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Understanding Tractor Power and Efficiency
Objectives• Define power (and associated terms) and describe
the types of power produced by a tractor.• Define specific power terms associated with
agricultural tractors, and make appropriate power calculations.
• Identify tractor power flow and associated losses.• Explain and calculate common power efficiencies
associated with agricultural tractors.• Interpret Nebraska/OECD Tractor Test reports.
Horsepower
• Arbitrary power unit developed by James Watt.
• 1-hp. is required to perform 33,000 ft.-lbs. / min.
• HP = F (lbs.) x D (ft.)
T (min.) x 33,000
Rotary Horsepower
• Measure of the hp developed by a rotating body (shaft, pulley, engine flywheel, etc,)
• Hp = To (lb-ft) x N (RPM)
5252
Tractor Power Ratings
• Indicated HP• Brake HP
– Gross
– Net
• Pto HP• Drawbar HP
Indicated HP• Maximum HP produced by
an engine.• Power generated in the
cylinder and applied to the head of the piston.
• Not “usable” power.
• Formula:
Brake HP• Sometimes called
“Engine HP.”• Common measure of
the power of stationary engines.
• Measured at the engine flywheel.– Gross BHP– Net BHP
PTO HP
• Power available at the PTO shaft of a tractor.– 540 RPM
– 1000 RPM
• Measured with a PTO dynamometer.
• Formula:
Drawbar HP
• DBHP is the power available for pulling a load attached to the drawbar.
• Formula
Tractor Power Flow
Chemical
Heat
Mechanical
PTO
Axle
Drawbar
Electrical
Hydraulic
Estimating Usable Tractor Power
• Bowers “86%” Rule.– Based on the
assumption that power transmission efficiency is 86% as power is transferred from one location or use condition to the next.
100 HP
86 HP
EHP PTOhp
Bowers “86%” Method
EHP = 75 HP
PTO HP =
Max DBHP - Concrete =
Max DBHP - Firm Soil =
Bowers “86%” Method
Usable DBHP - Firm Soil =
Usable DBHP - Tilled Soil =
Usable DBHP - Soft Soil =
Power Losses
75 EHP
30 DBHP
Engine Efficiencies
• Thermal Efficiency
• Mechanical Efficiency
• Volumetric Efficiency
Thermal Efficiency (et)
• Efficiency with which the engine converts potential chemical energy into useful mechanical energy.
et = Mechanical energy
Chemical energyx 100
et = Hp-hrs x 2545
Gal. / Hr x BTU/Galx 100
Typical value for No.2 Diesel fuel: 139,000 BTU/Gal.
Mechanical Efficiency (em)
• Ratio of brake horsepower (Bhp) to indicated horsepower (Ihp).
Em = BhpIhp
x 100
Mechanical efficiency ranges from 75 – 90%.
Ihp minus:>internal friction losses>pump, fan, alternator losses
Bowers “86%” Method
• Engine Power = 104 HP
• PTO Power =
• Max DBHP - Concrete =
• Max DBHP - Firm Soil =
• Usable DBHP - Firm Soil =
• Max DBHP - Tilled Soil =
• Max DBHP - Soft Soil = 42.1 hp
Volumetric Efficiency (ev)• Air-pumping efficiency of the engine.
– Ratio of volume of air-fuel mix brought in during intake to the piston displacement volume.
– Affected by:• Atmospheric pressure and temperature
• Induction system design
• Engine maintenance
• Engine rpm (piston speed)
• Naturally-aspirated engines = 75-85%
• Super- or Turbo-charged engines = 150-200%.
Nebraska/OECD Tractor Tests
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