Understanding Tractor Power and Efficiency. Objectives Define power (and associated terms) and...

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