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Shift up and throttle back to save tractor fuel

Mark Hanna, Stuart Birrell and Dana Petersen Published on 30 March 2012

Diesel fuel is the largest direct energy purchase for many farmers. Tractor operation accounts for much of this.

Higher-horsepower tractors that do a large percentage of their work as drawbar pull on tillage and seeding implements must efficiently transfer power from the engine to the drive wheels through a transmission.

Speed and torque available from the engine crankshaft is changed to a combination of drive axle speed and torque.

Drawbar tractor operations such as chisel plowing, field cultivating, planting and spraying require different amounts of drawbar pull or force, depending on the size of the implement and soil conditions.

Although a specific amount of power is available from any given tractor, various load requirements of farm operations frequently result in the tractor being only partially loaded for the amount of drawbar power available.

Power and friction
Fortunately, fuel economy of diesel engines is very forgiving for partial drawbar load operation if the transmission is shifted to a higher gear and the fuel supply throttle is adjusted to reduce fuel use.

When the engine speed (rpm) is greater than necessary, the excessive friction robs power from the engine. Therefore, a high throttle setting actually decreases the total “useful” power produced by the engine.

The concept of shifting up and reducing throttle setting to reduce engine speed for fuel savings is similar to what occurs with truck or automotive transmissions during highway travel.

At slower starting speeds, when greater force and higher torque is needed at the drive wheels to overcome inertia and accelerate the vehicle, the transmission transmits greater torque from the engine by reducing axle speed in a lower transmission gear.

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Once greater force is no longer required to accelerate the vehicle as it comes up to speed, engine power is shifted to higher transmission gears and the throttle (foot accelerator) is reduced.

New technology
Sophisticated transmissions available on some newer large and medium-sized tractors use electronic controls to automatically select gear and throttle setting depending on the drawbar load requirements and the travel speed selected by the operator.

Transmissions marketed as “infinitely” or “continuously” variable generally have the ability to automatically operate at the most fuel-efficient transmission setting for the selected speed and required load. New features such as this should be considered to improve overall fuel efficiency when deciding if an older tractor is nearing the end of its useful life in the farm operation.  FG

Stuart Birrell is a professor of Ag and Biosystems Engineering at Iowa State University, and Dana Petersen is program coordinator for the Iowa State University Farm Energy Initiative.

—Excerpts from Iowa State University Extension Farm Energy Fact sheet, March 2011

PHOTOS
TOP RIGHT: Drawbar tractor operations such as chisel plowing, field cultivating, planting and spraying require different amounts of drawbar pull or force, depending on the size of the implement and soil conditions.

BOTTOM RIGHT: The concept of shifting up and reducing throttle setting to reduce engine speed for fuel savings is similar to what occurs with truck or automotive transmissions during highway travel.Photos courtesy of Mark Hanna.

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Mark Hanna
Extension Ag Engineer
Iowa State University

 

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