EP0422277B1

EP0422277B1 - Combustion chamber and valve operating mechanism for multi-valve engine

Info

Publication number: EP0422277B1
Application number: EP89119004A
Authority: EP
Inventors: Isao Morishita
Original assignee: Yamaha Motor Co Ltd
Current assignee: Yamaha Motor Co Ltd
Priority date: 1989-10-12
Filing date: 1989-10-12
Publication date: 1994-03-09
Anticipated expiration: 2009-10-12
Also published as: DE68913740D1; DE68913740T2; EP0422277A1

Description

The invention relates to a cylinder head for an internal combustion engine comprising a lower surface defining in part a combustion chamber, at least three intake valves supported for reciprocation along axes that extend at acute angles to one side of a plane containing the axis of a cylinder bore of a cylinder block to which the cylinder head is to be attached, at least two exhaust valves supported for reciprocation along axes that extend at an acute angle to the other side of said plane, said intake and exhaust valves being operated by a common single camshaft supported through the cylinder head, wherein an area of the cylinder head spaced from the lower surface and between the portions supporting said intake and exhaust valves defines generally a coolant jacket which is partially defined by a bridging member defining an upper wall of said coolant jacket and extending substantially horizontally across a centre area of the cylinder head.
It has been recognized that the performance of an internal combustion engine can be significantly improved by increasing the number of valves serving each of the combustion chambers. By using a plurality of smaller diameter valves rather than individual large diameter valves, it is possible to increase the total effective valve flow area without a significant increase in the weight of the reciprocating masses. As a result, multiple valve engines are receiving considerable interest and application for a wide variety of purposes, such as powering motor vehicles.
However, as the number of valves per combustion chamber increases, the complexity of the engine also increases. In addition, the added port areas necessary to serve the multiple valves tends to complicate the ability to provide a rigid cylinder head arrangement that can withstand the loadings.
Moreover, an arrangement is preferred wherein the cylinder head is reinforced by at least one bridging member that extends transversely across the cylinder head in proximity to the combustion chamber in order to increase the stiffness of the cylinder head.
Although the use of such a bridging member or bridging members for reinforcing the cylinder head is particularly advantageous so as to provide a strong and yet lightweight construction, the use of bridging members by themselves can further complicate the overall configuration of the cylinder head and the formation of it.
Where multiple intake valves are utilized in conjunction with internal combustion engines, it has been generally the practice to dispose all of the intake valves on one side of the engine and all of the exhaust valves on the other side of the engine. This is generally done so as to simplify the porting for the engine and so that the ports for the intake and exhaust gases need not cross over each other. However, where the intake and exhaust valves are so disposed on opposite sides of the cylinder head, this has generally necessitated the use of separate camshafts for operating the intake and exhaust valves, respectively. Although the use of separate camshafts has certain advantages, it further complicates the engine. Not only does this tend to raise the overall height of the engine due to the positioning of the camshafts, but it also significantly complicates the timing arrangement for driving all of the camshaft from the crankshaft of the engine. Although the use of a single overhead camshaft for operating all of the valves is possible, the previously proposed arrangements of this type have given rise to certain other problems. For example, the placement of such added components as spark plugs and the like become a significant problem in connection with the use of single overhead camshafts for multiple valve engines.
In addition to the problems of component placement when all of the multiple valves of an engine are operated by a single camshaft, there also is a considerable problem in connection with adequate support for the camshaft. That is, where multiple valves are operated by a single camshaft, the operating loads exerted on the camshaft can be quite high. It is, of course, extremely important to support the camshaft in such a way that it will not flex and the valve operation will not be adversely affected.
Where multiple valves are operated for an engine from a single camshaft, it is generally the practice to employ some form of rocker arm arrangement for operating the valves from the camshaft so that the valves can be located in the combustion chamber at optimum positions and may reciprocate along the desired axes. However, where rocker arms are employed, it is also necessary to provide an arrangement for pivotally journaling the rocker arms. Although individual rocker arm supports have been proposed, these supports have a tendency of permitting the rocker arm to move transversely or rotate about an axis perpendicular to their normal pivotal axis and can adversely affect high speed performance. Therefore, it is desirable to mount the rocker arms for their pivotal movement on a rocker arm shaft which shaft will tend to maintain the movement of the rocker arm in its desired path. However, the use of the rocker arm shafts can give rise to other problems, particularly in the placement of other components of the engine.
In connection with the use of multiple valves, it is a desirable practice to employ three intake valves for each cylinder of the engine. In order to facilitate operation of the valves and their placement, it has been the practice to place two of these valves, the outermost valves, so they reciprocate about parallel acute axes to a plane containing the cylinder bore axis. The third, middle valve, is disposed outwardly toward the periphery of the cylinder bore from this plane and also reciprocates about an axis that is at an acute angle to the plane but which angle is less than the aforenoted angle. Although this arrangement is particularly useful, the placement of the valve heads previously proposed has been such as to require the formation of a fairly deep recess in the peripheral edge of the piston so as to provide clearance. This can present certain difficulties.
From US-A-4,624,222, an intake valve structure for an internal combustion engine is known which comprises a bridging member reinforcing the cylinder head. However, the rigidity of a camshaft arrangement for operating the valves should be increased further.
From GB-A 2 193 998 and DE-A-38 38 953, other cylinder head arrangements are known which, however, do not refer to specific aspects of multiple valve engines having at least three intake valves and at least two exhaust valves.
Accordingly, it is an objective of the present invention to provide a cylinder head of a multiple valve internal combustion engine that is highly reinforced and allows valve operation from a single camshaft without substantial deflection of same.
According to the present invention, the above objective is achieved in that the reinforcing bridging member defines a bearing portion for at least a centre bearing of the camshaft, the axis of which is displaced vis-a-vis said plane to the intake valve side of the cylinder head.
Preferred embodiments of the present invention are laid down in the further dependent claims.
In the following, the present invention is explained in greater detail by means of several embodiments thereof in conjunction with the drawings, wherein:

Figure 1 is a side elevational view of a motorcycle powered by an internal combustion engine constructed in accordance with an embodiment of the invention, with a portion broken away and shown in section.
Figure 2 is a cross-sectional view showing the top portion of the engine in cross-section taken along the line 2-2 of Figure 3.
Figure 3 is a top plan view of the cylinder head assembly with the cam cover removed but with components carried by it shown in phantom.
Figure 4 is a bottom plan view showing the cylinder head assembly, with the intake and exhaust valves being depicted in phantom.
Figure 5 is a cross-sectional view taken along the line 5-5 of Figure 2 and shows the engine mounting arrangement.
Figure 6 is a cross-sectional view, in part similar to Figure 2, and shows another embodiment of the invention. This view is taken generally along the line 6-6 of Figure 7.
Figure 7 is a top plan view of the cylinder head of this embodiment with the cam cover partially removed.
Figure 8 is a cross-sectional view of this embodiment and is taken along the line 8-8 in Figure 7.
Figure 9 is a cross-sectional view, in part similar to Figures 2 and 6, and shows yet another embodiment of the invention.
Figure 10 is a top plan view of the embodiment of Figure 9 with the cam cover removed.
Figure 11 is a top plan view, in part similar to Figure 10, showing yet another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to Figure 1, a motorcycle powered by an internal combustion engine constructed in accordance with an embodiment of the invention is identified generally by the reference numeral 11. The motorcycle 11 is depicted as a typical environment in which an engine embodying the invention may be employed. The invention has particular utility in conjunction with internal combustion engines used to power vehicles. However, it will be readily apparent to those skilled in the art that the invention may be practiced in conjunction with other engine applications.
The motorcycle 11 is comprised of a frame assembly, indicated generally by the refernce numeral 12, and which is of the welded up type including a head pipe 13 that journals a front fork assembly 14 for steering movement and which supports a front wheel 15 in a known manner. In a similar manner, a rear wheel 16 is supported by the frame assembly 12 by means including a trailing arm suspension 17.
An internal combustion engine, indicated generally by the reference numeral 18 and shown in more details in Figures 2 through 4, is supported within the frame assembly 12 by means including an engine mount 19 that is affixed to a gusset plate 21 which is affixed to or forms a part of the frame assembly 12 in a manner as will be later described by reference to Figure 5. The engine 18 is further supported in a suitable manner from a down tube 22 of the frame 12.
A fuel tank 23 is supported by the frame 12 above the engine 18 and supplies fuel to the engine in a known manner and specifically to carburetors 24 thereof. An air cleaner 25 supplies filtered air to the carburetors 24.
A seat 26 is supported on the rear of the frame assembly 12 behind the fuel tank 23 so as to accommodate a rider.
The engine 18, as is common with motorcycle practice, includes an integral crankcase, change speed transmission assembly 27 that drives the rear wheel 16 through a chain 28 in a known manner.
The engine 18 or at least portions of it are water cooled and for this purpose there is mounted a radiator 30 on the frame assembly 12 forwardly of the down tube 22. The construction of the motorcycle and its drive arrangement as thus far described may be considered to be conventional and, for that reason, further description of the motorcycle per se is not believed to be necessary to understand the construction and operation of the invention.
Referring now in detail to Figures 2 through 4, the construction of the engine 18 will be described in detail. Since the invention relates primarily to the cylinder head assembly and particularly the camshaft support and the valve actuating mechanism, only the top portion of the engine assembly has been depicted and will be described in conjunction with a single cylinder of the engine. In the illustrated embodiment, the engine is of the single cylinder type but it is to be understood that the invention can be practiced in combination with multiple cylinder engines and the adoption of the invention to such engines is believed to be well within the scope of those skilled in the art in view of the following description. Two such multi-cylinder embodiments will also be described by reference to Figures 9 and 11 and Figure 11.
The engine 18 is comprised of a cylinder block assembly 29 that defines one or more cylinder bores 31 by means of pressed in or otherwise inserted liners 32. A piston 33 is supported for recpirocation within the cylinder bore 31 and is connected by means of a connecting rod (not shown) to a crankshaft for driving it in a known manner.
A cylinder head assembly, indicated generally by the reference numeral 34, is affixed to the cylinder block 29 in a known manner and has a downwardly facing sealing surface 35 that is adapted to cooperate with a gasket (not shown) interposed between this surface and a corresponding upper surface of the cylinder block 29 for sealing purposes. Centrally of the sealing surface 35, the cylinder block is provided with a combustion chamber area 36 that is defined by a recess 37 of the cylinder head 34, the head of the piston 33 and cylinder bore 31. The volume of the chamber 36 varies, of course, with the reciprocation of the piston 33 and is shown in Figure 2 at its top dead center position.
In order to permit an intake charge to be delivered to the combustion chamber 36, there is provided a multiple intake valve assembly which lies generally on one side of a plane containing the axis C of the cylinder bore 31 (Figure 3) and which extends perpendicularly to the plane of Figure 2. This intake valve assembly includes a pair of intake valves 38 which are supported for reciprocation about parallel axes that are disposed at an acute angle to the aforenoted plane.
There is provided a further intake valve 39 that is supported by the cylinder head assembly 34 for reciprocation about an acute angle to the aforenoted plane but which angle is different than the angle of the intake valves 38. The disposition of the intake valves 38 and 39 is generally as set forth in United States Letters Patent 4,660,529, entitled "Four Cycle Engine", issued April 28, 1987 and assigned to the assignee of this application except as will be hereinafter noted. The disclosure of that application for the particular valve orientation and the advantages of it is incorporated herein by reference.
Each of the intake valves 38 and 39 is formed with a head portion 41 that cooperates with a respective valve seat 42 that is pressed into the cylinder head assembly 34 in a known manner. These valve seats are formed at the termination of an intake port 43 that extends through one side of the cylinder head assembly 34 from a single inlet as best shown in Figure 2 and which terminates at the individual valve seats 42.
Each of the valves 38 and 39 further has a stem portion 44 that is supported for reciprocation within a respective guide 45 formed in a portion 46 of the cylinder head 34. The angular disposition of the stems 44 is such that the tips thereof all lie on a line A as seen in Figure 3 when the valves 39 and 39 are closed.
The valves 38 and 39 are urged toward their closed positions by means of coil compression spring assemblies 47 that bear at their lower ends against a bearing plate 48 engaged with the cylinder head 34 or formed on the respective valve guide 45 and at their upper ends with a spring retainer 49 that is held to the stem 44 of the respective valve by a keeper assembly of a known type.
The valve placement shown in Patent 4,660,529 is such that the head of the intake valve 39, which is disposed furthest from the plane containing the cylinder bore axis, has the tip or toe portion of it disposed lower than or at the same point as the adjacent portions of the valves 38. At a result of this, it may be necessary to form a recess in the head of the piston 33 for clearance purposes. In accordance with a feature of this invention, the head 41 of this outermost valve 39 is raised so that its toe portion does not extend below the heads 41 of the valves 38 as may be clearly seen in Figure 2. As a result, a lesser recessed area is necessary in the head of the piston 33 for clearance purposes.
On the opposite side of the aforenoted plane, there are supported in the cylinder head assembly 34 a pair of exhaust valves 51 which have stem portions 52 that are reciprocally supported in the cylinder head assembly 34 by valve guides of the type aforedescribed. The valve stems 52 reciprocate about parallel axes which are disposed at an acute angle to the aforenoted plane and which acute angle is less than the acute angle of the pair of intake valves 38 and greater than the acute angle of the intake valve 39. The tips of the stems 52 lie along a line B (Figure 3) when the exhaust valves 51 are closed. These exhaust valves 51 control the flow of exhaust gases from the combustion chamber to respective exhaust ports 53 formed in the side of the cylinder head 34 opposite the intake passage 43. The valves 51 cooperate with valve seats 54 in a known manner so as to control this flow.
All of the intake valves 38 and 39 and both of the exhaust valves 51 are operated by means of a single overhead camshaft 55. The camshaft 55 is journaled, in a manner to be described, for rotation about a camshaft axis that extends parallel to the aforenoted plane and which is offset slightly from one side thereof and the cylinder bore axis C by the distance D2 toward the intake valves 38 and 39. However, the axes of reciprocation of the intake valves 38 and 39 all lie on one side of a plane containing the axis of rotation of the camshaft 55 while the axes of reciprocation of the exhaust valves 51 lies on the other side of this plane.
The camshaft 55, since it operates all of the valves 38, 39 and 51, must be very rigidly supported so as to resist any bending forces on it. To that end, the camshaft 55 is provided with spaced bearing portions adjacent its ends which are journaled within bearing surfaces 56 formed by the cylinder head 34. In addition, there is provided a central bearing portion on the camshaft inwardly of its ends that is journaled on a bearing surfaces 57 of the cylinder head assembly 34. The center of this camshaft bearing surface and the cylinder head bearing surface is offset a distance D1 from the cylinder bore axis C. The bearing surfaces 56 and 57 of the cylinder head cooperate with corresponding bearing surfaces formed by a combined bearing cap and cam cover assembly, indicated generally by the reference numeral 59 and which is affixed to the cylinder head 34 in a manner to be described.
One of the intake valves 38 and the intake valve 39 are operated by a single intake lobe 61 of the cam shaft 52, which lobe 61 is disposed between the bearing surfaces of the camshaft which cooperate with the cylinder head bearing surfaces 56 and 57 and the corresponding bearing portions of the cam cover, bearing cap 59. As a result, the construction is extremely rigid.
The cam lobe 61 cooperates with a follower portion 62 of a first intake rocker arm 63 that is journaled in the cam cover, bearing cap 59 on a rocker arm shaft 64. The rocker arm shaft 64 is journaled in a suitable manner in the cover 59. The rocker arm 63 has two extending arm portions 65. The portions 65 carry adjusting screws 66 that cooperate with the tips of the stem of one of the pair of intake valves 38 and the tip of the stem at the valve 39 so as to operate these valves.
The camshaft 55 is provided with a further cam lobe 67 that is disposed on the opposite side of the central bearing portion of the camshaft which cooperates with the cylinder head bearing surface 57 from the cam lobe 61. The cam lobe 67 is located between the bearing surfaces 55 and 56 and cooperates with a second intake rocker arm 68 that is also journaled on the rocker arm shaft 64. The rocker arm 68 is a single rocker arm and carries an adjusting screw 69 for operating the remaining valve of the intake valve pair 38.
The camshaft 55 is also provided with a pair of exhaust lobes 70 which lobes are each disposed between a respective one of the end bearing surfaces on the camshaft and the intake lobes 61 and 67. A pair of exhaust rocker arms, indicated generally by the reference numerals 71 each have follower portions 72 that cooperate with a respective of the exhaust cam lobes 70 for pivoting the rocker arms 71 about a rocker arm shaft 73 which, like the rocker arm shaft 64, is carried by the cam cover and bearing cap 59. Adjusting screws 74 carried at the outer ends 75 of the rocker arms 71 contact the tips of the exhaust valve stems 52 for operating the exhaust valves in a known manner.
The engine is also provided with a spark plug 76 for firing the charge in the combustion chamber 37. The spark plug 76 is received within a spark plug receiving recess 77 formed by aligned bores 78 and 79 formed in the cylinder head 34 and cam cover 59, respectively. A tapped hole 81 formed at the bottom of the cylinder head bore 78 receives the threads of the spark plug so that its gap will be positioned generally centrally of the combustion chamber 37.
The cylinder head 34 is also provided with a cooling jacket 82 through which coolant is circulated from a cooling jacket 83 of the cylinder block 29.
The camshaft 55 carries a sprocket 84 at one end which is driven at one-half crankshaft speed by a chain 85.
It should be noted that the provision of the cooling cavity 82 gives the cylinder head 34 a generally open V-type configuration. However, in accordance with the invention, there is provided a bridging member 86 that extends across the upper portion of the cooling cavity 82 and which serves as a bridging portion between the opposite sides of the cylinder head to additionally provide the bearing surface 57 for the center bearing portion of the camshaft 55. This bridging portion 86 also extends into the spark plug well 77 so as to provide good reinforcing for the cylinder head in this area. As a result of the use of this bridging member, which extends substantially across the center portion of the combustion chamber 36, the cylinder head will be quite strong.
As may be seen in Figures 2 and 3, the bearing cap, cam cover 59 is held to the cylinder head 34 by means of four spaced bolts 87. Certain of the bolts 87 extends through areas where the rocker arm shafts 67 and 73 also extend. In order to prevent interference, the rocker arm shafts are relieved in these areas as seen at 88 in Figure 2. In like manner, the exhaust rocker arm shaft 73 extends through the spark plug recess 77 and to prevent interference, a portion 89 of this rocker arm shaft is removed or relieved so as to permit the spark plug 76 to be inserted and removed without interference. As a result, the construction can be extremely compact and yet there is no sacrifice made for the good operation of the components or the ability to service them.
Referring now primarily to Figures 2 and 5, it will be seen that the bearing cap, cam cover 59 is provided with an upwardly extending lug portion 99 which is generally bifurcated to pass one of the hold-down bolts 87. This bifurcated lug 99 is formed with a pair of aligned bores through which a first bolt and nut assembly 91 extends so as to attach a pair of engine mounts 19 to the engine. A second bolt and nut assembly 92 passes through another series of holes in the engine mounts 19 so as to secure the engine mounts 19 and engine to the frame cross member or gusset 21, as aforedescribed.
In the embodiment of the invention as thus far described, the engine has been provided with a single spark plug for each cylinder or combustion chamber. Such an arrangement has particular utility; however, in large bore engines, it may be desirable to employ plural, such as two spark plugs. The use of plural spark plugs is advantageous to insure that the entire charge in the cylinder will be burned even under high speed running. By using multiple spark plugs, it is possible to insure that this result is obtained.
The invention, however, can be utilized in conjunction with engines having two spark plugs per cylinder and such an embodiment is shown in Figure 8 wherein the engine is identified generally by the reference numeral 101. Except for the use of the two spark plugs and the construction in the cylinder head for accommodating them, this embodiment is the same as the previously described embodiment. For that reason, components which are the same as those of the previously described embodiment have been identified by the same reference numerals and will be described again only insofar as is necessary to understand the construction and operation of this embodiment.
In this embodiment, there are provided two diametrically positioned spark plugs 102 and 103. The spark plug 102 is positioned in a spark plug recess 104 that is formed in the timing cover portion of the engine and passes between the flights of the drive chain 85. The spark plug 103, on the other hand, is disposed beyond the end of the camshaft 55 and is provided within a spark plug well 105 formed in this portion of the cylinder head. In all other regards, this embodiment is the same as the previously described embodiment, and for that reason further description of it is believed to be unnecessary to permit those skilled in the art to practice the invention.
In this embodiment, the bridging member 86 does not extend to the spark plug recess 77 because of the different placement of the spark plugs. However, the bridging member 86 still acts to reinforce the cylinder head as in the previously described embodiment.
In the embodiments of the invention as thus far described, the invention has been described in relation to a single cylinder engine. As has been previously noted, the invention is adaptable of use in multiple cylinder engines and one such embodiment is shown in Figures 9 and 10. Because this embodiment is generally similar to the previously described embodiments, those components which are the same or substantially the same have been identified by the same reference numerals and will not be described again in detail.
In this embodiment, an intake rocker arm shaft 151 and exhaust rocker arm shaft 152 are supported and journaled by means of bearing caps 153, 154 and 155 which are affixed to the cylinder head 34 in a conventional manner. As a result, the exhaust rocker arm shaft 152 can be positioned more inwardly from the spark plug recess 77 and it is not necessary to relieve the exhaust rocker arm shaft 152 for this reason. In a like manner, in this embodiment, the camshaft 55 is provided with spaced bearing portions that are journaled in the bearing caps 153, 154 and 155 in a more conventional manner. The camshaft 55 is provided with a thrust shoulder 156 that cooperates with thrust taking surfaces of the bearing cap 154 so as to take axial thrusts on the camshaft 55.
For each cylinder of the engine, the camshaft 55 is provided with a pair of intake lobes 157 that are disposed generally adjacent each other and a pair of exhaust lobes 158 that are disposed ouwardly of the intake lobes 157. A single intake rocker arm 159 for each cylinder has a pair of follower portions 161 that engage the intake cam lobes 157. The rocker arm 159 also has three extending arm portions 162, each of which operates a respective one of the intake valves.
In a similar manner, there is provided a single exhaust rocker arm 163 that has a pair of follower portions 164 that are engaged with the exhaust cam lobes 158. This rocker arm also has a pair of finger portions 165 that carry adjusting screws that cooperate with the exhaust valve stems to actuate them. In all other regards, this embodiment is the same as those previously described.
Figure 11 shows yet another embodiment of the invention which is generally similar to the embodiment of Figures 9 and 10 and which can be utilized in conjunction with a multiple cylinder engine. This embodiment differs from the embodment of Figures 9 and 10 only in that the rocker arms associated with the intake valves only have a single follower portion 201 that is engaged with a single cam lobe 202 on the camshaft 55 for operating the intake valves 38 and 39. In all other regards, this embodiment is the same as previously described embodiment and those components which are the same or substantially the same have been identified by the same reference numerals and further description of them and of this embodiment is not believed necessary to enable those skilled in the art to make and use the invention.
It should be readily apparent from the foregoing description that a number of embodiments of multiple valve, single overhead camshaft engines have been described, each of which is effective in achieving the goals of the invention as aforenoted.

Claims

Cylinder head (34) for an internal combustion engine comprising a lower surface (35) defining in part a combustion chamber (36), at least three intake valves (38,39) supported for reciprocation along axes that extend at acute angles to one side of a plane containing the axis of a cylinder bore (31) of a cylinder block (29) to which the cylinder head (34) is to be attached, at least two exhaust valves (51) supported for reciprocation along axes that extend at an acute angle to the other side of said plane, said intake and exhaust valves (38,39;51) being operated by a common single camshaft (55) supported through the cylinder head (34), wherein an area of the cylinder head (34) spaced from the lower surface (35) and between the portions supporting said intake and exhaust valves (38,39;51) defines generally a coolant jacket (82) which is partially defined by a bridging member (86) defining an upper wall of said coolant jacket (82) and extending substantially horizontally across a centre area of the cylinder head (34),
characterized in that
said bridging member (86) defines a bearing portion for at least a centre bearing of the camshaft (55), the axis of which is displaced vis-a-vis said plane to the intake valve side of the cylinder head (34).
Cylinder head as claimed in Claim 1, characterized by further including a spark plug receiving recess (77) formed in the cylinder head (34), said bridging member (86) merging into the portion of said cylinder head (34) defining said spark plug receiving recess (77).
Cylinder head as claimed in Claim 1 or 2, characterized by a pair of cam lobes (61,67) formed on the camshaft (55) on opposite sides of the portion of the camshaft (55) journaled by the bearing surface (57) formed by the bridging member (86).
Cylinder head as claimed in Claim 3, characterized in that the pair of cam lobes (61,67) operate the intake valves (38,39).
Cylinder head as claimed in Claim 3 or 4, characterized by a first rocker arm means (63 to 66,68,69)operated by the cam lobes (61,67) for operating the intake valves (38,39).
Cylinder head as claimed in Claim 5, characterized in that the first rocker arm means (63 to 66,68,69) includes at least two rocker arms (63,68).
Cylinder head as claimed in Claim 6, characterized in that the rocker arms (63,68) are pivotally supported upon a rocker arm shaft (64) and wherein the cylinder head is formed with a recess for passing a bolt (87) affixed to the cylinder head (34) and wherein the rocker arm shaft (64) passes across the recess and has a relief portion (88) so that the bolt (87) can be freely inserted into and removed from the recess.
Cylinder head as claimed in at least one of the preceding Claims 1 to 7, characterized in that a second rocker arm means (71 to 74) is provided operated by cam lobes (70) of the camshaft (55).
Cylinder head as claimed in Claim 8, characterized in that the second rocker arm means comprises two rocker arms (71) pivotally supported upon another rocker arm shaft (73).
Cylinder head as claimed in Claim 7, characterized in that the bolt (87) comprises a fastener for securing a cam cover (59) to the cylinder head (34).
Cylinder head as claimed in Claim 10, characterized in that the cam cover (59) carries the rocker arm shaft (64,73).
Cylinder head as claimed in at least one of the preceding Claims 1 to 11, characterized in that the rocker arm shaft (64,73) passes across the recess and has a relief portion (88) so that the bolt (87) or other member can be freely inserted into and removed from the recess.
Cylinder head as claimed in Claim 12, characterized in that the member comprises a spark plug (76).
Cylinder head as claimed in at least one of the preceding Claims 1 to 13, characterized by an internal combustion engine comprising a first pair of intake valves (38) supported by said cylinder head (34) for reciprocation about parallel axes that extend at an acute angle to one side of a plane containing the axis of the cylinder bore, a third intake valve (39) supported for reciprocation by said cylinder head (34) along an axis disposed at an acute angle to said plane, the angle of reciprocation of said third intake valve (39) being less than the angle of reciprocation of said first pair of intake valves (38), said third intake valve (39) being positioned at a greater distance from said plane than said first pair of intake valves (38) and having the portion of its head closest to said plane being disposed at a higher position relative to the associated cylinder bore than the heads of the first pair of intake valves (38).