Valve train arrangement for multiple valve engine

Abstract: A valve train arrangement for an internal combustion engine comprising a cylinder defining a bore (32) for receiving a piston, a cylinder head affixed relative to said cylinder and having a surface defining with said bore and the piston a combustion chamber, a pair of poppet valves (37,38) supported for reciprocation relative to said cylinder head about axes that are disposed non parallel to each other, one of said axes being inclined at an acute angle only to a first plane containing the axis of said bore, the other of said axes being inclined at an acute angle to said first plane and also at an acute angle to a second plane perpendicular to said first plane and passing through said bore axis, and a camshaft (55) supported for rotation about an axis parallel to said first plane, a first cam lobe (57) on said camshaft for operating said first valve, and a second cam lobe (56) on said camshaft for operating said second valve, said second cam lobe (56) having its cam surface extending in a direction inclined to the axis of rotation of said camshaft.

Description: This invention relates to a valve train arrangement for an internal combustion engine comprising a cylinder defining a bore for receiving a piston, a cylinder head affixed relative to said cylinder and having a surface defining with said bore and the piston a combustion chamber, four intake poppet valves operated from a single camshaft, whereby, at least first and second valves of the four intake poppet valves are supported for reciprocation relative to said cylinder head about axes that are disposed non parallel to each other, one of said axes being inclined at an acute angle only to a first plane containing the axis of said bore, the other of said axes being inclined at an acute angle to said first plane and also at an acute angle to a second plane perpendicular to said first plane and passing through said bore axis, said camshaft is supported for rotation about an axis parallel to said first plane, a first cam lobe on said camshaft operates said first valve through a first cam follower, and a second cam lobe on said camshaft operates said second valve through a second cam follower, said second cam lobe having its cam surface extending in a direction inclined to the axis of rotation of said camshaft. It has been basically understood that the performance of an internal combustion engine can be improved by improving the breathing of the intake charge into the combustion chamber and the exhaust charge from the combustion chamber. It is also well known that the breathing and volumetric efficiency of an engine can be improved by increasing the number of valves rather than merely providing a single extremely large intake valve and a single extremely large exhaust valve. By using multiple smaller valves, the inertia can be reduced and higher engine speeds obtained. However, there still is a significant problem in placing all of the components within the combustion chamber and also insuring that the combustion chamber has a proper configuration. In high performance engines at the present time, four valves per cylinder are now becoming increasingly common. Such arrangements all employ two intake valves and two exhaust valves per chamber. It has been proposed also to employ arrangements with five valves (three intake and two exhaust) so as to permit even further increases in performance. Although it was thought that five valves per cylinder might be the optimum number, considering the problems in connection with valve placement and valve actuation, it is now believed that the provision of six valves (four intake and two exhaust) can offer still further performance increases. However, there are a wide variety of problems in connection with the provision of so many valves in a single combustion chamber. In connection with the utilization of multiple valves, it is, of course, desirable to minimize the number of camshafts employed for operating all of the valves. Generally, it has been the practice with four and five valve per cylinder engines to employ two camshafts, one for operating the intake valves and one for operating the exhaust valves. However, when one camshaft is called upon to operate more than three valves, then the placement of the valves can be compromised. Specifically, if there ore four valves per cylinder operated by a single camshaft, it is normally the practice to align the valves so that they all reciprocate along axes that lie in a plane that will intersect or pass near the rotational centre of the camshaft axis. This means that the actual length of the camshaft and specifically the lobes require the valves to be all positioned so that the combustion chamber configuration tends to be large and provide large surface areas. This obviously reduces the possible compression ratio of the engine and, accordingly, its performance. In conjunction with the use of a single camshaft for operating multiple valves, it is frequently the practice to employ separate cam lobes for operating each individual valve or groups of valves. However, where there are multiple valves and the use of multiple cam lobes, then the rotational support for the camshaft presents some problem. That is, the highest axial loading on the camshaft occurs in the area of the cam lobes where they engage the valve actuating elements. However, if the cam lobes are all placed close to each other, it is difficult if not impossible to provide a bearing surface adjacent the cam lobes in order to take these side loadings. Conventional camshaft arrangements simply do not afford the opportunity to provide adequate bearing surfaces for the camshafts under these circumstances. GB-A-242 919 shows a six valve engine wherein a plurality of valves is operated by a camshaft which comprises conical cams having cam surfaces which extend inclined with respect to the axis of rotation of said camshaft. It is an objective of the present invention to provide an improved valve train arrangement for an internal combustion engine wherein the cam lobes can be spaced widely enough apart so as to afford adequate bearing surface, thus providing an improved bearing arrangement for the camshaft of an engine. In order to perform the afore-indicated objective, a valve train arrangement as indicated above is improved according to the present invention in that the single camshaft comprises the bearing surfaces between the first and second cam lobes, in that the surfaces of the first and second cam lobes engaged with the first and second cam followers are disposed offset from the centre of the first and second cam followers so as to provide a greater bearing length for the single camshaft between the first and second cam lobes, in that third and fourth valves of the four intake poppet valves and corresponding third and fourth cam lobes and followers are arranged symmetrically to the first and second valves, cam lobes and cam followers, in respect to the second plane and in that the first and third valves lie on the same side and at a further distance from the first plane than the second and fourth valves. Further preferred embodiments of the present invention are laid down in the other sub-claims. In the following embodiments of the present invetion are explained in further detail referring to the associated drawings, wherein: Figure 1 is a top plan view of a cylinder head assembly of an internal combustion engine constructed in accordance with an embodiment of the invention with the camshaft cover and certain elements of the valve train removed and other parts broken away and shown in section. Figure 2 is an enlarged cross sectional view taken along the line 2-2 of Figure 1. Figure 3 is a side elevational view, in part schematic, looking generally in the direction of the arrow 3 in Figure 2. Figure 4 is an enlarged top plan view with certain components shown in phantom for reference. Figure 5 is a bottom plan view of the cylinder head showing the valve and spark plug arrangement in accordance with another embodiment of the invention. Figure 6 is a bottom plan view, in part similar to Figure 5, and shows yet another embodiment of the invention. Figure 7 is a top plan view of a cylinder head constructed in accordance with another embodiment of the engine. Figure 8 is a top plan view showing the portion of the cylinder head associated with one cylinder and the valve parting arrangement in this embodiment. Figure 9 is a top plan view, in part similar to Figures 1, 7 with the cam cover removed, showing yet another embodiment of the invention. Figure 10 is a top plan view, in part similar to Figures 4 and 8 of this embodiment showing the valve placement and porting arrangement. Referring first to the embodiment of Figures 1 through 4, an internal combustion engine depicted in accordance with this embodiment is illustrated partially. Since the invention relates primarily to the construction of the cylinder head assembly the valve train, porting arrangement associated with it and camshaft drive only this portion of the engine has been shown in detail. It is to be understood, however, that the cylinder head assembly, which in this embodiment is identified generally by the reference numeral 31, is associated with a cylinder block having a plurality of aligned cylinder bores, shown in phantom in Figures 2 through 4 and identified generally by the reference numeral 32. In the illustrated embodiment, the engine is of the in line type. It is to be understood, however, that the invention can be utilized in conjunction with engines having other cylinder configurations. Also, certain facets of the invention can be utilized in conjunction with engines having cylinders that are not cylindrical bores. For that reason, the term "bore" as used in the specification and claims is intended to encompass cylinders having openings in which pistons are supported for reciprocation regardless of the cross sectional configuration. In the illustrated embodiment, the cylinder head assembly 31 is made up of a plurality of light alloy castings including a main cylinder head casting 33 in which recesses 34 are formed so as to define combustion chambers with the cylinder bores 32 and the pistons reciprocating therein. The pistons are not illustrated in the drawings. In addition to the main cylinder head casting 33, the cylinder head assembly further includes a cam carrier 35 which, as will be noted, contains the valve actuators and camshafts and a pair of cam covers, each indicated generally by the reference numeral 36. The cam covers 36, the camshaft carrier 35 and cylinder head 33 may be affixed to the associated cylinder block in any known manner. The cylinder head assembly 31 is provided with a set of four intake valves 37, 38, 39 and 41, each of which has stem portions that are slidably supported for reciprocation within a respective guide 42 pressed into the cylinder head portion 33. It will be noted that the stems of the center intake valves 38 and 39 reciprocate about respective axes that extend parallel to each other and which define a common plane that is disposed at an acute angle to a plane passing through the center of the bore 32, extending perpendicularly to the plane of Figure 2, and lying within a plane parallel to the plane of Figure 3. The outer or side intake valves 37 and 41 reciprocate about axes that are in a common plane in a direction parallel to the plane of reciprocation of the axes of reciprocation of the center intake valves 38 and 39 and which are disposed at an acute angle to the aforenoted plane of the cylinder bore. The acute angle of the side intake valves 37 and 41 relative to this plane is greater than the acute angle of the plane defined by the axes of reciprocation of the center intake valves 38 and 39. However, the axes of reciprocation of the side intake valves 37 and 41 also lie at an acute angle to a plane perpendicular to the aforenoted plane and passing through the cylinder axes. This perpendicular plane is parallel to the plane of Figure 2 and perpendicular to the plane of Figure 3. As a result of this acute angle, it should be noted that the tips of the stems of the valves 37 and 41 are disposed outwardly of the periphery of the cylinder 32. The intake valves 37, 38, 39 and 41, and specifically the head portions thereof, open and close respective valve seats formed by pressed in inserts. These valve seats defins intake ports which all are served by a common, siamese intake passage 43 that extends from an oval opening in a face 44 of the cylinder head to those valve ports. As may be best seen from Figure 4, the center intake valves 38 and 39 are much closer to the inlet opening Of the intake passage 43 and the gases flowing to the intake valves 38 and 39 have a straighter path than the situation with respect to the side intake valves 37 and 41. If it is desired to provide substantially uniform flow into the cylinder 32 through all of the intake ports served by the valves 37, 38, 39 and 41, then the valves 37 and 41 should be made with their heads of a larger diameter than the heads of the valves 38 and 39 as shown in Figure 4. Generally, if the size of the center intake valves 38, 39 equals to the size of the side intake valves 37, 41 over 50% amount of all air-fuel mixture which flows into the combustion chamber flows into the chamber from the center intake valves 38, 39. In the case of Figure 6, the mixture mainly flows into the chamber and the swirl effect is obtained to some extent, that is, the engine is tuned to high speed. On the other hand, in the case of Figure 4, the amount of the mixture from the respective intake valves is relatively averaged, because the size of the central intake valves 38, 39 is smaller, that is, the engine is tuned to middle speed. And in the case of Figure 8, the swirl effect is strengthened further than the case of Figure 6. The cylinder head assembly 31 also supports a second set of valves comprising exhaust valves 45 and 46 which lie generally on the opposite side of the first mentioned plane when these valves are in their closed position. The valves 45 and 46 have their stem portions supported for reciprocation within pressed in guides 47 and reciprocate along parallel axes that lie in a common plane that is disposed at an acute angle to the aforenoted plane. This acute angle is less than the acute angle of the valves 37 and 41 but greater than the acute angle of the valves 38 and 39. In the embodiment of Figures 1 through 4, the heads of the intake valves 37 and 41, although larger than the heads of the intake valves 38 and 39, are slightly smaller than or equal to the diameter of the heads of the exhaust valves 45 and 46. The exhaust valves 45 and 46 control the flow through one or more exhaust ports 48 formed in the side of the cylinder head portion 31 opposite to the intake port 43. With respect to the configuration of the intake port 43, it has been noted that it starts from a common opening but as it approaches the valves 37 and 38 and 39 and 41, it will branch into two portions 43a and 43b as best shown in Figure 3. A small dividing wall 48 extends between and divides these passageways 43a and 43b as may be best seen in Figure 4. The intake valves 37, 38, 39 and 41 are all operated by respective thimble tappets 51, 52, 53 and 54 that are slidably supported in bores formed in the cam carrier 35. These bores are disposed so as to be parallel to the respective valve stems 42 of the valves which they operate. As a result, the bores that support the tappets 52 and 53 have their central axes disposed in a common plane, that is at an acute angle to the first noted plane passing through the center of the cylinder bore, while the axes of reciprocation of the tappets 51 and 52 lie in a plane that is at an acute angle to this plane and also at an acute angle to the perpendicular plane aforenoted. As a result, the head portions of the tappets 51, 52, 53 and 54 will not all be in a common plane. Those of the tappets 52 and 53 are in a common plane, but those of the tappets 51 and 54 are skewed to this common plane. Coil compression springs and keepers act to urge the valves 37, 38, 39 and 41 toward their closed positions. The valves are opened by means of a camshaft assembly now to be described. An intake camshaft, indicated generally by the reference numeral 55 is journaled for rotation, in a manner to be described, by the cylinder head assembly 31 and specifically between the cam carrier 35 and bearing caps which will be described. The camshaft 55 rotates about an axis that is disposed parallel to the axis of rotation of the engine crankshaft (not shown). In this embodiment, the camshaft 55 is provided with individual can lobes 56, 57, 58 and 59 having a configuration to be described, each of which cooperates with a respective one of the thimble tappets 51, 52, 53 and 54 in a manner to be described. The cam carrier 35 is formed with individual integrally formed bearing surfaces that cooperate with bearing surfaces formed on the camshaft 55 between the respective cam lobes 56, 57, 58 and 59. Bearing caps 61 are affixed to the cam carrier 35 in a known manner. As has been previously noted, the pair of side intake valves 37 and 41 are disposed not only at an acute angle to the aforenoted first mentioned plane containing the axis of the cylinder bore, but also at an acute angle to a perpendicular plane. As a result, in order to achieve proper operation of the valves and a compact configuration, the thimble tappets 51 and 54 are disposed between the valves 37, 41 and the respective cam lobes 56 and 59. To further facilitate this operation and as best seen in Figures 1, 3 and 4, the cam lobes 56 and 59 are disposed so that their heel portions are, rather than cylindrical, as is typical with the normal tappets and specifically with the normal cam lobes 57 and 58, at an angle. In addition, the lobe portions 62 of these cams are disposed at an angle as shown in Figure 4 so that the lobes 62 of the cams 56 and 59 will engage the tappets 51 and 54 along a generally straight line and there will be very little sliding contact therebetween. As a result, very little wear will occur. Also, the cam lobes 56 and 59 may be disposed axially beyond the periphery of the cylinder bare 32. The heel portion at the cam lobes 56 and 59 is similarly tapered. Furthermore, in order to achieve a maximum bearing area and a larger bearing area that would be possible if the cam lobe configuration were more conventional, the cam lobes 56 and 59 are disposed so that their center points A are disposed outwardly by a distance α from the point of contact B with the thimble tappets 51 and 54. In a similar manner, the cam lobes 57 and 58 are disposed in an offset relationship so that their center points C are disposed at a distance β from the point of contact D with the thimble tappets 52 and 53. As a result, there can be a greater distance between the cam lobes 56 and 57, and 58 and 59 γ than if the contact was symmetric. The exhaust valves 45 and 46 are actuated in a generally similar manner to the intake valves 37, 38, 39 and 41. However, due to their alignment, the exhaust valves 45 and 46 are operated by respective thimble tappets 62 that are slidably supported within bores 63 formed in the cam carrier 35 and which bores have their center lines lying in a plane common to the plane of the axis of reciprocation of the valves 45 and 46. An exhaust camshaft 63 is journaled in an appropriate manner in the exhaust side of the cam cover 36 by means of bearings formed integrally in the cam carrier 45 and bearings formed by bearing caps 64 that are affixed in a suitable manner within this cam chamber. Consistent with the desire to provide high performance, the combustion chamber 34 may be provided with a pair of spark plugs 65 that are disposed, in this embodiment, with their gaps in side by side relationship aligned axially along the axis of rotation of the output shaft and lying substantially on the first mentioned plane containing the axis of the cylinder bore 32. The spark plugs 65 are accessible through spark plug wells 66 (Figure 1) formed centrally in the cylinder head assembly 31 and which may be opened through the area between the cam covers 36. Forwardly at one end of the cylinder head assembly 31, there is provided a chain case or timing case 67 in which a timing chain or belt 68 is contained that is driven from the output shaft of the engine in a known manner. This timing chain or belt 68 cooperates with suitable sprockets (not shown) attached to the camshafts 59 and 63 for driving them at one half of crankshaft speed, as is well known in this art. In the embodiment of Figures 1 through 4, the spark plugs 65 were disposed so that they were spaced apart from each other along the axis of rotation of the output shaft. As such, they are disposed between the side intake valves 37 and 41 and, accordingly, the placement of the spark plugs 65 limits the maximum size of the intake valves 41 and 37. In this regard, it should be noted that the intake valves 37 and 41 are disposed so that their peripheral edges are closer to the periphery of the cylinder bore 32 than the intake valves 38 and 39 and also than the exhaust valves 45 and 46. It is desirable to maintain the periphery off the valves 37 and 41 close to the axis of the bore 32 while moving the valves 38 and 39 somewhat inwardly from this periphery. If it is desired to further increase the diameter of the heads of the valves 37 and 41, then it may be desirable to reposition the spark plugs 65 so that, rather than lying on the first mentioned plane, they lie on the second mentioned plane. Such an embodiment is shown in Figure 5. Because of the similarity of this embodiment to the previously described embodiment, all components have been identified by the same reference numeral. The full illustration of the valve actuating mechanism is not believed to be necessary to understand how the invention can be employed in conjunction with this embodiment. It should be readily apparent from this figure that, not only can the intake valves 37 and 41 be enlarged in head diameter, but the same can be true with respect to the exhaust valves 45 and 46 without adversely effecting the cylinder head integrity or the positioning of the spark plugs. In the embodiments us thus far described, it should be readily apparent that the provision of a plurality of valves and specifically more than five valves per combustion chamber clearly complicates the cylinder head configuration and the difficulty of providing individual seat inserts for each of the valves. Figure 6 shows another embodiment of the invention wherein the cylinder head is provided with a common valve insert indicated generally by the reference numeral 81 which is provided with individual port openings 82 that serve each of the intake valves 38 and 39. As a result, the cylinder head configuration can be made more compact and the number of valve inserts can be substantially reduced. In this embodiment, the placement of the spark plugs 65 is the same as that shown in the embodiment of Figures 1 through 4. Also, in the illustrated embodiment of Figure 6, the intake valves 37, 38, 39 and 41 all have the same diameter as opposed to the use of larger intake valve heads for the valves 37 and 41 than the valves 38 and 39. Of course, it is to be understood that the valve size and spark plug location can be varied without departing from this embodiment of the invention. In the embodiments previously described the heads of the intake valves 38 and 39 have been smaller than or the same diameter as the head of the intake valves 37 and 41. Figures 7 and 8 show another embodiment of the invention in which the intake valves 38 and 39 have a larger diameter head than the intake valves 37 and 41. In all other regards and except for the arrangement for journaling the intake camshaft 55, this embodiment may be considered to be the same as the embodiment of Figures 1 through 4, the embodiment of Figure 5 or the embodiment of Figure 6. However, it is to be understood that the spark plugs 65 may be disposed either in the orientation of the embodiments of Figures 4 and 6 or the embodiment of Figure 5. In addition, for various reasons, this embodiment may be designed so that the heads of the intake valves 37 and 41 are substantially smaller than the heads of the intake valves 38 and 39 rather than vice versa, as in the embodiment of Figure 5 or wherein the heads are all the same diameter, as in the embodiment of Figure 6. Because of these similarities, the various components which have been described are identified by the same reference numerals. Further description of these components is not believed to be necessary in view of the foregoing description. In other words, the difference between the embodiment of Figures 7 and 8 and the embodiments of Figures 1 through 4, 5 and 6, has to do with the bearing arrangement for the intake camshaft 55 and, for that reason, only this difference will be described in conjunction with Figures 7 and 8. As was noted in conjunction with the embodiment of Figures 1 through 4, the arrangement of the bearing surfaces on the camshaft 55 can be varied by changing the spacing between the cam lobes 56, 57, 58 and 59. In the previously described embodiment of Figures 1 through 4, it was explained how the spacing between the cam lobes 56 and 57, and 58 and 59 can be increased. With regard to the embodiment of Figures 1 through 4, the spacing generally was such that individual bearing caps 61 could be provided between each of the cam lobe pairs 56, 57; 57, 58; and 58, 59. However, by carrying the description of Figure 3 to a further limit, it could be envisioned that the cam lobes 57 and 58 could be positioned closely adjacent each other and that a single bearing cap could be provided between the lobes 56 and 57 and the lobes 58 and 59 with no bearing cap between the lobes 57 and 58. It will soon from Figure 7 that the cam lobes 57 and 58 are placed quite close together and the cam lobes 56 and 57, and 58 and 59 are spaced more widely so as to provide bearing surfaces 101, 102, 103 and 104 to which the bearing caps (not shown) may be affixed for journaling the camshaft 55. There is also provided on the camshaft 55 a thrust shoulder 105 which cooperates with the cam carrier 35 to provide axial location. It will be seen that the cam lobes 60 of the exhaust camshaft 63 are quite widely spaced apart, as with the previously described embodiment, so as to provide bearing surfaces 106 and 107 to which bearing caps (not shown) may be affixed for journaling the exhaust camshaft 63. In addition, a thrust shoulder 108 is formed on the exhaust camshaft 63 and cooperates with the cam carrier 35 so as to provide axial location. Figures 11 and 12 show another embodiment of bearing arrangement for the intake camshaft. Since this embodiment is generally similar to the embodiment of Figure 7, components in this embodiment which are the same as in that embodiment have been identified by the same reference numerals and will not be described again, except insofar as is necessary to understand the construction and operation of this embodiment. In this embodiment, it will be noted that the cam lobes 57 and 58 are both offset away from the center or the cylinder bore so as to provide bearing areas 201 and 202 that are generally aligned with the centers of the cylinder bores and to which bearing caps (not shown) may be affixed so as to journal the intake camshaft 55. The principal of the offsetting has already been discussed and it is believed unnecessary to repeat it. In addition to the bearing surfaces 201 and 202, the cam carrier 35 provides end bearing surfaces 203 and 204 and a central bearing surface 205 to which bearing caps (not shown) may be affixed in a known manner. In addition, the bearing surface 205 cooperates with the thrust shoulder 105 of the intake camshaft 55 for its axial location.

Claims: A valve train arrangement for an internal combustion engine comprising a cylinder defining a bore (32) for receiving a piston, a cylinder head (31) affixed relative to said cylinder and having a surface defining with said bore (32) and the piston a combustion chamber, four intake poppet valves (37,38,39,41) operated from a single camshaft (55), whereby at least first (39) and second (41) valves of the four intake poppet valves (37,38,39,41) are supported for reciprocation relative to said cylinder head (31) about axes that are disposed non parallel to each other, one of said axes being inclined at an acute angle only to a first plane containing the axis of said bore (32), the other said axes being inclined at an acute angle to said first plane and also at an acute angle to a second plane perpendicular to said first plane and passing through said bore axis, said single camshaft (55) is supported for rotation about an axis parallel to said first plane, a first cam lobe (58) on said camshaft (55) operates said first valve (39) though a first cam follower (53), and a second cam lobe (59) on said camshaft (55) operates said second valve (41) through a second cam follower (54), said second cam lobe (59) having its cam surface extending in a direction inclined to the axis of rotation of said camshaft (55), characterised in that said single camshaft (55) comprises bearing surfaces between said first and second cam lobes (58,59); in that the surfaces of said first and second cam lobes (58,59) engaged with said first and second cam followers (53,54) are disposed offset from the centre of said first and second cam followers (53,54) so as to provide a greater bearing length for said single camshaft (55) between said first and second cam lobes (58,59); in that third and fourth valves (37,38) of the four intake poppet valves (37,38,39,41) and corresponding third and fourth cam lobes (56,57) are arranged symmetrically to the first and second valves (39,41), cam lobes (58,59) and cam followers (53,54), in respect to said second plane, and; in that said first and said third valves (39,38) lie on the same side and at a further distance from the first plane than the second and fourth valves (41,37). A valve train arrangement as claimed in claim 1, characterised in that the third and fourth as well as the first and second of the cam lobes (56, 57; 58, 59) are spaced apart further than the fourth and second of the cam lobes (57, 58) and the camshaft (55) is formed with bearing surfaces disposed between the first and second and between the third and fourth cam lobes (58, 59; 56, 57) for journaling the camshaft (55) for rotation. A valve train arrangement as claimed in claim 1, characterised in that the cam lobes (56, 57, 58, 59)are all spaced by equal distance from each other and bearing surfaces are formed between each of the cam lobes (56, 57, 58, 59) for rotatably journaling the camshaft (55). A valve train arrangement as claimed in at least one of the claims 1 to 3, characterised in that a spark plug (65) is disposed with its gap substantially on the axis of the cylinder bore (32). A valve train arrangement as claimed in at least one of the claims 1 to 3, characterised by a pair of spark plugs (65) having their gaps lying substantially along the first plane. A valve train arrangement as claimed in at least one of the claims 1 to 3, characterised by a pair of spark plugs (65) supported in the cylinder head (31) and having their gaps lying substantially along the second plane.

Inventors: ONODERA HIROAKI C O YAMAHA HAT, ONODERA, HIROAKI, C/O YAMAHA HATSUDOKI K. K.
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1990
Main classification: F01L1
IPC codes: F01L1, F02F1, F02B1, F02B23