Operational performance assessment of agricultural tractors

Field evaluation studies the effect of gear selection on the operational performance of agricultural tractors.

Tractors are machines equipped with elements, mechanisms and systems that, acting together, are responsible for providing power for the development of the most varied tasks in the field. The use of tractors to carry out agricultural tasks provided a series of advantages for the activity, however some aspects are unfavorable, such as the high cost of fuel, increased environmental pollution and dependence on non-renewable fuel sources.

The cost of fuel can, in some cases, exceed 50% of the total operating cost of the machine, making it a relevant expense in modern agricultural operations. The fuel consumption of agricultural machinery is generally presented as a function of the hour or area worked, with the first method prevailing in technical publications from manufacturers and specialized bodies. The engine's power and torque demand is a function of the characteristics of the operation developed and has a direct influence on hourly fuel consumption.

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The transmission is an essential system in agricultural tractors, as this mechanism makes it possible to adjust speed and torque for specific operating conditions. The correct selection of gear and working speed can be a simple and effective way to reduce the fuel consumption of agricultural tractors, as this action can make more efficient use of the power and torque available in the engine. The use of gears appropriate to the operation can also maintain the slippage of the tractor's wheels at appropriate levels, ensuring a more efficient and economical operation.

A group of researchers from the Federal University of Viçosa carried out a series of studies to evaluate the effect of gear selection and engine speed on the performance parameters of an agricultural tractor. In the studies, a John Deere tractor, model 5705, 4x2 with auxiliary front-wheel drive (TDA) and power of 62,56kW (85hp) in the engine at 2.250rpm was used. The tractor was equipped with 12.4 – 24 radial tires on the front axle and 18.4 – 30 on the rear axle, both from Goodyear’s Optitrac line.

The first test evaluated the effect of the selected gear and the traction force applied on fuel consumption, drawbar power and tractor tire slippage. Combinations between three gears (3rd A, 1st B and 2nd B) and three traction forces (8,83kN; 11,36kN and 15,50kN) were tested. The theoretical speeds obtained with each of the gears, at the engine speed of 2.250rpm, were: 3rd A – 1,07m/s, 1st B – 1,27m/s and 2nd B – 1,77m/s. The work was carried out on a concrete track, 40m long. The traction force was varied by changing the gears of a ballast tractor (Valtra Valmet brand, model 800), linked to the tested tractor using a steel cable.

The factors traction force and selected gear (speed) had a linear and positive effect on the hourly fuel consumption of the engine of the agricultural tractor tested, according to the response surface and the adjusted equation, presented in Figure 1A. Figure 1B presents the response surface and the adjusted model to describe the power demand on the drawbar as a function of the force and operating speed of the tractor. The tractor's power demand is the product between the traction force and the working speed, which explains the increase in hourly fuel consumption when the force and operating speed increase.

The slippage presented by the tractor's driving wheels was linearly and positively influenced by the traction force (Figure 2). The increase in speed provided an increase in the percentage of slippage, but less pronounced than the traction force. The slip percentage is a parameter strongly influenced by the traction force exerted by the machine. A 1m/s reduction in speed (gear with more torque) provides a 0,4053% reduction in the percentage of slippage of the driving wheels, increasing traction efficiency and optimizing fuel consumption.

In the second test, the effect of the selected gear and engine speed on hourly fuel consumption was evaluated. Combinations between three gears (3rd A, 1st B and 2nd B) and four engine speeds (1.000rpm, 1.500rpm, 2.000rpm and 2.500rpm) were tested, with the tractor exerting a constant traction force of 5kN.

In all gears evaluated, hourly fuel consumption increased with engine acceleration (Figure 3), which is basically explained by the greater flow of diesel oil into the cylinders to ensure engine operation at a given speed. 

In an agricultural operation, it is possible to combine gears and rotations that provide the same operational speed for the tractor, which can guarantee the same operational capacity for the mechanized set and potentially reduce fuel costs. This feature is already known to farmers, especially in lighter agricultural operations, where it is possible to combine higher (faster) gears with low engine speed and produce the appropriate speed for the operation with lower fuel consumption. The use of these economic combinations can provide reductions ranging from 10% to 40% in hourly fuel consumption, but it should be noted that only light and moderate agricultural operations allow the adoption of this procedure.

An inconvenience of using “gear up and rev down” is the decrease in the usable range of the engine's torque reserve, which is why it is the procedure most applied in light and moderate operations. Using less of the total torque reserve, the operator may have to perform many gear changes to pass through harder and more resistant terrain, thus losing operational efficiency.

The results of the tests presented highlight that it is extremely important to use appropriate gears for each operation performed and that the use of lower engine speeds can reduce fuel costs in light/moderate operations.

EXPERIMENT

In the experiment, the traction force was monitored using an Alfa Instrumentos brand load cell, with a capacity of 50kN. The actual speed of travel was monitored by a Dickey John Doppler effect radar, model Radar II, fixed to the tractor chassis. Fuel consumption was measured with an oval flowmeter, model LSF II. The rotation of the tractor's driving axles was monitored using inductive transducers associated with a reference system. The slippage of the tractor's driving wheels was obtained by calculating the percentage difference between the theoretical and effective speed. The instrumentation used in the experiment was connected to a Hottinger Baldwin Messtechnik data acquisition system.^(R) (HBM), model Spider8, using an acquisition rate of 50 data per second (50Hz).

Marconi Ribeiro Furtado Júnior, Instituto Federal Goiano; Haroldo Carlos Fernandes, Anderson Candido da Silva, Federal University of Viçosa; Daniel Mariano Leite, Federal University of Vale do São Francisco

Article published in issue 172 of Cultivar Máquina