WORK VEHICLE

Provided is a work vehicle capable of reducing the fuel consumption while maintaining the working efficiency. A wheel loader 1 comprising a vehicle body controller 5, 5A for controlling an engine 41, wherein the vehicle body controller 5, 5A limits an upper limit rotational speed of the engine 41 based on an accelerator pedal step-on amount α, a brake pedal step-on amount β, a torque converter speed ratio e which is a ratio of a rotational speed of a torque converter 42, a discharge pressure P1 of a loading hydraulic pump 44, and a stroke amount S1 of lift arm cylinders 22 or a stroke amount S2 of a bucket cylinder 24.

TECHNICAL FIELD

The present invention relates to a work vehicle equipped with a working device used in the work of excavating, carrying, and loading a work object such as earth and sand and minerals.

BACKGROUND ART

A work vehicle such as a wheel loader or a hydraulic excavator is equipped with a working device including an arm member rotationally movable in the vertical direction with respect to the vehicle body and a bucket rotationally movable in the vertical direction with respect to the arm member. When a hydraulic cylinder is supplied with a hydraulic oil, a rod extends or contracts, whereby the arm member and the bucket are driven, respectively. For example, in a state where the arm member is raised to the highest or the bucket is holding the load thereinto, when the rod extends or contracts to the limit and thus a piston reaches the stroke end, shock may occur.

In this respect, a hydraulic excavator disclosed in Patent Literature 1 is designed to, upon detecting that a piston of a hydraulic cylinder has approached the stroke end, reduce a discharge flow rate of a hydraulic pump and a rotational speed of an engine.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, if applying the technique for the hydraulic excavator disclosed in Patent Literature 1 to a wheel loader, the rotational speed of the engine is limited, for example, even in the excavation work in which the bucket may rotationally move upward to the limit (full tilt state) with respect to the arm member or in the loading work in which the arm member may rotationally move upward to the limit with respect to the vehicle body. This may cause delay in the actual operations and thus reduction in the working efficiency.

Therefore, an object of the present invention is to provide a work vehicle capable of reducing the fuel consumption while maintaining the working efficiency even when a hydraulic cylinder reaches the stroke end.

Solution to Problem

In order to achieve the object described above, the present invention provides a work vehicle comprising: a vehicle body provided with a plurality of wheels; an engine mounted on the vehicle body; a torque converter for amplifying a torque transmitted from the engine; an accelerator pedal for adjusting a rotational speed of the engine; a brake pedal for adjusting a braking force applied to the plurality of wheels; a working device attached to the vehicle body; a hydraulic cylinder for driving the working device; a loading hydraulic pump driven by the engine and supplying a hydraulic oil to the hydraulic cylinder; and a controller configured to control the engine, wherein the controller is configured to: limit an upper limit rotational speed of the engine based on a step-on amount of the accelerator pedal, a step-on amount of the brake pedal, a torque converter speed ratio that is a ratio of a rotational speed of the torque converter, a discharge pressure of the loading hydraulic pump, and a stroke amount of the hydraulic cylinder.

Advantageous Effects of Invention

According to the present invention, it is possible to reduce the fuel consumption while maintaining the working efficiency even when a hydraulic cylinder reaches the stroke end. The problems, configurations, and advantageous effects other than those described above will be clarified by explanation of the embodiments below.

DESCRIPTION OF EMBODIMENTS

Hereinafter, as one of the aspects of work vehicles according to each embodiment of the present invention, a wheel loader that excavates a work object such as earth and sand and minerals and loads the excavated work object into a dump truck and the like will be described.

<Configuration of Wheel Loader1>

Firstly, a configuration of a wheel loader1will be described with reference toFIG.1.

FIG.1is a side view illustrating appearance of a configuration example of the wheel loader1according to each embodiment of the present invention.

The wheel loader1is an articulated type work vehicle which is swiveled on a central portion of a vehicle body and steered thereby. Specifically, a front frame1A that is the front part of the vehicle body and a rear frame1B that is the rear part of the vehicle body are connected to each other by a center joint10to swivel in the left and right direction so that the front frame1A is bent in the left and right direction with respect to the rear frame1B.

The vehicle body is provided with four wheels11, namely, two of the wheels11are provided, as front wheels11A, on the left and right sides of the front frame1A, respectively, and the remaining two of the wheels11are provided, as rear wheels11B, on the left and right sides of the rear frame1B, respectively.FIG.1illustrates, among the pair of left and right front wheels11A and the pair of left and right rear wheels11B, only the front wheel11A and the rear wheel11B provided on the left side. In this connection, there are no limitation in the specific number of the wheels11to be provided on the vehicle body.

On a front portion of the front frame1A, a hydraulically driven working device2used for the loading work is provided. The working device2for loading includes a lift arm21with the proximal end portion attached to the front frame1A, two lift arm cylinders22for driving the lift arm21, a bucket23attached to the distal end portion of the lift arm21, a bucket cylinder24for driving the bucket23, and a bell crank25rotatably coupled to the lift arm21and serving as a link mechanism between the bucket23and the bucket cylinder24.

Each of the two lift arm cylinders22and the bucket cylinder24is one of the aspects of a hydraulic cylinder for driving the working device2. Although the two lift arm cylinders22are arranged side by side in the lateral direction of the vehicle body,FIG.1illustrates only one of the lift arm cylinders22which is disposed on the left side by a broken line.

The proximal end portion of the lift arm21is attached to the front frame1A, and when the two lift arm cylinders22are supplied with the hydraulic oil, the rods220expand or contract, respectively, whereby the lift arm21rotationally moves in the vertical direction with respect to the front frame1A. More specifically, the lift arm21rotationally moves upward with respect to the front frame1A in accordance with the extension of the rods220of the two lift arm cylinders22, and rotationally moves downward with respect to the front frame1A in accordance with the contraction of the rods220.

An arm angle sensor31serving as an angle sensor for detecting an angle θ1of the lift arm21with respect to the front frame1A (hereinafter, simply referred to as an “arm angle θ1”) is mounted on a portion of the lift arm21which is near the portion to which the front frame1A is coupled. The arm angle θ1detected by the arm angle sensor31is input to a vehicle body controller5which will be described later.

When the bucket cylinder24is supplied with the hydraulic oil, a rod240extends or contracts, whereby the bucket23rotationally moves in the vertical direction with respect to the lift arm21. More specifically, the bucket23tilts (rotationally moves upward with respect to the lift arm21) in accordance with the extension of the rod240of the bucket cylinder24, and dumps (rotationally moves downward with respect to the lift arm21) in accordance with the contraction of the rod240.

Note that the bucket23can be replaced with various attachments such as a blade, and in addition to the excavation work using the bucket23, the wheel loader1can perform various types of work such as dozing work, snow removing work, and the like.

A bucket angle sensor32serving as an angle sensor for detecting an angle θ2of the bucket23with respect to the lift arm21(hereinafter, simply referred to as a “bucket angle θ2”) is attached on a portion of the bucket23which is near the portion to which the lift arm21is coupled. As in the case of the arm angle θ1, the bucket angle θ2detected by the bucket angle sensor32is input to the vehicle body controller5which will be described later.

The rear frame1B includes an operator's cab12provided for an operator to get in, a machine room13for accommodating therein respective devices necessary for driving the wheel loader1, and a counterweight14for balancing the vehicle body with the working device2to prevent the vehicle body from tilting. On the rear frame1B, the operator's cab12is provided in the front thereof, the counterweight14is provided in the rear thereof, and the machine room13is provided between the operator's cab12and the counterweight14.

Next, the operations of the wheel loader1during the loading work will be described with reference toFIG.2toFIG.4.

FIG.2is a diagram for explaining the V-shade loading performed by the wheel loader1.FIG.3AtoFIG.3Care diagrams for explaining the excavation work performed by the wheel loader, and more specifically,FIG.3Aillustrates a scene in which the bucket23has been made thrust into a pile101,FIG.3Billustrates a scene in which the bucket23has scooped up the load, andFIG.3Cillustrates a scene in which the bucket23with the load therein has been raised upward.FIG.4is a diagram for explaining the dump approach operation performed by the wheel loader1.

Firstly, the wheel loader1travels forward toward the pile101to be excavated (arrow X1illustrated inFIG.2), and as illustrated inFIG.3A, makes the bucket23thrust into the pile101. Next, as illustrated inFIG.3B, in accordance with an operator's manipulation for raising the lift arm21while making the bucket23tilt or for raising the lift arm21after making the bucket23tilt, the wheel loader1scoops up a load such as earth and sand and minerals. Then, as illustrated inFIG.3C, in accordance with an operator's manipulation for raising the lift arm21, the bucket23is further raised upward with the load being loaded therein.

The series of operations illustrated inFIG.3AtoFIG.3Ccorresponds to the excavation work, and in the states illustrated inFIG.3BandFIG.3C, there are cases where the bucket23rotationally moves upward to the limit (full tilt state) with respect to the lift arm21so as to prevent the load being loaded therein from spilling off. In these cases, the rod240of the bucket cylinder24extends to the upper limit, and thus the stroke amount of the bucket cylinder24has reached a preset limit value. Upon completion of the excavation work, the wheel loader1moves rearward once to the original location (arrow X2illustrated inFIG.2)

Next, the wheel loader1moves forward toward a dump truck102which is a loading destination, and stops in front of the dump truck102to perform a dump approach operation (arrow Y1illustrated inFIG.2). Note thatFIG.2illustrates the wheel loader1in a state of being stopped in front of the dump truck102by a broken line.

In the dump approach operation, as illustrated inFIG.4, specifically, the operator steps on an accelerator pedal fully (full acceleration), and also performs the lifting operation to raise the lift arm21(state illustrated on the right side ofFIG.4). Next, the operator adjusts the vehicle speed by stepping on a brake pedal a little at the same time while further raising the lift arm21with maintaining the full acceleration state, so as to prevent the vehicle body from colliding with the dump truck102(state illustrated in the center ofFIG.4). Then, the operator further steps on the brake pedal to stop the vehicle body in front of the dump truck102, and makes the bucket23dump to discharge the load in the bucket23into the dump truck102(state illustrated on the left side ofFIG.4).

In the state illustrated inFIG.4, depending on the height of a vessel of the dump truck102, the lift arm21may rotationally move upward to the limit with respect to the front frame1A. In this case, the rods220of the two lift arm cylinders22extend to the upper limit, respectively, and thus the stroke amounts of the two lift arm cylinders22have reached preset limit values. Similarly, in the case of the dumping operation of the bucket23, the bucket23may rotatably move downward to the limit (full dump state) with respect to the lift arm21. In this case, the rod240of the bucket cylinder24contracts to the lower limit, and thus the stroke amount of the bucket cylinder24has reached a preset limit value.

Upon completion of the work of loading the load into the dump truck102, the wheel loader1moves rearward to the original location (arrow Y2illustrated inFIG.2). In this way, the wheel loader1performs the excavation work and the loading work by a method called “V-Shape loading” in which the wheel loader1reciprocates in a V-shape between the pile101and the dump truck102.

First Embodiment

Hereinafter, a drive system of the wheel loader1according to the first embodiment of the present invention will be described with reference toFIG.5toFIG.9.

(Overall Configuration of Drive System)

Firstly, the overall configuration of the drive system of the wheel loader1according to the first embodiment will be described with reference toFIG.5.

FIG.5illustrates a drive system configuration of the wheel loader1according to the first embodiment.

In the wheel loader1according to the present embodiment, the traveling of the vehicle body is controlled by a torque converter traveling drive system. The torque converter traveling drive system includes an engine41, an engine controller41A for controlling the engine41, a torque converter42coupled to an output shaft of the engine41, a transmission43coupled to an output shaft of the torque converter42, and a vehicle body controller5for controlling each of the devices including the engine controller41A, the torque converter42, and the transmission43.

The engine controller41A is connected to the vehicle body controller5via a CAN, and controls the engine41based on a signal output from the vehicle body controller5. Note that the engine controller41A and the vehicle body controller5are not necessarily provided separately, but one controller including the engine controller41A and the vehicle body controller5may be provided.

The torque converter42is a fluid clutch including an impeller, a turbine, and a stator, and has a function to amplify an output torque with respect to an input torque (torque transmitted from the engine41), that is, a function to make a torque ratio (=output torque/input torque) 1 or more.

The torque ratio decreases as a ratio of a rotational speed NE of the engine41(hereinafter, referred to as an “engine rotational speed NE”), which is also the rotational speed of an input shaft of the torque converter42, to a rotational speed NT of the output shaft of the torque converter42(hereinafter, referred to as a “torque converter output rotational speed NT”), that is, a torque converter speed ratio e which is a ratio of the rotational speed of the torque converter42(=rotational speed NT of the output shaft/rotational speed NE of the input shaft) increases. Thus, the torque converter42changes the rotational speed of the engine41, and then transmits it to the transmission43.

The engine rotational speed NE is detected by a first rotational speed sensor33provided on the output shaft of the engine41, and the torque converter output rotational speed NT is detected by a second rotational speed sensor34provided on the output shaft of the torque converter42, respectively, and are input to the vehicle body controller5.

The transmission43includes a clutch mechanism having a plurality of clutches and a gear mechanism having a plurality of transmission gears, and switches the traveling direction and speed stage of the vehicle body. That is, the transmission43changes the torque and rotational speed of the output shaft of the torque converter42, and then transmits them to the four wheels11.

In the torque converter traveling drive system, firstly, when the operator steps on an accelerator pedal71provided in the operator's cab12, the engine41rotates based on a step-on amount thereof, and then the input shaft of the torque converter42coupled to the output shaft of the engine41rotates in accordance with the rotation of the engine41. That is, operating the accelerator pedal71enables adjustment of the engine rotational speed NE.

A step-on amount α of the accelerator pedal71(hereinafter, referred to as an “accelerator pedal step-on amount α”) is proportional to the engine rotational speed NE, and the engine rotational speed NE increases as the accelerator pedal step-on amount α increases. Note that the accelerator pedal step-on amount α is detected, as a pedal opening degree, by a first step-on amount sensor35mounted on the accelerator pedal71, and then input to the vehicle body controller5.

When the input shaft of the torque converter42rotates, the output shaft of the torque converter42rotates via the oil inside the torque converter42. The transmission43changes the output torque from the torque converter42and then transmits it to the four wheels11, respectively, whereby the wheel loader1can travel.

In order to stop or decelerate the wheel loader1, the operator steps on a brake pedal72provided in the operator's cab12. The clutch mechanism of the transmission43is controlled based on a step-on amount β, of the brake pedal72(hereinafter, referred to as a “brake pedal step-on amount β”) to interrupt the transmission of the driving force to the four wheels11. That is, operating the brake pedal72enables adjustment of the braking force applied to the four wheels11. The brake pedal step-on amount β, is detected, as a brake secondary pressure, by a second step-on amount sensor36, and then input to the vehicle body controller5.

Furthermore, the wheel loader1is provided with, in addition to the traveling drive system of the vehicle body, a loading drive system for driving the working device2. The loading drive system includes a loading hydraulic pump44driven by the engine41to supply the hydraulic oil to the two lift arm cylinders22and the bucket cylinder24, respectively, a first directional control valve45provided between the loading hydraulic pump44and the two lift arm cylinders22, and a second directional control valve46provided between the loading hydraulic pump44and the bucket cylinder24.

The first directional control valve45controls the flow (flow rate and direction) of the hydraulic oil discharged from the loading hydraulic pump44and supplied to the two lift arm cylinders22. Similarly, the second directional control valve46controls the flow of the hydraulic oil discharged from the loading hydraulic pump44and supplied to the bucket cylinder24. A discharge pressure P1of the loading hydraulic pump44(hereinafter, referred to as a unit “discharge pressure P1”) is detected by a discharge pressure sensor37, and then input to the vehicle body controller5.

The first directional control valve45and the second directional control valve46are controlled based on command signals output from the vehicle body controller5, respectively. In the operator's cab12, an operation device73for operating the working device2(lift arm21and bucket23) is provided, and an operation signal corresponding to an operation amount of the operation device73is input to the vehicle body controller5. Then, the vehicle body controller5outputs a command signal based on the operation signal output from the operation device73.

Furthermore, in the present embodiment, the operator's cab12is provided with a monitor12A. The monitor12A displays, based on a display signal (notification signal) output from the vehicle body controller5, states of the devices such as a state of the engine41, which are necessary for the wheel loader1to perform the operations.

(Configuration of Vehicle Body Controller5)

Next, a configuration of the vehicle body controller5will be described with reference toFIG.6.

FIG.6is a functional block diagram illustrating functions of the vehicle body controller5.

The vehicle body controller5is configured such that a CPU, a RAM, a ROM, an HDD, an input I/F, and an output I/F are connected to each other via a bus. Various sensors such as the arm angle sensor31, the bucket angle sensor32, the first rotational speed sensor33, the second rotational speed sensor34, the first step-on amount sensor35, and the second step-on amount sensor36are connected to the input I/F, the engine controller41A and the monitor12A, etc. are connected to the output I/F.

In this hardware configuration, the CPU reads out a control program (software) stored in a recording medium such as the ROM, the HDD or an optical disc, and loads and executes the control program on the RAM, whereby the control program and the hardware collaborate to implement the functions of the controller5.

In the present embodiment, the controller5is described as a computer configured by a combination of software and hardware. However, the controller5is not limited thereto, and as one of the examples of configurations of other computers, an integrated circuit for implementing the functions of the control program executed on the side of the wheel loader1may be used.

The vehicle body controller5includes a data acquisition section51, a torque converter speed ratio calculation section52, a stroke amount calculation section53, a limitation condition determination section54, a storage section55, and a signal output section56.

The data acquisition section51is configured to acquire data relating to the accelerator pedal step-on amount α detected by the first step-on amount sensor35, the brake pedal step-on amount β, detected by the second step-on amount sensor36, the engine rotational speed NE detected by the first rotational speed sensor33, the torque converter output rotational speed NT detected by the second rotational speed sensor34, the discharge pressure P1detected by the discharge pressure sensor37, the arm angle θ1detected by the arm angle sensor31, and the bucket angle θ2detected by the bucket angle sensor32, respectively.

The torque converter speed ratio calculation section52is configured to calculate the torque converter speed ratio e (=NT/NE) based on the engine rotational speed NE and the torque converter output rotational speed NT acquired by the data acquisition section51. The stroke amount calculation section53is configured to calculate a stroke amount S1of the lift arm cylinders22(hereinafter, referred to as an “arm stroke amount S1”) based on the arm angle θ1acquired by the data acquisition section51, and a stroke amount S2of the bucket cylinder24(hereinafter, a “bucket stroke amount S2”) based on the bucket angle θ2acquired by the data acquisition section51, respectively.

In the present embodiment, the vehicle body controller5calculates the arm stroke amount S1based on the detected arm angle θ1and the bucket stroke amount S2based on the detected bucket angle θ2. However, the present embodiment is not limited thereto, but for example, a stroke sensor and the like which is capable of measuring a stroke amount of a hydraulic cylinder can be used to detect the stroke amount directly. In the case of using a stroke sensor and the like, it is necessary to separately attach new sensors to the lift arm cylinders22and the bucket cylinder24, on the other hand, using the existing arm angle sensor31and bucket angle sensor32enables reduction in the cost.

The limitation condition determination section54is configured to determine whether a first limitation condition and a second limitation condition are satisfied. The “first limitation condition” is a condition relating to the stall of the engine41, and includes that the accelerator pedal step-on amount α is equal to or more than a first step-on amount threshold value αth (α≥αth) and the brake pedal step-on amount β, is equal to or more than a second step-on amount threshold value βth (β≥βth), and in the present embodiment, further includes that the torque converter speed ratio e is equal to or less than a speed ratio threshold value eth (e≤eth).

Including that the torque converter speed ratio e is equal to or less than the speed ratio threshold value eth (e≤eth) in the first limitation condition enables the limitation condition determination section54(vehicle body controller5) to accurately determine a stall state of the engine41.

The first step-on amount threshold value αth corresponds to a first threshold value set based on an upper limit value of the accelerator pedal step-on amount, and is set to, for example, a value of about 90% of the upper limit value. The second step-on amount threshold value βth corresponds to a second threshold value set based on an upper limit value of the brake pedal step-on amount, for example, is set to a value of about 90% of the upper limit value. The speed ratio threshold value eth corresponds to a third threshold value set based on a torque speed ratio at the time when the engine41stalls, and is set to, for example, a value of about 0.2.

That is, when the first limitation condition is satisfied, the wheel loader1is in a state where, at least, it decelerates to the speed close to the speed at which it stops, and also revs up the engine41at the rotational speed close to the upper limit rotational speed (idling state).

The “second limitation condition” is a condition relating to the relief of the loading hydraulic pump44and the stroke end of the lift arm cylinders22or bucket cylinder24, and includes that the discharge pressure P1is equal to or more than a discharge pressure threshold value P1th (P1≥P1th), and the arm stroke amount S1is equal to or more than a first stroke amount threshold value S1th (S1≥S1th) or the bucket stroke amount S2is equal to or more than a second stroke amount threshold value S2th (S2≥S2th).

The discharge pressure threshold value P1th corresponds to a fourth threshold value set based on a main relief pressure of a drive circuit for driving the working device2, and is set to, for example, about 90% of the main relief pressure. The first stroke amount threshold value S1th and the second stroke amount threshold value S2th correspond to fifth threshold values set based on limit values of each stroke amount, and are set to, for example, about 90% of the limit values of each stroke amount.

That is, when the second limitation condition is satisfied, the wheel loader1is in a state where at least either the rods220of the two lift arm cylinders22or the rod240of the bucket cylinder24extends or contracts to a position close to the stroke end, and also the hydraulic oil discharged from the loading hydraulic pump44is likely to be relieved.

The storage section55is a memory, and retains the first step-on amount threshold value αth, the second step-on amount threshold value βth, the speed ratio threshold value eth, the discharge pressure threshold value P1th, the first stroke amount threshold value S1th, and the second stroke amount threshold value S2th.

When the limitation condition determination section54determines that the first limitation condition and the second limitation condition are satisfied, the signal output section56outputs a limitation signal for limiting an upper limit rotational speed of the engine41to the engine controller41A. Note that the upper limit rotational speed of the engine41may be the maximum rotational speed of the engine41, or the rotational speed which has been arbitrarily set.

In the present embodiment, when the limitation condition determination section54determines that the first limitation condition and the second limitation condition are satisfied, the signal output section56outputs, to the monitor12A, a display signal relating to a notification that the upper limit rotational speed of the engine41is being limited. The monitor12A is one of the aspects of a notification device for notifying that the upper limit rotational speed of the engine41is being limited. Note that the notification device is not necessarily a display device such as the monitor12A, and may be, for example, an audio device.

(Processing in Vehicle Body Controller5)

Next, a specific flow of processing executed in the vehicle body controller5will be described with reference toFIG.7. The advantageous operations and effects obtained by execution of this processing in the vehicle body controller5will be described with reference toFIG.8andFIG.9.

FIG.7is a flowchart illustrating a flow of the processing executed by the vehicle body controller5.FIG.8illustrates a graph showing the relation between the accelerator pedal step-on amount α and the engine rotational speed NE.FIG.9illustrates an example of the notification displayed on the monitor12A.

Firstly, the data acquisition section51acquires the accelerator pedal step-on amount α detected by the first step-on amount sensor35, the brake pedal step-on amount β, detected by the second step-on amount sensor36, the engine rotational speed NE detected by the first rotational speed sensor33, and the torque converter output rotational speed NT detected by the second rotational speed sensor34, respectively (step S501).

Next, the torque converter speed ratio calculation section52calculates the torque converter speed ratio e (=NT/NE) based on the engine rotational speed NE and the torque converter output rotational speed NT acquired in step S501(step S502).

Next, the limitation condition determination section54determines whether the accelerator pedal step-on amount α acquired in step S501is equal to or more than the first step-on amount threshold value αth and the brake pedal step-on amount β is equal to or more than the second step-on amount threshold value βth (step S503).

In step S503, when determining that the accelerator pedal step-on amount α is equal to or more than the first step-on amount threshold value αth and the brake pedal step-on amount β is equal to or more than the second step-on amount threshold value βth (α≥αth and β≥βth) (step S503/YES), the limitation condition determination section54subsequently determines whether the torque converter speed ratio e calculated in step S502is equal to or less than the speed ratio threshold value eth (step S504).

In step S504, when it is determined that the torque converter speed ratio e is equal to or less than the speed ratio threshold value eth (e≤eth) (step S504/YES), the data acquisition section51acquires the discharge pressure P1detected by the discharge pressure sensor37, the arm angle θ1detected by the arm angle sensor31, and the bucket angle θ2detected by the bucket angle sensor32, respectively (step S505).

Next, the stroke amount calculation section53calculates the arm stroke amount S1based on the arm angle θ1acquired in step S505and the bucket stroke amount S2based on the bucket angle θ2acquired in step S505, respectively (step S506).

Next, the limitation condition determination section54determines whether the discharge pressure P1acquired in step S505is equal to or more than the discharge pressure threshold value P1th, and the arm stroke amount S1calculated in step S506is equal to or more than the first stroke amount threshold value S1th or the bucket stroke amount S2is equal to or more than the second stroke amount threshold value S2th (step S507).

In step S507, when it is determined that the discharge pressure P1is equal to or more than the discharge pressure threshold value P1th, and the arm stroke amount S1is equal to or more than the first stroke amount threshold value S1th or the bucket stroke amount S2is equal to or more than the second stroke amount threshold value S2th (P1≥P1th and S1≥S1th, or P1≥P1th and S2≥S2th) (step S507/YES), the signal output section56outputs a limitation signal to the engine controller41A (step S508) and also outputs a display signal to the monitor12A (step S509). Then, the processing in the vehicle body controller5is ended.

When it is determined in step S503that the accelerator pedal step-on amount α is less than the first step-on amount threshold value αth or the brake pedal step-on amount β is less than the second step-on amount threshold value βth (α<αth or β<βth) (step S503/NO), when it is determined in step S504that the torque converter speed ratio e is more than the speed ratio threshold value eth (e>eth) (step S504/YES), or when it is determined in step S507that the discharge pressure P1is not equal to or more than the discharge pressure threshold value P1th or even if the discharge pressure P1is equal to or more than the discharge pressure threshold value P1th, the arm stroke amount S1is less than the first stroke amount threshold value S1th and the bucket stroke amount S2is less than the second stroke amount threshold value S2th (P1<P1th, or P1≥P1th, S1<S1th, and S2<S2th), the processing in the vehicle body controller5is ended (step S507/NO).

Thus, when the first limitation condition (at least in step S503) and the second limitation condition (step S507) are satisfied, as illustrated inFIG.8, the vehicle body controller5limits the upper limit rotational speed of the engine41from NE1to NE2(indicated by a chain line inFIG.8), whereby it is possible to eliminate the stall condition of the engine41and thus reduce the fuel consumption. In this case, in the working device2, since at least either the lift arm cylinders22or the bucket cylinder24is nearly in a stroke end state, and also the hydraulic oil of the loading hydraulic pump44is on the verge of the relief, limiting the upper limit rotational speed of the engine41does not interfere with the work. This enables the wheel loader1to reduce the fuel consumption while maintaining the working efficiency.

Furthermore, in the present embodiment, as illustrated inFIG.9, while the vehicle body controller5is limiting the upper limit rotational speed of the engine41, the monitor12A displays to that effect, thereby making it possible to alert the operator.

Second Embodiment

Next, a drive system of the wheel loader1according to a second embodiment of the present invention will be described with reference toFIG.10toFIG.13. InFIG.10toFIG.13, the components common to those described for the wheel loader1according to the first embodiment are provided with the same reference signs, and explanation thereof is omitted.

(Overall Configuration of Drive System)

Firstly, the overall configuration of the drive system of the wheel loader1according to the second embodiment will be described with reference toFIG.10andFIG.11.

FIG.10illustrates a configuration of the drive system of the wheel loader1according to the second embodiment of the present invention.FIG.11illustrates graphs each of which shows the relation between the engine41and the HST pump61.

In the wheel loader1according to the present embodiment, the traveling of the vehicle body is controlled by an HST traveling drive system which enables the wheel loader1to startup smoothly and stop without experiencing shocks. The HST traveling drive system includes, as illustrated inFIG.10, an HST pump61serving as a traveling hydraulic pump driven by the engine41, an HST charge pump61A for replenishing a pressure oil for controlling the HST pump61, an HST motor62serving as a traveling hydraulic motor connected to the HST pump61via a pair of pipes600A,600B by a closed-circuit.

The HST pump61is a swash plate type variable displacement hydraulic pump in which the displacement volume is controlled in accordance with a tilting angle (tilting amount). The tilting angle is adjusted by a pump regulator in accordance with a command signal output from the vehicle body controller5A. A tilting angle θ3of the HST pump61(hereinafter, simply referred to as a “tilting angle θ3”) is detected by a tilting angle sensor38, and then input to the vehicle body controller5A.

The HST motor62is a swash plate type variable displacement hydraulic motor in which the displacement volume is controlled in accordance with a tilting angle (tilting amount), and transmits the driving force of the engine3to the four wheels11. As in the case of the HST pump61, the tilting angle is adjusted by a motor regulator in accordance with a command signal output from the vehicle body controller5A. Note that a load pressure P2of the HST motor62(hereinafter, simply referred to as a “load pressure P2”) is detected by a pressure sensor39provided on the pipe600A which is one of the pipes600A,600B, and then input to the vehicle body controller5A.

In the HST traveling drive system, firstly, when the operator steps on the accelerator pedal71, the engine41rotates, and the HST pump61is driven by the driving force of the engine41. Next, the pressure oil discharged from the HST pump61flows into the HST motor62, thereby rotating the HST motor62. Then, the output torque from the HST motor62is transmitted to the four wheels11via an axle, whereby the vehicle body can travel.

As illustrated in the graph on the uppermost part and the graph on the middle part ofFIG.11, as the accelerator pedal step-on amount α increases and thus the engine rotational speed NE increases, the tilting angle θ3and input torque T of the HST pump61also increase proportionally. Accordingly, a discharge flow rate Q of the HST pump61increases, thereby increasing the flow rate of the pressure oil flowing from the HST pump61into the HST motor62. In this way, the rotational speed of the HST motor62increases, and thus the vehicle speed increases.

(Configuration of Vehicle Body Controller5A)

Next, a configuration of the vehicle body controller5A will be described with reference toFIG.12andFIG.13.

FIG.12is a functional block diagram illustrating functions of the vehicle body controller5A according to the second embodiment.FIG.13is a flowchart illustrating a flow of processing executed by the vehicle body controller5A according to the second embodiment.

As illustrated inFIG.12, the vehicle body controller5A according to the present embodiment includes a data acquisition section51A, a stroke amount calculation section53, a limitation condition determination section54A, a storage section55A, and the signal output section56. That is, the vehicle body controller5A according to the present embodiment does not include the torque converter speed ratio calculation section52which is included in the vehicle body controller5according to the first embodiment.

As illustrated inFIG.13, firstly, the data acquisition section51A detects the accelerator pedal step-on amount α detected by the first step-on amount sensor35, the brake pedal step-on amount β detected by the second step-on amount sensor36, the load pressure P2detected by the pressure sensor39, and the tilting angle θ3detected by the tilting angle sensor38, respectively (step S501A).

Next, the processing proceeds to step S503, and the limitation condition determination section54A determines whether the accelerator pedal step-on amount α acquired in step S501A is equal to or more than the first step-on amount threshold value αth and the brake pedal step-on amount β is equal to or more than the second step-on amount threshold value βth.

In step S503, when determining that the accelerator pedal step-on amount α is equal to or more than the first step-on amount threshold value αth and the brake pedal step-on amount β is equal to or more than the second step-on amount threshold value βth (α≥αth and (β≥βth) (step S503/YES), the limitation condition determination section54A subsequently determines whether the load pressure P2acquired in step S501A is equal to or more than a pressure threshold value P2th and the tilting angle θ3is equal to or more than a tilting angle threshold value θ3th (step S504A).

Here, the pressure threshold value P2th corresponds to a sixth threshold value set based on a relief pressure of the HST drive circuit, and is set to, for example, about 90% of the relief pressure. The tilting angle threshold value θ3th corresponds to a seventh threshold value set based on an upper limit value of the tilting angle of the HST pump61(tilting upper limit amount), and is set to, for example, about 90% of the upper limit angle.

That is, in the present embodiment, the first limitation condition includes that the accelerator pedal step-on amount α is equal to or more than the first step-on amount threshold value αth (α≥αth) and the brake pedal step-on amount β is equal to or more than the second step-on amount threshold value βth (β≥βth), and also that the load pressure P2is equal to or more than the pressure threshold value P2th (P2≥P2th) and the tilting angle θ3is equal to or more than the tilting angle threshold value θ3th (θ3≥θ3th).

In step S504A, when it is determined that the load pressure P2is equal to or more than the pressure threshold value P2th and the tilting angle θ3is equal to or more than the tilting angle threshold value θ3th (P≥P2th and θ3≥θ3th) (step S504A/YES), the processing proceeds to step S505and later. On the other hand, in step S504A, when it is determined that the load pressure P2is less than the pressure threshold value P2th or the tilting angle θ3is less than the tilting angle threshold value θ3th (P2<P2th or θ3<θ3th) (step S504A/NO), the processing in the vehicle body controller5A is ended.

Thus, in the case where the HST traveling drive system is employed as the traveling drive system of the wheel loader1, in the same manner as the torque converter traveling drive system, when the first limitation condition and the second limitation condition are satisfied, the vehicle body controller5A limits the upper limit rotational speed of the engine41. This enables the wheel loader1to reduce the fuel consumption while maintaining the working efficiency.

In the above, the present invention has been described with reference to each of the embodiments of the present invention. The invention is not limited to each of the embodiments described above, and various modifications may be made therein. For example, each of the embodiments are described in detail herein for the purpose of clarity and a concise description, and the present invention is not necessarily limited to those including all the features described above. Furthermore, some of the features according to a predetermined embodiment can be replaced with other features according to the separate embodiments, and other features can be added to the configuration of a predetermined embodiment. Still further, some of the features can include other features of the separate embodiments, be deleted, and/or replaced.

For example, in the embodiments described above, the wheel loader1has been described as one of the aspects of construction machines, however, the present invention is not limited thereto. For example, the present invention can be applied to other work vehicles such as forklifts.

Furthermore, as described in the embodiments above, there is no particular limitation on a driving method relating to the traveling of the wheel loader1. The present invention can be applied to either the torque converter type or the HST type.

REFERENCE SIGNS LIST