upstream from the part-duct. Alternatively, such a part-duct may incorporate an increasing cross-sectional area in the direction of flow of the medium. The medium will then have a reduced flow velocity and a lower dynamic pressure in the part- duct. This will lead to the static pressure decreasing upstream from the part-duct. A desired pressure distribution may be created in the inlet duct in a region upstream from the part-duct by appropriate positioning of a suitable number of part-ducts which have increasing and decreasing cross-sectional areas in the flow direction. With advantage, said flow guide means are arranged in a curved section of the inlet duct. To achieve a compact design of the apparatus, the venturi is preferably positioned parallel with a long side of the combustion engine. With such positioning of the venturi, the inlet duct requires downstream from the venturi at least one curved section to lead the medium to the engine's cylinders. Positioning the flow guide means in such a curved section is advantageous in that here they can counteract the flow instability caused by the presence of a curved section. The flow guide means in the curved section also have the advantage of usually reducing the medium's flow losses in this curved section.

According to another preferred embodiment of the present invention, said flow guide means incorporate at least one guide baffle. A suitably shaped guide baffle provides very effective control of the medium. A number of baffles arranged parallel in the inlet duct create a number of part-ducts arranged parallel which may be given an advantageous shape. If the baffles are arranged in a curved section, they have with advantage a

corresponding curved shape which guides the medium through the section. Said baffles preferably have a wing profile. A wing profile results in substantially optimum flow conditions.

According to another preferred embodiment of the present invention, said flow guide means are incorporated in a unit which can be fitted in the inlet duct. Such flow means may be incorporated in an insert package which may be fitted detachably or permanently in a curved section or at another suitable point in the inlet duct. Alternatively, said flow guide means may be incorporated in an integrated unit which incorporates part of the inlet duct. For example, a curved inlet duct section may be manufactured with integrated flow guide means. Such a unit may, for example, be made by die-casting.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention is described below by way of example with reference to the attached drawings, in which: Fig. 1 depicts schematically an apparatus for recirculation of exhaust gases in a supercharged diesel engine, Fig. 2 depicts schematically an apparatus according to a preferred embodiment of the invention and Fig. 3 depicts guide baffles and the curved section in Fig. 2 in more detail.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig. 1 depicts a supercharged diesel engine 1 with an apparatus which allows recirculation of part of the exhaust gases which are formed during the combustion processes in the engine's cylinders (EGR, Exhaust Gas Recirculation). The engine may for example be intended to power a heavy vehicle. Exhaust gases from the cylinders of the diesel engine 1 are led via a branched exhaust line 2 to a common exhaust line 3. The exhaust gases in the exhaust line 3, which have a pressure of about 2 bar, are led past a turbine 4 before

being discharged. The turbine 4, which is driven by the exhaust gases, drives the compressor 5. The compressor 5 compresses the air in an inlet duct 6 which leads air to the diesel engine 1. The compressor 5 compresses the air to a suitable pressure which may be about 3 bar. A cooler 7 is arranged in the inlet duct 6 to allow cooling of the compressed air. A return line 8 is designed to allow recirculation of exhaust gases from the exhaust line 3. The recirculating portion of the exhaust gases is intended to be mixed with the air in the inlet duct 6. The return line 8 incorporates a control valve 9 by which the recirculating exhaust gas flow can be shut off as necessary. To a certain extent the control valve 9 may also be used for controlling the proportion of exhaust gases led to the inlet duct 6. The return line 8 also incorporates a cooler 10 to allow cooling of the recirculating exhaust gases.

In the supercharged diesel engine 1, the air in the inlet duct 6 is at a higher pressure than the exhaust gases from the diesel engine 1. A venturi 11 is therefore used to make it possible to introduce the exhaust gases into the return line 8 leading to the inlet duct 6.

The functioning of the venturi 11 is explained in more detail below. After the mixing in of exhaust gases in the inlet duct 6, the resulting composite medium is led via a final inlet section 6'to a branched induction line 12 which allows supply of the medium to the engine's respective cylinders. The medium supplied to the cylinders of the diesel engine 1 comprises about 80% air and 20% exhaust gases. Adding exhaust gases to the air lowers the combustion temperature in the cylinders and hence also the content of nitrogen oxides NO, which are formed during the combustion process. Recirculation of exhaust gases is thus a relatively simple way of reducing the content of nitrogen oxides (NOX) in the exhaust gases. A venturi 11 is thus required in a supercharged combustion engine 1 to enable mixing in of exhaust gases in the inlet duct 6. The functioning of a venturi 11 depends on good flow quality, particularly downstream from the venturi. To assure the functioning of the venturi 11, conventional EGR systems therefore usually have a long straight section after the venturi 11. Such a straight section provides uniform and stable flow of the medium downstream from the venturi 11. The long straight section does mean, however, that conventional EGR systems are relatively space-consuming.

Fig. 2 depicts the venturi 11 and the final inlet duct 6'in more detail. The pressurised air in the inlet duct 6 flows initially through a first portion 11 a of the venturi 11. The first

portion 1 la has a continuously decreasing cross-sectional area. The velocity of the air in the first portion 1 la therefore increases progressively in the direction of flow while at the same time the static pressure of the air decreases. In a second portion lib of the venturi which has a minimum cross-sectional area, the static pressure of the air falls to a level which is lower than the pressure of the exhaust gases. The exhaust gases from the return line 8 are therefore drawn easily into the air flow in this second portion lib of the venturi.

The resulting composite medium comprising a mixture of air and exhaust gases flows thereafter through a third portion 11 c of the venturi which has a continuously increasing cross-sectional area. Here the medium has a progressively decreasing velocity and a rising static pressure. The pressurised medium is led thereafter to the respective cylinders of the diesel engine 1 via the final inlet duct 6'and the branched induction line 12. The final inlet duct 6'is in this case relatively short and incorporates a curved section 6".

Four wing-shaped guide baffles 13a-d are arranged in the curved section 6". The guide baffles 13a-d have a curvature which substantially corresponds to the curvature of the curved section 6". The advantages of the presence of the guide baffles 13a-d include reducing the medium's flow losses in the curved section.

The expanding flow channel in the third portion 11 c inevitably leads, however, to growth of a relatively thick boundary layer of the medium along the wall surface of the third portion 11 c. The boundary layer of the flowing medium is defined as the layer of the medium which exhibits a velocity of at most 80% of the free-flow velocity in the central part of the flow channel. An increasingly thick boundary layer along the wall surface of the third portion 11 c entails the risk of the flow of medium becoming unstable and of the third portion 11 c of the venturi therefore losing its pressure-raising function. The main purpose of the guide baffles 13a-d is to counteract the growth of the boundary layer in the venturi's third portion lie and to assure the functioning of the venturi 11 despite a relatively short final inlet duct 6'which also incorporates a curved section 6".

Fig. 3 depicts the guide baffles 13a-d in more detail. The guide baffles 13a-d are arranged substantially parallel in the curved section 6"of the final inlet duct 6'. The guide baffles 13a-d have a substantially parallel extent and are arranged at substantially constant spacings across the width of the inlet duct 6'. The guide baffles 13a-d thus divide the curved section 6"of the final inlet duct 6'into five substantially parallel part-

ducts cl 5. Each of the part-ducts cl 5 has an inlet cross-sectional area aine 5 and an outlet cross-sectional area agouti-s for the flowing medium. The guide baffles 13a-d may be arranged at different angles relative to one another to provide said part-ducts ci-s with increasing ! decreasing cross-sectional areas in the direction of flow of the medium. A decreasing cross-sectional area in the direction of flow of the medium leads to the medium having an increased flow velocity through the part-duct cl 5 and a higher dynamic pressure. This leads to the static pressure Ps being raised upstream from the particular part-duct ci-s. Alternatively, a part-duct cl 5 may incorporate an increasing cross-sectional area in the direction of flow of the medium, in which case the medium will have a reduced flow velocity and a lower dynamic pressure. This leads to the static pressure Ps being lowered upstream from the particular part-duct ci-s. The part-ducts ci-s may be given an appropriate shape to create a desired static pressure distribution Ps across the final inlet duct 6'in the region adjacent to the venturi's third portion 1 lc.

It is advantageous to achieve a static pressure distribution Ps with an extent which substantially corresponds to that depicted in Fig. 3. Such a static pressure distribution Ps results in a lower static pressure Ps in the flowing medium in the peripheral portion of the inlet duct and a higher static pressure Ps in the latter's central portion. The flowing medium will thus have a higher dynamic pressure and hence an increasing velocity in the peripheral portion of the final inlet duct 6'. This leads to equalisation of the medium's respective velocities in the peripheral portion and the central portion. Such equalisation of the medium's velocities in said portions counteracts growth of the boundary layer upstream in the third portion 1 lc of the venturi and hence the risk of unstable flow.

Guide baffles 13a-d forming suitably shaped part-ducts c5 can thus be used to achieve uniform and stable flow of the medium after the venturi's third portion 1 lc without having to arrange a long straight section after the venturi. This means that the apparatus can be of relatively compact design. As the medium is alternately drawn to different cylinders arranged at different points in the diesel engine 1 via the branched induction line 12, there is substantially continuous changing of direction of the medium's flow downstream from the guide baffles 13a-d. This makes it difficult to give the part-ducts cl- 5 a shape which provides an optimum static pressure distribution for flow of medium to all the cylinders. The shape of the part-ducts cl 5 has therefore to be experimented with so

that an acceptable static pressure distribution Ps is continuously maintained irrespective of which cylinder the medium is drawn towards at the time.

The guide baffles 13a-d may be incorporated in a insert package which can be fitted detachably or permanently at an appropriate point in the final inlet duct 6'. Alternatively, the guide baffles 13a-d may be incorporated in an integrated unit which incorporates the curved section 6"of the final inlet duct 6'. For example, such a curved section 6"may be manufactured as a die-cast unit.

The invention is in no way limited to the embodiments described in the drawings but may be varied freely within the scopes of the claims. For example, the guide baffles 13a-d may be arranged in substantially any desired quantity. The guide baffles 13a-d should in general be at least three in number so as to create four part-ducts. The guide baffles 13a-d need not be arranged in a curved section 6"but may also be arranged in a straight section of the final inlet duct 6'. It is not necessary to use the guide baffles 13a-d as flow guide means, as the flow guide means may be of substantially any functional shape. The shape and positioning of the flow guide means has therefore to be such as to create a pressure distribution in the final inlet duct 6'such as to counter growth of the boundary layer of the medium in the venturi's third portion 1 lc situated upstream.