SE1550729A1 - Exhaust Manifold - Google Patents

Abstract

16 ABSTRACT Herein an exhaust manifold (2) is disclosed. A first inlet passage (8) connects to a mainpassage (12) at a junction. An intersecting wall portion (20) between the first inlet passage(8) and the main passage (12) forms an upstream end portion of the junction. A first portion(22) of the first inlet passage (8) has a first inlet passage cross sectional area (24). The firstinlet passage cross sectional area (24) is smaller than a second inlet passage crosssectional area (28) of a second portion (26) of the first inlet passage (8) upstream of the firstportion (22) of the first inlet passage (8). The main passage (12) comprises a first mainpassage portion (30) at the intersecting wall portion (20). The first main passage portion (30)having a first main passage cross sectional area (32). The first main passage cross sectionalarea (32) is smaller than a second main passage cross sectional area (48) of a second mainpassage portion (40) upstream of the junction (18). Elected for publication: Fig. 3

Description

Exhaust I\/lanifold TECHNICAL FIELDThe present invention relates to an exhaust manifold for a four-stroke internal combustion engine.

BACKGROUND An exhaust manifold of an internal combustion engine conducts exhaust gas from usuallytvvo or more cylinders towards a common exhaust conduit for the two or more cylinders. Thecommon conduit leads to further parts of an exhaust system, such as e.g. a turbocharger, anexhaust gas cleaning system, and/or a silencer. Since the exhaust manifold is connected tomore than one cylinder of the combustion engine, it has to be ensured that the flows ofexhaust gas from the respective cylinders are directed towards an outlet end of the exhaust manifold.

WO 97/04222 discloses a collector for the primary pipes of an exhaust manifold of an internalcombustion engine. The collector comprises a plurality of primary pipes, each leading from acylinder of the combustion engine. An outlet end of each of the primary pipes has a cross-sectional area which is less than the main cross-sectional area of the primary pipe. Eachoutlet end of reduced cross-sectional area is in direct communication with a common cavityof the collector, at one end of the cavity. The reduction in area is sufficient to provide a pulseconversion effect so that the pressure waves of one cylinder do not negatively affect the gasflows of other cylinders. The area reduction is in the order of 5-10%.

US 5860278 discloses an exhaust manifold with several inlet ports and a combination ofdiffuser and nozzle portions associated with each inlet port for producing a high flow rate anda low pressure drop from entry to exit. The diffuser and nozzle portions are arranged inseries and designed into the manifold at each inlet port and slightly upstream of the nextdownstream positioned inlet port. The diffuser portion decreases the velocity of exhaustgases as they enter the manifold and are then turned substantially ninety degrees into alongitudinal direction of the manifold. This diffuser action slows the velocity and reducespressure losses as the turn of exhaust flow is accomplished. The next in series nozzleportion increases the velocity of exhaust gases to cause the exhaust flow to accelerate pasta neighboring downstream engine exhaust port.

Similarly, US 2006/236687 discloses an exhaust manifold with deflector members positioned between downstream gas inlets from cylinders of an internal combustion engine and a main 2 gas passage. The deflector members redirect the flow of exhaust gas from the inlets into thegeneral direction of the gas flowing in the main passage at an angle of less than 90 degreesas the exhaust gas enters the main passage. At the deflector member towards the inletpassage, the formed cross section area is no less than that of the inlet passage. At thedeflector member in the main passage, at least the same cross section area as an upstreaminlet passage is provided. As the upstream exhaust gas flow passes by the outside surface ofthe deflector member in the main passage, a low pressure area is created on the inletpassage side of the deflector.

SUMMARYlt is an object of the present invention to provide an exhaust manifold wherein inter-cylinderdisturbance of exhaust gas flow is at least alleviated.

According to an aspect of the invention, the object is achieved by an exhaust manifold for afour-stroke internal combustion engine, the exhaust manifold forming at least two inletpassages and one main passage having an outlet end arranged downstream of the at leasttwo inlet passages. A first inlet passage of the at least two inlet passages connects to themain passage at a junction. An intersecting wall portion between the first inlet passage andthe main passage forms an upstream end portion of the junction. A first portion of the firstinlet passage at the intersecting wall portion has a first inlet passage cross sectional area,and the first inlet passage cross sectional area is smaller than a second inlet passage crosssectional area of a second portion of the first inlet passage located upstream of the firstportion of the first inlet passage. The first inlet passage is connected to the main passagedownstream of a second inlet passage of the at least two inlet passages. The main passagecomprises a first main passage portion at the intersecting wall portion, the first main passageportion having a first main passage cross sectional area. The first main passage crosssectional area is smaller than a second main passage cross sectional area of a second main passage portion of the main passage located upstream of the junction. lt has been realised by the inventors that in addition to exhaust gas flow from one cylinder inan exhaust manifold towards an outlet of the exhaust manifold, a portion of exhaust gas willtend to flow also backwards in the exhaust manifold towards an upstream cylinder. Such flowbackwards will affect exhaust gas discharge from the upstream cylinder, which may reduceoverall available exhaust gas power to be utilised in e.g. a downstream turbochargerconnected to the outlet end of the exhaust manifold.

Accordingly, a charge of exhaust gas from a cylinder connected to the first inlet passage isaccelerated past the junction by the smaller first inlet passage cross sectional area, 3 compared to the second inlet passage cross sectional area. Moreover, the first main passagecross sectional area being smaller than the second main passage cross sectional areareduces the flow of exhaust gas in an upstream direction in the main passage towards thesecond inlet. Thus, exhaust gas from the first inlet passage disturbing flow of exhaust gasfrom the second inlet passage is reduced compared to if the first main passage crosssectional area were not reduced. As a result, the accelerated exhaust gas from the first inletpassage, when entering the main passage, is further subjected to a comparatively high flowresistance in the upstream direction, promoting flow of exhaust gas from the first inletpassage in a downstream direction in the main passage. Accordingly, the above mentionedobject is achieved.

The exhaust manifold is configured for conducting exhaust gas from each cylinder connectedto the at least two inlet passages into the main passage. The main passage forms a commonexhaust conduit for the cylinders connected to the at least two inlet passages. The mainpassage leads to further parts of an exhaust system. For instance, the outlet end of the mainpassage may be connected to e.g. a turbocharger, and/or an exhaust gas cleaning system, and/or a silencer.

According to embodiments, a main passage axis may extend centrally through the secondmain passage portion in a straight line past the junction and centrally through a third mainpassage portion of the main passage located downstream of the first main passage portion.

According to embodiments, a centre axis of the first inlet passage at the first portion of thefirst inlet passage may extend at an acute angle to an upstream direction of the mainpassage axis. ln this manner the exhaust gas emanating from a cylinder connected to thefirst inlet passage may be directed in a downstream direction in the main passage, towardsthe outlet end of the main passage. Moreover, the acceleration of the exhaust gas caused bythe reduced first inlet passage portion cross sectional area may be utilised to ensure that theexhaust gas may reliably pass the junction in the downstream direction, According to some embodiment the centre axis of the first inlet passage, at the first portion ofthe first inlet passage, may extend at an angle within a range of 30 - 45 degrees to theupstream direction of the main passage axis. ln this manner it may be ensured that theexhaust gas from the cylinder connected to the first inlet passage is safely directed in thedownstream direction in the main passage. l\/loreover, at an angle within such a range, theacceleration of the exhaust gas caused by the reduced first inlet passage portion crosssectional area may be utilised to ensure that the exhaust gas may reliably pass the junction 4 in the downstream direction, which may reduce the amount of exhaust gases flowing in anupstream direction past the junction towards the second in|et passage.

According to embodiments, the intersecting wall portion may form at least part of a deflectorarranged between the first in|et passage and the main passage. The deflector may extendtowards the main passage axis. ln this manner exhaust gas from the first in|et passage maybe directed in a downstream direction in the main passage by the deflector. l\/loreover,exhaust gas in the main passage, e.g. from the second in|et passage, may be directed awayfrom the junction by the deflector. Thus, the deflector may direct exhaust gas from both thefirst in|et passage and the second in|et passage.

According to embodiments, the deflector may comprise a first wall portion forming adelimiting surface of the main passage. The first wall portion may extend at an acute angle tothe upstream direction of the main passage axis being smaller than the angle of the centreaxis of the first portion of the first in|et passage to the upstream direction. ln this mannerexhaust gas in the main passage, e.g. from the second in|et passage, may be directed awayfrom the junction by the first wall portion of the deflector. lt has been discovered that an angleof the first wall portion smaller than the angle of the centre axis of the first portion of the firstin|et passage may be sufficient to reduce a tendency of the exhaust gas from the secondin|et passage to flow into the first in|et passage.

According to embodiments, the main passage may comprise a widening section extending atleast upstream of the first main passage portion, which widening section widens the mainpassage in a first direction. The first direction extends in a first cross sectional plane, whichfirst cross sectional plane extends through the first in|et passage, through at least a portion ofthe main passage, extends along the main passage axis, and includes the main passageaxis. The first direction is directed away from the first in|et passage. Since the wideningsection is arranged at least upstream of the junction in the main passage and widens in adirection away from the first in|et passage, exhaust gas from the first in|et passage flowingupstream in the main passage will flow into the widening section and form a circulating flowof exhaust gas therein. Accordingly, the first fraction of a charge of exhaust gas from the firstin|et passage will form a vortex in the widening section, which vortex reduces the effectiveflow area in the widening section for the remaining portion of the charge of exhaust gas fromthe first in|et passage. The effective flow area reduced by the vortex prevents further partialamounts of the exhaust gas from the first in|et passage to flow in an upstream direction fromthe junction. 5 The widening section may form a bulging portion of the main passage, which bulging portionbulges in a direction away from the first inlet passage. Whereas the widening section widensthe main passage in the first direction e.g. compared to the second main passage portion,the widening section, along a different cross-sectional direction, may have a smallerdimension than the second main passage portion.

According to embodiments, an inner surface of the main passage, opposite to the first inletpassage, at a transition between the widening section and the second main passage portionmay form an abrupt directional change, such that a vortex may be formed in exhaust gasesduring use of the exhaust manifold. ln this manner the abrupt directional change may assistin forming of the vortex in the widening section. Put differently, the inner surface may beprovided with an abrupt directional change at a point of the inner surface, to which a vortex inthe widening section extends. The abrupt directional change may be provided opposite to thefirst inlet passage, seen in the first cross sectional plane. The vortex in the widening sectionis formed by the first fraction of each charge of exhaust gas from the first inlet passage.

According to embodiments, the widening section may extend upstream of an end point of theintersecting wall portion over a distance having a length within a range of 1,2 - 1,7 times ahydraulic diameter of the second main passage portion. ln this manner a vortex may beformed in the widening section, which vortex reduces flow of exhaust gas upstream of thejunction, but which vortex does not affect flow of exhaust gas in a downstream direction fromthe junction to any particular extent. Seen in the first cross sectional plane, the wideningsection may extend upstream of the end point of the intersecting wall portion to the abruptdirectional change.

According to embodiments, the widening section may extend downstream of the end point ofthe intersecting wall portion over a distance having a length within a range of 2 - 3 times ahydraulic diameter of the second main passage portion. ln this manner the downstream endof the widening section does not promote flow of exhaust gas from the first inlet passage inthe upstream direction in the main passage.

Further features of, and advantages with, the present invention will become apparent whenstudying the appended claims and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS 6 Various aspects of the invention, including its particular features and advantages, will bereadily understood from the example embodiments discussed in the following detaileddescription and the accompanying drawings, in which: Fig. 1 illustrates a cross section through an exhaust manifold according to exemplaryembodiments adapted for use with a four-stroke internal combustion engine, and Figs. 2 and 3 illustrate cross sections through part of the exhaust manifold of Fig. 1.

DETAILED DESCRIPTION Aspects of the present invention will now be described more fully. Like numbers refer to likeelements throughout. Well-known functions or constructions will not necessarily be describedin detail for brevity and/or clarity.

Herein the term charge of exhaust gas refers to the exhaust gas discharged from onecombustion cycle of one cylinder. Since fuel combusts in the different cylinders of thecombustion engine at different angles of a crankshaft of the engine, charges of exhaust gasflow through different inlet passages of an exhaust manifold at different points in time. Theterm downstream relates to the flow direction in an exhaust manifold from a respectivecylinder to an outlet end of the exhaust manifold. The term upstream relates to the oppositeflow direction in the exhaust manifold.

Herein the term hydraulic diameter will be used for the purpose of comparing cross sectionalsizes and distances of various portions and sections of an exhaust manifold. The hydraulicdiameter is a commonly used term when calculating flow related parameters in noncircularpassages, such as tubes or channels. Using the hydraulic diameter, parameters of apassage may be calculated in the same manner as for a circular passage. The hydraulicdiameter, DH, is defined as: DH = 4A/P where A is the actual cross sectional area of a relevant passage portion, and P is the wettedperimeter, i.e. in this case the length of the inner perimeter, of the relevant cross-section.

Herein a cross sectional area of a passage portion is always calculated based on thehydraulic diameter of the relevant cross sectional area. Accordingly, herein a cross sectionalarea of a passage portion, AH, is defined as: AH = nDH2/4 7 This entails for instance that the actual cross sectional areas, A, of two passage portionsmay be the same, whereas the cross sectional areas, AH, based on the hydraulic diametersof the two passage portions may be of different sizes due to the two passage portions havingdifferent cross sectional shapes. lf not defined otherwise, herein a cross section of a passage portion extends perpendicularlyto a centre line of the relevant passage portion.

Fig. 1 illustrates a cross section through an exhaust manifold 2 according to exemplaryembodiments adapted for use with a four-stroke internal combustion engine. ln the illustratedembodiment, the exhaust manifold 2 is adapted to be attached to three cylinders of theinternal combustion engine (not shown). The internal combustion engine may be e.g. of atype having three or more cylinders arranged in a straight line, or of a type having cylindersarranged in a V configuration, such as in a V-6 or V-8 engine.

The exhaust manifold 2 forms a plurality of exhaust gas inlet branches 4 arranged in seriesand connected to a main branch 6. ln these embodiments, the exhaust manifold 2 comprisesthree exhaust gas inlet branches 4. ln alternative embodiments an exhaust gas manifold maycomprise only two exhaust gas inlet branches, or more than three inlet branches, dependinginter alia on the cylinder configuration of a relevant internal combustion engine. Each exhaustgas inlet branch 4 defines an inlet passage 8, 10, 14. The inlet passages 8, 10, 14 eachreceive a charge of exhaust gas from an associated exhaust opening (not shown) of acylinder head of the internal combustion engine. The main branch 6 defines a main passage12. The exhaust manifold 2 comprises walls having inner surfaces. The inner surfaces delimitthe inlet passages 8, 10, 14 and the main passage 12. Cross sectional areas of thepassages 8, 10, 14, 12 are delimited by the inner surfaces.

The exhaust manifold 2 forms at least two inlet passages 8, 10, 14 and one main passage12. The main passage 12 has an outlet end 16 arranged downstream of the at least two inletpassages 8, 10, 14.

The exhaust manifold 2 generally extends longitudinally with a closed fon/vard end portionand the open outlet end 16. When attached to the associated engine cylinder head, themanifold 2 is secured so as to align its inlet passages 8, 10, 14, with exhaust gas outletopenings of the cylinder head. As each exhaust valve opens for an associated cylinder, acharge of exhaust gas flows from the cylinder head into the associated inlet passage 8, 10,14, and from the inlet passage into the main passage 12. The outlet end 16 of the main 8 passage 12 may be connected to e.g. a turbocharger of the internal combustion engine. Theexhaust manifold 2 may be provided with non-shown flanges for connecting the exhaustmanifold 2 to the cylinder head, exhaust conduits, etc.

A first inlet passage 8 of the at least tvvo inlet passages connects to the main passage 12 atajunction 18. The at least two inlet passages 8, 10, 14 are connected in series to the mainpassage 12. The first inlet passage 8 is connected to the main passage 12 downstream of asecond inlet passage 10 of the at least two inlet passages 8, 10, 14. A third inlet passage 14of the at least two inlet passages 8, 10, 14 is connected to the main passage 12 downstreamof the first inlet passage 8. The third inlet passage 14 and its connection to the main passage12 are substantially identical to the first inlet passage 8 and its connection to the mainpassage 12. The difference between the second inlet passage 10, and the first and third inletpassages 8, 14 is that the second inlet passage 10 does not have any inlet passagearranged upstream thereof along the main passage 12. Accordingly, a charge of exhaust gasflowing from the second inlet passage 10 into the main passage 12 is subjected to differentflow conditions than charges of exhaust gas flowing from the first or third inlet passage 8, 14into the main passage 12.

The exhaust manifold 2 may be manufactured e.g. by sand casting. Different parts of theexhaust manifold 2 may be cast separately and thereafter joined to form the exhaustmanifold.

Figs. 2 and 3 illustrate cross sections through part of the exhaust manifold 2 of Fig. 1. I\/Iorespecifically, the cross sections of Figs. 2 and 3 illustrate a portion of the main passage 12and the first inlet passage 8 connecting thereto.

An intersecting wall portion 20 between the first inlet passage 8 and the main passage 12forms an upstream end portion of the junction 18. A first portion 22 of the first inlet passage 8at the intersecting wall portion 20 has a first inlet passage cross sectional area 24. A secondportion 26 of the first inlet passage 8 arranged upstream of the first portion 22 of the first inletpassage 8 has a second inlet passage cross sectional area 28. The second portion 26 of thefirst inlet passage 8 is in these embodiments arranged at an inlet 29 of the first inlet passage8. The first inlet passage cross sectional area 24 is smaller than the second inlet passagecross sectional area 28. Thus, also the hydraulic diameter of the first inlet passage crosssectional area 24 is smaller than the hydraulic diameter of the second inlet passage crosssectional area 28. 9 The main passage 12 comprises a first main passage portion 30 at the intersecting wallportion 20. The first main passage portion 30 has a first main passage cross sectional area32. The main passage 12 comprises a second main passage portion 40 upstream of thejunction 18. The second main passage portion 40 has a second main passage crosssectional area 48. The first main passage cross sectional area 32 is smaller than the secondmain passage cross sectional area 48. Thus, also the hydraulic diameter of the first mainpassage cross sectional area 32 is smaller than the hydraulic diameter of the second mainpassage cross sectional area 48.

The first portion 22 of the first inlet passage 8 may be positioned at an end portion 37 of theintersecting wall portion 20. Also the first main passage portion 30 may be positioned at theend portion 37 of the intersecting wall portion 20.

A main passage axis 38 extends centrally through the second main passage portion 40 in astraight line past the junction 18 and centrally through a third main passage portion 34downstream of the first main passage portion 30. The first main passage cross sectional area32 extends perpendicularly to the main passage axis 38.

A centre axis 42 of the first inlet passage 8 at the first portion 22 of the first inlet passage 8extends at an acute angle d to an upstream direction 44 of the main passage axis 38. Theangle d may be within a range of 30 - 45 degrees. Thus, a charge of exhaust gas flowingthrough the first inlet passage 8 is directed in a downstream direction 46 in the main passage12. Due to the reduction of the cross sectional area of the first inlet passage 8 from thesecond portion 26 of the first inlet passage 8 to the first portion 22 of the first inlet passage 8,the charge of exhaust gas is accelerated past the junction 18, in the downstream direction 46of the main passage 12. The acute angle d to the upstream direction 44 of the main passageaxis 38 entails that the acute angle d points in the downstream direction 46 of the main passage axis 38.

The smaller first main passage cross sectional area 32 compared to the second mainpassage cross sectional area 48 reduces the portion of a charge of exhaust gas from the firstinlet passage 8 flowing in the upstream direction 44 in the main passage 12.

According to embodiments, the first inlet passage cross sectional area 24 may have ahydraulic diameter within a range of 0,65 - 0,85 times a hydraulic diameter of the secondinlet passage cross sectional area 28. Preferably, the first inlet passage cross sectional area24 may have a hydraulic diameter within a range of 0,68 - 0,75 times the hydraulic diameter of the second inlet passage cross sectional area 28. ln this manner a charge of exhaust gasfrom a cylinder connected to the first inlet passage 8 is accelerated at a rate promoting flowof exhaust gas from the junction 18, in the downstream direction 46 in the main passage 12.

According to embodiments, the first main passage cross sectional area 32 may have ahydraulic diameter within a range of 0,8 - 0,99 times a hydraulic diameter of the second mainpassage cross sectional area 48. Preferably, the first main passage cross sectional area 32may have a hydraulic diameter within a range of 0,92 - 0,98 times the hydraulic diameter ofthe second main passage cross sectional area 48. ln this manner the flow of exhaust gasfrom the first inlet passage 8 in the upstream direction 44 in the main passage 12 is reducedcompared to if the first and second main passage portions 30, 40 had more similar crosssectional areas, while flow of exhaust gas from the upstream located second inlet passagetowards the outlet end of the main passage 12 is affected less negatively, or even not at all.

According to embodiments, the second inlet passage cross sectional area 28 and a thirdmain passage cross sectional area 36 of the third main passage portion 34 may havehydraulic diameters of substantially the same size. ln this manner exhaust gas pressure andflow from the first inlet passage 8 may remain consistent downstream of the junction 18 withthose at the inlet 29 of the first inlet passage 8 thus, providing consistent exhaust gasproperties towards the outlet end of the exhaust manifold 2. The hydraulic diameters of thesecond inlet passage cross sectional area 28 and the third main passage cross sectionalarea 36 may differ within the range of 1 - 3 %.

According to embodiments, the second main passage cross sectional area 48 may have asame hydraulic diameter as the third main passage cross sectional area 36. ln this mannerexhaust gas pressure and flow from the second inlet passage upstream of the first inletpassage 8 may remain consistent downstream of the junction 18 with those upstream of thejunction 18 thus, providing consistent exhaust gas properties towards the outlet end of theexhaust manifold 2.

According to some embodiments a hydraulic diameter of a cross sectional area of the outletend 16 of the exhaust manifold 2, see Fig. 1, may have substantially a same hydraulicdiameter as those of the second and third main passage cross sectional areas 48, 36. Thus,consistent exhaust gas properties may be maintained along portions of the main passage 12and towards the outlet end 16 of the exhaust manifold 2. Mentioned purely as an example,the second and third cross sectional areas 48, 36 of the exhaust manifold 2 may havehydraulic diameters of 42 mm, for a V8 diesel engine having a total displacement of 16 litres. 11 The intersecting wall portion 20 forms at least part of a deflector 50 arranged between thefirst inlet passage 8 and the main passage 12. The deflector 50 extends into the mainpassage 12. That is, the deflector 50 extends towards the main passage axis 38. Thedeflector 50 comprises a first wall portion 52 forming a delimiting surface of the mainpassage 12. The first wall portion 52 extends at an acute angle ß to the upstream direction44 of the main passage axis 38 smaller than the angle d of the centre axis 42 of the firstportion 22 of the first inlet passage 8 to the upstream direction 44. ln case the first wallportion 52 is slightly curved, e.g. as in the illustrated embodiments, this feature entails that ageneral outline of the first wall portion 52 extends at an acute angle ß to the upstreamdirection 44 of the main passage axis 38 smaller than the angle oi. For instance, a tangent ofsuch a slightly curved wall portion may extend at an acute angle ß to the upstream direction44 of the main passage axis 38 smaller than the angle oi. The angle ß may be at least 1degree. The acute angle ß to the upstream direction 44 of the main passage axis 38 entails that the acute angle ß points in the downstream direction 46 of the main passage axis 38.

The main passage 12 comprises a widening section 54 extending at least upstream of thefirst main passage portion 30. The widening section 54 widens the main passage 12 in a firstdirection 56. The first direction 56 is directed away from the first inlet passage 8 in a firstcross sectional plane. The first cross sectional plane extends through the first inlet passage8, through at least a portion of the main passage 12, extends along the main passage axis38, and includes the main passage axis 38. The first cross sectional plane also includes thecentre axis 42 of the first inlet passage 8. That is, the cross sections of Figs. 2 and 3 areplaced in the first cross sectional plane. ln these embodiments, the widening section 54 alsoextends downstream of the first main passage portion 30. The widening section 54 mayextend between the second and third main passage portions 40, 34.

A portion of a charge of exhaust gas from the first inlet passage 8 will flow upstream in themain passage 12, despite the first main passage portion 30 having a smaller cross sectionalarea than the second main passage portion 40. The widening section 54 will cause theexhaust gas to form a vortex 58 in the main passage 12, upstream of the first passageportion 30. The vortex 58 reduces the effective flow area 60 of the main passage 12. Thus,for a latter portion of a charge of exhaust gas from the first inlet passage 8, the vortex 58reduces the flow of exhaust gas in a direction upstream of the first main passage portion 30.

An inner surface 61 of the main passage 12, opposite to the first inlet passage 8 seen in thefirst cross sectional plane, at a transition between the widening section 54 and the second 12 main passage portion 40 forms an abrupt directional change 63, such that a vortex is formedin exhaust gases during use of the exhaust manifold 2. The vortex in the widening section isformed by the first fraction of each charge of exhaust gas from the first inlet passage 8. Theinner surface 61 is an inner surface of a second wall portion 62 extending along the wideningsection 54 and the second main passage portion 40. Seen in the first cross sectional plane,the widening section 54 extends upstream of an end point 64 of the intersecting wall portion20 over a distance having a length within a range of 1,2 - 1,7 times a hydraulic diameter ofthe second main passage portion 40. The widening section 54 may extend upstream of theend point 64 to the abrupt directional change 63. Seen in the first cross sectional plane, thewidening section 54 extends downstream of the end point 64 of the intersecting wall portion20 over a distance having a length within a range of 2 - 3 times a hydraulic diameter of thesecond main passage portion 40. Suitably, a transition between the widening section 54 andthe third main passage portion 34 is gradual. The gradual transition between the wideningsection 54 and the third main passage portion 34 promotes a laminar flow of exhaust gas inthe downstream direction 46. Thus, no vortex is formed at the gradual transition, as opposedto at the abrupt directional change 63 at the upstream end of the widening section 54 where a vortex is formed.

The end portion 37 of the intersecting wall portion 20 comprises the end point 64 in the firstcross sectional plane. ln Fig. 3, outlines 66 of three inner perimeters of the main passage 12 according to anembodiment are illustrated.

This invention should not be construed as limited to the embodiments set forth herein. Aperson skilled in the art will realize that different features of the embodiments disclosedherein may be combined to create embodiments other than those described herein, withoutdeparting from the scope of the present invention, as defined by the appended claims.

As used herein, the term "comprising" or "comprises" is open-ended, and includes one ormore stated features, elements, steps, components or functions but does not preclude thepresence or addition of one or more other features, elements, steps, components, functionsor groups thereof.

Claims (13)

1. An exhaust manifold (2) for a four-stroke internal combustion engine, the exhaust manifold(2) forming at least two inlet passages and one main passage (12) having an outlet end (16)arranged downstream of the at least two inlet passages, wherein a first inlet passage (8) of the at least two inlet passages connects tothe main passage (12) at ajunction (18), wherein an intersecting wall portion (20) between the first inlet passage (8) andthe main passage (12) forms an upstream end portion of the junction (18), wherein a first portion (22) of the first inlet passage (8) at the intersecting wallportion (20) has a first inlet passage cross sectional area (24), and wherein the first inlet passage cross sectional area (24) is smaller than asecond inlet passage cross sectional area (28) of a second portion (26) of the first inletpassage (8) located upstream of the first portion (22) of the first inlet passage (8), characterised in that the first inlet passage (8) is connected to the main passage (12) downstream ofa second inlet passage (10) of the at least two inlet passages, wherein the main passage (12) comprises a first main passage portion (30) atthe intersecting wall portion (20), the first main passage portion (30) having a first mainpassage cross sectional area (32), and wherein the first main passage cross sectional area (32) is smaller than asecond main passage cross sectional area (48) of a second main passage portion (40) of themain passage (12) located upstream of the junction (18).

2. The exhaust manifold (2) according to c|aim 1, wherein a main passage axis (38) extendscentrally through the second main passage portion (40) in a straight line past the junction(18) and centrally through a third main passage portion (34) of the main passage (12) locateddownstream of the first main passage portion (30).

3. The exhaust manifold (2) according to c|aim 2, wherein a centre axis (42) of the first inletpassage (8) at the first portion (22) of the first inlet passage (8) extends at an acute angle (oi)to an upstream direction (44) of the main passage axis (38), preferably at an angle (oi) withina range of 30 - 45 degrees.

4. The exhaust manifold (2) according to any one of claims 2 or 3, wherein the intersectingwall portion (20) forms at least part of a deflector (50) arranged between the first inletpassage (8) and the main passage (12), and wherein the deflector (50) extends towards themain passage axis (38). 14

5. The exhaust manifold (2) according to claims 2 - 4, wherein the deflector (50) comprises afirst wall portion (52) forming a delimiting surface of the main passage (12), and wherein thefirst wall portion (52) extends at an acute angle (ß) to the upstream direction (44) of the mainpassage axis (38) being smaller than the angle (d) of the centre axis (42) of the first portion (22) of the first inlet passage (8) to the upstream direction (44).

6. The exhaust manifold (2) according to any one of the preceding claims, wherein the mainpassage (12) comprises a widening section (54) extending at least upstream of the first mainpassage portion (30), which widening section (54) widens the main passage (12) in a firstdirection (56), the first direction (56) extending in a first cross sectional plane, which firstcross sectional plane extends through the first inlet passage (8), through at least a portion ofthe main passage (12), extends along the main passage axis (38), and includes the mainpassage axis (38), and wherein the first direction (56) is directed away from the first inlet passage (8).

7. The exhaust manifold (2) according to claim 6, wherein an inner surface (61) of the mainpassage (12), opposite to the first inlet passage (8), at a transition between the widening section (54) and the second main passage portion (40) forms an abrupt directional change(63), such that a vortex is formed in exhaust gases during use of the exhaust manifold (2).

8. The exhaust manifold (2) according to claim 6 or 7, wherein the widening section (54)extends upstream of an end point (64) of the intersecting wall portion (20) over a distancehaving a length within a range of 1,2 - 1,7 times a hydraulic diameter of the second mainpassage portion (40).

9. The exhaust manifold (2) according to any one of claims 6 - 8, wherein the wideningsection (54) extends downstream of an end point (64) of the intersecting wall portion (20)over a distance having a length within a range of 2 - 3 times a hydraulic diameter of thesecond main passage portion (40).

10. The exhaust manifold (2) according to any one of the preceding claims, wherein thesecond portion (26) of the first inlet passage (8) is arranged at an inlet (29) of the first inletpassage (8), and wherein the first inlet passage cross sectional area (24) has a hydraulicdiameter within a range of 0,65 - 0,85 times a hydraulic diameter of the second inlet passagecross sectional area (28), preferably within a range of 0,68 - 0,75 times the hydraulicdiameter of the second inlet passage cross sectional area (28).

11. The exhaust manifold (2) according to claim 10, wherein the first main passage crosssectional area (32) has a hydraulic diameter within a range of 0,8 -0,99 times a hydraulicdiameter of the second main passage cross sectional area (48), preferably within a range of0,92 - 0,98 times the hydraulic diameter of the second main passage cross sectional area(48).

12. The exhaust manifold (2) according to claim 10 or 11, wherein the second inlet passagecross sectional area (28) and a third main passage cross sectional area (36) of the third mainpassage potion (34) have hydraulic diameters of substantially the same size.

13. The exhaust manifold (2) according to any one of the preceding claims, wherein thesecond main passage cross sectional area (48) has a same hydraulic diameter as a thirdmain passage cross sectional area (36) of the third main passage portion (34).