Patent Publication Number: US-6213820-B1

Title: Control for watercraft engine

Description:
PRIORITY INFORMATION 
     This application is based on and claims priority to Japanese Patent Application No. 11-44465 filed Feb. 23, 1999, the entire contents of which is hereby expressly corporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to alarm control systems for engines. More specifically, the present invention relates to alarm control systems for lubrication systems of engines of outboard motors. 
     2. Description of Related Art 
     Outboard motors pose unique challenges to engine designers due to their orientation and the rotation of the engines about a tilting and trimming axis during operation. One such challenge involves supplying lubricant to the moving components of the engine during a variety of operating conditions. Because the orientation of the engine changes during use, accurately sensing a level of lubricant remaining in an lubricant pan becomes difficult, if not impossible. Accurate monitoring of the lubricant is desirable to ensure that the engine is not run dry of lubricant because of a leak or a clogged passage. 
     In some outboard motors, the engine has a pressure sensor that detects a decrease in lubricant by evaluating the operating pressure within the lubrication system. If the pressure falls to a level indicative of a malfunction, then a buzzer or other alarm immediately sounds. One difficulty in such sensors is determining whether the low pressure is indicative of an actual problem or, rather, is indicative of a sudden change in operating conditions. For instance, due to the viscous nature of oil as a lubricant, the pressure of the lubricant does not vary as rapidly as engine speed. Accordingly, upon rapid acceleration, the lubricant pressure may incorrectly indicate a low pressure and a nonexistent malfunction. 
     Some engine designers have remedied these false alarm problems by setting the sensor to indicate a problem only when the pressure falls below a minimum pressure that corresponds to an adequate supply of lubricant during idle speed operation. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention includes the realization that when a lubrication alarm system of an outboard motor is configured trigger an alarm only when the oil pressure falls below a predetermined minimum oil pressure for idle speed operation, the engine may operate at higher speeds with inadequate lubrication, thereby reducing the durability and life span of the engine. For example, although an outboard motor may generate sufficient minimum oil pressure at idle so as to prevent a conventional oil pressure alarm system from being triggered, the engine may operate at higher speeds with an inadequate flow rate of oil. This condition may be produced by a number of various causes such as, for example but without limitation, a leak, or a partial or complete blockage of one of the lubricant galleries within the engine. Although the engine may generate sufficient oil pressure at idle, a leak or a blockage within the engine may cause the oil pressure to fall below the appropriate pressure for the corresponding engine speed above idle. Thus, conventional systems do not adequately address the problems associated with lubricant pressure irregularities at engine speeds above idle. 
     A need therefore exists for a lubrication system for an outboard motor which is able to better identify inadequate lubricant pressure at engine speeds above idle and emit appropriate alarms when the lubricant pressure falls below a desired pressure. 
     In accordance with one aspect of the present invention, a lubricant pressure alarm system for use in a marine engine, includes an engine speed sensor, a lubricant pressure sensor, and an alarm threshold calculator configured to calculate a threshold pressure based on the engine speed. The alarm system is configured to emit an alarm signal when a lubricant pressure within the engine falls below the alarm threshold. By providing a lubrication alarm system as such, the present invention ensures that an operator is adequately informed of inadequate lubrication flows during high speed operation. 
     According to another aspect of the invention, the lubrication alarm system is configured so as to emit an alarm if the lubricant pressure within the engine fluctuates above a predetermined fluctuation rate during operation. Thus, by configuring the lubrication alarm system as such, the lubrication system informs an operator of a potential malfunction of the lubrication system. For example, if a vehicle is operated in a rough manner, liquid lubricant in an lubricant pan of the engine may be violently sloshed within the lubricant pan. Such movement of the liquid lubricant may cause the pressure to fluctuate rapidly as the lubricant inlet in the lubricant pan becomes exposed and resubmerged in the liquid lubricant, allowing air to enter the lubricant inlet and the lubricant pump. As the inlet repeatedly becomes exposed above the level of liquid and resubmerged below the liquid lubricant, the pressure in the lubricant system fluctuates due to the air entering the system. Thus, by configuration the lubricant alarm system to emit an alarm when the pressure in the lubricant system fluctuates above a predetermined rate, the operator of the associated vehicle is informed of the interruption in lubricant delivery, and thus may stop the engine or slow the engine speed so as to prevent damage to the engine. 
     Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features of the invention will now be described with reference to the drawings of preferred embodiments of the present lubricant alarm system. The illustrated embodiment of the lubricant alarm system is intended to illustrate, but not to limit the invention. The drawings contain the following figures: 
     FIG. 1 is a perspective view showing a watercraft propelled by an outboard motor constructed in accord a preferred embodiment of the present invention. 
     FIG. 2 is a schematic view showing the outboard motor including an engine. The engine, in part, and an ECU are shown generally in the upper half of the figure. The outboard motor, in part, and the watercraft are shown in the lower half of the figure. The ECU, a power supply system, fuel injection system and lubrication system link the two views together. The outboard motor and associated watercraft are illustrated in phantom. 
     FIG. 3 is an elevational side view of the powerhead of the outboard motor shown in FIG.  2 . An upper and lower protective cowling are shown in section. 
     FIG. 4 is a top plan view of the engine shown in FIG.  3 . The upper protective cowling is detached one half of the lower cowling is omitted. 
     FIG. 5 is a partial sectional view of the engine shown in FIG. 3 illustrating an interior of a crankcase and an lubricant pan of the engine. 
     FIG. 6 is a graph illustrating a relationship between engine speed and lubricant pressure in the 
     FIG. 7 is a schematic representation of a lubrication alarm unit constructed in accordance with embodiment of the present invention. 
     FIG. 8 is a flow diagram of a lubrication system control routine. 
     FIG. 9 is a graph illustrating lubricant pressure over time during a state of operation of an engine. 
     FIG. 10 is a graph illustrating lubricant pressure over time at particular engine speeds of conventional outboard motors. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
     With initial reference to FIG. 1, an outboard motor  10  for powering a watercraft  12  is illustrated. The outboard motor  10  advantageously has a lubrication alarm system arranged and configured in accordance with certain features, aspects, and advantages of the present invention. The outboard motor  10  provides an exemplary environment in which the control system has particular utility. The lubrication alarm system of the present invention may also find utility in applications having engines that experience rapid fluctuations in lubrication system pressures and reservoirs that may experience significant sloshing or reorientation, such as, for example but without limitation, personal watercraft, small jet boats, offroad vehicles, circle track racing vehicles, and heavy construction equipment. 
     With reference to FIG. 2, in the illustrated embodiment, the outboard motor  10  comprises a drive unit  14  and a bracket assembly  16 . Although schematically shown in FIGS. 1 and 2, the bracket assembly  16  comprises a swivel bracket and a clamping bracket. The swivel bracket supports the drive unit  14  for pivotal movement about a generally vertically extending steering axis. The clamping bracket, in turn, is affixed to a transom  18  of the watercraft  12  and supports the swivel bracket for pivotal movement about a generally horizontally extending axis. A hydraulic tilt system can be provided between the swivel bracket and clamping bracket to tilt up or down the drive unit  14 . If this tilt system is not provided, the operator may tilt the drive unit  14  manually. Since the construction of the bracket assembly  16  is well known in the art, a further description is not believed to be necessary to enable those skilled in the art to practice the invention. 
     As used throughout this description, the terms “forward,” “front” and “fore” mean at or to the side of the bracket assembly  16 , and the terms “rear,” “reverse” and “rearwardly” mean at or to the opposite side of the front side, unless indicated otherwise. 
     As seen in FIG. 1, the associated watercraft  12  is a power boat. The watercraft  12  has a hull  20  that defines a deck  22 . A pair of seats  24  are disposed in the forward most area of the deck  22 . One of the seats  24  is provided for the operator and is positioned near a steering wheel  26  that is rotatably mounted on a control mast  28 . The steering wheel  26  is coupled to the bracket assembly  16  of the outboard motor  10  so that the operator can remotely steer the motor  10  to the left and right. 
     With reference to FIGS. 1-3, the drive unit  14  will now be described in detail. The drive unit  14  includes a drive shaft housing  32 , and a lower unit  34 . The power head  30  is disposed atop the drive unit  14  and includes an engine  36 , a top protective cowling  38  and a bottom protecting cowling  40 . The cowlings  38 ,  40 , define a cowling assembly  42 . 
     The engine  36  operates on a four stroke combustion principle and powers a propulsion device. As seen in FIG. 2, the engine  36  has a cylinder block  44 . In the illustrated embodiment, the cylinder block  44  defines four cylinder bores  46  which are generally horizontally extending and spaced generally vertically from each other. As such, the engine  36  is an L4 (in-line 4 cylinder) type. A piston  48  reciprocates in each cylinder bore  46 . It is to be noted that the engine may be of any type (v-type, w-type), may have other numbers of cylinders and/or may operate under other principles of operation (two-cycle, rotary, or diesel principles). 
     A cylinder head assembly  50  is affixed to one end of the cylinder block  44  and defines four combustion chambers  52  with the pistons  48  and the cylinder bores  46 . The other end of the cylinder block  44  is closed with a crankcase member  54  (FIG. 3) defining a crankcase chamber. 
     With reference to FIG. 2, a crankshaft  56  extends generally vertically through the crankcase chamber. The crankshaft  56  is connected to the pistons  48  by connecting rods  58  and rotates with the reciprocal movement of the pistons  48  within the cylinder bores  46 . The crankcase member  54  is located at the forward most position of the power head  30 , and the cylinder block  44  and the cylinder head assembly  50  extend rearwardly from the crankcase member  54 . 
     The engine  36  includes an air induction system  60  and an exhaust system  62 . The air induction system  60  is configured to supply air charges to the combustion chambers  52 . The induction system  60  includes a plenum chamber member  64  which defines a plenum chamber  66  therein. Four main intake passages  68  extend from the plenum chamber  66  to a corresponding number of intake ports  70  formed on the cylinder head assembly  50 . 
     The intake ports  70  are opened and closed by intake valves  72 . When the intake ports  70  are opened, air from the intake passages  68  and intake ports  70  flows into the combustion chambers  52 . 
     The plenum chamber member  64  is positioned on the port side of the crankcase member  54 . The plenum chamber member  64  has an inlet opening (not shown) that opens to the interior of the cowling assembly  42  at its front side. The plenum chamber member  64  functions as an intake silencer and/or a collector of air charges. The air intake passages  68  extend rearwardly from the plenum chamber  66  along the cylinder block  44  and curve toward the intake ports  70 . The respective intake passages  68  are vertically spaced apart from each other. 
     With reference to FIG. 3, the air intake passages  68  are defined by duct sections  74 , throttle bodies  76 , and runners  78 . The duct sections  74  are formed integrally with the plenum chamber member  64 . 
     As shown in FIG. 3, the upper two throttle bodies  76  are integrated with each other. The upper two intake runners  78  are also integrated with each other at their fore portions and then forked into two portions. The lower two throttle bodies  76 , as viewed in FIG. 3, and the corresponding lower two intake runners  78  have the same construction as the upper two throttle bodies  76  and intake runners  78 , respectively. 
     The respective throttle bodies  76  support throttle valves  80  (FIG. 2) therein for pivotal movement about axes  81  (FIG. 4) of valve shafts extending generally vertically. The valve shafts are linked together to form a single valve shaft assembly  82  that passes through the throttle bodies  76 . 
     The throttle valves  80  are operable via a throttle cable  84  (FIG. 3) and a non-linear control mechanism  86 . The throttle cable  84  is connected to a throttle/shift lever  88  (FIG.  1 ) that is positioned aside of the control mast  28 , so as to be operable by an operator of the watercraft  12 . 
     With reference to FIG. 3, the non-linear control mechanism  86  includes a first lever  90  and a second lever  92  joined together with each other by a cam connection  94 . The first lever  90  is pivotally connected to the throttle cable  84  and also to a first pin  96  which is affixed to the crankcase member  54 . The first lever  90  has a cam hole  98  at the opposite end of the connection with the throttle cable  84 . The second lever  92  is generally shaped as the letter “L” and pivotally connected to a second pin  100  which is affixed to the crankcase member  54 . The second lever  92  has a pin  102  that reciprocates within the cam hole  98 . The other end of the second lever  92  is connected to a control rod  104 . The control rod  104 , in turn, is pivotally connected to a lever member which is connected to the throttle valve shaft assembly  82  via a torsion spring  106  that urges the control rod  104  to the position shown in FIG.  3 . At this position of the control rod  104 , the throttle valve  80  is in a closed position wherein almost no air charge can pass through the air intake passages  68 . 
     When the throttle cable  86  is operated by the throttle/shift lever  88 , the first lever  90  pivots about the first pin  96  in a counter-clockwise direction, as viewed in FIG.  3 . The second lever  92 , then pivots about the second pin  100  in a clockwise direction. Since the cam follower pin  102  of the second lever  92  reciprocates in the cam hole  98 , the second lever  92  moves according to the shape of the cam hole  98 . Thus, the second lever  92  pushes the control rod  104  against the bias force of the torsion spring  106  to open the throttle valves  80 . When the throttle cable  84  is released, the control rod  104  returns to the initial position by the biasing force of the spring  106  and the throttle valves  80  are closed again. 
     A throttle valve position sensor  108  is arranged atop of a throttle valve shaft assembly  82 . A signal from the position sensor  108  is sent to an ECU  110  via a throttle position data line  112  for use in controlling various aspects of engine operation including, for example, but without limitation, fuel injection control which will be described later. The signal from the throttle valve position sensor  108  corresponds to the engine load in one aspect as well as the throttle opening. The ECU  110  is mounted on the left side of the engine  36  and also will be described in detail later. 
     The air induction system  60  further includes a bypass passage or idle air supply passage that bypasses the throttle valves  80 , although it is omitted in FIG.  3 . The engine  36  also preferably includes an idle air adjusting unit (not shown) which is controlled by the ECU  110 . 
     With reference to FIG. 3, the cowling assembly  42  generally completely encloses the engine  36 . The upper cowling  38  is detachably affixed to the bottom cowling  40  so that an operator can access the engine  36  for maintenance or other purposes. The upper cowling  38  has an air intake compartment  110  defined between a top surface  112  of the upper cowling  38  and cover members  114 . Each air intake compartment  110  has an air inlet duct  116  that connects the space in the compartment  110  and the interior of the cowling assembly  42 . 
     In operation, air is introduced into the air intake compartments  110  and enters the interior of the cowling assembly  42  through the air inlet ducts  116 . The air then passes through the inlet opening of the plenum chamber member  64  and enters the plenum chamber  66 . During idle of the engine  36 , an air charge amount is controlled by the throttle valves  80  to meet the requirements of the engine  36 . The air charge then flows through the runners  78  and to the intake ports  72  (FIG.  2 ). 
     As described above, the intake valves  72  are provided at the intake ports  70 . When the intake valves  72  are opened, the air is supplied to the combustion chambers  52  as an air charge. Under the idle running condition, the throttle valves  80  are generally closed. The air, therefore, enters the ports  70  through the idle air adjusting unit (not shown) which is controlled by the ECU  110 . The idle air charge adjusted in the adjusting unit is then supplied to the combustion chambers  52  via the intake ports  70 . 
     The exhaust system  62  is configured to discharge burnt charges or exhaust gasses outside of the outboard motor  10  from the combustion chambers  52 . Exhaust ports  118  are defined in the cylinder head assembly  50  and are opened and closed by exhaust valves  120 . When the exhaust ports  118  are opened, the combustion chambers  52  communicate with a single or multiple exhaust passages  122  which lead the exhaust gasses downstream through the exhaust system  62 . 
     An intake camshaft  124  and an exhaust camshaft  126  are provided to control the opening and closing of the induction valve  72  and exhaust valves  120 , respectively. The camshafts  124 ,  126  extend approximately vertically and parallel with each other. The camshafts  124 ,  126  have cam lobes that act against the valve  72 ,  120 , at predetermined timings to open and close the respective ports. The cam shafts  124 ,  126  are journaled on the cylinder head assembly  50  and are driven by the crankshaft  56  via a camshaft drive unit. In the illustrated embodiment, the camshaft drive unit is positioned at the upper end of the engine  36 , as viewed in FIG.  3 . 
     With reference to FIG. 4, the camshaft drive unit includes sprockets  128 ,  130  mounted to an upper end of the camshafts  124 ,  126 . The crankshaft  56  also includes a sprocket  132  at an upper end thereof. A timing belt or chain  134  is wound around the sprockets  128 ,  130 ,  132 . As the crankshaft  156  rotates, the cam shafts  124 ,  126  are thereby driven. 
     With reference to FIG. 2, the engine  36  also includes a fuel injection system  136 . The fuel injection system  136  includes four fuel injectors  138  which have injection nozzles exposed to the intake ports  70  so that injected fuel is directed toward the combustion chambers  52 . A main fuel supply tank  140  is part of the fuel injection system and is placed in the hull  20  of the associated watercraft  12 . Although any place on the deck  22  is available, in the illustrated embodiment, the fuel tank  140  is positioned at a rear left side of the deck  22 . 
     Fuel is drawn from the fuel tank  149  by a first low pressure pump  142  and a second low pressure pump  144  through a first fuel supply conduit  146 . The first low pressure pump  142  is a manually-operated pump. The second low pressure pump  144  is a diaphragm-type pump operated by one of the intake and exhaust camshafts  124 ,  126 . In the illustrated embodiment, the second low-pressure fuel pump  144  is mounted on the cylinder head assembly  50  (FIG.  3 ). 
     A quick disconnect coupling (not shown) is preferably provided in the first fuel conduit  146 . A fuel filter  148  is positioned in the conduit  146  at an appropriate location. 
     From the low pressure pump  144 , fuel is supplied to a vapor separator  150  through a second fuel supply conduit  152 . In the illustrated embodiment, the vapor separator  150  is affixed to the lower two intake runners  78 , as viewed in FIG.  3  and between the intake runner  78  and the cylinder block  44 . At the vapor separator end of the conduit  152 , a float valve is provided which is operated by a float  154  so as to maintain a uniform level of the fuel contained in the vapor separator  136 . 
     A high pressure fuel pump  156  is provided within the vapor separator  136  and pressurizes fuel within the vapor separator  150 . The high-pressure fuel pump  156  is connected with the fuel injectors  138  through a fuel delivery conduit  158 . Preferably, the conduit  158  itself forms a fuel rail connecting the fuel injectors  158  with the high-pressure fuel pump  156 . The high-pressure fuel pump  156  is driven by an electric motor  160  that is directly connected to the pump  156  at its lower end, as viewed in FIG.  2 . The electric motor  160  is activated by the ECU  110  and is controlled via a fuel pump control line  162 . 
     A fuel return conduit  164  is also provided between the fuel injectors  138  and the vapor separator  150 . Excess fuel that is not injected by the injector  138  returns to the vapor separator  150  through the conduit  164 . A pressure regulator  166  is mounted on the vapor separator  150  at the end of the return conduit  164  to limit the pressure of the fuel delivered to the fuel injectors  138 . The flow generated by the return of unused fuel from the fuel injectors aids in cooling the fuel injectors. 
     In operation, a predetermined amount of fuel is sprayed into the intake ports  70  via the injection nozzles of the fuel injectors  138 . The timing and duration of the fuel injection is dictated by the ECU  110 . The fuel charge delivered by the fuel injectors  138  then enters the combustion chambers  52  with an air charge at the moment the intake valves  72  are opened. Since the fuel pressure is regulated by the pressure regulator  166 , a duration during which the nozzles of the injectors  138  are opened is a factor determined by the ECU  110  to measure an amount of fuel to be injected by the fuel injectors  138 . The duration and the injection timing are thus controlled by the ECU  110  through a fuel injector control line  168 . Preferably, the fuel injectors  138  are operated by solenoids  170 , as is known in the art. Thus, the fuel injector control line  168  signals the solenoids  170  to open according to the timing and duration determined by the ECU  110 . 
     The engine  36  further includes an ignition system, indicated generally by the reference numeral  172 . Four spark plugs  174  are fixed on the cylinder head assembly  50  and exposed into the respective combustion chambers  52 . The spark plugs  174  ignite an air/fuel charge at a certain timing as determined by the ECU  110  to burn the air/fuel charge therein. For this purpose, the ignition system  172  includes an ignition coil  176  interposed between the spark plugs  174  and the ECU  110 , along a spark plug control line  178 . 
     As seen in FIGS. 3 and 4, a flywheel assembly  180  is affixed to an upper end of the crankshaft  56 . A cover member  182  covers the flywheel assembly  180 , sprockets  128 ,  130 ,  132 , and the belt  134  so as to prevent debris and/or other foreign materials from becoming entrained in the sprockets  128 ,  130 ,  132  and to protect an operator from the moving components when the upper cowling  38  is removed. The flywheel assembly  180  includes an AC generator that generates electric power. The generated AC power is led to a battery  184  (FIG.  2 ), through a rectifier that rectifies the AC power to DC power. The battery  184  accumulates electrical energy therein and also supplies it to electrical equipment including the ECU  110 , solenoids  170 , and ignition coil  176 . 
     A negative pole  186  of the battery  184  is grounded, while the positive pole  188  is coupled to the ECU  110 , the solenoids  170 , and the ignition coil  176  through a power supply line  190 . A main relay  192  is provided between the power supply line  190  and the ECU  110 . A main switch  194  is provided to activate the main relay  192  and the ECU  110 . Preferably, the main switch  194  is placed on the control mast  28  at the right-hand side of the steering wheel  26  (FIG. 1) so as to be easily accessible by an operator. 
     While not illustrated, the engine  36  also can include a recoil starter to drive the flywheel assembly  180  when starting the engine  36 . A starter motor can be employed in addition or in the alternative to the recoil starter for the same purpose. The use of a starter motor is preferred when the present invention is employed with larger size engines. The recoil starter is operated by an operator of the watercraft  12  when the operator wants to start the engine  36 . For example, the starter motor may be activated when the main switch  194  is actuated by the operator of the watercraft  12 . 
     With reference to FIG. 1, the battery  184  is located in the hull  20  of the associated watercraft  12 . Like the fuel tank  140 , the battery  184  may be placed at any position on the deck  22 , however, in the illustrated embodiment, it is positioned at the rear right side of the deck  22 . Desirably, a display panel  196  is provided forwarded from the steering wheel  26  facing the chair  24  provided behind the steering wheel  26 . Various instruments may be provided in the display panel  196  to provide the operator with various information regarding engine operation, including, for example but without limitation, engine speed, fuel level, lubricant pressure, engine temperature, and watercraft speed. 
     As seen in the lower half of FIG. 2, the driveshaft housing  32  depends from the power head  30  and supports a driveshaft  200  which is driven by the crankshaft  56  of the engine  36 . The driveshaft  200  extends generally vertically through the driveshaft housing  32 . The driveshaft housing  32  also defines internal passages which form portions of the exhaust system  62 . 
     The lower unit  34  depends from the driveshaft housing  32  and supports a propeller shaft  202  which is driven by the driveshaft  200 . The propeller shaft  202  extends generally horizontally through the lower unit  34 . In the illustrated embodiment, the propulsion device includes a propeller  204  that is affixed to an outer end of the propeller shaft  202  and is thereby driven. 
     A transmission  206  is provided between the driveshaft  200  and the propeller shaft  202 . The transmission  206  couples together the two shafts  200 ,  202  which lie generally normal to each other (i.e., at a 90° angle) with bevel gear combination. 
     A switchover mechanism is provided for the transmission  206  to shift rotational directions of the propeller  204  between forward, neutral and reverse. The switchover mechanism includes a shift cam (not shown), a shift rod  208  and shift cable  210  (FIG.  3 ). The shift rod  208  extends generally vertically through the driveshaft housing  32  and the lower unit  34 , while the shift cable  210  extends outwardly from the lower cowling  40  and is connected to the throttle/shift lever  88  that is operable by the operator when the operator wants to shift the transmission directions. 
     The lower unit  34  also defines an internal passage that forms a discharge section of the exhaust system  62 . At engine speed above idle, the majority of the exhaust gasses are discharged to the body of water surrounding the outboard motor  10  through the internal passage and finally through a hub of the propeller  204 . 
     The engine  36  also has a lubrication system  212 , which is schematically represented in FIGS. 2 and 5. The lubrication system  212  is provided for lubricating certain portions of the engine  36 , such as, for example but without limitation, the pivotal joints of the connecting rod  58  with the crankshaft  56  and with the piston  48 , the cam shaft  124 ,  126 , the bearings journaling the crankshaft  56  within the crankcase and the walls of the cylinder bores  46 . 
     A lubricant reservoir  214  is disposed at an appropriate location in the driveshaft housing  32 . Lubricant in the reservoir  214  is drawn therefrom by an lubricant pump  216 . In the illustrated embodiment, the lubricant pump  216  is driven by the crankshaft  56 . However, the lubricant pump  216  may alternatively be driven by the crank shaft  200  or an electric motor (not shown). Lubricant from the lubricant pump  216  is directed to a lubricant supply line  218  and is delivered to various portions of the engine which benefit from circulating lubricant. After the lubricant has passed through the various engine galleries, the lubricant collects in an lubricant pan  219  (FIG. 5) provided at a lower end of the crank case. Lubricant returns to the lubricant pump  216  via a return line  220 . Thus, the lubrication system  212  is formed as a closed loop. As shown in FIG. 5, an exhaust guide  221  is provided at the lower end of the engine  36 , between the pan  219  and the lubricant pump  216 . The operation and control of the lubrication system  212  will be described in more detail below. 
     The outboard motor  10  also includes a cooling system for cooling heated portions in the engine  36  such as the cylinder block  24  and a cylinder head assembly  55 . In the illustrated embodiment, a water jacket  222  is provided in the cylinder block  44 . A water pump  224  is provided for supplying cooling water to the various water jackets which may be included in the engine  36 , including the water jacket  222 . The water pump  224  is driven by the driveshaft  200 . Although not shown, a water inlet is provided in the lower unit  34  to draw cooling water from the body of water surrounding the motor  36 . The water is supplied to the water jackets through a water supply conduit  226 . 
     As noted above, the ECU  110  controls engine operations including fuel injection from the fuel injectors  138  and firing of the spark plugs  174 , according to various control maps stored in the ECU  110 . In order to determine appropriate control scenarios, the ECU utilizes maps and/or indices stored within the ECU  110  with reference to data collected from various sensors. For example, the ECU  110  may refer to data collected from the throttle valve position sensor  108  and other sensors provided for sensing engine running conditions, ambient conditions or conditions of the outboard motor  10  that will affect engine performance. 
     In the illustrated embodiment, there is provided, associated with the crankshaft  56 , plural crankshaft angle position sensors  228  which, when measuring crankshaft angle versus time, output a crankshaft rotational speed signal or engine speed signal to the ECU  110 . The crankshaft position sensors  228  define a pulse generator that produces pulses which are, in turn, converted to an engine speed within the ECU  110  or another separate converter (not shown). 
     A combustion condition or oxygen (O 2 ) sensor  230  senses the in-cylinder combustion conditions by sensing the residual amount of oxygen in the combustion products at a point in time approximately when the exhaust port is opened. The output from the oxygen sensor  230  is output to the ECU  110  via an oxygen sensor data line  231 . 
     A water temperature sensor  232  is connected to the cylinder block  44  so as to communicate with the water jacket  222 . The water temperature sensor  232  is configured to sense the temperature of water flowing through the water jacket  222  and to output a water temperature signal to the ECU  110  via a water temperature data line  234 . 
     A lubricant temperature sensor  236  and a lubricant pressure sensor  238  are connected to the engine at positions appropriate for sensing lubricant temperature and pressure, respectively. For example, with reference to FIG. 5, the lubricant sensor pressure  238  may be positioned so as to communicate with an lubricant gallery  240  which includes a number of branch passages  242  which distribute lubricant flowing therethrough to a plurality of bearings  244  which support and journal the crankshaft  256 . A lubricant temperature data line  246  and a lubricant pressure data line  248  connect the ECU  110  with the lubricant temperature sensor  236  and the lubricant pressure sensor  238 , respectively. 
     The above noted sensors correspond to merely some of those conditions which may be sensed for purposes of engine control and it is, of course, practicable to provide other sensors such as an intake air pressure sensor, intake air temperature sensor, an engine height sensor, a trim angle sensor, a knock sensor, a neutral sensor, a watercraft pitch sensor, a shift position sensor and an atmospheric temperature sensor in accordance with various control strategies. 
     With reference to FIGS. 2 and 5, the lubricant system  212  will be described in more detail. As noted above, the lubricant supply system  212  generally comprises a lubricant pump  214  and a plurality of lubricant passages, conduits and galleries through which lubricant is supplied to various moving components of the engine  36 . The system  212  also includes the lubricant pan  218 , or a return reservoir, such that lubricant may drain from the moving components of the engine  36  and into the pan  218 . While the illustrated embodiment features a lubricant pan  218 , it is anticipated that the present invention may be used with engines featuring a dry-sump arrangement as well as the illustrated wet-sump arrangement. 
     In the illustrated embodiment, lubricant is drawn from within the lubricant pan  218  through a pick-up  250 . As is known in the art, the pick-up  250  may be provided with a mesh straining cover to remove some of the larger impurities prior to being cycled through the lubrication system  212 . 
     Preferably, the lubricant is then pumped by the lubricant pump  214  through a high-pressure pressure regulator, or a pressure regulating valve (not shown). The lubricant is then delivered to the various engine components, including, for example, the bearings  244  in any suitable manner. At an uppermost portion of the lubricant passages in the illustrated embodiment, the lubricant pumped by the pump  214  communicates with the pressure sensor  238 , as illustrated in FIG.  5 . The pressure sensor  238  is preferably configured to generate an output signal which is proportional to the lubricant pressure sensed by the sensor  238 , e.g., as the pressure sensed increases, an output voltage of the sensor  238  also increase proportionately. More preferably, the sensor  238  is configured to generate a signal that has a linear relationship with the lubricant pressure sensed. 
     The lubricant is preferably supplied to the camshafts  124 ,  126  and allowed to return to the lubricant pan  218  via return passages  220 . The sensor  238  also may be positioned in any of a number of other locations along the lubricant passages. 
     As shown in FIG. 5, in the illustrated embodiment, lubricant from the lubricant pump  216  is directed through the supply passage  240  to an lubricant filter  252 . The lubricant filter removes impurities from the lubricant formed therein and returns lubricant to the lubricant gallery  240  for distribution to the various engine components. 
     With reference to FIG. 7, the outboard motor  10  of the present embodiment features an alarm control system  254 . The alarm control system  254  samples signals provided by a variety of sensors adapted to convey information about the engine&#39;s operational condition. In the illustrated embodiment of FIG. 7, the alarm control system  254  samples signals provided by the lubricant pressure sensor  238  and the crank angle position sensors  228 . 
     The alarm control system  254  also includes an alarm pressure threshold calculator  256  which is configured to calculate an alarm pressure threshold. The calculator  256  is preferably configured to generate a pressure threshold P TX  which corresponds to a lubricant pressure which is greater than a minimum lubricant pressure required to protect the engine  36  at the engine speed detected by the crank angle position sensors  228 . 
     As shown in FIG. 7, the alarm pressure threshold calculator  256  receives engine speed data from the crank angle position sensors  228 . The data received from the crank angle position sensors  228  may be in the form of a voltage received directly from the crank angle position sensors  228 , may be converted to an engine speed by the alarm pressure threshold calculator  156  of the ECU  110 , or may be in the form of a digital signal produced by a converter (not shown) which is indicative of a rotational speed of the crankshaft  56 . 
     Preferably, the alarm pressure threshold calculator  256  determines the alarm pressure threshold by comparing the engine speed with a control map stored in the lubricant alarm system  254 . For example, FIG. 6 illustrates a map of alarm threshold pressures as a function of engine speed. The vertical axis of the graph of FIG. 6 indicates lubricant pressure and the horizontal axis indicates engine speed. Line  258  of FIG. 6 indicates a minimum lubricant pressure required to protect the engine  36  over the engine speed range N 1  to N 2 . The minimum required pressure at engine speed N 1  is a lubricant pressure of P R1 . The minimum required lubricant pressure at engine speed N 2  is P R2 . Also shown in FIG. 6 is a line  260  which represents an alarm pressure threshold P TX  which is greater than the minimum lubricant pressure required for a particular engine speed. For example, the alarm threshold pressure P T1  s greater than the minimum required lubricant pressure P R1 . Similarly, the alarm pressure threshold P T2  at engine N 2  is greater than the minimum required lubricant pressure P R2 . 
     As shown in FIG. 6, the vertical difference between the minimum required pressure line  258  and the alarm pressure threshold line  260  remains constant along the length of the lines  258 ,  260  by a distance of ΔP T . However, it is to be noted that the minimum required pressure line  258  may be represented as a curve according to the lubrication requirements of a particular engine. Additionally, the alarm pressure threshold line  260  may be represented as a curve having a nonuniform offset ΔP T  from the minimum required lubricant pressure line  258 . However, regardless of the shape of the alarm pressure threshold line  260 , it is advantageous for the alarm pressure threshold P TX  to be greater than the minimum required lubricant pressure P RX  for any given engine speed. 
     As shown in FIG. 7, the alarm control  254  includes a comparator  256 . The comparator is connected to the alarm pressure threshold calculator  256  and the lubricant pressure sensor  238 . The comparator is configured to compare output from the alarm pressure threshold calculator  256  and the lubricant pressure sensor  238  so as to determine if the lubricant pressure P X  in the engine  36  is less than the alarm pressure threshold P TX  determined by the alarm pressure threshold calculator  256 . The comparator  262  is also connected to an alarm control  264  which may be connected to a visual alarm  266 , an auditory alarm  268  and/or a disable unit  270 . 
     The alarm control  264  may be configured to control the alarms  266 ,  268 ,  270  individually or sequentially. For example, the alarm control  264  may be configured to initiate an alarm by first initializing the visual alarm  256 . The visual alarm  256  may comprise a warning indicator such as an alarm lamp  272  (FIG. 1) mounted in the display panel  196 . Optionally, the alarm control  264  may then initiate the auditory alarm  268  which may comprise an audible tone emitted from a noise generator such as a buzzer (not shown) mounted in the vicinity of the operator&#39;s seat  24  (FIG.  1 ). Further, the alarm control  264  may initialize the disable unit  270  which may be configured to partially or completely disable the engine  36  by causing at least one of the ignition system, the fuel system, or the power system of the engine  36  to partially or completely cease ignition of fual/air charges in the combustion chambers  52 . 
     In operation, the alarm system  254  receives an engine speed signal from at least one of the crank angle position sensors  228  as indicated in FIG.  7 . The alarm pressure threshold calculator  256  then calculates an alarm pressure threshold P TX  based on the speed signal from the crank angle position sensor  228  and based on a control map, such as the map shown in FIG.  6 . 
     The comparator  266  receives the alarm pressure threshold P TX  from the alarm pressure threshold calculator  256  as well as a lubricant pressure signal P X  from the lubricant pressures sensor  238 . The comparator  262  compares the alarm pressure threshold signal P TX  to the lubricant pressure signal P X  and determines whether the lubricant pressure P X  in the engine  36  with the alarm pressure threshold signal P TX . If the lubricant pressure P X  is less than the alarm pressure threshold P TX , the comparator  262  signals the alarm control  264  to initiate an alarm sequence. As noted above, the alarm control  264  may control the alarms  266 ,  268 ,  270 , individually or sequentially. 
     Optionally, the alarm control system  254  may be configured to detect an undesirable fluctuation of lubricant pressure in the lubrication system  212 . For example, with reference to FIG. 9, a lubricant pressure fluctuation in the engine  36  is illustrated therein. The graph of FIG. 9 includes a vertical axis indicating lubricant pressure in the engine  36  and the horizontal axis indicates time. 
     During operation of the engine  36 , lubricant pressure P X  within the engine  36  may fluctuate as a result of the operating conditions. However, certain malfunctions within the engine  36  may cause the lubricant pressure  36  to fluctuate to an undesirable degree. For example, during operation of a watercraft such as the watercraft  12  with the outboard motor  10  attached thereto lubricant within the lubricant pan  218  (FIG. 5) may be splashed within the lubricant pan  218  thereby cause the lubricant to move in and out of contact with the collector  250 . When the collector  250  is not in contact with liquid lubricant, air enters the supply line  218  to the lubricant pump  216  which interrupts a flow of lubricant through the lubrication system. As air bubbles travel through the various engine galleries and conduits within the engine  36 , the lubricant pressure within the engine  36  will fluctuate. For example, as shown in FIG. 9, as air bubbles pass by the lubricant pressure sensor  238 , the lubricant pressure P X  sensed by the lubricant pressure sensor  238  will fluctuate rapidly over time. Additionally, as the air travels through the lubrication system, various components of the engine  36  may be inadequately lubricated. Thus, the alarm control system  254  is desirably configured to detect undesirable fluctuations in the lubricant pressure P X  which may be indicative of inadequate lubrication within the engine  36 . 
     As shown in FIG. 9, the fluctuation in lubricant pressure P X  within the engine  36  is sensed by lubricant pressure sensor  238  over time. For example, at time T 1  the lubricant pressure sensor  238  detects a lubricant pressure P 1  in the engine  36 . Subsequently, the lubricant pressure sensor  238  senses lubricant pressure P 2  at time T 2 , pressure P 3  at time T 3 , and lubricant pressure P 4  at time T 4 . Each fluctuation ΔP F  is defined as the absolute value of the difference from a current lubricant pressure P X  to a previous detected lubricant pressure P (X−1) . For example, a pressure fluctuation ΔP F  from time T 1  to time T 2  would be the absolute value of the difference of P 2  and P 1 , i.e., 
     
       
         
           |P 
           2 
           −P 
           1 
           |=ΔP 
           F 
         
       
     
     It is to be noted that during normal operation of the outboard motor  10 , there will be acceptable fluctuations in lubricant pressure. However, it is preferable that the alarm control system  254  is configured to detect and respond to pressure fluctuations above the predetermined pressure fluctuation alarm threshold ΔP A . 
     Thus, the predetermined pressure fluctuation alarm threshold ΔP A  is set at a pressure difference which would be indicative of inadequate lubricant flow in the engine  36 , such as for example but without limitation, pressure fluctuations caused by air flowing through the lubrication system  212  in the engine  36 . Thus, if a pressure fluctuation occurs in the lubrication system  212 , the alarm control system  254  may initiate an alarm, or may record the fluctuation for further computations. 
     For example, the comparator  262 , or another separate comparator (not shown) may be configured to compare a present lubricant pressure P X  with a previous lubricant pressure P (X−1) . The comparator  262  may calculate the absolute value of the difference between lubricant pressure P X  and lubricant pressure P (X−1) . For example, the comparator  262 , with reference to FIG. 9, may calculate the absolute value of the difference between lubricant pressure P 1  and lubricant pressure P 2  as pressure fluctuation ΔP 1-2 . If the pressure fluctuation ΔP 1-2  is greater than a predetermined pressure alarm threshold ΔP A , the comparator  262  records data indicating a pressure fluctuation greater than the predetermined pressure fluctuation threshold ΔP A  has been exceeded at a time corresponding to the fluctuation, i.e., ΔP 1-2 . 
     Preferably, the comparator  262 , or another component (not shown) of the alarm control system  254  tallies the number of pressure fluctuations which exceed the predetermined pressure fluctuation alarm threshold ΔP A  over a period of time and records the number of such fluctuations as F P . 
     Preferably, the comparator, or another component of the alarm control system  254 , compares the number of unacceptable pressure fluctuations F P  with the predetermined pressure fluctuation rate threshold F PT . The predetermined pressure fluctuation rate threshold F PT  indicates the maximum number of unacceptable pressure fluctuations that may occur for a predetermined period of time. For example, the pressure fluctuation threshold may be set at a rate such as two per second, for example. Thus, if the alarm control system  254  detects more than two unacceptable pressure fluctuations in one second, the alarm control system  254  emits an alarm. 
     For example, if the comparator  262  detects three unacceptable pressure fluctuations in one second, i.e., F P =3, where the predetermined pressure fluctuation rate threshold F PT =2, the comparator  262  will signal the alarm control  264  to emit an alarm. As noted above, the alarm control  264  may operate the alarms  266 ,  268 ,  270  individually or sequentially. 
     The comparator  262  and the alarm pressure threshold calculator  256  may be a comparator, a calculator, a logic circuit board or the like. The illustrated embodiment features visual alarms, auditory alarms, and disabling arrangements. Of course, tactile alarms and other alarms suitable to transmit information regarding an undesirable characteristic of engine performance may be used. Visual alarms may include, without limitation, lights and gauges. Auditory alarms may include, without limitation, buzzers, bells, sirens, and the like. Disabling arrangements may, as will be recognized, selectively disable combustion within selected combustion chambers in order to slow engine speed or completely stop engine operation in any suitable manner. 
     FIG. 8 illustrates a control subroutine  280  for practicing the present alarm scheme for the engine  36 . The control routine  280  is initiated when the engine  36  is running. As shown in FIG. 8, the control routine  280  may start at a step S 1  where it is determined whether the engine is running. If the engine is running, the program moves on to a step S 2 . Alternatively, the control subroutine  280  may operate at all times when the engine  36  is running 
     At the step S 2 , the alarm system  254  reads the engine speed. For example, the alarm system  254 , may receive a signal from the crank angle position sensors  228 , or from a translator which translates the signal from the crank angle position sensors  228  into another signal for further processing by the alarm system  254 . After the alarm system  254  has read the engine speed, N X , the control subroutine  280  moves on to a step S 3 . 
     At the step S 3 , the control subroutine  280  detects the lubricant pressure P X  in the engine  36 . After the lubricant pressure P X  has been detected, the control subroutine  280  moves on to a step S 4 . 
     At the step S 4 , the control subroutine  280  calculates an alarm pressure threshold P TX  based on the engine speed N X , as described above with respect to the alarm control system  254 . After the alarm pressure threshold P TX  has been determined, the control subroutine  280  moves on to a Step S 5 . 
     At the step S 5 , it is determined whether the lubricant pressure P X  is less than the alarm pressure threshold P TX . Alternatively, the control subroutine  280  may determine whether the lubricant pressure P X  is less than or equal to the alarm pressure threshold P TX , as is apparent to one of ordinary skill in the art. If the lubricant pressure P X  is less than the alarm pressure threshold P TX , the control subroutine  280  moves on to a step S 6 . 
     At the step S 6 , the control subroutine initiates an alarm. As noted above, the alarm system  254  may include at least one of a visual alarm  266 , an auditory alarm  268 , and a disable unit  270 . Additionally, the control routine  280  may operate the alarms  266 ,  268 ,  270  individually or sequentially. 
     If, however, at the step S 5 , it is determined that the lubricant pressure P X  is equal to or greater than the alarm pressure threshold P TX , the control subroutine may return to step S 2  and repeat. Alternatively, if it is determined at the step S 5 , that the lubricant pressure P X  is greater than the alarm pressure threshold P TX , the control subroutine may move on to a step S 7 . 
     At the step S 7 , it is determined whether fluctuation of the current engine lubricant pressure P X  has changed from the previously read engine lubricant pressure P (X−1)  more than a predetermined amount ΔP A . Preferably, the predetermined pressure change ΔP A  is set at a pressure change which would be indicative of a lubricant system malfunction, as described above with respect to the alarm control system  254 . Thus, if it is determined that the change in lubricant pressure, e.g., the absolute value of P X −P (X−1)  is greater than ΔP A , the control routine  280  moves on to a step S 8 . 
     At the step S 8 , it is determined whether the engine lubricant pressure has fluctuated more than a predetermined number of times over a predetermined time period. If it is determined, at the step S 8  that the number of lubricant pressure fluctuations F P  above the predetermined pressure differential ΔP A  is greater than the predetermined lubricant pressure fluctuation threshold F PT , the control routine  280  moves on to step S 6  and initiates at least one of alarms  266 ,  268 ,  270 , as noted above. 
     If, however, it is determined at the steps S 7  or S 8  that the requirements stated therein are not satisfied, the control routine  280  returns to step S 2  and repeats. 
     It is to be noted that the alarm control system  254  may be in the form of a hard wired feedback control circuit, as schematically represented in FIG.  7 . Alternatively, the alarm control system  254  may be constructed of a dedicated processor and a memory for storing a computer program configured to perform the steps S 1 -S 8 . Additionally, the alarm control system  254  may be constructed of a general purpose computer having a general purpose processor and the memory for storing the computer program for performing the routine  280 . Preferably, however, the alarm control system  254  is incorporated into the ECU  110 , in any of the above-mentioned forms. 
     By constructing the alarm control system  254  as such, the present invention provides for enhanced prevention of engine damage caused by insufficient lubricant flow. For example, with reference to FIG. 10, lubricant pressure fluctuations during an engine speed fluctuation scenario is shown therein. The graph in FIG. 10 includes lubricant pressure plotted on the left-hand side vertical axis and is plotted as a solid line on the graph. The right-hand side vertical axis of the graph indicates engine speed plotted as a broken line. The horizontal axis of FIG. 10 indicates elapsed time. 
     The graph of FIG. 10 illustrates an example of engine speed fluctuation of a conventional outboard motor. The engine speed of the outboard motor  10  starts at V 1  at time T 0 ′, increases to engine speed S 2  at time T 1 ′, and returns to speed S 1  at time T 2 ′. When the lubrication system of a conventional outboard motor is operating properly, the lubricant pressure P′ increases and decreases proportionally with engine speed V. However, due to the viscous nature of lubricant, the pressure of lubricant does not vary as rapidly as engine speed. For example, as shown in FIG. 10, the curve labeled as P′ A  indicates the lubricant pressure within an outboard motor which is operating properly. Thus, as shown in FIG. 10, lubricant pressure P′ A  increases as the engine speed increases from engine speed S 1  to S 2  and decreases again as the engine speed drops from engine speed S 2  to engine speed S 1 . However, due to the nature of lubricants such as oil, the lubricant pressure P′ A  drops to a minimum point  272  before rising again to a proper lubricant pressure appropriate for the engine speed S 1 . 
     In certain conventional outboard motors, lubricant pressure alarms have been calibrated to emit an alarm if the lubricant pressure drops below a pressure P′ T1 . However, since under normal operation, lubricant pressure within an outboard motor may drop below this threshold down to a minimum point  272  during normal operation, such conventional outboard motors may erroneously emit an alarm when no malfunction is actually present. Thus, other conventional outboard motors have been known to include alarms which are calibrated to emit an alarm only when the lubricant pressure within the engine drops below a pressure P′ T2  which is lower than P′ T1 , thus avoiding the emission of an alarm when the lubricant pressure in the outboard motor drops to a minimum point, such as minimum point  272 . 
     However, one aspect of the present invention involves a realization that lubrication system alarms which only operate so as to emit an alarm when the lubricant pressure within the engine drops below a single predetermined threshold suffer from the drawback that other unacceptable pressure fluctuations may not trigger the lubricant pressure alarm. For example, FIG. 10 illustrates an lubricant pressure drop along line P′ B  where the lubricant pressure in an engine drops rapidly from a normal lubricant pressure along line P′ A  to zero. In this case, an alarm would be sounded in an outboard motor which uses a predetermined alarm threshold pressure P′ T1  or P′ T2 . However, the alarm would not be emitted until lubricant pressure P′ drops below the corresponding thresholds. Thus, for the time period while the lubricant pressure is dropping along line P′ B , the engine will be inadequately lubricated and suffer damage. Additionally, if the lubrication system of the engine experiences a partial lubricant pressure reduction such as illustrated by the line P′ C , the lubricant pressure alarm may not be triggered at all. 
     For example, with a lubricant pressure alarm set at the threshold P′ T2 , a pressure drop along the line P′ C  would not trigger the corresponding alarm. Finally, if a lubricant pressure within an outboard motor fluctuates similarly to the fluctuation illustrated in FIG. 9, without extending below the pressure thresholds P′ T1  or P′ T2  illustrated in FIG. 10, those corresponding alarms would not be triggered, despite the inadequate flow of lubricant through the engine. 
     Thus, by constructing the lubricant pressure alarm control system  254  in accordance with the present invention, undesirable reductions in lubricant pressure within the engine  36  are more accurately identified and an operator is informed more readily regarding undesirable lubricant pressures within the engine, thus enhancing the durability and lifespan of the engine  36 . 
     Of course, the foregoing description is that of certain features, aspects and advantages of the present invention to which various changes and modifications may be made without departing from the spirit and scope of the present invention. Moreover, a watercraft may not feature all objects and advantages discussed above to use certain features, aspects and advantages of the present invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. The present invention, therefore, should only be defined by the appended claims.