Patent Publication Number: US-7219635-B2

Title: Four-cycle engine and system for detecting phase difference of four-cycle engine

Description:
TECHNICAL FIELD 
   The present invention generally relates to a four-cycle engine, and particularly to a four-cycle engine configured to detect a phase difference between a crankshaft and a camshaft. The present invention also relates to a system for detecting the phase difference between the crankshaft and the camshaft in the four-cycle engine. 
   BACKGROUND 
   In general, four-cycle engines are classified into various types according to structures of valve systems. Personal watercraft or small vehicles are typically equipped with engines constructed in such a manner that camshafts are mounted above cylinders. Such engines are referred to as single overhead camshaft (SOHC) engines, which include a single camshaft, and double overhead camshaft (DOHC) engines, which include two camshafts. 
   In the SOHC type engine and the DOHC type engine, a crankshaft is coupled to camshaft(s) through a timing chain or a timing belt so that rotation of the crankshaft is transmitted to the camshaft(s). More specifically, the timing chain (or timing belt) is installed around a crank sprocket (or crank pulley) mounted on an end portion of the crankshaft, and a cam sprocket (or cam pulley) mounted on an end portion of the camshaft. Since the cam sprocket has twice as many teeth as those of the crank sprocket, the rotation of the crankshaft is transmitted to the camshaft such that the number of rotations of the camshaft becomes half as many as that of the crankshaft. 
   During assembly of the engine, first, the crankshaft provided with the crank sprocket is accommodated into a crankcase, and the camshaft provided with the cam sprocket is accommodated into a cylinder head. Then, the timing chain is installed around the crank sprocket and the cam sprocket, and a tensioner is caused to come into contact with the timing chain to apply a suitable tension to the timing chain. The cam pulley and the timing belt are incorporated into the engine according to a similar procedure. 
   The crankshaft rotates in cooperation with reciprocation of a piston coupled to the crankshaft through a connecting rod, while an intake valve and an exhaust valve operate in association with the rotation of the camshaft(s), causing an intake port and an exhaust port to open and close. In the four-cycle engine, the reciprocation of the piston is transmitted to the intake and exhaust valves through the crankshaft and the camshaft so that the piston and the intake and exhaust valves operate in association with each other. 
   It is necessary that the piston and the intake and exhaust valves operate in association with each other at suitable timings. More specifically, it is necessary that the intake and exhaust valves operate to open or close at timings at which the reciprocating piston is in a predetermined position. By allowing the piston and the intake and exhaust valves to suitably operate in association with each other, strokes (intake, compression, expansion, and exhaust strokes) in the interior of a combustion chamber are carried out correctly. As a result, high engine performance is obtained. Therefore, during assembly of the engine, it is necessary to incorporate a crankshaft and camshaft(s) with a correct phase difference (phase angle) between them. 
   However, since the timing chain is installed around the crank sprocket and the cam sprocket and then the tensioner is incorporated during assembly of the engine as described above, relative positions of the crankshaft and the camshaft may deviate from desired positions, that is, a phase difference between them may vary from a correct value, by application of the tension from the tensioner to the timing chain. As a result, engine performance may be degraded. 
   Japanese Laid-Open Patent Application Publication No. Hei. 11-129963 and No. Hei. 11-201011 disclose an engine equipped with a sensor configured to detect protrusions of a pulser rotor mounted on a crankshaft or a camshaft in order to detect which of the strokes the engine is traveling, and to thereby set suitable ignition timings. 
   In such an engine, it is possible to detect the phase difference between the crankshaft and the camshaft by using the pulser rotor and the sensor. But, the detecting precision is low because they are intended to detect which of the strokes the engine is traveling as described above. If the phase difference between the crankshaft and the camshaft is one tooth of the cam sprocket, it may be undetectable. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above described conditions, and an object of the present invention is to provide a four-cycle engine that is capable of detecting phase difference between a crankshaft and a camshaft with relatively high accuracy, and a system for detecting the phase difference of the four-cycle engine. 
   According to one aspect of the present invention, there is provided a four-cycle engine comprising a crankshaft provided with a first gear; a camshaft provided with a second gear; an endless rotation transmission that is installed around the first and second gears and is configured to transmit rotation of the crankshaft to the camshaft; a crank phase detecting device configured to detect a rotational phase of the crankshaft that is obtained by dividing a phase corresponding to one rotation of the crankshaft by a number that is equal to or more than a half of teeth of the second gear of the camshaft; and a cam phase detecting device configured to detect at least one rotational phase of the camshaft. 
   In such a configuration, the phase difference between the crankshaft and the camshaft is detected with high accuracy based on a signal detected by the crank phase detecting device and a signal detected by the cam phase detecting device. Since the crank phase detecting device is configured to detect the rotational phase of the crankshaft that is obtained by dividing a phase corresponding to one rotation of the crankshaft by a number that is equal to or more than a half of the teeth of the second gear of the camshaft, and the camshaft typically rotates once while the crankshaft rotates twice, the crank phase detecting device is capable of detecting the phase difference of one tooth of the second gear of the camshaft. The phase difference is detected in such a manner that a computing device may be communicatively coupled to the engine and may analyze the signal from the crank phase detecting device and the signal from the cam phase detecting device. 
   The crank phase detecting device may include a pulser rotor provided at a peripheral region thereof with a plurality of protrusions arranged in a circumferential direction thereof and a crank angle sensor configured to detect the protrusions, and the protrusions of the pulser rotor may be arranged at a predetermined pitch angle that is equal to or less than twice as large as a pitch angle of the teeth of the second gear of the camshaft. 
   The crank phase detecting device is easily configured by using the pulser rotor and the crank angle sensor. By arranging the protrusions at the peripheral region of the pulser rotor at the predetermined pitch angle that is equal to or less than twice as large as the pitch angle of the teeth of the second gear of the camshaft, the crank phase detecting device is able to detect the phase difference corresponding to one tooth of the second gear as described above. 
   Two adjacent protrusions of the plurality of protrusions of the pulser rotor may be spaced apart from each other to have a pitch angle that is equal to or more than twice as large as the predetermined pitch angle. In such a configuration, the protrusions arranged on the pulser rotor are numbered, assuming that the protrusions, spaced apart from each other to have a larger pitch angle, are reference protrusions. By comparing the signal from crank phase detecting device to the signal from the cam phase detecting device, information indicating a number representing an advanced angle or a retarded angle corresponding to the phase difference is obtained. It shall be understood that the information indicating how the protrusions are numbered or the number representing the advanced or retarded angle is obtained by analysis in the computing device communicatively coupled to the engine. 
   The cam phase detecting device may include a rotor having at least one protrusion at a peripheral region thereof and a cam angle sensor configured to detect the protrusion of the rotor. Thus, the cam phase detecting device may be easily configured by using the rotor and the cam angle sensor. 
   The cam phase detecting device may include an air-intake pressure sensor configured to detect an air-intake pressure of the engine. Thus, the cam phase detecting device may be easily manufactured to include an air-intake pressure sensor. In this case, the phase of the camshaft is detectable by detecting a rising of the air-intake pressure or the like. In an engine equipped with the air-intake pressure sensor configured, for example, to set suitable ignition timings, an undesirable increase in the number of components may be avoided. 
   The camshaft may include a first camshaft configured to drive an intake valve and a second camshaft configured to drive an exhaust valve. The cam phase detecting device may include a first cam phase detecting device configured to detect at least one rotational phase of the first camshaft and a second cam phase detecting device configured to detect at least one rotational phase of the second camshaft. With such a configuration, the phase difference between the crankshaft and the camshaft is detectable with high accuracy in a DOHC four-cycle engine. 
   The first cam phase detecting device and the second cam phase detecting device may be each configured to include a rotor having at least one protrusion at a peripheral region thereof and a cam angle sensor configured to detect the protrusion of the rotor. With such a configuration, the cam phase detecting device for detecting the phase of the camshaft for driving the intake valve is easily configured by using the rotor and the cam angle sensor in the DOHC type four-cycle engine. 
   The first cam phase detecting device may include an air-intake pressure sensor configured to detect an air-intake pressure of the engine, and the second cam phase detecting device may include a rotor having at least one protrusion at a peripheral region thereof and a cam angle sensor configured to detect the protrusion of the rotor. With such a configuration, the phase of the camshaft for driving the intake valve is detected by using the air-intake pressure sensor and the phase of the camshaft for driving the exhaust valve is detected by using the rotor and the cam angle sensor. 
   The first and second gears may include sprockets and the endless rotation transmission may include a chain. The first and second gears may include toothed pulleys and the endless rotation transmission may include a toothed belt. 
   According to another aspect of the present invention, there is provided a system for detecting a phase difference in a four-cycle engine, comprising a four-cycle engine including a crankshaft provided with a first gear; a camshaft provided with a second gear; an endless rotation transmission that is installed around the first and second gears and is configured to transmit rotation of the crankshaft to the camshaft; a crank phase detecting device configured to detect a rotational phase of the crankshaft that is obtained by dividing a phase corresponding to one rotation of the crankshaft by a number that is equal to or more than a half of teeth of the second gear of the camshaft; and a cam phase detecting device configured to detect at least one rotational phase of the camshaft; and a phase difference detecting device configured to detect a phase difference between the crankshaft and camshaft based on a signal received from the crank phase detecting device and a signal received from the cam phase detecting device. 
   In such a configuration, the phase difference between the crankshaft and the camshaft may be detected with high accuracy in the four-cycle engine. 
   The phase difference detecting device may be configured to compare a phase of the camshaft that is predetermined with respect to the signal from the crank phase detecting device to a phase of the camshaft that is indicated by the signal from the cam phase detecting device to thereby detect the phase difference of the camshaft with respect to the crankshaft. 
   The phase difference detecting device may include a display configured to display the phase difference of the camshaft in a form of a numeric value of advanced or retarded teeth of the second gear of the camshaft. In such a configuration, the phase difference is detectable accurately in assembling of the engine. 
   The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view showing a construction of an entire personal watercraft according to an embodiment of the present invention; 
       FIG. 2  is a plan view of the personal watercraft of  FIG. 1 ; 
       FIG. 3  is a front view of a construction of an engine mounted in the personal watercraft of  FIG. 1 , a part of which is cut away to illustrate a construction of a valve system; 
       FIG. 4  is a view showing a structure of a pulser rotor equipped in the engine of  FIG. 3 ; 
       FIG. 5  is a view schematically showing a configuration of a system for detecting a phase difference between a crankshaft, and camshafts respectively configured to drive an intake valve and an exhaust valve; 
       FIG. 6  is a flowchart showing an example of an operation of a phase difference detecting device included in the system of  FIG. 5 ; 
       FIG. 7  is a timing chart showing examples of a signal output from a crank angle sensor and input to the phase difference detecting device of  FIG. 5  and a signal output from a cam angle sensor and input to the phase difference detecting device; 
       FIG. 8  is a timing chart showing examples of a signal output from the crank angle sensor and input to the phase difference detecting device of  FIG. 5  and a signal output from the air-intake pressure sensor and input to the phase difference detecting device; 
       FIG. 9  is an enlarged front view of another engine configured to detect rotational phases of the camshafts using the rotor and the cam angle sensor, showing a region surrounding a cylinder head; and 
       FIG. 10  is a front view of a construction of another engine including pulleys and a timing belt, a part of which is cut away to illustrate a construction of a valve system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, an embodiment of a four-cycle engine will be described with reference to the drawings. By way of example, a four-cycle engine mounted in a water-jet propulsion personal watercraft will be described. As used herein, the term “rightward” and “leftward” refers to rightward and leftward as a body of the watercraft is viewed from rear. 
   Turning now to  FIGS. 1 and 2 , a body  1  of the watercraft includes a hull  2  and a deck  3  covering the hull  2  from above. A line at which the hull  2  and the deck  3  are connected over the entire perimeter thereof is called a gunnel line  4 . In  FIG. 1 , the gunnel line  4  is located above a waterline L (indicated by two-dotted line in  FIG. 1 ) of the personal watercraft in a state and extends substantially in parallel with the waterline L. 
   As indicated by a broken line of  FIG. 2 , a deck opening  3 A, which has a substantially rectangular shape as seen from above, is formed at a substantially center section of the deck  3  in the upper portion of the body  1  such that its longitudinal direction corresponds with the longitudinal direction of the body  1 . A seat  7  is removably mounted over the deck opening  3 A. An engine room  6  is provided in a space defined by the hull  2  and the deck  3  below the deck opening  3 A. In the engine room  6 , a four-cycle engine (hereinafter referred to as an engine) E is mounted. 
   As shown in  FIG. 1 , the engine E is mounted such that a center axis of a crankshaft  10  extends along the longitudinal direction of the body  1 . A rear end of the crankshaft  10  is coupled to a pump shaft  12  of the water jet pump P through a propeller shaft  11 . Therefore, the crankshaft  10  is configured to rotate integrally with the pump shaft  12 . An impeller  13  is attached on the pump shaft  12 . The impeller  13  is covered with a cylindrical pump casing  15  on the outer periphery thereof. 
   A water intake  16  is provided on the bottom of the hull  2 . The water is taken in through the water intake  16  and is fed to the water jet pump P through a water passage  17 . The water jet pump P causes the impeller  13  to pressurize and accelerate the water and then causes fairing vanes  14  to guide the water behind the impeller  13 . The water jet pump P ejects the water through a pump nozzle  18  having a cross-sectional area that is gradually reduced rearward, and from an outlet port  19  provided on the rear end of the pump nozzle  18 . As the resulting reaction, the personal watercraft obtains a propulsion force. 
   As shown in  FIGS. 1 and 2 , a bar-type steering handle  20  is connected to a steering nozzle  21  positioned behind the pump nozzle  18  through a cable (not shown). The steering nozzle  21  is pivotable rightward and leftward around a pivot (not shown). The steering handle  20  cooperates with the steering nozzle  21 . When the rider rotates the handle  20  clockwise or counterclockwise, the steering nozzle  21  pivots toward the opposite direction so that the ejection direction of the water being ejected through the pump nozzle  21  can be changed, and the watercraft can be correspondingly turned to any desired direction while the water jet pump P is generating the propulsion force. 
   As shown in  FIG. 1 , a bowl-shaped reverse deflector  23  is provided on an upper portion on the rear side of the steering nozzle  21  such that it is vertically pivotable around a pivot shaft  24  that is oriented horizontally. As shown in  FIGS. 1 and 2 , a reverse switching lever  27  is attached to the body  1  in front of the right handle  20  and is configured to switch between forward travel and rearward travel. 
     FIG. 3  is a front view of a construction of the engine E mounted in the personal watercraft of  FIG. 1 , a part of which is cut away to illustrate a construction of a valve system. The engine E illustrated in  FIG. 3  is a double overhead camshaft (DOHC) type four-cycle four-cylinder engine. 
   As shown in  FIG. 3 , the engine E includes a crankcase  31  that is provided with an oil pan  30  on a lower portion thereof and is divided into two parts vertically arranged, a cylinder block  32  connected to an upper portion of the crankcase  31  and configured to accommodate a piston (not shown) reciprocatable therein, a cylinder head  33  that is connected to an upper portion of the cylinder block  32  and is configured to substantially accommodate a camshaft (first camshaft)  55  configured to drive an intake valve  55 C for taking in air and a camshaft (second camshaft)  56  configured to drive an exhaust valve  56 C for exhausting a gas, and a cylinder head cover  34  provided to cover the cylinder head  33  from above. 
   Engine mounts  31 A are mounted to right and left outer wall portions of the crankcase  31 . The engine E is mounted to the body  1  (see  FIG. 1 ) in such a manner that the engine mounts  31 A are fastened to an inner bottom surface of the hull  2  (see  FIG. 1 ) with dampers (not shown) sandwiched between them. The engine E is placed within the engine room  6  (see  FIG. 2 ) in such a manner that each cylinder  35  including the cylinder block  32 , the cylinder head  33 , and so on is oriented to extend vertically. 
   An oil pump  40  is housed in the oil pan  30 . An oil filter  41  is attached to the right outer wall portion of the crankcase  31  and configured to remove unwanted substances from the oil. The oil pump  40  pumps oil from the oil pan  30  to flow the oil into engine components of the engine E through the oil filter  41  and oil paths (not shown). A breather pipe  42  extends outward from an upper portion of the cylinder head cover  34  and then downward along a left wall portion of the engine E, and is connected to an oil separator  43  secured to a left wall portion of the cylinder block  32  by fasteners. Oil mist generated in a cam chamber formed inside the cylinder head cover  34  is guided to the oil separator  43  through the breather pipe  42 , and is separated into liquid oil and a gas therein. An air-intake pipe  44  extends outward from a right wall portion of the cylinder head  33 . Air is taken into the engine room  6  (see  FIG. 2 ) from outside the watercraft and is guided to a combustion chamber (not shown) of the engine E through the air-intake pipe  44 . 
   A front portion of the crankcase  31 , a front portion of the cylinder block  32 , a front portion of the cylinder head  33 , and a front portion of the cylinder head cover  34  respectively have double-walled structures. A chain tunnel  36  is formed between wall portions of the double-walled structure and configured to extend vertically. In  FIG. 3 , an outer (front) wall portion is omitted from the wall portions of the double-wall structure forming the chain tunnel  36  to illustrate an internal structure of the chain tunnel  36 . 
   The crankshaft  10  is housed in the crankcase  31 . A front end portion  10 A of the crankshaft  10  extends through a rear wall portion  36 A of the chain tunnel  36  and protrudes into the chain tunnel  36 . Two crank sprockets (first gear)  50  each having a plurality of teeth  50 A (17 teeth in this embodiment) are mounted on the front end portion  10 A of the crankshaft  10  and are configured to rotate integrally with the crankshaft  10 . In  FIG. 3 , only an outer (front) crank sprocket is illustrated. A drive pump sprocket  45  is mounted on the oil pump  40  mounted within the oil pan  30 . A pump drive chain  46  is installed around the inner (rear) crank sprocket  50  and the pump sprocket  45 . The oil pump  40  is driven in cooperation with rotation of the crankshaft  10 . 
   The camshaft  55  configured to drive the intake valve  55 C and the camshaft  56  configured to drive the exhaust valve  56 C are positioned between an upper portion of the cylinder head  33  and a lower portion of the cylinder head cover  34 . The camshafts  55  and  56  are mounted in such a manner that their axial direction is parallel to the longitudinal direction of the crankshaft  10  and the camshaft  55  is located on the right side of the camshaft  56 . The camshaft  55  and the camshaft  56  are provided with a cam  55   b  and a cam  56 B corresponding to each cylinder  35  of the engine E, respectively. The cams  55 B and  56 B drive the intake valve  55 C and the exhaust valve  56 C (indicated by broken lines of  FIG. 3 ), causing intake and exhaust ports (not shown) of the engine E to open and close. 
   A front end portion  55 A of the camshaft  55  extends through the rear wall portion  36 A of the chain tunnel  36  and protrudes into the chain tunnel  36 . A cam sprocket (second gear)  57  is mounted on the front end portion  55 A of the camshaft  55  and is configured to rotate integrally with the camshaft  55 . A front end portion  56 A of the camshaft  56  extends through the rear wall portion  36 A of the chain tunnel  36  and protrudes into the chain tunnel  36 . A cam sprocket (second gear)  58  is mounted on the front end portion  56 A of the camshaft  56  and is configured to rotate integrally with the camshaft  56 . 
   The cam sprocket  57  and the cam sprocket  58  of this embodiment are of a disc shape. Teeth  57 A having 34 teeth and teeth  58 A having 34 teeth which are twice as many as 17 teeth of the crank sprocket  50  are respectively formed at peripheral regions of the cam sprocket  57  and the cam sprocket  58  in such a manner that they are arranged at equal intervals in the circumferential direction of the sprockets  57  and  58  so as to protrude radially outward. A timing chain (endless rotation transmission)  60  is installed around the outer (front) crank sprocket  50 , the cam sprocket  57 , and the cam sprocket  58  in mesh with the teeth  50 A,  57 A, and  58 A. In this construction, the rotation of the crankshaft  10  is transmitted through the timing chain  60 , causing the camshaft  55  and the camshaft  56  to rotate. In the engine E of this embodiment, the crankshaft  10  rotates clockwise in  FIG. 3 , causing the timing chain  60 , the camshaft  55 , and the camshaft  56  to rotate clockwise. 
   A movable chain slack guide  61  and a fixed chain guide  62  are mounted in the interior of the chain tunnel  36 . The chain slack guide  61  vertically extends on the right side of the timing chain  60 . The chain slack guide  61  is pivotally mounted at a lower end portion thereof to a region of a wall of the crankcase  31  near and above the crank sprocket  50 . An upper end portion of the chain slack guide  61  is positioned near and below the cam sprocket  57 . A tensioner  65  is mounted on a right wall portion of the cylinder head  33  and is configured to bias an upper portion of the chain slack guide  61  to the left. The tensioner  65  supports the timing chain  60  from the right with the chain slack guide  61  interposed between them, to apply a suitable tension to the timing chain  60 . 
   The fixed chain guide  62  vertically extends on the left side of the timing chain  60  in the interior of the chain tunnel  36  from a position near the left side of the crank sprocket  50  to a position below and near the cam sprocket  58 . The chain guide  62  supports the timing chain  60  from the left by a groove (not shown) formed on a right side portion thereof to extend in a longitudinal direction thereof. A left portion of the timing chain  60  is accommodated in the groove of the chain guide  62 . The timing chain  60  is movable along the groove. 
   A crank phase detecting device  70  is mounted in the vicinity of the front end portion  10 A of the crankshaft  10  in the interior of the chain tunnel  36  and is configured to detect a rotational phase of the crankshaft  10 . The crank phase detecting device  70  includes a pulser rotor  71  configured to rotate integrally with the crankshaft  10 . The pulser rotor  71  is of a disc shape and is provided with a plurality of protrusions  72  at a peripheral region thereof. The crank phase detecting device  70  further includes a crank angle sensor  73  attached to the rear wall portion  36 A of the chain tunnel  36  in the interior of the chain tunnel  36 . The crank angle sensor  73  is positioned close to the peripheral region of the pulser rotor  71 . The crank angle sensor  73  is configured to detect a distance between the sensor  73  and the peripheral region of the pulser rotor  71  rotatable integrally with the crankshaft  10 , and to output a signal P (see  FIGS. 7 and 8 ) having a pulse each time the crank angle sensor  73  detects that the protrusion  72  is present in its front. 
     FIG. 4  is a view showing a structure of the pulser rotor  71 . As shown in  FIG. 4 , a number of (22 in this embodiment) protrusions  72  of a substantially rectangular shape are formed at the peripheral region of the pulser rotor  71  of a disc shape so as to protrude radially outward. The protrusions  72  are arranged in the circumferential direction of the pulser rotor  71  at an equal pitch angle of 15 degrees, except two predetermined adjacent protrusions  72   a  and  72   b  arranged to be spaced apart from each other at an angle of 45 degrees, which is three times as large as 15 degrees (obtained by dividing 360 degrees by 24). A recess  74   a  is positioned between the protrusion  72   a  (located counterclockwise relative to the protrusion  72   b ) and a protrusion  72   c  (located adjacent the protrusion  72   a  and counterclockwise relative to the protrusion  72   a ). The recess  74   a  is deeper than the remaining recesses  74  formed between adjacent protrusions  72 . 
   As shown in  FIG. 3 , a cam phase detecting device  80  is mounted in the vicinity of the front end portion  56 A of the camshaft  56  configured to drive the exhaust valve  56 C and is configured to detect a rotational phase of the camshaft  56 . The cam phase detecting device  80  includes a rotor  81  mounted on the front end portion  56 A of the camshaft  56  in the interior of the chain tunnel  36 . The rotor  81  is configured to rotate integrally with the camshaft  56  and is provided with a protrusion  82  at a peripheral region thereof. The cam phase detecting device  80  further includes a cam angle sensor  83  attached to the left wall portion of the cylinder head  33 . The cam angle sensor  83  is positioned a predetermined distance apart from the peripheral region of the rotor  81 . The cam angle sensor  83  is configured to detect a distance between the sensor  83  and the peripheral region of the rotor  81  configured to rotate integrally with the camshaft  56  and to output a signal Q (see  FIG. 7 ) having a pulse each time the cam angle sensor  83  detects that the protrusion  82  is present in its front. 
   An air-intake pressure sensor  85  is attached to the air-intake pipe  44  extending from the right wall portion of the cylinder head  33  and is configured to detect an air pressure in the interior of the air-intake pipe  44 . The air-intake pipe  44  extends rightward and upward from the right wall portion of the cylinder head  33  and is curved at a position to extend downward. The air-intake pressure sensor  85  is attached to an outer region of a curved portion  44 A of the air-intake pipe  44 . The air-intake pressure sensor  85  is configured to detect an air pressure in the interior of the air-intake pipe  44  that vary according to an operation of the intake valve  55 C, and to output a signal R (see  FIG. 8 ) regarding the detected air pressure. 
   A procedure for detecting a phase difference between the crankshaft  10  and the camshaft  55  or the camshaft  56  in the engine E of this embodiment will be described.  FIG. 5  is a view schematically showing a configuration of a system  100  configured to detect the phase difference. As shown in  FIG. 5 , the system  100  is configured in such a manner that the engine E is communicatively coupled to the phase difference detecting device  90  positioned outside the engine E through signal lines. The phase difference detecting device  90  may be incorporated in a computing device, for example, electrical control unit (ECU)  99  (see  FIG. 1 ) which is typically built in the body  1  of the personal watercraft. Alternatively, the phase difference detecting device may be incorporated into a remotely connected computing device that is configured to be linked to each of the sensors  73 ,  83 ,  85 . The computing device may be, for example, a hand-held portable computing device for ease of use in the manufacturing process. 
   The phase difference detecting device  90  includes a processor  91 , RAM  92 , ROM  93 , input interfaces  94  to  96 , and an output interface  97 . The processor  91  computes data loaded from the RAM  92  or the ROM  93  or data externally input through the input interfaces  94  to  96  and outputs computed data. The RAM  92  temporarily stores the computed data from the processor  91  or the data externally input. The ROM  93  contains various programs to enable the processor  91  to operate. 
   The input interface  94  is coupled to the crank angle sensor  73  of the crank phase detecting device  70  through a signal line  94   a . The input interface  95  is coupled to the cam angle sensor  83  of the cam phase detecting device  80  through a signal line  95   a . The input interface  96  is coupled to the air-intake pressure sensor  85  of a cam phase detecting device through a signal line  96   a . A digital display  98  is coupled to the output interface  97  through a signal line  97   a , and is configured to display alphanumeric or other messages in accordance with an instruction from the processor  91 . 
     FIG. 6  is a flowchart showing an example of an operation of the phase difference detecting device  90 . As shown in  FIG. 6 , when the engine E starts-up (S 1 ), the signal P is output from the crank angle sensor  73  and input to the phase difference detecting device  90  through the signal line  94   a  (S 2 ), and the signal Q is output from the cam angle sensor  83  and input to the phase difference detecting device  90  through the signal line  95   a  (S 3 ). Based on the signals P and Q, the phase difference detecting device  90  determines whether or not the phase difference between the crankshaft  10  and the camshaft  56  is correct (S 4 ). 
     FIG. 7  is a timing chart showing an example of the signals P and Q input to the phase difference detecting device  90 . As shown in  FIG. 7 , the signal P from the crank angle sensor  73  generates a pulse P 1  in ON-state for a time period h 1  continuously at intervals of a relatively short time period h 2 , and then turns to OFF-state for a relatively long time period h 3  (h 3 &gt;h 2 ), which is followed by the pulse P 1  generated continuously at intervals of the time period h 2 . 
   Each pulse P 1  is generated each time the crank angle sensor  73  detects that any one of the protrusions  72  of the pulser rotor  71  is present near the sensor  73  during rotation of the crankshaft  10 . The time period h 1  is a time period required for one protrusion of the protrusions  72  to pass through the front of the crank angle sensor  73 . The time period h 2  is a time period required for one protrusion of the protrusions  72  (e.g., protrusion  72   a  of  FIG. 4 ) and its adjacent recess  74  (e.g., recess  74   a  of  FIG. 4 ) to pass through the front of the crank angle sensor  73 . The time period h 3  is a time period that elapses from when the crank angle sensor  73  detects the protrusion  72   b  until the crank angle sensor  73  detects the protrusion  72   a  spaced 45 degrees apart from the protrusion  72   b . As can be seen from  FIG. 7 , a time period h 4  elapses from when the crank angle sensor  73  detects the protrusion  72   a  ( FIG. 4 ) first until the crank angle sensor  73  detects the protrusion  72   a  next, and is equal to a time period required per rotation of the crankshaft  10 . 
   The phase difference detecting device  90  counts each pulse P 1 . Specifically, after detecting that the pulse P is in OFF-state for the time period h 3 , the phase difference detecting device  90  sequentially counts the pulses P 1  generated continuously at intervals of the time period h 2  that is shorter than the time period h 3  in such a manner that the detecting device  90  counts from “1” to “22” corresponding to 22 protrusions of the protrusions  72  of the pulser rotor  71  and assign numbers to each counted protrusion, e.g. “No. 1, No. 2 . . . .” After that, when detecting that the pulse P is in OFF-state for the time period h 3  again, the phase difference detecting device  90  resets the count, and re-counts the pulses P 1 . In this manner, the phase difference detecting device  90  detects the rotational phase of the crankshaft  10  at a pitch angle of 15 degrees. 
   As described above, the phase difference detecting device  90  compares the time period h 2  to the time period h 3  to detect the first pulse P 1  (No. 1 pulse P 1 ), namely, signal waveform generated when the phase difference detecting device  90  detects the presence of the protrusion  72   a  in  FIG. 4 . While the time periods h 2  and h 3  vary according to an engine speed of the engine E, the phase difference detecting device  90  is able to reliably determine that the time period h 2  is shorter than the time period h 3 , because the pulser rotor  71  of the engine E of this embodiment is constructed in such a manner that the protrusions  72   a  and  72   c  are apart from each other at a pitch angle of 15 degrees, while the protrusions  72   a  and  72   b  are spaced apart from each other at a pitch angle of 45 degrees. Furthermore, since the recess  74   a  between the protrusions  72   a  and  72   c  is deeper than the recess  74  between other adjacent protrusions  72 , the waveform corresponding to the recess  74   a  is deeper than the waveform corresponding to the recess  74  in the signal P from the crank angle sensor  73  (see  FIG. 7 ). Since the pulse P 1  corresponding to the protrusion  72   a  has a potential difference between ON-state and OFF-state that is larger than that of other protrusions  72 , the crank angle sensor  73  is able to detect the protrusion  72   a  with high accuracy. 
   The signal Q from the cam angle sensor  83  generates a rectangular pulse Q 1  in ON-state for a time period h A  continuously at intervals of a relatively long time period h B . Each pulse Q 1  is generated each time the cam angle sensor  83  detects that the protrusion  82  of the rotor  81  mounted on the camshaft  56  is present near the sensor  83  during rotation of the camshaft  56 . The time period h A  is a time period required for the protrusion  82  to pass through the front of the cam angle sensor  83 . The time period h B  is a time period required per rotation of the camshaft  56 , and is twice as long as the time period h 4  required per rotation of the crankshaft  10 . 
   The ROM  93  (see  FIG. 5 ) of the phase difference detecting device  90  contains programs to determine whether or not the crankshaft  10  and the camshaft  56  are rotating with a correct phase difference between them, based on the signals P and Q. The step S 4  of  FIG. 6  is implemented by the operation of the processor  91  based on the programs. Upon detecting the pulse Q 1  (rising of the pulse Q 1  in this embodiment) generated in the signal Q from the cam angle sensor  83 , the processor  91  obtains numbers of pulses P 1  generated in the signal P from the crank angle sensor  73 . By way of example, as shown in  FIG. 7 , upon detecting the pulse Q 1  in the signal Q from the cam angle sensor  83  at time t 1 , the processor  91  obtains numbers ( 5 ,  6 ) of the pulse P 1  ( 5 ) and the pulse P 1  ( 6 ) which are generated in the signal P from the crank angle sensor  73  near the time t 1 . Then, the processor  90  determines whether or not the numbers ( 5 ,  6 ) match preset serial numbers. The reason why the processor  90  obtains the serial numbers is that the pulse Q 1  and the pulse P 1  are typically output with a slight time lag. 
   If it is determined that the obtained numbers ( 5 ,  6 ) are smaller than the preset numbers, the processor  90  determines that the phase of the camshaft  56  is ahead of the phase of the crankshaft  10  by the difference in numbers (S 5 ), and sends a predetermined signal to the digital display  98  through the output interface  97  and the signal line  97   a . The digital display  98  receives the signal, and displays a message stating that the phase of the camshaft  56  is ahead of a correct angle, and an advanced angle (S 12 ). According to the message and the advanced angle displayed on the digital display  98 , an operator or the like re-installs the timing chain  60 . In this manner, the camshaft  56  is easily set to have a correct phase difference with respect to the crankshaft  10 . 
   Assuming that the preset numbers are ( 6 ,  7 ), the processor  90  may determine that the phase difference is correct if the cam angle sensor  83  detects the protrusion  82  of the rotor  81  as indicated by a broken line in the signal Q of  FIG. 7  while the crank angle sensor  73  is detecting a sixth protrusion of the protrusions  72  (corresponding to a pulse P 1  ( 6 ) in  FIG. 7 ) and a seventh protrusion of the protrusions  72  (corresponding to a pulse P 1  ( 7 ) in  FIG. 7 ). On the other hand, if the obtained numbers are ( 5 ,  6 ), the processor  90  may determine that the phase of the camshaft  56  is ahead of the phase of the crankshaft  10  by 15 degrees. 
   As described with reference to  FIG. 4 , the engine E of this embodiment is configured such that the protrusions  72  of the pulser rotor  71  are arranged at an equal pitch angle of 15 degrees. Since the cam sprocket  58  rotates once while the pulser rotor  71  rotates twice, the phase difference corresponding to one protrusion of the protrusions  72  of the pulser rotor  71  corresponds to the phase difference of 7.5 (15/2) degrees in terms of the rotational angle of the cam sprocket  58 . The cam sprocket  58  has 34 teeth in total which are arranged at a pitch angle of about 10.5 degrees. Therefore, the system  100  is able to detect the phase difference corresponding to one tooth of the teeth  58 A of the cam sprocket  58 . The digital display  98  of  FIG. 5  is configured to display how many teeth  58 A of the cam sprocket  58  are ahead of or behind the protrusions  72  of the pulser rotor  71 . Thus, the phase difference of the camshaft  56  is accurately detectable. 
   If it is determined that the obtained numbers ( 5 ,  6 ) are larger than the preset numbers in step S 4  of  FIG. 6 , the processor  90  determines that the phase of the camshaft  56  is behind the phase of the crankshaft  10  by the difference in numbers (S 6 ), and sends a predetermined signal to the digital display  98 . The digital display  98  receives the signal and displays a message stating that the phase of the camshaft  56  is behind a correct angle and a retarded angle (S 12 ). 
   If it is determined that the obtained numbers ( 5 ,  6 ) match the preset numbers in step S 4  of  FIG. 6 , the processor  90  determines that the camshaft  56  and the crankshaft  10  are set with a correct phase difference, and then receives the signal R from the air-intake pressure sensor  85  (S 7 ). Based on the signal R from the air-intake pressure sensor  85  and the signal P input from the crank angle sensor  73  in step S 2 , the phase difference detecting device  90  determines whether or not a phase difference between the crankshaft  10  and the camshaft  55  is correct (S 8 ). 
     FIG. 8  is a timing chart showing examples of the signals P and R input to the phase difference detecting device  90 . The signal P in  FIG. 8  is identical to the signal P from the crank angle sensor  73  that is illustrated in  FIG. 7  and therefore, will not be further described. As shown in  FIG. 8 , the signal R from the air-intake pressure sensor  85  has a substantially constant value that continues for a predetermined time period and then varies to form a negative-pressure wave R 1  with a negative peak for a time period h X  by opening and closing the intake valve  85 , which is repeated at intervals of a time period h Y  required per rotation of the camshaft  55 . The time period h Y  is twice as long as the time period h 4  required per rotation of the crankshaft  10 . 
   The ROM  93  (see  FIG. 5 ) included in the phase difference detecting device  90  contains programs to determine whether or not the crankshaft  10  and the camshaft  55  are rotating with a correct phase difference, based on the signals P and R. The step S 8  of  FIG. 6  is implemented by an operation of a processor  91  based on the programs. 
   Upon detecting the presence of the negative-pressure wave R 1  (appropriate points that are references such as a rising of the negative pressure) in the signal R from the air-intake pressure sensor  85 , the processor  91  obtains numbers of the pulse P 1  generated in the signal P from the crank angle sensor  73 . Then, the processor  91  determines whether or not the obtained numbers match preset serial numbers, in the same manner that the phase difference between the crankshaft  10  and the camshaft  56  is detected, which will not be further described. 
   If it is determined that the obtained numbers are smaller than the preset numbers, the processor  91  determines that the phase of the camshaft  55  is ahead of the phase of the crankshaft  10  by the difference in numbers (S 10 ), and sends a predetermined signal to the digital display  98  through the output interface  97  and the signal line  97   a . The digital display  98  receives the signal, and displays a message stating that the phase of the camshaft  55  is ahead of a correct angle, and an advanced angle, based on the received signal (S 12 ). 
   If it is determined that the obtained numbers are larger than the preset numbers, the processor  91  determines that the phase of the camshaft  55  is behind the phase of the crankshaft  10  by the difference in numbers (S 11 ), and sends a predetermined signal to the digital display  98 . The digital display  98  receives the signal, and displays a message stating that the phase of the camshaft  55  is behind a correct angle, and a retarded angle, based on the received signal (S 12 ). 
   If it is determined that the obtained numbers match the preset numbers in step S 8 , the processor  91  determines that the camshaft  55  and the crankshaft  10  are set with a correct phase difference, and repeats the operation in the step S 2  and the following steps. If it is determined that the camshaft  55  and the crankshaft  10  are set with a correct phase difference in step S 8 , the processor  91  may cause the digital display  98  to display a message stating that they are set with the correct phase difference. 
   In the engine E of this embodiment, as described above, the cam sprocket  57  and the cam sprocket  58  have 34 teeth, respectively, while the pulser rotor  71  mounted on the crankshaft  10  have 22 protrusions arranged at a pitch angle of 15 degrees in the circumferential direction thereof. Therefore, the system  100  is able to detect the phase difference corresponding to one tooth of the teeth  57 A and one tooth of the teeth  58 A of the cam sprockets  57  and  58 , respectively. 
   The system  100  is able to detect the phase difference corresponding to one tooth of the teeth  57 A or one tooth of the teeth  58 A with high accuracy so long as the crank phase detecting device  70  is capable of detecting a rotational phase of the crankshaft  10  which is obtained by dividing the phase (360 degrees) corresponding to one rotation of the crankshaft  10  by a number that is equal to or more than a half of the teeth of the cam sprocket (second gear)  57  or the cam sprocket (second gear)  58 . In other words, it is necessary that the pitch angle of the protrusions  72  of the pulser rotor  71  be equal to or less than twice as large as the pitch angle of the teeth  57 A of the cam sprocket  57  or the teeth  58 A of the cam sprocket  58 . 
   While the engine E is configured in such a manner that the rotational phase of the camshaft  55  is detected by using the air-intake pressure sensor  85  and the rotational phase of the camshaft  56  is detected by using the rotor  81  and the cam angle sensor  83 , the rotational phase of the camshaft  55  and the rotational phase of the camshaft  56  may alternatively be detected by using the rotor  81  and the cam angle sensor  83 . 
     FIG. 9  is an enlarged front view showing a region surrounding the cylinder head  33  of an engine E 1  configured to detect the rotational phase of the camshaft  55  and the rotational phase of the camshaft  56  by using the rotor  81  and the cam angle sensor  83 . As shown in  FIG. 9 , the cam phase detecting device  80  is mounted in the vicinity of the front end portion  56 A of the cylinder head  56  and is configured to detect the phase of the camshaft  56  configured to drive the exhaust valve  56 C. The cam phase detecting device  80  includes the rotor  81  and the cam angle sensor  83 . The configuration of cam phase detecting device  80  is similar to that described with reference to  FIG. 3 . 
   A cam phase detecting device  110  is mounted in the vicinity of the front end portion  55 A of the camshaft  55  and is configured to detect the rotational phase of the camshaft  55  configured to drive the intake valve  55 C. The configuration of the cam phase detecting device  110  is similar to that of the cam phase detecting device  80 . The cam phase detecting device  110  includes a rotor  111  mounted on the front end portion  55 A of the camshaft  55  and a cam angle sensor  113  attached to a right wall portion of the cylinder head  33  in the interior of the chain tunnel  36 . The rotor  111  is provided with a protrusion  112  at a peripheral region thereof. The cam angle sensor  113  is configured to detect a distance between the sensor  113  and the peripheral region of the rotor  111  configured to rotate integrally with the camshaft  55  and to output a signal having a pulse to outside each time the sensor  113  detects that the protrusion  112  is present in its front. 
   When the system  100  is configured using the engine E 1 , the cam angle sensor  113  of the cam phase detecting device  110  is coupled to the input interface  96  (see  FIG. 5 ) of the phase difference detecting device  90 . The signal from the cam angle sensor  113  has a waveform identical to that of the signal Q of  FIG. 7 . Therefore, in the engine E 1 , the phase difference detecting device  90  is capable of detecting the phase difference between the crankshaft  10 , and the camshaft  55  and the camshaft  56  with high accuracy. 
   The method of detecting the phase difference in the engine E 1  is identical to that described with reference to  FIGS. 5 to 7 , and will not be further described. In addition, the other configuration and components (not shown) of the engine E 1  are identical to those of the engine E of  FIG. 3 , and will not be further described. 
   While the engine E and the engine E 1  are each configured to include the crank sprocket  50 , the cam sprocket  57 , the cam sprocket  58 , and the timing chain  60 , they may alternatively be configured to include pulleys and a timing belt. 
     FIG. 10  is a front view of a construction of an engine E 2  including the pulleys and the timing belt, a part of which is cut away to illustrate a construction of a valve system. As shown in  FIG. 10 , two crank pulleys (first gear or toothed pulley)  120  each having a plurality of teeth (17 teeth in this embodiment)  120 A are mounted on the front end portion  10 A of the crankshaft  10  and are configured to rotate integrally with the crankshaft  10 . In  FIG. 10 , only the outer (front) crank pulley  10  is illustrated. The oil pump  40  equipped in the interior of the oil pan  30  includes a drive pump pulley  121 . A drive belt  122  is installed around the inner (rear) crank pulley  120  and the pump pulley  121 . The oil pump  40  is driven in cooperation with the rotation of the crankshaft  10 . 
   A cam pulley (second gear or toothed pulley)  125  is mounted on the front end portion  55 A of the camshaft  55  and is configured to rotate integrally with the camshaft  55 . A cam pulley (second gear or toothed pulley)  126  is mounted on the front end portion  56 A of the camshaft  56  and is configured to rotate integrally with the camshaft  56 . 
   The cam pulleys  125  and  126  of this embodiment are of a disc shape and are respectively provided at peripheral regions thereof with teeth  125 A and teeth  126 A. Each of cam pulleys  125  and  126  are provided with 34 teeth, twice as many as the 17 teeth of the crank pulley  120 . The teeth  125 A and the teeth  126 A are arranged at equal intervals in the circumferential direction thereof so as to protrude radially outward. A timing belt (endless rotation transmission or toothed belt)  130  is installed around the outer (front) crank pulley  120 , the cam pulley  125  and the cam pulley  126  in mesh with the teeth  120 A,  125 A, and  126 A. In this construction, the rotation of the crankshaft  10  is transmitted through the timing belt  130 , causing the camshafts  55  and  56  to rotate. In the engine E 2  of this embodiment, the crankshaft  10  rotates clockwise, causing the timing belt  130  and the camshafts  55  and  56  to rotate clockwise. 
   Two tension idler pulleys (hereinafter referred to as tensioners)  131  and  132  are rotatably mounted to a front portion of the cylinder block  32  in the interior of the chain tunnel  36 . The tensioners  131  and  132  are disposed so that their center axes are oriented in the longitudinal direction of the crankshaft  10 . 
   A peripheral portion  131 A of the right tensioner  131  is configured to contact, from rightward, a portion  130 A of the timing belt  130  movable between the crank pulley  120  and the cam pulley  125 , thereby pressing the portion  130 A toward a center of the engine E 2  in the rightward and leftward direction. A peripheral portion  132 A of the left tensioner  132  is configured to contact, from leftward, a portion  130 B of the timing belt  130  movable between the crank pulley  120  and the cam pulley  126 , thereby pressing the portion  130 B toward the center of the engine E 2  in the rightward and leftward direction. The tensioners  131  and  132  apply a suitable tension to the timing belt  130 . 
   As in the engine E of  FIG. 3 , the engine E 2  is equipped with the crank phase detecting device  70  including the pulser rotor  71  and the crank angle sensor  73 , the cam phase detecting device  80  including the rotor  81  and the cam angle sensor  83 , and the cam phase detecting device including the air-intake pressure sensor  85 . The configuration of these components is identical to that of the engine E of  FIG. 3 , and therefore will not be further described. 
   In the engine E 2  thus constructed, the cam pulley  125  and the cam pulley  126  respectively have 34 teeth, and the pulser rotor  71  mounted on the crankshaft  10  has 22 protrusions at a pitch angle of 15 degrees. In such a configuration, the phase difference detecting device  90  may detect the phase difference corresponding to one tooth  125 A of the cam pulley  125  and one tooth  126 A of the cam pulley  126 . 
   The system  100  is able to detect the phase difference corresponding to one tooth  125 A or  126 A with high accuracy so long as the crank phase detecting device  70  is capable of detecting a rotational phase of the crankshaft  10  that is obtained by dividing the phase corresponding to one rotation (360 degrees) of the crankshaft  10  by a number that is equal to or more than a half of the teeth of the cam pulley  125  or the cam pulley  126 . In other words, it is necessary that the pitch angle of the protrusions  72  of the pulser rotor  71  be equal to or less than twice as large as the pitch angle of the teeth  125 A of the cam pulley  125  or the teeth  126 A of the cam pulley  126 . 
   In the configuration for transmitting the rotation from the crankshaft  10  to the camshafts  55  and  56 , an idler sprocket or an idler pulley may be mounted to relay the rotation. The present invention is applicable to the single overhead camshaft (SOHC) type engine as well as the DOHC type engine. 
   While the engines E, E 1 , and E 2  mounted in the personal watercraft have been described in this embodiment, the present invention is applicable to engines for other purposes, such as engines mounted in motorcycles, small four-wheeled automobiles or generators. 
   As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.