Construction machine

1. A construction machine comprising:

an engine driving a plurality of hydraulic pumps configured to supply operating oil to a hydraulic actuator;

a wheel coupled to the engine through a transmission;

a torque converter that is a fluid coupling and interposed between the engine and the transmission;

a lock-up device configured to switch fixing and unfixing between an output shaft of the torque converter and an input shaft of the torque converter;

a main pump serving as one of the hydraulic pumps and coupled to and driven by the torque converter, the main pump being configured to supply the operating oil to a boom cylinder configured to actuate a boom, a bucket cylinder configured to actuate a bucket, and a steering cylinder;

a fan pump serving as one of the hydraulic pumps and coupled to and driven by the torque converter;

a fan motor supplied with the operating oil from the fan pump through a fan circuit and configured to cool the engine;

an exhaust adjustment mechanism adjusting a flow rate of exhaust from the engine;

a control device controlling the exhaust adjustment mechanism;

a circulation line extending from the main pump through a cargo circuit to a tank;

a release line branching from the circulation line at a position upstream of the cargo circuit, the release line extending to the tank, a relief valve being provided on the release line;

an on-off valve provided on the circulation line at a position upstream of the cargo circuit and downstream of a branch point of the release line; and

a steering circuit branching from the circulation line at a position upstream of the on-off valve and connected to the steering cylinder, wherein:

the control device determines whether or not a condition in which an actual revolution speed of the engine is a set revolution speed or more and a load factor of the engine calculated from an amount of fuel injected to the engine is a set rate or less is satisfied;

when the condition is satisfied, and therefore, the control device determines that the construction machine is traveling downhill, the control device i) controls the exhaust adjustment mechanism such that the exhaust adjustment mechanism executes exhaust braking, ii) controls the fan circuit such that a revolution speed of the fan motor becomes a predetermined value or more, iii) controls the on-off valve such that the on-off valve closes the circulation line, and iv) controls the lock-up device such that the lock-up device fixes the output shaft of the torque converter to the input shaft of the torque converter;

when the actual revolution speed of the engine is less than a second set revolution speed that is the set revolution speed or less, and an actual vehicle speed is less than a set vehicle speed that is the allowable vehicle speed or less, or when an accelerator pedal is stepped on, the control device i) controls the exhaust adjustment mechanism such that the exhaust adjustment mechanism cancels the exhaust brake, ii) controls the fan circuit such that the revolution speed of the fan motor becomes a revolution speed having a value before the revolution speed of the fan motor becomes the predetermined value or more, iii) controls the on-off valve such that the on-off valve opens the circulation line, and iv) controls the lock-up device such that the lock-up device unfixes the output shaft of the torque converter from the input shaft of the torque converter.

2. The construction machine according to claim 1 , further comprising a turbocharger including a compressor and a turbine, the compressor and the turbine being connected to the engine, wherein:

the turbine is a variable nozzle turbine; and

the exhaust adjustment mechanism changes an opening degree of a nozzle of the turbine to adjust a flow rate of exhaust from the engine.