What is it and why is torque reserve so important?

If you have ever had contact with agricultural work, especially with the use of tractors, or have been looking for the best tractor to purchase, you must have heard about Torque Reserve. After all, what is torque reserve? Why is it so important when choosing a tractor?

In the tractor manufacturers' catalogue, each model has its characteristics specified, such as power, torque, rotation and torque reserve, but after all, what is this reserve?

Torque reserve is a present and important feature, especially in agricultural tractors. It represents the “elasticity” of the engine in a work operation in which the engine loses rotation due to additional effort, but recovers it through an increase in engine torque. Roughly speaking, when the engine starts to lose rotation during its work, its torque increases up to a limit at which we call maximum torque and finally the governor (mechanical or electronic) works at maximum output.

It is not a rule, but normally in heavy work operations, it is recommended that the tractor works at nominal speed, close to the maximum speed to be more practical, at this point the maximum power is found, but not the maximum torque, as is possible. See the engine curve graph below:

Numerically speaking, the torque reserve represents a ratio of torques, in other words, it represents in percentage values, the increase in torque from the torque at maximum power to the maximum torque, it is in this rotation range that we carry out almost all agricultural operations.

The torque reserve can be calculated using the following equation:

How are these curves obtained?

Have you ever heard of a dynamometer? There is a special model used to obtain torque and power curves for tractors from the tractor's power take-off. It is nothing more than an instrumented dynamometric brake that brakes the engine indirectly through the power take-off, obtaining torque and rotation data, the power is due to the product of these two factors.

The theoretical foundation is based on the Prony Brake, according to the following scheme:

The belt brakes the axis of radius “r” (engine flywheel or socket outside the tractor) and resulting in effort to the arm supported on the scale, generating a resulting force “F”, the rotation reading is obtained by an inductive sensor at From then on, the program can calculate torque, rotation, power and even plot the curves.

Currently, the most modern dynamometers have hydraulic brake systems and clutches with water cooling systems and electronic data collection systems that deliver engine curve data more reliably.

By Marcos Okuno (Campinas State University)