Working machine

A working machine comprising:

• an engine;
• a travelling device;
• a hydro static transmission (HST) pump composed of a tilted-plate type variable capacity pump driven by the engine;
• an HST motor connected to the HST pump and a pair of gear-changing oil passages to form a closed circuit with the HST pump and the pair of gear-changing oil passages, the HST motor being configured to be driven by a discharge oil from the HST pump to drive the travelling device;
• a pilot pump driven by the engine;
• a travel operation device configured to control a tilted plate of the HST pump due to a pilot oil discharged by the pilot pump;
• a pressure control valve configured to control a travel primary side pressure, the travel primary side pressure being a pressure of a primary side of the travel operation device;
• an acceleration operating member for increasing and decreasing an engine speed of the engine to determine a target engine speed;
• and
• a control unit configured to control the pressure control valve, the control unit including:
• a no-load characteristic line showing a relationship, under a condition where the engine is unloaded, between the travel primary side pressure and a real engine speed that is an actual speed of the engine;
• and
• a drop characteristic line provided for each of the target engine speeds determined by the acceleration operating member, the drop characteristic line showing a relationship, under a condition where a predefined load or more is applied to the engine, between the travel primary side pressure and the real engine speed, and the drop characteristic line including:
• a first characteristic line portion used for decreasing the travel primary side pressure on a basis of a difference between the real engine speed and the target engine speed, the target engine speed being a speed of the engine determined by the acceleration operating member;
• and
• a second characteristic line portion used for decreasing the travel primary side pressure on a basis of a difference between the real engine speed and the target engine speed and at a rate smaller than a rate for decreasing the travel primary side pressure in the first characteristic line portion,
• wherein the control unit is configured to control the pressure control valve on a basis of the no-load characteristic line when the engine is unloaded and to control the pressure control valve on a basis of the first characteristic line portion and the second characteristic line portion when a predetermined load or more is applied to the engine.