Tractor power: engine, drawbar and PTO
Tractor power from engine to drawbar: brake, PTO and drawbar power, efficiencies, wheel slip, tractive efficiency and fuel use, with worked sums.
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A tractor's brochure may advertise 60 kilowatts, yet a plough behind it gets far less. Power is lost at every step: in burning fuel, in the transmission, and above all where the tyres meet soft soil. Understanding the different power ratings of a tractor, and where the losses occur, lets farmers and engineers match tractors to implements, ballast them correctly, avoid wasting fuel and choose the right machine for the job.
Tractor power, efficiencies and traction are standard topics in farm machinery and agricultural engineering courses and exams.
The tractor's main systems
- Engine: almost always a four-stroke diesel (compression ignition) engine, valued for torque and fuel efficiency.
- Clutch, gearbox and transmission: mechanical, power-shift or continuously variable transmissions (CVT).
- Differential and final drives to the wheels; front-wheel assist or four-wheel drive for traction.
- Power take-off (PTO): a splined shaft at the rear (sometimes front), turning at standard speeds of 540 or 1,000 revolutions per minute, to drive implements such as rotavators, mowers, balers and pumps.
- Hydraulic system and three-point linkage: the three-point hitch with draft control, developed by Harry Ferguson in the 1920s and 1930s, lifts and controls mounted implements and transfers part of their weight to the rear wheels for traction.
- Tyres and ballast: for grip and stability.
Power ratings
| Power | Where measured | Typical share |
|---|---|---|
| Fuel (input) power | energy in fuel burned per second | 100% |
| Indicated power | developed in the cylinders | about 40–45% of fuel energy |
| Brake power | at the engine flywheel or crankshaft | about 80–85% of indicated (mechanical efficiency) |
| PTO power | at the PTO shaft | about 85–90% of brake |
| Drawbar power | pull × speed at the hitch | about 55–85% of PTO, depending on soil |
Brake power is calculated from engine torque and speed:
Drawbar power is pull (draft) multiplied by travel speed: P (kW) = F (kN) × v (m/s).
Standard tractor tests, begun at the University of Nebraska in 1920 and now coordinated internationally by the OECD Tractor Codes, measure PTO power, drawbar performance, fuel consumption, hydraulic capacity and noise, so buyers can compare tractors fairly.
Traction: wheel slip and tractive efficiency
On soil, tyres must slip a little to generate pull. Wheel slip is:
Best tractive efficiency usually occurs at about 8 to 15 per cent slip on tilled soil (lower on firm ground). Too little slip suggests excess ballast, wasting power in rolling resistance and compacting the soil; too much wastes fuel and tyre wear.
Tractive efficiency = drawbar power ÷ axle power. ASABE data give typical ratios of drawbar to PTO power of roughly 0.87 on concrete, 0.72 on firm soil, 0.67 on tilled soil and 0.55 on soft soil for two-wheel-drive tractors; four-wheel drive and tracks do better in soft conditions.
Fuel use
Diesel engines turn about 35 to 40 per cent of fuel energy into brake power at good loads. Specific fuel consumption at full load is typically about 0.22 to 0.26 kg of fuel per kWh of brake power, rising sharply at light loads. Matching the tractor to the implement, using the right gear (gear up, throttle down for light work), correct tyre pressure and ballast all save fuel.
From engine to drawbar
A tractor engine produces a torque of 300 N·m at 2,000 rpm.
- Brake power = 2π × 2,000 × 300 ÷ 60,000 ≈ 62.8 kW (about 84 hp).
- PTO power at 90 per cent = 56.5 kW.
- Drawbar power on tilled soil at 0.67 of PTO ≈ 37.9 kW.
- At a working speed of 6 km/h (1.67 m/s), the maximum pull = 37.9 ÷ 1.67 ≈ 22.7 kN.
Fuel. At 0.25 kg per kWh and full load: 62.8 × 0.25 ≈ 15.7 kg per hour; with diesel at 0.84 kg per litre, about 18.7 litres per hour.
Wheel slip. The rear wheels' rolling circumference is 5 m. In 10 revolutions the tractor should travel 50 m, but travels only 43 m:
- Slip = (50 − 43) ÷ 50 × 100 = 14 per cent, within the efficient range for tilled soil.
Choosing tractor size
The right size depends on the heaviest regular job, usually primary tillage, and the area to be covered in the available time window. Required drawbar power = implement draft × speed; divide by the drawbar-to-PTO ratio for the soil to find the PTO power needed, then add a margin of 10 to 20 per cent. An oversized tractor wastes capital and compacts soil; an undersized one works slowly and wears out.
Key points
- Tractors use diesel engines, transmissions, PTO (540 or 1,000 rpm), hydraulics and the three-point linkage (Ferguson).
- Power ratings decrease from fuel to indicated, brake, PTO and drawbar power.
- Brake power = 2π N T ÷ 60,000 (kW); drawbar power = pull × speed.
- Wheel slip of about 8–15 per cent on tilled soil gives the best tractive efficiency.
- Drawbar-to-PTO ratio falls from about 0.87 on concrete to 0.55 on soft soil.
- Specific fuel consumption is about 0.22–0.26 kg per kWh at full load; match tractor and implement to save fuel.
What the exams ask
- School-leaving and entrance papers: parts of a tractor, and the uses of PTO and hydraulics.
- Degree and research-fellowship entrances: power ratings and efficiencies, calculations of brake and drawbar power, wheel slip, tractive efficiency and fuel consumption.
- Agricultural engineering examinations: tractor testing, traction theory, ballasting, tractor–implement matching, and selection.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does the same tractor pull less in soft soil?
Tyres sink and slip more, and more power is used just to roll the tractor through the soil, leaving less for pull at the drawbar.
What is PTO horsepower?
The power available at the power take-off shaft, the usual basis for advertising and comparing tractor power, as it is easy to measure precisely on a test bench.
Does adding ballast always help?
Up to a point. Ballast reduces slip, but too much increases rolling resistance, fuel use and soil compaction. Adjust ballast to keep slip in the recommended range for the job.
Sources
- Goering, C. E., Stone, M. L., Smith, D. W. and Turnquist, P. K. Off-Road Vehicle Engineering Principles. ASAE, 2003.
- ASABE. Agricultural Machinery Management Data (ASAE D497). American Society of Agricultural and Biological Engineers.
- Liljedahl, J. B., Turnquist, P. K., Smith, D. W. and Hoki, M. Tractors and Their Power Units, 4th edition. Van Nostrand Reinhold, 1989.
- OECD. OECD Standard Codes for the Official Testing of Agricultural and Forestry Tractors. OECD, updated regularly.
- Hunt, D. and Wilson, D. Farm Power and Machinery Management, 11th edition. Waveland Press, 2016.
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