Unit 2: Tractors and their control systems - Subjective Questions

Agricultural tractors can be classified as follows:

1. According to purpose:

• General-purpose tractor: Used for ploughing, harrowing, sowing, transport, and other common farm operations.
• Row-crop tractor: Designed with high ground clearance and adjustable wheel tread for intercultural operations in row crops.
• Orchard tractor: Compact and streamlined to operate safely under low branches and between closely spaced trees.
• Garden tractor: A small tractor used for gardening, lawn maintenance, and light-duty work.
• Industrial tractor: Designed for material handling, construction, and earth-moving operations.

2. According to construction:

• Wheeled tractor: Moves on pneumatic wheels and is suitable for most agricultural operations.
• Crawler or track-type tractor: Moves on continuous tracks and provides high traction with low ground pressure.

3. According to drive:

• Two-wheel drive: Engine power is transmitted mainly to the rear wheels.
• Front-wheel assist: Front wheels receive additional power when required.
• Four-wheel drive: Power is supplied to all four wheels for greater traction.

4. According to power rating:

• Small tractor: Generally used on small farms and for light operations.
• Medium tractor: Suitable for normal tillage, sowing, and transport work.
• Large tractor: Used for heavy tillage, large implements, and extensive farming.

The selection of an economical tractor depends on matching tractor capacity with farm requirements. Important factors include:

• Farm size: A small farm generally requires a lower-power tractor, while a large farm can economically utilize a higher-power tractor.
• Cropping pattern: The number and type of crops determine the frequency and timing of tractor operations.
• Soil condition: Heavy and hard soils require greater drawbar power than light soils.
• Type of operations: Primary tillage needs more power than sowing, spraying, or intercultural work.
• Implement compatibility: The tractor must have suitable drawbar, PTO, and hydraulic capacities for available implements.
• Annual use: Higher annual utilization reduces fixed cost per hour.
• Purchase price and financing: Initial cost, interest rate, loan period, and subsidies affect ownership cost.
• Fuel efficiency: A tractor with lower specific fuel consumption reduces operating cost.
• Repair and maintenance: Availability and cost of spare parts, service facilities, and skilled mechanics must be considered.
• Resale value: A reliable tractor with good market demand usually has a higher salvage value.

The most economical tractor is not necessarily the least expensive one; it is the tractor that completes farm operations on time at the lowest total cost per unit of work.

Fixed costs are incurred because the tractor is owned, even when it is not being used. They include:

• Depreciation
• Interest on investment
• Insurance
• Taxes
• Housing or shelter cost

Operating costs are incurred only when the tractor is used. They include:

• Fuel cost
• Lubricant cost
• Repair and maintenance cost
• Operator wages
• Cost of consumable parts

Annual depreciation by the straight-line method may be estimated as:

where is annual depreciation, is purchase price, is salvage value, and is economic life in years.

The total hourly tractor cost is:

where is total cost per hour, is total annual fixed cost, is annual hours of use, and is operating cost per hour.

Increasing annual utilization usually lowers the fixed cost per hour, although fuel and other operating costs continue to depend on actual use.

The air cleaner removes dust, chaff, and other abrasive particles from the air entering the engine. Clean air prevents rapid wear of the cylinder, piston rings, and valves.

A dry-type air cleaner generally consists of:

• A pre-cleaner or dust bowl
• An outer safety cover
• A pleated paper filter element
• A secondary or safety element
• Sealing gaskets
• An air restriction indicator

Working:

1. Air first enters the pre-cleaner, where centrifugal action separates larger dust particles.
2. The air then passes through the pleated paper element.
3. Fine particles are trapped on the outer surface and within the pores of the filter medium.
4. Clean air flows through the central passage to the intake manifold.
5. A restriction indicator warns the operator when the element becomes excessively clogged.

Maintenance precautions:

• Clean or replace the element at the recommended interval.
• Do not wash a paper element unless the manufacturer permits it.
• Inspect the element for holes and damaged seals.
• Prevent dust from entering the clean-air side during servicing.

Oil-bath air cleaner:

• Incoming air is directed toward a reservoir containing oil.
• Sudden change in direction causes heavier dust particles to enter the oil.
• Oil-wetted mesh traps finer particles.
• It is robust and suitable for dusty field conditions.
• It requires regular cleaning and maintenance of the correct oil level.
• Excess oil or an incorrect oil grade may restrict airflow or allow oil to enter the engine.

Dry-type air cleaner:

• It uses a pleated paper or synthetic filter element.
• It generally provides high filtration efficiency with low maintenance.
• It is lighter and easier to service than an oil-bath cleaner.
• A restriction indicator can be used to determine the correct service time.
• A damaged or poorly sealed element may permit dust to enter the engine.

Comparison:

The oil-bath type tolerates severe dust and repeated servicing but needs more routine attention. The dry type is compact, efficient, and convenient but requires careful handling and timely element replacement. Modern tractors commonly use dry filters with primary and safety elements.

A tractor diesel fuel system stores, cleans, meters, pressurizes, and injects fuel into the engine cylinders.

Main components:

• Fuel tank and filler cap
• Sediment bowl or water separator
• Fuel lines
• Feed or lift pump
• Primary and secondary fuel filters
• Fuel injection pump
• Governor
• High-pressure pipes
• Injectors
• Leak-off or return line

Working sequence:

1. Diesel is drawn from the fuel tank through the sediment bowl, where water and heavy impurities settle.
2. The feed pump supplies fuel at low pressure through the filters.
3. Filtered fuel enters the injection pump.
4. The injection pump meters the required quantity and raises it to high pressure at the correct time.
5. High-pressure fuel travels through rigid pipes to each injector.
6. The injector atomizes the fuel and sprays it into the combustion chamber.
7. Excess fuel and leakage from the injectors return to the tank through the return line.
8. The governor adjusts the quantity of fuel supplied according to engine load and selected speed.

Correct injection timing, pressure, atomization, and fuel cleanliness are essential for efficient combustion.

Fuel injection pump:

• Meters the required quantity of diesel for each cycle.
• Raises the fuel to the pressure required for injection.
• Supplies fuel to each cylinder in the correct firing order.
• Begins and ends injection at the proper time.
• Distributes nearly equal fuel quantities among the cylinders.

Governor:

• Maintains the selected engine speed as load changes.
• Increases fuel delivery when engine speed falls under load.
• Reduces fuel delivery when the load decreases and speed rises.
• Limits maximum engine speed to prevent damage.
• Helps maintain steady PTO and field operating speeds.

Injector nozzle:

• Opens when fuel pressure exceeds its preset opening pressure.
• Atomizes fuel into fine droplets.
• Produces the required spray pattern and penetration.
• directs fuel to the correct region of the combustion chamber.
• Closes rapidly to prevent dribbling after injection.

These three components must operate in coordination to obtain complete combustion, good fuel economy, easy starting, and low exhaust smoke.

Air locking occurs when air enters the low-pressure or high-pressure fuel circuit and interrupts the continuous supply of diesel to the injection pump or injectors. Because air is compressible, the system may fail to develop the pressure needed for injection.

Common causes:

• Empty fuel tank
• Loose fuel-pipe connections
• Leaking seals or damaged pipes
• Replacement of filters without proper priming
• Opening the fuel system during repairs

Symptoms:

• Engine fails to start
• Engine starts and then stops
• Irregular running or misfiring
• Loss of power
• Absence of fuel at an injector connection

Bleeding procedure:

1. Fill the tank with clean diesel and open the fuel shut-off valve.
2. Loosen the bleed screw on the primary filter.
3. Operate the hand-priming lever until bubble-free fuel flows, then tighten the screw.
4. Repeat the process at the secondary filter and injection pump bleed points.
5. If required, loosen injector-pipe nuts slightly and crank the engine until fuel appears without bubbles.
6. Tighten the injector connections and start the engine.

Fuel must be contained safely, and the operator must never place a hand near a high-pressure injection leak.

The forced-circulation liquid cooling system maintains the engine within its efficient operating-temperature range.

Main components:

• Water jackets around the cylinders and cylinder head
• Centrifugal water pump
• Radiator
• Cooling fan
• Thermostat valve
• Pressure cap
• Hoses and connecting passages
• Coolant-temperature indicator
• Expansion or recovery tank, where provided

Working:

1. The water pump circulates coolant through the engine water jackets.
2. Coolant absorbs heat from the cylinder walls, combustion chamber, and cylinder head.
3. When the engine is cold, the thermostat remains closed and coolant circulates through a bypass passage, allowing rapid warm-up.
4. At the specified temperature, the thermostat opens and directs hot coolant to the radiator.
5. Coolant flows through radiator tubes and transfers heat to the fins.
6. Air moved by the fan and tractor motion carries the heat away.
7. Cooled liquid returns to the pump and the cycle repeats.
8. The pressure cap raises the coolant boiling point and controls flow to the recovery tank.

The system prevents both overheating and excessive cooling, improving combustion, lubrication, and engine life.

Possible causes of overheating include:

• Low coolant level or coolant leakage
• Blocked radiator core or dirty fins
• Loose, worn, or broken fan belt
• Faulty water pump
• Thermostat stuck in the closed position
• Defective radiator pressure cap
• Scale, rust, or sludge in cooling passages
• Incorrect injection timing
• Engine overload or operation in an unsuitable gear
• Low engine-oil level
• Damaged cylinder-head gasket

Preventive and corrective measures:

• Check the coolant level only after allowing a hot engine to cool.
• Repair leaking hoses, clamps, radiator joints, seals, and gaskets.
• Clean dust and chaff from radiator fins using suitable low-pressure air or water.
• Adjust or replace the fan belt according to the specified tension.
• Test the thermostat and replace it if defective.
• Inspect pump operation and pressure-cap condition.
• Flush the system and use the recommended coolant mixture.
• Maintain correct fuel-injection timing and engine-oil level.
• Reduce load or select a lower gear when the tractor is overloaded.

The radiator cap should never be removed suddenly from a hot pressurized system because escaping steam and coolant can cause severe burns.

A tractor hydraulic system uses pressurized fluid to raise, lower, and control mounted or remote implements.

Main components:

• Hydraulic-fluid reservoir or transmission housing
• Suction strainer and return filter
• Engine-driven hydraulic pump
• Pressure-relief valve
• Control or directional valve
• Hydraulic cylinder and piston
• Rock shaft and lift arms
• Hydraulic pipes and hoses
• Remote couplers, where provided

Working principle:

1. The pump draws hydraulic fluid from the reservoir through a strainer.
2. It converts mechanical input into hydraulic flow.
3. The control valve directs pressurized fluid to the hydraulic cylinder.
4. Fluid pressure acting on the piston produces force according to:

where is piston force, is fluid pressure, and is piston area.

5. Piston movement rotates the rock shaft and raises the lift arms.
6. For lowering, the control valve allows fluid to return to the reservoir while the implement descends.
7. The relief valve limits maximum pressure and protects components from overload.

A hydraulic system transmits force effectively because the fluid is nearly incompressible and confined within a closed circuit.

Position control:

• Maintains the implement at a selected height or position relative to the tractor.
• The operator selects the lift-arm position using a control lever.
• It is suitable for implements whose working depth is controlled by wheels or where a fixed height is required.
• Common applications include transport, mowing, and operating mounted equipment above the ground.

Draft control:

• Maintains nearly uniform implement draft or pulling resistance.
• A sensing mechanism detects changes in soil resistance through the top link or lower links.
• When draft becomes excessive, the hydraulic system raises the implement slightly.
• When draft decreases, the implement is lowered to restore the selected load.
• It is useful for ploughing and other soil-engaging operations.

Mixed control:

• Combines position and draft responses.
• It limits excessive depth variation while still responding to changes in soil resistance.
• It is useful in fields with variable soil conditions where pure draft control may produce uneven depth.

Thus, position control regulates implement location, draft control regulates pulling load, and mixed control balances both requirements.

1. Implement fails to lift:

• Causes: Low oil level, clogged suction strainer, pump failure, open control valve, or severe internal leakage.
• Remedies: Refill with specified fluid, clean the strainer, inspect the pump, and repair leaking valves or seals.

2. Slow or weak lifting:

• Causes: Worn pump, incorrect fluid viscosity, blocked filter, low engine speed, or relief valve opening too early.
• Remedies: Use correct fluid, replace filters, test pump output, and adjust or replace the relief valve.

3. Jerky operation:

• Causes: Air in the system, low fluid level, restricted suction line, or contaminated oil.
• Remedies: Correct the oil level, tighten suction connections, bleed the system, and replace contaminated fluid.

4. Implement drops under load:

• Causes: Leaking cylinder seals, worn control valve, or external hose leakage.
• Remedies: Replace seals, service the valve, and repair hoses or fittings.

5. Excessive noise or overheating:

• Causes: Cavitation, incorrect oil, blocked passages, continuous relief-valve operation, or excessive load.
• Remedies: Remove suction restrictions, use the recommended oil, clean the system, and avoid overloading.

Hydraulic pressure should be released before disconnecting any line or fitting.

Functions of the lubrication system:

• Reduces friction and wear between moving surfaces.
• Carries heat away from pistons, bearings, and other components.
• Forms a sealing film between piston rings and cylinder walls.
• Removes contaminants and transports them to the oil filter.
• Protects metal surfaces against corrosion and rust.
• Cushions shock loads in bearings and valve-train parts.
• Reduces operating noise.

Properties of a good engine oil:

• Suitable viscosity: It must flow during cold starting while maintaining an adequate film at operating temperature.
• High viscosity index: Its viscosity should change as little as possible with temperature.
• Oxidation stability: It should resist sludge, varnish, and acid formation.
• Detergency and dispersancy: It should keep deposits and fine particles suspended.
• Anti-wear ability: It should protect heavily loaded surfaces.
• Low pour point: It should remain fluid at low temperatures.
• High flash point: It should resist excessive evaporation and ignition.
• Corrosion resistance: It should protect engine parts from chemical attack.

Only oil of the viscosity grade and service classification recommended by the tractor manufacturer should be used.

In a full-pressure lubrication system, an oil pump supplies filtered oil under pressure to the principal engine components.

Main components:

• Oil sump
• Pickup pipe and strainer
• Gear-type or rotor-type oil pump
• Pressure-relief valve
• Oil filter
• Main oil gallery
• Drilled oil passages
• Oil-pressure indicator or gauge
• Oil cooler, where fitted

Working:

1. Engine oil is stored in the sump.
2. The pump draws oil through the pickup strainer.
3. The pump forces oil through the filter and into the main oil gallery.
4. Pressurized oil flows to the crankshaft main bearings, connecting-rod bearings, camshaft bearings, and valve mechanism.
5. Oil reaching the crankshaft passes through drilled passages to lubricate the crankpins.
6. Cylinder walls, pistons, and timing gears receive oil by splash, spray, or directed jets.
7. After lubrication and cooling, oil drains back to the sump by gravity.
8. The relief valve prevents excessive pressure by bypassing oil when pressure exceeds the set value.

Adequate pressure, correct oil level, and effective filtration are necessary to maintain a continuous lubricating film.

Full-flow filtration:

• All oil delivered by the pump passes through the filter before reaching the main oil gallery.
• It continuously protects bearings from circulating contaminants.
• The filter must have sufficient flow capacity to avoid excessive pressure loss.
• A bypass valve allows oil to reach the engine if the filter becomes blocked or the oil is very cold.

Bypass filtration:

• Only a small portion of the pump output passes through the filter at a time.
• Filtered oil generally returns to the sump.
• It can use a finer filter medium because the flow rate is lower.
• Complete cleaning of the oil takes place gradually.

Importance of filters:

• Remove metal particles, carbon, dust, and sludge.
• Reduce abrasive wear and deposit formation.
• Extend engine and oil service life.

Importance of valves:

• The filter bypass valve prevents oil starvation when filter resistance is excessive.
• The pump pressure-relief valve limits system pressure and protects the pump, filter, seals, and passages.

A bypass valve protects oil flow, but operation with unfiltered oil should be temporary; a blocked filter must be replaced promptly.

The clutch connects and disconnects engine power from the transmission. It permits smooth starting, gear changing, stopping without shutting down the engine, and controlled engagement of the load.

Working of a friction clutch:

• With the pedal released, springs press the pressure plate against the clutch disc and flywheel.
• Friction locks these members together, transmitting engine torque to the gearbox input shaft.
• When the pedal is depressed, the release bearing operates release levers.
• The pressure plate moves away from the clutch disc, interrupting power flow.
• Pedal free play ensures that the release bearing is not continuously loaded.

Single-plate clutch:

• Uses one driven friction disc.
• Generally interrupts power to both the transmission and PTO together.
• It is simple, compact, and relatively inexpensive.

Dual clutch:

• Uses separate clutch sections for transmission drive and PTO drive.
• Partial pedal movement disconnects the transmission while the PTO continues operating.
• Full pedal movement disconnects both transmission and PTO power.
• It improves control when using PTO-driven machines such as mowers or balers.

Incorrect free play can cause clutch slip, incomplete disengagement, overheating, and rapid wear.

The tractor transmission modifies engine torque and speed and delivers power to the driving wheels.

Power-flow sequence:

1. Engine flywheel: Receives power from the crankshaft.
2. Clutch: Engages or interrupts power flow to the transmission.
3. Gearbox: Provides different speed ratios and reverse motion. Lower gears reduce speed and increase wheel torque.
4. Main shaft or output shaft: Carries gearbox output toward the rear axle.
5. Differential: Divides torque between the two driving wheels while allowing them to rotate at different speeds during turns.
6. Differential lock: Temporarily locks both axle shafts together when one wheel loses traction.
7. Final drive: Provides an additional speed reduction and torque increase through bull gears, planetary gears, or similar arrangements.
8. Rear axles and wheels: Transmit the final torque to the ground.

The relationship between speed and transmission ratio may be expressed as:

where is the speed ratio, is input speed, and is output speed. A larger reduction ratio produces lower wheel speed and higher available torque, neglecting losses.

Differential:

The differential consists mainly of a crown wheel, differential casing, spider or pinion gears, and side gears connected to the axle shafts. It performs two functions:

• Divides transmission torque between the left and right driving wheels.
• Allows the outer wheel to rotate faster than the inner wheel when the tractor turns.

Without differential action, tyre wear, steering difficulty, and transmission stress would increase during turns.

Differential lock:

• Mechanically connects the two axle shafts or differential members.
• Forces both driving wheels to rotate together.
• Improves traction when one wheel slips in mud or loose soil.
• Must generally be disengaged before turning or operating on firm ground.

Final drive:

• Located between the differential and driving wheels.
• Provides the last stage of speed reduction.
• Increases torque delivered to the wheels.
• Reduces the torque load on upstream transmission components.
• May use bull gears, spur gears, or planetary reduction units.

Together, these units provide high tractive torque, permit smooth turning, and improve tractor mobility under variable field conditions.

Sliding-mesh transmission:

• Gears are moved directly into mesh to select a ratio.
• It has simple construction and low cost.
• Gear shifting may be noisy and requires skill because gear speeds must be matched.

Constant-mesh transmission:

• Gear pairs remain continuously in mesh.
• Dog clutches select the required gear.
• It offers easier engagement and less gear-tooth damage than sliding mesh.
• The clutch may still be required for changing ratios.

Synchromesh transmission:

• Gears remain in mesh, and synchronizers equalize the speeds of engaging members.
• It allows smoother and quicker gear changes.
• It is more complex and expensive than basic mechanical systems.

Hydrostatic transmission:

• Uses a variable-displacement hydraulic pump and hydraulic motor.
• Provides stepless speed variation and easy reversal.
• It is convenient for operations requiring frequent speed changes.
• It can have lower mechanical efficiency and higher cost than a simple gear transmission.

Mechanical gear transmissions are efficient and durable for heavy drawbar work, whereas hydrostatic transmission provides superior control and convenience for variable-speed operations.