Prediction of the maximum available traction boundary for HST-based agricultural tractors

The traction force of an agricultural tractor with a hydrostatic transmission (HST) is governed jointly by the engine, the hydrostatic unit (HSU) and the gearbox; when these are not matched, part of the engine power cannot be converted into traction within the working speed range. This study proposes a method for determining the boundary of the traction available from an HST-based tractor at the component-matching stage. The boundary was constructed from four limits: the engine power and torque-rise characteristics, the maximum allowable pressure of the HSU with its experimentally measured efficiency map, the gearbox ratios, and tire slip. The workable area was derived for three gear ranges of a 43.3 kW tractor, selected so that the governing limit differed between cases, and verified by measuring the traction force during plowing with a six-component load cell. Where each limit governed, the measured traction force reached 86–96% of the predicted boundary on average and exceeded it only transiently, whereas the predicted vehicle speed deviated by up to 22.7% at high HSU pressure owing to a pressure-dependent loss of volumetric efficiency that a single coefficient corrected. The method allows the engine, HSU and gearbox to be matched to a target traction force.