Prime movers: where the power comes from
A prime mover is the part that makes power for a machine. It turns fuel or electricity into spinning (rotary) power.
- Diesel engine: tractors, combines, big pumps. Strong and saves fuel.
- Petrol engine: small sprayers, brush cutters, small pumps. Light and easy to start.
- Electric motor: tube-wells, chaff cutters, dairy machines. Clean, but it needs a power line or battery.
Power is measured in kW (kilowatt). Old books use horsepower (hp). 1 hp is about 0.75 kW.
The 4-stroke engine
- Intake: piston goes down, the inlet valve opens, air comes in.
- Compression: both valves shut, piston goes up and squeezes the air. It becomes very hot (diesel).
- Power: diesel is sprayed in and lights by itself. The hot gas pushes the piston down. This is the only stroke that gives power.
- Exhaust: the exhaust valve opens, piston goes up and pushes the smoke out.
A petrol engine does the same, but it takes in a petrol-air mix and a spark plug lights it. The crank turns the up-down push into a spin. A heavy flywheel keeps the spin smooth between power strokes.
Tractors: parts and how power flows
The tractor is the main power unit of a farm. It pulls, pushes, lifts and spins other machines.
- Engine: usually diesel.
- Clutch: joins or cuts the engine from the gearbox, so you can change gear or stop.
- Gearbox: picks speed and pulling force.
- Rear axle and big wheels: move the tractor. Big lugs grip the soil.
- PTO (power take-off): a spinning shaft at the back that runs implements like a rotavator.
- Hydraulics and 3-point hitch: lift, lower and hold an implement. The drawbar pulls a trailer.
- Steering, brakes, seat and ROPS (roll-over protection) for the driver.
Types: 2-wheel (walking) tractor for small plots, 4-wheel tractor for most farms, crawler (track) tractor for wet or heavy soil.
Gear rule: power = force × speed. With the same power, doubling the speed halves the pull.
Implements: the tools on the tractor
An implement is a tool joined to a tractor. Each job has its own implement.
- Plough: cuts and turns the soil (primary tillage).
- Harrow, cultivator, rotavator: break clods and make a fine seedbed (secondary tillage). The rotavator is driven by the PTO.
- Seed drill / planter: puts seed in rows at the right depth and gap.
- Sprayer: sprays pesticide or liquid fertiliser.
- Trailer, harvester, baler: carry or harvest.
Implements are mounted (lifted on the hitch), semi-mounted or trailed (pulled on their own wheels). Choose by the job, soil and the tractor kW. An implement that is too big makes the engine labour and waste fuel.
Fuels and lubricating oils
Fuels
- Diesel for most tractors and pumps. Keep it clean: dirt blocks the filter and injectors.
- Petrol for small engines. Never mix it up with diesel.
- Others: biodiesel, ethanol blends, CNG and electricity (batteries).
Store fuel in closed, labelled drums, away from fire. Fill the tank with the engine OFF.
Lubricating oils
A lubricant forms a thin film between moving parts. It does 5 jobs: reduces friction, cools, cleans, seals and protects from rust.
- Engine oil: in the engine.
- Gear (transmission) oil: in the gearbox and axle.
- Hydraulic oil: in the lift system.
- Grease: thick, for bearings and joints.
Check the oil level before work and change oil and filters at the time in the manual. Old oil is dirty and thin, and parts wear fast.
Key formulas and definitions
- Power = force × speed (at the drawbar: F = P ÷ v)
- Engine cycle: Intake → Compression → Power → Exhaust (4 strokes = 2 crank turns)
- 1 hp ≈ 0.75 kW
- Field-work rule: low gear = slow + strong pull; high gear = fast + weak pull
Worked examples
1. A tractor gives 40 kW at the drawbar and moves at 4 km/h. What is the pulling force?
Speed = 4 ÷ 3.6 = 1.11 m/s. Force = P ÷ v = 40 000 W ÷ 1.11 m/s = 36 000 N = 36 kN.
2. The same tractor is shifted to 8 km/h. Find the new pulling force.
Speed = 8 ÷ 3.6 = 2.22 m/s. Force = 40 000 ÷ 2.22 = 18 000 N = 18 kN. Double speed gave half the force.
3. A 4-stroke engine runs at 1800 turns per minute. How many power strokes does one cylinder give per minute?
One power stroke needs 2 turns of the crank. 1800 ÷ 2 = 900 power strokes per minute.
4. A tractor engine is 60 hp. Write it in kW.
60 × 0.75 = 45 kW (about).
5. A plough needs a pull of 30 kN and the tractor gives 40 kW at the drawbar. What is the highest speed it can keep in km/h?
v = P ÷ F = 40 000 ÷ 30 000 = 1.33 m/s. In km/h: 1.33 × 3.6 = 4.8 km/h.
6. A farmer sees the engine oil is milky and below the mark. Name two things he should do.
Stop the engine. Milky oil means water has leaked in, so call the mechanic and change the oil and filter. Do not run it until fixed. Always refill to the correct mark with the right grade.
Common mistakes
- Thinking all 4 strokes give power. Only the power stroke does; the other three use energy.
- Mixing up petrol and diesel in the tank. This can wreck the engine.
- Running the engine with low oil because "it still works". Wear and heat grow quickly.
- Using a big heavy implement on a small tractor. The engine labours and fuel is wasted.