Sub-Concept: Power Transmission
Agricultural Application: The agricultural industry relies on the efficient transmission of power in
mechanical systems.
Exercise: Comparing the Thermal Efficiency of Gasoline and Diesel Tractors
Applied Principle(s): Understanding heat engines, Thermal efficiency of engines
Goals:
Preparation Time:
Materials: (per group)
References: Refer to any high school physics textbook
2. Provide each student with an instruction/data sheet, and allow the class time to read the entire
activity and attached information sheet before proceeding.
3. Provide each group with the necessary materials.
4. Allow the students to proceed through the activity, providing assistance as necessary.
5. When all groups have finished, discuss the activity questions and results as a class. Discuss the
practical applications of the knowledge gained through the activity.
AGRISCIENCE EXERCISE
Introductory Information
Internal combustion engines are devices that turn the heat energy produced by burning fuels into
mechanical power. Unfortunately, this conversion process is not perfect. A large percentage of
the BTUs of heat energy produced by the burning fuel is never transformed into useful mechanical
power.
The thermal efficiency (TE) of an engine is the ratio of the mechanical power (horsepower-hours)
produced compared to the power value (BTUs) of the fuel consumed. The formula for
calculating the TE of an engine is given below:
TE = Hp-hrs x 2545 x 100
(gal/hr)(BTU/lb)(lbs/gal)
where, TE = thermal efficiency, as a percent
Hp-hrs = Horsepower produced x hours operated
2545 = a constant (BTUs of heat energy equivalent to one hp-hr of mechanical power
BTU/lb = BTUs per pound of fuel (averages around 20,000)
lbs/gal = weight per gallon of fuel, in pounds (varies by fuel)
Part A: Calculating the thermal efficiency of the International Hydro 70 gasoline tractor. For
purposes of this activity, assume that both fuels have a heat value of 20,000 BTUs per pound.
1. Locate the appropriate test report (Test 1154).
2. Locate the "Maximum Power and Fuel Consumption" section of the report and record the
following information:
Maximum pto-hp (rated engine speed) = ________________
Fuel consumption (gal/hr) during maximum pto-hp test = ____________
3. Calculate the number of Hp-hrs produced by multiplying maximum pto-hp times one hour (max
pto-hp x 1 hr = x hp-hrs). Record the number of hp-hrs in the blank:
hp-hrs = ___________
4. Locate the "Fuel, Oil and Time" entry in the right-hand column of the report. Determine the
per gallon weight of the fuel used in the test and record this information below:
Weight of fuel (lbs/gal) = ______________
5. Calculate the thermal efficiency of the diesel tractor using the information obtained and the
formula provided in the "Introductory Information" section. Show the TE formula and
calculations. Draw a box around your final answer.
PART B: Calculating the thermal efficiency of the International Hydro 70 diesel tractor.
1. Locate the appropriate test report (Test 1155).
2. Locate the "Maximum Power and Fuel Consumption" section of the report and record the
following information:
Maximum pto-hp (rated engine speed) = ____________
Fuel consumption (gal/hr) during maximum pto-hp test = ______________
3. Calculate the number of Hp-hrs produced by multiplying maximum pto-hp times one hour (max
pto-hp x 1 hr = x hp-hrs). Record the number of hp-hrs in the blank:
hp-hrs = _________________
4. Locate the "Fuel, Oil and Time" entry in the right-hand column of the report. Determine the
per gallon weight of the fuel used in the test and record this information below:
Weight of Fuel (lbs/gal) = _________________
5. Calculate the thermal efficiency of the LP-gas tractor using the information obtained and the
formula provided in the "Introductory Information" section. Show the TE formula and all
calculations. Draw a box around you final answer.
PART C: Discussion Questions
Answer the following discussion questions based on the activity you have just completed. Be
prepared to share your conclusions with the rest of the class.
1. Approximately what percentage of the heat energy in a gallon of fuel was "wasted" by the
diesel tractor? _________ By the gasoline tractor? __________
2. Where do you suppose this "wasted" heat energy is lost?
3. What do you think could be done to make tractors more thermally efficient?
4. Which of the two fuels is most widely used in agricultural tractors? Why?
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