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The choice of tractor configuration is of primary importance in tillage operations for the optimisation of traction performance, i.e. for limiting slip which involves energy loss. To a great extent, this aspect affects the fuel consumption and the time required for soil tillage. Tyre inflation pressure and wheel load are both easily managed parameters which play a significant role in controlling the traction performance of a tractor. The present study aimed to investigate the influence of tyre inflation pressure and wheel load on the traction performance of a mechanical front wheel drive MFWD tractor (65 kW engine power) on an agricultural clay (C) Vertic Cambisol on the basis of results of traction tests and simulations with a semi-empirical soil-tyre interaction model adapted for MFWD vehicles. The traction tests were carried out using four tractor configurations with two tractor weights (40.8 kN and 50.2 kN) and two tyre inflation pressures (60 kPa and 160 kPa). Traction performance was considered in terms of drawbar pull, traction coefficient, tractive efficiency, power delivery efficiency and specific fuel consumption in relation to wheel slip. A decrease in tyre pressure and an increase in wheel load resulted in higher drawbar pull however, only the former produced improvements in terms of coefficient of traction, tractive efficiency, power delivery efficiency and specific fuel consumption, while the only significant benefit resulting from the latter was a reduction in specific fuel consumption at a tyre pressure of 160 kPa and a slip of under 15%.

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High slip of tractor traction tyres causes topsoil damage in terms of soil cutting effect with the formation of a strengthless layer strongly exposed to erosion and an underlying layer where shear deformations contribute to the alteration of soil structure functionalities. The cutting effect is clearly indicated by longitudinal topsoil shear displacement. In spite of a recognized need for limiting the slip of tractor tyres, no theoretical approaches have been presented so far to indicate a range where no topsoil damage occurs. In this paper mechanica l conditions along the soil-tyre contact surface which lead to topsoil cutting were analysed with a soil-tyre interaction model and discussed on the basis of traction tests with a M FWD tractor on an agricultural silt loam Calcaric Fluvisol. The longitudinal topsoil shear displacement was measured for a slip ranging between 5% and 48%. An evident topsoil failure took place as soon as the shear stress along the soil-tyre contact approached the soil strength. Values of slip at which this condition was reached were identified for three tractor configurat ions. These slip values should be regarded as indicative limits not to be exceeded in tillage operations in order to avoid topsoil damage in t he conditions considered.

The main purpose of this study was to evaluate the effect that mechanical stresses acting under the slipping driving wheels of agricultural equipment have on the soil's pore system and water flow process (surface runoff generation during extreme event). The field experiment simulated low slip (1%) and high slip (27%) on a clay loam. The stress on the soil surface and changes in the amounts of water flowing from macropores were simulated using the Tires/tracks And Soil Compaction (TASC) tool and the MACRO model, respectively. Taking a 65 kW tractor on a clay loam as a reference, results showed that an increase in slip of the rear wheels from 1% to 27% caused normal stress to increase from 90.6 kPa to 104.4 kPa at the topsoil level, and the maximum shear contact stress to rise drastically from 6.0 kPa to 61.6 kPa. At 27% slip, topsoil was sheared and displaced over a distance of 0.35 m. Excessive normal and shear stress values with high slip caused severe reductions of the soil's macroporosity, saturated hydraulic conductivity, and water quantities flowing from topsoil macropores. Assuming that, under conditions of intense rainfall on sloping land, a loss in vertical water flow would mean an increase in surface runoff, we calculated that a rainfall intensity of 100 mm h -1 and a rainfall duration of 1 h would increase the runoff coefficient to 0.79 at low slip and to 1.00 at high slip, indicating that 100% of rainwater would be transformed into surface runoff at high slip. We expect that these effects have a significant impact on soil erosion and floods in steeper terrain (slope > 15°) and across larger surface areas (> 16 m 2 ) than those included in our study.

An analytical model to simulate the traction performance of mechanical front wheel drive MFWD tractors was developed at the Agroscope Reckenholz-Tänikon ART. The model was validated via several field tests in which the relationship between drawbar pull and slip was measured for four MFWD tractors of power ranging between 40 and 123 kW on four arable soils of different texture (clay, clay loam, silty loam, and loamy sand). The pulling tests were carried out in steady-state controlling the pulling force along numerous corridors. Different configurations of tractors were considered by changing the wheel load and the tyre pressure. Simulations of traction performance matched experimental results with good agreement (mean error of 8% with maximum and minimum values of 17% and 1% respectively). The model was used as framework for developing a new module for the excel application TASCV3.0.xlsm, a practical computer tool which compares different tractor configurations, soil textures and conditions, in order to determine variants which make for better traction performance, this resulting in saving fuel and time, i.e. reducing the costs of tillage management.

This study aimed to investigate the influence of the mechanical behaviour of the soil surface on the traction performance and the fuel consumption of an agricultural tractor, both in qualitative and in quantitative terms, in order to increase the consciousness about the major role of the soil mechanical response in the optimisation of the energy aspects involved in the traction developed by a tractor and promote the development of new strategies to reduce costs of tillage management and improve agricultural sustainability. The traction performance of a 65 kW MFWD tractor at tyre pressures of 60 and 160 kPa was compared on four Swiss agricultural soils: a clay with corn stubbles, a clay loam with wheat stubbles, a silty loam and a loamy sand both with corn stubbles. Tests performed with a bevameter pointed out noticeable differences in the mechanical behaviour of the soils. According to such differences, the drawbar pull on the four soils was significantly disparate with differences in maximal values of about 16% at a tyre pressure of 60 kPa and up to 37% at a tyre pressure of 160 kPa. Simulations with a semi-empirical tractor-soil interaction model also showed dissimilarities in traction coefficient, motion resistance, and traction efficiency. Measurements of the fuel consumption pointed out the presence of a narrow slip range where the specific fuel consumption SFC is minimised. This range doesn’t vary significantly among the considered soils as well as with the tyre pressure and doesn’t differ very much from the range where the power delivery efficiency is maximised. The SFC differed for almost 20% among the considered soils at a tyre pressure of 60 kPa and for ca. 10% at a tyre pressure of 160 kPa. The increase in tyre pressure from 60 to 160 kPa produced an increment in SFC up to 16%. The results of this study clearly pointed out how the traction performance is a characteristic of the tractor-soil system and not of the tractor only, therefore, a proper knowledge of the soil mechanical behaviour should aid in developing strategies oriented towards reducing fossil fuel consumption.