In a work vehicle, when an engine load is equal to or lower than a rated engine torque, the engine torque is matched to the engine load in a high-speed control area in a torque chart. For instance, a target engine speed is set according to the setting of a fuel dial and the high-speed control area associated with this target engine speed is set.
Alternatively, the high-speed control area is set according to the setting of a fuel dial and the target engine speed associated with this high-speed control area is set. The engine load and the engine torque are matched in this high-speed control area.
Many operators generally set a target engine speed at or around a rated engine speed so as to improve an operating quantity. A low fuel-consumption area, namely a fuel-efficient area, usually exists in a middle-speed area or a high-torque area on an engine torque chart. Therefore, a high-speed control area defined between a non-load high-idle speed and a rated speed does not correspond to an efficient area in terms of fuel consumption.
In order to drive an engine in the fuel-efficient area, a typically-known control device presets the value of a target engine speed and the value of a target engine output torque, which values correspond to each other, for each of plural selectable operation modes (see, for instance, Patent Document 1). With the use of such a control device, when an operator selects, for instance, a second operation mode, the engine speed can be set lower than that in a first operation mode, and therefore the fuel consumption can be improved.
However, according to the above-described operation mode switching, the operator needs to operate the operation mode switching each time to improve the fuel consumption. Further, in a situation where the engine speed in the second operation mode is set at a value simply reduced relative to the engine speed in the first operation mode, selection of the second operation mode leads to the following problem. The maximum speed of a working device (hereinafter referred to as a work equipment) of a work vehicle is decreased as compared to that in the first operation mode. As a result, an operating quantity in the second operation mode becomes smaller than that in the first operation mode.
[Patent Document 1] JP-A-10-273919