Abstract:
An engine control device can include a throttle valve for adjusting an amount of air supplied to an engine, operation amount detecting sensors for detecting an operation amount of a throttle lever, a motor for driving the throttle valve between open and closed positions according to detection values of the operation amount detecting sensor  21   a  or the like, and throttle opening detecting sensors for detecting an opening of the throttle valve is provided with a limp-home mechanism for keeping the throttle valve in a mechanically neutral position when an abnormality occurs. When the throttle valve is in the mechanically neutral position, an ignition timing control can be switched in a stepwise manner over a predetermined time period from a regular ignition timing control to a limp-home ignition timing control.

Description:
PRIORITY INFORMATION 
   This application is based on and claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2005-007553, filed on Jan. 14, 2005, the entire contents of which is hereby expressly incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Inventions 
   The present inventions relate to an engine control device including ignition timing control functions. 
   2. Description of the Related Art 
   Water jet propulsion boats, some of which are referred to as “personal watercraft” are provided with a throttle lever disposed in the vicinity of a grip of the steering handlebars. The engines of such watercraft propel the watercraft according to operation of the throttle lever. The heading of the watercraft is changed according to operation of the handlebars. 
   Recently, several personal watercraft manufacturers have incorporated into such watercraft, systems which can raise engine output when an operator operates the steering handlebars by a large amount while not operating the throttle lever. For example, such a system is disclosed in Japanese Patent Publication JP-A-2001-329881. This type f system can improve steerability of the water jet propulsion boat when it docks while coasting, or the like. 
   Some of such personal watercraft include throttle opening detection means for detecting the opening of a throttle valve that opens and closes according to operation of the throttle lever as well as steering angle detection means for detecting the steering angle of the steering handlebars. These watercraft can also include running speed detection means for detecting the running speed of the watercraft, and engine output control means for controlling the engine output. 
   The engine output control means can be configured to increase the engine output when a value corresponding to the throttle opening detected by the throttle opening detection means is not more than a predetermined value, a value corresponding to the steering angle detected by the steering angle detection means is not less than a predetermined value, and a value corresponding to the running speed detected by the running speed detection means is not less than a predetermined value. 
   SUMMARY OF THE INVENTIONS 
   An aspect of at least one of the embodiments disclosed herein includes the realization that, in the conventional watercraft noted above, in some cases, engine speed exceeds or does not reach a target speed during a throttle/steering assist operation, due to variations in the manufacture of the engine, variations in the operation of an actuator for opening and closing the throttle valve, or the like. For example, after prolonged use, a throttle valve or throttle valve shaft can become sticky, and thus will not move as fast as when the throttle valve was new. To solve such problems, strict predetermined settings, accuracy and the like in the associated mechanical designs can be used, however, such results in cost increases and more complicated and time-consuming maintenance procedures. On the other hand, adjustment of ignition timing during such operational modes can be used to compensate for the change in performance of the engine components, thereby preventing the occurrence of variation in engine speed by controlling ignition timing without cost increase and degraded maintainability. 
   Thus, in accordance with an embodiment, an engine control device comprises a throttle opening detection device configured to detect an opening of a throttle valve. An operation load detection device is configured to detect an operation load of steering handlebars. A rotational speed detection device is configured to detect a speed of an engine. An engine speed control device is configured to increase the speed of an engine when a value detected by the throttle opening detection device is smaller than a predetermined value and when a value detected by the operation load detection device is greater than a predetermined value. Additionally, an ignition timing control device is configured to compute a difference between a detection value from the rotational speed detection device and a preset target engine speed and to adjust ignition timing of an engine based on the computed value, when the engine speed is increased by the control of the engine speed control device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the inventions, features, aspects, and embodiments will become more apparent upon reading the following detailed description and with reference to the accompanying drawings of an embodiment that exemplifies the invention. The drawings comprise eight figures. 
       FIG. 1  is a side elevational view of a small watercraft with an engine control device according to an embodiment. 
       FIG. 2  is a plan view of a steering handle of the watercraft of  FIG. 1 . 
       FIG. 3  is a schematic diagram of the engine control device. 
       FIG. 4  is a block diagram of the engine control device. 
       FIG. 5  is a flowchart illustrating a program that can be used in conjunction with the engine control device, or other engine control devices for providing a steering assist mode engine operation, in accordance with an embodiment. 
       FIG. 6  is a graph illustrating an exemplary multi-dimensional basic ignition timing map that can be used in conjunction with the program of  FIG. 5  and/or some embodiments of the engine control device of  FIGS. 1–4 . 
       FIG. 7  is a graph illustrating an exemplary relationship between engine speed and basic ignition timing. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment is described below with reference to the drawings.  FIG. 1  shows a personal water-jet propulsion watercraft  10  with an engine control device  20 . The embodiments disclosed herein are described in the context of a personal watercraft because these embodiments have particular utility in this context. However, the embodiments and inventions herein can also be applied to other marine vessels, such as and small jet boats, as well as other vehicles. 
   With reference to  FIG. 1 , in the boat  10 , a boat body  11  is formed of a deck  11   a  and a lower hull  11   b . A steering handle  12  is provided in the upper part of the boat body  11  at a portion, forwardly of the center. A seat  13  is provided in the upper part of the boat body  11  at about the middle of the boat  10 . The steering handle  12 , also shown in  FIG. 2 , is mounted to the upper end of a steering shaft  12   a  provided in the boat body  11 , for rotation about or with the steering shaft  12   a.    
   With reference to  FIG. 2 , in the vicinity of a grip  12   b  on the right side (on the starboard side) of the steering handle  12  is provided a throttle lever  21  for rotation about its base-end side portions, although other configurations can also be used. The throttle lever  21  is movable toward the grip  12   b  through a driver&#39;s operation, with, for example, an operator&#39;s finger, although other configurations can also be used. When the lever  21  is not being depressed, the lever is held separated from the grip  12   b , by a spring, for example. In the base section of the throttle lever  21  is provided an amount-of-operation sensor  21   a  for detecting the amount of operation (amount of rotation) of the throttle lever  21 . The sensor  21   a  can be any type of sensor. One type of sensor that can be used is, for example, but without limitation, a rheostat configured to output a voltage indicative of the angular position of the lever  21   a , however, other sensors can also be used. 
   The watercraft  10  can also include a steering angle sensor  22  configured to detect an angular displacement of the steering handle  12 . For example, the steering angle sensor  22  can be mounted in the vicinity of the steering shaft  12   a  and configured to detecting the rotational angle of the steering shaft  12   a  (steering handle  12 ), and or the angular velocity of the steering handle. However, other sensors can also be used. 
   With reference to  FIG. 1 , in the bottom of the body  11  at the forward portion thereof is disposed a fuel tank  14  for storing fuel. In the bottom of the hull  11   b  at the middle portion thereof is disposed an engine  23 . 
   To the engine  23  are connected an intake device  15  for sending a mixture of air and fuel supplied from the fuel tank  14 , and an exhaust device  16  for discharging exhaust gas delivered from the engine  23 , from the rear end of the body  11  to the outside. 
   In the illustrated embodiment, the engine  23  is a four-stroke, four-cylinder type. However, this is merely one type of engine that can be used with the present inventions. Engines having a different number of cylinders, other cylinder arrangements, various cylinder orientations (e.g., upright cylinder banks, V-type, and W-type), and operating on various combustion principles (e.g., four stroke, crankcase compression two-stroke, diesel, and rotary) are all practicable for use with the inventions disclosed herein. As shown in  FIG. 3 , the engine  23  draws in through an intake valve  23   a , a mixture of fuel and air from the intake device  15  provided on the side of the intake valve  23   a . Additionally, the engine  23  discharges exhaust gasses through an exhaust valve  23   b , to the exhaust device  16  provided on the side of the exhaust valve  23   b.    
   The fuel and air mixture supplied into the engine  23  through the intake valve  23   a  is ignited through activation of an ignition device. The ignition device can be any type of ignition device or system. In the illustrated embodiment, the ignition device comprises an ignition or “spark” plug  24   a  provided on the engine  23 . 
   When the air/fuel mixture is ignited, the mixture combusts and thus causes a piston  23   c  provided in the engine  23  to make an up and down movement. The movement of the piston  23   c  causes a crankshaft  23   d  to rotate. The crankshaft  23   d  is connected to an impeller shaft  17  through a direct connection or through one or a plurality of additional shafts. As such, the rotational force of the engine  23  is transmitted to the impeller shaft  17  to rotate an impeller (not shown) that is connected to the impeller shaft  17 . 
   The impeller is part of a propulsion device  18  disposed at the rear end of and generally along a longitudinal centerline of the body  11 . The rotation of the impeller produces a propulsive force for the water-jet propulsion boat  10 . That is, the propulsion device  18  is provided with a water induction port  18   a  open toward the bottom of the boat body  11 . A water ejection port (not shown) propulsion device  18  is open at the stem. Water introduced from the water induction port  18   a  is jetted out from the water ejection port through rotation of the impeller to produce a propulsive force for the boat body  11 . 
   A steering nozzle  19  is attached to the rear end of the propulsion device  18 . The steering nozzle  19  is configured to deflect the water jet ejected from the propulsion device  18  so as to change a running direction of the watercraft  10  to the left and right by moving its rear portion to the left and right in response to the operation of the steering handle  12 . 
   The intake device  15  can include an intake pipe  15   a  connected to the engine  23 , a throttle body  15   b  connected to the upstream end of the intake pipe  15   a , and the like. Air outside the boat body  11  is drawn through an intake air duct  15   c  and an intake box  15   d . The flow of such air can be regulated by opening/closing of a throttle valve  25  provided in the throttle body  15   b . As air flows through the intake pipe  15   a ,it is mixed with fuel. The fuel can be supplied from the fuel tank  14  through a fuel feed device made up of an injector  26   a  and the like. This type of fuel injection system is known as “port” fuel injection. Other types of fuel injection systems, such as, for example, but without limitation, direct fuel injection, can also be used. 
   The exhaust device  16  includes an exhaust pipe  16   a  that can comprises bent pipes connected to the engine  23 . A tank-like water lock  16   b  can be connected to the rear end of the exhaust pipe  16   a , so as to suppress water from flowing upstream through the exhaust system. 
   An exhaust pipe  16   c  can be connected to the rear of the water lock  16   b . The exhaust pipe  16   a  can extend initially forwardly. In the illustrated engine, one exhaust pipe  16   a  is provided for each cylinder of the engine  23 . These individual exhaust pipes  16   a  are grouped together on the starboard side of the engine  23 , and extend around the front side of the engine  23  toward the port side. On the front or port side of the engine  23 , the individual pipes  16   a  can be merged together to form a common exhaust passage. The common exhaust passage can extend further rearwardly from the read side of the engine  23 . However, this is merely one exemplary configuration of the exhaust system. Other configurations can also be used. 
   The rear end of the common exhaust passage  16   a  is in communication with the front of the water lock  16   b . The exhaust pipe  16   c  extends rearward from the rear top surface of the water lock  16   b . The exhaust pipe  16   c  first extends upwardly from the rear top surface of the water lock  16   b  and then downwardly to the rear, the downstream end of which opens to an undersurface of the body  11 , near the rear end. One such undersurface can be a side wall of a tunnel in which the propulsion device  18  is disposed. The exhaust device  16  discharges exhaust gas to the outside, with outside water or the like prevented from entering into the engine  23 . 
   In the illustrated embodiment, the throttle valve  25  is formed in the shape of a disc, to the central portion (in its diametric direction) of which is fixed a rotational shaft  25   a . The rotational shaft  25   a  is supported inside the throttle body  15   b  for rotation, to one end of which is connected a motor  27 . Therefore, the throttle valve  25  rotates in normal and reverse directions on the rotational shaft  24   a  in association with the rotation of the motor  27 , to open/close the intake air passage in the throttle body  15   b.    
   With reference to  FIG. 4 , the watercraft  10  can also include a limp home mechanism (not shown). Such a limp home mechanism can be configured to act as a means of performing a limp home function when the motor  27  fails to adjust throttle opening to a desired amount. For example, but without limitation, the motor  27  might fail to properly adjust the throttle valve  25  due to an energization abnormality or the like due to breaking of wire of the motor  27  and the like. 
   The engine control device  20  according to this embodiment is provided, in addition to the foregoing devices, with various devices such as an electronic control unit  30  (hereinafter referred to as an ECU) and the like and various kinds of sensors. In the fuel tank  14 , a filter  14   a , a fuel pump  26   bc  and a pressure control valve  26   c  are disposed. The pressure control valve  26   c  is connected to the injector  26   a , controlling the pressure of fuel whose impurities are filtered and removed by the filter  14   a  and which is fed to the injector  26   a  through operation of the fuel pump  26   b.    
   Also, an ignition coil  24   b  is connected to each ignition plug  24   a . The ignition coil  24   b  sends electric current to the ignition plugs  24   a  in accordance with ignition timing. In this manner, the ignition plugs  24   a  discharge electricity to ignite fuel. 
   In the vicinity of the crankshaft  23   d  in the engine  23  is provided a rotational speed sensor  28   a  for detecting the rotational speed of the crankshaft  23   d . In the body of the engine  23  is provided a temperature sensor  28   b  for detecting the temperature of the engine body. In the vicinity of the rotational shaft  24   a  of the throttle valve  25  is provided a throttle opening sensor  29  (see  FIG. 4 ) for detecting the opening of the throttle valve  25 . 
   In the intake pipe  15   a  are provided an intake air pressure sensor  31   a  for detecting the intake air pressure inside the intake pipe  15   a  and an intake air temperature sensor  31   b  for detecting the intake air temperature inside the intake pipe  15   a . In the exhaust pipe  16   a  is provided an exhaust air-fuel ratio sensor  32  for detecting the air-fuel ratio in the exhaust pipe  16   a.    
   The ECU  30  can include a CPU  35   a , a RAM  35   b , a ROM  35   c , a timer  35   d  and various kinds of circuitry devices (not shown), as shown in  FIG. 5 . Detection signals can be input from an amount-of-operation sensor  21  a which indicates the condition of operation of the throttle lever  21  and the rotational speed sensor  28   a  which indicates the rotating condition of the engine  23 . The ECU  30  can be configured to process the detection signals from these sensors based on a control map stored in the ROM  35   c , transfers control signals to the injector  26   a , ignition coil  24   b , motor  27 , fuel pump  26   b , pressure control valve  26   c  and the like, for the control of fuel injection or ignition timing, as well as of opening of the throttle valve  25 . That is, the ECU  30  functions as the mode judgment means, ignition timing control device and fuel injection control device according to one embodiment. 
   The ECU  30  can be connected to a battery  36  through a power source line L. An ignition switch  37  can be disposed in the power source line L. The ignition switch  37  can be turned on or off by the operation of the boat operator. The electric power is supplied to the ECU  30  when the ignition switch  37  is turned on. Also, the battery  36  can be connected to the motor  27  through a power source line L 1 . A relay  38  is disposed in the power source line L 1 . The relay  38  is switched on or off based upon signals sent from the ECU  30 . When the relay  38  is switched off, the power supply path (the power source line L 1 ) to the motor  27  is disconnected to stop supplying the power to the motor  27 . 
   During operation, an operator can start the water-jet propulsion boat  10 . If the driver sets a main switch (not shown) ON and also manipulates the ignition switch  37  to an ON position, then the watercraft  10  is brought into condition for running; the engine  23  is running at least at idle speed. In this case, the relay  38  is set to be ON and the motor  27  is in the condition for operation. The watercraft  10  runs in a direction according to operator&#39;s operation of the steering handlebars  12  at a speed according to operator&#39;s operation of the throttle lever  21 . 
   More specifically, operating the steering handlebars  12  causes the steering nozzle  19  to pivot right and left, so that the running direction of the water jet propulsion boat  10  is determined. Squeezing the throttle lever  21  toward the grip  12   b  increases a throttle opening so that the watercraft  10  accelerates. On the other hand, releasing the throttle lever  21  so as to allow it to move away from the grip  12   b  reduces the throttle opening so that the watercraft  10  decelerates. 
   Control of the engine  23  can be performed according to a program illustrated in flowchart form in  FIG. 5 . The program can optionally be repeated at given time intervals after the ignition switch  37  is set ON. 
   The program first starts at the Step  100 . Then, the program proceeds to the Step  102 , where an engine speed n detected by the rotational speed sensor  28   a  is read. A value indicative of engine speed n can be temporarily stored in the RAM  35   b . Next, the program proceeds to the Step  104 . 
   In the Step  104 , an operation load S of the steering handlebars  12  detected by the operation load sensor  22  is read. This value can also be stored in the RAM  35   b . The program then proceeds to the Step  106 . 
   In the Step  106 , a process of reading and storing an opening α of the throttle valve  25  detected by the throttle opening sensor  29  in the RAM  35   b  can be executed. Then the program proceeds to the Step  108 . 
   In the Step  108 , basic ignition timing θ can be computed. Regarding the basic ignition timing θ, for example, for example, an exemplary multi-dimensional map for computing regular ignition timings is shown in  FIG. 6 . Such a map can be predetermined and stored in the ROM  35   c . The basic ignition timing θ illustrated in  FIG. 6  can be determined based upon values of an engine speed (rpm) and a throttle opening (%), in a manner well known in the art. That is, the map for computing the regular ignition timing is made based upon relationships between the basic ignition timing and the throttle opening both at a certain engine speed and relationships between the basic ignition timing and the engine speed at a certain throttle opening. 
   In this manner, an optimum basic ignition timing θ when the throttle opening α and the engine speed n are certain values can be computed. The basic ignition timing θ is computed from the engine speed n obtained in the process of the Step  102  and the throttle opening α obtained in the process of the Step  106 . The computed value of the basic ignition timing θ can be stored in the RAM  35   b . As used herein, the term “basic ignition timing θ” is intended to mean the timing at which each ignition plug  24   a  ignites the mixture supplied to the engine  23 , and is indicated by an angle before “0 degrees” which is defined as the top dead center (TDC) of the piston  23   c . An example of the relationships between the basic ignition timing θ and the engine speed n during a typical acceleration of the watercraft  10  is shown in  FIG. 7 . 
   The program then proceeds to the Step  110  to compute a basic fuel injection amount Q. Regarding the basic fuel injection amount Q, a map for computing a basic fuel injection amount (not shown) can be predetermined and stored in the ROM  35   c . The basic fuel injection amount Q can be based upon a map for computing the basic fuel injection amount. The basic fuel injection amount Q means the injection amount of fuel that is supplied to the engine  23  from each injector  26   a . The map for computing the basic fuel injection amount can be prepared as a three or more dimensional map that shows the relationships among the engine speed n, the throttle opening α and the fuel injection amount Q. 
   Thus, a basic fuel injection amount Q that can achieve an optimum engine speed n can be computed from the engine speed n obtained in the process of the Step  102  and the throttle opening α obtained in the process of the Step  106 . The computed value of the basic fuel injection amount Q can also be stored in the RAM  35   b . Also, on this occasion, although not depicted in  FIG. 5 , a computation of fuel injection timing can be conducted together with the computation of the basic fuel injection amount Q. 
   The program then proceeds to the Step  112 , where the CPU  35   a  determines whether or not the throttle opening α determined in the process of the Step  106  is smaller than a preset value α 0 . The preset value α 0  is set to be an opening of the throttle valve  25  to which the throttle valve  25  pivots when the operator only slightly squeezes the throttle lever  21  toward the grip  12   b . When the throttle opening α is greater than the preset value α 0 , it is determined that the operator is intentionally operating the throttle lever  21  to accelerate the watercraft  10 . On the other hand, when the throttle opening α is smaller than the preset value α 0 , it is determined that the operator is not intentionally operating the throttle lever  21 . 
   For example, in the Step  112 , a determination is made as to whether or not the water jet propulsion boat  10  is coasting, which can be used to decide whether or not the watercraft  10  should be in a steering-assist control mode. When the throttle opening α is greater than the preset value α 0 , then “NO” is the result of determination in the Step  112  and the program proceeds to the Step  114 . 
   In the Step  114 , the injector  26   a  injects the fuel based upon the basic fuel injection amount Q and the basic fuel injection timing both computed in the process of the Step  110 . Additionally, a process to control the ignition timing of the ignition plug  24   a  is conducted based upon the basic ignition timing θ computed in the process of the Step  108 . In this manner, the engine  23  starts normal operation. The program then proceeds to the Step  116  and temporarily ends and/or returns. 
   The program is again started from the Step  100  and proceeds to the Step  102 . Thereafter, the same processes as the steps  102 – 112  can be conducted to renew the obtained values of the engine speed n, the operation load S and the throttle opening α and to compute a basic ignition timing θ, a basic fuel injection amount Q and a basic fuel injection timing corresponding to the values. Those renewed values can be stored in the RAM  35   b.    
   With reference again to the Step  112 , a determination is made as to whether or not the throttle opening α is smaller than the preset value α 0 . While the throttle opening α is greater than the preset value α 0 , a normal fuel injection and ignition process continues in the Step  114 . In this scenario, fuel injection and ignition are executed based on the renewed values of the basic ignition timing θ, basic fuel injection amount Q and basic fuel injection timing. 
   On the other hand, when the throttle opening α is smaller than the preset value α 0 , then “YES” is the result of the determination in the Step  112  and the program proceeds to the Step  118 . In the Step  118 , a determination is made as to whether or not the operation load S of the steering handlebars  12  (detected by the operation load sensor  22  in the execution of the preceding Step) is greater than a preset value S 0 . The preset value S 0  can be set to a value greater than the operation load applied when the operator turns the steering handlebars  12  during normal operation of the boat. When the operation load S is greater than the preset value S 0 , control changes from normal mode to a steering assist mode. 
   For example, the steering assist mode control can be a control mode or method in which the speed of the engine  23  is increased when the steering handlebars  12  are turned by a greater amount than normal. In some embodiments, the steering assist mode can be triggered when the operator turns the handlebars  12  to a maximum position and applies additional pressure on the handlebars  12 . With either configuration, the watercraft  10  temporarily produces additional thrust so that steerability is improved. 
   In the present embodiment, when the operation load S for normal steering is “0” for example, the preset value S 0  is set to “0”. When the steering handlebars  12  are turned to a maximum steering angle position where the operation load S exceeds “0,” the control can be changed to a steering assist mode. Alternatively, the preset value S 0  may be an operation load S at which a steering angle achieves a predetermined angle. In such case, thrust produced by the water jet propulsion boat  10  can be increased according to the amount of the operation load S. 
   When the operation load S is smaller than the preset value S 0 , then “NO” is the result of the determination of the Step  118  and the program proceeds to the Step  114  and the foregoing processes are repeated thereafter. On the other hand, when “YES” is the result of the determination of the Step  118 , the program proceeds to the Step  120 . 
   In the Step  120 , a determination is made as to whether or not the engine speed n determined in the process of the Step  102  is higher than a preset value n 0  in the most recent execution of the program. The preset value n 0  can be set as a threshold value to determine whether or not correction is required for the engine speed, and when the engine speed n is not higher than the preset value n 0 , it is determined that no correction is required. 
   When the engine speed n is not higher than the preset value n 0  in the Step  120 , “NO” is the result of the determination and thus the program proceeds to the Step  114 . On the other hand, when the engine speed n is higher than the preset value n 0 , “YES” is the result of the determination and the program proceeds to the Step  122 . 
   In the Step  122 , a determination is made as to whether or not a value of an actuator is greater than a preset value. For example, but without limitation, the value of the actuator can be computed based on the speed of the motor  27 , an opening of the throttle valve  25  or the like. In this embodiment, a value can be computed based on the speed of the motor  27 . The preset value in this case is also set as a threshold value to determine whether or not correction is required for the engine speed. 
   When the value of the actuator is not greater than the preset value in the Step  122 , “NO” is the result of this determination and the program proceeds to the Step  114 . On the other hand, when the value of the actuator is greater than the preset value, “YES” is the result and the program proceeds to the Step  124 . 
   In the Step  124 , a process of determining a difference Δne between the values of the engine speed n and a target engine speed n 1  is executed. Then in the Step  126 , a P-term correction value Θp is computed according to the computed difference Δne. The P-term correction value Θp can be determined based on Table 1 below. 
   
     
       
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Δne(1) 
               . . . 
               Δne(n) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               P-term 
               Θp1 
               . . . 
               Θpn 
             
             
                 
               correction 
             
             
                 
                 
             
           
        
       
     
   
   For example, in the Table 1, the difference Δne is differentiated into difference Δne( 1 ) to difference Δne(n), and the P-term correction value Θp corresponding to each difference value is differentiated as P-term correction value Θp 1  to P-term correction value Θpn. In addition, in the Table 1, the difference Δne( 1 ) is smaller than the difference Δne(n), and the P-term correction value Θp 1  is smaller than the P-term correction value Θpn, and the P-term correction value Θp is set as the value in proportion to the corresponding difference Δne. 
   The P-term correction value Θp thus becomes greater for a greater difference between the values of the engine speed n and the target engine speed n 1 . In some embodiments, when the difference Δne is a positive value, ignition timing is corrected toward a retarded timing (fewer degrees before TDC). Meanwhile, when the difference Δne is a negative value, or when the value of the engine speed n is smaller than the target engine speed n 1 , the ignition timing is adjusted toward a more advanced timing (more degrees before TDC). 
   Then, the program proceeds to the Step  128 , where an I-term correction value Θi is computed according to the difference Δne. The I-term correction value Θi can be determined based on Table 2 below. 
   
     
       
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
               Δne(1) 
               . . . 
               Δne(n) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               I-term 
               Θi1 
               . . . 
               Θin 
             
             
                 
               correction 
             
             
                 
                 
             
           
        
       
     
   
   In the Table 2, the difference Δne is differentiated into difference Δne( 1 ) to difference Δne(n), and the I-term correction value Θi is differentiated into I-term correction value Θ)i 1  to I-term correction value Θin. In addition, in the Table 2, the difference Δne( 1 ) is smaller than the difference Δne(n), and the I-term correction value Θi 1  is smaller than the I-term correction value Θin, and the I-term correction value Θi is set as the value integrated by the corresponding difference Δne. The I-term correction value Θi thus becomes greater for a greater difference between the values of the engine speed n and the target engine speed n 1 . In some embodiments, when the difference Δne is a positive value, ignition timing is corrected toward a more retarded timing. On the other hand, when the difference Δne is a negative value, the ignition timing is adjusted toward a more advanced timing. 
   Then the program proceeds to the Step  130 , where a steering assist mode ignition timing correction value Θpi is computed based on the P-term correction value Θp determined in the process of the Step  126  and the I-term correction value Θi determined in the process of the Step  128 . The steering assist mode ignition timing correction value Θpi can be computed by the following equation 1.
 
Θ pi ( n )=Θ p ( n )+Θ i   Equation 1
 
   Further, the I-term correction value Θi can be expressed by the following equation 2, and the computation of the equation 2 can be executed at predetermined short time intervals. More specifically, the steering assist mode ignition timing correction value Θpi can be determined as the sum of the P-term correction value Θp and the I-term correction value Θi.
 
Θ i=Θi ( n− 1)+Θ i ( n )  Equation 2
 
   In the Step  130 , when the computing process of the steering assist mode ignition timing correction value Θpi is completed, the program proceeds to the Step  132 . In the Step  132 , a determination is made as to whether or not the steering assist mode ignition timing correction value Θpi determined in the process of the Step  130  is smaller than a maximum correction value Θmax. The maximum correction value Θmax can be the value which represents an upper limit for which the engine speed n can be corrected by the steering assist mode ignition timing correction value Θpi. 
   When the steering assist mode ignition timing correction value Θpi is not smaller than the maximum correction value Θmax, it is determined that the engine speed sensor  28   a  or the like has failed or another failure has occurred, causing the engine speed n detected by the engine speed sensor  28   a  to be significantly greater than the target engine speed n 1 . 
   When the steering assist mode ignition timing correction value Θpi is smaller than the maximum correction value Θmax, then “YES” is the result of the determination in the Step  132  and the program proceeds to the Step  134 , where determination is made as to whether or not the steering assist mode ignition timing correction value Θpi determined in the process of the Step  130  is greater than a minimum correction value Θmin. The minimum correction value Θmin is the value which represents a lower limit for which the engine speed n can be corrected by the steering assist mode ignition timing correction value Θpi. When the steering assist mode ignition timing correction value Θpi is not greater than the minimum correction value Θmin, it is determined that the speed sensor  28   a  or the like has failed or another failure has occurred, as with when the steering assist mode ignition timing correction value Θpi is greater than the maximum correction value Θmax. 
   When the steering assist mode ignition timing correction value Θpi is greater than the minimum correction value Θmin, then “YES” is the result in the Step  134  and the program proceeds to the Step  136 . In the Step  136 , a process can be executed in which fuel is injected from the injector  26   a  based on the values of the basic fuel injection amount Q and the basic fuel injection timing computed in the process of the Step  110  in the most recent execution of the program. Additionally, the ignition timing of the ignition plug  24   a  can be controlled for ignition, based on the value of the basic ignition timing θ computed in the process of the Step  108  plus or minus the steering assist mode ignition timing correction value Θpi determined in the process of the Step  130 . 
   While the engine control device  20  is in steering assist mode, “YES” is the result of the determination in the steps  120 ,  122 ,  132  and  134 , the fuel injection and the ignition are repeated considering the steering assist mode ignition timing correction value Θpi computed in each case. In this event, when the steering assist mode ignition timing correction value Θpi is a positive value, the basic ignition timing θ is adjusted toward a more retarded timing. On the other hand, when the steering assist mode ignition timing correction value Θpi is a negative value, the basic ignition timing θ is adjusted toward a more advanced timing. 
   Meanwhile, when trouble occurs in the devices configuring the engine control device  20  and “NO” is the result of the determination in the Step  132  or  134 , the program proceeds to the Step  138 . In the Step  138 , a process in the event of abnormality is executed. 
   This process can be a limp-home control operation, for example. The limp-home control can be executed in a way such that a mechanism (not shown) brings the opening of the throttle valve  25  to a preset throttle opening α when the relay  38  is deactivated and the motor  27  is not operable, and the relay  38  is deactivated by the control of the ECU  30 . The opening of the throttle valve  25  thus achieves the preset opening for the limp-home control. 
   Thus, in the Step  138 , a process of injecting fuel from the injector  26   a  and controlling the ignition timing of the ignition plug  24   a  based on preset values of the fuel injection amount, the fuel injection timing and the ignition timing is executed as the process for the limp-home control. Then, after the fuel injection and the ignition are completed, the program proceeds to the Step  116  to end and/or return. The program starts again from the Step  100 , and as long as “NO” is the result of the determination in the Step  132  or  134 , the process for the limp-home control of the Step  138  is executed. 
   As described above, in the engine control device  20  in accordance with this embodiment, since the steering assist mode ignition timing correction value Θpi is computed based on known PI control and used for the steering assist mode control, variation in the engine speed n caused by mechanical tolerance and ambient conditions can be restricted in a simple manner. Therefore, the occurrence of variation in the engine speed n can be prevented, without cost increase and poor maintainability due to a complicated structure and increased accuracy requirement of the engine  23 , so that appropriate thrust is produced from the engine. 
   Further, in the engine control device  20 , the steering assist mode ignition timing correction value Θpi has the maximum correction value Θmax and the minimum correction value Θmin, and the ignition timing correction control is executed when the steering assist mode ignition timing correction value Θpi is between the maximum correction value Θmax and the minimum correction value Θmin. This can prevent the correction process from being endlessly repeated when the variation occurs in the engine speed n due to mechanical trouble or the like, and allows control for such event. Further, in the watercraft  10  in accordance with this embodiment, since the engine speed n when the engine is in steering assist mode is controlled to approximate the preset target engine speed n 1 , stable thrust can be provided and steerability is improved. 
   The engine control device  20  according to the present invention is not limited to the embodiment described above and can be practiced with proper modifications. For example, in the foregoing embodiment, the steering assist mode ignition timing correction value Θpi is computed based on the PI control, but this may be computed using PID control which considers a derivative-term correction value. This allows more accurate correction control. Further, the steps  120  and  122  in the flowchart of  FIG. 5  can be omitted or only one of them can be used. Also, the arrangement, structure and the like of the portions which configure the engine control device according to the present invention may be modified as appropriate within the technical scope of the present invention. 
   With respect to the use of PI or PID control, since the correction control using the known proportional-plus-integral control is used for the steering assist mode control, variation in engine speed caused by mechanical tolerance, ambient conditions and the like can be restricted in a simple manner. Thus, the occurrence of variation in the engine speed can be prevented, without cost increase and poor maintainability due to a complicated structure and increased accuracy requirement of the engine, so that appropriate thrust is produced from the engine. 
   Additionally, as noted above with reference to at least some of the embodiments disclosed herein, the present control device can utilize a correction value for the ignition timing correction control, which is executed by the ignition timing control device based on the value computed based on the difference between the detection value from the rotational speed detection device and the preset target engine speed, can have an upper limit and a lower limit, and the ignition timing correction control can be executed when the correction value is between the upper limit and the lower limit. 
   This can prevent the correction process from being endlessly repeated when the engine speed fails to become lower or higher than the target speed due to mechanical trouble or the like. For example, in at least some of the embodiments disclosed herein, when the correction value is greater than the upper limit or smaller than the lower limit, it is determined that a device such as a sensor has some failure, and alternative control, for example, limp-home control, is executed. 
   Further, although the present inventions have been disclosed in the context of certain preferred embodiments, features, aspects, and examples, it will be understood by those skilled in the art that the present inventions extend beyond the specifically disclosed embodiments to other alternative embodiments and/or uses and obvious modifications and equivalents thereof. In addition, while a number of variations have been shown and described in detail, other modifications, which are within the scope of the present inventions, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or subcombinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the present inventions. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the present inventions. Thus, it is intended that the scope of the present inventions herein disclosed should not be limited by the particular disclosed embodiments described above.