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 100 HP EHP  PTOhp 86 HP • 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. 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. Mechanical energy et = et = Chemical energy x 100 Hp-hrs x 2545 x 100 Gal. / Hr x BTU/Gal Typical value for No.2 Diesel fuel: 139,000 BTU/Gal. Mechanical Efficiency (em) • Ratio of brake horsepower (Bhp) to indicated horsepower (Ihp). Em = Bhp Ihp x 100 Ihp minus: >internal friction losses Mechanical efficiency ranges >pump, fan, alternator losses 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%. Nebraska/OECD Tractor Tests