Abstract:
An automatic cruise control method for vehicles is initiated by firstly selecting an automatic cruise mode. Then a current vehicle speed is detected and the current vehicle speed is compared with a target speed. Next, a new target speed is obtained according to the comparison and an amount of fuel to be injected is determined according to the new target speed .

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cruise control method for a vehicle. More particularly, the invention relates to a cruise control method that can improve drive efficiency of a vehicle when the vehicle runs up a slope in an automatic cruise control mode. 
     2. Description of the Related Art 
     Generally, in an automatic cruise mode, vehicle speed is maintained at a constant level by an automatic cruise control system. 
     FIG. 1 shows such a conventional automatic cruise control system. 
     An automatic cruise control system comprises a vehicle speed sensor  10  for detecting a vehicle speed, a throttle position sensor  11  for detecting a throttle opening, a cruise switch  12  for setting vehicle speed at a predetermined level according to a driver&#39;s will, an electronic control unit  13  for controlling a vehicle speed to the predetermined level by receiving signals from the vehicle speed sensor  10  and the throttle position sensor  11 , and a fuel injecting part  14  for injecting fuel according to a control signal transmitted from the electronic control unit  13 . 
     When a cruise control mode is selected by operating the cruise control switch  12 , the electronic control unit  13  controls the fuel injection part  14  according to vehicle speed and throttle opening signals transmitted from the vehicle speed and throttle position sensors  10  and  11 . That is, by controlling an amount of fuel being injected, the vehicle speed is maintained at a constant level. 
     FIG. 2 shows graphs illustrating an amount of fuel to be injected with respect to vehicle speeds according to various road conditions during an automatic cruise control mode. 
     As shown in the drawing, when the vehicle runs on a level ground to point A, since the difference between a current vehicle speed and a target speed is very low, the amount of fuel injected is not varied substantially. That is, the fuel is supplied according to a speed difference between a current speed and a target speed. 
     However, as the vehicle goes up a slope, since the vehicle speed is reduced, the amount of fuel to be injected is increased. Particularly, when the vehicle reaches a mid-point of the slope, although the amount of fuel delivery is maximized, it is very difficult to increase the vehicle speed to the target speed. 
     In addition, when the vehicle goes down the slope, since the vehicle speed increases above the target speed, the fuel injection is stopped. In this state, when the vehicle again meets an upward slope, the vehicle speed is reduced since the coasting speed obtained when the vehicle goes down the slope cannot be maintained. 
     That is, when the vehicle goes up the slope, the vehicle speed is not increased to the target speed even when the fuel is injected at its maximum rate. In addition, when the vehicle goes down the slope, since the coasting speed cannot be maintained, the drive effect in cruise control mode deteriorates. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to solve the above problems. 
     It is an objective of the present invention to provide a cruise control method for improving drive effect when a vehicle goes up and down hills while in a cruise control mode. 
     To achieve the above objective, the present invention provides an automatic cruise control method for vehicles, comprising the steps of selecting an automatic cruise mode; detecting a current vehicle speed; comparing the current vehicle speed with a target speed; determining a new target speed according to the comparison; and controlling an amount of fuel to be injected according to the new target speed. 
     The step of comparing the current vehicle speed with the target speed comprises the step of determining if the current speed is lower than the target speed. 
     The step of determining a new target speed comprises, if the current speed is lower than the target speed, the steps of obtaining a speed difference between the target speed and the current speed, and establishing the new target speed by adding the speed difference to the target speed. 
     The amount of fuel to be injected is determined by a value obtained by integrating the new target speed. 
     The automatic cruise control method may further comprise the step of, if the current speed is higher than the target speed, stopping an injection of fuel. 
     The automatic cruise control method may further comprise the step of setting a current speed as the new target speed after the step of stopping an injection of fuel. 
    
    
     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 schematic block diagram illustrating a conventional automatic cruise control system; 
     FIG. 2 is graphs illustrating an amount of fuel to be injected with respect to vehicle speeds according to various road conditions using a conventional automatic cruise control system. 
     FIG. 3 is a flow chart illustrating a cruise control method according to a preferred embodiment of the present invention; and 
     FIG. 4 is graphs illustrating an amount of fuel to be injected with respect to different vehicle speeds according to various road conditions using an automatic cruise control method according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 3 shows a flow chart illustrating an automatic cruise control method according to a preferred embodiment of the present invention. 
     As shown in FIG. 3, when the drive mode is set on an automatic cruise mode (step  20 ), a current vehicle speed is detected (step  21 ). In step  22 , it is determined whether or not the current vehicle speed is less than a cruise speed (a target speed) set by a driver. 
     If the current speed is less than the target speed, a speed difference between the current speed and the target speed is calculated in step  23 . A new target speed is calculated in step  24  by adding the speed difference determined in step  23  to the previous target speed, and in step  25  fuel is injected according to the new target speed. In step  26 , it is determined whether or not the cruise mode is turned off. 
     If the current speed of step  21  is not less than the target speed, as determined in step  22 , fuel injection is stopped in step  27 , and in step  28 , a new target speed is set at the current speed. In step  29 , it is then determined whether or not the new target speed is higher than a current speed. If the new target speed is higher than the current speed, step  25  is implemented so that into the engine for obtaining the new target speed fuel is injected into the engine for obtaining the new target speed. If the new target speed is not found to be higher than the current speed, step  27  is once again invoked. 
     As described above, when the driver turns on a cruise switch, the electronic control unit  13  begins controlling the vehicle speed in the automatic cruise control mode (step  20 ). 
     During automatic cruise control operation, the vehicle speed sensor  10  detects current speed and transmits the corresponding signals to the electronic control unit  13  (step  21 ). 
     Then, the electronic control unit  13  determines whether or not the detected current speed is less than the target speed (cruise speed) (step  22 ). If the current speed is less than the target speed, the electronic control unit  13  determines that the vehicle is encountering resistance as when it goes up a slope, and thus that the vehicle speed is reducing (see FIG.  4 ). The electronic control unit then calculates a speed difference between the current speed and the target speed using a pre-set program (step  23 ). 
     Next, a new target speed is calculated by adding the speed difference to the previous target speed as shown in FIG. 4 (step  24 ), then fuel is injected by controlling the fuel injection part  14  (step  25 ). At this point, the amount of fuel to be injected is determined by a value obtained by proportionally integrating the new target speed. 
     In other words, the fuel injection requirements are calculated for achieving the new target speed. Typically this calculation is accomplished by the electronic control unit  13 . 
     In this state, if the vehicle goes down a slope as shown in FIG. 4, the electronic control unit  13  determines if a current speed detected by the vehicle speed sensor  10  is higher than the target speed (step  22 ). 
     If the current speed is higher than the target speed, the electronic control unit  13  determines that the vehicle is going down the slope, and thus that the vehicle speed is increasing. Accordingly, the electronic control unit  13  controls the fuel injection part  14  such that it does not inject fuel (step  27 ). 
     In addition, the electronic control unit  13  assigns the current speed as the new target speed (step  28 ), then, in step  29 , determines if the new target speed is higher than a current speed. 
     If the new target speed is lower than the current speed, the electronic control unit  13  determines that the vehicle is still going down the slope, so it controls the fuel injection part  14  such that it does not inject fuel. If the new target speed is higher than the current speed, the electronic control unit determines that the vehicle is going up the slope, so fuel is injected as in step  25 . At this point, the amount of fuel to be injected is determined by a value found by proportionally integrating a final target speed determined as in step  24 . 
     As described above, in the present invention, since the target speed is varied in accordance with road conditions, drive effect of a vehicle in the automatic cruise mode can be greatly improved. 
     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.