DE69131403T2 - Internal combustion engine - Google Patents

Description

The present invention relates to the field of V-type internal combustion engines.

More specifically, the invention relates to a V-type internal combustion engine having the features of the preamble of claim 1. An engine of this type is known from US-A-4,592,314.

Recently, five-valve engines having three intake valves and two exhaust valves per cylinder head have been increasingly used in order to increase the loading efficiency and hence the power of the engine. On the other hand, difficulties arise in distributing such an increased number of intake and exhaust valves above the combustion chamber of each cylinder of such an engine. With regard to the design of the cylinder head of such an engine, care should be taken to keep the height of the engine as low as possible, while ensuring sufficient cooling capability of the cooling jacket arrangement extending around the cylinder head. In view of the valve actuating mechanism for such an engine, each intake or exhaust valve has a valve lifter at the upper end of the valve stem of each of the valves which is urged by lobes of an associated camshaft, and a biasing valve spring is installed between a valve retaining clip secured on the upper portion of each valve stem and a valve spring seat provided on the inner wall portion of the cylinder head for each of the intake and exhaust valves.

Engines, such as a five-valve engine, which have a greater number of intake valves than exhaust valves for each cylinder often use exhaust valves which, due to their fewer number relative to the intake valves, are designed to be larger in diameter to ensure the necessary cross-sections of the exhaust passageways. Accordingly, the mass of such an exhaust valve exceeds that of the intake valve, and exhaust valves which are heavier than intake valves have been used in many cases.

Furthermore, the valve lift of the exhaust valves has been set to be greater than that of the intake valves. In view of the above-mentioned conditions, the preloading valve spring for the exhaust valve requires a larger diameter and a larger spring constant in order to urge the heavier exhaust valve to its closed position without any malfunction, and the length of the valve spring must be greater than that of the intake valve springs to enable the valve lift of the exhaust valves to be increased. The increased length contributes to a considerable increase in the overall height of the machine.

In any case, it is desirable to reduce the height of the engine as much as possible. In view of the above-mentioned requirements, a limited engine height requires that the valve spring seat of the exhaust valve be lowered in order to maintain the necessary valve lift. However, a lowered valve spring seat normally consumes some space required for the cooling arrangement of the cylinder head, in particular for the cooling jacket on the exhaust side near the exhaust passageway for the exhaust gas. Consequently, lowering the position of the valve spring seat on the exhaust side results in a smaller cooling jacket at that area and reduced cooling effectiveness. On the other hand, the valve lift of the exhaust valve would be insufficient.

Furthermore, on the intake valve side, the increased number of them results in a very narrow space available only between the adjacent intake valves, and the distance between them becomes considerably small, resulting in problems in ensuring the space for accommodating the valve lifters of the intake valves.

In addition to the problems noted above, particularly for V-type engines, the camshaft drive mechanism comprising an intermediate gear, an intermediate sprocket, or an intermediate pulley (depending on the type of transmission used to drive the camshaft from the crankshaft) should not unnecessarily restrict the space inside the V-bank for locating and servicing auxiliary devices located therein. Accordingly, components of the camshaft drive transmission or housings therefor should not protrude into the space between the two banks of a V-type internal combustion engine.

Accordingly, it is an object of the present invention to provide a V-type engine having an improved and more compact layout which increases the space within the V-bank arrangement between the pair of cylinder banks so as to accommodate auxiliary devices therein.

According to the present invention, this object is achieved by an internal combustion engine having the features of claim 1.

According to a preferred embodiment of the invention, an intermediate shaft is associated with each of the cylinder banks for driving the respective pair of intake and exhaust camshafts, each of the intermediate shafts being supported for rotation about an axis which is mounted parallel to the intake and exhaust camshafts and laterally offset with respect to the center plane of the respective cylinder bank towards the exhaust side thereof.

According to another preferred embodiment of the invention, each of the intermediate shafts is provided with an intermediate gear in engagement with a pair of cam gears fixed to the respective pair of intake and exhaust camshafts. Preferably, the intermediate gears are in engagement with a common crankshaft output gear for driving both intermediate gears, the crankshaft output gear being driven by a crankshaft gear fixed to the crankshaft.

According to another preferred embodiment of the invention, the intermediate shafts are connected to the respective pair of intake and exhaust camshafts via drive belts and are connected to the camshaft via another drive belt.

Further preferred embodiments of the invention are specified in the further dependent claims.

In the following, the present invention will be explained and illustrated in more detail using preferred embodiments of the invention in conjunction with the accompanying drawings, wherein:

Fig. 1 is a front view of a V-type internal combustion engine having a gear camshaft drive mechanism according to a first embodiment of the present invention,

Fig. 2 shows a front view of a V-type internal combustion engine having a Belt drive mechanism for the camshafts according to another preferred embodiment of the invention,

Fig. 3 is a schematic front view of a V-type internal combustion engine having a geared drive mechanism similar to Fig. 1 according to yet another embodiment of the present invention,

Fig. 4 is a front sectional view of a head portion of a right-hand cylinder of the machine shown in Fig. 3,

Fig. 5 shows a plan view of the upper head of the cylinder head of the engine according to the embodiment of Figs. 3 and 4.

Hereinafter, a V-type four-stroke internal combustion engine having five valves for each cylinder will be explained with reference to the accompanying drawings. First, the basic structure of the engine will be explained with reference to Fig. 1.

In Fig. 1, which schematically shows a front view of the engine 1, a cylinder block having a pair of cylinder banks arranged in a V-bank arrangement is shown. The cylinder block defines a plurality of cylinders or liners 6 arranged in the shape of a V as seen in the direction of the crankshaft shown in Fig. 1. A piston 7 is fitted in each cylinder 6 and is connected via a connecting rod 8 to a crankshaft 9, as usual.

As can be seen from Fig. 1, the valve actuating mechanism comprising an intake camshaft 24 and an exhaust camshaft 25 is constructed so as to avoid any obstruction of the internal space between the V-banks by the housing of the cylinder head for each bank of cylinders.

More specifically, as indicated in Figs. 1 and 2, which is a front view of a V-type four-stroke engine similar to that of Fig. 3, the distance A of an axis 24a of the intake camshaft 24 from a central plane C containing the axis of the cylinders of the cylinder bank as well as the axis of a crankshaft 9 is such that it is smaller than the distance B of the axis 25a of the exhaust camshaft 25 from this plane (c). Similarly, an intermediate sprocket of the camshaft drive system, such as the intermediate gear 35, is adapted to engage a pair of cam gears 24b, 25b attached to the respective intake and exhaust camshafts 24, 25, are rotatably supported by the cylinder head laterally offset by an amount D from the center plane C to avoid housing parts of the cylinder head protruding into the V-space adapted to accommodate auxiliary equipment such as an alternator 54 therein. In this way, a compact engine structure can be obtained, with the intake camshaft being arranged inwardly close to the V-space of the engine, while the exhaust camshaft 25 is arranged along the outside of the cylinder head 4 oppositely with respect to the center plane C.

In the embodiment of Fig. 1, the valve drive system includes a gear drive arrangement to drive both the intake and exhaust cam gears 24b, 25b from the intermediate gear 35 mounted on an intermediate gear shaft 35a, which in turn is driven by another gear drive structure comprising a crankshaft output gear. Of course, the valve actuating mechanism and camshaft drive chain may not only comprise gear trains, but may also be implemented by timing belts or timing chains which provide associated transmission elements, such as cam sprockets and cam pulleys on the camshafts, and which use an intermediate sprocket or pulley in place of the intermediate gear 35.

The embodiment according to Fig. 2 corresponds to that of the previous embodiment of Fig. 2 with regard to the asymmetrical offset of the camshafts 24, 25 on either side of the center plane C as well as with regard to the corresponding offset in the amount D of the intermediate pulley 42. In the case of Fig. 2, the drive belts or timing chains are used to drive the camshaft 24, 25 via respective cam sprockets or cam pulleys 24b, 25b from the intermediate sprocket or pulley 42b, establishing a camshaft drive train connected via the intermediate shaft 35a to another drive train driven by the crankshaft 9 in a conventional manner.

By means of the above-mentioned structure, the receiving space within the V-space of the machine can be used for maintenance or for arranging auxiliary equipment such as for example the alternator 54, which also helps to limit the height of the machine.

Hereinafter, the internal structure of a V-type engine will be explained in detail with reference to Figs. 3 to 5, which illustrate a third embodiment having generally the same layout as the previous embodiments.

In Fig. 3, which schematically shows a front view of the engine 1, there is shown a cylinder block 2 having a pair of cylinder banks arranged in a V-bank arrangement. The cylinder block defines a plurality of cylinders or liners 6 arranged in the shape of a V as seen in the direction of the crankshaft shown in Fig. 3. A piston 7 is fitted in each cylinder 6 and is connected via a connecting rod 8 to a crankshaft 9 as usual. The cylinder head 4 of each cylinder bank of the V-type engine is a mirror-symmetrical structure composed of an upper head 11 and a lower head 10 respectively. The lower head 10 defines combustion spaces 12 which in turn form a combustion chamber for each cylinder 6 defined by the front surface of the respective piston 7 slidably received therein.

As shown in Fig. 4, the combustion chamber 12 of each cylinder 6 has three intake ports 12a, 12b and 12c as well as two exhaust ports 12d and 12e arranged along the circumference of the combustion chamber 12, the central portion of which is formed with an insertion opening 121 adapted to receive an ordinary spark plug therein. The exhaust ports 12d and 12e are led out to the outer wall 10b of the cylinder head 4 extending along the side circumference of the V-shaped cylinder bank by means of the exhaust ports 13d and 13e. The intake ports 12a, 12b and 12c are led out to a wall 10a of the cylinder head 4, which is arranged on the inside of the V-shaped cylinder bank, by means of intake channels 13a, 13b, 13c which connect to each other via an extension portion 11c which extends through the upper head 11 and upwards therefrom. The connection portion 13f is shaped to be elliptical with the major diameter oriented parallel to the crankshaft axis. A mounting opening 11d for receiving a fuel injection valve 30 is provided so as to extend through a portion of the central intake passage 13b. A slide valve 39 for opening and closing the connecting portion 13f is arranged in the extension portion 11c of the intake passages, and an air nozzle 40 is connected to this extension portion 11c. In order to prevent dust or the like from entering the air nozzle 40, a cover 41 is provided.

As shown in Fig. 4, a cooling jacket for circulating cooling water from the cylinder block through the cylinder head is shown to be provided in the lower head 10. The cooling water jacket and the internal structure of the cylinder head are constructed to cover the combustion chamber 12. This cooling water jacket is composed of a water jacket 31a on the inlet side extending from the area of the inlet channels 13a, 13b and 13c to the side of the inner wall 10a of the lower head 10, another cooling jacket 31b arranged on the outlet side extending from the area of the outlet channels 13d and 13e to the outer side wall 10b of the lower head 10, and a central cooling jacket 31c extending substantially between the inlet channels 13a, 13b, 13c and the outlet channels 13d and 13e. The structure and arrangement of the different sections 31a, 31b, 31c of the water jacket arrangement are clearly shown in Fig. 4.

Communication holes 31e are drilled to communicate with upper portions of both the central cooling jacket 31c and the intake valve jacket 31a and extend laterally offset from the intake ports 13a and 13c, respectively. According to this structure, the cooling water of the cooling circuit flows from the cooling water jacket of the cylinder block 2 (not shown) into the cooling jacket 31b on the exhaust side of the cylinder head 4 and subsequently flows through the central jacket 31c and into the cooling jacket 31a arranged on the intake side of the cylinder head 4. From the intake side jacket 31a, the water is circulated so as to be discharged via the drain outlet 31d. At the beginning of each coolant circulation, any air present at the top portion of the central jacket 31c is discharged into the cooling jacket 31a on the inlet side via the communication openings 31e.

The inlet and outlet valves 14, 15 each have a valve stem 14b, 15b with Valve plates 14a, 15a at their lower end portions, which are suitable for opening or closing the intake ports 12a, 12b, 12c and the exhaust ports 12d and 12e, respectively. As can be seen from Figs. 4 and 5 (and also from Fig. 1), the lower portion, i.e. the valve plate 14a of the side intake valves associated with the intake ports 12a and 12c, crosses the median plane C containing the axis of the cylinders 6 of the cylinder bank, as well as the axis of the crankshaft 9. The upper end portion of the valve stems 14b, 15b of the intake and exhaust valves 14, 15 are arranged in guide openings 11a, 11b defined in the upper head 11. These guide holes 11a and 11b, as shown in more detail in Fig. 5, are formed in a unitary structure establishing a radially connected double (exhaust side) or triple (intake side) structure, respectively. Accordingly, the diameters of these guide holes 11a, 11b are sufficiently large to eliminate any boundary wall area between adjacent guide holes 11a, 11b on the intake or exhaust side. Furthermore, cast-in intake and exhaust inserts 16, 17 form liners for the guide holes 11a, 11b as a reinforcing structure, preferably made of a material different from that of the cylinder head to provide increased strength of the inserts 16, 17. In this way, the intake and exhaust inserts 16 and 17 form sliding holes for slidably receiving the intake and exhaust tappets 18, 19, respectively, which are a bottomed cylindrical shape in which the upper end of each valve stem 14b, 15b is engaged with the respective inside bottom portion of the intake and exhaust tappets 18, 19 via a cushion, respectively. Near the upper end of each valve stem 14b and 15b, a spring retainer 20, 21 is installed, which is adapted to retain the urging springs 22, 23 of the intake and exhaust valves 14, 15, respectively. Both valve pressing springs 22 and 23 of the intake and exhaust valves 14, 15 respectively are of a concentric double structure and extend between the holders 20 and 21 and the associated valve seats 12g and 12h formed on the lower head 10 of the cylinder head 4 respectively. By means of the valve springs 22 and 23, the intake and exhaust valves 14, 15 are kept pressed in a direction for closing the intake and exhaust ports. The intake valves 14 and the exhaust valves 15 of each row of the V-type engine are operated by an intake camshaft 24 and an exhaust camshaft 25, respectively, which establish a rotating contact with each intake tappet 18 and each exhaust tappet 19. Bearing portions formed on the upper head 11 and cam caps secured by bolts form bearings for both camshafts 24, 25. The intake valve 14 and the exhaust valve 15 are moved downward by depressing the intake tappet 18 and the exhaust tappet 19 by the corresponding cam lobes of the camshafts 24 and 25, respectively. As indicated in Fig. 4, the outer diameter of the valve spring 22 of the intake valve is set to be substantially equal to the outer diameter of the associated intake tappet 18 and that of the valve spring retainer 20, and the position thereof is set to be located adjacent to the lower end of the intake valve tappet 18 so that the upper end of the intake valve spring 22 is positioned substantially at the lower end of the intake tappet 18. The spring seat 12g of the intake valve spring 22 is provided at a correspondingly suitable position of the internal structure of the lower head 10 of the cylinder head 4. Due to the above-mentioned arrangement, the valve operating mechanism on the intake valve side has a structure in which the valve spring 22 of the intake valve 14 is a double-stacked arrangement on the intake tappet 18. In this way, an increase in the diameter of the intake tappet 18 of each of the intake valves 14 can be avoided and therefore the intake tappets 18 can be arranged closely adjacent to each other within the available space without any problems.

Further, as similarly shown in Fig. 4, the outer diameter of the valve spring 23 of each of the exhaust valves 15 and the corresponding outer diameter of the associated valve spring retainer 21 are set to be slightly smaller than the inner diameter of the exhaust tappet 17 which, like the intake tappet 18, is of a downwardly open, bottomed, cylindrical structure. Furthermore, the height positions of the valve spring retainer 21 of the exhaust valve 15 and the spring seat 12h on an inner wall of the lower head 10 of the cylinder head 4 are set such that an upper end portion of the valve spring 23 which biases the respective exhaust valve 14 to its closed position projects into the exhaust tappet 17. Accordingly, the valve actuation system is at the Exhaust camshaft side of a so-called cup structure in which the upper region of the valve spring 23 of the exhaust valve is covered by the associated exhaust tappet 19, which forms a receiving space for the upper end of the valve spring 23.

As can be seen from Fig. 4, the distance L2 from the axis of the exhaust camshaft 25 to the spring seat 12h of the exhaust valve spring 23 is set to be smaller than the distance L1 from the intake camshaft 24 to the associated valve seat 129 of the intake valve spring 22. Accordingly, the distance L2' between the exhaust valve spring seat 12h and the valve seat is set to be larger than the corresponding distance L1' on the intake side, while the distance of each axis of the intake and exhaust camshafts 24, 25 to the associated valve seat is equal to each other. Due to this dimensional layout, it is possible to make the exhaust side cooling jacket 31b larger while at the same time keeping the overall height of the engine within acceptable limits. Accordingly, the cooling efficiency can be improved if the cooling jacket on the exhaust side of the cylinder head can be enlarged to obtain a larger cooling capacity than conventional.

Furthermore, in this embodiment of the present invention, the number of intake valves 14 (three) exceeds that of exhaust valves 15 (two), resulting in an increased diameter of the exhaust valve (15) to ensure the necessary cross-section of the exhaust port area. Furthermore, the valve lift of the exhaust valves 15 is also set to be larger than that of the intake valves 14. In view of this dimensional aspect (resulting in an increased weight of the exhaust valves compared to the intake valves), stronger exhaust valve springs 23 must be used, which entails a larger spring constant and a longer length of the exhaust valve spring 23 compared to the intake valve spring 22. According to this arrangement, the danger of a higher position of the exhaust camshaft 25 and consequently a larger overall engine height due to the structure of the arrangement of the exhaust tappet 19 and the exhaust valve spring 23 is overcome by the upper valve spring receiving structure for the exhaust valve 15. The alternative can also be avoided, namely to increase the height of the machine in the conventional way by lowering the Valve seats of the exhaust valves to provide sufficient space to accommodate the stronger valve springs, which would result in an adverse consequence of insufficient cooling efficiency for the exhaust side of the cylinder head since the size of the water cooling jacket 31b on the exhaust side would be reduced. A relatively high area of the valve seat 12h for the exhaust valve spring 23 can be ensured by adopting a so-called cup structure for the exhaust valve side to enable the distance L2 between the axis of the exhaust camshaft 25 and the associated valve spring seat 12h of the exhaust valves 15 to be reduced, but the distance L2' on the exhaust side between the exhaust port 12e, 12d and the valve spring seat 12h is increased, which fulfills the objectives of low engine height and unaffected cooling efficiency.

On the intake side, the valve spring retainer 20 is preferably arranged close to the lower edge portion of the associated valve tappet 18 in order to maintain the necessary diameter of the three closely spaced valve tappets 18 for the three intake valves 14 in this embodiment as low as possible so that they can be arranged without any difficulty, allowing sufficient space for the insert 16 in the cylinder head without weakening the engaging area between the bores lined by the insert 16. In particular, when the cylinder head is made of a light metal alloy, such as an aluminum alloy, this problem becomes important.

Also, the distance L1 between the axis of the intake camshaft 24 and the associated intake valve spring seat 12g may exceed the corresponding distance L2 on the exhaust side, such a design facilitating the elimination of the intake valve springs 22 is not obligatory, but both distances L1 and L2 on the intake and exhaust sides may be equal to each other. In the latter case, the length of the valve springs 23 for the exhaust valve 15 may be further increased and the cooling capabilities of the exhaust side cooling jacket 31b may be improved by increasing the size thereof.

As can be seen from the embodiment of Fig. 3-5, as far as these have been explained so far, the cooling jacket formed in the cylinder head 4 has two side jackets 31a, 31b on the intake and exhaust sides of the lower head 10 of the cylinder head 4 as well as a central cooling jacket 31c. Furthermore, according to an embodiment where the distance L2' on the exhaust side exceeds the distance L1' on the intake side, the exhaust spring seat 12h formed by an integral wall portion of the lower head 10 of the cylinder head 4 is further away from a lower surface 10a of the cylinder head 4 which meets with the cylinder block 2. Accordingly, a larger volume can be secured for the cooling jacket 31b on the exhaust side of the cylinder side 4. Depending on the further design of the cylinder head, the volume of the exhaust side cooling jacket 31b may still exceed that of the intake side cooling jacket 31a. Nevertheless, the axis of rotation of the intake and exhaust camshafts 24, 25 is located at approximately the same distance above the lower surface 10c of the cylinder head 4.

In view of the problems of arranging a plurality of intake valves (or exhaust valves) closely adjacent to one another in order to increase the loading efficiency of the engine, for example using three intake valves (and two exhaust valves), or a further increased number of intake or exhaust valves, it is particularly important that the bearing structure for reciprocally supporting the valves (having tappets 18, 19 at the upper ends of the valve stems of the intake and exhaust valves 14, 15) comprises inserts or liners 16, 17, preferably received as an integral structure in an opening of the cylinder head, the inserts 16, 17 for reciprocally supporting the associated tappets 18, 19 of the intake and exhaust valves 14, 15 not only having a plurality of bores which are formed by the associated tappets of the intake and exhaust valves 14, 15, but they are also made of a material different from that of the cast cylinder head 4. In this way, a very strong and reinforced support structure for the valve tappets 18, 19 is obtained and multiple valves can be used on the inlet or exhaust side without presenting problems of ensuring sufficient strength of the material between adjacent valve tappets on the inlet or exhaust side. In this way, an improved cylinder head arrangement can be obtained which allows the use of facilitated by multiple plungers positioned close to each other but supported by a reinforced support structure.

Claims (13)

1. A V-type internal combustion engine comprising a cylinder block (2) having a pair of cylinder banks arranged in a V-bank arrangement to define a V-space therebetween with at least one cylinder bore (6) defined in each cylinder bank, a cylinder head (4) secured to each of the cylinder banks, and a pair of intake and exhaust camshafts (24, 25) rotatably secured in each of the cylinder heads (4) for actuating a plurality of intake valves (14) and exhaust valves (15), respectively, the distance (B) of the axis of the exhaust camshaft (25) from a center plane (C) containing the cylinder axis of the respective cylinder and the axis of the crankshaft (9) being greater than the distance (A) of the respective intake camshaft (24) from the center plane (C), characterized in that the Intake camshafts (24) are arranged on the sides of the cylinder heads (4) facing the V-space between the cylinder banks, the intake and exhaust valves (14, 15) being biased to their closed positions by valve springs (22, 23), the valve springs being supported by spring seats (12g, 12h) provided on the cylinder head (4), and that the distance (L2') between the exhaust spring seats (12h) and the exhaust ports (12e) formed in a lower surface of the cylinder head (4) is greater than the distance (L1') between the intake spring seats (12g) and the respective intake ports (12c). 2. Internal combustion engine according to claim 1, characterized in that an intermediate shaft (35a) is associated with each of the cylinder banks for driving the respective pair of intake and exhaust camshafts (24, 25), each of the intermediate shafts (35a) being supported rotatably about an axis which is parallel to the intake and exhaust camshafts (24, 25) and laterally offset with respect to the center plane (C) of the respective cylinder bank to the exhaust side thereof. 3. Internal combustion engine according to claim 1 or 2, characterized in that each of the intermediate shafts (345a) is provided with an intermediate gear (35) in engagement with a Pair of cam gears (24b, 25b) attached to the respective pair of intake and exhaust camshafts (24, 25). 4. Internal combustion engine according to claim 3, characterized in that the intermediate gears (35) are in engagement with a common crankshaft output gear (33) for driving both intermediate gears (35). 5. Internal combustion engine according to claim 4, characterized in that the crankshaft output gear (33) is driven by a crankshaft gear (9a) which is attached to the crankshaft (9). 6. Internal combustion engine according to claim 2, characterized in that the intermediate shafts (35a) are connected to the respective pair of intake and exhaust camshafts (24, 25) via drive belts (42) and are connected to the crankshaft (9) via another drive belt. 7. Internal combustion engine according to at least one of claims 1 to 6, characterized in that the auxiliary devices, in particular an alternator (54), are arranged in the V-space defined between the pair of cylinder rows. 8. Internal combustion engine according to at least one of claims 1 to 7, characterized in that the intake and exhaust camshafts (24, 25) actuate the intake and exhaust valves (14, 15) via valve tappets (18, 19), and that the valve springs (22, 23) extend between spring seats (12g, 12h) provided on the cylinder head (4) and spring retaining parts (20, 21) which are fastened to the shafts (14b, 15b) of the intake and exhaust valves (14, 15), respectively. 9. Internal combustion engine according to claim 8, characterized in that the spring retaining parts (20) of the intake valves (14) are arranged close to a lower end of the associated intake valve tappets (18), with the upper end of the intake valve springs (22) held outside the intake valve tappets (18) at the lower end thereof, while the valve spring retaining parts (19) of the exhaust valves (15) are accommodated together with the upper end of the associated exhaust valve springs (23) inside the exhaust valve tappets (19). 10. Internal combustion engine according to at least one of claims 8 or 9, characterized in that the distance (L2) between the axis of the exhaust valve actuating camshaft (25) and the spring seats (121) formed on a lower head (10) of the cylinder head (4) is less than or equal to the distance (L1) between the axis of the intake valve actuating camshaft (24) and the spring seats (12g) of the intake valve (14). 11. Internal combustion engine according to at least one of claims 1 to 10, characterized in that the cylinder head (4) is provided with a cooling jacket which has a central cooling jacket (31c) and cooling jackets (31a, 31b) on the inlet and outlet sides of the cylinder head, each for circulating coolant through the cylinder head. 12. Internal combustion engine according to at least one of claims 1 to 11, characterized in that the axes of rotation of the intake and exhaust camshafts (24, 25) are arranged at approximately the same distance from the lower surface (10a) of the cylinder head (4), the intake and exhaust camshafts (24, 25) being arranged at approximately the same height in relation to the cylinder block (2). 13. Internal combustion engine according to at least one of claims 1 to 12, characterized in that three intake valves (14), which comprise a pair of side intake valves and a central intake valve, and a pair of exhaust valves (15) are provided per cylinder, with a lower end region (14a) of the side intake valves crossing the central plane (C).