In the operation or selection of tractors, it is helpful to understand how they are designed to operate. A general explanation of how a tractor utilizes engine horsepower and torque is outlined in this section.
Manufacturers specify the power output of their tractors and the ratings are usually certified by an independent laboratory. Tractors manufactured in the United States are tested at the Nebraska Tractor Test Laboratory, http://tractortestlab.unl.edu/. The tests provide information on maximum horsepower, pulling capacities at different speeds, and fuel efficiencies.
Horsepower is the rate of doing work. Work is moving a weight over a distance. Early power measurements were derived by a horse moving a weight over a distance in a given time, thus horsepower. It was defined as 33,000 foot-pounds per minute. Torque is the force that causes a twisting effort in a rotating shaft such as a tractor axle and wheels or a power-take-off (PTO) shaft. PTO horsepower is determined by the amount of torque in foot-pounds of force (ft-lbf) and shaft speed in revolutions per minute (rpm). The term “power,” a more universally accepted term today, is used in the remaining discussion rather than horsepower. Key terms used in describing tractor operation are in italics.
Each tractor model has a rated power that has been measured at the rated engine speed. Engine and governor design determine the rated engine speed. The rated power is the power that the engine will develop at the rated speed, with a fully open throttle. When the engine is producing the rated power at the rated speed, the governor will be fully open and providing maximum fuel flow to the engine. For most tractors, the rated power is also the maximum power. However, some newer tractors have a maximum power that is greater than the rated power. Electronic governors and new engine designs allow the power to increase as the engine speed drops below the rated speed.
Tractor engine power can be used in several ways. The most common uses are the PTO shaft and the drawbar. PTO power is easy to accurately measured, and is the most common way of describing tractor power. PTO power is about 96 to 98 percent of engine power.
Drawbar power is power available to pull an implement. This power is transmitted through the axles and wheels to the soil. Drawbar power is highly variable and depends on the traction between the wheels and the soil surface. Drawbar power is often 50 to 75 percent of PTO power. Draft force is the force needed to pull an implement.
Figure 1 shows the power and torque
curves as a function of engine speed for a typical tractor engine. The
maximum engine power is developed at the rated speed of 2200 rpm.
High-idle speed of 2400 rpm is the engine speed at full throttle with
the transmission in park. As a load is applied to the engine (tractor
plowing), engine speed decreases and the governor continues to open until it
is fully open at 2200 rpm. At this point the engine is developing maximum
power (maximum acres per hour). Governor action of 200 rpm is common.
Increasing the load reduces the engine speed and power. Torque, however, is
increasing as speed is decreasing until 1400 rpm. This increase in torque
is called torque rise. The torque rise is what provides a tractor’s
lugging ability; the ability to increase drawbar pull to overcome hard spots
in a field without stopping. Lugging the engine is acceptable as long as it
is not too severe or occurs too frequently. If tractor engine overheating
occurs, a lower gear should be used.
For light to moderate loads, the graph shows a part-throttle operation which is more fuel efficient than full-throttle operation. A higher gear may be necessary, so that ground speed remains the same. Modern tractor engines are designed for variable speeds.
Kotzabassis, C., H.T. Wiedemann, and
S.W. Searcy. 1994. Tractor energy conservation. Fact Sheet No. L-5085.
Texas Agricultural Extension Service. College Station, TX 77843-2117. 5pp.
Stud-link Anchor Chain Specifications1
1 Values are subject to change
and weights are approximate.
Table 1. Mechanical techniques to prevent the regrowth of nine different brush species.1
1Based on grubbing studies listed in Table 2.
Table 2. Efficacy of the low-energy grubber operating in seven different brush species.1
1Adapted from Wiedemann (1997).
Wiedemann, H.T. 1997. Factors to consider when sculpting brush: Mechanical treatment options, pp 88-95, In: D. Rollins, D.N. Ueckert, and C.G. Brown (eds.), Proc.: Brush Sculptors Symposium, 21-22 August, Uvalde, TX and 17-18 Sept., Abilene, TX. Texas Agri. Exp. Sta. Pub., San Angelo, TX, 150 pp.
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