Patent Publication Number: US-2007105465-A1

Title: Watercraft Having a Four Stroke Engine with a Supercharger

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a continuation of U.S. Non-provisional Application No. 09/794,238, which was filed on Feb. 28, 2001, and which issued as U.S. Pat. No. 6,626,140 on Sep. 30, 2003. That application relates to and relies for priority on U.S. Provisional Application No. 60/185,703, filed on Feb. 29, 2000, and U.S. Provisional Application No. 60/257,174, filed on Dec. 22, 2000. All three applications are incorporated herein by reference. In addition, this application is related, but does not claim priority, to U.S. Pat. No. 6,544,086 (Ser. No. 09/794,219 to Tscherne et al.), No. 6,390,869 (Ser. No. 09/794,240 to Korenjak et al.), No. 6,601,528 (Ser. No. 09/794,237 to Bilek et al.), No. 6,591,819 (Ser. No. 09/794,215 to Tscherne et al.), and No. 6,415,759 (Ser. No. 09/794,245 to Ohrenberger et al.), the contents of all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to a new engine for use in, for example, personal watercraft. In particular, the present invention relates to a new four-stroke in-line engine that was developed with a view to the future stricter environmental and emission regulations. The engine has a supercharger for enhancing engine performance.  
     BACKGROUND OF THE INVENTION  
      There is a very popular type of watercraft known as a “personal watercraft” which is designed to be operated primarily by a single rider. Although this type of watercraft is commonly employed for single riders, frequently provisions are made for accommodating additional passengers although the maximum number of passengers is more limited than conventional types of watercraft.  
      This type of watercraft is also generally quite sporting in nature and normally accommodates at least the rider on a type of seat in which the rider sits in a straddle fashion. The passenger&#39;s area is frequently open through the rear of the watercraft so as to facilitate entry and exit of the rider and passengers to the body of water in which the watercraft is operating, as this type of watercraft is normally ridden with passengers that are wearing swimming suits.  
      These personal watercraft are generally quite small so that they can be conveniently transported from the owner&#39;s home to a body of water for its use. Because of the small size, the layout of the components is extremely critical, and this gives rise to several design considerations that are peculiar to this type of watercraft. However, due to its sporting nature it is also desirable if the watercraft is powered by an engine and propulsion device that are not only efficient but also generate sufficient power.  
      Traditionally, two-cycle engines have been used to power watercraft, including personal watercraft. These engines have the advantage that they are fairly powerful, relatively lightweight, and compact.  
      One particular disadvantage to the two-cycle engine is its emission content. Two-cycle engines generally exhaust larger quantities of hydrocarbons and other pollutants than four-cycle engines due to cylinder charging inefficiencies and the combustion of lubricating oil among other things. When measures are taken to reduce emissions of the two-cycle engine, other generally undesirable consequences can result, such as an increase in the weight of the engine, a reduction of its power output or the like. With concern for the environment and increasingly strict emissions requirements being instituted by various governing bodies. There is motivation to provide a power plant that reduces exhaust emissions while retaining other advantageous characteristics such as compactness, low weight and high power output.  
      Four-cycle engines are commonly used as power plants in other applications, such as automobiles. These engines have the advantage that their emissions output are generally desirably lower as compared to a two-cycle engine for a given power output. These engines are typically larger than two-cycle engines and present numerous spatial issues when located in a personal watercraft.  
      Superchargers are used to enhance engine performance. To date, the present inventors are not aware of the use of a supercharger in an engine for a personal watercraft. U.S. Pat. No. 5,634,422 to Kobayashi et al., entitled “Personal Watercraft With V-Type Engine,” U.S. Pat. No. 5,647,779 to Nanami, entitled “Manifold and Water Trap System For A Marine Engine,” U.S. Pat. No. 5,839,930 to Nanami et al., entitled “Engine Lubricating System For Watercraft,” and U.S. Pat. No. 5,846,102 to Nitta et al., entitled “Four-Cycle Engine For A Small Jet Boat” disclose various engines for personal watercraft. None of these references disclose the use of a supercharger.  
     OBJECT OF THE INVENTION  
      It is an object of the present invention to provide a four stroke, in-line engine having a compact construction.  
      It is another object of the present invention to provide a four stroke, in-line engine having a modular construction to permit the interchange of parts between various engine models.  
      It is another object of the present invention to provide a four stroke, in-line engine having improved exhaust emission characteristics.  
      It is another object of the present invention to provide a four stroke engine having a narrow and low profile.  
      It is another object of the present invention to provide a four stroke engine having a low profile valve actuation assembly for controlling the operation of the intake and exhaust valves.  
      It is another object of the present invention to provide a cylinder head having a low profile to reduce engine height.  
      It is another object of the present invention to offset the placement of the intake valves and exhaust valves with respect to a vertical axis within the cylinder head to reduce engine height.  
      It is another object of the present invention to provide an improved spark plug mounting assembly for easy access within the cylinder head.  
      It is another object of the present invention to provide a Y-shaped intake rocker arm assembly providing compact construction.  
      It is yet another object of the present invention to provide a four stroke engine having an improved oil collection system and oil holding tank.  
      It is another object to provide a four stroke engine which combines a closed loop cooling system and an open loop cooling system for enhanced cooling of the engine in accordance with the present invention.  
      It is another object to provide an open loop cooling system for cooling an exhaust manifold in accordance with the present invention, wherein the open loop cooling system enhances cooling of the crankcase and cylinder head.  
      It is another object to provide an open loop cooling system for cooling an exhaust manifold in accordance with the present invention, wherein the open cooling system lowers the temperature of the exhaust manifold such that the exhaust manifold functions as a heat sink for the crankcase and cylinder head.  
      It is another object of the present invention to provide a closed loop cooling system for selectively cooling the crankcase and cylinder head of the four stroke engine.  
      It is another object of the present invention to provide a closed loop cooling system having a selectively operable heat exchanger.  
      It is another object of the present invention to provide a supercharger for enhanced engine performance.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to a supercharged four stroke internal combustion engine. The supercharged engine includes a crankcase having a crankshaft rotatably mounted therein and a cylinder head connected to the crankcase. The crankcase and the cylinder head form at least three combustion cylinder. Each cylinder includes at least one intake valve and at least one exhaust valve. A valve actuation assembly operates the intake and exhaust valves. An air intake passageway is operatively coupled to the cylinders through the intake valves. An air intake manifold is connected to the cylinder head and operatively connected to the air intake passageways. A power take off housing is located on one end of the crankcase. The crankshaft terminates within the power take off housing. The engine includes a supercharger for boosting air intake to the air intake manifold. The supercharger is mounted to the power take off and operatively connected to the crank shaft within the power take off housing.  
      In accordance with the present invention, the air intake manifold is preferably formed from a plastic material and includes a central air passageway. The central air passageway is operatively connected to the supercharger. The air intake manifold further includes a flame arrester located within the central air passageway. The flow of air from the supercharger travels through the central air passageway and the flame arrester.  
      In accordance with the present invention, the supercharger includes a mounting portion. The mounting portion is positioned within a mounting opening in the power take off housing. The supercharger Further includes an inlet portion having an inlet opening. The inlet portion is connected to the mounting portion. An air passageway extends from the inlet opening to the air intake manifold.  
      The supercharger further includes a blower located within the inlet portion for directing a stream of air to the air intake manifold. The blower includes a blower drive shaft, which is rotatably mounted within the mounting portion. The blower drive shaft is operatively connected to the crank shaft through a connection assembly.  
      The connection assembly dampens the transmission of vibrational from the crankshaft to the blower drive shaft. The connection assembly includes a blower drive pinion located on one end of the blower drive shaft and a biased intermediate member located on the one end of the blower drive shaft. The biased intermediate member applies a force on the blower drive pinion such that the blower drive pinion is engaged with a rotating member secured to the crankshaft.  
      The rotating member includes a plurality of rotating gears. One of the rotating gears engages the blower drive pinion. Another rotating gear engages an engine starting mechanism. One of the rotating gears drives a balance shaft located within the crankcase.  
      The present invention is also directed to a personal watercraft. The personal watercraft includes a hull, a seating assembly, and a four stroke internal combustion engine secured to the hull below the seating assembly. The engine includes a supercharger for boosting air intake to the air intake manifold.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein:  
       FIG. 1  is a downward rear schematic perspective view of a left side of an overhead camshaft aspirated engine in accordance with the present invention;  
       FIG. 2  is a downward rear schematic perspective view of a right side of the engine of  FIG. 1 ;  
       FIG. 3  is a downward front schematic perspective view of the left side of the engine of  FIG. 1 ;  
       FIG. 4  is a downward front schematic perspective view of the right side of the engine of  FIG. 1 ;  
       FIG. 5  is a rear end view of the engine of  FIG. 1  illustrating one possible positioning of the engine within a personal watercraft;  
       FIG. 6  is a downward rear schematic perspective view of a variation of the engine of  FIG. 1  having a supercharger;  
       FIG. 7  is a rear end view of the engine of  FIG. 6 ;  
       FIG. 8  is a partial cross-sectional end view of the crankcase and cylinder head housing in accordance with the present invention;  
       FIG. 9  is a bottom view illustrating the upper crankcase of the engine in accordance with the present invention;  
       FIG. 10  is a top view of the lower crankshaft illustrating the positioning of the crankshaft and the balance shaft;  
       FIG. 11  is a right side partial schematic sectional view of the engine of  FIG. 6 ;  
       FIG. 12  is a partial schematic sectional view of the piston, valves and valve actuator assembly in accordance with the present invention;  
       FIG. 13  is a partial overhead schematic view of the rocker arm assemblies of the valve operating assembly for operating the intake and exhaust valves;  
       FIG. 14  is an end cross sectional view of one of the exhaust rocker arm assemblies and a portion of the intake rocker arm assembly taken along section line  14 - 14  of  FIG. 13 ;  
       FIG. 15  is a cross sectional view of the operative end of the rocker arm assemblies showing a collapsed position of the hydraulic adjuster on the left side and an extended position of the hydraulic adjuster on the right side;  
       FIG. 16  is a right side cross sectional view of the valve operating assembly located within the cylinder head having the camshaft in cross section;  
       FIG. 17  is another right side cross sectional view of the valve operating assembly located within the cylinder head;  
       FIG. 18  is an end cross sectional view illustrating the spark plug assembly within the cylinder head;  
       FIG. 19  is a cross sectional view illustrating the placement of the cylinder head cover on the cylinder head;  
       FIG. 20  is a cross sectional view of the engine of  FIG. 1  through one cylinder of the engine;  
       FIG. 21  is a schematic perspective view of the exhaust manifold in accordance with the present invention;  
       FIG. 22  is a longitudinal cross sectional view of a portion of the exhaust manifold of  FIG. 21 ;  
       FIG. 23  is a side cross sectional view of a portion of the exhaust manifold of  FIG. 21 ;  
       FIG. 24  is a schematic view of the exhaust manifold and open loop cooling system in accordance with the present invention;  
       FIG. 25  is a schematic diagram of the cooling system for the engine in accordance with the present invention;  
       FIG. 26  is a rear perspective view of a right side of the air intake and fuel injection system for the engine in accordance with the present invention;  
       FIG. 27  is a cross sectional view of the air intake and fuel injection system of  FIG. 26  taken along a longitudinal axis of the system;  
       FIG. 28  is a side cross sectional view of the air intake and fuel injection system of  FIG. 26  through a swing pipe;  
       FIG. 29  is a variation of the air intake and fuel injection system of  FIG. 28  illustrating a cooling jacket within the swing pipe;  
       FIG. 30  is a front perspective view of a right side of the air intake and fuel injection system for the engine having a supercharger in accordance with the present invention;  
       FIG. 31  is a cross sectional view of the air intake and fuel injection system of  FIG. 30  taken along a longitudinal axis of the system;  
       FIG. 32  is a rear view of the engine illustrating the power take off lid and cooling system in accordance with the present invention and the oil filter housing in partial cross section;  
       FIG. 33  is a side cross sectional view of a thermostat and pump assembly of a portion of the cooling system and a lubrication pump of the lubrication assembly in accordance with the present invention;  
       FIG. 34  is a partial schematic/partial side cross sectional view of an oil filter unit in accordance with the present invention;  
       FIG. 35  is a schematic diagram illustrating the oil channel system for the lubrication system for the cylinder head housing;  
       FIG. 36  is a cross sectional side view of the power take off assembly for the engine illustrating the generator assembly in accordance with the present invention;  
       FIG. 37  is another cross sectional side view of the power take off assembly for the engine illustrating the starter assembly in accordance with the present invention;  
       FIG. 38  is a cross sectional side view of the power take off assembly having a supercharger for the engine in accordance with the present invention;  
       FIG. 39  is a partial schematic/partial sectional view of the cam chain tensioner in accordance with the present invention;  
       FIG. 40  is a schematic view of the blow-by ventilation system and suction pump in accordance with the present invention;  
       FIG. 41  is a schematic view of the blow-by ventilation system and suction pump of  FIG. 38  having the suction pump cover removed;  
       FIG. 42  is a schematic view of the engine management system for the engine in accordance with the present invention;  
       FIG. 43  is a schematic perspective view of the exhaust manifold according to an alternative embodiment;  
       FIG. 44  is a cross sectional view of a portion of the exhaust manifold of  FIG. 43 ;  
       FIG. 45  is a schematic diagram of the cooling system for the engine in accordance with the present invention for use in connection with the exhaust manifold of  FIG. 43 ;  
       FIG. 46  is a cross sectional view of the cyclone of the blow-by ventilation system;  
       FIG. 47  is a partial overhead cross sectional view of the engine of  FIG. 6  having a cut away of the balance shaft and the power take off assembly;  
       FIG. 48  is an overhead view of the valve train;  
       FIG. 49  is a partial side cross sectional view of the balance shaft and power take off assembly; and  
       FIG. 50  is a side view of the engine of  FIG. 1  illustrating one possible positioning of the engine within a personal watercraft.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A four-stroke three cylinder in-line engine  1  in accordance with the present invention is illustrated generally in  FIGS. 1-4 . The engine  1  in accordance with the present invention will be described in connection with a personal watercraft  5 , shown in cross-section in  FIG. 5 . A variation of the engine  1  is illustrated in  FIGS. 6 and 7 . The engine  2  shown in  FIGS. 6 and 7  includes a supercharger. The engines  1  and  2  are adapted to be installed below a raised pedestal having a seating bench of the personal watercraft  5  inside the hull  4 , as shown in  FIGS. 5 and 50 . With this arrangement, the oil filter cannot be placed on the lower side of the engine or of its crankcase, respectively, if it is to be accessible for maintenance purposes because the hull  4  would prevent access to the oil filter. To address this, the oil filter is installed at the power take off side of the engine, to be easily accessible from above. The access through the seating area at present is the only access to the engine.  
      While designed for use in personal watercraft, it is contemplated that the engine  1  (or engine  2 ) can be used in all terrain vehicles, snowmobiles, boats and other vehicles with minor modifications. For example, the cooling system for the exhaust manifold must be modified for non-marine applications. Further, while the embodiments shown disclose an engine positioning with the power take off to the rear of the engine, the orientation can be altered to have the power take off to the front or to the side depending on the specific vehicle or specific application.  
     Engine Configuration  
      The engine  1  includes a crankcase  10 . A cylinder head housing  20  is connected to the crankcase  10  to form a plurality of combustion chambers. The crankcase  10  and cylinder head housing  20  are inclined with respect to a vertical axis, as shown in  FIGS. 5 and 8 . This arrangement provides sufficient space for the air intake and fuel injection system  40  while maintaining an overall reduced engine profile. The engines illustrated and described herein include three cylinders. The present invention, however, is not limited to three cylinders; rather, it is contemplated that a greater or fewer number of cylinders are considered to be well within the scope of the present invention. For example, a single cylinder version of the engine may be employed in a fishing boat. Two or three cylinder versions of the engine may be employed in a personal watercraft. A four cylinder version of the engine may be employed in a jet boat. Four or more cylinders are considered to be well within the scope of the present invention.  
      The engine  1  or  2  provides for the location of various engine components including, but not limited to the starter assembly, the generator, the oil pump, coolant pump and other devices at one end of the engine in the power take off assembly  50 , described below and shown in  FIGS. 33, 36 ,  37  and  38 . This unique construction and layout of components permits the use of similar parts and engine components for one, two, three and four cylinder versions of the engine. Furthermore, this arrangement permits the addition of additional cylinders on the end of the engine opposite the power take off assembly. The layout of the parts is the same. Minimal redesign of these components is necessary when increasing or reducing the number of cylinders.  
      The engine  1  contemplated herein includes an exhaust manifold  30  that is secured to one side of the cylinder head housing  20  and an air intake and fuel injection system  40 . The air intake and fuel injection system  40  is secured to an opposite side of the cylinder head housing  20  in the area above the cylinder head housing  20 .  
      The present invention, however, is not limited to having a fuel injection system; rather, it is contemplated that the engine can instead be equipped with a carburetor.  
      A power take off assembly  50  is located on an end of the cylinder block  10  within the hull  4 . The power take off assembly  50  defines the rear side of the engine when located within the personal watercraft  5 . The engine  1  or  2  further includes a lubrication system  60  as shown in  FIG. 11 . The engine  1  further includes a blow-by ventilation system  70 , as shown in  FIG. 11 , and an engine cooling system  80 , as shown in  FIG. 25 .  
      An engine  2  is shown in  FIGS. 6 and 7 , which is a variation of the engine  1 . The engine  2  has substantially the same configuration as the engine  1 . The engine  2  further includes a supercharger  90 . The use of a supercharger for an engine for use in a personal watercraft is a new development, which is described in greater detail below. The engine  1  can be converted with minor modification to the engine  2  having a supercharger  90 . In particular, as described below, the supercharger  90  is attached to an opposite end of the intake manifold  41  as compared to the normally aspirated engine  1 . The ignition and induction parameters of the engine may be modified to enhance engine performance when the supercharger  90  is used. It is also contemplated that the compression ratio of the engine may have to be altered to accommodate the supercharger  90 . In accordance with the present invention, it is contemplated that the engines  1  and  2  will be produced on the same assembly line.  
      Because it is contemplated that the engine in accordance with the present invention will be used in marine applications, the exterior surfaces of the engines  1  or  2  will be provided with a suitable coating to reduce corrosion and the direct exposure of the engine to the elements. The individual components of the engines  1  and  2  will now be described in greater detail.  
     Crankcase  
      As illustrated in  FIG. 8 , the crankcase  10  contains a plurality of passageways and compartments formed therein. Furthermore, the crankcase  10  is formed with vertical partitions, as shown in  FIGS. 9 and 10 , which separate the individual crank chambers, described below and external fins located on the crankcase  10 . These vertical partitions and external fins increase the strength of the crankcase  10 . The spaced apart vertical fins provide additional strength for an upper crankcase  13  of the crankcase  10  while minimizing the weight. The vertical partitions increase engine strength and separate the crank chambers  121  in the upper and lower crankcases  13  and  12 . The vertical partitions also secure the upper and lower crankcases together using suitable fasteners. The fasteners extend through bores in the vertical partitions from a lower end of the lower crankcase to the upper crankcase. The fasteners also serve to secure the bearings, described below, within the vertical partitions. The crankcase  10  is preferably formed from a cast aluminum alloy (e.g. AlSi) for both strength and weight considerations. The crankcase  10  is preferably die cast. The present invention, however, is not limited to the use of aluminum alloys; rather, other materials including but not limited to steels, alloys and composites are considered to be well within the scope of the present invention provided the materials have sufficient strength for use in engine applications.  
      The crankcase  10  includes an upper crankcase  13  containing the cylinder block and a lower crankcase  12 . A balance shaft  115  and a crankshaft  123  are located at the union between the lower crankcase  12  and the upper crankcase  13 . An oil tank  11  formed in a bottom portion of the lower crankcase  12 , as shown in  FIG. 8 . The oil tank  11  has a generally u-shaped configuration that partially surrounds a lower portion of a crankcase  12 . The oil tank  11  is located on both the bottom and side of the engine to house the necessary volume of oil while maintaining the engine&#39;s reduced profile such that oil is located on the bottom of the crankcase and the side of the crankcase  10 . An interior of the upper crankcase  13  and the lower crankcase  12  are connected to the oil tank  11  through outlet openings  111 , as shown in  FIGS. 8 and 11 . A channel  112  extends from each opening  111  to an upper portion  113  formed in the lower crankcase  13 . The oil collected from the crank chamber  121  flows through outlet openings  111  and channels  112 , then enters the upper channel portion  113  and returns to the oil tank  11 . This oil then flows under the influence of gravity downward into a lower portion  114  of the oil tank  11 .  
      A balance shaft  115  extends through the crankcase  10 . The balance shaft  115  and the crankshaft  123  are located at the union of the lower crankcase  12  and the upper crankcase  13 . To prevent oil from flowing from upper channel portion  113  and contacting the balance shaft  115 , an optional baffle assembly is located within the upper portion  113 . The balance shaft  115  is provided to counteract the moment generated by rotation of the crankshaft  123 , shown in  FIG. 10 . This arrangement produces mass balancing of the first order. The balance shaft  115  and the crankshaft  123  extend in a parallel relationship, as shown in  FIG. 10 . The balance shaft  115  is rotatably mounted within a bore  1132  that extends through the crankcase  10 , as shown in  FIGS. 9 and 10 . Suitable bearing assemblies are provided for smooth rotation of the balance shaft  115 . The bearing assemblies are fixed using the fasteners described above. Preferably, the balance shaft  115  should be mounted in an anti-friction shell bearing but, alternatively, roller bearings can also be used. The balance shaft  115  is operatively connected by gear  1151  to the crankshaft  123  through gear  1231 . This connection is preferably located within the power take off assembly  50  on one end of the crankcase  10 .  
      The oil tank  11  forms a portion of a dry sump lubrication system. The lubrication system and the operation of the same will be described in greater detail below.  
      As  FIGS. 9 and 10  illustrate, the crankcase  10  includes at least one crank chamber  121  and in the preferred embodiment includes one isolated crank chamber for each engine cylinder. In accordance with the presently disclosed embodiments of engines  1  and  2 , three crank chambers  121  are provided. Each crank clamber  121  includes an outlet opening  111  connected to the channel  112 , described above. A bore  122  extends through the crankcase  10  and each of the crank chambers  121 , as shown in  FIGS. 9 and 10 . A crankshaft  123  is received therein, as shown in  FIG. 10 . The crankshaft  123  can be a one-piece forging, cast or assembled depending upon the engine application. For example, a cast crankshaft may be used in low performance applications. The crankshaft  123  is rotatably mounted within a bore  122 . Suitable bearing assemblies are provided for smooth rotation of the crankshaft  123 .  
      As shown in  FIG. 25 , a cylinder  124  extends through the crankcase  10  above each of the crank chambers  121 . In accordance with the present invention, the engines  1  and  2  each include three cylinders  124 , as shown in  FIG. 11 . A piston  1241  is slidably received within the cylinder  124 . The piston  1241 , shown in  FIG. 11 , reciprocates axially within the cylinder  124  as is known. The piston  1241  is connected to the crankshaft  123  through a connecting rod  1242  and piston pin  1243  to convert axial movement of the pistons  1241  to rotational movement of the crankshaft  123  and vice-versa. A cooling passageway  125  extends around the cylinders  124 , as shown in  FIG. 25 . The cooling passageway  125  is connected to the engine cooling system  80  further described below. As shown in  FIG. 25 , the cooling passageway  125  extends substantially around the perimeter of the cylinders. This passageway has a generally U-shaped configuration.  
      At present, the cylinder liners are formed with grey cast iron. The upper crankcase  13  is then cast around the liners. The upper crankcase  13  may be formed from under-eutectic AlSi (e.g. cast-AlSi 9)(with 9% silicon). The interior of the cylinder liners may then be honed. The use of grey cast iron increases the weight of the crankcase  13 . It is desirable to eliminate the use of the cylinder liners. With this in mind, it is contemplated that the cylinder liners may be eliminated. Instead, an interior surface of the upper crankcase  13  can include a thermal coating to reduce friction. This coating may be applied plasma spraying or other suitable process. Alternatively, AlSi-alloys (alloys of aluminum and silicon) are used to form the liners for the cylinders  124 . The cylinder liners may be formed from over-eutectic AlSi with primary silicon grains therein (e.g. AlSi 19) (with 19% silicon) to minimize friction and wear. The crankcase  10  may be formed from under-eutectic AlSi (e.g. cast-AISi 9) (with 9% silicon). The cylinder liners are assembled to the cylinder block during the casting of the upper crankcase  13 . Beforehand, a binding layer consisting of eutectic AlSi 12 (with 12% silicon) is thermally sprayed (e.g. plasma sprayed) onto the outer wall of the liner to provide a better bond and a better heat-removal property (high heat transfer coefficient) between the liner and the cylinder block  10 . Alternatively, the cylinder liners can also be inserted into the cylinder block of the upper crankcase  13  mechanically with a force fit. It is also contemplated that the cylinder block  10  can be formed from over-eutectic AlSi (e.g. AlSi 19) without the need for separate cylinder liners. With this arrangement, however, the cylinder is more difficult to machine, more expensive and thus, is not presently preferred. In such a liner-less embodiment, the cylinders can be optionally provided with a surface coating for enhanced wear and friction properties. It is contemplated that the pistons  1241  may be formed of aluminum coated with iron.  
     Cylinder Head Housing  
      The cylinder head housing  20  is secured to the upper end of the crankcase, as shown in  FIG. 8 . The cylinder head housing  20  is bolted to the crankcase and provides a combustion chamber  2001  above each cylinder  124 . A pair of exhaust valves  21  and a pair of intake valves  22  are mounted in each combustion chamber  2001 . As shown in  FIG. 11 , the pair of exhaust valves  21  are located on one side of the cylinder head housing  20  and the pair of intake valves  22  are located on an opposite side of the cylinder head housing  20 . The present invention, however, is not limited to a pair of exhaust valves and a pair of intake valves; rather, a single exhaust valve and a single intake valve may be employed. Furthermore, more than two intake and exhaust valves may be provided. Furthermore, any combination of intake and exhaust valves is contemplated provided each cylinder includes more intake valves than exhaust valves.  
      As shown in  FIG. 8 , the intake valves  22  and the exhaust valves  21  are disposed at an angle with respect to the vertical axis of the engine  1  or  2 . This reduces the height of the cylinder head housing  20 , which reduces the overall height of the engine  1  or  2 .  
      The cylinder head housing  20  further includes at least one exhaust passageway  23  for each combustion chamber  2001  extending through the cylinder head housing  20 , as shown in  FIGS. 8, 12  and  13 . The passageway  23  includes a pair of siamesed exhaust ports  231  that connect the exhaust passageway  23  to the chamber  2001 , as shown in  FIGS. 12 and 13 . Each of the pair of exhaust valves  21  is positioned in one of the respective exhaust ports  231  to selectively open and close the ports  231  at predetermined intervals to permit the removal of exhaust gases from the chamber  2001 . An opposite end of the exhaust passageway  23  has an opening  232 , as shown in  FIG. 14 , that is operatively connected to the exhaust manifold  30 . The exhaust manifold  30  is secured to the cylinder head housing  20  using suitable fasteners on a downwardly facing side of the cylinder head housing  20 , as shown  FIG. 5 .  
      The cylinder head housing  20  further includes at least one intake passageway  24  for each cylinder  124  extending through the cylinder head housing  20 , as shown in  FIGS. 8, 12  and  13 . The passageway  24  includes a pair of siamesed intake ports  241  that connect the intake passageway  24  to the chamber  2001 . Each of the pair of intake valves  22  is positioned in one of the intake ports  241  to selectively open and close the ports  241  at predetermined intervals to permit the influx of fuel and air into the chamber  2001 . An opposite end of the intake passageway  24  has an opening  242 , as shown in  FIG. 14 , that is operatively connected to the air intake and fuel injection system  40 . The air intake and fuel injection system  40  is secured to the cylinder bead housing  20  opposite the exhaust manifold  30  using suitable fasteners on an upwardly facing side of the cylinder head housing  20 , as shown in  FIG. 5 . While the intake and exhaust ports are shown as being siamesed, they can alternatively remain separated until connected to the respective intake and exhaust manifolds. The cylinder head housing  20  includes a spark plug  27  that is located in a central inclined position, as described in greater detail below.  
     Valve Operating Assembly  
      A valve operating assembly illustrated in  FIGS. 8 and 12 - 17  operates the intake valves  22  and exhaust valves  21  in accordance with predetermined engine operating parameters. The valve operating assembly is located within the cylinder head housing  20  and is driven by the crankshaft  123 . As discussed in greater detail below in connection with the power take off assembly  50 , the crankshaft  123  extends from the crankcase  10  into a power take off housing  59 . A gear assembly  54  is secured to the crankshaft  123  within the power take off housing  59  and includes a chain gear  542 .  
      A cam shaft  29  is rotatably mounted within the cylinder head housing  20 . One end of the cam shaft  29  extends into a control chain chamber  202  within the cylinder head housing  20 . The control chain chamber  202  extends into the cylinder block of the upper crankcase, as shown in  FIG. 48 , and enters the power take off assembly  50 . A cam gear  551  is operatively coupled to a chain gear  542  by a control chain  55 , which extends around both the gear  551  and gear  542 . The control chain  55  extends through the control chain chamber  202  into the power take off assembly  50 . The cam gear  551  and chain gear  542  are sized to have a 2 to 1 relationship.  
      The camshaft  29  is rotatably mounted to the cylinder head housing  20  in a position between the intake and exhaust valves  21  and  22 . Suitable bearing assemblies are provided for the smooth operation and rotation of the camshaft  29  within the cylinder head housing  20 . As shown in  FIG. 12 , a plurality of cam lobes  291  and  292  are provided along the camshaft  29  to operate the valves  21  and  22  in each cylinder. A cam lobe  291  provides the necessary motion to operate the intake valves  22  through the rocker arm assembly  25 . A pair of cams  292  provide the necessary motion to operate the exhaust valves  21  through the rocker arm assemblies  26 . A cam  291  and a pair of cams  292  are positioned over each cylinder, as shown in  FIGS. 16 and 17 . The cams  291  and  292  are oriented on the camshaft  29  to produce a predetermined timing for opening and closing the valves  21  and  22 . The orientation of the cams  291  and  292  vary for each cylinder such that all cylinders do not operate at the same time, rather the cylinders operate in a predetermined sequence. While the camshaft  29  is illustrated with a solid construction, it is contemplated that the camshaft  29  may have a hollow construction. Furthermore, the camshaft may be forged, cast or assembled.  
      The valve operating assembly includes a Y-shaped intake rocker arm assembly  25  that operates both of the pair of intake valves  22 , as shown in  FIG. 13 , in response to the cam lobe  291 . The valve operating assembly further includes a pair of exhaust rocker arm assemblies  26  that operate the pair of exhaust valves  21 , as shown in  FIG. 13 , in response to cam lobes  292 . The intake rocker arm assembly  25  is a forked assembly rocker arm having a pair of valve operating arms  251  and  252 . One operating arm  251  operates one of the intake valves  22  and the other operating arm  252  operates the other intake valve  22 . The fork like shape of the rocker arm assembly  25  provides access to the spark plug assembly  27  positioned within the cylinder head housing  20 . The spark plug assembly  27  will be described in greater detail below. The fork like shape of the rocker arm assembly  25  reduces the overall width of the necessary assemblies to operate the valves for each cylinder.  
      In an effort to reduce the weight of the rocker arm assemblies  25  and  26 , the rocker arm assemblies  25  and  26  may be produced from an aluminum alloy (AlSi) by forging or casting. The present invention, however, is not limited to rocker arm assemblies formed from aluminum; rather, it is contemplated that other materials including but not limited to steel and alloys of the same may be cast or forged to form the rocker arm assemblies  25  and  26 .  
      The rocker arm assemblies  25  and  26  are rotatably mounted on a rocker arm support axle  28  in a position between the intake and exhaust valves  21  and  22 . The stationary support axle  28  is mounted to the cylinder head by a plurality of fastener assemblies  281 , as shown in  FIGS. 16 and 17 . The fastener assemblies  281  may include screw type fasteners, pin fasteners or other similar fastener assemblies for securing the support axle  28  within the cylinder head housing  20  and preventing its rotation. The rocker arm support shaft  28  is mounted to the cylinder head housing  20 . The axle  28  is laterally offset and vertically spaced from the camshaft  29 , as shown in  FIGS. 12, 14  and  18 . This arrangement results in a compact construction that reduces the overall height of the cylinder head housing  20 . It is contemplated that the axle  28  may be located on the vertical axis of the cylinder or adjacent to the same.  
      The camshaft  29  is operatively connected to the crankshaft  123 , as described below. The cam gear associated with the crankshaft gear are sized to have a 2 to 1 relationship. The angled intake and exhaust valves  21  and  22  provide an enlarged area within the cylinder head housing  20  between the valves in which to locate the cam shaft, axle and the rocker arm assemblies  25  and  26 . This also provides sufficient space to maintain the 2 to 1 relationship between the cam gear and the crankshaft gear without increasing the height of the cylinder head housing  20 .  
      The rocker arm assembly  25  wil now be described in greater detail, reference being made to  FIGS. 12 and 14 . As described above, the rocker arm assembly  25  has a pair of operating arms  251  and  252 . A free end of each of the pair of operating arms  251  and  252  is positioned over a respective intake valve  22  and includes an hydraulic adjuster  253  for contacting the intake valve  22 . The hydraulic adjuster  253  abuts the upper surface of the valve stem of the intake valve  22 . The hydraulic adjuster  253  is located within a cavity in the respective arm  251  and  252 . Passageways  2512  and  262  extend from the cavities, respectively, to the rocker arm support axle  28 , as shown in  FIGS. 8, 12 , and  14 . The passageways are hydraulically linked to the rocker arm support axle  28 . The rocker arm support axle  28  includes a central passageway through which a supply of hydraulic fluid (preferably lubricant from the lubricant system) or other suitable lubricant flows. The fluid passes from the central passageway through radial openings  282  to the passageways. The fluid flows through the passageways to the cavities where it biases the hydraulic adjuster  253  into contact with the intake valve  22 . The fluid insures that the hydraulic adjuster  253  is always in contact with the intake valve  22  such that zero lash exists between the valve and hydraulic adjuster  253 . This insures that the entire motion of the cam  291  is transferred to the intake valves  22  to facilitate their opening and closing. Although fluid is used to bias the hydraulic adjuster  253  into engagement with the valves  22  in the embodiment illustrated, it is contemplated that a screw adjuster assembly or other mechanical assembly can be provided to perform the same operation.  
      An opposite end of the rocker arm assembly  25  includes a cam follower  254 . The follower  254  may include a roller assembly having bearings that is rotatably mounted to the rocker arm assembly  25 . The follower  254  travels along the cam  291 , which causes the rocker arm assembly  25  to pivot about the rocker support axle  28 . The motion of the cam  291  is transferred to open ad close the intake valves  22 . Fluid from the central passageway  281  may be directed through another passageway, not shown, in the rocker arm assembly  25  to provide a supply of fluid to lubricate the follower assembly  254  to provide for smooth operation. The present invention, however, is not limited to the roller followers described herein; rather, it is contemplated that other followers including but not limited to sliding blocks may be utilized to follow the cam  291 .  
      The rocker arm assembly  25  has a compact angled construction, as shown in  FIG. 14  so as to allow for a narrow and low construction. Similarly, the low arrangement of the camshaft  29  and associated drive chain wheel, which also does not project beyond the cylinder head housing  20 , as seen in  FIGS. 16 and 17  assists in constructing an engine with a narrow and low profile.  
      As seen in  FIGS. 8, 12  and  14 , the camshaft  29  and the support axle  28  are offset relative to the longitudinal axis of the cylinder. The camshaft  29  is offset to provide room for the spark plug assembly  27 , described below. Both the camshaft  29  and the support axle  28  are located closer to the exhaust valves  21  than the intake valves  22 . The offset nature of the support axle  28  increases the overall length of the intake rocker arm assembly  25 . This increases the lever arm of the intake rocker arm assembly  25  and maximizes the force (within the size constraints of the cylinder head housing  20 ) applied to operate both intake valves  22  with one rocker arm assembly. The intake and exhaust valves are disposed at an angle with respect to the cylinder axis. In principle, however, also other geometries (e.g. with a central arrangement of the camshaft  29 ) are conceivable. Alternatively, the rocker arm support axle  28  may be located closer towards the intake valves so as to make the forked operating arms  251  and  252 —which are heavy due to this construction—shorter and thus less heavy. With this arrangement, the location of the camshaft  29  should also be relocated to maintain the lever arm of the intake rocker arm assembly  25 .  
      The rocker arm assemblies  26  will now be described in greater detail. Each exhaust rocker arm assembly  26  has the same construction. A free end of the rocker assembly  26  is positioned over a respective exhaust valve  21  and includes a hydraulic adjuster  263  for contacting the exhaust valve  21 . The hydraulic adjuster abuts the upper surface of the valve stem of the exhaust valve  21 . Like the hydraulic adjuster  253 , the hydraulic adjuster  263  is located within a cavity  261 . A passageway  262  extends from the cavity  261  to the rocker arm support axle  28 . The passageway  262  is hydraulically linked to the rocker arm support axle  28  through radial openings  282 . The fluid flows through the passageway  262  to the cavity  261  where it biases the operating assembly  263  into contact with the exhaust valve  21 . The fluid ensures that the hydraulic adjuster  263  is always in contact with the exhaust valve  21  such that zero lash exists between the valve and hydraulic adjuster  263 . This insures that all motion of the cam  292  is transferred to the exhaust valve  21  to facilitate opening and closing. Although fluid is used to bias the hydraulic adjuster  263  into engagement with the valve  21 , it is contemplated that a mechanical assembly (e.g. a screw adjuster) may be provided to perform the same operation.  
      An opposite end of the exhaust rocker arm assembly  26  includes a cam follower  264 . The follower  264  has a similar construction to the follower assembly  254 , described above. The rocker arm assembly  26  also has a compact angled construction, as shown in  FIG. 14  so as to allow for a narrow and low construction.  
      The construction of the hydraulic adjusters  253  and  263  will now be described in greater detail in connection with  FIG. 15 . The hydraulic adjusters  253  and  263  have the same construction. The hydraulic valve adjusters  253  and  263  are maintenance free and require no adjustment. The hydraulic adjuster  263  is positioned within the cavity  261 . The hydraulic adjuster  263  includes an inner stationary piston  2631  and an outer movable piston  2632 , which is located between the cavity  261  and the inner stationary piston  2631 . The inner stationary piston  2631  includes a central cavity  2633  that is in communication with the cavity  261 , as shown in  FIG. 15 .  
      An opposite end of the piston  2631  includes an aperture  2634  such that the cavity  2633  is in fluidic communication with a cavity  2635  in the piston  2632 . A ball and seat check valve  2636  selectively closes the aperture  2634 . A valve contacting cap  2637  is pivotably mounted on an end of the piston  2632 . The cap  2637  contacts the valve stem of the exhaust valve  22  when the piston  2632  is in an extended position, as shown in the right side of  FIG. 15 .  
      In operation, hydraulic fluid flows through channel  262  into the cavity  261 . After the cavities  261  and  2633  have filled with fluid, the valve  2636  opens to permit the flow of fluid into cavity  2635  through aperture  2634 . As the cavity  2635  fills with hydraulic fluid, the piston  2632  extends to the position shown in the right side of  FIG. 15 . The spring assembly  2638  is located in the cavity  2635 . The downward travel of the piston  2632  is limited by contact with the valve stem and a seal  2639  that is secured to one end of the piston  2632  and is slidably received around the piston  2631 . When in the normal downward steady state position, the contacting cap  2637  contacts the valve stem such that motion of the rocker arm assembly is transferred to the valve to open the valve at predetermined locations of the camshaft  29 . After engine shut off, a sufficient amount of fluid is maintained in the cavity  2635  to maintain the outer movable piston  2632  in engagement with the corresponding valve stem.  
       FIGS. 16 and 17  illustrate an axial section through the camshaft  29  and the rocker arm support axle  28 . The camshaft  29  is mounted in a bearing bracket  293  with two collars  294  and  295 . Lubricant is supplied to the clearance region between these two collars  294  and  295 . By means of this double plain bearing in the respective bearing bracket  293 , the bearing becomes very rigid and the dynamic changing loads occurring during operation can be accommodated efficiently. Mounting of the camshaft  29  is effected by inserting it in from one end of the cylinder head housing  20  near the power take off end of the engine. The camshaft  29  is secured by a plate positioned within the cylinder head housing  20  against axial shifting. The plate extends through a vertical slot located within the cylinder head housing  20 . The plate may be further used to orient the axle  28  within the cylinder head housing  20 . It is also contemplated that a pin may be used to secure the camshaft against axial shifting. The pin may be located in a slot or groove extending around the perimeter of the camshaft.  
      Although the operation of the intake valves  22  and exhaust valves  21  has been described in connection with rocker arm assemblies  25  and  26 , other assemblies are contemplated for operating the valves. For example, the valves may be electromagnetically operated. Alternatively, the valves may be hydraulically operated using a slave piston/master piston arrangement. Furthermore, the Y-shaped rocker may be used to actuate the exhaust valves. Individual rocker arms may be used to operate intake valves. With this arrangement, the location of the spark plug assembly  27  must be relocated. It is also contemplated that gas springs may be used to bias the valves into a closed position when high rotation speeds are desired for high rpm output. It is also contemplated that a variable valve train may be substituted to vary the timing of the valve operation.  
     Spark Plug Assembly  
      The spark plug assembly  27  will now be described in greater detail in connection with  FIG. 18 . A spark plug  271  is connected by threaded engagement to the cylinder head housing  20 , as shown in  FIG. 18  such that an electrode portion of the spark plug  271  extends into the cylinder. The spark plug assembly  27  is located between the intake valves  22  and the exhaust valves  21  closer to the intake valves  21  because the intake side of the engine is cooler than the exhaust side of the engine. It is desirable to isolate the spark plug  271  from the remainder of the cylinder head housing  20 , which contains oil therein. A tube assembly  272  surrounds the spark plug  271 . The tube assembly  272  is preferably formed from a die cast plastic. It, however, is contemplated that other light weight materials may be used to form the tube assembly  272  so long as the tube assembly  272  isolates the spark plug  271  from the oil-carrying portions of the cylinder head housing  20 . It is preferable that the spark plug assembly  27  be inclined at an angle with respect to the central axis of the cylinder. The angle between the spark plug assembly and the intake valves is small (e.g. 3° is preferable). The angle, however, may be zero.  
      Each tube assembly  272  is sealingly inserted into a pedestal  273  on the cylinder head housing  20 , which forms a socket for the spark plug  271 . A slight compression fit between the tube  272  and a bore in the pedestal  273  can provide a sealing engagement between the two components although this sealing engagement can also be augmented by providing an o-ring between the two compartments. On an outer end, a seal  274  is vulcanized onto the tube assembly  272  which effects the sealing between the tube assembly  272  and a cylinder head cover  275 . Alternatively, the seal  274  can be provided as a separate component between the tube  272  and cover  275 . Use of the tube  272  provides for a lighter weight head assembly and also simplifies the casting of the cylinder head since the isolating tube is not cast as part of the cylinder head. The tube assembly  272  accommodates a plastic body spark plug connector  276  in which the ignition coil or the spark transformer are cast. In this way, the path of the high voltage to the spark plug  271  can be kept extremely short. From the outside, only a low voltage is supplied to the plastic body spark plug connector  276  and the ignition coil contained therein. The plastic body spark plug connector  276  and the spark plug  271  can easily be removed through the tube assembly  272 . The plastic body spark plug connector  276  abuts the inner side of the tube assembly  272 . A venting assembly is provided to enable venting from the spark plug region towards the environment. A splash water screen  2763  is attached to the plastic body  276 .  
      A cylinder head cover  275  is attached to the cylinder head housing  20  using a plurality of fastener elements  2751 , as shown in  FIG. 19 . The cylinder head cover  275  is preferably formed from aluminum or some synthetic material. The connection between the cylinder head housing  20  and the cylinder head cover  275  is acoustically decoupled. An elastomeric gasket  2753  is positioned between the cylinder head housing  20  and the cylinder head cover  275  to provide a seal between the two components. The gasket  2753  has a protruding portion  2754  that is configured to sealingly engage a slot  2755  in the cylinder head cover  275 . This engagement maintains the gasket in a desired position between the cylinder head housing  20  and the cylinder head cover  275  and helps prevent the gasket  2753  from dislocating and causing leaks. In addition, the elastomeric gasket also reduces and prevents a direct sound propagation from the cylinder head housing  20  to the cylinder head cover  275  thereby reducing overall noise emanating from the engine. A further elastomeric gasket  2752  is provided between the fastener element  2751  and cylinder head cover  275  to seal the connection therebetween and also block direct sound propagation from the cylinder head housing  20  to the cylinder head cover  275  through the fastener  2751 . With this arrangement, the cylinder head cover  225  is isolated from the cylinder head housing  20 .  
     Exhaust Manifold  
      A preferred embodiment of the exhaust manifold  30  will now be described in connection with  FIGS. 21-24 . The exhaust manifold  30  includes a first manifold  31  and a second manifold  32 , as shown in  FIG. 24 . The first manifold  31  is connected to one side of the cylinder head housing  20 . It is preferably located on the smaller downward facing side of the cylinder head housing  20  because it does not require as much space as the induction system  40 , described below. The first manifold  31  includes at least one exhaust passageway  311  that is operatively coupled to each exhaust passageway  23  in the cylinder head housing  20 . Each exhaust passageway  311  connects to a main exhaust passageway  312 , which extends in a direction towards the power take off assembly  50 . With this arrangement, exhaust gases exit the cylinder head housing  20  through each exhaust passageway  23  when the respective exhaust valves  21  are opened. The exhaust gases then travel through the exhaust passageway  311  to the main exhaust passageway  312 .  
      The first manifold  31  is connected at the end nearest the power take off assembly  50  to the second manifold  32 . The second manifold  32  includes a main exhaust passageway  321 . The exhaust gases travel through the main exhaust passageway  321  into the muffler system  33 .  
      Due to U.S. Government regulation, it is necessary to cool the exhaust components to limit the temperature of these components below a threshold value. It is desirable to cool the exhaust gases as the gases pass through the exhaust manifold  30  and an associated muffler system  33 . The muffler system  33  preferably includes a first muffler  331  directly connected to the exhaust manifold  30  and a second muffler  332  connected to the first muffler  331 .  
      The first and second manifolds  31  and  32  are equipped with an open loop cooling system for cooling the manifolds  31  and  32  and the exhaust gases contained therein. Each manifold  31  and  32  has a double jacket construction that permits cooling water to flow around the interior of the manifolds  31  and  32  without mixing with the exhaust gases. The first manifold  31  is preferably cast. The second manifold  32  is preferably formed from stainless steel.  
      The first manifold  31  has an inner manifold  313  and an outer manifold  314 , as shown in  FIGS. 22 and 23 . The spacing between the inner and outer manifolds  312  and  314  forms a cooling passageway  315 . The inner and outer manifolds  313  and  314  are interconnected at various points along the manifold. The cooling passageway  315  has a generally unshaped configuration when viewed from a vertical cross section such that it surrounds the main passageway  311  on the top, bottom and at least one side. The cooling water enters the passageway  315  through at least one inlet  316 . The cooling water then travels through the cooling passageway  315  and exits through at least one outlet  317 .  
      The second manifold  32 , as shown in  FIG. 24 , also has an inner manifold  322  and an outer manifold  323 . The spacing between the inner and outer manifolds  322  and  323  forms a cooling passageway  324 , therebetween. The cooling passageway  324  substantially surrounds the main exhaust passageway  321 . The cooling water enters the cooling passageway  324  through at least one inlet  325  located near the connection between the first manifold  31  and the second manifold  32 . The cooling water exits the cooling passageway through at least one outlet  326  located near the point where the second manifold  32  enters the first muffler  331 .  
      The cooling system for the exhaust manifold  30  and muffler system  33  is an open loop cooling system. Cooling water is supplied to the first and second manifolds  31  and  32  by a jet pump of the propulsion unit of the personal watercraft  5 , which draws cooling water from the body of water in which the personal watercraft  5  is operating. An open loop cooling system can be used for the exhaust manifold  30  because the geometry of the cooling jacket for the exhaust manifold  30  is relatively simple with larger passageways. There is less concern for the clogging of these passageways. On the contrary, the geometry of the cooling system for the cylinder head housing  20  and crankcase  10  is more complex with smaller passageways. There is a greater concern about clogging that may occur when using a coolant drawn from outside the watercraft  5 . As such, a closed loop cooling system is preferred for the cylinder head housing  20  and crankcase  10 .  
      The cooling passageways  315  and  324  sufficiently cool the manifolds  31  and  32 . The temperature of the exhaust gases, however, remains too high. It must be further cooled before venting to the atmosphere or released into the water. It is desirable to cool the exhaust gases as the exhaust gases enter the first muffler  331 . At least one injection nozzle  34  is located adjacent the end of the main exhaust passageway  321 , such that a stream of cooling water is injected into the exhaust stream as the exhaust stream enters the first muffler  331 . Although it is preferable that the at least one injection nozzle  34  be located within the muffler  331 , it is contemplated that the injection nozzles  34  may be located within the main exhaust passageway  323 .  
      It is possible for the personal watercraft  5  to overturn or rollover during operation. It is desirable to prevent the cooling water used to cool the exhaust gases from traveling within the main exhaust passageways  314  and  323  to the cylinder head housing  20 . The design of the second manifold  32  and the connection between the second manifold  32  and the first muffler  331  prevent the return of the cooling water to the cylinder head housing  20 .  
      The second manifold  32  terminates within the first muffler  331  at a central location. The outlet opening for the main exhaust passageway  323  is spaced from the top, bottom and side walls of the first muffler  331 . With this arrangement, cooling water that has accumulated within the first muffler  331  should not enter the main exhaust passageway  323  because the cooling water should travel along the sides of the first muffler  331  (spaced from the outlet) when rollover occurs.  
      In the event that some cooling water enters the main exhaust passageway  323 , the configuration of the second manifold  32  prevents passage of cooling water to the cylinder head housing  20 . The second manifold  32  contains a unshaped bend or gooseneck portion that traps the cooling water. With this arrangement in a rollover condition, the cooling water must first travel downward from the first muffler  331  through the bend or gooseneck portion and then upward before entering the first manifold  31 . The change in direction of the main exhaust passageway  323  in the gooseneck portion essentially prevents any cooling water from entering the first manifold  31  or the cylinder head  32 .  
      The present invention is not limited to the above-described gooseneck portion for preventing water from entering the first manifold  31  at the cylinder head  20 ; rather, other geometries that produce a similar effect are considered to be well within the scope of the present invention.  
      An alternative embodiment of the exhaust manifold will now be described in connection with  FIGS. 43 and 44 . The exhaust manifold  300  is connected to one side of the cylinder head housing  20 . Like the manifold  30  described above, the manifold  300  is preferably located on the smaller downward facing side of the cylinder head housing  20 . The exhaust manifold  300  includes at least one exhaust passageway  310  that is operatively coupled to each exhaust passageway  23  in the cylinder head housing  20 . Each exhaust passageway  310  connects to a main exhaust passageway  320 . The exhaust gases exit the cylinder head housing  20  through each exhaust passageway  23  when the respective exhaust valves  21  are opened. The exhaust gases then travel through the exhaust passageway  310  to the main exhaust passageway  320 . The main exhaust passageway  320  first directs the exhaust gases toward the front of the personal watercraft, then in an opposite direction through knee bend  330  toward the rear of the personal watercraft. The exhaust gases may then exit the exhaust manifold  300  to a muffler system and/or water trap. The muffler system may include a pair of mufflers.  
      In this alternative arrangement, the exhaust manifold  300  also has a double jacket construction that permits cooling water to flow around the exhaust gases without mixing the cooling water and the exhaust gases. The double jacket construction includes an inner manifold  340  and an outer manifold  350 , which create a cooling chamber  370  therebetween. Webs  360  separate the cooling chamber  370  into a first portion  3701  and a second portion  3702 , as shown in  FIG. 22 . The cooling water passes through the cooling chambers  3701  and  3702 , as shown in  FIG. 44 .  
      Like the manifold  30  the exhaust manifold cooling system is an open loop cooling system. As such, a jet pump of the propulsion unit draws cooling water from the body of water in which the personal watercraft  5  is operating, shown in  FIG. 44 . The cooling water is supplied to the exhaust manifold  300  through a primary inlet port  510  located in the bend  330  of the exhaust manifold  300 , as shown in  FIGS. 43 and 45 . The cooling water then flows through the first chamber portion  3701  until it connects with the second chamber  3702  at the rear portion of the exhaust manifold  300 . The cooling water then flows back through the second chamber  3702  until it is discharged through the outlet port  3520  back into the body of water, as shown in  FIG. 45 . Thus, the separation of the chamber  370  into two portions  3701  and  3702  that are interconnected only at an end of the exhaust manifold distant from the cooling intake and outlet ports provides for a U-shaped cooling circuit in the manifold, enhancing the cooling efficiency of the manifold.  
      These cooling arrangement maintain the exhaust manifolds  30  and  300  at a lower temperature than the cylinder head housing  20  and the cylinder block  10 . As a result, the exhaust manifolds  30  and  300  function as a heat sink, withdrawing heat from the cylinder head housing  20  and the cylinder block  10 . This reduces the cooling requirements placed on the closed loop cooling system  80 , described below. The coolant in the exhaust manifold (e.g. the water drawn from the body of water) has a lower temperature than the coolant for the closed loop cooling system, described below.  
      At least one temperature sensor  39  is located in the muffler to measure the temperature of the exhaust gases, as shown in  FIG. 42 . The exhaust manifold  300  is equipped with an injection cooling system, which supplies additional cooling water to the exhaust manifold. A first injection nozzle  381  sprays cooling water directly into the exhaust passageway  320  in a direction away from the cylinder head housing  20 . A second injection nozzle  383  sprays cooling water directly into the exhaust passageway  320  also in a direction away from the cylinder head housing  20 . The location of the nozzles in the knee of the exhaust manifold prevents the backward travel of the cooling water into the cylinder head. The combined open loop cooling system with the injection cooling system functions to cool both the exhaust manifold and the exhaust gases within the manifold.  
     Air Intake and Fuel Injection System  
      The air intake and fuel injection system or induction system  40  will now be described in connection with  FIGS. 26-31 . The system  40  is connected to the cylinder head housing  20  opposite the exhaust manifold  30 . The air intake into the engine  1  or  2  is effected from within the hull of the personal watercraft  5  via an air box, not shown, but disclosed in U.S. Provisional Patent Application No. 60/224,355, filed on Aug. 11, 2000, entitled “WATERCRAFT HAVING AIR/WATER SEPARATING DEVICE” and U.S. Provisional Patent Application No. 60/229,340, filed on Sep. 1, 2000, entitled “PERSONAL WATERCRAFT HAVING IMPROVED FUEL, LUBRICATION AND AIR INTAKE SYSTEMS” the specifications of which are incorporated specifically herein by reference. The air box comprises an air inlet in the form of a snorkel, a water separator unit and a muffler unit. The air box is located apart from the engine and connected to the engine via a tube or hose to prevent water from entering the air intake system.  
      The air flows through the tube connecting the air box with the engine, and then passes to an air intake manifold or plenum  41 , illustrated in  FIGS. 26-31 . The air manifold  41  is preferably formed from a plastic material. The present invention, however, is not limited to the use of a plastic material; rather, metals, high strength alloys and other suitable synthetic materials may be used.  
      The air manifold  41  has a symmetrical geometry. With this arrangement, air flow into the air manifold  41  can be provided at either end of the air manifold  41 , thereby enabling use of the same air manifold  41  in either a normally aspirated engine  1  or a supercharged engine  2 , which engines have different flow paths for air into the air intake manifold. In the normally aspirated engine, the air from a throttle (if the engine has fuel injection) or a carburetor (if the engine does not have fuel injection) flows into one end of the air manifold  41 , as shown for example in  FIG. 4 . Preferably, this end faces the airbox to shorten the distance and the pressure loss between the intake manifold and the airbox.  
      Irrespective of which end of the air manifold is used to intake air, in a fuel injection version of the engine, the air manifold  41  includes a throttle body  411  containing a throttle at the plenum inlet to regulate the flow of air into the manifold  41 . The degree of opening of the throttle of the throttle body  411  is controlled by the engine management system  200 , as shown in  FIG. 42 . The throttle body  411  further includes a by-pass idle valve  4111 . The by-pass idle valve  4111  is preferably controlled by a stepper motor that controls the cross sectional opening of the by-pass idle valve  4111  and the amount of air flowing through it. Alternatively, it is contemplated that the idle valve  4111  may include an electromagnetically operated valve. The operation of the by-pass idle valve  4111  is controlled by the engine management system  200 . The engine management system operates the stepper motor based on the engine speed to adjust it to a given threshold value. In normal operation, the idle valve  4111  is open when the throttle of the throttle body  411  is closed. This permits the flow of a predetermined amount of air into the manifold  41  during an engine idling less than the normal air intake into the air manifold  41 . The idle valve  4111  is not fully closed when the throttle of the throttle body  411  is open. In a normal full load steady state operating condition, the idle valve  4111  is partly but not entirely open. This provides a reserve of intake air used for transient engine operating conditions (e.g., a rapid deceleration phase). The stepper motor is operated such that the maximum amount of air can be drawn into the air manifold  41  so that the air/fuel mixture is not too high. The location of the throttle body  411  is different for the normally aspirated engine  1  and the supercharged engine  2 . It is contemplated that the throttle body  411  may be replaced by a carburetor in a non-fuel injected version of the engine.  
      The air manifold  41  further includes at least one swing pipe  412  for each cylinder. Each swing pipe  412  is operatively connected to the respective intake passageway  24  to supply air to the combustion chambers through intake openings  241 . The flow pattern of the air within the air manifold  41  is indicated by the arrows in  FIGS. 27-29  and  31 . As shown, the air enters the air manifold  41  via the throttle body  411 . The air passes radially through a cylindrical flame arrester  42  and then flows through each swing pipe  412  to the respective intake passageway  24 .  
      The flame arrester  42  in the air manifold  41  prevents backfire of flames from entering the engine compartment interior within the hull of the personal watercraft. The flame arrester  42  includes a perforated inner pipe  421  and a pleated porous outer shell  422 . In accordance with the present invention, the location of the flame arrester  42  is advantageous. The flame arrester  42  is located within the central passageway in the air manifold  41 . As such, the flame arrester  42  is located between the swing pipe  412  and the air inlet. In the event of a backfire, this location is advantageous because all flames are caught by the flame arrester  42  before passage to the air inlet (i.e., the throttle or the supercharger). Thus, backfire flame cannot reach outside of the engine, especially important when the engine is installed on a watercraft or aircraft where an engine compartment fire can be more disastrous than in an automobile. Although a cylindrical flame arrester  42  is illustrated, it is also contemplated that the flame arrester may be in the form of a flat plate or an arcuate member.  
      The air manifold  41  is constructed to withstand the build up of back pressure in the event of a backfire. The manifold  41  is configured such that the back pressure is dissipated within the swing pipe  412 . To prevent failure or cracking of the manifold in the event of a significant build up of back pressure, a pressure relief valve may be provided. The pressure relief valve may be made integral with an end cap  413 , which is secured to an end of the air manifold  41 , as shown in  FIG. 27 . The end cap  413  may be integrally formed with the air manifold  41 .  
      In the supercharger version of the engine  2 , the supercharger  90  and the throttle body  411  are interconnected between the air box and the air manifold  41 . The throttle body  411  is located between the air manifold  41  and the supercharger  90 . The supercharger assembly  90 , however, is connected to an opposite end of the air manifold  41 , as shown in  FIGS. 30 and 31 . The location of the throttle body  411  is also relocated to this end. As such, the air manifold  41  is designed such that the throttle body  411  and the pressure relief valve, if provided, can be located on either end of the manifold  41  to provide increased flexibility such that the same manifold geometry can be used for either the supercharger version or the normally aspirated version of the engine.  
      The intake manifold  41  also includes at least one drainage port. The drainage plug is removably located within the drainage port. In the event that water enters the interior of the intake manifold  41 , the plugs can be removed to drain the water. Alternatively, a hose can be connected to the drainage port having a valve at an opposite end for more controlled drainage. Furthermore, it is contemplated that an automatically operated drainage valve may be provided to drain the air manifold upon engine shutdown.  
      It is contemplated that the air manifold  41  may include a cooling jacket  49  along an exterior wall of the air manifold  41 , as shown in  FIG. 29 . The cooling jacket  49  cools the air within the air manifold  41  and, more particularly, the swing pipe  412  before entering the combustion chambers. The cooling of the intake air is especially useful for a supercharge version of the engine because the operation of the supercharger (by compressing) the air increases the temperature of the air. The cooling jacket may be linked to the open loop cooling system.  
      The air intake and fuel injection system  40  further includes a fuel injection assembly  43 . The fuel injection assembly  43  includes a common fuel rail  431 . The fuel rail  431  extends along an upper portion of the intake manifold  41 , as shown in  FIGS. 26, 27 ,  30  and  31 . It is preferred that the pressure of the fuel into the fuel rail  431  be regulated by the fuel supply assembly  203  located in the fuel tank  204 . In an arrangement where the fuel supply is not controlled in the fuel tank, an optional pressure control valve  432  is located at one end of the fuel rail  431 . The pressure control valve  432  is provided to control fuel pressure within the fuel injection assembly  43 . In this arrangement, a separate fuel return line is required.  
      At least one fuel injection nozzle  434  extends from the fuel rail  431  to the each swing pipe  412  adjacent the connection to each intake passageway  24 . A fuel injection nozzle  434  is provided for each engine cylinder. The swing pipe  412  extends along the sides of the fuel injection nozzle  434 . This increases air flow ar und the injection nozzle  434  such that no pockets of reduced air flow are produced adjacent the nozzle  434  because reduced air flow may produce residue on the wall of the swing pipe adjacent the nozzle, which could reduce performance and flow of fuel into the cylinder chamber. Additionally, to prevent the formation of pockets, the nozzles  434  may extend into the swing pipe  412 . Fuel from the injection nozzle  434  is mixed with the air within the swing pipe  412  as the air enters the intake passageway  24 . The fuel injection nozzles  434  are electromagnetically controlled by the engine management system  200  so that the nozzles  434  are independently and sequentially operated.  
     Power Take Off Assembly  
      The power take off assembly  50  of the engine  1  or  2  will now be described in connection with  FIGS. 32-34  and  36 . The crankshaft  123 , described above, extends from one end of the crankcase  10 , as shown in  FIG. 33 . The rotation motion of the crankshaft  123  is transferred to a drive shaft  51 . A threaded connecting assembly  52  is secured to the end of the crankshaft  123 . The threaded connecting assembly  52  includes a plurality of teeth  521  that extend around an inner periphery of one end of the connecting assembly  52 . The teeth  521  are adapted to mate with complementary teeth  511  on the drive shaft  51 . As shown in  FIGS. 36 and 37 , the teeth  511  have a generally arcuate shape. Although a generally linear tooth arrangement is considered to be well within the scope of the present invention, the arcuate tooth is preferred. The arcuate arrangement allows for slight angular deviations between the crankshaft  123  and the drive shaft  51 . This is especially important when the crankshaft  123  and the drive shaft  1  are not in exact alignment or when the personal watercraft is operated in extreme conditions, such as, for example, when jumping waves. The use of the threaded connecting assembly  52  is also advantageous. In the event of wear resulting from non-exact alignment, only the connecting assembly  52  need be replaced.  
      The arcuate teeth  511  of the connecting assembly  52  are lubricated with engine oil. The oil is supplied from a first crankshaft main bearing  1232  via hollow bores  1233  in the crankshaft  123 . The oil then flows to the arcuate teeth  511 . This arrangement reduces engine maintenance because the operator no longer needs to grease the connection between the crankshaft and the drive shaft. The lubrication is performed by the lubrication system of the engine. The power take off housing  59  seals the components contained therein with the power take off assembly  50 . Thus, protecting these components from exposure to marine conditions.  
      The connecting assembly  52  includes a sealing extension  522 , wherein the extension  522  extends along a portion of the drive shaft  51 . An o-ring seal  523  or other suitable sealing member is positioned between the sealing extension  522  of the connecting assembly  52  and the drive shaft  51 . There is no relative rotational movement between the drive shaft  51  and the connecting assembly  52 . As such, there are no rotational stresses on the o-ring seal  523 . The sealing extension  522  and the o-ring  523  prevents lubricant from escaping from the engine. A labyrinth sealing arrangement may be provided between the sealing extension  522  and the power take off housing  59  to prevent the passage of lubricant from the power take off assembly  50  around the drive shaft  51 . Alternatively, a screw or worm conveyor may be provided, which conveys lubricant back to the power take off assembly. At least one bore may be provided to form a shortcut such that the oil is drawn into the screw conveyor.  
      Additionally, the sealing of the drive shaft  51  with respect to the outside is effected by a sealing assembly  53 . The sealing assembly  53  includes several sealing elements that can be used alone or in combination. The sealing assembly  53  includes flexible bellows  531 , a shaft seal ring  532 , and sealing rings  533 . The flexible bellows  531  connects the power take off housing  59  with an external bearing carrier race  5311 , which in turn is rotatably mounted on the drive shaft  51  via two self lubricating antifriction bearings (rolling bearings)  5312  and a bearing carrier inner race  5313 . Sealing between the two bearing carrier races  5311  and  5313  is effected by the shaft sealing ring  532 . The sealing rings  533  (in the form of polymeric o-rings) act as a seal between the bearing carrier inner race  5313  and the drive shaft  51 . The sealing rings  533  also ensure a reliable fit between the two parts. A safety ring or clip  534  secures the bearing carrier inner race  5313  on the drive shaft  51  against any axial displacement. This may also be accomplished using a step formed in the drive shaft  51 . The flexible bellow  531  is clamped to the power take off housing  59  and the external bearing carrier race  5311  by clamps  5314  and  5315 , respectively.  
      Alternatively, the antifriction bearings  5312  are lubricated with engine oil. The oil is supplied from a first crankshaft main bearing  1232  via hollow bores  1233  in the crankshaft  123 . The oil flows through the arcuate teeth  511  to the antifriction bearings  5312  and finally returns between the power take off housing  59  and the connecting assembly  52  into the interior of the engine. With this arrangement, a second flexible seal is provided in the event the flexible bellow  531  fails.  
      The power take off assembly  50  further includes a gear assembly  54 , as shown in  FIGS. 36 and 37 . The gear assembly  54  includes a main gear  541  secured to the crankshaft  123  for driving the balance shaft  115 , a chain gear  542  integrally connected to the main gear  541  for driving a cam control chain  55 , and a large gear  543 . It is contemplated that the chain gear  542  may be a separate component that is either force fit, fastened to or integrated into the crankshaft  123 . The large gear  543  includes at least a first gear  5432  for engagement with a starter  56  through intermediate gear  561 , as shown in  FIG. 37 A  second gear  5431  may be secured to the large gear  543  if the engine  2  is so equipped for driving a supercharger  90 , as described below and shown in  FIG. 38  For reducing the number of required parts for the engine family, a single gear  543  having both gears  5431  and  5432  may be used in either the blown or normally aspirated engines. It is also contemplated that the large gear  543  is formed as a single gear such that a portion of each tooth of the gear is used to drive the supercharger and another portion is used to drive the starter.  
      Linking the intermediate gear  561  for the starter assembly  56  to the crankshaft  123  through the gear  543  results in a reduction of the engine profile. A thrust screw drive within the intermediate gear  561  for the starter assembly  56  allows for an automatic engagement of a drive pinion  562  with the first gear  5432  during the starting procedure. The intermediate gear  561  moves the drive pinion  562  into engagement with the first gear  5432  against the bias of a return spring  563 . At least one dampening spring  564  is provided to absorb vibration. After the starters operation is complete, the thrust screw drive disengages such that the return spring  563  biases the drive pinion  562  out of engagement with the first gear  5432 . The drive pinion  562  is mounted to a pinion shaft  565  that is connected to the starter assembly  56  such that rotational movement generated by the starter assembly  56  is transferred to the drive pinion  562 . The pinion shaft  565  is slidably and rotatably received within a recess in the power take off housing  59 .  
      As illustrated in  FIG. 36 , a generator assembly  57  is also part of the power take off assembly  50 . The generator assembly  57  includes a magnet wheel  571  connected to the gear assembly  54 , as shown in  FIG. 36  using suitable fasteners. The generator assembly  57  is a permanently excited 3-phase generator, in which permanent magnets  572 , which are fastened to magnet wheel  571 , rotate around a stator  573 . The stator  573  is fixed to the inner side of the power take off housing  59 . The location and arrangement of the generator assembly  57  provides for easy encapsulation because of reduced wiring requirements. The magnet wheel  571  rotates around the stationary coils. This arrangement is advantageous because it eliminates the need for rotating coil members and also in view of possible repair work. Furthermore, it reduces the weight of the rotating masses. Additionally, the magnet wheel  571  is constructed as an extrusion-molded part.  
      The rotational speed of the crankshaft  123  is measured by an engine or crankshaft speed sensor  58  located within the power take off housing  59 . A cup shaped actuator  544  is secured to the gear assembly  54  between the large gear  543  and the magnet wheel  571  of the generator assembly  57 . The actuator  544  extends between the gear  543  and wheel  571  and between the sensor  58  and the wheel  571 , as shown in  FIG. 36 . The actuator  544  includes a plurality of teeth extending around the perimeter thereof. A predetermined number of teeth are missing at predetermined locations along the perimeter. The sensor  58  detects the absence of the teeth as the actuator  544  rotates. The speed of the crankshaft and engine speed can be determined from this.  
      Alternatively, it is contemplated that the magnet wheel  571  may include at least one conductor piece mounted therein. The conductor piece triggers the crankshaft or engine speed sensor  58 . Instantaneous values of the crankshaft position can be received therefrom and the angular speed (rotational speed) is then calculated by the engine management system  200 , described below. The angular resolution is 10°, i.e. during rotation of the crankshaft  123 , after every 10° of rotation, a pulse is sent by the crankshaft position sensor to the control device. It is contemplated that the present invention is not limited to an angular resolution of 10°; rather, angular resolutions greater than and less than 10° are considered to be well within the scope of the present invention.  
      The arrangement of the components within the power take off housing  59  results in a more compact engine design. As described above, the engine components are located on the power take off end. The power take off housing  59  protects these elements from the marine conditions in which the personal watercraft operates. Furthermore, a common drive assembly connected to the crankshaft  123  is provided to drive these components without the need for numerous belts and other connections. Additional features and benefits of the power take off assembly  50  will be described below in connection with the description of the lubricating system  60 , the blow-by ventilation system  70 , engine cooling system  80  and supercharger  90 .  
     Lubricating System  
      The lubricating system  60  will now be described in greater detail in connection with  FIGS. 8, 11 ,  12 ,  14 - 16  and  32 - 35 .  
      The engines  1  and  2  have a dry-sump lubricating system  60 . The lubrication system  60  includes the oil tank  11 , described above and shown in  FIG. 8 . The oil collected in the crank chambers  121  emerges therefrom via outlet openings  111  into a channel  112 . The oil then flows to the upper portion  113  of the oil tank  11  adjacent the balance shaft  115 . From there, the oil flows back by gravity to the bottom of the oil tank  11 , where the oil is collected and stored.  
      From the oil tank  11 , the oil is conveyed to an oil cooling assembly  86 , shown in  FIGS. 23 and 25 , by an oil pump  61 , as shown in  FIGS. 25 and 33  through integrated channels in the lower crankcase  12 . The oil pump  61  is integrated into the power take off housing  59  and is coaxially disposed and driven by the balance shaft  115  via a connecting shaft  612 . The connecting shaft  612  is received within a suitable recess within the end of the balance shaft  115  such that rotation movement of the balance shaft  115  is transferred to the drive shaft  612 . The oil pump  61  is preferably a troichoid pump. It is preferred that the oil be sucked from the bottom of the oil tank  11 . Furthermore, it is also preferred that the oil be removed from a more centrally located pickup position within the tank  11 , rather than the front or rear of the tank  11 . This is a preventative measure to avoid air entrapment in extreme operating conditions (extreme acceleration and deceleration modes). The oil cooling assembly  86  is designed as a plate-type cooler and is fixed onto the cylinder block  10 . To cool the engine, water is used in a closed cooling system  80 , described in greater detail below.  
      From the oil cooling assembly  86 , the oil is conveyed to the oil filter unit  62 , as shown in  FIGS. 32 and 34  through integrated channels in the lower crankcase  12 . The oil filter unit  62  has an oil filter casing  621  that is integrated to the power take off housing  59 . The oil filter unit  62  is closed at one end by a removable oil filter cover  622 . Located within the oil filter casing  621  is an annular oil filter  623  and a valve rod  624 . One end of the valve rod  624  is connected with the oil filter cover  622 . The valve rod  624  is secured to the cover by a suitable fastener. The valve rod  624  acts as a fastener to secure the cover  622  to the filter casing  621 . The other end of the valve rod  624  extends into a drainage opening  625 . When the valve rod  624  is pulled out of the drainage opening  625 , the oil which has remained in the filter casing  621  can automatically drain through the drainage opening  625 . Alternatively, the oil filter cover  622  may be configured as a screw lid.  
      Unlike conventional oil filter units where the overflow valve is integrated in the upper region of the filter cover  622 , the oil filter unit  62  includes an external overflow valve  626  and a bypass duct  627 . In the event that the oil filter unit  62  is clogged, a direct connection is formed between an inlet channel  628  and an outlet channel  629  of the oil filter unit  62 . This arrangement has the advantage that the oil does not flow around a dirty oil filter. Thus, no dirt particles can contaminate the oil circuit.  
      The filtered oil is then supplied to the engine  1  or  2  for lubricating the various components through the main oil gallery in the upper crankcase  13  of the crankcase  10 , as illustrated in the oil circuit in  FIGS. 8 and 11 .  
      One aspect of the lubricating system  60  relates to the return of the oil from the crank chambers  121  in the upper crankcase  12  into the integrated oil tank  11 . The oil is pushed out of the crankcase. This is effected by a differential pressure acting between the crank chambers  121  and the oil tank  11  and the induction system, respectively. This differential pressure is a result of the pressure pulses caused by the pistons  1241  in the crank chambers  121 . It is also partially due to a consequence of a “Blow-By” effect, which refers to cylinder pressure losses. The piston  1241  does not provide a 100% sealing on the cylinder wall, so part of the combustion gas caused during combustion leaks past the cylinder downwardly into the lower crankcase  12 . This so-called blow-by gas creates additional pressure in the crank chambers  121  below the pistons  1241  and is dependent on the load and the rotational speed of the engine. However, on account of the above-mentioned blow-by effect, the overall effect results in a pressure that is always above the pressure between the air box and the throttle body. The return of the blow-by gas is described in greater detail below in connection with the blow-by ventilation system  70 .  
      The rotational movement of the crankshaft  123  is also utilized to carry oil to the outlet openings  111 , and here two effects are to be found. First, by the direct contact of the crank webs  1231  with the oil, in case of direct wetting, there occurs an entrainment effect as a consequence of the shearing forces. Second, with smaller amounts of oil in the crank chambers  121 , if there is no direct contact between crank web  1231  and oil, gas forces will occur which likewise drive the oil to the respective outlet openings  111 . At the base of the crank chambers  121 , in the vicinity of the outlet openings  111 , stripper edges may be arranged which strip the oil from the crank webs  1231 .  
      To enable an optimum utilization of the above-described effect for the oil return, the three crank chambers  121  (discussed above) in the crankcase  12  are hermetically separated from each other, and each crank chamber  121  is equipped with a separate outlet opening  111  for the oil. Thus, the pressure in one chamber is not affected by the pressure in the other chambers. The cross-sections of the channel system for the oil return following the outlet openings  111  are dimensioned suitably (i.e. not too large) so as to ensure the conveyance of the oil back to the oil tank  11  on account of the differential pressure, without the risk of a pressure equalization between oil tank  11  and crankcase  12 . Alternatively, the channels can also unify, so that one single channel  112  leads to the oil tank  11 . The arrangement should be designed such that no oil “short-circuit” and no pressure balance will occur between the individual crank chambers  121 , i.e. oil must not be permitted to flow directly from one crank chamber  121  into another chamber.  
      The return channels  112  for the oil return from the three hermetically closed crank chambers  121  to the oil tank  11  may be realized by channels cast into the lower crankcase  12  which enter the oil tank  11  adjacent the union between the upper crankcase  13  and the lower crankcase  12 . Alternately, they may be realized by separate ducts, in particular hoses or tubes. As such, normally hoses are only used in connection with external oil tanks. In the present “in-case oil tank,” hoses can be avoided. To prevent an undesired flow-back of oil from the oil tank  11  to the crank chambers  12  and—in consequence—a flooding of the crank chambers in extreme inclined positions or in flip-over position of the personal watercraft  5 , non-return valves (not illustrated) may be installed in the channels  112 .  
      To remove the lubricating oil which has collected in the region close to the bottom of the crank case  12  adjacent the bottom of the power take off housing  59 , a separate suction pump  71  is provided. Like the oil pump  61 , the suction pump  71  is coaxially arranged along and driven by the balance shaft  115 . The pump  71  is preferably a troichoid pump. The pump  71  is located on an opposite end of the balance shaft  115  when compared to the pump  61 . The oil is conveyed from the bottom of the power take off housing  59  through a duct  126  cast into the lower crankcase  12  to the suction pump  71 . Alternatively, it is contemplated that the blow-by gas created in the crank chamber  121  adjacent the power take off housing  59  is fed into the power take off housing  59  to provide pressure to remove the oil from the bottom of the power take off housing  59  near the bottom of the crank case.  
      The oil collected in the bottom of each crank chamber  121  exits through the opening  111 . The oil is then driven through the channel  112  back to the oil tank  11  by the blow-by gas pressure. The oil collected inside the power take off housing  59  is removed by a suction pump  71  or other suitable pumping assembly. The oil flows through a channel  126 , shown in  FIGS. 11, 41  and  49 , again integrated into the lower crankcase  12  from the power take off side to the opposite side, where the suction pump  71  is mounted, as shown in  FIGS. 40 and 41 . The oil passes through an oil sieve  72  before it enters the suction pump  71  and is finally conveyed back through a U-shaped channel  711  to the oil tank  11 , as shown in  FIGS. 11 , and  40 . It is contemplated that the channel  711  is integrated in the housing of the suction pump  71 .  
      Regarding the oil circuit, it is added that cooling and lubrication of the pistons  1241  and liners are effected by aid of spraying nozzles  64  at the lower side of the piston  1241 , as shown in  FIG. 8 . Oil is supplied to the nozzles  64  from the main oil gallery  65 . The spray nozzle  64  is positioned such that the jet reaches the piston lower side not only in the lower dead center position illustrated, but also in the upper dead center position.  
       FIGS. 8 and 35  illustrate one possible oil channel system  63  in the region of the cylinder head housing  20  by way of a schematic 3D representation. Other systems are contemplated to be well within the scope of the present invention. The oil is conveyed to the cylinder head housing  20  through at least one ascending duct  631  in the upper crankcase  13 . The ascending duct  631  is connected to the main oil gallery  65 . The oil enters cylinder head housing  20  from the ascending duct  631  through a transverse bore  632 . In the ascending duct  631 , a throttle  6311  is installed which restricts the amount of oil flowing therethrough. In addition, a check valve  6312  is disposed in the ascending duct  631 , which blocks the oil conduit as soon as the engine  1  or  2  is stopped. As such, a certain amount of oil can be stored in the channels in the cylinder head housing  20 . This stored oil is particularly useful during a cold start since lubrication can be initiated rapidly therewith and provided to the valve train sooner to prevent damage to the valve train.  
      Connecting bores  633  branch off of the tansverse bore  632  and connect the latter with the bores  634 . The bores  634  also receive the cylinder head fastening screws. The oil rises upwardly in the annular gap between the cylinder head screw and the corresponding bores  634 . The oil then enters into a V-shaped channel section  635  formed by two obliquely downwardly directed bores  6351  and  6352 . From the ascending branch  6352  of the V-shaped channel section  635 , the oil directly enters into the interior of the hollow rocker arm support axle  28 . From there, the oil is directed to the bearing places of the rocker arm assemblies  25  and  26  via the radial openings  282 , as shown in  FIG. 14 . Also, the oil is admitted to the operating assemblies  253  and  263 . It is contemplated that other channel systems and arrangements are well within the scope of the present invention provided the channel systems conduct lubricant from the main oil gallery  65  to the support axle  28 .  
      Lubricant is supplied to the camshaft  29  via bearing bracket  293 , described above, through bore  636 .  
      Below the camshaft  29 , the oil may accumulate in a small basin in which the lobes  291  and  292  of the camshaft  29  may be immersed for lubricating purposes. The lubricant within the cylinder head housing  20  collects in a depression under the camshaft  29  adjacent the cylinder closest to the power take off assembly  50 . The oil from the other cylinders within the cylinder head flows to the depression through passageways  295 , which interconnect the areas in the cylinder head adjacent the other cylinders. The oil exits the cylinder head housing  20  through an inclined passageway into the control chain chamber  202  where it flows into the power take off assembly  50 . This lubricant contributes to the lubrication of the gears and supercharger  90  (if present) within the power take off assembly  50 .  
     Blow-By Ventilation System  
      The engines  1  and  2  are preferably equipped with a blow-by ventilation system  70  for separating oil from the vented blow-by gas. A preferred form of the blow-by ventilation system  70  is illustrated in  FIGS. 3, 4 ,  11 ,  40 ,  41  and  46 .  
      The blow-by gas originating from the combustion chambers  124  due to leakage between the pistons  1241  and cylinder liners first accumulates in the (sealed) crank chambers  121  and from there it flows together with the oil through the channels  112  to the oil tank  11 , where it accumulates and mixes in the upper portion  113  of the oil tank  11  with any gas in the oil tank  11  from the power take off assembly  5 d. From the oil tank  11 , the gas mixture is then conveyed through a channel  712  (in the housing of the suction pump  71  and the lid of the sieve  72 ), shown in  FIG. 40  to a shutoff and pressure relieve valve  73 , which is open in normal engine operation. The pressure relief valve  73  includes a valve rod  731  that moves the valve  73  between open and closed positions by a solenoid assembly  77 . In the event that the solenoid assembly  77  is not operational, the pressure relief valve  73  includes a spring assembly  732  that permits the opening of the valve  73  in the event of a build up of pressure within the tank  11 .  
      The gas mixture from the oil tank  11  is split into two partial flows: a first portion flows back to the cylinder head chamber within the cylinder head housing  20  through a passageway  74 , shown in  FIGS. 40 and 41 . A second portion is vented tangentially into an oil separator  75  designed as a cyclone. In the cyclone, the gas mixture is separated from oil by centrifugal forces due to the swirling of the gas/oil mixture in the cyclone. The cleaned gas mixture leaves the cyclone through a central pipe  751 . The cleaned gas mixture then passes a second shutoff and pressure relief valve  76  and is finally conveyed to the air intake between the airbox and the throttle body  411 , where it merges with the fresh air drawn in by the engine.  
      The shutoff and pressure relief valve  76  is also mounted on the valve rod  731  and is also actuated by the solenoid  77 . With this arrangement, the valves  73  and  76  operate simultaneously. The valves  73  and  76  are closed by drawback springs  732  and  761  when the solenoid  77  is not activated and they are open when the solenoid  77  is activated. With this arrangement, the engine is sealed, preventing oil leaks when the engine is shut down. In normal (upright vehicle) engine operation, the solenoid  77  is activated and the valves  73  and  76  are opened respectively. However, in the event of a roll-over of the vehicle, the valves are closed instantly to prevent oil from entering the induction system  40  and/or the airbox and leaking into the environment. The closure of valve  73  prevents oil from accumulating in the cylinder head housing  20  in a roll-over event. This would cause a temporary lack of oil in the oil tank  11 , when the personal watercraft  5  has returned to a normal upright position and could result in an undersupply of lubricant to the engine, which may result in severe damage to the engine  1  or  2 . The valves  73  and  76  are also closed when the engine is shut down.  
      A pressure sensor or sensor switch may be provided in the oil tank  11  or in the channel  712  to sense the pressure within the tank  1 l. If the oil pressure exceeds a certain threshold value, the engine management system  200  operates in an emergency mode (e.g. limp home function). The engine management system operates the engine at a reduced speed. The engine management system also interacts with other onboard computer systems to notify the operator of the engine malfunction. Additionally, the pressure sensor can be used to detect oil leakage in the lubrication circuit.  
      The gas mixture enters the upper portion of the cyclone  75  through the opening  755 . As such, the gas mixture tangentially enters the cyclone  75 . Oil droplets within the gas mixture are thrust against the inner wall of the cyclone  75  as a result of centrifugal forces within the cyclone  75 .  
      The separated oil then flows down the inner wall of the cyclone  75  towards opening  752 ; collects in the bottom of the cyclone  75 ; and exits the cyclone  75  through an opening  752  into a channel  753  integrated in the sieve lid  721 , and merges with the oil flow from the power take off assembly  50  in front of the oil sieve  72 , to be conveyed back to the oil tank  11 . Within the channel  753  there is provided a throttle  754  which ensures that a sufficient height negative pressure (vacuum) can build up in the suction port of the suction pump  71 , so that the power take off housing  50  is drained reliably in all operating conditions. In a cold start condition (when the oil is very viscous) the throttle  754  may even be closed by an additional valve (not shown) especially at idling speed to guarantee the aforesaid requirement.  
      An oil filler tube  78  is integrated to the cyclone  75 . A cap  781  is provided for closing the filler tube  78 . Fresh oil flows down the filler tube  78  into a channel  722  integrated in the sieve lid  721 . The oil enters a U-shaped duct through a port  715 , shown in  FIG. 40 , in the housing of the suction pump, merges with the oil from the power take off assembly  50  and is finally conveyed to the oil tank  11 .  
      In the preferred embodiment, the valves  73  and  76 , the cyclone  75  and the oil filler tube  78  are assembled to form a single unit.  
      In accordance with the blow-by gas ventilation system  70  described herein, a slight vacuum (underpressure, negative pressure, subpressure) is generated in the interior in the power take off assembly  5 q and within the cylinder head housing  20 . As a result, no oil or contaminated blow-by gas can escape to the environmnent.  
     Engine Cooling System  
      An engine cooling system  80  will now be described in connection with  FIGS. 25, 32  and  33 . The engine cooling system  80  is a closed system utilizing a coolant such as glycol, water or a mixture of them. The present invention, however, is not limited to these coolants; rather, it is contemplated that other cooling liquids are considered to be well within the scope of the present invention. The cooling circuit of the engine cooling system  80  is illustrated in  FIG. 25 . The closed loop cooling system  80  cooperates with the open loop cooling arrangement described above in connection with the exhaust manifold  30  to effectively cool the engines  1  and  2 .  
      The engine cooling system  80  includes a pump assembly  81  located on one end of the engine  1  or  2 , as shown in  FIG. 32 .  
      As illustrated in  FIG. 33 , the pump assembly  81  is arranged externally of the power take off housing  59 . The power take off housing  59  and pump lid  611  together form the pump casing. It is designed as a rotary pump and consists of an impeller  811  which is located, screwed or attached onto the end of the connecting shaft  612 , which projects from the power take off housing  59 . The connecting rod  612  also drives the oil pump  61 . Impeller  811  is driven by connecting rod  612 . The connecting rod  612  also drives the oil pump  61 . The pump assembly  81  also includes a pump lid  812 , which is fastened to the power take off housing  59  and forms the pump casing in cooperation therewith. The pump assembly  81  has a one piece housing having an integrated thermostat.  
      As shown in  FIG. 25 , the coolant flows from the pump assembly  81  through a passageway  82  to the cylinder block of the upper crankcase  13 . The passageway  82  includes a main passageway  821  and a by-pass passageway  822 . The passageways  821  and  822  direct coolant to the cooling passageway  125  in the cylinder block. The coolant flows along the exterior of the cylinders  124 , as shown in  FIG. 25 . With this arrangement, the coolant travels in a generally U-shaped manner along a side of the cylinders  124  adjacent the intake manifold; around the end of the cylinder furthest from the power take off assembly  50  and then along the side of the cylinders adjacent the exhaust manifold in a direction back towards the power take off assembly  50 . At the same tire, the coolant is directed in an upward direction towards the cylinder head housing  20 . The by-pass passageway  822  reduces the load on the main passageway  821  and improves the flow pattern in the cooling passageway  125  at an end portion of the cooling passageway  125  opposite the inlet. The coolant from the by-pass passageway  822  mixtures with the coolant in the coolant passageway  125  to reduce the temperature of the coolant in the end portion of the cooling passageway  125 . Furthermore, the entry of coolant into the cooling passageway  125  from the by-pass passageway  822  improves the upward flow of coolant into the cylinder head housing  20 . It is preferred that the passageways  821  and  822  are integrally formed in the power take off housing  59  and crankcase  10 . It, however, is contemplated that the passageways may be hoses connecting the components to one another.  
      From the upper crankcase  13 , the coolant then passes upwardly to the cylinder head housing  20  through bores  131  in a head gasket  130  positioned between the upper crankcase  13  and cylinder head housing  20 , as schematically illustrated in  FIG. 25 . The bores  131  are located on the exhaust manifold side of the gasket  130 . These bores  130  act as throttles to adjust the flow of coolant into the cylinder head housing  20 . Additional small bores are located on the intake manifold side of the gasket  130 . These bores vent air trapped within the passageway  125  into the cylinder head housing  20 . The coolant first passes over the exhaust side of the cylinder head toward the intake side of the cylinder head before exiting the cylinder head housing  20  through a common passageway.  
      From the cylinder head housing  20 , the coolant is then conveyed through a hose to a thermostat  83  through an inlet passageway  817  located on the pump assembly  81 , as shown in  FIGS. 25 and 32 . As illustrated in  FIG. 33 , the thermostat  83  is directly mounted on the pump lid  812 . The thermostat  83  comprises a two-part thermostat casing  831  and  832  including hose connections and a temperature-sensitive valve  833 , which automatically opens if a predetermined temperature threshold value is exceeded. The coolant then flows through outlet passage  816  to a heat exchanger  84  (shown schematically in  FIG. 25 ), where the coolant is cooled by exchanging heat to the atmosphere. This can be in the form of a cooling plate exposed to the body of water. The cooling plate may be located in a lower portion of the hull of the personal watercraft  5 . The cooling plate is described in U.S. Provisional Patent Application Ser. No. 60/160,819, filed Oct. 21, 1999 entitled “WATERCRAFT WITH CLOSED-LOOP HEAT EXCHANGER,” and U.S. patent application Ser. No. 09/691,129, filed Oct. 19, 2000 entitled “WATERCRAFT HAVING A CLOSED COOLANT CIRCULATING SYSTEM WITH A HEAT EXCHANGER THAT CONSTITUTES AN EXTERIOR SURFACE OF THE HULL” the specifications of which are incorporated herein specifically by reference. The coolant is then returned to the pump assembly  81  through an inlet  815 .  
      The primary purpose of the cooling system  80  is to cool the engine  1  or  2  during operation. The operation of the cooling system  80  is temporarily modified during engine start-up so that the engine quickly reaches an optimal operating temperature. During initial engine start-up, the thermostat  83  deactivates the heat exchanger  84 . As such, the coolant is not cooled prior to reentry into the pump assembly  81 ; rather, the coolant returns directly from the inlet  817  into the coolant pump  81 .  
      The cooling system  80  furthermore includes an oil cooling assembly  86 . The oil cooling assembly  86  is connected to pump assembly  81  and thermostat  83 . With this arrangement, a portion of the coolant from the pump assembly  81  is directed to the oil cooling assembly  86  through passageway  861  to cool the engine oil. After passing through the oil cooling assembly  86 , the coolant returns to the thermostat  83  via return passageway  862 . The coolant from the passageway  862  enters the thermostat housing in the vicinity of the inlet  817 . The oil cooling assembly  86  preferably is a plate-type cooler and disposed on the side of the lower crankcase  12 . The coolant, which heats sooner than the oil, is used to heat the engine oil during engine start-up.  
      The cooling system  80  further includes a temperature sensor  87 , which is linked to the engine management system, shown in  FIGS. 25 and 42 . As shown in  FIG. 25 , an expansion reservoir  88  is provided in the return from the cylinder head housing  20  to the thermostat  83 , as shown in  FIG. 23 . The expansion reservoir  88  adjusts for expansion of the cooling fluid within the system  80 . The expansion reservoir  88  further a refill port  881  for refilling the system  80 . The reservoir  88  further provides a venting function for removing air from the cooling system  80 . In this manner, the interconnecting duct between the reservoir  88  and the cylinder head housing  20  has to be linked to the highest point in the cylinder head housing  20  to prevent the formation of an air barrier which could cause overheating.  
     Supercharger Assembly  
      As discussed above, the engines in accordance with the present invention may include a supercharger  90 . The engine  2  having a supercharger  90  is illustrated in  FIGS. 6, 7 ,  30 ,  31  and  38 . The supercharger  90  is provided to increase the air intake and enhance engine performance. The preassembled supercharger  90  is plugged in a corresponding port  591 , as shown in  FIG. 33 , in the power take off housing  59  and sealed with sealing rings  592 , as shown in  FIG. 38 . It is contemplated that a turbocharger may be used in connection with the present invention. The supercharger, however, provides improved operating characteristics when compared to the turbocharger. Furthermore, the turbocharger produces additional heat as compared to the supercharger, which places increased demands on the cooling systems.  
      The supercharger  90  includes a cast housing  91 , which is preferably formed from a metal, however, it may be formed from a high strength plastic or other suitable material. The housing  91  includes an inlet portion  911 . The inlet portion  911  is operatively connected to the airbox (not shown). Air enters the supercharger  90  through the inlet portion  911 . Located within the housing  91  adjacent the inlet portion  911  is an impeller  92 , which operates to draw air into the supercharger from the airbox. An air passageway  912  extends around the impeller  92  to collect the air compressed by the impeller. The air passageway  912  is connected to the intake manifold  41  through the throttle body  411 . The housing  91  further includes a mounting portion  913  that extends backward from the inlet portion  911 . The mounting portion  913  is received within the port  591  in the power take off housing  59  and sealed with at least one sealing assembly  592 .  
      As shown in  FIG. 38 , a blower drive shaft  922  extends through the mounting portion  913  and inlet portion  911 . The blower drive shaft  922  is rotatably mounted within the housing  91  with at least one bearing assembly  921 . A drive pinion  93  is coupled to the blower drive shaft  922 . It is preferred that this be a non-positive coupling. As such, the drive pinion  93  is non-positively connected with the blower shaft  922  via an intermediate element  94  by a biasing spring force, which is preferably supplied by a spring assembly  95 . The spring assembly  95  includes a plurality of cup springs. Other spring assemblies and means for providing a connection that can slip under high torque to prevent damage to the impeller or other components, however, are considered to be well within the scope of the present invention. The drive shaft  922  includes splines to prevent rotational movement of the intermediate element  94  with respect to the drive shaft  922 . The shaft  922  includes a lubrication passageway that delivers lubricant to the drive pinion  93  to reduce wear. The lubrication passageway is connected to the lubrication system. The connection between the drive pinion  93  and the intermediate element  94  is formed as a plane frictional surface. This unique connection assembly can dampen the rotational and torsional vibrations transmitted by the crankshaft  123 .  
      The supercharger  90  is operatively coupled by the drive pinion  93  to the gear assembly  54  through gear  5431 . The supercharger  90  preferably includes a cooling jacket connected to the open or closed loop cooling system to cool and prevent failure of the supercharger  90 . The cooling of the supercharger  90  improves engine performance.  
      In accordance with the present invention, the supercharger  90  preferably utilizes a low-cost rotary (radial or radial-axial) blower. The present invention, however, is not limited to these blowers; rather, it is contemplated that a positive displacement blower (e.g. a Rootes or Wankel blower) may be employed. Furthermore, the supercharger  90  may be used for separating a certain water content from the intake air.  
     Control Tensioner  
      In accordance with the present invention, the engines  1  and  2  are preferably equipped with a control tensioner for controlling the tension within chain  55 . The present invention, however, is not limited for use with a chain; rather, it is contemplated that the control tensioner can be used with other flexible linkages, including but not limited to belts. A mechanical chain tensioner  100  is illustrated in  FIG. 39 . The tensioner  100  includes a driving element  101 . The driving element  101  preferably includes a spring assembly. The spring assembly is preferably a rotationally active helical pressure spring. The spring assembly  101  is rotationally biased by aid of a thread cap  102 . The spring includes a spring ender  1011  that engages a slot  1021  in thread cap  102 . The thread cap  102  is externally screwed into a retainer  103 . The spring assembly  101  is received at one end in a blind hole bore of a hollow adjustment element  104  which is screwed into a thread bore of the retainer  103 . The spring also includes a spring end  1012  that engages a slot  1042  in adjustment element  104 . The overlapping thread engagement of adjustment element  104  with retainer  103  is designed to be relatively long. As oil gets into this threaded connection, it provides a small damping effect to the adjustment element  104  due to vibrations of the cam chain. This small damping effect is enhanced if the thread overlap is kept relatively long. The external thread of the adjustment element  104  preferably includes multiple threads and it is designed such that it is borderline self-locking in the retainer  103 . This design must take into account the presence of oil between the threads, which reduces friction, when determining the necessary inclination of the threads. If the inclination is too small (very self locking), a strong spring force is required to overcome the locking action of the threads. It is desirable to avoid unnecessary tension on the chain to avoid wear and decreases in the lifetime of the chain. The self tensioning action is effected by the interaction of the chain vibration and the borderline self locking of the threads. That is, it will maintain its extended position under normal loads but can retract a distance under high loads to prevent damage to the cam chain. For instance, if automatic adjustment occurs when the engine is cold, upon reaching operation temperature, the aluminum cylinder and head have expanded more than the steel cam chain and can create too high of a tension in the chain. The borderline self locking feature allows the plunger to retract slightly before chain tension becomes so high as to damage the chain. The adjustment element  104  is rotationally driven by the spring assembly  101  if the tension of the chain  55  slackens and is axially outwardly displaced. The adjustment element  104  acts via a balancing arcuate intermediate piece  105  on a tensioning rail  106 . The chain tensioner  100  enables a later adjustment by aid of the combined biasing and fixing element  102  if the chain  55  undergoes elongation.  
      The thread piece  102 , the retainer  103  and adjustment element  104  preferably are made of synthetic material because of the smaller thermal elongation encountered as compared to aluminum. The adjustment element  104  includes a steel insert  1041  on one end to reduce wear.  
      In accordance with the present invention, the engines  1  and  2  described herein are not limited to the mechanical chain tensioner  100 ; rather, other tensioner assemblies are contemplated to be well within the scope of the present invention. For example, a hydraulic tensioner may be used. The mechanical tensioner  100 , however, has numerous advantages over this hydraulic counterpart. First, the mechanical tensioner  100  can be manufactured at a lower cost and does not require a complicated oil supply.  
     Engine Control Unit  
      The operation of the engine  1  or  2  is controlled by an engine management system  200 , as shown in  FIG. 42 . The engine management system  200  includes an electronic control unit  201  monitors and controls the operation of various engine components including but not limited to ignition, the fuel pump, the fuel injection assembly, the air intake, engine cooling, engine speed, engine lubrication, exhaust gas in the muffler in response to input from various sensors and monitors located with the engines  1  and  2 . It is contemplated that the electronic control unit  201  may further control functions, such as, e.g., realization of a departing lock, realization of a start/stop control, and the identification of authorized personal watercraft users. The electronic control unit  201  further communicates with the other computer systems on the personal watercraft for the control of instruments, non engine watercraft functions and service needs.  
      The engine management system  200  also controls the gas pump  203  in the gas tank  204 , which includes a coarse filter  2041  and a float assembly  2042 .  
      The gas pump  203  has an associated pressure regulator  2043 , such that a constant gas pressure is mechanically provided. From there, a retumless fuel system  205  leads to the injection nozzles or valves  434  seated on the fuel rail  431 . These injection nozzles  434  inject the fuel in the form ofjets in the air in the intake passageway. The engine management system  200  controls the operation of the nozzles  434  such that there is sequential injection, wherein each cylinder has an individual injection (i.e., no group injection). The injection amount is determined by the engine control device  201  on the basis of the applied characteristic fields by the pulse width, i.e. by the duration of the injection time.  
      A returnless fuel system  205  prevents the fuel from heating due to the engine heat, as could otherwise be the case with a fuel return from the engine to the fuel tank.  
      The engine management system  200  also includes various sensors, such as the temperature sensor  39  in the exhaust muffler, an air temperature sensor  43  attached to the intake manifold  41  and a water temperature sensor  87 .  
      A knock sensor  206  senses at an early time the knocking critical for the engine—which has a high specific performance level. The knock sensor  206  includes a piezo quartz element, which measures the solid-borne acoustic signals at the cylinder block and transmits the corresponding signals to the electronic control unit  201 . The latter has a detection software to detect a possible knocking combustion and to cause a correction in a manner known per se, by ignition angle displacement.  
      The sensors further include the crankshaft position sensor  207 . A corresponding rotary position sensor  208  is associated with the camshaft. By aid of this camshaft sensor  208 , it is recognized whether the crankshaft is present in the angle range of 0 to 360° or in the range of 360 to 720°, which is possible via the camshaft because the latter rotates at half the rotational speed of the crankshaft. For the sake of simplicity, the camshaft sensor  208  is directly associated with the chain wheel  551  at the camshaft.  
      For load measurement, the actual load of the engine is calculated by the intake manifold pressure measured by sensor  210  and engine speed measured from the crankshaft  123  in the power take off assembly  50 . A throttle potentiometer  209  is used for corrections and a limp home function. In the event the engine is operating in a limp home function (e.g., broken intake air pressure sensor), the engine control unit  201  communicates with another onboard computer system to notify the operator via an instrument panel that the engine is operating in a limp home function. A pressure sensor  210  is arranged in the suction pipe to sense the absolute pressure, which is especially useful for the engine  2  containing the supercharger assembly  90  and for all operation modes with slightly opened or closed throttle valve. Thus, there is no direct air amount or air mass measurement, but auxiliary parameters are used therefor.  
      Finally, for the sake of completeness, various voltage checks should be mentioned which are carried out by the electronic control unit  201 , e.g. for the supply voltage of the injection valves, which is useful insofar as the board voltage on the personal watercraft  5  may very well fluctuate.  
      It will be apparent to those skilled in the art that various modifications and variations may be made without departing from the scope of the present invention. Thus, it is intended that the present invention covers the modifications and variations of the invention, provided they come within the scope of the appended claims and their equivalents.