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 HeatMechanicalPTO 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 (e t ) Efficiency with which the engine converts potential chemical energy into useful mechanical energy. e t = Mechanical energy Chemical energy x 100 e t = Hp-hrs x 2545 Gal. / Hr x BTU/Gal x 100 Typical value for No.2 Diesel fuel: 139,000 BTU/Gal.

Mechanical Efficiency (e m ) Ratio of brake horsepower (Bhp) to indicated horsepower (Ihp). E m = Bhp Ihp 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 (e v ) 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 = %.

Nebraska/OECD Tractor Tests