Fuel consumption with continuously variable transmission (CVT)

With the advancement of agriculture in Brazil, there has been an increase in productivity, mainly due to new technologies. Rural producers are increasingly demanding because they have become highly dependent on agricultural mechanization, and the agricultural tractor is a fundamental element in raising production standards.

Agricultural tractors are powered by engines that transform thermal energy into mechanical energy and then transfer it to the axle, which moves the wheels and provides power through the drawbar, enabling them to pull and tow agricultural implements, also providing the power needed for some to work while stationary.

Basically, tractors move through engine and transmission management, but the engine is always coupled to a transmission system known as a gearbox, which is designed to provide rotation and/or torque.

Transmissions are basically classified as: mechanical (sliding and synchronized gears), hydrostatic and hydrodynamic. However, there is the automatic transmission that allows changing without the use of the clutch, that is, the input shaft remains in solidarity with the engine rotation without interrupting the transmission of power.

During the 50s, the use of the CVT system began (Continuously variable transmission) in some agricultural machines, and harvesters led this use, as in addition to being responsible for moving the implement, it was also applied in the mechanical system for trailing grains, however, it was not used on a large scale.

Thus, with the advancement of technology in automobiles, automated transmissions also began to be adopted more frequently in agricultural tractors in Brazil, and have gradually been winning over producers and operators due to their convenience, comfort and durability. These automatic transmissions are classified as semi-automatic power shift e full power shift gear changes are made by pressing just one button without the need to use the clutch or are completely mechanized, respectively.

But the transmission that has been proliferating and attracting a lot of attention is the CVT, which is equipped with a control system that makes it possible to adjust the transmission ratio and engine speed, which provides operation with the maximum point of efficiency in fuel consumption, therefore, it is capable of offering greater engine power with maximum efficiency, without the use of gears.

Its greater durability occurs because it is not necessary to press the clutch pedal to make gear changes, that is, it allows for precise and smooth gear changes. Remember that there is a clutch still present in the system, but it is not necessary for the operator to use it. With a simple gear lever, the operator can choose automatic gear changes or even increase or decrease them sequentially with a single button.

Basically, the most common CVT transmission is the friction type, which operates based on two bodies that attract each other in contact at points of the distance, varying the rotation wedges and thus allowing friction to transfer all the energy generated from one body to the other. Sometimes a third body may be contained in the system, which may be a wheel or a belt.

Benefits and features include the elimination of “gear shift shock,” making gear changes smoother; reduced fuel consumption; keeps the engine in its best power range regardless of the speed it is traveling; less power loss; does not “search” for gears as speed decreases, making it easier to work on sloping terrain, in addition to being attractive from an environmentalist point of view due to the reduction in air pollution.

The need to develop CVT technology in agriculture was with the intention of reducing fuel consumption, as it is one of the highest costs in developed agricultural operations, such as subsoiling, plowing and harrowing, which is also directly linked to the suitability of the tractor, type of implement, depth and quantity of operation, types of soil, among others.

Fuel consumption in some agricultural operations is also influenced by ballast, the force demanded by the drawbar, the type of tire used, the working slope, the use of auxiliary front wheel drive, the speed of travel, and others, in addition to observing the gear selection during work.

Based on all the factors that influence the fuel consumption of an agricultural tractor, engine speed is directly important, as increasing speed requires increasing rotation, which results in a drop in torque and consequently an increase in consumption.

Diesel engines, i.e. Otto cycle engines, have greater efficiency and torque because they have a greater capacity to admit oxygen, resulting in more time to complete the combustion cycle. Therefore, they operate at lower RPMs and tend to consume less fuel. However, agricultural engines have higher fuel consumption when they reach maximum power RPM and the accelerator is at maximum, so consumption tends to decrease as RPM drops.

Therefore, the purpose of the CVT transmission is to select a constant engine speed with the transmission offering the possibility of changing the speed so that it is always appropriate for the operation being carried out, thus, it is possible to maintain a low speed and consequently greater savings in fuel consumption.

Field Test

To evaluate the relationship between engine speed and hourly fuel consumption in a tractor with CVT transmission, a study was carried out using a New Holland test tractor, model T 7070 with nominal power of 167 kW (227 hp), 6-cylinder turbocharged intercooled diesel engine, fuel system common rail, CVT transmission (Continuously variable transmission), 4x2 MFWD 4WD.

The tires used were Trelleborg, being at the front model 600/65 R28 with a pressure of 12 psi (82 kPa) with 40% hydraulic ballast and at the rear 710/70 R38 with a pressure of 14 psi (96 kPa) with 40% hydraulic ballast (water), both single and with a load capacity of 4789 kg (46,97 kN) on the front axle and 8819 kg (86,49 kN) on the rear axle, which provided anticipation of 2,0%. The total weight of the tractor was 12.280 kg (120,43 kN) being 5172 kg (50,73 kN) at the front and 8819 kg (86,49 kN) at the rear.

In the load simulation evaluations on the drawbar, a 340 hp tractor was used, which was connected to the test tractor through a steel cable and performed controlled braking, starting with the test tractor in motion until reaching the desired loads. Data collection was provided through a load cell allocated to the drawbar and instantly sent to a printed circuit board data acquisition system designed in Proteus 8.1 software (Labcenter Eletronics), with the acquisition frequency being one hertz, and the data was subsequently stored on a hard drive (external HD).

The collected data were subjected to a polynomial regression analysis with the aid of SigmaPlot software (Systat Software, 2006), which is an interactive graphical assistant that offers the option of modeling and visualizing technical data.

After analyzing the data, it was possible to observe that the tractor achieved its highest engine speed when the drawbar had a force of 6000 kgf and consequently there was an increase in hourly fuel consumption (CHC) as greater engine power is required to increase torque.

It was possible to conclude in the research on CVT transmission that hourly fuel consumption is influenced by both engine speed and the force on the drawbar, that is, both place demands on the engine.

With the increase in force on the drawbar, it was possible to notice that the rotation per minute responded in a linear manner, that is, it is necessary to increase the engine rotation to obtain greater torque to pull the load, that is, the higher the rotation, the greater the tractor torque.

* By Yasser Alabi Oiole, Leonardo Leonidas Kmiecik, Marcos Cristiano Machioski, Thiago Xavier da Silva e Samir Paulo Jasper (Lata – Agricultural Tractor Adaptation Laboratory)