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UPMSP Vocational Automobile study materials
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UPMSP Vocational Automobile practice questions

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Question 1Automobile Fundamentals and Workshop Safety

In an automobile workshop, which personal protective equipment is most important when grinding metal parts on a bench grinder?

  1. Loose-fitting overalls
  2. A dust mask only
  3. Cotton hand gloves
  4. Safety goggles with a face shield Correct answer.

Explanation

Grinding throws high-speed metal chips and sparks toward the face and eyes, so impact-rated eye protection combined with a face shield is essential. Gloves can catch in rotating machinery, and a dust mask alone does not protect the eyes.

Question 2Automobile Fundamentals and Workshop Safety

A four-stroke petrol engine completes one full working cycle in how many revolutions of the crankshaft?

  1. One revolution
  2. Half a revolution
  3. Two revolutions Correct answer.
  4. Four revolutions

Explanation

The four strokes are intake, compression, power and exhaust, and each stroke corresponds to half a crankshaft revolution. Therefore the complete cycle takes two revolutions of the crankshaft.

Question 3Automobile Fundamentals and Workshop Safety

Which instrument is used to measure the internal diameter of a cylinder bore accurately?

  1. Telescopic gauge with micrometer Correct answer.
  2. Feeler gauge
  3. Outside micrometer
  4. Vernier caliper

Explanation

A telescopic gauge transfers the bore diameter to an outside micrometer, giving an accurate internal measurement. A vernier caliper can measure bore openings but with less precision for deep bores.

Question 4Automobile Fundamentals and Workshop Safety

What is the primary function of the clutch in a manual-transmission vehicle?

  1. To increase engine torque
  2. To connect and disconnect engine power from the transmission smoothly Correct answer.
  3. To cool the gearbox oil
  4. To reduce engine noise

Explanation

The clutch engages and disengages the engine from the transmission so the vehicle can start from rest and change gears smoothly. It does not multiply torque or perform cooling duties.

Question 5Automobile Fundamentals and Workshop Safety

In a lead-acid car battery, which acid is used as the electrolyte?

  1. Hydrochloric acid
  2. Nitric acid
  3. Sulphuric acid Correct answer.
  4. Acetic acid

Explanation

A lead-acid battery uses dilute sulphuric acid as the electrolyte between lead and lead-dioxide plates. The other acids listed do not produce the reversible lead-acid reaction.

Question 6Automobile Fundamentals and Workshop Safety

Which type of fire extinguisher should be used on a petrol fire in the workshop?

  1. Wet chemical
  2. Carbon dioxide only for deep fat
  3. Foam or dry powder (ABC) Correct answer.
  4. Water

Explanation

Petrol is a Class B flammable liquid fire, so foam or ABC dry powder extinguishers are appropriate. Water spreads burning fuel and must never be used on petrol.

Question 7Automobile Fundamentals and Workshop Safety

What does the abbreviation TDC mean in engine terminology?

  1. Timing Delay Control
  2. Top Dead Centre Correct answer.
  3. Torque Distribution Coupling
  4. Total Displacement Capacity

Explanation

TDC stands for Top Dead Centre, the position where the piston is at its highest point in the cylinder. Ignition and valve timing are often referenced to this position.

Question 8Automobile Fundamentals and Workshop Safety

A mechanic is about to work under a vehicle raised on a hydraulic trolley jack. What is the correct safe practice?

  1. Ask a colleague to hold the vehicle steady
  2. Lower the vehicle onto wooden blocks balanced on the jack
  3. Place axle stands under the chassis before working Correct answer.
  4. Leave the jack holding the load and work quickly

Explanation

A hydraulic jack can lose pressure suddenly, so the vehicle must be supported on rated axle stands before anyone works underneath. The jack is only a lifting device, not a support.

Question 9Automobile Fundamentals and Workshop Safety

In a four-stroke engine, during which stroke does the intake valve open and the piston move downward?

  1. Exhaust stroke
  2. Compression stroke
  3. Intake stroke Correct answer.
  4. Power stroke

Explanation

During the intake stroke the piston moves down and the intake valve is open, drawing the air-fuel mixture into the cylinder. The other strokes involve compression, combustion or expulsion of gases.

Question 10Automobile Fundamentals and Workshop Safety

What is the main purpose of the radiator in a liquid-cooled engine?

  1. To filter impurities from the coolant
  2. To store coolant for later use
  3. To increase coolant pressure
  4. To transfer heat from the coolant to the surrounding air Correct answer.

Explanation

The radiator passes hot coolant through tubes with fins so airflow removes heat and lowers coolant temperature. It does not store coolant or filter it.

Question 11Automobile Fundamentals and Workshop Safety

A vehicle pulls to one side while braking. Which fault is the most likely cause?

  1. Worn spark plugs
  2. Low engine oil level
  3. Unequal braking force between left and right wheels Correct answer.
  4. Incorrect tyre inflation on the spare wheel

Explanation

If one brake caliper or wheel cylinder applies more force than the other, the vehicle will pull toward the stronger side during braking. Engine oil and spark plugs do not affect braking balance.

Question 12Automobile Fundamentals and Workshop Safety

Which type of gearbox allows gear changes without the driver operating a clutch pedal?

  1. Constant-mesh gearbox
  2. Synchromesh manual gearbox
  3. Sliding-mesh gearbox
  4. Automatic transmission Correct answer.

Explanation

An automatic transmission changes ratios through hydraulic or electronic control, so the driver does not operate a clutch pedal. Manual gearboxes, whether sliding-mesh, constant-mesh or synchromesh, require clutch operation.

Question 13Automobile Fundamentals and Workshop Safety

What is the function of the alternator in a vehicle charging system?

  1. To convert mechanical energy into electrical energy for the battery and loads Correct answer.
  2. To store electrical charge chemically
  3. To convert electrical energy into mechanical rotation
  4. To regulate tyre pressure

Explanation

The alternator is driven by the engine belt and generates alternating current, which is rectified to charge the battery and supply electrical loads. It does not store charge itself.

Question 14Automobile Fundamentals and Workshop Safety

A tyre is marked 195/65 R15. What does the number 65 represent?

  1. Tyre width in millimetres
  2. Aspect ratio as a percentage of section width Correct answer.
  3. Maximum speed rating
  4. Rim diameter in inches

Explanation

In this size code, 195 is the section width in millimetres and 65 is the aspect ratio, meaning sidewall height is 65 percent of the width. The R indicates radial construction and 15 is the rim diameter in inches.

Question 15Automobile Fundamentals and Workshop Safety

Which suspension component is primarily responsible for absorbing road shocks?

  1. Shock absorber (damper) Correct answer.
  2. Leaf spring
  3. Track rod
  4. Anti-roll bar

Explanation

The shock absorber damps the oscillation of the spring, converting motion into heat so road shocks do not cause continued bouncing. The spring supports load, while the damper controls movement.

Question 16Automobile Fundamentals and Workshop Safety

A workshop compressor is used to inflate a tyre. What safety check should be made before connecting the air line to the valve?

  1. Confirm the regulator pressure is set to the tyre's recommended value Correct answer.
  2. Remove the safety valve
  3. Open the drain valve fully
  4. Disconnect the pressure gauge

Explanation

The regulator must be set to the tyre's recommended pressure so the tyre is not over-inflated, which can cause a burst. Checking the gauge and regulator before connection prevents over-pressurisation.

Question 17Automobile Fundamentals and Workshop Safety

Which of the following is the correct way to store a used lead-acid battery in the workshop?

  1. Upright in a designated acid-resistant area away from drains Correct answer.
  2. In a sealed metal bin with other waste
  3. Lying on its side in a walkway
  4. On a shelf above the workbench near tools

Explanation

Batteries should be kept upright in a designated, acid-resistant area away from drains and ignition sources to prevent leakage and environmental contamination. They should never be stored in walkways or with general waste.

Question 18Automobile Fundamentals and Workshop Safety

What does a compression test on a petrol engine primarily indicate?

  1. The condition of the piston rings, valves and head gasket Correct answer.
  2. The battery's state of charge
  3. The tyre tread depth
  4. The quality of the engine oil

Explanation

A compression test measures cylinder pressure during cranking, revealing leakage through worn rings, leaking valves or a blown head gasket. It does not assess oil quality, battery charge or tyres.

Question 19Automobile Fundamentals and Workshop Safety

In a hydraulic braking system, what is the purpose of the brake fluid reservoir?

  1. To generate braking pressure
  2. To hold hydraulic fluid for the braking circuit Correct answer.
  3. To cool the brake discs
  4. To lubricate the brake pads

Explanation

The reservoir stores hydraulic brake fluid so the master cylinder always has a supply and can compensate for fluid movement and minor loss. It does not generate pressure or lubricate pads.

Question 20Automobile Fundamentals and Workshop Safety

A vehicle's engine oil pressure warning lamp stays on after starting. Which action is most appropriate?

  1. Add coolant to the radiator
  2. Stop the engine immediately and check the oil level and system Correct answer.
  3. Ignore it if the engine sounds normal
  4. Continue driving to the workshop at high speed

Explanation

A persistent oil-pressure warning indicates possible loss of lubrication, which can destroy bearings within minutes. The safe action is to stop the engine, check the oil level and investigate the system before running it again.

Question 21Automobile Fundamentals and Workshop Safety

Why is a torque wrench preferred over a standard spanner when tightening cylinder-head bolts?

  1. It removes damaged threads
  2. It tightens bolts faster
  3. It applies a controlled and even clamping force to the specified value Correct answer.
  4. It measures bolt length

Explanation

Cylinder-head bolts must be tightened to a specified torque, usually in a sequence, to ensure even clamping and prevent warping or gasket failure. A torque wrench limits applied torque to the set value.

Question 22Automobile Fundamentals and Workshop Safety

In a four-stroke engine, valve overlap refers to which condition?

  1. Both valves remaining closed during compression
  2. Both intake and exhaust valves being open near the end of exhaust and start of intake Correct answer.
  3. The intake valve staying open throughout the power stroke
  4. The exhaust valve opening twice per cycle

Explanation

Valve overlap occurs when the intake and exhaust valves are both open briefly around top dead centre, helping scavenge exhaust gases and improve cylinder filling. It is a designed timing feature, not a fault.

Question 23Automobile Fundamentals and Workshop Safety

A technician finds the brake pedal feels spongy and travels further than usual. Which fault is the most likely cause?

  1. Air trapped in the hydraulic circuit Correct answer.
  2. Worn tyre tread
  3. A clogged air filter
  4. Low coolant level

Explanation

Air in the hydraulic circuit is compressible, so pedal effort is partly absorbed and the pedal feels spongy with increased travel. The remedy is bleeding the brakes to remove the air.

Question 24Automobile Fundamentals and Workshop Safety

Why must a vehicle's wheels be balanced after fitting a new tyre?

  1. To change the tyre's speed rating
  2. To increase tyre pressure
  3. To improve fuel octane
  4. To prevent vibration caused by uneven mass distribution Correct answer.

Explanation

Uneven mass around the wheel creates vibration at speed, which stresses bearings and suspension and reduces comfort. Balancing adds or removes small weights so the mass is evenly distributed.

Question 25Engine Systems and Components

In a four-stroke diesel engine, which component directly opens the intake and exhaust valves at the correct moment?

  1. The flywheel ring gear
  2. The camshaft lobes acting through lifters and pushrods or directly on buckets Correct answer.
  3. The oil pump drive gear
  4. The crankshaft counterweights

Explanation

The camshaft is driven at half crankshaft speed and its lobes are ground to open each valve at the designed point in the cycle. Motion is transferred through lifters, pushrods and rockers in OHV designs, or directly onto buckets in OHC designs. The crankshaft and flywheel only store and deliver torque, they do not time valve events.

Question 26Engine Systems and Components

A technician measures the cylinder bore of an engine and finds it is 0.30 mm larger than the standard size with visible vertical scratches. Which repair is normally required?

  1. Rebore to the next oversize and fit matching oversize pistons and rings Correct answer.
  2. Replace only the piston rings with standard rings
  3. Hone lightly and reuse the original pistons
  4. Fit a thicker head gasket to compensate

Explanation

Bore wear of this magnitude plus scoring exceeds what honing can correct, so the cylinder must be bored to the next available oversize. Oversize pistons and rings restore the correct piston-to-bore clearance and ring end gap. Standard rings in a worn bore will not seal and will cause blow-by and oil consumption.

Question 27Engine Systems and Components

What is the primary function of the crankshaft in a reciprocating engine?

  1. To atomise the fuel before combustion
  2. To convert the reciprocating motion of the pistons into rotary motion and deliver torque Correct answer.
  3. To filter the engine oil
  4. To open the valves in sequence

Explanation

The connecting rods push on the crankpins, and the offset of these pins from the main axis turns linear piston travel into rotation. The crankshaft also carries the flywheel and pulley, delivering torque to the transmission. Valve actuation, oil filtration and fuel atomisation belong to other systems.

Question 28Engine Systems and Components

During a compression test on a petrol engine, one cylinder reads 6 bar while the others read 11 bar. Adding a spoonful of oil to that cylinder raises the reading to 10 bar. What does this indicate?

  1. Incorrect ignition timing
  2. Worn piston rings or cylinder bore Correct answer.
  3. A leaking head gasket between cylinders
  4. A burnt exhaust valve

Explanation

The oil temporarily seals the gap between rings and bore, so a large rise after oiling points to ring or bore wear. A valve or gasket leak would not be sealed by oil and the reading would stay low. This wet-versus-dry comparison is the standard way to separate ring leakage from valve leakage.

Question 29Engine Systems and Components

Which type of piston ring is fitted in the lowest groove and mainly controls oil film on the cylinder wall?

  1. Keystone ring
  2. Top chrome ring
  3. Oil control ring Correct answer.
  4. Compression ring

Explanation

The oil control ring sits in the lowest groove and its rails scrape excess oil from the bore back to the sump through drain holes. Compression rings in the upper grooves seal combustion pressure. Keystone and chrome rings are types of compression ring, not oil control rings.

Question 30Engine Systems and Components

A diesel engine produces white smoke continuously after warm-up. Which cause should be investigated first?

  1. A blocked air filter restricting flow
  2. Late injection timing or low compression causing unburnt fuel Correct answer.
  3. Excessive engine oil dilution
  4. A faulty radiator cap

Explanation

Persistent white smoke after warm-up is typically vaporised but unburnt fuel, often from retarded injection timing, low compression or faulty injectors. Checking timing and compression addresses the root cause. Black smoke would suggest over-fuelling and blue smoke would suggest oil burning.

Question 31Engine Systems and Components

What is the purpose of the thermostat in an engine cooling system?

  1. To maintain coolant temperature within the designed operating range by controlling flow Correct answer.
  2. To pressurise the cooling system above atmospheric pressure
  3. To drive the water pump impeller
  4. To circulate coolant through the radiator at all times

Explanation

The thermostat is a wax-operated valve that stays closed while the engine is cold, allowing rapid warm-up, then opens to route coolant to the radiator. It modulates flow to hold the engine near its design temperature. Pressurisation is the job of the radiator cap and circulation is done by the pump.

Question 32Engine Systems and Components

Which component in a diesel fuel injection system pressurises fuel and delivers it to each injector in firing order?

  1. The fuel filter
  2. The feed pump
  3. The injection pump Correct answer.
  4. The lift pump

Explanation

The injection pump both generates high pressure and meters the quantity of fuel delivered to each injector at the correct time. The feed or lift pump only supplies low-pressure fuel from the tank. The filter removes contaminants and does not create injection pressure.

Question 33Engine Systems and Components

A valve clearance measured with a feeler gauge is 0.15 mm on an engine whose specification is 0.25 mm. What is the likely effect if left uncorrected?

  1. The valve spring will break immediately
  2. The valve will not close fully, causing compression loss
  3. The valve will open early and close late, causing backfiring into the intake
  4. The valve will open late and close early, reducing volumetric efficiency Correct answer.

Explanation

Excessive clearance delays the opening and hastens the closing of the valve, shortening the time-area it is open. This reduces breathing efficiency and can increase mechanical noise. Insufficient clearance, by contrast, holds the valve off its seat and causes compression loss.

Question 34Engine Systems and Components

In a turbocharged engine, what drives the turbine wheel?

  1. The exhaust gas leaving the engine Correct answer.
  2. An electric motor
  3. The intake air pressure
  4. A belt from the crankshaft

Explanation

Exhaust gas energy spins the turbine, which is on a common shaft with the compressor. The compressor then raises intake air density. Belt-driven and electrically driven compressors exist but are not what drives a conventional turbocharger turbine.

Question 35Engine Systems and Components

Which condition most directly causes detonation (knocking) in a petrol engine?

  1. A cold engine at idle
  2. A clogged fuel filter at light load
  3. Excessive ignition advance combined with high load Correct answer.
  4. Fuel octane rating higher than required

Explanation

Detonation occurs when end-gas auto-ignites before the flame front arrives, and excessive advance or high cylinder pressure promotes it. High-octane fuel resists this and is not a cause. Cold idle and light-load filter restriction do not create the pressure and temperature needed for knock.

Question 36Engine Systems and Components

What is the function of the flywheel in an engine assembly?

  1. To increase compression pressure
  2. To filter metal particles from the oil
  3. To regulate the coolant temperature
  4. To store rotational energy and smooth out power pulses between firing strokes Correct answer.

Explanation

The flywheel's mass carries the engine through the non-power strokes and reduces speed fluctuation. It also provides the friction surface for the clutch and the ring gear for starting. Compression, oil filtration and cooling are handled by other parts.

Question 37Engine Systems and Components

A mechanic finds engine oil contaminated with coolant and a milky emulsion on the dipstick. Which defect is the most probable cause?

  1. An incorrect spark plug gap
  2. A failed head gasket or cracked cylinder head Correct answer.
  3. A worn oil control ring
  4. A loose fan belt

Explanation

Coolant entering the oil galleries or crankcase through a failed head gasket or crack produces the milky emulsion typical of oil mixed with water. This also pressurises the cooling system. Ring wear, belt slip and plug gap do not create a coolant-to-oil path.

Question 38Engine Systems and Components

Which type of bearing is used for the big-end and main journals of a typical automotive crankshaft?

  1. Needle roller bearing
  2. Thin-wall precision shell bearing Correct answer.
  3. Ball bearing
  4. Taper roller bearing

Explanation

Crankshaft main and big-end bearings are thin-wall steel shells lined with bearing material, supplied in undersize grades for reground journals. They rely on a hydrodynamic oil film rather than rolling elements. Ball, roller and needle bearings are used elsewhere, such as gearboxes and wheel hubs.

Question 39Engine Systems and Components

An engine has correct compression, good spark and fuel delivery, but will not start. The timing light shows the spark occurs after top dead centre on the compression stroke. What is the most likely fault?

  1. The radiator is blocked
  2. The ignition timing is retarded or the distributor/cam timing is incorrect Correct answer.
  3. The engine oil grade is too thin
  4. The fuel pump delivers too much pressure

Explanation

A spark occurring after TDC on compression means combustion begins too late to sustain starting. This points to retarded ignition timing or a camshaft/distributor installed a tooth off. Fuel pressure, cooling and oil grade do not shift ignition timing.

Question 40Engine Systems and Components

What is the main reason for fitting a balance shaft in some four-cylinder engines?

  1. To cool the exhaust valves
  2. To reduce the valve clearance
  3. To cancel second-order reciprocating forces that cause vibration Correct answer.
  4. To increase the compression ratio

Explanation

In-line four-cylinder engines have unbalanced second-order forces from piston and rod motion. Counter-rotating balance shafts running at twice crankshaft speed cancel these forces and reduce vibration. They do not affect compression, valve clearance or valve cooling.

Question 41Engine Systems and Components

Which component reduces the pressure pulsations in the lubrication system and protects the filter from damage when oil is cold?

  1. The oil cooler
  2. The dipstick
  3. The oil pressure relief valve Correct answer.
  4. The oil strainer

Explanation

The relief valve opens when pressure exceeds a set value, bypassing oil back to the sump or pump inlet. This protects the filter and galleries from over-pressure, especially with cold, thick oil. The strainer removes large particles and the cooler controls temperature.

Question 42Engine Systems and Components

A diesel engine has low power, black smoke and a knocking sound under load. Which fault best explains all three symptoms together?

  1. Faulty injectors delivering poorly atomised fuel Correct answer.
  2. A stuck-open thermostat
  3. A leaking vacuum advance diaphragm
  4. An undercharged battery

Explanation

Poor atomisation from worn nozzles delays and incomplete combustion, producing black smoke, lost power and diesel knock. The other options affect ignition advance, cooling and starting respectively and cannot explain the combination. Checking injector spray pattern and pop pressure is the logical next step.

Question 43Engine Systems and Components

What does the term 'valve overlap' describe in a four-stroke engine?

  1. The distance the valve lifts off its seat
  2. The period when both intake and exhaust valves are open around top dead centre Correct answer.
  3. The clearance between valve stem and guide
  4. The angle between two camshaft lobes

Explanation

Overlap is the crank angle during which both valves are open, occurring as the exhaust closes and the intake begins to open near TDC. It uses exhaust gas inertia to help scavenge the chamber and draw in fresh charge. Lift, guide clearance and lobe spacing are separate parameters.

Question 44Engine Systems and Components

Which statement best describes the operation of a common rail diesel injection system?

  1. Injection timing is fixed by a mechanical governor and cannot vary
  2. Fuel pressure is generated only during each injection stroke by a cam-driven plunger
  3. Fuel is mixed with air in the intake manifold by a venturi
  4. A high-pressure pump stores fuel in a rail, and the ECU controls injector opening electronically Correct answer.

Explanation

In a common rail system, a high-pressure pump charges an accumulator rail, and the ECU fires solenoid or piezo injectors electronically. This separates pressure generation from injection timing, allowing multiple injections per cycle and flexible timing. Older systems used a cam-driven plunger for each injection event.

Question 45Engine Systems and Components

What is the purpose of the radiator cap's pressure valve?

  1. To filter rust particles from the coolant
  2. To raise the boiling point of the coolant by maintaining system pressure Correct answer.
  3. To open the thermostat at a set temperature
  4. To circulate coolant through the heater core

Explanation

By holding the system above atmospheric pressure, the cap raises the coolant boiling point and prevents cavitation and boil-over. Its vacuum valve also allows air or coolant back in as the system cools. Circulation, filtration and thermostat operation are separate functions.

Question 46Engine Systems and Components

During a cylinder leakage test, air escapes continuously from the radiator filler neck with the piston at TDC on compression. Which defect does this indicate?

  1. A leaking exhaust valve
  2. Worn piston rings
  3. A leaking head gasket or cracked head Correct answer.
  4. A leaking intake valve

Explanation

Air entering the cooling system from the combustion chamber means the separation between them is breached, typically by a head gasket failure or crack. Intake and exhaust valve leaks would be heard at the throttle body or tailpipe. Ring leakage would be heard at the oil filler cap.

Question 47Engine Systems and Components

Which material property makes cast iron a common choice for cylinder blocks?

  1. Good wear resistance and vibration damping with low cost Correct answer.
  2. Low thermal expansion compared with aluminium
  3. High ductility for deep drawing
  4. High electrical conductivity

Explanation

Cast iron offers excellent wear resistance, damping of noise and vibration, and low cost, which is why many blocks and liners are made from it. Aluminium is used where low weight matters but needs liners or treated bores. High ductility for drawing is a property of sheet steel, not cast iron.

Question 48Engine Systems and Components

A petrol engine idles roughly and a vacuum gauge connected to the intake manifold reads a steady low value with slight needle fluctuation. Which fault fits this reading?

  1. A vacuum leak at the intake manifold gasket Correct answer.
  2. A fully open throttle at high speed
  3. A correctly adjusted idle mixture
  4. A new and correctly sealing exhaust system

Explanation

A low, fluctuating manifold vacuum at idle typically indicates a leak admitting unmetered air downstream of the throttle, leaning the mixture and disturbing idle quality. A healthy engine shows a high steady reading at idle. Wide-open throttle and a sealed exhaust do not produce this pattern.

Question 49Engine Systems and Components

What is the main function of the exhaust gas recirculation (EGR) valve in a diesel engine?

  1. To increase maximum power output
  2. To lower combustion temperature and reduce nitrogen oxide formation Correct answer.
  3. To heat the intake air for easier starting
  4. To increase the compression ratio

Explanation

Recirculating a portion of exhaust gas into the intake dilutes the fresh charge and lowers peak combustion temperature. Lower temperature suppresses the formation of nitrogen oxides. EGR does not raise power, change compression ratio or act as an intake heater.

Question 50Engine Systems and Components

In a four-stroke SI engine, what is the primary function of the camshaft?

  1. To open and close the intake and exhaust valves at the correct time Correct answer.
  2. To pressurise the fuel before it is delivered to the injectors
  3. To convert reciprocating motion of the pistons into rotary motion
  4. To maintain uniform crankshaft speed by storing kinetic energy

Explanation

The camshaft carries lobes that actuate the valve train, opening each valve at the correct point in the four-stroke cycle and allowing the valve spring to close it. Its rotational speed is half the crankshaft speed in a four-stroke engine, which synchronises valve events with piston position. It does not itself convert piston motion into rotation or store energy.