Abstract:
The control system establishes an engine warm-up protocol based, at least in part, on elapsed time from engine start. The control system provides a reduced reliance on engine temperature as a basis for determining an appropriate engine idle speed. The control system thus reduces the likelihood of unstable idling conditions when, for example, inadequate cooling water or extremely cold cooling water is being supplied to the engine.

Description:
RELATED APPLICATION  
       [0001]     This application claims priority to Japanese application Serial No. JP2003-188020, filed on Jun. 30, 2003, and JP2004-123202, filed on Apr. 19, 2004, the entire contents of which are hereby expressly incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates marine engines and, more particularly, relates to marine engines used in motors designed for low speed trolling operation.  
         [0004]     2. Description of the Related Art  
         [0005]     Outboard motors frequently propel watercraft while running at an engine speed slightly above or slightly below a neutral idle engine speed. Such operation is commonly called trolling. During trolling, a conventional engine control unit (ECU) for the outboard motor seeks to achieve a target engine idling speed. The ECU may manipulate a secondary air valve that opens and closes an air bypass around the main throttle valve such that the idling engine speed, or trolling engine speed, can be adjusted higher or lower.  
         [0006]     In some instances, the target engine speed is determined based upon a reference engine speed stored in memory and is able to be adjusted based upon operator input. In other words, a reference engine speed is used unless that reference engine speed is increased or decreased by manual input from an operator of the outboard motor.. In many instances, the reference engine speed is determined based upon a detected engine operating temperature with the reference engine speed generally decreasing as the engine operating temperature increases.  
         [0007]     Outboard motors are typically water-cooled. Since watercraft are designed to float upon bodies of water, the surrounding water is a convenient source of cooling water for outboard motors. Thus, open loop cooling systems are common within the industry. The open loop cooling systems, however, sometimes deliver water that is substantially colder than the engine was designed and the colder water can retard the warming up of the engine. In such arrangements, the assumed engine temperature may be higher than the actual engine temperature. Thus, the ECU may be fooled into believed a warmed-up condition has been achieved and may set the idle speed lower than desired for the actual engine operating temperature. The lower idle speed can cause the engine to stall due to the relatively higher than expected friction forces in the engine due to the lower temperature.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, a control system for a marine engine is desired in which stable idle speed operation can be maintained even if the engine has not achieved a truly warmed up operating temperature.  
         [0009]     The preferred embodiments of the present control system for outboard motor have several features, no single one of which necessarily is solely responsible for their desirable attributes. Without limiting the scope of this control system as expressed by the claims that follow, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of the Preferred Embodiments,” one will understand how the features of the preferred embodiments provide advantages, which may include the reducing the likelihood of unstable idling conditions even when the engine temperature changes before the engine is completely warmed-up, the allowance for the watercraft operator to at least increase the engine idle speed without creating unstable idling conditions, even when the engine is not completely warmed-up, the allowance for the watercraft operator to set the target engine idle speed after stable idling conditions have been established, the assurance that the engine warms-up completely regardless of any changes in the engine temperature or in the reference engine idle speed at the engine start, the automatic reset of the input engine idle speed when the engine speed is a predetermined value or higher and the automatic reset of the input engine idle speed when the engine is stopped.  
         [0010]     One aspect of the present invention involves a control system for an outboard motor that comprises an engine. The outboard motor is adapted to propel a watercraft with thrust produced by an engine-driven propeller. The control system comprises an operability sensor and at least one engine idle sensor. The operability sensor is adapted to detect whether the watercraft is operable. The engine idle sensor is adapted to detect whether the engine is idling. The control system further comprises apparatus adapted to determine an elapsed time after an engine start, and apparatus adapted to determine a reference engine idle speed based on the elapsed time after an engine start and to set the reference engine idle speed. The control system further comprises a controller adapted to adjust an engine idle speed during idle speed running based on the reference engine idle speed, when the operability sensor detects that the watercraft is operable and the engine idle sensor detects that the engine is idling.  
         [0011]     Another aspect of the present invention involves a control system for a marine engine. The marine engine comprises an engine body defining at least one cylinder bore in which a piston reciprocates. A cylinder head is secured to a first end of the engine body for closing the cylinder bore. The cylinder head defines, with the piston and the cylinder bore, a combustion chamber. An intake passage is in selective fluid communication with the combustion chamber and is configured to provide air for an air/fuel mixture to the combustion chamber. An air induction system is configured to supply air to the intake passage. At least one sensor is configured to monitor engine running conditions. An engine control unit is configured to determine an elapsed time after an engine start and further configured to control an engine idle speed based upon the engine running conditions and the elapsed time.  
         [0012]     A further aspect of the present invention involves a method of operating a marine engine. The marine engine is adapted for driving a marine propulsion device. The method comprises the steps of determining at least one actual engine running condition, determining an elapsed time after an engine start, setting a reference engine idle speed based upon the elapsed time, reading an input engine idle speed, comparing the reference engine idle speed to a preset engine idle speed, setting a target engine idle speed to be one of the reference engine idle speed or the input engine idle speed, and adjusting an actual engine idle speed to be equal to the target engine idle speed.  
         [0013]     Another aspect of the present invention involves a control system for a marine engine. The marine engine is adapted to propel a watercraft with thrust produced by an engine-driven propeller. The control system comprises an operability sensor adapted to detect whether the watercraft is operable. At least one engine idle sensor is adapted to detect whether the engine is idling. An apparatus is adapted to determine an elapsed time after an engine start. Another apparatus is adapted to determine a reference engine idle speed based on the elapsed time after an engine start. A controller is adapted to adjust an engine idle speed during idle speed running based on the reference engine idle speed when the operability sensor detects that the watercraft is operable and the engine idle sensor detects that the engine is idling.  
         [0014]     An additional aspect of the present invention involves a marine engine for a watercraft. The engine comprises an engine body that defines at least one cylinder bore in which a piston reciprocates. A cylinder head is secured to a first end of the engine body for closing the cylinder bore and defines with the piston and the cylinder bore a combustion chamber. An intake passage is in selective fluid communication with the combustion chamber and is configured to provide air for an air/fuel mixture to the combustion chamber. An air induction system is configured to supply air to the intake passage. At least one sensor is configured to monitor engine running conditions. An engine control unit is configured to determine an elapsed time after an engine start and further is configured to control an engine idle speed based upon the engine running conditions and the elapsed time.  
         [0015]     An aspect of the present invention also involves a method of operating an outboard motor for a watercraft. The outboard motor comprises an engine for driving a marine propulsion device. The method comprises determining at least one actual engine running condition; determining an elapsed time after an engine start; setting a reference engine idle speed based at least in part upon the elapsed time; reading an input engine idle speed; comparing the reference engine idle speed to a preset engine idle speed; setting a target engine idle speed to be one of the reference engine idle speed and the input engine idle speed; and adjusting an actual engine idle speed to be equal to the target engine idle speed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The preferred embodiments of the present control system for outboard motor, illustrating its features, will now be discussed in detail. These embodiments depict the novel and non-obvious control system shown in the accompanying drawings, which are for illustrative purposes only. These drawings include the following figures, in which like numerals indicate like parts:  
         [0017]      FIG. 1  is a schematic right-side elevation view of an outboard motor including a preferred embodiment of the present engine control unit;  
         [0018]      FIG. 2  is a schematic view of the interior of the outboard motor and engine control unit of  FIG. 1 ;  
         [0019]      FIG. 3  is a flowchart that diagrams a preferred embodiment of a method for controlling engine idle speed, such as the present control system might carry out;  
         [0020]      FIG. 4  is a graph illustrating an example of the relationship between elapsed time and reference engine idle speed in the present control system;  
         [0021]      FIG. 5  is a graph illustrating an example of the relationship between engine temperature and target engine idle speed immediately after an engine start in the present control system; and  
         [0022]      FIG. 6  is a flowchart that diagrams another preferred embodiment of a method for controlling engine idle speed, such as the present engine control system might carry out. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]      FIG. 1  illustrates, in a schematic view, an outboard motor  10  including the present engine control system. While the present invention is described in the context of an outboard motor, certain features, aspects and advantages can be used with other types of marine engines, including but not limited to those used in stern drive applications, inboard/outboard applications, personal watercraft applications, jet boat applications and the like.  
         [0024]     The illustrated outboard motor  10  is mounted to the rear of a watercraft hull  12 . In the illustrated embodiment, swivel and clamp brackets  14  mount the outboard motor  10  to the hull  12 . The brackets  14  enable the motor  10  to rotate about a substantially vertical axis, such that the motor  10  is able to steer the watercraft  12 . The brackets  14  also enable the motor  10  to tilt relative to the hull  12  along a substantially horizontal axis, such that a lower portion of the motor  10  can be moved clear of obstacles as the watercraft  12  is put into and taken out of a body of water, or can be trimmed during operation of the watercraft, for instance. Those of skill in the art will appreciate that alternative apparatus may be used to mount the outboard motor  10  to the hull  12 .  
         [0025]     With continued reference to  FIG. 1 , the outboard motor  10  includes a housing comprising a top cowling  16 , an upper casing  18  and a lower casing  20 . The top cowling  16  contains an engine  22 . A drive shaft  24  extends downward from the engine  22 , through the upper casing  18  and into the lower casing  20 . A lower end of the drive shaft  24  is operably connected to a propeller  26 . The engine  22  produces power, or drive torque, which the drive shaft  24  transmits to the propeller  26 . The propeller  26  produces thrust to propel the watercraft  12  across a body of water.  
         [0026]     A water pump  28 , which is attached to an intermediate portion of the drive shaft  24 , draws in water from the body of water surrounding the watercraft  12 . The water pump  28  supplies the drawn-in water to the engine  22  in order to cool the engine  22 . The water pump  28  then discharges the water to the body of water surrounding the watercraft  12 . In some arrangements, a closed loop cooling system can be used instead of the above-described open loop cooling system.  
         [0027]     A steering rod  30  preferably extends forward from a portion of the body of the outboard motor, such as, for instance, the top cowling  16 . A watercraft operator (not shown) can apply lateral torque to the steering rod  30  to rotate the motor  10  relative to the hull  12  about a substantially vertical axis. As the motor  10  rotates, the propulsive force supplied by the propeller  26  guides the watercraft  12  in the desired direction.  
         [0028]     An end portion of the steering rod  30  preferably includes an accelerator grip  32 . By twisting the accelerator grip  32 , the watercraft operator can control the operating speed of the engine  22 . For example, to make the watercraft  12  accelerate, the operator twists the accelerator grip  32  in a first direction. The twisting motion preferably controls the opening and closing of a throttle valve  58 , which is described in detail below, in any suitable manner. The control mechanism may be purely mechanical, such as cables running from the accelerator grip  32  to the throttle valve  58 . Alternatively, the control mechanism may be electronic.  
         [0029]     An end of the illustrated accelerator grip  32  includes an idle speed control switch  34 . The idle speed control switch  34  preferably controls the opening and closing, or the degree thereof, of a secondary air valve  86 , or idle speed control valve, which is described in detail below. The control mechanism may be purely mechanical, such as cables running from the accelerator grip  32  to the throttle valve  58 . Alternatively, the control mechanism may be electronic. Moreover, the engine operating speed and the engine idle speed can be controlled from controls located elsewhere on the watercraft, such as near a captain&#39;s seat.  
         [0030]     The illustrated top cowling  16  further comprises a shift switch  36  for selecting one of forward, reverse or neutral modes of a transmission (not shown). Other operating options also can be provided. In the preferred arrangement, when the switch  36  occupies the forward position, the propeller  26  spins in a first direction to drive the watercraft  12  forward; when the switch  36  occupies the reverse position, the propeller  26  spins in a second direction to drive the watercraft  12  backward; and when the switch  36  occupies the neutral position, the propeller  26  does not spin, regardless of the engine speed.  
         [0031]      FIG. 2  illustrates, in a schematic view, the engine  22  of  FIG. 1 , including a preferred embodiment of an exemplary control system. The illustrated engine  22  runs on the four-stroke combustion cycle, and includes a cylinder body  38 , a crankshaft  40 , a piston  42 , a combustion chamber  44 , an intake passageway  46 , an intake valve  48 , an exhaust passageway  50 , an exhaust valve  52 , a spark plug  54  and an ignition coil  56 .  
         [0032]     At the inlet side, the intake passageway  46  includes a throttle valve  58  that controls the volume of intake airflow to the combustion chamber  44 . As the air intake volume increases, the engine speed accelerates, and as the intake volume decreases, the engine speed decelerates.  
         [0033]     Downstream from the throttle valve  58 , the intake passageway  46  comprises a fuel injector  60 . A fuel tank  62  supplies fuel to the injector  60  in any suitable manner. In the illustrated arrangement, a primary pump  64  transfers the fuel from the fuel tank  62  through a low-pressure filter  66 . A low-pressure fuel pump  68  then transfers the fuel to a secondary fuel tank  70 . Finally, a high-pressure fuel pump  72  transfers the fuel through a suction filter  74  and into the injector  60 . Water supplied by the water pump  28  can be used to cool the fuel after it has been pressurized by the high-pressure fuel pump  72 .  
         [0034]     In the illustrated arrangement, a stator coil  76  mounted to the drive shaft  24  generates electric power. The electric power passes through a regulator  78  to be stored in a battery  80 . The battery  80  is connected to a starter motor  82 . The starter motor  82 , drawing power from the battery  80 , starts the engine  22  when desired by the operator. The motor  82  may include a kill switch (not shown) for cutting power to the engine  22 , such as in emergency situations.  
         [0035]     A surge tank  84  positioned between the throttle valve  58  and the intake passageway  46  receives air passing through the throttle valve  58 . The air entering the surge tank  84  passes into the intake passageway  46  to be supplied to the combustion chamber  44 . A secondary air valve  86  regulates a volume of secondary air flowing into the surge tank  84 . The secondary air bypasses the throttle valve  58  and flows directly into the surge tank  84 . Preferably, the bypassed air flows through a bypass passage  87  and the secondary air valve  86  controls the air flow through the bypass passage  87 .  
         [0036]     The secondary air alters idling conditions of the engine  22 . Specifically, during idle, the throttle valve  58  either is closed or substantially closed and, as the secondary air valve  86  opens, the volume of secondary air flow supplied to the engine increases. The increased airflow acts to increases the engine idle speed. Vice versa, as the secondary air valve  86  closes and the volume of secondary air flow decreases, the idle speed of the engine decreases.  
         [0037]     The secondary air valve  86  may, for example, comprise an electromagnetic solenoid valve. In such a valve, as the amount of electric current supplied to the solenoid increases, the displacement of an armature increases, thus opening the valve  86 . Other suitable valve arrangements also can be used. In some configurations, a needle valve, a small butterfly valve or the like can be used.  
         [0038]     In the illustrated arrangement, an engine control unit (ECU)  88  controls the operating conditions of the engine  22 , including the opening and closing of the secondary air valve  86 . The ECU  88  may include a processing unit (not shown) such as a microcomputer or an operation circuit. Furthermore, while a single structure is illustrated, in some arrangements the ECU  88  may comprise a number of discrete processing units or controllers that operate in a coordinated manner. It also is to be noted that the control system may be in the form of a hard wired control circuit. Alternatively, the control system may be constructed of a dedicated processor and a memory for storing a computer program configured to perform the steps recited below. Additionally, the control system may be constructed of a general purpose computer having a general purpose processor and the memory for storing the computer program for performing the desired routines. Preferably, however, the control system is incorporated into the ECU  88 , in any of the above-mentioned forms.  
         [0039]     The illustrated ECU  88  receives inputs for engine control from various sensors. For example, these sensors may include a crank angle sensor  90 , a cooling water temperature sensor  92 , a throttle opening sensor  94 , a hydraulic pressure sensor  96 , an intake air temperature sensor  98  and/or an intake air pressure sensor  100 .  
         [0040]     The crank angle sensor  90  detects the rotational angle, or phase, of the drive shaft  24 . The crank angle sensor  90  may also detect the rotational speed of another rotating shaft, such as the drive shaft  24 , for example but without limitation. The selected shaft preferably rotates at the same or a proportional speed to the engine speed. Other suitable structures and arrangements also can be used to detect the speed at which the engine is operating. For instance, signals from a flywheel magneto can be used.  
         [0041]     The cooling water temperature sensor  92  detects the temperature of the cooling water, which provides a proxy for the temperature inside the cylinder body  38 . Other structures and arrangements also can be used to sense the operating temperature of the engine. For instance, sensors can be positioned within the exhaust system, sensors can be positioned on selected components of the engine or the like.  
         [0042]     The throttle opening sensor  94  detects the degree of openness of the throttle valve  58 . Other suitable structures and arrangements can also be used to sense operator demand. For instance, position of an input device, such as the twist grip, for instance, can be sensed. In some embodiments, the intake air flow rate or pressure can be sensed.  
         [0043]     The hydraulic pressure sensor  96  detects hydraulic pressure generated by a hydraulic pump (not shown). In some arrangements, this sensor can be used as a proxy for engine speed assuming that the hydraulic pressure will increase with engine speed increases.  
         [0044]     The intake air temperature sensor  98  detects the temperature of the air entering the throttle valve  58 . The intake air pressure sensor  100  detects the pressure of the air in the surge tank  84 . These sensors can be positioned in other regions of the intake system.  
         [0045]     In order to determine appropriate engine operation control scenarios, the ECU  88  preferably uses control maps and/or indices stored within the ECU  88  in combination with data collected from these and other various input sensors. For example, the shift switch  36  and the idle speed control switch  34  may transmit output signals to the ECU  88 . In addition to the previously mentioned sensors, the ECU&#39;s various input sensors also can include, but are not limited to, a throttle lever position sensor and an oxygen (O 2 ) sensor. It should be noted that the above-identified sensors merely correspond to some of the sensors that can be used for engine control and it is, of course, practicable to provide other sensors, such as a knock sensor, a neutral sensor, a watercraft pitch sensor, a shift position sensor and an atmospheric temperature sensor. The selected sensors can be provided for sensing engine running conditions, ambient conditions or other conditions of the engine or associated watercraft.  
         [0046]     After receiving input signals from the sensors and the various other sources, the ECU  88  outputs control signals to various engine components. For example, the ECU  88  may output control signals to the fuel injector  60 , the ignition coil  56 , and/or the secondary air valve  86 . The ECU also may output signals to lights, buzzers and gauges for feedback to the operator.  
         [0047]     The ECU  88  executes various processing operations to control the operating conditions of the engine  22 , including secondary air valve opening control.  FIG. 3  illustrates a flowchart of a preferred processing operation that computes a secondary air valve opening command value and outputs it as a command signal to the secondary air valve  86 . This processing operation may, for example, be executed as a timer interrupt process at intervals of prescribed sampling time, ΔT. ΔT may equal, for example but without limitation, approximately 10 milliseconds.  
         [0048]     In the processing operation illustrated in  FIG. 3 , at the first step S 1  following initialization, the ECU  88  determines whether or not the engine  22  is stopped. This determination may be based on, for example, a reading from the crank angle sensor of any change in the crank angle. If there is no change in the crank angle over the sampling interval, then the engine  22  is stopped. If the engine  22  is determined to be stopped, the process moves on to step S 17 , which is described in detail below. If the engine  22  is determined to be running, however, the process moves on to step S 2 .  
         [0049]     At step S 2 , the ECU  88  determines the engine speed. This determination may be based on, for example, input from the crank angle sensor  90 . Other suitable techniques for determining engine speed, by proxy or otherwise, also can be used. The process then moves on to step S 3 .  
         [0050]     At step S 3 , the ECU  88  determines whether or not the watercraft  12  is operable. This determination may be based on, for example, whether or not the shift switch  36  occupies one of the forward or reverse positions. In some arrangements, the position of a clutching assembly can be sensed. In other arrangements, movement of the propeller shaft can be sensed. Yet other arrangements can use any other suitable technique for determining if the watercraft is operable. If the watercraft  12  is inoperable, the process moves on to step S 11 , which is described in detail below. However, if the watercraft  12  is operable, the process moves on to step S 4 .  
         [0051]     At step S 4 , the ECU  88  determines whether or not the opening of the throttle valve  58  is zero or substantially zero. In other words, a determination is made as to whether the throttle valve is in a “closed” position. This determination may be based on, for example, input from the throttle opening sensor  94  or input from a proxy, such as an operator-controlled input device (e.g., a twist grip position) for example but without limitation. If the throttle opening is not zero, meaning that the engine  22  is not idling, the process moves on to step S 11 . However, if the throttle opening is zero, meaning that the engine  22  is idling, the process moves on to step S 5 .  
         [0052]     At step S 5 , the ECU  88  determines the elapsed time since the last engine start. For example, the ECU  88  may include a timer (not shown) that resets each time the engine  22  is started. Alternatively, the ECU  88  may compute the elapsed time since the last engine start by multiplying the number of times that the processing operation has been executed since the last engine start by the prescribed sampling time, ΔT. Those of skill in the art will appreciate that the elapsed time could also be determined in other ways.  
         [0053]     After the ECU  88  has determined the elapsed time since the last engine start, the process goes on to step S 6 . At step S 6 , the ECU  88  sets a reference engine idle speed. The reference engine idle speed is based on the elapsed time since the last engine start, and is set in accordance with a control map or table of values. For example, the control map  102  of  FIG. 4  plots the relationship between the reference engine idle speed and the elapsed time since the last engine start. The control map  104  of  FIG. 5  plots the relationship between the appropriate engine idle speed immediately after an engine start (indicated as “engine idle speed at start” in  FIG. 5 ) and the engine temperature.  
         [0054]     In accordance with a control map, such as the one illustrated in  FIG. 5  for example but without limitation, the ECU  88  determines an appropriate engine idle speed immediately after the engine  22  is started. The ECU  88  makes this determination based on the engine temperature. Engine temperature may be detected by the cooling water temperature sensor  92 , or any of the other configurations described above. Moreover, other suitable techniques for sensing engine temperature can be used. As the control map of  FIG. 5  illustrates, the engine idle speed is configured to decrease as the engine temperature increases. The engine  22  thus tends to idle at a higher speed when the engine temperature is relatively low. The low temperature increases the viscosity of the engine oil, which generates greater friction. The higher idle speed helps to overcome the greater friction, leading to advantageous idling conditions.  
         [0055]     After the ECU  88  determines an appropriate engine idle speed, the ECU  88  then sets the actual engine idle speed to be approximately equal to the determined value. As  FIG. 4  illustrates, the engine idle speed preferably decreases at a constant rate as the elapsed time from the engine start increases. In this manner, fluctuations in the engine temperature do not adversely change the idle speed of the engine. Due to the decrease in speed over time, the engine idle speed eventually reaches a preset engine idle speed  106  (see  FIG. 4 ). Thereafter, the engine idle speed preferably remains at the preset engine idle speed  106 .  
         [0056]     The preset engine idle speed  106  is the desired engine idle speed after the engine has warmed-up. Therefore, whether or not the engine warm-up has been completed can be determined by comparing the reference engine idle speed to the preset engine idle speed  106 . If the two values are equal, engine warm-up is complete. If the reference engine idle speed is greater than the preset engine idle speed  106 , engine warm-up is not yet complete. The time required for the warm-up to be completed can also be computed from the engine idle speed immediately after the engine start, and the predetermined rate at which the reference engine idle speed decreases.  
         [0057]     Once the ECU  88  sets the reference engine idle speed, the process moves on to step S 7 . At step S 7 , the ECU  88  reads an input engine idle speed from the idle speed control switch  34 . The process then moves on to step S 8 .  
         [0058]     At step S 8 , the ECU  88  determines whether or not the engine  22  has warmed-up completely. As described above, the ECU  88  makes this determination by comparing the reference engine idle speed to the preset engine idle speed. If the warm-up is complete, the process goes on to step S 9 . If the warm-up is not complete, the process goes on to step S 10 .  
         [0059]     At step S 9 , the warm-up is complete, so the ECU  88  sets the input engine idle speed, which was read at step S 7 , as the target engine idle speed during idle speed running. Then, the process goes on to step S 15 , which is described in detail below.  
         [0060]     At step S 10 , the warm-up is not complete, so the ECU  88  sets the greater of the reference engine idle speed, which was set at step S 6 , or the input engine idle speed, which was read at step S 7 , as the target engine idle speed during idle speed running. Then, the process goes on to step S 15 , which is described in detail below.  
         [0061]     Meanwhile, at step S 3  or step S 4  the operating process may follow a different path from that described above. For example, at step S 3  the ECU  88  may receive an input that indicates that the shift switch  36  occupies the neutral position. Alternatively, at step S 4  the ECU  88  may receive an input that indicates that the throttle opening is not zero. In either of these scenarios, the process bypasses step S 5  and moves to step S 11 .  
         [0062]     At step S 11  the ECU  88  determines the engine temperature. For example, the cooling water temperature sensor  92  may output the engine temperature to the ECU  88 , as described above. The process then goes on to step S 12 . At step S 12 , the ECU  88  sets the target engine idle speed based upon the engine temperature, in accordance with a control map such as the one illustrated in  FIG. 5 . The process then goes on to step S 13 .  
         [0063]     At step S 13 , the ECU  88  determines whether or not the engine speed is greater than or equal to a preset value. In some arrangements, the preset value can correlate to a speed indicative of the watercraft being moved at speeds significantly above trolling speeds. The preset value can be stored within a memory location accessible by the ECU  88 . In this manner, the operator is free to move the watercraft from trolling location to trolling location without altering the idle speed set in step S 12  (see S 14 ). If the engine speed is greater than or equal to the preset value, the process goes on to step S 14 . If the engine speed is less than the preset value, the process goes on to step S 15 .  
         [0064]     At step S 14 , the input engine idle speed is reset (initialized). Then, the process goes on to step S  15 .  
         [0065]     At step S 15 , the ECU  88  sets a secondary air valve opening command value. This value is based on the engine speed, which was read at step S 2 , and the target engine idle speed during idle speed running, which was set at step S 9  or step S 10 , or the target engine idle speed, which was set at step S 12 . The secondary air valve opening command value may depend upon the prevailing secondary air valve opening condition and the prevailing engine speed. In such a case, the secondary air valve opening command value may be set to a secondary air valve opening target value that achieves the target engine idle speed. Once the ECU  88  has set the secondary air valve opening command value, the process goes on to step S 16 .  
         [0066]     At step S 16 , the ECU  88  outputs the secondary air valve opening command value to the secondary air valve  86 . Then, the process returns to the main program.  
         [0067]     Meanwhile, at step S 1  the ECU  88  may have determined that the engine is stopped. In such an event, the process moves on to step S 17 . At step S 17  the input engine idle speed is reset (initialized). Then, the process returns to the main program.  
         [0068]     The processing operation illustrated in  FIG. 3  and described above determines that the watercraft  12  is in a state of idle speed running (e.g., trolling) when the watercraft  12  is operable (step S 3 ) and the throttle opening is substantially zero (step S 4 ). According to this processing operation, the ECU  88  controls the engine idle speed during trolling (e.g., idle speed movement of the watercraft) based on the reference engine idle speed (steps S 5 -S 10 , S 15  and S 16 ). As illustrated in  FIG. 4 , the reference engine idle speed decreases at a predetermined rate with a lapse of time after the engine  22  is started. The rate of decrease of the reference engine idle speed is independent of engine temperature. Therefore, the processing operation illustrated in  FIG. 3  greatly reduces the likelihood of unstable idling conditions even when the engine temperature changes before the engine  22  is completely warmed-up.  
         [0069]     The processing operation illustrated in  FIG. 3  sets the target engine idle speed during idle speed running to be the greater of the reference engine idle speed or the input engine idle speed (step S 10 ). Therefore, this processing operation allows the watercraft operator to at least increase the engine idle speed without creating substantial unstable idling conditions, even when the engine  22  is not completely warmed-up.  
         [0070]     After the engine  22  has warmed-up completely (step S 8 ), the processing operation illustrated in  FIG. 3  sets the input engine idle speed as the target engine idle speed during idle speed running (step S 9 ). Therefore, this processing operation allows the watercraft operator to set the target engine idle speed after substantially stable idling conditions have been established.  
         [0071]     Rather than relying on the temperature of the cooling water flowing through the engine  22 , the processing operation illustrated in  FIG. 3  assumes that the engine warm-up is complete when the reference engine idle speed reaches the preset engine idle speed  106  (step S 8 ). Therefore, this processing operation substantially increases the likelihood that the engine will warm-up completely regardless of any changes in the engine temperature (as approximated by the cooling water temperature) or in the reference engine idle speed at the engine start.  
         [0072]     The processing operation illustrated in  FIG. 3  sets the reference engine idle speed immediately after an engine start based on the engine temperature (steps S 11  and S 12 ). Therefore, this processing operation reduces the likelihood of unstable engine conditions and ensures complete engine warm-up.  
         [0073]     When the engine speed is a preset value or higher, the processing operation illustrated in  FIG. 3  resets (initializes) the input engine idle speed. Stated otherwise, the input engine idle speed will not be reset unless the preset value is exceeded. Therefore, this processing operation greatly reduces the likelihood that the input engine idle speed will be reset by the watercraft operator. Such resets might ordinarily happen when the operator causes the watercraft  12  to alternately move and stop while looking for a favorable fishing spot, or when the operator runs the watercraft  12  while monitoring the displayed engine speed to maintain it below the preset speed.  
         [0074]     When the engine  22  is stopped, the processing operation illustrated in  FIG. 3  resets (initializes) the input engine idle speed. This step in the processing operation would require the operator to manually input the desired engine idle speed upon each subsequent starting of the engine  22 .  
         [0075]      FIG. 6  illustrates another preferred processing operation that computes a secondary air valve opening command value and outputs it as a command signal to the secondary air valve  86 . This embodiment is compatible with the general configuration of an outboard motor  10  with a watercraft engine control system illustrated in  FIGS. 1 and 2 . Further, like the processing operation of  FIG. 3 , this processing operation also can be executed as a timer interrupt process at intervals of a prescribed sampling time, ΔT. ΔT may equal, for example, approximately 10 milliseconds.  
         [0076]     In the flowchart of  FIG. 6 , many steps are identical to certain steps in the processing operation of  FIG. 3 . However, the order of steps in  FIG. 6  differs from that of  FIG. 3 . At the first step S 21 , the ECU  88  determines whether or not the engine  22  is stopped, as with step S 1  of  FIG. 3 . If the engine  22  is stopped, the process goes on to step S 32 , which is explained in detail below. If the engine  22  is running, the process goes on to step S 22 .  
         [0077]     At step S 22 , the ECU  88  determines the engine speed, as with step S 2  of  FIG. 3 . Then, the process goes on to step S 23 . At step S 23 , the ECU  88  determines whether or not the engine speed read at step S 22  is greater than or equal to a predetermined value, as with step S 13  of  FIG. 3 . If the engine speed is greater than or equal to the predetermined value, the process goes on to step S 33 , which is explained in detail below. If the engine speed is less than the predetermined value, the process goes on to step S 24 .  
         [0078]     At step S 24 , as with step S 5  of  FIG. 3 , the ECU  88  determines the elapsed time since the last engine start. Then, the process goes on to step S 25 .  
         [0079]     At step S 25 , as with step S 6  of  FIG. 3 , the ECU  88  computes and sets the reference engine idle speed based on the elapsed time since the last engine start. Then, the process goes on to step S 26 .  
         [0080]     At step S 26 , the ECU  88  determines the input engine idle speed, as with step S 7  of  FIG. 3 . Then, the process goes on to step S 27 .  
         [0081]     At step S 27 , as with step S 8  of  FIG. 3 , the ECU  88  determines whether or not the engine  22  is completely warmed-up. Again, this determination is based upon whether or not the reference engine idle speed is equal to the preset engine idle speed. If the engine  22  is completely warmed-up, the process goes on to step S 28 . If not, the process goes on to step S 29 .  
         [0082]     At step S 28 , the ECU  88  sets the input engine idle speed read at step S 26  as the target engine idle speed during idle speed running, as with step S 9  of  FIG. 3 . Then, the process goes on to step S 30 .  
         [0083]     Meanwhile, at step S 29 , the ECU  88  sets either the reference engine idle speed set at step S 25  or the input engine idle speed read at step S 26 , whichever is higher, as the target engine idle speed during idle speed running. This step is analogous to step S 10  of  FIG. 3 . Then, the process moves on to step S 30 .  
         [0084]     Meanwhile, if it was determined at step S 21  that the engine is stopped, then the process advances to step S 32 . At step S 32 , the ECU  88  determines whether or not the engine stop switch, or kill switch, is in an ON state. If the kill switch is in an ON state, the process goes on to step S 33 . At step S 33 , the input engine idle speed is reset (initialized). Then, the process returns to the main program. However, If the kill switch is not in an ON state, the process goes on to step S 34 .  
         [0085]     At step S 34 , the ECU  88  determines the engine temperature, as with step S 11  of  FIG. 3 . Then, the process goes on to step S 35 .  
         [0086]     At step S 35 , as with step S 12  of  FIG. 3 , the ECU  88  sets a target engine idle speed based on the engine temperature. Then, the process goes on to step S 30 .  
         [0087]     At step S 30 , as with step S 15  of  FIG. 3 , the ECU  88  sets a secondary air valve opening command value based on the engine speed read at step S 22 , and the target engine idle speed during idle speed running set at step S 28  or step S 29 , or the target engine idle speed set at step S 35 . Then, the process goes on to step S 31 .  
         [0088]     At step S 31 , the ECU  88  outputs the secondary air valve opening command value to the secondary air valve  86 , as with step S 16  of  FIG. 3 . Then, the process returns to the main program.  
         [0089]     According to this processing operation, the input engine idle speed is reset (initialized) when the engine  22  is stopped and the kill switch is in an ON state. Such conditions prevail when the operator intentionally stops the engine  22 . This processing operation reminds the watercraft operator that the input engine idle speed is reset after the engine  22  is intentionally stopped.  
         [0090]     The above presents a description of the best mode contemplated for carrying out the present control system for outboard motor, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains to make and use this control system. This control system is, however, susceptible to modifications and alternate constructions from that discussed above that are fully equivalent. Consequently, this control system is not limited to the particular embodiments disclosed. On the contrary, this control system covers all modifications and alternate constructions coming within the spirit and scope of the control system as generally expressed by the following claims, which particularly point out and distinctly claim the subject matter of the control system. The steps of the control routines set forth above can be combined, separated, and reordered while still embodying certain features, aspects and advantages of the present invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.