Torque Rise Explained

Grainews wanted to know more about the concept of “torque rise” in tractors. Does it count as real horsepower or is it just fancy salesmanship? Editor Jay Whetter put together a few questions for Clint Schroer, communications manager for Cummins’ off-highway division. Here are the questions and Schroer’s answers:

1. How does an engine achieve torque rise? What is going on inside the engine to provide this extra output? The amount of torque is determined by the quantity of fuel delivered to the cylinders. By injecting a larger quantity of fuel at an intermediate engine speed, a higher torque can be achieved than what is available at the engine’s rated speed and load. The difference between the amount of torque at this intermediate speed (called torque peak) and the torque available at rated power is called torque rise. For example, if an engine develops 1,000 pound-feet of torque at rated power, and 1,400 pound-feet at torque peak, it has a torque rise of 40 per cent. (See the graph for another example.) The amount of fuel delivered is directly related to and determined with electronic calibrations and fuel curves today.

2. Why do we hear more about torque rise now than in the past? Is this a reflection of the more electronic controls on the engine? Torque rise has always been an important feature and customer consideration for an ag tractor, in both mechanical and electronic engines. However, electronic controls are more responsive (almost immediately) to torque demands and make it possible to achieve a significant “power boost” below rated RPM. This was not possible with mechanical engines.

3. Why is torque rise important in the operation of a tractor? With sufficient torque rise, a tractor can be “lugged down” from rated RPM to torque peak RPM without danger of stalling, and easily pull through tough field conditions without changing gears. Without torque rise, the engine would quickly lose RPM and stall since there would be no available torque back-up as the engine is lugged below rated RPM.

4. When shopping for a 4WD tractor, what should a farmer look for in torque rise features? Make sure to match the tractor’s rated hp and torque to the job requirements. This match of rated hp and torque allows the engine to get maximum fuel economy and will not be under or overloaded. The torque rise will then allow the operator to keep consistent speeds or performance if the job requirements suddenly become more demanding.

At torque peak, the cylinder pressure is at its highest, and the fuel consumption correspondingly reaches it’s highest point, so prolonged engine operation at torque peak is best avoided.

Most modern 4WD tractor feature “boost power” as well as significant torque rise. For example, the Cummins QSX15-535 features rated power of 535 hp at 2,100 RPM, but with a maximum power of 589 hp at 1,800 RPM. In addition, the engine is able to achieve 40 per cent torque rise at 1,400 RPM.

By definition, the torque continues to rise as the engine is lugged down from rated RPM to torque peak RPM. From the QSX example above, the amount of torque will reach its maximum (peak) at 1,400 RPM. If the operator continues to lug the engine down, both the torque and RPM will drop rapidly, until either the load is decreased, or the engine stalls.



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