Patent Publication Number: US-6705264-B2

Title: Valve control for outboard motor engine

Description:
PRIORITY INFORMATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/470,845, filed Dec. 23, 1999 now abandoned, which claims priority from Japanese Patent Application No. 10-365,909, filed Dec. 24, 1998, and was laid-open on Jul. 4, 2000 as Japanese Laid-Open Application No. 2000-186516; the entire contents of these applications are hereby expressly incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an engine valve actuating system for an outboard motor and more particularly to an improved arrangement for achieving variable valve actuation (timing and/or lift) in the operation of an engine valve. 
     2. Description of Related Art 
     There is an increasing emphasis on obtaining more effective emission control, better fuel economy and, at the same time, continued increase in power output in outboard motors. Accordingly, four-cycle engines have started to replace two-cycle engines in outboard motors. It is difficult, however, to arrange all the components of a four-cycle engine into the limited space of an outboard motor cowling. 
     It is also desirable to achieve good emission control, fuel economy and high power output over the entire speed and load ranges of the outboard motor. In automotive four-cycle engines, there have been proposed a wide variety of devices to permit the engine characteristics to be adjusted when running so as to obtain optimum performance across the entire speed and load range. One such device is a variable valve actuating mechanism, which includes both changing valve timing and/or the valve lift. However, variable valve actuating mechanisms are typically complex and are not compact. Accordingly, because of the size constraints of an outboard motor, it previously has been difficult to employ variable valve actuating mechanisms in an outboard motor. 
     A need therefore exists for an engine with a variable valve actuating mechanism that is simply constructed and compact in structure. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention involves an engine comprising an output shaft and at least one cylinder having a cylinder axis. The output shaft and the cylinder are arranged such that a central plane, which contains the cylinder axis, either lies parallel to or contains an axis about which the output shaft rotates. A plurality of ports communicating with the cylinder and a plurality of valves selectively open and close the ports. At least a first valve is disposed on a first side of the central plane and at least a second valve is disposed on a second side of the central plane. A valve actuating mechanism comprises a camshaft having a plurality of cams and a pair of adjacent first and second rockers. A first support pivotally supports the first and second rockers. Each rocker has cam side arm with a following surface engaged with one of the cams to pivot the rocker about the first support. The first rocker has first and second bores and cam side arm with an operator that directly engages the first valve. The first bore slideably supports a first member and the second bore slideably supports a second member. The first support includes a first passage that communicates with the second bore. The second rocker further includes a first engagement surface that engages the first member. The second member is arranged to engage the first member when an actuating pressure is supplied to the first passage such that movement of the second rocker is transmitted to the first rocker. The valve actuating mechanism also includes at least a third rocker. The third rocker has cam side arm with a following surface, which engages another one of the cams to pivot the third rocker about a second support, and a valve side arm with an operator that directly engages the second valve. 
     Another aspect of the present invention involves an engine including a valve actuating mechanism comprising a camshaft with at least two adjacent intake cams and at least one exhaust cam. An intake rocker support supports a pair of adjacent, pivotally-supported first and second intake rockers. Each intake rocker has a cam side arm with intake following surface that is engaged with one of the intake cams for pivoting the intake rocker about the intake rocker support. The first intake rocker has a cam side arm with an operating portion that directly engages an intake valve of the engine. A first member is slideably supported within a first bore of the first intake rocker, a second member is slideably supported within a second bore of the first intake rocker. A first passage is located within the intake rocker support and communicates with the second bore. The second intake rocker further includes a first engagement surface that engages the first member. The second member selectively engages the first member when an actuating pressure is supplied to the first passage such that movement of the second intake rocker is transmitted to the first intake rocker. The valve actuating mechanism additionally comprises at least one exhaust rocker having a cam side arm with an exhaust following surface engaged with the exhaust cam for pivoting the exhaust rocker about an exhaust rocker support. The exhaust rocker support lies generally parallel to the intake rocker support and is on a side of the camshaft opposite the intake rocker support. 
     In accordance with an additional aspect of the present invention, a valve actuating mechanism is provided for an engine. A camshaft is located inside a cam cover and is driven by a crankshaft of the engine. The camshaft includes a plurality of cams. An intake rocker shaft extends along one side of the camshaft and an exhaust rocker shaft extends along generally an opposition side of the camshaft inside the cam cover. Intake and exhaust rockers are supported by the respective intake and exhaust rocker shafts for transmitting cam rotation to corresponding valves of the engine. Means is provided to selectively couple one of the valves to one of a pair of adjacent cams on the camshaft. The cams of the pair have the cams of the pair have different shapes to vary an operating characteristic of the corresponding valve. In one preferred mode, the cams have different lifts. 
     Additional aspects, features and advantages will be understood by the following description of several preferred embodiments of the present engine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-noted and other features, aspects and advantages of the present engine and valve actuating mechanism will now be described with reference to the drawings of preferred embodiments which are intended to illustrate and not to limit the invention. The drawings comprise 9 figures. 
     FIG. 1 is a side elevational view of an outboard motor which can embody an engine (shown in phantom) that is configured in accordance with a preferred embodiment of the present invention, the outboard motor being mounted to the transom of a watercraft (shown partially); 
     FIG. 2 is a top plan and partial cross-sectional view along line  2 — 2  in FIG. 1, with an upper cowling of the outboard motor shown substantially in phantom; 
     FIG. 3 is a rear, top, and right (i.e., starboard) side perspective view of the engine shown in FIGS. 1 and 2; 
     FIG. 4 is a rear, top, and left (i.e., port) side perspective view of a valve actuating mechanism having certain features and advantages according to a preferred embodiment of the present invention; 
     FIG. 5 is an exploded view of the valve actuating mechanism of FIG. 4; 
     FIG. 6A is a schematic cross-sectional view of the valve actuating mechanism of FIG. 4 in an unlocked position; 
     FIG. 6B is a schematic cross-sectional view of the valve actuating mechanism of FIG. 4 in a locked position; 
     FIG. 7 is a cross-sectional view of a locking mechanism of the valve actuating mechanism take on long line  7 — 7  of FIG. 6A; 
     FIG. 8 is a graph showing the potential effects of the valve actuating mechanism on engine torque; and 
     FIG. 9 is a top plan and partial cross-sectional view of an engine configured in accordance with another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     Embodiments of an improved internal combustion engine that includes a variable valve actuating mechanism will now be described in detail. The variable valve actuating mechanism enables the engine to produce high torque across large ranges of speeds and loads. As compared to prior art variable valve actuating mechanisms, the present mechanism uses fewer parts and less space. This reduction in size is particularly important for engines with space limitations, such as, for example, outboard motors. Accordingly, the present variable valve actuating mechanism is illustrated and described in the context of an outboard motor; however, certain aspects of the present invention can be used with engines of other types of vehicles, as well as with other types of prime movers. 
     With reference to FIG. 1, an outboard motor is identified generally by reference numeral  10 . The outboard motor  10  is shown as being attached to an associated watercraft hull, indicated generally by the reference numeral  12  and shown partially in cross-section. The outboard motor  10  is shown attached to a transom  14  of the hull  12  in a manner that will be described below. 
     The outboard motor  10  is comprised of a powerhead, indicated generally by the reference numeral  16 . The powerhead  16  includes a bottom cowling portion  18  and an upper cowling portion  20  that is detachably connected to the bottom cowling portion  18  in a known manner. The upper cowling portion  20  is formed from a suitable material, such as a molded fiberglass reinforced resin or the like. The upper cowling portion  20  has a lower peripheral edge  22  that is held in a sealing engagement with the lower cowling portion  18  by a suitable latching device (not shown). 
     The lower cowling portion  18  preferably has an opening at its bottom portion through which an upper portion of an exhaust guide member or support member  23  extends. The exhaust guide member  23  preferably is made of an aluminum-based alloy. The bottom cowling member  18  and the exhaust guide member  23  together generally form a tray. An engine  24 , which is indicated generally by the reference numeral  24  and which has a construction that will be described later in more detail, is placed onto this tray and is affixed to the exhaust guide member  23 . The exhaust guide member in this manner supports the engine  24 . The exhaust guide member  23  also has an exhaust discharge passage through which burnt charges (e.g., exhaust gases) from the engine  24  are routed as described below. 
     The protective cowling  20  encircles the internal combustion engine  24 . In the illustrated embodiment, the engine  24  is a V 6 , four-stroke engine. However, those skilled in the art will readily appreciate that several aspects of the variable valve actuating mechanism can be used with a variety of engines with different cylinder configurations (e.g., in-line or slanted) and/or with more or less cylinders (e.g., four cylinders). 
     As shown in FIG. 2, the engine  24  includes cylinder block  26  which includes a pair of cylinder banks  28 ,  30  arranged in a V-type configuration. The cylinder banks  28 ,  30  are closed at their rear ends (i.e., the end farthest from the transom  14  of the boat) by cylinder head assemblies  32 ,  34  in a manner which will be described. Cam covers  36 ,  38  are affixed to the cylinder head assemblies  32 ,  34 , respectively, and enclose respective cam chambers  39 ,  41  in which the valve actuating mechanisms contained. In the illustrated embodiment, these valve actuating mechanisms include a single overhead camshaft for each cylinder head assembly  32 ,  34 , as described in greater detail below. 
     A crankcase member  40  is affixed to the end of the cylinder block  26  opposite the cylinder heads  36 ,  38 . As such, the crankcase member  40  defines a crankcase  42  having an upper surface  43  (FIG.  3 ), and in which a crankshaft  44  is rotatably journaled. As is typical with outboard motor practice, the engine  24  is mounted in the powerhead  16  so that the crankshaft  44  rotates about a generally vertically extending axis. This facilitates coupling to a driveshaft  46  (FIG.  1 ). 
     As shown in FIG. 1, the driveshaft  46  extends into and is journaled within a driveshaft housing, indicated generally by the reference numeral  48 , and which is enclosed in its upper end by the tray (i.e., by the exhaust guide  23  and bottom cowling member  18 ). This driveshaft housing  48  includes an outer housing casing  50 . The exhaust guide  23  thus is interposed between the engine  24  and the upper end of the driveshaft housing  48  within the lower cowling member  18 . 
     The driveshaft  46  extends into a lower unit  52 , wherein it drives a conventional bevel gear, forward, neutral and reverse transmission, indicated generally by the reference numeral  54  and shown only schematically. The transmission  54  is shown in a schematic fashion, and any known type of transmission may be employed. 
     The transmission  54  drives a propeller shaft  56  which is journaled within the lower unit  52  in a known manner. A hub  58  of a propeller  60  is coupled to the propeller shaft  56  for providing a propulsive force to the watercraft hull  12 . 
     A steering shaft (not shown) is attached to the outer housing casing  50  by an upper bracket assembly  62  and a lower bracket assembly  64 . The steering shaft is supported for steering movement within a swivel bracket  66  so as to pivot about a vertical steering axis. The steering axis is juxtaposed to and disposed slightly forward of the driveshaft  46 . A tiller or steering arm  68  is affixed to the upper end of the steering shaft for steering the outboard motor  10  through an arc  70  (FIG.  2 ). The swivel bracket  66  is connected by a pivot pin  72  to a conventional clamping bracket, indicated generally by the reference numeral  74  and partially depicted. The pivot pin  72  permits tilt and trim movement of the swivel bracket  66  and outboard motor  10  relative to the transom  14  of the hull  12 . This tilt and trim movement is indicated by the arc  76  (FIG.  1 ). 
     A hydraulic tilt and trim mechanism  78  can be pivotally connected between the swivel bracket  66  and the clamping bracket  74  for effecting the tilt and trim movement, and for permitting the outboard motor  10  to pop up when an underwater obstacle is struck. As is well known, these types of hydraulic mechanisms  78  permit the outboard motor  10  to return to its previous trim adjusted position once such an underwater obstacle is cleared. 
     With reference to FIG. 2, the construction of the engine  24  will now be described in more detail. As has been noted, the illustrated engine  24  is of the V-type and, accordingly, the cylinder block  26  is formed with a pair of angularly related cylinder banks  28 ,  30 , each of which is formed with a plurality of horizontally-extending cylinder bores  80 ,  82 . The cylinder bores  80 ,  82  may be formed from thin liners that are either cast or otherwise secured in place within the cylinder banks  28 ,  30 . Alternatively, the cylinder bores  80 ,  82  may be formed directly in the base material of the cylinder banks  28 ,  30 . If a light alloy casting is employed for the cylinder banks  28 ,  30 , such liners can be used. 
     In the illustrated embodiment, the cylinder banks  28 ,  30  each include three cylinder bores  80 ,  82 . Since the engine  24  is a V-type engine, the cylinder bores  80 ,  82  in each cylinder bank preferably are staggered with respect to one another. Thus, as shown in FIG. 3, the uppermost cylinder bore in the left cylinder bank  30  (left as shown in FIG. 2) is at an elevation higher than the uppermost cylinder bore in the right cylinder bank  28  (right as shown in FIG.  2 ). 
     With reference to FIG. 2, pistons  84 ,  86  are supported for reciprocation in the cylinder bores  80 ,  82 , respectively. Piston pins  88 ,  90  connect the pistons  84 ,  86  to respective connecting rods  92 ,  94 . The connecting rods  92 ,  94 , as is typical in V-type practice, may be journaled in side-by-side relationship on adjacent throws of the crankshaft  44 . That is, pairs of cylinders,  80 ,  82 , one from each cylinder bank  28 ,  30 , may have the big ends of their connecting rods  92 ,  94  journaled in side-by-side relationship on adjacent crankshaft throws. This is one reason why the cylinder bores  80 ,  82  of the cylinder banks  28 ,  30  are staggered relative to each other. In the illustrated embodiment, however, separate throws are provided for the cylinders of each cylinder bank  28 ,  30 . The throw pairs are nevertheless disposed between main bearings (not shown) of the crankshaft  44  to maintain a compact construction. 
     The cylinder head assemblies  32 ,  34  are provided with individual recesses  98 ,  100  which cooperate with the respective cylinder bores  80 ,  82  and heads of the pistons  84 ,  86  to form the combustion chambers. These recesses  98 ,  100  are surrounded by a lower cylinder head surface that is planar and held in sealing engagement with either the cylinder banks  28 ,  30  or with the cylinder head gaskets (not shown) interposed therebetween, in a known manner. These planar surfaces of the cylinder head assemblies  32 ,  34  may partially override the cylinder bores,  80 ,  82  to provide a squish area, if desired. The cylinder head assemblies  32 ,  34  are affixed in any suitable manner to the cylinder banks  28 ,  30 . 
     Because of the angular inclination between the cylinder banks  28 ,  30 , as is typical with V-type engine practice, a valley  102  is formed between the cylinder head assemblies  32  and  34 . An induction system for the engine, indicated generally by the reference numeral  104 , is positioned in part in the valley  102 . The induction system  104  includes intake passages  106 ,  108  that extend from a surface of the respective cylinder head assemblies  32 ,  34  to valve seats formed on the combustion chamber recesses  98 ,  100 . A single intake passage and port may be formed for each combustion chamber recess  98 ,  100  or, alternatively, there may be multiple valve seats for each recesses  98 ,  100 . 
     Poppet-type intake valves  114 ,  116  are slideably supported in the cylinder head assemblies  32 ,  34  in a known manner, and have their head portions engageable with the valve seats so as to control the flow of the intake charge into the combustion chambers through the intake passages  106 ,  108 . The intake valves  114 ,  116  are biased toward their closed position by coil compression springs  115  (see FIG.  4 ). The intake valves  114 ,  116  are operated by single overhead camshafts  118 ,  120 , respectively, which are journaled in the cylinder head assemblies  32 ,  34 . The rotational axes of the camshafts  118 ,  120  are generally parallel to the axis of the crankshaft  44  (i.e., generally vertical). The manner in which the intake valves  114 ,  116  are opened and closed by the camshafts  118 ,  120  will be described later. 
     The intake camshafts  118 ,  120  are driven by the crankshaft  44  via a camshaft drive mechanism, which is not shown. Such camshaft drive mechanisms are well known in the art and they can be considered to be conventional. Thus, a further description of the camshaft drive mechanism is not believed necessary for one of ordinary skill in the art to use the present valve actuating mechanism. 
     A flywheel-magneto assembly  121  is disposed at the upper end of and connected to the crankshaft, as best understood from FIG. 3. A flywheel cover desirably covers the flywheel-magneto assembly  121 . 
     On the outer side of the respective cylinder bank  26 ,  28 , each cylinder head assembly  32 ,  34  is connected with one or more exhaust passages  124 ,  126  (FIG.  2 ). Each exhaust passage  124 ,  126  emanate from one or more valve seats formed in the cylinder head recesses  98 ,  100 , and cooperates with exhaust systems for discharging exhaust gasses to the atmosphere through a path that will be described later. 
     As shown in FIG. 2, exhaust valves  130 ,  132  are supported for reciprocation in the cylinder head assemblies  32 ,  34 , respectively, in a manner similar to the intake valves  114 ,  116 . The exhaust valves  130 ,  132  are biased toward their closed positions by coil compression springs  115  (see FIG.  4 ). The exhaust valves  130 ,  132  like the intake valves  114 ,  116  are opened and closed by the single overhead camshafts  118 ,  120 . The manner in which the exhaust  130 ,  132  valves are opened and closed by the camshafts  118 ,  120  will be described later. 
     With reference to FIGS. 1 and 2, the engine  24  discharges exhaust gases through the exhaust manifolds  123 ,  125 , and down into a silencing arrangement provided with an internal expansion chamber in the driveshaft housing  48  through exhaust pipes  127 ,  129  (see FIG.  4 ). The exhaust pipes  127 ,  129  extend from the exhaust manifolds  123 ,  125 , respectively. The exhaust pipes  127 ,  129  extend into an expansion chamber formed at the rear of the driveshaft housing (not shown). The expansion chamber terminates at its lower end in an exhaust gas discharge formed in the lower unit  52  for delivering the exhaust gases to the atmosphere, through the body of water in which the associated watercraft is operating. Although the preferred embodiment illustrates an exhaust passage through the hub, any type of conventional above-the-water exhaust gas discharge can be used with the outboard motor. For example, the exhaust discharge may include an underwater, high speed exhaust gas discharge and an above the water, low speed exhaust gas discharge. 
     The induction system  104  for the engine  24  is discussed with reference to FIGS. 2-4. As is typical with outboard motor practice, the powerhead  16 , and specifically the main cowling portion  20 , is formed with at least one air inlet opening (not shown). The air inlet opening desirably is configured so as to permit copious amounts of air to flow into the interior of the protective cowling while at the same time inhibiting water entry. Any of the known inlet type devices can be utilized for this purpose. 
     In conjunction with the induction system  104  for the engine  24 , it is desirable to provide a relatively large plenum area that supplies the individual cylinders through respective runners. The use of a plenum area is desired so as to minimize the interference from one cylinder to the others. This presents a particular space problem, particularly in conjunction with outboard motors where space is at a premium. Therefore, the induction system  104  is designed so as to provide a large plenum volume and still maintain a compact construction. Furthermore, construction is such that servicing of the engine is not significantly affected. 
     As shown in FIGS. 2 and 3, the cowling member  20  forms an engine compartment  144  around the engine  24 . The induction system includes an air inlet device  146 , positioned adjacent the crankcase chamber  42  of the engine  24 . The inlet device  146  includes at least one orifice (not shown) configured to allow air from the engine compartment  144  to enter the inlet device  146 . The inlet device  146  also includes an outlet  148  connected to an induction passage  150 . 
     The induction passage  150  extends between the inlet device and a throttle device  152 . The induction passage  150  is connected to the throttle device by a flange assembly  154 . The flange assembly  154  is formed of a plurality of plates and fasteners that are configured to form a substantially air tight fluidic connection between the air induction passage  150  and the throttle device  152 . 
     The throttle device  152  in the illustrated embodiment includes a throttle body  156  and a throttle valve (not shown) journaled within the throttle body  156 . Of course, other types of throttle devices also can be used. The throttle valve is operated by a remote actuator. By utilizing a single throttle device  152  for the induction system, the overall construction of the induction system  104  can be significantly simplified. 
     As shown in FIG. 3, the throttle device  152  is positioned below the intake runners  170 ,  172  and above the exhaust pipes  127 ,  129 . In the illustrated embodiment, the throttle body  156  is disposed above the point at which the exhaust pipes  127 ,  129  merge together. The throttle body  156  is attached to a branch portion  158  of the induction passage  150  via a flange assembly  160  which may be constructed identically to flange assembly  154 . The branch portion  158  includes a junction portion  164  downstream from the flange  160 . 
     The junction portion  164  divides the induction passage  150  into a first branch passage and a second branch passage. The first branch passage extends from the junction portion  164  to the second plenum chamber  168 . The second branch passage extends forwardly from the junction portion  164  and along a forward side of the throttle device, then curves upwardly to the first plenum chamber  162 . As such, the junction portion  164  divides the air flow emanating from the throttle device  152  so as to feed the plenum chambers  162 ,  168  with substantially equal flows of air. 
     With reference to FIG. 2, the plenum chambers  162 ,  168  overlie at least a portion of the cam covers  36 ,  38  and are mounted thereon by mounting posts (not shown) which have threaded fasteners, so as to provide a rigid assembly. As shown in FIG. 3, the plenum chambers  162 ,  168  extend substantially the full length of the respective cylinder banks  28 ,  30 , and thus provide a substantial volume for the inducted air. 
     With reference to FIG. 3, each plenum chamber  62 ,  68  communicates with a plurality of runners  170 ,  172 , respectively. The runners  170 ,  172  extend transversely across the upper portion of the engine valley area  102  and curve downwardly so as to communicate with the respective intake passages  106 ,  108  formed in the head assemblies  32 ,  34 . A connection plate  171  connects the runner  170 ,  172  to the intake passages  106 ,  108 . The runners  170 ,  172  are in direct alignment with the passages  106 ,  108  formed in the head assemblies  32  and  34 . The runners  170 ,  172  thus communicate with respective intake passages  106 ,  108  formed in the cylinder head assembly  32 ,  34  that are disposed on an opposite side of the valley from the respective plenum chambers  62 ,  68 . 
     Thus, this arrangement provides not only a large effective plenum chamber volume, since each plenum chamber  162 ,  168  serves only three cylinders, but also provides relatively long runners  170 ,  172  that extend from the plenum chambers  162 ,  168 , to the cylinder head induction passages  106 ,  108 . The length of these runners  170 ,  172  can be tuned relative to the volume so as to provide the desired charging effect in the induction system  104 . The described arrangement with the long runners  170 ,  172  is particularly effective at midrange speeds. 
     As seen in FIG. 2, the illustrated engine  24  is provided with a manifold type fuel injection system. The fuel injection system includes the plurality of fuel injectors  174 , one fuel injector  174  for each cylinder head induction passage  106 ,  108 . The fuel injectors  174  are disposed in the area between the reentrant positions of the runners  170 ,  172  and hence, are protected by these runners, since they are partially surrounded by them, while at the same time being accessible. Thus, air may flow over the injectors  174  so as to cool the injectors  174  along with the air flowing through the runners  106 ,  108 . Preferably, the injectors  174  are of the electrically operated type embodying solenoid actuated valves. 
     The injectors  174  for the respective cylinder banks  28 ,  30  are mounted in a manifold flange which is contiguous with the flow passages  106 ,  108 . Hence, the fuel spray from the injectors  174  can easily mix with the air flowing into the combustion chambers  98 ,  100  so as to provide a good mixture distribution. Other types of charge formers, however, can be used with the present engine. Such charge formers include, without limitation, direct injection fuel injectors and carburetors. 
     The injectors  174  have their tip inlet portions received in a fuel rail  180  that extends vertically through the area encompassed by the runners  170 ,  172  and is thus protected by the runners  170 ,  172 . The fuel rail  180  has two flow passages, one for the fuel injectors  174  of the cylinder bank  28 , and one for the fuel injectors  174  of the cylinder bank  30 . As such, the flow passages within the fuel rail  180  are in side-by-side relationship and accommodate the crossover relationship of the injectors  174 . 
     A suitable fuel supply system is provided for supplying fuel to the fuel rail  180 . Such fuel systems are well known in the art and they can be considered to be conventional. Thus, a further description of the fuel delivery system is not necessary for one of ordinary skill in the art to understand the present engine. 
     With reference to FIG. 3, sparkplugs  181  are mounted in the cylinder head assemblies  32 ,  34 . Although not illustrated in the figures, the spark plugs  181  are mounted with their electrodes (i.e., gaps) extending into the recesses  98 ,  100  (FIG.  2 ). The sparkplugs  181  are fired by suitable ignition system. 
     As shown in FIG. 3, the overall height of the engine  24  is reduced by positioning the throttle device  152  below the runners  170 ,  172 . In addition, with the throttle device  152  mounted at a position between the induction runners  170 ,  172  and the exhaust pipe, the present engine design effectively utilizes a large dead space which has gone unused in known outboard motors with V-type engines. 
     As discussed above, one advantage stemming from positioning the throttle device  152  at least partially below the upper surface of the crankcase  42 , and the thus resulting reduction in the overall height of the engine, is that a tight fitting cowling may be fit over the engine which is shorter in overall height than a known conventional cowling. As discussed above, since the upper portion or the powerhead of an outboard motor is subjected to significant airflow during certain operation conditions, it is desirable to shape the upper cowling so as to minimize the frontal area of the cowling. By reducing the frontal area of the cowling the aerodynamic drag on the watercraft using the outboard motor  10  is therefore reduced. 
     The variable valve actuating mechanism will now be described with reference to FIGS. 2,  4 ,  5 , and  6 . As best seen in FIGS. 2 and 4, the intake valves  114 ,  116  and the exhaust valves  130 ,  132  are controlled by single overhead cam shafts  118 ,  120 . As mentioned above, the camshafts  118 ,  120  in the illustrated embodiment are suitably journaled within the cylinder head assemblies  32 ,  24  for rotation about a generally vertical camshaft axis that is generally parallel to the crankshaft axis. 
     As best seen in FIG. 5, each camshaft  118 ,  120  preferably has five cam lobes per cylinder. The construction of the valve actuating mechanism for each cylinder preferably is substantially the same. Accordingly, the following description focuses on one of the valve actuating mechanisms associated with the port-side camshaft  118 . Unless indicated otherwise, the valve actuating mechanisms for the other cylinders have the same construction. 
     In the illustrated embodiment, the two outer cam lobes are the exhaust cams  202 . Associated with exhaust cams  202  are the exhaust valve rocker  210 , which are journaled on a common exhaust rocker shaft  212 . The exhaust rocker shaft  212  is suitably supported within the cylinder head assemblies  32 ,  34 . The axis of the exhaust rocker shaft  212  lies generally parallel to the camshaft  118  axis and preferably is offset to one side of the camshaft  118  towards the exhaust valves  130 ,  132 . 
     The exhaust rockers  210  include cam side arms  222  that extend from the rocker shaft  212  towards the camshaft  118 . At the tip of each cam side arm  222  is a follower surface or roller  226  that cooperates with the exhaust cam lobes  202  for pivoting the corresponding exhaust rocker  210  about the rocker shaft  212 . The exhaust rockers  210  also include valve side arms  224  that extend from the rocker shaft  212  towards the exhaust valves  130 ,  132 . Adjusting screws  228  carried by valve side arms  224  contact the tips of the exhaust valves  130 ,  132  for actuating the exhaust valves in a known manner. As mentioned above, the exhaust valves  130 ,  132  are biased in a closed position by coil compression springs  115 . The coil compression springs  115  also bias the cam side arms  222  towards the cam shaft  118  so that the rocker follower surface  226  maintains engagement with the exhaust cam lobes  202 . 
     The middle three cam lobes comprise the low lift intake cam  204 , the high lift intake cam  206 , and the medium lift intake cam  208 . Associated with the intake cams  204 ,  206 ,  208  are the low, high, and middle intake rockers, indicated generally by the reference numerals,  214 ,  216 ,  218 . These intake rockers  214 ,  216 ,  218  are journaled on a common intake rocker shaft  220  that is suitably supported within the cylinder head assemblies  32 ,  34 . The axis of the intake rocker shaft  220  lies generally parallel to the axes of the camshaft  118  and the exhaust rocker shaft  212 . Preferably, the intake rocker shaft  220  lies on a side of the camshaft  118  opposite the exhaust rocker shaft  212  and towards the intake valves  114 ,  116 . 
     As may be best seen from FIGS. 4 and 5, the low and medium cams lobes  204 ,  208  and their cooperating intake rockers  214 ,  218  are each associated with one of the intake valves  114 . The high cam lobe  206  and its cooperating intake rocker  216  are not directly associated with an intake valve. However, as will be described below, the high cam rocker  216  can be selectively coupled to either the low or medium rockers  214 ,  218 . 
     The low and medium intake rockers  214 ,  218 , like the exhaust rockers, have cam side arms  222 . At the end of each cam side arm  222  are followers or rollers  226 , which are engaged with the low and medium cam lobes  204 ,  208  for pivoting the low and medium intake rockers  214 ,  218  about the intake rocker shaft  220 . The low and medium intake rockers  214 ,  218  also include valve side arms  224  that extend from the intake rocker shaft  220  towards the intake valves  114 . Adjusting screws  228  carried by the valve side arms  224  contact the tips of the intake valves  114  for actuating the intake valves in a known manner. As with the exhaust valves, the intake valves  114  are biased in a closed position by coil compression springs  115 . The coil compression springs  115  also bias the cam side arms  222  towards the cam shaft  118  so that rocker follower surface  226  maintains engagement with the low and medium cam lobes  204 ,  208 . Thus, the low and medium intake rockers  214 ,  218  generally operate as conventional rockers for the valve actuation during such time as the high rocker  216  is not coupled to either of the rockers  214 ,  218 . This coupling method will be described later. 
     At this point, it should be noted that the low, high and medium cam lobes  204 ,  206 ,  208  are preferably of different lifts and diameters. The cam lobes  204 ,  206  can also be configured to provide slightly different timing. Preferably, the high cam lobe  206  preferably has a higher lift and larger diameter than that of the low and medium cam lobe  204 ,  206 . More preferably, the medium cam lobe  206  has a higher lift than the low cam lobe  204 . That is, in one preferred arrangement, the low cam lobe  204  has a lift L 1 , high cam lobe have a lift L 2  and the medium can has a lift L 3  and L 1 &lt;L 2 &lt;L 3 . 
     The mechanism for selectively coupling the high intake rocker  216  to operate the low and medium intake rockers  214 ,  218  will now be described with particular reference to FIGS. 5,  6 A and  6 B. FIG. 6A show the coupling mechanism, which is indicated generally by the reference numeral  232 , in the disengaged condition so that the low intake rocker  214  and medium intake rocker  216  operate without any control or interference from the high intake rocker  216 . Under this condition, the low and medium cam lobes  204 ,  208  and low and medium intake rockers  214 ,  218  control the degree of maximum opening (L 1 ) and timing of opening of the intake valves  114  with the fully-opened position being shown in FIG.  6 A. 
     As best seen in FIG. 5, the low and medium intake rockers  214 ,  218  have boss portions  230  that extend from the valve side arm  224  towards the high intake rocker  216 . Cylindrical bores  231  are formed in the boss portions  230 . A coupling plunger member  234  is slideably supported within each bore  231 . The head or top portion of each coupling plunger member  234  is engaged by an adjusting screw  236 . The adjusting screws  236  extend through threaded holes  238  formed in wing shaped protrusions  235  that extend from the cam side arm  222  of the high intake rocker  216  towards the low and medium intake rockers  214 ,  218 . 
     As may be best seen in FIGS. 5 and 6A, the lower end of each boss portion  230  is at least partially closed by a cap  240  which braces a biasing spring  242  that acts on the lower end of each coupling plunger member  234 . This spring  242  keeps the coupling plunger member  234  and specifically its top surface in constant engagement with the adjusting screw  236 . It should be apparent, however, that if desired, some clearance can be maintained between each adjustment screw  236  and the top surface of corresponding coupling plunger member  234 . 
     Each coupling plunger member  234  is formed with a bore  244  that extends from a flat surface  245  formed on a side thereof by a machined recess. Received within the bore  244  is a return spring arrangement that is comprised of a pair of end caps  246 ,  247  that are urged apart by a coil compression spring  248 . 
     In the uncoupled state when only the low and medium cams  204 ,  208  are operating the valves  114 , this compression spring  248  causes one end cap  247  to be urged to a position where it sits flush with the flat surface  245  of the coupling plunger member  234 . Under this condition the end cap  247  generally abuts a slideable locking member  250 . 
     Each locking member  250  is slideably supported within a bore  252  that extends through another boss of the low and medium intake rockers  214 ,  218 . The boss is formed just below the respective journal of the low and medium intake rockers  214 ,  218  on the intake rocker shaft  220 . The outer end of each bore  252  is closed by a closure plug  254  and in the uncoupled state, the locking member  250  generally floats between closure plug  254 . 
     The cooperation of the locking member  250  with the flat surface  245  of the coupling plunger member  234  permits reciprocation of the coupling plunger member  234  in the bore  231  (see also FIG.  7 ). Accordingly, when the high cam lobe  206  causes the high intake rocker  216  to begin its lift, the coupling plunger members  234  will be driven downwardly in the bores  231 . Under this condition, the low and medium intake rockers  214 ,  218  will experience no additional movement, and thus there is lost motion under this operation. In other words, movement of the high intake rocker  216  is not transmitted to the intake valves  114 . 
     It should be noted that in the retracted position of the locking members  250  in the uncoupled state, gap  256  are provided between each locking member  250  and the respective closure plug  254 . Each gap  256  communicates with an oil control passage  258   a ,  258   b  that extends through the rocker shaft  220  to the low and medium intake rockers  214 ,  218  respectively. Second passages  260  extend through the low and medium intake rockers  214 ,  218  to connect each oil control passage  258   a ,  258   b  to the respective gap  256 . The rocker shaft  220  contains a plurality of lumens or passages  258  of which the first and second passages  258   a ,  258   b  form a part; however, in one variation the rocker shaft  220  is hollow and a single central passage communicates with both the first and second passages  258   a ,  258   b  that branch off the central passage  258 . 
     Hydraulic fluid pressure may be exerted selectively through one or both of the passages  258   a ,  258   b  to the respective gap  256  in accordance with a desired control strategy. One such strategy will be described later with reference to FIG.  7 . Another control strategy, which can be used with a mechanism employing only one control passage  258 , is to have the valves actuated by (1) the low and medium cams  204 ,  208 ; and/or (2) just the high cam  206 . The hydraulic fluid pressure applied to each gap  256  is sufficient to overcome the spring force applied by the respective spring  248  within the bore  244  of the coupling plunger  234  so as to actuate the locking member  250 . When actuated, the locking member  250  is disposed partially in the bore  244  of the coupling member  234  and partially in the second bore  252  of the intake rocker  214 ,  218 . The coupling plunger  234  thus cannot move relative to the body of the intake rocker  214 ,  218 . 
     When both control passages  258   a ,  258   b  are pressurized, each locking plunger  250  registers with the engagement bore  244  and acts on the retainer member  246  to force it to inwardly compress the spring  248 . At this time, the high intake rocker  216  will be coupled to the low and medium rockers  214 ,  218 . Because of its greater lift and timing, it will actually control the opening of the valves  114  so as to provide a greater lift under this coupled condition as clearly shown in FIG.  6 B. As explained below, the control passages  258   a ,  258   b  can be separately pressurized to provide a number of control modes for the valve actuating mechanism. 
     When the hydraulic pressure in the passages  258   a ,  258   b  and gap  256  is relieved, the spring  248  will urge the locking member  250  back to its disengaged position as shown in FIG.  6 A. 
     Accordingly, this simple and relatively small variable valve actuating mechanism provides at least four modes of valve actuation. In a first mode, the control passages  258   a ,  258   b  are not pressurized. Therefore, as illustrated in FIG.  6 A and described above, the locking members  250  in both the low and medium rockers  214 ,  218  are not engaged with the engagement bore  244 . Movement of the coupling plunger member  234  that is caused by the movement of the high rocker  216  is absorbed by the spring  242  and is not transmitted to the low and medium rockers  214 ,  218 . Accordingly, the lift amount (L 1 ) and timing of the intake valves  114  are controlled by the low and medium cam lobes  204 ,  208 . It should be noted that varying types of lift arrangements may be employed and different lift ratios and/or valve timing between the two valves. That is the lift and/or timing of the valve operated by the low cam lobe  204  may be the same or different than the medium cam  208 . 
     In a second mode, pressure is only applied to the control passage  258   b  that communicates with the medium intake rocker  218 . Accordingly, as illustrated in FIG. 6B, the locking member  250  is engaged with the engagement bore  244  of the medium intake rocker  218 . As a result, the coupling plunger  234  cannot freely move within the bore  231  and movement of the high intake rocker arm  216  is transmitted to the medium intake rocker arm  218 . Therefore, the lift and timing of the intake valves  114  are respectively controlled by the low cam  204  and the high cam  206 . 
     In a third mode, pressure is only applied to the control passage  258   a  that communicates with the low intake rocker  214 . Accordingly, the locking member  250  is engaged with the engagement bore  244  of the low intake rocker  214 . As a result, the coupling plunger  234  cannot freely move within the bore  231  and movement of the high intake rocker arm  216  is transmitted to the low intake rocker arm  214 . Therefore, the lift and timing of the intake valves  114  are respectively controlled by the high cam  206  and the medium cam  204 . 
     In a fourth mode, pressure is applied to both control passages  258   a ,  258   b . Accordingly, the locking members  250  in both the low and medium intake rockers  214 ,  218  are engaged with the engagement bores  244 . As a result, the coupling plungers  234  cannot freely move within the bores  231  and movement of the high intake rocker arm  216  is transmitted to the low and medium intake rocker arms  214 ,  218 . Therefore, the lift and timing of the intake valves  114  are respectively controlled by the high cam  206 . 
     FIG. 8 illustrates the effects on engine performance that can be achieved using the present valve actuating mechanism. The dashed line  300  represents the typical torque performance of an engine without a variable valve timing. As is typical, torque decreases sharply at high and low engine speeds because of the inherent design compromises that are made when choosing valve lift and timing. 
     The solid line  302  represents the improved torque performance that can be achieved when using the present valve actuating mechanism. To achieve the improved performance, the valve actuating mechanism can be operated in the first mode during low speed operation. In this mode, the lift and timing of the intake valves  114 ,  116  are controlled by the low and medium cams  204 ,  208 . During medium speed operation, the valve actuating mechanism can be operated in the second or third mode. That is, the lift and timing of the intake valves  114 ,  116  are controlled by the low and high cams  204 ,  206  or the medium and high cams  208 ,  206 . During high speed operation, the valve actuating mechanism can be operated in the fourth mode wherein the lift and timing of the intake valves  114 ,  116  are controlled by the high intake cam  206 . Accordingly, as is evident from FIG. 8, a relatively flat torque curve can be achieved. 
     FIG. 9 illustrates an engine configured in accordance with another preferred embodiment of the present invention. In this embodiment, the plenum chambers  162 ,  168  have a compact shape. Specifically, the plenum chambers  162 ,  168  lie within line C, which extends from the corners of the cam covers  36 ,  38  of the cylinder head at an angle that is not greater than approximately 30 degrees and preferably less than 15 degrees. This arrangement reduces the size of the engine  24  and the length of the intake pipe  150 , which can increase pumping loses. Nevertheless, engine performance can be maintained because of the valve actuating mechanism described above. 
     Certain objects and advantages of the invention have been described above for the purpose of describing the invention and the advantages achieved over the prior art. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. Furthermore, although this invention has been described in terms of certain preferred embodiments, other embodiments that will be apparent to those of ordinary skill in the art are intended to be within the scope of this invention. Accordingly, the scope of the invention is intended to be defined by the claims that follow.