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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.
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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 Heat Electrical Mechanical Hydraulic PTO Axle Drawbar Chemical
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 EHP PTOhp 86 HP
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 =
30DBHP Power Losses 75 EHP
Engine Efficiencies • Thermal Efficiency • Mechanical Efficiency • Volumetric Efficiency
Mechanical energy et = x 100 Chemical energy Hp-hrs x 2545 et = x 100 Gal. / Hr x BTU/Gal Thermal Efficiency (et) • Efficiency with which the engine converts potential chemical energy into useful mechanical energy. Typical value for No.2 Diesel fuel: 139,000 BTU/Gal.
Bhp Em = x 100 Ihp Mechanical Efficiency (em) • Ratio of brake horsepower (Bhp) to indicated horsepower (Ihp). Ihp minus:>internal friction losses>pump, fan, alternator losses Mechanical efficiency ranges from 75 – 90%.
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%.
