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