Patent Publication Number: US-6910450-B2

Title: Variable valve timing structure for outboard motor engine

Description:
PRIORITY INFORMATION 
   This application is based on and claims priority to Japanese Patent Applications No. 2000-163084, filed May 31, 2000 and No. 2000-163285, filed May 31, 2000, the entire contents of which are hereby expressly incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to a variable valve timing structure, and more particularly relates to a variable valve timing structure for an outboard motor. 
   2. Description of Related Art 
   A typical outboard motor comprises a power head and a housing unit depending from the power head. The power head includes an internal combustion engine that drives a marine propulsion device (e.g., a propeller) through a driveshaft and a propulsion shaft, which are both journaled within the housing unit. The marine propulsion device is attached to the end of a propulsion unit, which extends from housing unit and is in a submerged position. 
   There is an increasing emphasis on obtaining more effective emission control, better fuel economy and, at the same time, continued high or higher power output in outboard motors. Accordingly, four-cycle engines have started to replace two-cycle engines in outboard motors. However, it is difficult to arrange all the components of a four-cycle engine into the limited of space of an outboard motor cowling. 
   It is also desirable to achieve good emission control, fuel economy and high power output during the entire speed and load range 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 during operation 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. The valve timing usually is advanced in the high engine speed range to effect higher charging efficiency and higher performance. At lower engine speeds, the timing typically is delayed to effect higher combustion efficiency, fuel economy and good emission control. 
   Typically, such variable valve actuating mechanisms are hydraulically operated. The working fluid for operating the mechanism is typically provided by the lubrication system of the motor. The pressure of the working fluid is used to actuate various parts of the variable valve actuating mechanism. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention involves the recognition that the lubricant in the lubrication system typically contains vapors and/or bubbles. These vapors can adversely affect the operation of the variable valve actuating mechanism. For example, the vapors in the lubricant tend to rise. As such, the vapors tend to collect in the upper portions of lubricant passages. This can result in uneven flow of the lubricant, which can adversely effect the operation of the variable valve actuating mechanism. 
   As such, there is a need for an improved variable valve actuating mechanism that reduces the adverse effects of vapors in the working fluid. Such a mechanism should also be configured to minimize the number of parts, to reduce the size of the engine and to facilitate assembly and maintenance. 
   Therefore, one aspect of the present invention is an internal combustion engine for an outboard motor that comprises at least one combustion chamber formed by at least a engine body, a cylinder head assembly and a piston that moves relative to the engine body and the cylinder head assembly. A crankshaft extends in a generally vertical direction and is coupled to the piston such that movement of the piston causes the crankshaft to rotate. A port is in communication with the combustion chamber. A valve is moveable between open and closed positions of the port. A camshaft is journaled for rotation and extends generally parallel to the crankshaft. The camshaft includes at least one cam configured to open and close the valve. A rotor is attached an upper end of the camshaft and is positioned for at least partial rotation within a housing. The rotor defines at least a first space and a second space within said housing. A driven member is coupled to the housing. A drive member is coupled to an upper end of the output shaft. The drive member is coupled to the driven member such that rotation of the drive member is transmitted to the driven member. A control valve is positioned within a common hydraulic passage having a first opening and a second opening. A first hydraulic passage is in communication with the first space and the first opening and a second hydraulic passage in communication with the second space and second opening. The control valve is configured to selectively open and close the first and second openings such that hydraulic fluid is preferentially supplied to either the first space or the second space. The control valve is positioned generally along an axis that is perpendicular to the camshaft. 
   Another aspect of the present invention is an internal combustion engine for an outboard motor that comprises at least one combustion chamber formed by at least a engine body, a cylinder head assembly and a piston that moves relative to the engine body and the cylinder head assembly. A crankshaft extends in a generally vertical direction and is coupled to the piston such that movement of the piston causes the crankshaft to rotate. A port is in communication with the combustion chamber. A valve is moveable between open and closed positions of the port. A camshaft is journaled for rotation and extends generally parallel to the crankshaft. The camshaft includes at least one cam configured to open and close the valve. A rotor is attached an upper end of the camshaft and is positioned for at least partial rotation within a housing. The rotor defines at least a first space and a second space within said housing. A driven member is coupled to the housing. A drive member is coupled to an upper end of the output shaft. The drive member is coupled to the driven member such that rotation of the drive member is transmitted to the driven member. A control valve is positioned within a common hydraulic passage having a first opening and a second opening. A first hydraulic passage is in communication with the first space and the first opening and a second hydraulic passage in communication with the second space and second opening. The control valve is configured to selectively open and close the first and second openings such that hydraulic fluid is preferentially supplied to either the first space or the second space. The first and second openings are positioned generally at a common engine elevation. 
   Yet another aspect of the present invention is 37. An internal combustion engine for an outboard motor comprising an engine body, a piston movable relative to the engine body, a crankshaft that extends in a generally vertical direction and is journaled for rotation by the piston, the engine body, the piston and a cylinder head assembly together defining a combustion chamber, a port in communication with the combustion chamber, a valve movable between open and closed positions of the port, a camshaft that extends generally parallel to the crankshaft and is journaled for rotation to actuate the valve in a set angular position, a variable valve timing mechanism arranged to set the valve actuator to an angular position between a first angular position and a second angular portion, the first angular position being advanced as compared to the second angular position, the variable valve timing mechanism comprising a setting section, a supply section and a control section, the section comprising a control valve that is disposed on along an axis that is generally perpendicular to the camshaft. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will now be described with reference to the drawings of a preferred embodiment which is intended to illustrate and not to limit the invention. The drawings comprise 13 figures. 
       FIG. 1  is a side elevational view of an outboard motor having certain features and advantages according to the present invention. 
       FIG. 2  is a sectional port side view of a power head of the outboard motor. An engine of the power head is also shown in section. A camshaft drive mechanism is omitted in this figure except for an intake driven sprocket. 
       FIG. 3  is a top plan view of the power head. 
       FIG. 4  is a rear view of the power head. The cowling assembly is shown in section taken along the line  4 — 4  of FIG.  2 . 
       FIG. 5  is an enlarged, sectional side view of a portion of the engine that includes a variable valve timing (VVT) mechanism having certain features and advantages according to the present invention. 
       FIG. 6  is a cross-sectional view of the VVT mechanism taken along the line  6 — 6  of FIG.  5 . 
       FIG. 7  is a cross-sectional view of the VVT mechanism taken along the line  7 — 7  of  FIG. 5   
       FIG. 8  is an enlarged, sectional side view of another arrangement of a variable valve timing (VVT) mechanism having certain features and advantages according to the present invention. 
       FIG. 9  is a cross-sectional view of the VVT mechanism of  FIG. 8  taken along line  9 — 9  of FIG.  8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     FIGS. 1-4  illustrate an overall construction of an outboard motor  30  that employs an internal combustion engine  32  and a variable valve timing mechanism that are configured in accordance with certain features, aspects and advantages of the present invention. The engine and variable valve timing mechanism are described in the context of an outboard motor because the engine and variable valve timing mechanism have particular utility in this context. However, certain features, aspects and advantages of the present invention may find utility with other types of marine drives, land vehicles and/or stationary engines. 
   With initial reference to  FIG. 1 , the illustrated outboard motor  30  comprises a drive unit  34  and a bracket assembly  36 . The bracket assembly  36  supports the drive unit  34  on a transom  38  of an associated watercraft  40 . With the watercraft  40  resting on the surface  41  of a body of water, the bracket assembly  36  is configured to place a marine propulsion device of the outboard motor  30  in a submerged position. The bracket assembly  36  preferably comprises a swivel bracket  42 , a clamping bracket  44 , a steering shaft (not shown) and a pivot pin  46 . 
   The steering shaft typically extends through the swivel bracket  42  and is affixed to the drive unit  34  by top and bottom mount assemblies  43 . The steering shaft is pivotally journaled for steering movement about a generally vertically extending steering axis defined within the swivel bracket  42 . The clamping bracket  44  comprises a pair of bracket arms that are spaced apart from each other and that are affixed to the watercraft transom  38 . The pivot pin  46  completes a hinge coupling between the swivel bracket  42  and the clamping bracket  44 . The pivot pin  46  extends through the bracket arms so that the clamping bracket  44  supports the swivel bracket  42  for pivotal movement about a generally horizontally extending tilt axis defined by the pivot pin  46 . The drive unit  34  thus can be tilted or trimmed about the pivot pin  46 . 
   As used through this description, the terms “forward,” “forwardly” and “front” mean at or to the side of the outboard motor where the bracket assembly  36  is located, and the terms “rear,” “reverse,” “backwardly” and “rearwardly” mean at or to the opposite side of the front side, unless indicated otherwise or otherwise readily apparent from the context use. 
   A hydraulic tilt and trim adjustment system  48  preferably is provided between the swivel bracket  42  and the clamping bracket  44  to tilt the swivel bracket  42  and the drive unit  34  relative to the clamping bracket  44 . Otherwise, the outboard motor  30  can have a manually operated system for tilting the drive unit  34 . Typically, the term “tilt movement”, when used in a broad sense, comprises both a tilt movement and a trim adjustment movement. 
   The illustrated drive unit  34  comprises a power head  50  and a housing unit  52 , which includes a driveshaft housing  54  and a lower unit  56 . The power head  50  is disposed atop the drive unit  34  and includes an internal combustion engine  32  that is positioned within a protective cowling  60  that preferably is made of plastic. Preferably, the protective cowling  60  defines a generally closed cavity  62  (see  FIG. 2 ) in which the engine  32  is disposed. The protective cowling assembly  60  preferably comprises a top cowling member  64  and a bottom cowling member  66 . In one arrangement, the top cowling member  64  is detachably affixed to the bottom cowling member  66  by a coupling mechanism so that a user, operator, mechanic or repair person can access the engine  32  for maintenance and/or for other purposes. 
   With particular reference to  FIG. 2 , the top cowling member  64  preferably has a rear intake opening  72  formed on its rear and top portion. A rear intake member  74  with a rear air duct  76  is affixed to the top cowling member  64  to form a rear air intake space  78  with the rear top portion of the top cowling member  64 . As best seen in  FIG. 4 , the rear air duct  74  is disposed on the starboard side of the rear intake member  74 . 
   With continued reference to  FIG. 2 , the top cowling member  64  defines a recessed portion  82  at a front end thereof. An opening  84  is defined proximate the recessed portion  82  on the starboard side. An outer shell  86  covers the recessed portion  82  to define a front air intake space  88 . A front air duct  90  is affixed to the recessed portion  82  of the top cowling member  64  to be placed over the opening  84  and to communicate with the closed cavity  62 . The air duct  90  has a plurality of apertures  92 , each of which preferably is cylindrical. Ambient air thus is drawn into the closed cavity  62  through the rear intake openings  72  and then through the air duct  76  and front air duct  90 . The top cowling member  64  also can taper in girth toward its top surface, which is in the general proximity of the air intake opening  72 . 
   The bottom cowling member  66  preferably has an opening  96  at its bottom portion through which an upper portion of an exhaust guide member  98  (see  FIG. 1 ) extends. The exhaust guide member  98  preferably is made of aluminum alloy and is affixed atop the driveshaft housing  54 . The bottom cowling member  66  and the exhaust guide member  98  together generally form a tray. The engine  32  is placed onto this tray and is affixed to the exhaust guide member  98 . The exhaust guide member  98  also has an exhaust passage through which burnt charges (e.g., exhaust gases) from the engine  32  are discharged. 
   The engine  32  in the illustrated embodiment preferably operates on a four-cycle combustion principle. With reference to  FIG. 3 , the engine  32  has a cylinder block  102 . The illustrated cylinder block  102  defines four cylinder bores  104  which extend generally horizontally and are generally vertically spaced from one another. As used in this description, the term “horizontal” and “horizontally” mean that the subject portions, members or components extend generally parallel to the water line  41  when the associated watercraft  40  and the drive unit  34  are placed in the position shown in FIG.  1 . The term “vertically” in turn means that portions, members or components extend generally normal to those that extend horizontally. It should be appreciated that the illustrated type of engine merely exemplifies one type of engine on which various aspects and features of the present invention can be suitably used. Engines having other number of cylinders, having other cylinder arrangements, and operating on other combustion principles (e.g., crankcase compression two-stroke or rotary) also can employ various features, aspects and advantages of the present invention. 
   A piston  106  is positioned for reciprocal movement in each cylinder bore  104 , as is a well-known in the art. A cylinder head assembly  108  is affixed to one end of the cylinder block  102  for closing the cylinder bores  104 . The cylinder head assembly  108  preferably defines four combustion chambers  110  together with the associated pistons  106  and cylinder bores  104 . Of course, the number of combustion chambers can vary, as indicated above. A crankcase member  112  closes the other end of the cylinder bores  104  and defines a crankcase chamber  114  together with the cylinder block  102 . A crankshaft or output shaft  118  extends generally vertically through the crankcase chamber  114  and is journaled for rotation by several bearing blocks in a suitable arrangement. Connecting rods  120  couple the crankshaft  118  with the respective pistons  106  in a well-known manner. Thus, the crankshaft  118  can rotate with the reciprocal movement of the pistons  106 . 
   Preferably, the crankcase member  112  is located at the most forward position, with the cylinder block  102  and the cylinder head assembly  108  extending rearward from the crankcase member  112 . Generally, the cylinder block  102 , the cylinder head assembly  108  and the crankcase member  112  together define an engine body  124 . Preferably, at least these major engine portions  102 ,  108 ,  112  are made of an aluminum alloy. The aluminum alloy advantageously increases strength over cast iron while decreasing the weight of the engine body  96 . 
   With particular reference to  FIGS. 2-5 , the engine  32  further comprises an air induction system or device  126  for supplying air to the combustion chambers  110 . The air induction system  126  draws the air from the cavity  62  to the combustion chambers  110 . The air induction system  126  preferably comprises eight intake ports  128 , four intake passages  130  and a single plenum chamber  132 . In the illustrated arrangement, two intake ports  128  are allotted to one combustion chamber  110  and also to one intake passage  130 . The intake ports  128  are defined in the cylinder head assembly  108 . Intake valves  134  are slidably disposed at the cylinder head assembly  108  to move between an open position and a closed position. Bias springs  136  ( FIG. 5 ) can be used to urge the intake valves  134  toward the respective closed positions and can be secured in position on the respective valve stems by retainers  138  that are affixed to the valves  134 . When each intake valve  134  is in the open position, the intake passage  130  that is associated with the intake port  128  communicates with the associated combustion chamber  110 . 
   Each intake passage  130  preferably is defined by an intake manifold  140 , a throttle body  142  and an intake runner  144 . The intake manifold  140  and the throttle body  142  preferably are made of aluminum alloy, while the intake runner  144  can be made of plastic. As best seen in  FIG. 3 , a portion of the intake runner  144  extends forwardly. The respective portions of the intake runners  144  define the plenum chamber  132  together with a plenum chamber member  146  that preferably is made of plastic. The plenum chamber  132  has an air inlet  148  such that air in the closed cavity  62  can be drawn into the plenum chamber  132  through the air inlet  148  before flowing through the respective intake passages  130 . The plenum chamber  132  promotes uniform air flow between the intake passages  130  and acts as an intake silencer. The intake passage  130  (i.e., the intake manifold  140  or the intake runner  144 ) preferably includes an intake pressure sensor (not shown) to sense the pressure in the intake passage  130 . Preferably, the respective intake passages  130  are similarly sized such that every passage  130  will operate at substantially equal pressure. 
   Each throttle body  142  has a throttle valve  152  journaled for pivotal movement about an axis of a valve shaft  154  that extends generally vertically. The valve shaft  154  links the all of the valves  152  to enable simultaneous valve movement. The valve shaft  154  is operable by the operator through an appropriate conventional throttle valve linkage. The throttle valves  152  are movable between an open position and a closed position to regulate the amount of air flowing through the air intake passages  130 . Normally, the greater the opening degree, the higher the rate of airflow and the higher the engine speed. In order to bring and maintain idle speed, the throttle valves  152  are almost closed but preferably not completely closed to ensure a stable idle speed and to prevent sticking of the throttle valves  152 . Preferably, a throttle position sensor (not shown) is disposed atop the valve shaft  154  to sense the position of the throttle valves  152 . 
   The air induction system  126  preferably includes an idle air delivery device that bypasses the throttle valves  152  and extends from the plenum chamber  132  to the respective intake passages  130 . Idle air thus may be delivered to the combustion chambers  110  through the idle air delivery device when the throttle valves  152  are substantially closed. The idle air delivery device preferably includes an idle air passage that is branched from the respective intake passages, an idle valve and an idle valve actuator. The idle valve preferably is a needle valve that can move between an open position and a closed position. The idle valve actuator actuates the idle valve to a certain position to adjust an amount of the idle air flowing into the combustion chambers. 
   The engine  32  also includes an exhaust system that routes burnt charges (i.e., exhaust gases) from the combustion chambers  110  to a location outside of the outboard motor  30 . Each cylinder bore  104  preferably has two exhaust ports (not shown) defined in the cylinder head assembly  108 . The exhaust ports are selectively opened and closed by exhaust valves. A structure of each exhaust valve and an arrangement of the exhaust valves are substantially the same as the intake valve and the arrangement thereof, respectively. 
   An exhaust manifold (not shown) preferably is formed next to the exhaust ports and extends generally vertically. The exhaust manifold communicates with the combustion chambers  110  through the exhaust ports to collect exhaust gases therefrom. The exhaust manifold is coupled with the foregoing exhaust passage of the exhaust guide member  98  (see FIG.  1 ). When the exhaust ports are opened, the combustion chambers  110  thus communicate with the exhaust passage through the exhaust manifold. 
   With particular reference to  FIGS. 2 ,  3  and  5 , a valve cam mechanism or valve actuator  170  preferably is provided for actuating the intake valves  134  and the exhaust valves. In the illustrated embodiment, the valve cam mechanism  170  includes an intake camshaft  172  and an exhaust camshaft  174  that extend generally vertically. The camshafts  174  are journaled for rotation by the cylinder head assembly  108  and an upper bearing cap  176  and a lower bearing cap  178 . Preferably, at least the upper bearing cap  176  is formed by a single integral member, which supports the intake and the exhaust cam shafts  172 ,  174 . A camshaft cover  179  is affixed to the cylinder head assembly  108  to cover the camshafts  172 ,  174 . As best seen in  FIG. 5 , each camshaft  172 ,  174  has cam lobes  180  to push valve lifters  182  that are affixed to the respective ends of the intake valves  134  and exhaust valves. The cam lobes  180  repeatedly push the valve lifters  182  at timing that is in proportion to the engine speed with the rotation of the camshafts  172 ,  174  to actuate the intake valves  134  and the exhaust valves. A method for controlling the timing will be described below. 
   A camshaft drive mechanism  186  is provided for driving the valve cam mechanism  170 . As best seen in  FIG. 3 , an intake driven sprocket  188  is positioned atop the intake camshaft  172  and an exhaust driven sprocket  190  is positioned atop the exhaust camshaft  174 . A drive sprocket  192  is also positioned atop the crankshaft  118 . A timing chain or belt  194  is wound around the driven sprockets  188 ,  190  and the drive sprocket  192 . The crankshaft  118  thus drives the respective camshafts  172 ,  174  through the timing chain  194  in a timed relationship. In other words, the sprockets  188 ,  190 ,  192  are all connected such that the sprockets  188 ,  190 ,  192  rotate in a generally fixed relationship with each other. Because the camshafts  172 ,  174  generally rotate at half of the speed of the rotation of the crankshaft  118  in the four-cycle combustion principle, the diameter of the driven sprockets  188 ,  190  is preferably twice as large as a diameter of the drive sprocket  192 . 
   The engine  32  preferably has a port or manifold fuel injection system. The fuel injection system preferably comprises four fuel injectors  198  (see  FIG. 4 ) with one fuel injector allotted for each of the respective combustion chambers  110 . Each fuel injector  198  preferably has an injection nozzle directed toward the associated intake passage  130  adjacent to the intake ports  134 . The fuel injectors  198  spray fuel into the intake passages  130  under control of an electronic control unit (ECU) that preferably is mounted on the engine body  124  at an appropriate location. The ECU controls the timing and duration of injection by the fuel injectors  198  so that the nozzles spray a proper amount of the fuel per combustion cycle. Of course, the fuel injectors  198  can be disposed for direct cylinder injection and carburetors can replace or accompany the fuel injectors  198 . 
   The engine  32  further comprises an ignition or firing system. Each combustion chamber  110  is provided with a spark plug  202  that is connected to the ECU through an igniter such that ignition timing is also controlled by the ECU. Each spark plug  202  has electrodes that are exposed to the associated combustion chamber and are spaced apart from each other with a small gap. As is well known, the spark plugs  202  make a spark between the electrodes to ignite an air/fuel charge in the combustion chamber  110  at selected ignition timing under control of the ECU. In some arrangements, glow plugs can be used. 
   In the illustrated engine  32 , the pistons  106  reciprocate between top dead center and bottom dead center. When the crankshaft  118  makes two rotations, the pistons  106  generally move from top dead center to bottom dead center (the intake stroke), from bottom dead center to top dead center (the compression stroke), from top dead center to bottom dead center (the power stroke) and from bottom dead center to top dead center (the exhaust stroke). During the four strokes of the pistons  106 , the camshafts  172 ,  174  make one rotation and actuate the intake valves  134  and the exhaust valves to open the intake ports  128  during the intake stroke and to open exhaust ports during the exhaust stroke, respectively. Of course, other engine operating cycles also can be used. 
   Generally, at the beginning of the intake stroke, air is drawn through the air intake passages  130  and fuel is injected into the intake passages  130  by the fuel injectors  198 . The air and the fuel thus are mixed to form the air/fuel charge in the combustion chambers  110 . Slightly before or during the power stroke, the respective spark plugs  202  ignite the compressed air/fuel charge in the respective combustion chambers  110 . The engine  32  thus continuously repeats the four-cycle combustion process. 
   During engine operation, heat builds in the engine body  124 . The engine  32  thus includes a cooling system to cool the engine body  124 . The outboard motor  30  preferably employs an open-loop type water cooling system that introduces cooling water from the body of water surrounding the motor  30  and then discharges the water to the water body. The cooling system includes one or more water jackets defined within the engine body  124  through which the introduced water runs to absorb heat from the engine body  124 . As best seen in  FIG. 3 , the cooling system preferably includes a water discharge pipe  206  that extends from an outer surface of the engine body  124 . A thermostat chamber  208  is defined at a location where the discharge pipe  206  is connected to the engine body  124  to enclose a thermostant  210  ( FIG. 2 ) that controls flow of the discharged cooling water. When water temperature is relatively low (e.g., immediately after the engine  32  is started up), the thermostat  210  inhibits the water from flowing out so that the engine  32  can be warmed up quickly. 
   The engine  32  also preferably includes a lubrication system. Although any type of lubrication systems can be applied, a closed-loop type system is employed in the illustrated embodiment. The lubrication system comprises a lubricant tank defining a reservoir cavity preferably positioned within the driveshaft housing  54 . An oil pump is provided at a desired location, such as atop the driveshaft housing  54 , to pressurize the lubricant oil in the reservoir cavity and to pass the lubricant oil through a suction pipe toward desired engine portions through lubricant delivery passages. The engine portions that receive lubrication include, for example, the crankshaft bearings, the connecting rods  120  and the pistons  106 . Portions  214  of the delivery passages ( FIG. 2 ) can be defined in the crankshaft  118 . Lubricant return passages also are provided to return the oil to the lubricant tank for re-circulation. 
   With reference to  FIGS. 2 and 4 , a flywheel assembly  216  preferably is positioned above atop the crankshaft  118  and is mounted for rotation with the crankshaft  118 . The flywheel assembly  216  preferably comprises a flywheel magneto or AC generator that supplies electric power to various electrical components, such as the fuel injection system, the ignition system and the ECU. A protective cover  218  extends over a majority of the top portion of the engine  32  to cover the portion including the fly wheel assembly  216  and the camshaft drive mechanism  186 . The protective cover  218  preferably has a rib  219  ( FIG. 4 ) that prevents air from flowing directly toward the portion of the engine  32  that has the air induction system  126  (i.e., the starboard side of the engine  32 ). The protective cover  218  also preferably has a second rib  220  ( FIG. 2 ) that inhibits the air from flowing directly toward a front portion of the engine body  124 . The ribs  219 ,  222  advantageously form an air flow path that moves around the engine body  124  in a manner that can also cool the engine body  124 . 
   With reference again to  FIG. 1 , the driveshaft housing  54  depends from the power head  50  to support a driveshaft  222  which is coupled with the crankshaft  118  and which extends generally vertically through the driveshaft housing  54 . The driveshaft housing  54  preferably defines an internal section (not shown) of the exhaust system that leads the majority of exhaust gases to the lower unit  56 . Preferably, an idle discharge section (not shown) is branched off from the internal section to discharge idle exhaust gases directly out to the atmosphere through a discharge port (not shown) that is formed on a rear surface of the driveshaft housing  54 . 
   The lower unit  56  depends from the driveshaft housing  54  and supports a propulsion shaft  226  that is driven by the driveshaft  222 . The propulsion shaft  226  extends generally horizontally through the lower unit  56  and is journaled for rotation. A propulsion device is attached to the propulsion shaft  226 . In the illustrated arrangement, the propulsion device is a propeller  228  that is affixed to an outer end of the propulsion shaft  226 . The propulsion device, however, can take the form of a dual counter-rotating system, a hydrodynamic jet, or any of a number of other suitable propulsion devices. 
   A transmission  232  preferably is provided between the driveshaft  222  and the propulsion shaft  226 , which lie generally normal to each other (i.e., at a 90° angle). The transmission  232  couples together the two shafts  222 ,  226  with bevel gears, as is well known in the art. The outboard motor  30  preferably has a switchover or clutch mechanism that allows the transmission  232  to change the rotational direction of the propeller  228  among forward, neutral or reverse. 
   With general reference to  FIGS. 2-4  and with particular reference to  FIGS. 5-7 , a variable valve timing mechanism (herein “VVT mechanism”)  240  having certain aspects, features and advantages according to the present invention will now be described. 
   The VVT mechanism  240  preferably is configured to set the intake camshaft  172  to an angular position that is between a first angular position and a second angular position with respect to the intake driven sprocket  188 . At the first angular position, the intake camshaft  172  opens and closes the intake valves  134  at the most advanced timing. At the second angular position, the intake camshaft  172  opens and closes the intake valves  134  at the most delayed timing. Any angular position between both the first and second angular position is delayed with respect to the first angular position and is advanced with respect to the second angular position. 
   The VVT mechanism  240  preferably is hydraulically operated. As best seen in  FIG. 5 , the illustrated VVT mechanism  240  comprises a setting section  242 , a fluid supply section  244  and a control section  246 . As will be explained in more detail below, the setting section  242  sets the intake camshaft  172  at a certain angular position with respect to the intake driven sprocket  188  in response to a rate of working fluid flow that is allotted to each of two spaces of the setting section  242 . The fluid supply section  244  preferably supplies the working fluid to the setting section  242 . Preferably, the working fluid is a portion of the lubricant from the lubrication system. Of course in some arrangements, a separate hydraulic circuit can be formed. In such arrangements, a separate pump can be used. The control section  246  selects the amount of the working fluid allotted to each of the two spaces and preferably is under the control of the ECU. 
   With particular reference to  FIGS. 5 and 6 , the setting mechanism  242  preferably includes an outer housing  250  and an inner rotor  252 . The illustrated outer housing  250  is affixed to the intake driven sprocket  188  by three bolts  254  and preferably forms at least one chamber  256  and more preferably three chambers  256 , which can be positioned between the three bolts  254 . The inner rotor  252  is affixed atop of the intake camshaft  172  by a bolt  258  and preferably has at least one vane  260  pivotably placed within each of the respective chambers  256  of the housing  250 . In the illustrated arrangement, the inner rotor  252  has three vanes  260  that extend radially and are spaced apart from each other by angle of approximately 120 degrees. The sides of each vane  260  divide the respective chambers  256  such that define a first space  262  and a second space  264 . Seal members  266  preferably are carried by the respective vanes  260  and abut on an inner surface of the housing  250  so as to substantially separate the first and second spaces  262 ,  264  from each other. 
   The respective first spaces  262  communicate with one another through respective pathways  270  and a ditch  272  that is formed around the bolt  258 , while the respective second spaces  264  communicate with one another through respective pathways  274  and a ditch  276  that is also formed around the bolt  258 . The ditches  272 ,  276  in the illustrated arrangement generally are configured as a substantially circular flow path around the bolt and are axially offset from one another. A pathway  278  extends from the ditch  272  to a bottom portion of the rotor  252 . A cover member  280  is affixed to the outer housing  250  by screws  282  to cover the bolt  258 . 
   With particular reference to  FIGS. 5 and 7 , the fluid supply section  244  preferably includes a supply passage  284  (see also  FIG. 2 ) and a first and second passages  286 ,  288 . The supply passage  284  and the first and second passages  286 ,  288  communicate with one another through the control section  246 . The supply passage  284  preferably has a passage portion  284   a  ( FIG. 5 ) defined in the cylinder head assembly  108  and a passage portion  284   b  ( FIG. 2 ) defined in the bearing cap  176 . 
   The supply passage  284  communicates with the lubrication system so that a portion of the lubricant is supplied to this VVT mechanism  240 . Because the illustrated passage portion  284   a  is formed by a drilling process in the illustrated embodiment, a closure member  290  closes one end of the passage portion  284   a.    
   The first and second passages  286 ,  288  preferably are defined in a top portion of the camshaft  172  and the upper bearing cap  176 . A portion of the first passage  286  is formed in the camshaft  172  and includes a pathway  292  that extends vertically and communicates with the pathway  278  that communicates with the ditch  272  of the first space  262 . The ditch  294  advantageously places the pathway  292  in fluid communication with a pathway  300  regardless of the angular orientation of the camshaft  172 . A portion of the second passage  288  formed in the camshaft  172 , in turn, includes a pathway  296  that extends vertically and communicates with the ditch  274  of the second space  264 . As shown in  FIG. 5 , a portion of the first delivery passage  286  formed in the bearing cap  176  includes a pathway  300  that extends generally vertically and horizontally and communicates with the ditch  294 , while a portion of the second delivery passage  288  formed in the bearing cap  176  includes a pathway  302  that extends generally vertically and horizontally and communicates with the ditch  298 . The inlet ends of the pathways first and second delivery passages  286 ,  288  selectively communicate with a common chamber  304  of the control section  246  through a first inlet port  306  and a second inlet port  308 , respectively. 
   A seal member  310  is inserted between the cylinder head assembly  108 , the camshaft  172  and the bearing cap  176  to inhibit the lubricant from leaking out. It should be noted that  FIGS. 5 and 7  show the delivery passages  286 ,  288  in a schematic fashion and that the passages  286 ,  288  preferably do not actually merge together. 
   The control section  246  preferably includes an oil control valve (OCV)  314 . The OCV  314  comprises a housing section  316  and a cylinder section  318 . Both the housing and cylinder sections  316 ,  318  preferably are positioned in the upper bearing cap  176 . The sections  316 ,  318  preferably also extend through a hole of the camshaft cover  179 . The camshaft cover preferably  179  includes a lip  319  around the opening. A bellow  320 , preferably made of rubber, is provided between the housing section  316  and the lip  319  of the camshaft cover  179  to close and seal the through-hole. 
   The cylinder section  318  defines the common chamber  304  that communicates the supply passage  284  and the first and second delivery passages  286 ,  288 . The cylinder section preferably includes a drain  289  that, in the illustrated arrangement, is open to the interior of the camshaft cover  179  although in other arrangements the drain  289  can be connected to other portions of the lubrication system. The housing section  316  preferably encloses a solenoid type actuator, although other types of actuators can also be used. 
   A rod  324  extends into the common chamber  304  from the housing  316  and is axially movable therein. The illustrated rod  324  has a first valve  326  and a second valve  328  and a pair of guide portions  330 . The valves  326 ,  328  and the guide portions  330  have an outer diameter that is larger than an outer diameter of the rod  324  and approximately equal to an inner diameter of the cylinder  318 . The rod  324  defines an internal passage  334 , which extends through the rod  324 , and apertures  336   a ,  336   b ,  336   c , which communicate with the passage  334  and the common chamber  304  to allow the lubricant to escape through the drain  289  through an opening  335  as will be explained in more detail below. A coil spring  338  is retained at an end of the cylinder  318  opposite to the housing section  316  to urge the rod  324  toward the solenoid. 
   The solenoid actuates the rod  324  under control of the ECU so that the rod  324  can take several axial positions in the chamber  304 . More specifically, the solenoid is configured to preferably push the rod  324  step by step toward certain positions as the ECU commands. If the desired position is closer to the solenoid than the present position, then the solenoid does have to actuate the rod  324  and the coil spring  338  cam push the rod  324  back to the desired position. 
   To direct lubricant to the first space  262 , the rod  324  is moved to the left of the position shown in FIG.  7 . In this position, the first passage  286  is in communication with the supply passage  284  while the second valve  328  substantially isolates the second passage  288  from the supply passage  284 . In this manner, lubricant can flow into the first space  262  while the lubricant in the second space  264  can escape to the drain  289 . For example, in the illustrated arrangement, the lubricant in the second passage  288  can flow into the aperture  336   c  through passage  334  and to the drain  289 . To direct lubricant to the second space  264 , the rod  324  is moved to the right from the position shown in FIG.  7 . In this position, the second passage  288  is in communication with the supply passage  284  while the first valve  326  substantially isolates the first passage  286  from the supply passage  284 . In this manner, lubricant can flow into the second space  264  while the lubricant in the first space  262  can escape through the drain  289 . That is, the lubricant in the first passage  286  can flow into the aperture  336   b  and through passage  334  into the drain  289 . In a “neutral” position, which is illustrated in  FIG. 7 , the first and second valves  326 ,  328  cover the first and second passages  286 ,  288 . As such, in this position, the lubricant in the first and second spaces  262  cannot escape and the position of the inner rotor  252  is fixed. 
   In the manner described above, the degree to which the inlet ports  306 ,  308  are closed or opened determines the amount of the lubricant that is allotted to the first and second passages  286 ,  288  and to the first and second spaces  262 ,  264  in the setting section  242  described above. The amount of the lubricant supplied to the first and second spaces  262 ,  264  thus determines an angular position of the camshaft  172  with respect to the intake driven sprocket  188 . If more lubricant is allotted to the first space  262  than to the second space  264 , the camshaft  172  is set closer to the most advanced position, and vise versa. 
   The operation of the illustrated VVT mechanism  240  will now be described in more detail. When the engine  32  is running, the rotation of the crankshaft  118  is transmitted to the exhaust camshaft  174  through the exhaust driven sprocket  190  and the timing chain  194 . In a similar manner, the rotation of the crankshaft is also transmitted to the intake camshaft  172  through the timing chain  194 , intake driven sprocket  188  and the VVT mechanism  240 . Preferably, the intake and exhaust camshafts  172 ,  174  rotate at a predetermined speed (e.g., one half of the speed of the crankshaft  118 ). 
   As mentioned above, the outer housing  250  of the VVT mechanism  240  is coupled to and thus rotated by the intake driven sprocket  188 . The rotation of outer housing  250  is transmitted to the inner rotor  252  through the lubricant in the chambers  256  of the housing  250 . The inner rotor  252 , in turn, is affixed to atop the intake camshaft  172  such that the rotation of the inner rotor  252  is transmitted to the intake camshaft  172 . When the intake camshaft  172  is rotated, the intake valves  134  are opened and closed at an appropriate timing by the intake cams  180  formed in the intake camshaft  172 . Therefore, by selectively supplying lubricant to the first and second spaces  262 ,  264  inside the VVT mechanism  240 , the phase of the intake camshaft  172  with respect to the intake driven sprocket  188  can be adjusted and, thus, the timing of the opening and closing of the intake valves  134  can be controlled. 
   The control section  246  selectively supplies and removes lubricant to/from the first and second spaces  262 ,  264  as described above. Lubricant is supplied from the lubricant pump or an additional pump to the common chamber  304  of the control section  246  through the lubricant passages  284 . From the common chamber  304 , the lubricant is selectively supplied to the delivery passages  286 ,  288 , by alternately opening and closing or by partially blocking the inlet ports  306 ,  308  with the rod  324  of the OCV  314 . As mentioned above, the ECU controls the movement of the rod  324 . 
   When the lubricant is supplied to the first delivery passage  286 , lubricant is supplied to the first space  262  through the lubricant passages  292 ,  278 ,  270 , lubricant is removed from the second space  264  and the inner rotor  252  rotates to the clockwise direction relative to the outer housing  250  as shown in FIG.  6 . When lubricant is supplied to the second delivery passage  288 , lubricant is supplied to the second space  264  through the lubricant passages  298 ,  296   274  and lubricant is removed from the first space as described above. The inner rotor  252  rotates relative to the outer housing  250  in the counterclockwise direction as shown in FIG.  6 . As such, the phase of the intake camshaft  172  which rotates together with the inner rotor  252  can be adjusted and the opening-and-closing timing of the intake valves  134  can be advanced or delayed. To set the inner rotor  252  at a particular position, the first and second passages  286 ,  288  are closed by the first and second valves  326 ,  328  as shown in FIG.  7 . 
   An advantage of the illustrate arrangement is that the since the OCV  314  is generally positioned along a substantially horizontal axis, which in the illustrated arrangement, is also generally perpendicular to the intake camshaft  172 . This arrangement is advantageous for several reasons. For example, the lubricant in the lubricant system may have vapors (i.e., bubbles) mixed into the lubricant. As mentioned above, if the OCV  314  is positioned along a substantially vertical axis, these vapors can tend to rise and can be preferentially directed to one of the two supply passages  286 ,  288 . This can alter the amount of lubricant that is supplied to the first and second spaces  262 ,  264 , which in turn, can cause inaccuracies in the phase angle of the inner rotor  252  with respect to the outer housing  250  and the timing of the opening and closing of the intake valves  134 . By arranging the common chamber and such that the inlet ports  306 ,  308  are located substantially at the same elevation, the lubricant supplied to the first and second spaces  262 ,  264  is more consistent as the vapors are not preferentially directed to either the first or the second passages  286 ,  288 . 
   Another advantage of the illustrated arrangement is that, in the illustrated arrangement, the OCV  314  is positioned near the upper end of the intake camshaft  172 . More preferably, the OCV  313  is positioned in the upper bearing cap  176 , which supports the intake camshaft  172  and, in the illustrated arrangement, the exhaust cam shaft  174 . This position reduces the distance between the OCV  314  and the setting section  242 , which is located atop the intake cam shaft  172 . As such, the length of the various lubricant passages, which preferably are also located in the upper bearing cap  176 , of the fluid supply section  244  can be reduced. The shortened distances increases the responsiveness of the VVT  240  to the position changes of the OCV  314 . 
   Another advantage of the illustrated arrangement is that the OCV  314  positioned generally along an axis that extends across the engine  32  from the right side to the left side. This provides for a compact size of the engine  32 . 
   It should be appreciated that, although in the illustrated arrangement the VVT  240  is provided for the intake valves  134 , in a modified arrangement a VVT  240  of a similar arrangement can be provided instead, or in addition, for the exhaust valves. 
     FIGS. 8 and 9  illustrate a modified arrangement of the VVT  240  having certain features and advantages according to the present invention. In this arrangement, the VVT  240  includes a lubricant filter  300 . The lubricant filter  300  preferably is located on a contact face  302  between the upper bearing cap  176  and the cylinder head assembly  108 . More specifically, a lubricant filter bore  304  is provided in the supply passage  284  for supporting the filter  300 . The bore  304  has an opening on the contact face  302 . 
   An advantage of this arrangement is that it provides for a simplified assembly. For example, the filter  300  can be inserted into the bore  304  and then the upper bearing cap can coupled to the cylinder head assembly  108 . In a similar manner, the filter can be easily replaced or checked by uncoupling the cylinder head assembly  108  and the upper bearing cap  176  to expose the filter  300 . It should be appreciated that in a modified arrangement, the bore can be positioned in the cylinder head assembly such that the filter is positioned in the cylinder head assembly. In such an arrangement, the bore would have an opening on the contact face of the cylinder head assembly. 
   Of course, the foregoing description is that of a preferred construction having certain features, aspects and advantages in accordance with the present invention. Various changes, combinations, sub-combinations and modifications may be made to the above-described arrangements without departing from the spirit and scope of the invention, as defined by the appended claims.