Soil preparation and tire inflation

Studies show that high tire inflation pressures reduce the work capacity and increase the fuel consumption of agricultural tractors, in addition to providing less power, low drawbar performance and lower operational efficiency of the set. Evaluating the best inflation of the wheelsets and considering the conditions and surface of the ground is essential to obtain the highest performance from the set.

The high costs of fuel, parts, maintenance and environmental care increase the need for greater efficiency in agricultural operations. Therefore, new alternatives for managing agricultural machinery on rural properties must be researched and implemented. When carrying out an agricultural operation, tires with low inflation pressure tend to have a greater contact area with the ground and thus provide the tractor with greater tractor capacity. With the correct internal pressure and adequate ballasting of the wheelsets, it is possible to obtain greater performance and extend the useful life of the tire, in addition to minimizing problems with loss of traction, excessive slippage and increased fuel consumption.

The internal inflation pressure of the tires and the working speeds selected for a given operation affect operational aspects of the tractor related to its performance, such as tire slippage and kinematic relationship, parameters that are important in evaluating the performance of an agricultural tractor. On the property, the drawbar, as it is very versatile, is the most used for transmitting power to implements. However, it can result in poor performance due to excessive slippage, type of soil preparation and poor weight distribution over the wheelsets. Skating refers to the slippage between the surface of the tread and the surface of the ground, being a determining factor for traction to occur more easily. A certain amount of slippage must occur to prevent excessive wear on tractor parts and tires. In general, between 10% and 15% is acceptable for firm soils, which may vary according to the surface of the soil and road surface.

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Agricultural tractors with auxiliary front-wheel drive (TDA) have a rigid connection between the front and rear axle when the operator turns on the TDA. With the system connected, the front axle rotates at a higher speed than the rear axle, which is called kinematic ratio. Tractors with wheels of different diameters bring additional complications, as normally a differential is not incorporated between the two axles of the tractor to compensate for the kinematic differences produced between the front and rear wheels. For this, in some tractor models the user has alternatives to change the kinematic relationship.

There are several conditions that can influence the tractor's traction, and thus, the quality and efficiency of the services performed in the field. The main ones are the properties and condition of the soil surface. Among the conditions we have the type of soil, the moisture content of that soil, the coverage and its slope. In studies evaluating the operational performance of an agricultural tractor in areas with different types of vegetation cover, it was shown that the greater amount of dry matter on the soil surface tends to increase slip rates and, thus, reduce tractor efficiency. Specific fuel consumption is also a consistent indicator for evaluating a tractor's performance and the use of high tire inflation pressures can reduce work capacity and significantly increase fuel consumption.

Thus, an experiment was carried out in a red latosol at an altitude of 1143 meters. Four tire inflation pressures were evaluated (69; 103,5; 124,2 and 138kPa) and three soil surface conditions: direct planting (firm soil without disturbance), medium harrowing (soil prepared with harrowing), conventional tillage (soil prepared with two plowing harrows followed by a leveling harrow), with four repetitions. The specific fuel consumption, kinematic relationship and slippage of the front and rear wheels of the tractor were evaluated. The area used for each assessment was 50 meters long and four meters wide, spaced 10 meters apart, so that the tractor could stop at the end of each assessment and subsequently enter into working mode before starting the next assessment.

An 4 hp 2x85 TDA TL 83 New Holland tractor was used, with diagonal tires: 14.9-24 at the front and 18.4-30 at the rear, with liquid ballast of 75% of its internal volume, totaling a total tractor weight of 4165 kg. During the evaluations, the tractor worked at 2200 rpm in II-2 gear, providing a speed of 5,6 km/h always with TDA turned off. 

To quantify the energy demand, a graduated cylinder filled with fuel was used with an outlet for the feed pump intake and a return from the injection pump. In this way, the consumption in the test tube was directly read. Then, the specific fuel consumption was calculated. To evaluate the slip rate of the front and rear wheels and the kinematic relationship, markings were made on the flanks of the wheelsets and the number of revolutions of the front and rear wheels on the left side were recorded, taking the seated operator as a reference.

The data obtained were subjected to analysis of variance and subsequent mean test (Tukey 5%).

There was a significant influence of the soil surface condition on all characteristics evaluated; specific fuel consumption, kinematic relationship and slippage of the front and rear wheels of the tractor (Table 1), and also inflation pressures, front wheel slippage and specific fuel consumption.

The soil, when mobilized (medium harrow and conventional preparation), provided lower values ​​of kinematic ratio and front wheel slippage. This can be explained by the lower rolling resistance imposed on the tire by the fragmented soil, thus reducing the slip generated by the rear axle pushing the front axle, which has TDA turned off. Thus, the front wheel, slipping less and completing more turns in relation to the rear wheel, consequently reduces the kinematic ratio.

On the other hand, the ground with a flatter surface, without large undulations, resulted in lower fuel consumption and rear wheel slippage. Reducing the slippage of the rear wheel, which is responsible for exerting the tractor force that moves the tractor, reduces specific fuel consumption, as a greater distance is covered in less time.

As for inflation pressure, there was no significant change in the kinematic relationship and rear wheel slippage. In other words, the range of pressure variations evaluated here did not provide changes to the wheelsets capable of significantly altering the kinematic relationship or rear wheel slippage. However, the increase in inflation pressure reduced the slippage of the front wheel (Figure 1), which can be explained by the lower rolling resistance of the front wheelset with the decrease in the tire-to-ground contact area, which occurs with the increase in inflation pressure. .

Specific fuel consumption increased linearly with the increase in tire inflation pressure, being 17% higher at 138 compared to 69 kPa (Figure 2). Studies show that high tire inflation pressures reduce the work capacity and increase the fuel consumption of agricultural tractors. Furthermore, it provides the tractor with less power, low drawbar performance, and lower overall operational efficiency.

Finally, it can be concluded that greater soil disturbance causes an increase in fuel consumption and decreases the kinematic ratio. Fuel consumption increases linearly with the increase in tire inflation pressure. And the kinematic relationship and rear wheel slip are not influenced by the tire inflation pressure.

João de Deus Godinho Júnior, Vítor Resende Vedovelo Litordi, Lucas Caixeta Vieira, Renato Adriane Alves Ruas, Federal University of Viçosa

Article published in issue 172 of Cultivar Máquina