Gear Up and Throttle Back to Save Fuel

Farm Energy November 17, 2015 Print Friendly and PDF
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"Gear up and throttle back" is a fuel-saving practice suitable for light drawbar loads (less than 65% of full power) when reduced PTO speed is not a problem. For the most efficient operation, a tractor's engine should be operated near its rated capacity. However, many field operations (such as light tillage, planting, cultivating, and hay raking) do not require full tractor power. This is especially true when older implements, which were sized for a smaller tractor, are used with higher horsepower tractors. Also, many operations should be performed at a fixed field speed.

Gear up and throttle back when the field operation requires less than 65% of the tractor engine's full power. Photo: Gene Alexander, NRCS, South Dakota

For these lighter operations, a substantial amount of fuel can be saved by shifting to a higher gear and slowing the engine speed to maintain the desired field speed; thus, the term "gear up and throttle back." An example of this procedure is shifting a manual transmission car or truck from second to third gear while reducing the throttle setting to maintain travel speed. The goal of gearing up and throttling back is not to increase operation speeds or to do work more quickly but to save fuel.

General Guidelines for Gear Up and Throttle Back

  • Consider "gear up and throttle back" on light load operations (typically those requiring less than 65% of full engine power).
  • Select a higher gear to maintain travel speed and implement productivity while reducing engine RPM.
  • Do not overload the engine. Check the engine response to the throttle setting and drawbar load.
  • Remove extra ballast. Extra weight is used to properly ballast a tractor for field operations to achieve the optimal traction. Heavy draft loads require more ballast than lighter loads. If it is convenient for light loads, remove extra ballast to reduce rolling resistance and improve fuel economy as well as reduce the potential for soil compaction.

Normally, "gear up and throttle back" can be used when loads require less than 65% of a tractor's power. It is generally safe to reduce engine RPM by 20% to 30% of the rated RPM. Check the operator's manual for specific recommendations for your tractor.

There is no justification for operating either turbocharged or naturally aspirated engines at full throttle when full drawbar horsepower is not required.

Most tractor manufacturers indicate that the "gear up and throttle back" practice is suitable for their tractors and recommend the practice for fuel savings. Further, this practice could decrease maintenance, downtime, and expenses generally incurred from over-speeding mechanical equipment.

Definitions: Work, Power, Energy, and Efficiency

  • Work is defined as moving a weight or a force over a distance. "Foot-pound" (abbreviated ft-lbf) is a common unit of measurement for work. For example, to lift a 55-pound object 10 feet would require 550 ft-lbf of work. In the case of a tractor, if a force (i.e., drawbar pull) of 3,300 pounds is needed to pull a disk and the disk is pulled 10 feet, then 33,000 ft-lbf of work would be done.
  • Power is the amount of work done in a given period of time. If the 55-pound object was lifted 10 feet in 1 second, the power required would be 550 ft-lbf/sec. Similarly, if it takes 1 minute to pull the disk 10 feet, the power required is 33,000 ft-lbf/min. The unit of measurement for power is horsepower. One horsepower equals 550 ft-lbf/sec or 33,000 ft-lbf/min. Both of the examples required 1 horsepower to complete the task.
  • Energy is the capacity to do work. For tractors, gallons of fuel consumed is a measure of the amount of energy used.
  • Efficiency can be measured as the amount of work done divided by the amount of energy used. For tractors, horsepower-hour (hp-hr) is the standard measure of work done. One hp-hr is 1 horsepower expended over 1 hour, which is equivalent to 1,980,000 ft-lbf of work. Horsepower-hours per gallon (hp-hr/gal) of fuel is a common measure of tractor engine efficiency. Hp-hr/gal can be calculated from either PTO (power-take-off) or drawbar power. The hp-hr/gal values from the PTO tests will be higher than the drawbar observations due to transmission inefficiencies.

Specific fuel consumption (hp-hr/gal) is not generally affected by engine size and can be used to compare the fuel efficiency of different sizes of tractors. Higher values of hp-hr/gal indicate greater fuel efficiency in the same way that higher miles per gallon indicate a better fuel economy for highway vehicles.

For diesel tractor engines, 13.5 hp-hr/gal would be an average fuel efficiency for drawbar loads, while a very efficient tractor can achieve 18.5 hp-hr/gal for loads on the PTO. Increased fuel efficiency is the advantage of "gear up and throttle back" practice.

Drawbacks to Consider

There are a few drawbacks with "gear up and throttle back."

  • When engine speed is reduced, reaction time of the tractor hydraulics will be slower, and PTO speed is correspondingly reduced.
  • When PTO speed is reduced, the PTO-driven device may have unacceptable performance and/or reduced productivity.
  • For some load conditions, reduced PTO speeds can reduce the PTO-driven unit's life and cause failure of drive lines.

Do Not Overload the Tractor

When using the practice of "gear up and throttle back," the most important thing to remember is NOT to overload or lug the engine. Overloading the engine requires the engine to produce more torque at a low engine speed than it is designed for. Excessive black exhaust smoke is one indication of an overloaded diesel engine.

To check the engine for overloading, work the tractor for a short time at the desired speed and throttle setting. Then, rapidly open the throttle. If the engine readily picks up speed, it is not overloaded, and the original throttle setting is suitable. If the engine does not respond quickly, shift down a gear or increase the engine speed. Again, check for engine overload at the new settings.

Remember, fuel consumption and specific fuel consumption can vary widely for individual tractor models. Consult University of Nebraska Tractor Test Reports for your specific tractors when making an efficiency selection. Keep accurate records of the fuel usage of all tractors under a variety of operating conditions. With accurate records, an equipment system manager will be able to select the most economical tractor for a specific operation.

Summary

The fuel saving practice of "gear up and throttle back" involves reducing engine speed to 70% to 80% of rated engine speed, and shifting to a faster gear to maintain the desired field speed and implement productivity. This practice is suitable for light drawbar loads (less than 65% of full power) when reduced PTO speed is not a problem. Remember, DO NOT overload the engine.

If you "gear up and throttle back" whenever possible, you will be on your way toward getting the most for your fuel dollars.

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Contributors to This Article

Source: Adapted from Gear Up and Throttle Down — Saving Fuel. Grisso, R and Pitman, R. Virginia Cooperative Extension, 2009.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.