Unit 2: Tractors and their control systems

SOL223 — Farm Machinery And Power 10 min read

I. Orientation

A tractor is a self-propelled power unit designed to provide drawbar pull, belt power, and power through a power take-off (PTO) for agricultural operations. Its engine converts the chemical energy of fuel into mechanical power, while control systems regulate air, fuel, temperature, lubrication, hydraulic pressure, and transmission of power.

A. Defining properties and operating principles

Power conversion: The engine produces brake power at the crankshaft; part of this power is transmitted to the wheels, PTO, and hydraulic pump.
Control systems: Each system maintains a required operating condition, such as clean intake air, correct fuel delivery, safe coolant temperature, oil pressure, hydraulic pressure, and suitable travel speed.
Useful tractor power: Drawbar power is lower than engine power because of transmission, rolling, and soil-contact losses.
Basic performance relation:
TEXT
```
  Power (kW) = Force (kN) × Speed (m/s)
```
Force is the pulling force available at the drawbar, and speed is travel speed.
Operating assumption: Efficient operation requires matching tractor size, implement load, soil condition, speed, and field capacity.

II. Tractor classifications and economics — Types and cost effectiveness

A. Tractor classifications and economics

Tractors are classified according to construction, traction arrangement, use, and power; their economic value depends on the work completed per unit of ownership and operating cost.

Classification by construction:
- Wheeled tractors: Use rubber tyres and are suitable for transport, tillage, sowing, and general farm work.
- Crawler tractors: Use continuous tracks, giving high traction and low ground pressure; they are useful on soft or uneven land.
- Walking or two-wheel tractors: Have a small engine and handle-operated controls; they suit small holdings and light implements.
Classification by drive:
- Two-wheel drive (2WD): Power is normally delivered to the rear wheels; it is economical for dry, level fields.
- Four-wheel drive (4WD): All wheels receive power, improving traction during heavy draft work or wet conditions.
Classification by use: General-purpose, row-crop, orchard, garden, industrial, and specialized tractors are designed around clearance, turning radius, implement compatibility, and visibility.
Classification by power: Engine power is commonly stated in kilowatts (kW) or horsepower (hp).
TEXT
```
  1 hp ≈ 0.746 kW
```
Economic cost: Fixed costs include depreciation, interest, insurance, housing, and taxes; variable costs include fuel, lubricants, repairs, maintenance, and labour.
Cost per hour:
TEXT
```
  Cost per hour = Fixed cost per hour + Variable cost per hour
```
A tractor used for too few hours has a high fixed cost per hour.
Field capacity:
TEXT
```
  Theoretical field capacity (ha/h) = Width (m) × Speed (km/h) / 10
```
Effective field capacity is lower because of turning, adjustment, overlap, and interruptions.

III. Air cleaning system — Protecting the engine intake

A. Air cleaning system

The air cleaning system removes dust and abrasive particles from air before it enters the cylinders, preventing rapid wear of the piston, rings, cylinder liner, and valves.

Purpose: A diesel engine may require several thousand litres of air for each litre of fuel burned; field air often contains soil dust, especially during tillage.
Dry-type cleaner: A paper filter element traps dust as air passes through it. A primary element performs most filtration, while a safety element protects the engine if the primary element is damaged.
Oil-bath cleaner: Air changes direction through oil, causing heavier dust to settle; the remaining particles are trapped in an oil-wetted mesh.
- Maintenance: Clean the bowl and mesh and refill with the specified oil to the marked level.
Pre-cleaner: A centrifugal or vortex pre-cleaner removes larger particles before they reach the main element.
Restriction indicator: A blocked filter increases intake restriction. The indicator signals service when the pressure drop exceeds the manufacturer’s limit.
Effect of blockage: Reduced air supply causes incomplete combustion, black smoke, reduced power, increased fuel consumption, and higher exhaust temperature.
Service rule: Never operate without the filter or clean a paper element by damaging it with excessive compressed-air pressure.

IV. Fuel supply system — Metering and injection

A. Fuel supply system

The fuel supply system stores, filters, pressurizes, meters, and injects diesel fuel into the engine at the correct quantity and timing.

Fuel path:

TEXT

  Tank → sediment bowl → primary filter → lift pump → fine filter
       → injection pump → high-pressure line → injector → cylinder

Fuel tank: Stores fuel and usually includes a vented cap, outlet, drain, and sediment space; water and dirt must be removed during periodic draining.
Fuel filters: The primary filter removes larger contamination and water; the secondary filter removes fine particles that could damage injection components.
Lift pump: Also called the transfer or feed pump, it moves fuel from the tank to the injection pump at low pressure.
Injection pump: Meters fuel for each cylinder and raises it to very high pressure. Timing determines when injection begins relative to piston position.
Injector: Atomizes fuel into fine droplets and produces the required spray pattern for rapid mixing with compressed hot air.
Governor: Regulates fuel delivery as load changes, maintaining selected engine speed and preventing overspeed.
Air in the system: Air causes irregular running, misfiring, or failure to start. Bleeding removes air from filter housings and injection lines according to the service procedure.
Fuel quality: Clean fuel, correct storage, and timely filter replacement protect precision parts such as the pump plunger and injector nozzle.

V. Cooling system — Controlling engine temperature

A. Cooling system

The cooling system removes excess heat from the engine so that operating temperature remains high enough for efficient combustion but low enough to prevent damage.

Main components: Radiator, water pump, fan, thermostat, hoses, coolant passages, pressure cap, and expansion or recovery tank.
Heat-transfer path: Coolant absorbs heat around the cylinders and cylinder head, then releases it through radiator tubes and fins to moving air.
Thermostat: Remains closed during cold starting to speed warm-up and opens at its calibrated temperature to allow radiator circulation.
Water pump: Maintains coolant flow; a worn impeller, damaged seal, or loose belt can cause overheating.
Radiator: Heat is transferred from coolant to air. Bent fins, external dirt, internal scale, or low coolant reduce heat transfer.
Fan and shroud: Increase airflow through the radiator, especially at low tractor speed. A shroud improves the fan’s effectiveness.
Pressure cap: Raises coolant boiling point by maintaining system pressure; it must never be removed while the engine is hot.
Overheating causes: Low coolant, blocked fins, slipping fan belt, defective thermostat, overloaded engine, or incorrect injection timing.
Overcooling: A missing or open thermostat can keep the engine below its designed temperature, increasing fuel consumption, wear, and deposits.
Coolant protection: Correct antifreeze concentration prevents freezing and reduces corrosion; plain water alone may promote scale and corrosion.

VI. Hydraulic system — Fluid power for implements

A. Hydraulic system

The tractor hydraulic system uses pressurized fluid to lift, control, and sometimes drive implements through the three-point hitch and remote hydraulic outlets.

Operating principle: A pump creates fluid flow; resistance to that flow produces pressure.
TEXT
```
  Hydraulic power (kW) = Pressure (kPa) × Flow (L/min) / 60,000
```
Pressure is system pressure and flow is pump delivery.
Reservoir: The transmission or a separate tank stores hydraulic oil and allows heat dissipation, deaeration, and sediment collection.
Pump: Usually a gear, piston, or vane pump; it supplies flow when the engine drives it.
Control valve: Directs oil to raise, hold, or lower the hitch or to operate a remote cylinder or motor.
Relief valve: Opens at the set maximum pressure to protect the pump, hoses, and cylinders from overload.
Cylinder: Converts fluid pressure into linear force.
TEXT
```
  Force (N) = Pressure (Pa) × Piston area (m²)
```
Three-point hitch: Uses lower links, top link, lift arms, and hydraulic links to attach and position implements.
Draft and position control: Draft control responds to soil resistance; position control maintains the selected implement height.
Maintenance: Use specified clean oil, replace filters, inspect hoses for leaks, and lower implements before servicing. Pressurized oil can penetrate skin.

VII. Lubrication system — Reducing friction and wear

A. Lubrication system

The lubrication system supplies clean oil to moving engine parts, reducing friction, carrying away heat, sealing the piston-cylinder interface, and removing contaminants.

Oil sump: Holds the lubricant at the bottom of the engine; the dipstick indicates the acceptable oil level between minimum and maximum marks.
Oil pump: Draws oil through a strainer and delivers it under pressure to galleries and bearings.
Pressure-fed lubrication: Main bearings, connecting-rod bearings, camshaft bearings, and often turbocharger bearings receive oil through drilled passages.
Splash and spray lubrication: Oil thrown from moving parts lubricates cylinder walls, gears, and other components not directly connected to pressure galleries.
Oil filter: Removes metallic particles, carbon, and dirt. A bypass valve may allow limited flow if the filter becomes blocked, preventing complete oil starvation.
Pressure relief valve: Limits excessive oil pressure, especially when oil is cold and highly viscous.
Lubricant functions:
- Friction reduction: Separates surfaces with an oil film.
- Cooling: Carries heat from bearings and piston undersides.
- Cleaning: Suspends contaminants until captured by the filter or removed during an oil change.
- Sealing: Helps seal combustion gases between piston rings and cylinder liner.
Oil viscosity: A higher-viscosity oil resists flow more strongly; the correct grade must match ambient temperature and manufacturer requirements.
Low oil pressure: May result from low oil level, worn bearings, a damaged pump, blocked strainer, or excessive oil dilution. Continued operation can seize the engine.

VIII. Transmission system — Matching engine power to field work

A. Transmission system

The transmission carries engine torque to the driving wheels while providing different speed ratios, reverse motion, neutral, and sometimes independent PTO operation.

Power path: