Patent Publication Number: US-10316494-B2

Title: Working machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase application of International Patent Application No. PCT/EP2014/073608, filed Nov. 3, 2014, which claims priority to Japanese Patent Application No, 2013-229269, filed Nov. 5, 2013, both of which are incorporated by reference herein in their entireties for all purposes. 
     TECHNICAL FIELD 
     The present invention relates to a working machine having a traveling hydraulic pressure motor. 
     BACKGROUND ART 
     In recent years, energy saving (reduction in fuel consumption) has become an important issue in working machines. Moreover, in excavators, it is requested to suppress an engine rotation speed as low as possible in an idling mode or during an excavation operation (normal operation) in a relatively light load. 
     Here, merely lowering a rated engine rotation speed itself is not possible when traveling performance needs to be ensured. That is, if the rated engine rotation speed is decreased, a maximum pump flow rate decreases to decrease a traveling speed, and a maximum engine output decreases to decrease traveling ability under power-demanding conditions such as on slope land. 
     Thus, there is a demand for a controller capable of reducing an engine rotation speed during a normal operation state and securing a traveling speed and traveling power. 
     From such a viewpoint, as a technique of easily realizing low fuel consumption and low noise during an excavation operation while securing traveling performance, an excavator engine controller in which engine rotation speed control means selects isochronous control when a traveling state is detected and selects droop control when the traveling state is not detected so that an engine rotation speed during an operation is lower than the engine rotation speed (rated engine rotation speed) during traveling is disclosed (see Patent Literature 1). 
     Further, an engine controller is disclosed in which, when the input from travel operation means is detected, a second target rotation speed which is a predetermined target engine rotation speed during traveling is compared with a first target rotation speed which is a minimum engine rotation speed at which a target pump discharge flow rate is realized with maximum pump displacement and the engine rotation speed is controlled by selecting the larger target rotation speed, that is, the smaller engine rotation speed is selected when a machine is not traveling (see Patent Literature 2). 
     Further, as an engine controller capable of controlling driving of an engine efficiently with low fuel consumption, an engine controller which detects an operation state of a lever with pilot pressure, detects the type and combination of hydraulic actuators operated from the operation state, applies a target engine rotation speed (upper limit) set in advance individually according to the type and combination, and sets an upper limit of an engine rotation speed in a travel-only mode so as to be higher than the lower limit of the engine rotation speed is disclosed (see Patent Literature 3). 
     Patent Literature 1: Japanese Patent Application Publication No. 2007-255414 
     Patent Literature 2: Japanese Patent Application Publication No. 2009-074406 
     Patent Literature 3: Japanese Patent Application Publication No.2011-157931 
     As described above, although techniques of implementing change of an engine rotation speed between during operations and during traveling have been known in the art, the conventional engine controllers cannot deal with power-demanding traveling conditions such as on slope land or during spin-turning. 
     DISCLOSURE OF THE INVENTION 
     The present invention has been made in view of such a problem, and an object thereof is to provide a working machine capable of reducing fuel consumption during a normal operation state and securing traveling speed and power under severe traveling conditions. 
     An invention according to claim  1  is a working machine including: a vehicle body that includes a traveling hydraulic pressure motor and travels using the traveling hydraulic pressure motor; a working unit that includes a working hydraulic pressure actuator and is mounted on the vehicle body operated by the working hydraulic pressure actuator; an engine that is mounted on the vehicle body; a variable displacement hydraulic pressure pump that is driven by the engine so as to supply an operating fluid to the traveling hydraulic pressure motor and the working hydraulic pressure actuator; and a controller that decreases an engine rotation speed to be lower than a rated rotation speed during a normal operation state in a state where the traveling hydraulic pressure motor is stopped, that controls the engine rotation speed so that a pump flow rate is adjusted corresponding to a target traveling speed during a light load state where an engine load generated when operating the traveling hydraulic pressure motor is lower than a threshold, and that controls the engine rotation speed and a pump control output torque so that engine output power is increased from that during the normal operation state and the light load, during a heavy load state where the engine load is higher than the threshold. 
     An invention according to claim  2  is the working machine according to claim  1 , in which the controller has: a normal operation mode where during the normal operation state, a normal operation mode engine rotation speed, which is lower than the rated rotation speed, is selected as an engine rotation speed command value and a normal torque is selected as a target pump control torque; a light load traveling mode where during the light load state, a light load traveling mode engine rotation speed, which is different from the normal operation mode engine rotation speed, is selected as the engine rotation speed command value and the normal torque is selected as the target pump control torque; and a heavy load traveling mode where during the heavy load state, a heavy load traveling mode engine rotation speed, which is higher than the normal operation mode engine rotation speed, is selected as the engine rotation speed command value and a torque increased from the normal torque of the normal operation mode and the light load traveling mode is selected as the target pump control torque. 
     An invention according to claim  3  is the working machine according to claim  1  or  2 , in which the traveling hydraulic pressure motor is provided on each of left and right sides of the vehicle body, a plurality of variable displacement hydraulic pressure pumps are provided so as to correspond to these traveling hydraulic pressure motors and include a pump swash plate as displacement varying means, and the controller maintains a pump swash plate of each hydraulic pressure pump during traveling at a maximum swash angle and variably controls the engine rotation speed. 
     An invention according to claim  4  is the working machine according to claim  2  or  3 , in which the controller has: a first threshold for changing the engine rotation speed from a light load traveling mode engine rotation speed to a heavy load traveling mode engine rotation speed when the engine load during traveling increases; and a second threshold, which is lower than the first threshold, for controlling to return the engine rotation speed from the heavy load traveling mode engine rotation speed to the light load traveling mode engine rotation speed when the engine load decreases. 
     An invention according to claim  5  is the working machine according to any one of claims  1  to  4 , in which the engine load is determined based on at least one measured amounts of a pump load pressure discharged from the hydraulic pressure pump, a traveling load pressure applied to the traveling hydraulic pressure motor, a fuel discharge amount of the engine, and a cylinder internal pressure of the engine, or the fuel discharge amount and the rotation speed of the engine. 
     According to the invention disclosed in claim  1 , in the working machine in which the working unit operated by the working hydraulic pressure actuator is mounted on the vehicle body having the traveling hydraulic pressure motor, the controller decreases the engine rotation speed to be lower than the rated rotation speed during the normal operation state, controls the engine rotation speed so that the pump flow rate is adjusted so as to correspond to the target traveling speed during the light load state, and controls the engine rotation speed and the pump control output torque so that the engine output power is increased from that of during the normal operation state and the light load state during the heavy lead state. Thus, during the normal operation state, it is possible to reduce the fuel consumption by decreasing the engine rotation speed. During the light load state, it is possible to secure a predetermined traveling speed by adjusting the pump flow rate so as to correspond to the target traveling speed. Further, during the heavy load state, the engine rotation speed and the pump control output torque are controlled so that the engine output power is increased from that during the normal operation state and the light load state. Therefore, it is possible to secure the traveling speed and the output power under severe traveling conditions such as during traveling on slope lane or during spin-turning. As a result, it is possible to reduce the fuel consumption during the normal operation state of the working machine and to secure the traveling speed and the output power under severe traveling conditions. 
     According to the invention disclosed in claim  2 , the normal operation mode during the normal operation state where the vehicle stops traveling, the light load traveling mode during the light load state, and the heavy load traveling mode during the heavy load state are selected, and different combinations of the engine rotation speed and the target pump control torque are selected for respective modes. Thus, the control ideal for each mode can be performed easily. 
     According to the invention disclosed in claim  3 , the controller variably controls the engine rotation speed while maintaining the pump swash plates of the plurality of hydraulic pressure pumps provided so as to correspond to the traveling hydraulic pressure motors on both left and right sides of the vehicle body to the maximum swash angle. Thus, it is possible to control the pump flow rates discharged from the respective hydraulic pressure pumps appropriately. Since the pump swash plates of the respective hydraulic pressure pumps are maintained to the maximum swash angle, it is possible to improve the pumping efficiency. Further, since a state where the pump swash plates reach their physical limits is created, a difference between the pump flow rates of two variable displacement pumps occurs rarely, and the ability to travel straight can be secured. 
     According to the invention disclosed in claim  4 , with switching of an engine rotation speed being implemented between the rising period and the falling period of the pump load pressure along a hysteresis curve on the basis of two steps of first and second thresholds, it is possible to prevent oscillatory behavior of the engine rotation speed and to improve stability of control. 
     According to the invention disclosed in claim  5 , it is possible to select an output torque calculated from at least one of the measured amounts that can be detected by sensors provided in an existing working machine among the pump load pressure, the traveling load pressure, the fuel discharge amount of the engine, and the cylinder internal pressure of the engine, or the fuel discharge amount and the rotation speed of the engine that can be detected by sensors provided in an existing working machine and to determine the engine load based on the output torque. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a characteristic diagram of an engine rotation speed and an engine output power, illustrating an embodiment of engine control in a working machine according to the present invention; 
         FIG. 2  is a flowchart illustrating how the engine control switches modes; 
         FIG. 3  is a flowchart illustrating how the engine control determines an engine rotation speed command value; 
         FIG. 4  is a flowchart illustrating how the engine control determines a torque command value; 
         FIG. 5  is a side view of the working machine; and 
         FIG. 6  is a block diagram illustrating a configuration example associated with the engine control. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, the present invention will be described in detail according to an embodiment illustrated in  FIGS. 1 to 6 . 
     In this embodiment, a case where engine rotation speeds are set such that (normal operation mode engine rotation speed)&lt;(light load traveling mode engine rotation speed)&lt;(heavy load traveling mode engine rotation speed) is described by way of an example. However, the engine rotation speeds are not necessarily set such that the light load traveling mode engine rotation speed is between the normal operation mode engine rotation speed and the heavy load traveling mode engine rotation speed. The engine rotation speeds may be set such that (light load traveling mode engine rotation speed)≤(normal operation mode engine rotation speed)&lt;(heavy load traveling mode engine rotation speed) or that (normal operation mode engine rotation speed)&lt;(heavy load traveling mode engine rotation speed)≤(light load traveling mode engine rotation speed). However, description of these cases is not provided. 
     Engine rotation speeds may be set such that (normal operation mode engine rotation speed)&lt;(light load traveling mode engine rotation speed) when a pump flow rate is prioritized and that (normal operation mode engine rotation speed)&lt;(heavy load traveling mode engine rotation speed) when engine output power is prioritized. 
       FIG. 5  illustrates an excavator  1  as a working machine, and a vehicle body  2  of the excavator  1  has a configuration in which an upper slewing body  4  is rotatably provided on a lower traveling body  3  movable by traveling hydraulic pressure motors (hereinafter referred to as traveling motors  3   m ) provided on both left and right sides of the vehicle body  2 , and a working unit  5  is mounted on the upper slewing body  4 . 
     An engine and pump unit  6  that supplies operating oil to hydraulic actuators (the traveling motor  3   m , a slewing motor  5   m , a boom cylinder  5   a , a stick cylinder  5   b , and a bucket cylinder  5   c ) that drive the vehicle body  2  and the working unit  5  is mounted on the vehicle body  2 . The slewing motor  5   m , the boom cylinder  5   a , the stick cylinder  5   b , and the bucket cylinder  5   c  are working hydraulic actuators  5   m ,  5   a ,  5   b , and  5   c  as working hydraulic pressure actuators. 
       FIG. 6  illustrates a schematic configuration of an engine and pump controller  7  that controls the engine and pump unit  6 . The engine and pump controller  7  controls a displacement (swash angle) of a variable displacement hydraulic pressure pump (hereinafter referred to as a variable displacement pump  11 ) that supplies operating oil as operating fluid to a hydraulic circuit  10  such as a control valve that controls the hydraulic actuators (the traveling motor  3   m  and the like) and controls an engine rotation speed (engine speed) and engine output power of an engine  12  that drives the variable displacement pump  11 . Two variable displacement pumps  11  are provided so as to correspond to the traveling motors  3   m  on the left and right sides of the vehicle body. 
     The engine  12  includes a rotation speed sensor  13  that detects an engine rotation speed and a fuel injector  14  that controls fuel injection pressure, a fuel injection point in time (timing), and a fuel injection amount (period). The rotation speed sensor  13  and the fuel injector  14  are connected to an engine controller  15  for fuel injection control. The fuel injector  14  includes an electronic governor or the like for setting engine rotation speeds. 
     The variable displacement pump  11  includes a pump regulator  16  that includes a pump swash plate as pump displacement varying means and controls a tilt angle (hereinafter referred to as a swash angle) of the pump swash plate indirectly with the aid of an electromagnetic proportional valve  16   s  or directly without the aid of the electromagnetic proportional valve  16   s , a swash angle sensor that detects the swash angle controlled by the pump regulator  16  as a pump displacement control position, and a pump pressure sensor  18  as load pressure detecting means for detecting pump load pressure (that is, pump discharge pressure) used for determination of an engine load. The swash angle sensor  17  and the pump pressure sensor  18  are connected to a machine controller  19 . 
     The pump pressure sensor  18  as the load pressure detector detects working load pressure as pump load pressure during operations in a non-traveling state and detects the larger one of traveling load pressure and working load pressure applied to the traveling motor  3   m  as the pump load pressure during traveling and provides data used when determining whether a present control mode is a light load traveling mode or a heavy load traveling mode described later. A pressure sensor may detect high-pressure-side pressure on a traveling hydraulic circuit as the traveling load pressure. 
     Moreover, as parameters for determining an engine load, at least one of the measured amounts such as traveling load pressure applied to the traveling motor  3   m  (when working load pressure is small only), a fuel discharge amount of the engine  12 , and cylinder internal pressure of the engine  12  or an output torque calculated from the fuel discharge amount and the rotation speed of the engine  12  in addition to the pump load pressure discharged from the variable displacement pump  11  may be used. These parameters are measured amounts that can be detected by the sensors provided in an existing working machine or state amounts (the output torque) that can be calculated from the measured amounts detected by the sensors provided in a working machine. The engine load can be determined based on the measured amounts or state amounts. 
     A lever or pedal-type operating tool  20  operated by an operator of the excavator  1  and a dial engine governor (dial accelerator)  21  as engine rotation speed setting means are connected to the machine controller  19 . 
     The operating tool  20  is a joystick or a remote control valve. A joystick-type operating tool controls a control valve of the hydraulic circuit  10  according to an operation amount indirectly with the aid of an electromagnetic proportional valve according to pilot control, and a remote control valve-type operating tool directly controls the control valve of the hydraulic circuit  10  according to the operation amount. The operation amounts (including the presence of operations) of various hydraulic actuators are converted into electrical signals and are input to the machine controller  19 . 
     The dial engine governor  21  has multi-step dial values and can select a predetermined engine rotation speed for each dial value, which includes a normal operation mode engine rotation speed, a light load traveling mode engine rotation speed, a heavy load traveling mode engine rotation speed, and the like described later. 
     The engine controller  15  and the machine controller  19  are connected and exchange information with each other. The engine controller  15  and machine controller  19  will be referred to as a controller  22 . 
     The controller  22  has a normal operation mode during a normal operation state, a light load traveling mode during a light load state, and a heavy load traveling mode during a heavy load state. 
     In the normal operation mode, a normal operation mode engine rotation speed lower than a rated rotation speed is selected as an engine rotation speed command value, and a normal torque is selected as a target pump control torque. 
     In the light load traveling mode, a light load traveling mode engine rotation speed different from the normal operation mode engine rotation speed is selected as the engine rotation speed command value, and a normal torque is selected as the target pump control torque. 
     In the heavy load traveling mode, a heavy load traveling mode engine rotation speed higher (or different from the normal operation mode engine rotation speed and the light load traveling mode engine rotation speed) than the normal operation mode engine rotation speed is selected as the engine rotation speed command value, and a torque increased from the normal torque in the normal operation mode and the light load traveling mode is selected as the target pump control torque. 
     The engine and pump controller  7  has a function of controlling the engine rotation speed and output power of the engine  12  with the aid of the engine controller  15  and the fuel injector  14  based on an engine rotation speed command value and a torque command value determined based on a travel operation state of the operating tool  20 , the dial value of the dial engine governor  21 , and the pump load pressure state detected by the pump pressure sensor  18 , adjusting the pump regulator  16  of the variable displacement pump  11  with the aid of an electromagnetic proportional valve (not illustrated) according to an electrical signal output from the machine controller  19 , and controlling the output and the upper limit output of the variable displacement pump  11 . 
       FIG. 1  is a characteristic diagram illustrating an engine rotation speed and engine output power associated with control of reducing the fuel consumption during the normal operation state in a state (non-traveling state) where the traveling motor  3   m  is stopped and securing a traveling speed and maximum output power in a traveling state. 
     In the example illustrated in  FIG. 1 , HE- 1  indicates the characteristics of an excavator having relatively low output power, HE- 2  indicates the characteristics of an excavator having relatively high output power, and in both excavators, a normal operation mode engine rotation speed during the normal operation state is lower than their rated rotation speed. That is, in order to reduce fuel consumption, the engine rotation speed is suppressed as low as possible. When the engine rotation speed is decreased to the normal operation mode engine rotation speed, the maximum flow rate of the variable displacement pump  11  and the maximum output power of the engine are decreased, which results in reduction in fuel consumption. 
     Moreover, during a light load state, for example, when the traveling motor  3   m  is operated so that the vehicle body  2  travels straight on a flat ground, the engine rotation speed is controlled such that the pump flow rate is adjusted so as to correspond to a target traveling speed. That is, isochronous control is performed so that the engine rotation speed is increased to a light load traveling mode engine rotation speed in order to secure the traveling speed and the rotation speed is maintained (first step). 
     In this case, when the engine rotation speed is variably controlled to an appropriate value while maintaining the pump swash plates of the two variable displacement pumps  11  provided so as to correspond to the traveling motors  3   m  on both left and right sides of the vehicle body  2  to their maximum swash angle, the pump flow rates discharged from the variable displacement pumps  11  can be controlled appropriately. Moreover, when the pump swash plates of the variable displacement pumps  11  are maintained to their maximum swash angles, pumping efficiency can be improved. Further, since a state where the pump swash plates reach their physical limits is created, a difference between the pump flow rates of two variable displacement pumps occurs rarely, and the ability to travel straight can be secured. 
     Further, during a heavy load state where more power than during the light load state is demanded, both an engine rotation speed and a pump control output torque are controlled so that engine output power is maximized. That is, when a travel load increases, the engine rotation speed is changed to a heavy load traveling mode engine rotation speed (second step) in order to increase the upper limit engine output power and the upper limit pump control output power is changed. 
     In this case, the engine rotation speed is controlled so that the engine rotation speed changes along a hysteresis curve rather than along an oscillatory curve such that the engine rotation speed is changed from the first step to the second step if the pump load pressure during traveling exceeds a first threshold A and the engine rotation speed returns from the second step to the first step if the pump load pressure is lower than a second threshold B, which is lower than the first threshold A. 
       FIG. 2  is a flowchart illustrating how the controller  22  switches modes, which will be described below. 
     (Step S 1 ) 
     It is determined whether a travel operation is detected. The travel operation may be detected by a pressure switch or a pressure sensor detecting a travel pilot pressure of operating a travel control valve or may be detected by a mechanical switch directly detecting the input of a travel lever. 
     (Step S 2 ) 
     When the travel operation is not detected, it is determined that the present control mode is a normal operation mode. 
     (Step S 3 ) 
     When the travel operation is detected, the pump load pressure detected by the pump pressure sensor  18  is detected. In the case of a travel-only operation, a traveling load pressure detected by a pressure sensor provided in a traveling hydraulic circuit may be used. 
     When the travel operation is performed, although the present control mode is changed from the normal operation mode to the traveling mode, since two traveling modes of a light load traveling mode and a heavy load traveling mode are present depending on the pump load pressure, a threshold for distinguishing the two modes is determined. In this case, since a system becomes unstable if the measured pump load pressure fluctuates frequently across a threshold, the first and second thresholds A and B having hysteresis characteristics are set. 
     That is, the controller  22  has the first threshold A at which the engine rotation speed is modified from the light load traveling mode engine rotation speed to the heavy load traveling mode engine rotation speed when the pump load pressure during traveling increases and the second threshold B at which the engine rotation speed is decreased from the heavy load traveling mode engine rotation speed to the light load traveling mode engine rotation speed when the pump load pressure decreases. The controller  22  uses the first threshold A when the pump load pressure increases and uses the second threshold B, which is lower than the first threshold A, when the pump load pressure decreases. In this way, the engine rotation speeds in the rising period and the falling period of the pump load pressure are switched along a hysteresis curve using the two steps of first and second thresholds A and B. Thus, it is possible to prevent oscillatory behavior of the engine rotation speed and to improve stability of control. These thresholds A and B are parameters that can be adjusted depending on model. 
     (Step S 4 ) 
     It is determined whether an average pump load pressure detected by the pump pressure sensor  18  and the like is higher than the threshold A or B. That is, it is determined whether the increasing pump load pressure is higher than the first threshold A and whether the decreasing pump load pressure is higher than the second threshold B. 
     (Step S 5 ) 
     When the pump load pressure is lower than the threshold A or B, it is determined that the present control mode is a light load traveling mode. That is, when the increasing pump load pressure is lower than the first threshold A or the decreasing pump load pressure is lower than the second threshold B, it is determined that the present control mode is a light load traveling mode. 
     (Step S 6 ) 
     When the pump load pressure is higher than the threshold A or B, it is determined that the present control mode is a heavy load traveling mode. That is, when the increasing pump load pressure is higher than the first threshold A or the decreasing pump load pressure is higher than the second threshold B, it is determined that the present control mode is a heavy load traveling mode. 
       FIG. 3  is a flowchart illustrating how the controller  22  determines an engine rotation speed command value, which will be described below. 
     An optimal traveling engine rotation speed command value for each of the normal operation mode, the light load traveling mode, and the heavy load traveling mode is provided, and the command value is set as a parameter of each model. 
     (Step S 11 ) 
     It is determined whether the present control mode is a normal operation mode, a light load traveling mode, or a heavy load traveling mode. 
     (Step S 12 ) 
     When the present control mode is a normal operation mode, the normal operation mode engine rotation speed is determined. 
     (Step S 13 ) 
     In the case of step S 12 , the normal operation mode engine rotation speed is output from the machine controller  19  to the engine controller  15  as an engine rotation speed command value. 
     (Step S 14 ) 
     When it is determined in step S 11  that the present control mode is a light load traveling mode, it is determined whether the dial value of the dial engine governor  21  is the maximum value or a value close to the maximum value, or is lower than these values. (Step S 15 ) 
     When it is determined in step S 14  that the dial value of the dial engine governor  21  is the maximum value (for example, “10”) or a value (for example, “9” or “8”) close to the maximum value, the light load traveling mode engine rotation speed is determined as the engine rotation speed command value and is output from the machine controller  19  to the engine controller  15 . It is assumed that the light load traveling mode engine rotation speed is a value which realizes a target traveling speed. 
     The light load traveling mode engine rotation speed command value is calculated according to the following equation.
 
(Target traveling motor rotation speed)×(Traveling motor displacement)=(Traveling flow rate)
 
(Traveling flow rate)/(Pump flow rate)=(Pump rotation speed)
 
(Pump rotation speed)/(Engine-pump reduction ratio)=(Engine rotation speed)
 
     (Step S 16 ) 
     When it is determined in step S 14  that the dial value of the dial engine governor  21  is not the maximum value or a value close to the maximum value, mode switching is implemented and then the normal operation mode engine rotation speed is determined as the engine rotation speed command value and is output from the machine controller  19  to the engine controller  15 . During the mode switching, a delay circuit or a ramp function for smoothing the control may be used. 
     (Step S 17 ) 
     When it is determined in step S 11  that the present control mode is a heavy load traveling mode, it is determined whether the dial value of the dial engine governor  21  is the maximum value or a value close to the maximum value, or is lower than these values. 
     The dial value of the dial engine governor  21  is set such that the “light load traveling mode engine rotation speed” is set as the command value when the dial value is equal to or larger than a predetermined dial value only and the “normal operation mode engine rotation speed” is set as the command value when the dial value is smaller than the predetermined dial value. 
     (Step S 18 ) 
     When it is determined in step S 17  that the dial value of the dial engine governor  21  is the maximum value (for example, “10”) or a value (for example, “9” or “8”) close to the maximum value, the heavy load traveling mode engine rotation speed is determined as the engine rotation speed command value and is output from the machine controller  19  to the engine controller  15 . It is assumed that the heavy load traveling mode engine rotation speed is a value which can use the maximum output power (=(Torque)×(Rotation speed)) on an engine torque curve. 
     (Step S 19 ) 
     When it is determined in step S 17  that the dial value of the dial engine governor  21  is not the maximum value or a value close to the maximum value, mode switching is implemented and then the normal operation mode engine rotation speed is determined as the engine rotation speed command value and is output from the machine controller  19  to the engine controller  15 . 
     In steps S 15  and S 16  and steps S 18  and S 19 , when switching the rotation speed command values, a delay circuit or a ramp function for smoothing the control may be used. 
     Although in steps S 14  and S 17  of selecting the conditions for the engine rotation speed command value, illustrated in  FIG. 3 , a case where the dial value of the dial engine governor  21  is set to the maximum value or a value close to the maximum value is distinguished from a case where the dial value is set to a value smaller than these values, such conditional determination may not be performed. 
     For example, when it is determined in step S 11  that the present control mode is the light load traveling mode, the light load traveling mode engine rotation speed only may be determined as the engine rotation speed command value. Moreover, when it is determined in step S 11  that the present control mode is the heavy load traveling mode, the heavy load traveling mode engine rotation speed only may be determined as the engine rotation speed command value. 
       FIG. 4  is a flowchart illustrating how the controller  22  determines a torque command value, which will be described below. 
     (Step S 21 ) 
     It is determined whether the present control mode is a normal operation mode or a light load traveling mode, or a heavy load traveling mode. 
     (Step S 22 ) 
     When the present control mode is the normal operation mode or the light load traveling mode, a target pump control torque is determined as a normal torque. 
     (Step S 23 ) 
     In the case of step S 22 , the normal torque is output as a pump torque command value. 
     (Step S 24 ) 
     It is determined whether the dial value of the dial engine governor  21  is a maximum value or a value close to the maximum value. When the dial value is smaller than the maximum value or the value close to the maximum value, the heavy load traveling mode is switched to the light load traveling mode or the normal operation mode. During the mode switching, a delay circuit or a ramp function for smoothing the control may be used. 
     (Step S 25 ) 
     When the dial value of the dial engine governor  21  is equal to or larger than the maximum value or the value close to the maximum value, a maximum torque is used as a pump torque command value. In short, during the heavy load traveling mode, a target pump control torque is maximized. The maximum torque is set as a parameter of each model. 
     In step S 24 , the target pump control torque value may be determined as the maximum torque only when the dial value of the dial engine governor  21  is set to the maximum value “10”, and the torque may be switched to the normal torque when the dial value is “9” or smaller. Alternatively, the maximum torque may be determined as the pump torque command value when the dial value is the value “9” close to the maximum value or is “8” or larger, and the torque may be switched to the normal torque when the dial value is smaller than “8” or “7”. 
     Although in steps S 24 , a case where the dial value of the dial engine governor  21  is set to the maximum value (“10”) or a value (“9” or “8”) close to the maximum value is distinguished from a case where the dial value is set to a value smaller than these values, such conditional determination of step S 24  may not be performed. 
     For example, when it is determined in step S 21  that the present control mode is the heavy load traveling mode, the maximum torque only may be determined as the torque command value. 
     Next, how the engine rotation speed and torque are controlled in the following three modes depending on an operation state of the excavator will be described. 
     (1) During Normal Operation (Normal Operation Mode) 
     Mode switching condition: A state where no travel operation is performed is created (see  FIG. 2 ). 
     Engine rotation speed: The “normal operation mode engine rotation speed” lower than the rated rotation speed is selected as the engine rotation speed command value (see  FIG. 3 ). 
     Torque: The “normal torque” lower than the maximum value is selected as the target pump control torque (see  FIG. 4 ). Actually, the engine output power is controlled by controlling the pump torque. 
     (2) During Light Load (Light Load Traveling Mode) 
     Mode switching condition: A state where a travel operation is performed is detected and the pump load pressure is lower than the second threshold B (see  FIG. 2 ). Although the pump pressure detected by the pump pressure sensor  18  is used as the pump load pressure, the motor pressure detected by a pressure sensor as another pump load detecting means for detecting the high-pressure-side pressure on a traveling motor circuit may be used. 
     Engine rotation speed: The “light load traveling mode engine rotation speed” higher than the “normal operation mode engine rotation speed” and lower than “heavy load traveling mode engine rotation speed” is selected as the command value (see  FIG. 3 ). It is assumed that the light load traveling mode engine rotation speed is a value which can realize the target traveling speed (specification value). The maximum speed during the travel operation is always the target traveling speed (specification value). 
     The dial value of the dial engine governor  21  may be set such that the “light load traveling mode engine rotation speed” is set as the command value when the dial value is equal to or larger than a predetermined dial value only and the “normal operation mode engine rotation speed” defined in (1) is set as the command value when the dial value is smaller than the predetermined dial value (see  FIG. 3 ). 
     Torque: The “normal torque” lower than the maximum value is selected as the target pump control torque (see  FIG. 4 ). 
     (3) During Heavy Load (Heavy Load Traveling Mode) 
     Mode switching condition: A state where a travel operation is performed is detected and the pump load pressure is higher than the first threshold A (see  FIG. 2 ). 
     Engine rotation speed: The “heavy load traveling mode engine rotation speed” higher than the “normal operation mode engine rotation speed” and the “light load traveling mode engine rotation speed” is selected as the command value (see  FIG. 3 ). The heavy load traveling mode engine rotation speed is a rotation speed value at which the maximum output power on an engine torque curve is obtained. 
     The dial value of the dial engine governor  21  may be set such that the “heavy load traveling mode engine rotation speed” is set as the command value when the dial value is equal to or larger than a predetermined dial value only and the “normal operation mode engine rotation speed” defined in (1) is set as the command value when the dial value is smaller than the predetermined dial value (see  FIG. 3 ). 
     Torque: The “maximum torque” is selected as the target pump control torque (see  FIG. 4 ). A value unique to each model is set as the target torque. 
     The dial value of the dial engine governor  21  may be set such that the target torque is set to the maximum value when the dial value is equal to or larger than a predetermined dial value only and the normal torque defined in (1) is set as the command value when the dial value is smaller than the predetermined dial value (see  FIG. 4 ). 
     Here, if only one threshold is used rather than using different thresholds A and B, when the pump load pressure is near the threshold, the control mode may switch continuously between the two modes ( 2 ) and ( 3 ) in a short period of time and the engine rotation speed may fluctuate and exhibit oscillatory behavior. In order to prevent this, the threshold of the pump load pressure has hysteresis characteristics such that the first threshold A associated with the pump load pressure when the control mode switches from the mode ( 2 ) to the mode ( 3 ) and the second threshold B associated with the pump load pressure when the control mode switches from the mode ( 3 ) to the mode ( 2 ) is set to be lower than the first threshold A. 
     As described above, in the excavator  1  in which the working unit  5  operated by the working hydraulic actuators  5   m ,  5   a ,  5   b , and  5   c  is mounted on the vehicle body  2  having the traveling motor  3   m , the controller  22  decreases the engine rotation speed to be lower than the rated rotation speed during the normal operation state, controls the engine rotation speed so that the pump flow rate is adjusted so as to correspond to the target traveling speed during the light load state, and controls the engine rotation speed and the pump control output torque so that the engine output power is maximized during the heavy load state. Thus, during the normal operation state, it is possible to reduce the fuel consumption by decreasing the engine rotation speed. During the light load state, it is possible to secure a predetermined traveling speed by adjusting the pump flow rate so as to correspond to the target traveling speed. Further, in the heavy load, the engine output power is maximized by controlling the engine rotation speed and the pump control output torque. Therefore, it is possible to secure the traveling speed and the output power under severe traveling conditions such as during traveling on slope lane or during spin-turning. As a result, it is possible to reduce the fuel consumption during the normal operation state of the excavator  1  and to secure the traveling speed and the output power under severe traveling conditions. 
     Moreover, the normal operation mode during the normal operation state where the vehicle stops traveling, the light load traveling mode during the light load state, and the heavy load traveling mode during the heavy load state are selected depending on the operation state of the excavator  1 , and different combinations of the engine rotation speed and the target pump control torque are selected for respective modes to control the engine rotation speed and the torque. Therefore, the control ideal for each mode can be performed easily, and it is possible to reduce the fuel consumption during the normal operation state and to secure the traveling performance. 
     INDUSTRIAL APPLICABILITY 
     The present invention is industrially applicable to business operators associated with manufacturing and selling working machines. 
     EXPLANATION OF REFERENCE NUMERALS 
       1 : Excavator as working machine 
       2 : Vehicle body 
       3   m : Traveling hydraulic pressure motor (Traveling motor) 
       5 : Working unit 
       5   m ,  5   a ,  5   b ,  5   c : Working hydraulic actuator as working hydraulic pressure actuator 
       11 : Hydraulic pressure pump (Variable displacement pump) 
       12 : Engine 
       22 : Controller