Predicting Tractor Diesel Fuel Consumption
Written by admin on August 27, 2010 – 12:08 am -Robert “Bobby” Grisso, Extension Engineer, Biological Systems Engineering, Virginia Tech; John V. Perumpral, Professor Emeritus, Biological Systems Engineering, Virginia Tech; David Vaughan, Professor, Biological Systems Engineering, Virginia Tech; Gary T. Roberson, Associate Professor and Extension Specialist, North Carolina State University; Robert Pitman, Superintendent, Eastern Virginia Agriculture Research and Extension Center, Virginia Tech
General relationships capable of predicting tractor diesel fuel consumption are very useful for budget and management purposes but may not have the ability to compare fuel consumption for several potential engine configurations, such as turbocharging and air densification components. The objective of this factsheet is to examine several methods that use field measurements and Nebraska Tractor Test Laboratory results to estimate fuel consumption. Using these equations, farmers can estimate and compare the fuel savings for different operating and loading conditions.
Introduction
According to Siemens and Bowers (1999), “Depending on the type of fuel and the amount of time a tractor or machine is used, fuel and lubricant costs will usually represent at least 16 percent to over 45 percent of the total machine costs. …” Thus, fuel consumption plays a significant role in the selection and management of tractors and equipment. Currently, most budget models use a simplified method for estimating diesel fuel consumption. Better estimates representing actual field operations are needed to compare machinery management strategies.
The worth of a tractor is measured by the amount of work that can be accomplished and the cost associated with completing the task. Drawbar power is defined by pull (or draft) and travel speed. An ideal tractor would convert all fuel energy into useful work at the drawbar. However, due to power losses, not all fuel energy is converted into useful work.
Efficient operation of farm tractors includes (1) maximizing the fuel efficiency of the engine and the mechanical efficiency of the drivetrain, (2) maximizing tractive advantage of the traction devices, and (3) selecting an optimum travel speed for a given tractor-implement system. This factsheet focuses on methods to estimate and improve fuel efficiency of a diesel power unit.
The Nebraska Tractor Test Laboratory (NTTL) has a long history of testing tractors and disseminating power and fuel consumption data. The NTTL is the official testing station for agricultural tractors in the United States. Tractors manufactured in the United States and other countries are tested, and NTTL publishes the test results. During standardized tests, the power is calculated and the corresponding fuel consumption is measured. The power at the power takeoff (PTO) is calculated from the torque and speed at the PTO. Drawbar power is calculated from the drawbar pull and the forward speed of the tractor. For more details and for a sample test report, see Using Tractor Test Data for Selecting Farm Tractors, Virginia Cooperative Extension publication 442-072.
Terminology
Tractor manufacturers specify power output at several tractor locations, such as power takeoff, drawbar, hydraulic outlets, and electrical outlets. For each tractor model, the rated power output is measured at the rated engine speed. Typically, this power is measured at the PTO, and in this factsheet, it is referred to as “rated PTO power.” For most modern tractors, the rated power will not be the maximum available power. Most modern engines often produce more power because they can be operated at speeds other than rated speeds.
Fuel consumption is measured by the amount of fuel used during a specific time period. The most common measure of the energy efficiency of a tractor is referred to here as “specific volumetric fuel consumption” (SVFC), which is given in units of gallons per horsepower-hour (gal/hp-h). Specific volumetric fuel consumption is generally not affected by engine size, and it is used to compare the energy efficiencies of tractors with different size engines and under different operating conditions. SVFC for diesel engines typically ranges from 0.0476 to 0.1110 gal/hp-h.
For ease of computation, the reciprocal of SVFC is often used and is referred to here as “specific volumetric fuel efficiency” (SVFE) with units of horsepower-hours per gallon (hp-h/gal), with corresponding ranges from 12 to 21 hp-h/gal.
The NTTL reports the SVFE for several drawbar load tests, rated PTO speed, and varying PTO power tests. Figure 1 shows sample data from an NTTL report. The SVFE for this test is shown under the columns labeled with units of hp-h/gal (kW-h/L). For example, at rated engine speed, the tractor shown in figure 1 developed 115.96 PTO horsepower with an SVFE of 17 hp-h/gal.
The data measured in NTTL Report 1725 (shown in figure 1) is used to demonstrate the computation for equation (4). For the drawbar performance at “75% of Pull at Maximum Power,” the engine speed was 2,190 rpm, and the SVFE was 12.80 hp-h/gal. The corresponding test with a reduced throttle setting had an engine speed of 1,665 rpm and an SVFE of 14.63 hp-h/gal. The SVFC was calculated as 0.0781 gal/hp-h for full throttle and 0.0684 gal/hp-h for the reduced throttle test. The decrease in SVFC was 12.4 percent, while the engine speed was reduced by 24 percent. Similarly, the “50% of Pull at Maximum Power” tests resulted in a 24 percent reduction in engine speed and a 15.8 percent decrease in SVFC.
Tractor Fuel Efficiency Improvements
Newer tractors are generally more efficient than models produced 20 years ago. Improvements in fuel efficiency during the last 20 years prove this point. Figure 2 shows the average and maximum specific fuel consumption of tractors tested from 1980 to 2000. Models tested in 2000 averaged 16.5 hp-h/gal, compared to an average of 14.5 hp-h/gal for models tested in 1980. The fuel savings of 10 to 15 percent became possible because of improved engine/transmission design and improved ability to match tractors and implements for given field conditions. Today’s tractors have more electronic controls for more efficient delivery of power to the PTO, drawbar (for pulling), and hydraulic lifts and controls.
The reduction in fuel efficiency seen during years 1991, 1998, and 2004 (figure 2) is attributed to the new emission requirements that went into effect for off-road vehicles. Even though Environmental Protection Agency (EPA) regulations initially challenged engine designers, fuel efficiency has improved significantly since then. All tractors are not equal in fuel consumption. The maximum value line in figure 2 represents the fuel efficiency for the most fuel-efficient tractor for that year. The fuel consumption data is an important consideration during the selection and purchase of a tractor.
Fuel Consumption Estimates
ASABE Standards (2006, 2009) are widely used for estimating fuel consumption for budget preparations. The most widely used relationship for estimating fuel consumption in gallons per hour (gal/h) is
QAVG = a? • PPTO (1)
where
QAVG = average diesel consumption (gal/h),
PPTO = rated PTO power (hp),
a? = 0.044 gal/hp-h.
Bowers (2001, e-mail correspondence) stated that equation (1) was developed based on PTO power test results from the Nebraska Tractor Test reports during the mid-1970s. The fuel consumption (gal/h) over the varying PTO power tests (approximately 100 percent, 85 percent, 65 percent, 45 percent, 20 percent, and 0 percent of rated PTO power) were averaged, then the average was divided by the rated PTO power. For this reason, the annual fuel consumption estimates using this method give fuel consumption based on the assumption that the tractor is operated under the same load pattern for equal time. Due to this assumption, this method underestimates fuel consumption.
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