CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Korea patent Application No. 99-62083, filed on Dec. 24, 1999. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a control method for a diesel engine, and more particularly, to a control method for a diesel engine in which a vehicle speed limit is increased after determining that the vehicle is travelling on a downgrade, thereby enabling the momentum of the downgrade to be used to propel the vehicle. 
     (b) Description of the Related Art 
     The main structural elements of a diesel engine for converting heat energy into mechanical energy are not significantly different from those of the gasoline engine. However, the processes of fuel supply and fuel combustion are performed differently in the diesel and gasoline engines. In particular, only air is supplied during the intake stroke, and after compression to a high compression ratio of 15-22:1, resulting in an increase in temperature to roughly 500-600° C., fuel injected into the combustion chamber is ignited as a result of the high temperature generated therein (i.e., the fuel undergoes self-combustion). 
     Accordingly, a fuel injection system is needed in the diesel engine. Also, diesel fuel, which easily self-combusts, must be used. The fuel injection system of the diesel engine includes a fuel tank, a fuel pipe, a fuel supply pump, a fuel filter, an injection pump, a high pressure pipe, and an injection nozzle. Fuel is supplied to the injection nozzle by first passing through these elements in the order listed. 
     The injection pump is driven by an engine crankshaft to pressurize fuel to a high pressure and inject the fuel into the combustion chamber through the injection nozzle. The fuel is injected into the combustion chamber at a predetermined pressure and at the appropriate time. Together with the shape of the combustion chamber, injection timing, injection period, fuel injection amount, injection state (i.e., spray travel distance or penetration force), distribution state, degree of atomization, etc., these are all important factors in determining the combustion state. A governor and timer are provided in the injection pump to enable the variation of fuel injection amounts and fuel injection timing. 
     The fuel injection amount and fuel injection timing can be mechanically controlled or electronically controlled using a microcomputer. These parameters are now, for the most part, controlled electronically. Also, much research is being conducted to enable more precise control of fuel injection amount and timing. 
     Diesel engines are more often found in large vehicles such as buses and trucks. As a safety precaution, a speed limit device is typically provided to prevent the vehicle from travelling over a predetermined speed of, for example, a legal speed limit. That is, when the predetermined speed is exceeded, operation of the accelerator pedal has no effect in that the fuel supply to the engine is cut-off. 
     In both the vehicle equipped with the mechanical and electronic control of the fuel injection amount and fuel injection timing, it is possible to disengage such control. In the case of the diesel engine that is electronically controlled, it is possible to remove or disconnect the vehicle speed sensor, thereby disenabling fuel injection amount and timing control. However, when the vehicle speed sensor is removed or disconnected, various other electronic controls are also disabled, such as the automatic cruise control. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to solve the above problems. 
     It is an object of the present invention to provide a control method for a diesel engine in which the vehicle speed limit is automatically increased to a specific level to enable an increase in speed by the operation of the accelerator pedal. 
     To achieve the above object, the present invention provides a method for controlling a diesel engine comprising the steps of determining if a vehicle is travelling on a downgrade; outputting a predetermined fuel supply cut-off signal when the vehicle is travelling on a downgrade; determining an error between a vehicle speed limit and a present vehicle speed; setting an aiming speed as a first predetermined value; determining an aiming error; determining if the aiming error is greater than 0; setting an enable vehicle speed as a second predetermined value when the aiming error is greater than 0; setting the enable vehicle speed to 0 when the aiming error is less than 0; subtracting the error between the vehicle speed limit and the present vehicle speed from the enable vehicle speed to thereby derive an enable error; determining if the enable error is greater than 0; setting an offset vehicle speed Off_set as a third predetermined value when the enable error is greater than 0; setting the offset vehicle speed to 0 when the enable error is not greater than 0; determining if an accelerator pedal position has reached or is greater than a fourth predetermined value; adding the offset vehicle speed to the vehicle speed limit to thereby obtain a new vehicle speed limit when the accelerator position has reached the fourth predetermined value; determining if the present vehicle speed is less than the new vehicle speed limit; outputting a fuel supply control signal when the present vehicle speed is less than the new vehicle speed limit; and outputting a fuel supply cut-off control signal when the present vehicle speed is greater than the new vehicle speed limit. 
     According to a feature of the present invention, it is determined if the vehicle is travelling on a downgrade when the vehicle speed exceeds the vehicle speed limit for over a predetermined duration of time. 
     According to another feature of the present invention, the first and second predetermined values are stored in a pre-installed program and are respectively 6 kph and 4 kph. 
     According to still another feature of the present invention, when the enable error is greater than 0, it is determined that the vehicle has reached the end of the downgrade. 
     According to still yet another feature of the present invention, the third predetermined value is stored in a pre-installed program and is 20 kph. 
     According to still yet another feature of the present invention, the fourth predetermined value is 50%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
     FIG. 1 is a block diagram of a diesel engine control system to which the present invention is applied; 
     FIG. 2 is a flow chart of a control method for a diesel engine according to a preferred embodiment of the present invention; and 
     FIG. 3 shows various graphs of vehicle speed and fuel supply in relation to time as a vehicle travels on a downgrade. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1 shows a block diagram of a diesel engine control system to which the present invention is applied. 
     The diesel engine control system comprises a detection unit  10  including a vehicle speed sensor  11  for detecting vehicle speed and an accelerator pedal position sensor  12  for detecting a position of an accelerator pedal; a control unit  20  for controlling fuel supply; and a fuel injector  30  operated according to signals output by the control unit  20  to supply fuel and cut off the supply of the same. 
     The control unit  20  receives signals output by the detection unit  10  to determine vehicle speed and accelerator pedal position, and compares the vehicle speed with a predetermined vehicle speed limit. When the vehicle speed exceeds the vehicle speed limit, the control unit  20  outputs a fuel supply cut-off signal. Using the detected vehicle speed, the control unit  20  also determines if the vehicle is travelling on a downgrade, as well as when an end of the downgrade has been reached. When the vehicle is travelling on a downgrade, an offset vehicle speed is established and it is determined if the position of the accelerator pedal has reached or exceeded a third predetermined value. When the position of the accelerator pedal is at or exceeds the third predetermined value, the offset vehicle speed is added to the predetermined vehicle speed limit to generate a new vehicle speed limit. Next, the control unit  20  determines if the detected vehicle speed is less than the new vehicle speed limit. When the detected vehicle speed is less than the new vehicle speed limit, the control unit  20  outputs a fuel supply control signal, and when the detected vehicle speed is greater than the new vehicle speed limit, the control unit  20  outputs a fuel supply cut-off control signal. 
     FIG. 2 shows a flow chart of a control method for a diesel engine according to a preferred embodiment of the present invention. 
     The control unit  20 , which performs control of the diesel engine using a pre-installed program, receives driving detection signals from the detection unit  10  in step S 100 . That is, the vehicle speed sensor  11  and the accelerator pedal position sensor  12  detect vehicle speed and accelerator pedal position, respectively, then outputs corresponding detection signals to the control unit  20 . 
     The control unit  20  receives and reads the detection values of vehicle speed and acceleration pedal position from the detection unit  10 , then determines if the vehicle speed exceeds a vehicle speed limit stored in a memory of the pre-installed program in step S 110 . When the vehicle speed exceeds the vehicle speed limit, the control unit  20  determines that the vehicle is travelling on a downgrade (see FIG.  3 ), after which the control unit  20  outputs a predetermined fuel supply cut-off signal to the fuel injector  30  in step S 120 . The fuel injector  30  is driven by the fuel supply cut-off signal output by the control unit  20  to thereby discontinue the supply of fuel to the engine. 
     Next, the control unit  20  subtracts the vehicle speed limit from the presently detected vehicle speed and determines an error between the vehicle speed limit and the vehicle speed in step S 130 . The control unit  20  then sets an aiming speed as a first predetermined value (e.g., 6 kph) stored in the memory of the pre-installed program in step S 140 . 
     After step S 140 , the control unit  20  subtracts the aiming speed from the error between the vehicle speed limit and the vehicle speed to derive an aiming error Am_Err in step S 150 . The control unit  20  then determines if the aiming error Am_Err is greater than 0 in step S 160 . 
     When the aiming error Am_Err is greater than 0, the control unit  20  sets an enable vehicle speed as a second predetermined value (e.g., 4 kph) stored in the memory of the pre-installed program in step S 170 . However, when the aiming error Am_Err is less than 0, the control unit  20  sets the enable vehicle speed to 0 in step S 171 . After either step S 170  or S 171 , the control unit  20  subtracts the error between the vehicle speed limit and the present vehicle speed from the enable vehicle speed to thereby derive an enable error En_Err in step S 180 . 
     Subsequently, the control unit  20  determines if the enable error En_Err is greater than 0 in step S 190 . When the enable error En_Err is greater than 0, the control unit  20  sets an offset vehicle speed Off_set as a third predetermined value (e.g., 20 kph) stored in the memory of the pre-installed program in step S 200 . At this time, it is determined that the vehicle has reached the end of the downgrade. 
     However, when the enable error En_Err is not greater than 0 in step S 190 , the control unit  20  determines that the vehicle is still on the downgrade such that the offset vehicle speed Off_set is set to 0 in step S 201 . Either after step S 200  or S 201 , the control unit  20  determines if the accelerator pedal position has reached or is greater than, by operation of the driver, a fourth predetermined value in step S 210 . When the accelerator pedal position is less than the fourth predetermined value, the control process is ended. 
     When the accelerator position has reached the fourth predetermined value, the control unit  20  adds the offset vehicle speed Off_set to the vehicle speed limit stored in the pre-installed program to thereby obtain a new vehicle speed limit New_Speed in step S 220 . The control unit  20  then determines if the present vehicle speed is less than the new vehicle speed limit New_Speed in step S 230 . 
     When the present vehicle speed is less than the new vehicle speed limit New_Speed, the control unit  20  outputs a fuel supply control signal to the fuel injector  30  such that the vehicle increases to a speed corresponding to the position of the accelerator pedal operated by the driver in step S 240 . However, when the present vehicle speed is greater than the new vehicle speed limit New_Speed, the control unit  20  outputs a fuel supply cut-off control signal to the fuel injector  30  in step S 250 . The fuel injector  30  acts accordingly. 
     In the control method of the present invention as described above, after determining when the vehicle is driving on a downgrade, the vehicle speed limit is increased to a specific level to enable an increase in speed by the operation of the accelerator pedal. Accordingly, the momentum of a downgrade can be used. 
     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.
A method for controlling a diesel engine, is provided where when a vehicle is traveling on a downgrade, an enable vehicle speed is set according to a calculated aiming error, and a vehicle speed limit is modified based on an accelerator pedal position and by an offset value that is set according to an enable error calculated based on the present vehicle speed limit, the present vehicle speed, and the enable vehicle speed.