I85PREP
⌘ K
WBBSE Madhyamik Vocational Automobile study materials
WBBSE Madhyamik Vocational Automobile study resources

WBBSE Madhyamik Vocational Automobile practice questions

One question at a time. Build your WBBSE Madhyamik Vocational Automobile understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Automobile Workshop Safety and Tools

A mechanic must loosen a tightly rusted nut on a wheel hub. Which spanner should be selected to avoid rounding the nut corners?

  1. A pipe wrench with serrated jaws
  2. A ring (box-end) spanner of the exact size Correct answer.
  3. An adjustable wrench opened slightly wider than the nut
  4. A double-ended open-ended spanner of the exact size

Explanation

A ring spanner grips all six flats of the nut, so the load is spread and the corners are much less likely to round off when high torque is needed on a rusted fastener. Open-ended and adjustable tools contact only two flats and tend to slip under heavy load.

Question 2Automobile Workshop Safety and Tools

In an automobile workshop, which fire extinguisher is the correct first choice for a small petrol fire near the fuel tank?

  1. Water (stored pressure) extinguisher
  2. Wet chemical extinguisher
  3. Dry powder (ABC) extinguisher Correct answer.
  4. Foam (AFFF) extinguisher

Explanation

Petrol is a Class B flammable liquid, and dry powder extinguishes it by smothering and interrupting the flame chemistry without conducting electricity. Water spreads burning fuel, and wet chemical is designed for cooking oils, not hydrocarbons.

Question 3Automobile Workshop Safety and Tools

Before working under a vehicle raised on a hydraulic trolley jack, what is the correct safety procedure?

  1. Ask a helper to keep pumping the jack handle
  2. Place approved axle stands under the chassis and lower the jack onto them Correct answer.
  3. Leave the jack holding the load and chock one rear wheel
  4. Slide a wooden plank under the sump for extra support

Explanation

A hydraulic jack is a lifting device only; a seal failure can drop the vehicle instantly. The load must be transferred to rated axle stands placed on firm, level ground at the manufacturer's lifting points before anyone goes underneath.

Question 4Automobile Workshop Safety and Tools

Which personal protective equipment is essential when grinding a metal component on a bench grinder?

  1. Safety goggles or a face shield plus eye protection Correct answer.
  2. Ear plugs alone
  3. Cotton gloves and a dust mask only
  4. A welding helmet with a dark filter

Explanation

Grinding throws high-speed metal particles and sparks, so impact-rated eye protection or a full face shield is mandatory. A welding helmet's dark filter would make the work invisible, and gloves near a rotating wheel create an entanglement hazard.

Question 5Automobile Workshop Safety and Tools

A torque wrench reads 60 N·m on its scale. What does this value represent?

  1. The force applied at the handle in newtons
  2. The tensile load inside the bolt in newtons
  3. The turning moment about the fastener axis in newton-metres Correct answer.
  4. The angle the fastener turns in degrees

Explanation

Torque is the product of force and perpendicular distance from the pivot, so its unit is the newton-metre and it measures turning moment about the fastener axis. It is not a direct measure of bolt tension or of the angle turned.

Question 6Automobile Workshop Safety and Tools

Which tool is designed to remove a broken stud whose shank is embedded in a cylinder head?

  1. A hacksaw blade
  2. A centre punch
  3. A screw extractor (easy-out) Correct answer.
  4. A cold chisel

Explanation

A screw extractor has a reverse-tapered, fluted body that bites into a drilled pilot hole and turns the remnant out counter-clockwise. Chisels, punches and hacksaw blades cannot grip the inside of a drilled stud.

Question 7Automobile Workshop Safety and Tools

Which item is a standard part of a basic first-aid box kept in an automobile workshop?

  1. A set of feeler gauges
  2. Sterile gauze and adhesive dressings Correct answer.
  3. A can of brake fluid
  4. A bottle of engine oil

Explanation

A workshop first-aid box must contain materials for treating cuts, burns and eye injuries, such as sterile gauze, adhesive dressings and eye wash. Engine oil, brake fluid and feeler gauges are workshop consumables or tools, not first-aid supplies.

Question 8Automobile Workshop Safety and Tools

When using a two-post hydraulic lift in a workshop, where should the lifting pads be positioned?

  1. Under the exhaust pipe and fuel tank
  2. At the manufacturer's designated lift points on the chassis Correct answer.
  3. Anywhere along the floor pan between the wheels
  4. Under the doors so the body is supported

Explanation

Vehicle makers specify reinforced lift points that can carry the vehicle's weight without bending panels or crushing lines. Placing pads under doors, exhaust or the fuel tank can cause costly damage or a dangerous drop.

Question 9Automobile Workshop Safety and Tools

A technician needs to measure the diameter of a valve stem to check wear against the service limit. Which instrument gives the most suitable precision?

  1. A vernier caliper with 0.1 mm graduations
  2. A try square
  3. An outside micrometer graduated to 0.01 mm Correct answer.
  4. A steel rule graduated in millimetres

Explanation

Valve stem wear limits are typically a few hundredths of a millimetre, so an outside micrometer reading to 0.01 mm is appropriate. A steel rule, coarse caliper or try square cannot resolve such small differences reliably.

Question 10Automobile Workshop Safety and Tools

A socket is marked 13 mm and is being used on a bolt head measured at 13 mm across flats. What is the correct fit?

  1. The socket should be hammered onto the bolt
  2. The socket should be at least 2 mm larger for clearance
  3. The socket should be a snug fit with no visible play Correct answer.
  4. The socket size refers to the thread diameter, not the head

Explanation

Socket sizes denote the distance across the flats of the fastener head, so a 13 mm socket fits a 13 mm head snugly. Oversize sockets slip and round the corners, and hammering damages both tool and fastener.

Question 11Automobile Workshop Safety and Tools

Why should compressed air never be used to clean dust off a mechanic's overalls or skin?

  1. It removes dust too slowly to be useful
  2. It may force air and debris into the body and cause serious injury Correct answer.
  3. It always leaves an oily residue on clothing
  4. It is prohibited only because it wastes electricity

Explanation

Compressed air at workshop pressure can inject air, metal particles or solvents through the skin, causing air embolism or eye injury. Dust should be removed with a brush, vacuum or low-pressure nozzle held well away from the body.

Question 12Automobile Workshop Safety and Tools

A hydraulic floor jack feels spongy and slowly sinks under load. What is the most appropriate action?

  1. Top up the reservoir with engine oil and continue using it
  2. Place a wooden block on the saddle to increase height
  3. Remove the jack from service and have it repaired or replaced Correct answer.
  4. Pump the handle several times before each lift to compensate

Explanation

A jack that sinks is losing hydraulic pressure through a failed seal or valve, and no operating technique makes it safe. It must be tagged out of service and repaired by a competent person; topping up with the wrong oil can worsen seal damage.

Question 13Automobile Workshop Safety and Tools

Which property makes a hammer with a loose head dangerous, and what is the correct remedy?

  1. The head becomes magnetised, so it should be demagnetised
  2. The handle becomes too heavy, so it should be shortened
  3. The face becomes too hard, so it should be ground softer
  4. The head may fly off during a swing, so the wedge must be refitted or the hammer replaced Correct answer.

Explanation

A loose head can separate from the handle under the high acceleration of a swing and strike someone nearby. The head must be re-secured with a proper wedge or, better, the hammer withdrawn and replaced.

Question 14Automobile Workshop Safety and Tools

What is the primary purpose of a bench vice's swivel base?

  1. To allow the workpiece to be rotated to a convenient working angle Correct answer.
  2. To prevent the jaws from marking soft metals
  3. To increase the clamping force on the workpiece
  4. To store spare jaw inserts

Explanation

The swivel base lets the whole vice body be rotated and locked so the work can be presented at a comfortable angle. Clamping force comes from the screw, and jaw protection comes from soft jaw caps.

Question 15Automobile Workshop Safety and Tools

A vehicle's battery is being disconnected for electrical work. Which sequence reduces the risk of sparks and short circuits?

  1. Disconnect both terminals simultaneously with two spanners
  2. Disconnect the positive terminal while the ignition is on
  3. Disconnect the negative terminal first, then the positive Correct answer.
  4. Disconnect the positive terminal first, then the negative

Explanation

With the negative terminal removed first, the spanner cannot bridge the positive post to the earthed body and cause a short or spark. Reconnection is done in the reverse order, negative last.

Question 16Automobile Workshop Safety and Tools

Why is a torque wrench normally returned to its lowest setting after use?

  1. To keep the internal spring unstressed so the calibration stays accurate Correct answer.
  2. To prevent the socket from rusting
  3. To make the scale easier to read next time
  4. To allow the wrench to be used as a breaker bar

Explanation

A micrometer-style torque wrench stores energy in a spring, and leaving it compressed for long periods causes the spring to take a set and the calibration to drift. Backing it off to the minimum setting preserves accuracy.

Question 17Automobile Workshop Safety and Tools

A technician is about to arc weld a bracket on a vehicle fitted with an engine control unit. What precaution protects the electronics?

  1. Connect the welder earth clamp to the vehicle's painted body panel
  2. Disconnect the battery and unplug the ECU, and earth the welder close to the work Correct answer.
  3. Leave the ignition switched on so the ECU stays active
  4. Increase the welding current to finish the job faster

Explanation

Welding current can travel through the vehicle's wiring and destroy sensitive electronics, so the battery is disconnected and the earth clamp is placed as close as possible to the weld. A distant or poor earth forces current through other paths.

Question 18Automobile Workshop Safety and Tools

Which practice correctly prevents a vehicle from rolling while it is being pushed into the workshop bay?

  1. All helpers push on the same side of the vehicle
  2. One person steers while others push, with a chock ready and the handbrake released Correct answer.
  3. The handbrake is left fully applied during pushing
  4. The gear lever is left in a forward gear while pushing

Explanation

Pushing requires the handbrake released and a driver at the wheel to steer and brake, with chocks available to stop a runaway. Pushing on one side only makes the vehicle veer, and a gear or handbrake fights the movement.

Question 19Automobile Workshop Safety and Tools

A vernier caliper reads 0.02 mm on its main scale with the vernier's 10th division aligned. If the main scale shows 12 mm, what is the reading?

  1. 12.00 mm
  2. 12.02 mm
  3. 12.20 mm Correct answer.
  4. 12.10 mm

Explanation

With a least count of 0.02 mm, the tenth vernier division aligning adds 10 × 0.02 = 0.20 mm to the main scale value of 12 mm, giving 12.20 mm. Reading the division number as 0.02 instead of multiplying it is a common error.

Question 20Automobile Workshop Safety and Tools

A bolt specified for 90 N·m is being tightened with a wrench whose handle is extended by a 0.5 m cheater bar. Why is this dangerous?

  1. The applied torque rises far above the specification and can shear or yield the bolt Correct answer.
  2. The bolt becomes magnetic and loses its strength
  3. The extension reduces torque and the bolt stays loose
  4. The cheater bar changes the thread pitch of the bolt

Explanation

Torque equals force times lever arm, so lengthening the handle multiplies the moment applied for the same hand force, easily exceeding the specified 90 N·m. Over-torque stretches the bolt beyond yield and can snap it or damage the thread.

Question 21Automobile Workshop Safety and Tools

A grinding wheel shows a slight crack and a dull grey glaze on its face. What is the correct action before further use?

  1. Increase the wheel speed to break off the glazed layer
  2. Use the wheel as it is because the glaze improves cutting
  3. Ring-test the wheel, dress the face, and discard it if the crack is confirmed Correct answer.
  4. Soak the wheel in water overnight to remove the crack

Explanation

A cracked bonded wheel can burst at operating speed, so it must be ring-tested and rejected if damaged; glazing is corrected by dressing the face to expose fresh abrasive. Running a suspect wheel or speeding it up risks a catastrophic failure.

Question 22Automobile Workshop Safety and Tools

A CO2 extinguisher is discharged in a small enclosed inspection pit where a technician is working. Why is this hazardous?

  1. It raises the temperature of the pit rapidly
  2. It displaces oxygen and can cause asphyxiation in a confined space Correct answer.
  3. It conducts electricity through the metal pit walls
  4. It reacts with petrol to form a flammable gas

Explanation

Carbon dioxide smothers fire by excluding oxygen, and in a confined pit it can quickly reduce breathable oxygen to a dangerous level. Personnel should evacuate and ventilate the space after discharge before re-entering.

Question 23Automobile Workshop Safety and Tools

Two bolts of the same thread and grade are tightened, one dry and one lubricated, both to the same torque figure. Which statement is correct?

  1. The lubricated bolt develops higher preload because less torque is lost to friction Correct answer.
  2. Neither bolt develops preload until the engine is started
  3. The dry bolt develops higher preload because friction adds clamping force
  4. Both bolts develop identical preload because the torque is the same

Explanation

Only part of the applied torque becomes bolt tension; the rest is lost to thread and under-head friction. Lubrication lowers friction, so more of the same torque turns into preload, which is why specifications usually state whether threads are dry or oiled.

Question 24Automobile Workshop Safety and Tools

During a road test a technician notices the brake pedal sinks slowly to the floor while held at a stop. Which workshop check best identifies the cause?

  1. Adjust the handbrake cable and retest the pedal
  2. Top up the fluid and continue the road test without further checks
  3. Replace the brake pads and retest the pedal
  4. Inspect for external fluid leaks and check whether the pedal holds pressure when the line is clamped Correct answer.

Explanation

A pedal that creeps down under steady pressure indicates a hydraulic leak, either external or past a master cylinder seal. Clamping lines to isolate sections and checking for leaks locates the faulty circuit before any road use.

Question 25Automotive Engine Fundamentals

In a four-stroke petrol engine, the stroke during which the intake valve remains open and the piston moves from TDC to BDC is called the

  1. compression stroke
  2. suction stroke Correct answer.
  3. power stroke
  4. exhaust stroke

Explanation

The suction (intake) stroke draws the air-fuel mixture into the cylinder as the piston travels downward from top dead centre to bottom dead centre with the intake valve open. The other three strokes occur after the charge is trapped and the valves close.

Question 26Automotive Engine Fundamentals

The component that converts the reciprocating motion of the piston into rotary motion of the crankshaft is the

  1. connecting rod Correct answer.
  2. flywheel
  3. camshaft
  4. cylinder liner

Explanation

The connecting rod links the piston pin to the crankpin, so as the piston moves up and down the rod swings and turns the crankshaft. This is the standard mechanism for converting reciprocating motion into rotation.

Question 27Automotive Engine Fundamentals

The correct firing order for a typical inline four-cylinder engine with cylinders numbered 1 to 4 from the front is

  1. 1-3-4-2 Correct answer.
  2. 1-2-3-4
  3. 1-4-3-2
  4. 1-2-4-3

Explanation

Inline four-cylinder engines commonly fire 1-3-4-2 so that power strokes are evenly spaced through 720 degrees of crankshaft rotation and balance is maintained. Firing 1-2-3-4 in sequence would place power strokes on adjacent cylinders and load the crank unevenly.

Question 28Automotive Engine Fundamentals

In a diesel engine, ignition of the fuel charge occurs because

  1. the fuel is ignited by the hot compressed air Correct answer.
  2. the glow plug fires continuously during running
  3. the injector contains an ignition coil
  4. a spark plug fires just before top dead centre

Explanation

A diesel engine compresses only air to a high pressure and temperature, then injects fuel which self-ignites on contact with that hot air. No spark is used during normal running.

Question 29Automotive Engine Fundamentals

The main function of the radiator in a water-cooled engine cooling system is to

  1. transfer heat from the coolant to the air Correct answer.
  2. pressurise the cooling system
  3. lubricate the water pump seal
  4. circulate coolant through the block

Explanation

Hot coolant from the engine passes through the radiator tubes, and airflow across the fins removes heat before the coolant returns to the engine. The radiator is therefore the main heat exchanger of the system.

Question 30Automotive Engine Fundamentals

Which part of the engine lubrication system removes solid particles from the circulating oil?

  1. Oil pump
  2. Oil cooler
  3. Oil filter Correct answer.
  4. Dipstick

Explanation

The oil filter traps metal particles, carbon and sludge so that clean oil reaches the bearings and other moving parts. The pump only moves oil and the cooler only controls temperature.

Question 31Automotive Engine Fundamentals

The valve clearance in an engine is measured between the

  1. valve guide and the valve stem
  2. valve face and the valve seat
  3. spring retainer and the cylinder head
  4. valve stem tip and the rocker arm Correct answer.

Explanation

Valve clearance is the small gap between the top of the valve stem and the rocker arm or tappet, set to allow for thermal expansion. It is checked with a feeler gauge when the valve is fully closed.

Question 32Automotive Engine Fundamentals

A compression gauge reading is low on one cylinder but normal on the others. Adding a little oil to that cylinder raises the reading. This most probably indicates

  1. an incorrectly set valve clearance
  2. worn piston rings Correct answer.
  3. a leaking head gasket
  4. a burnt exhaust valve

Explanation

When oil temporarily seals the gap between worn rings and the cylinder wall, compression rises, which points to ring or bore wear. A valve or gasket leak would not be sealed by adding oil.

Question 33Automotive Engine Fundamentals

The purpose of the thermostat in the engine cooling system is to

  1. remove air bubbles from the coolant
  2. drive the radiator cooling fan
  3. keep the coolant pressure constant
  4. regulate coolant flow to maintain operating temperature Correct answer.

Explanation

The thermostat stays closed while the engine is cold, allowing quick warm-up, then opens at its rated temperature to let coolant flow to the radiator. In this way it holds the engine near its designed operating temperature.

Question 34Automotive Engine Fundamentals

In a battery ignition system, the component that steps up the low battery voltage to the high voltage needed at the spark plug is the

  1. condenser
  2. ballast resistor
  3. ignition coil Correct answer.
  4. distributor

Explanation

The ignition coil has primary and secondary windings, and the sudden collapse of primary current induces a very high voltage in the secondary winding. That high voltage is then fed to the spark plug.

Question 35Automotive Engine Fundamentals

A technician finds that the engine cranks normally but will not start, and a spark test shows a strong blue spark at the plug. The next logical check is

  1. the starter motor brushes
  2. the radiator cap pressure
  3. fuel delivery to the cylinder Correct answer.
  4. the battery terminal voltage

Explanation

A strong spark confirms that the ignition side is working, so the fault is likely on the fuel or air side. Checking whether fuel reaches the cylinder is therefore the logical next step.

Question 36Automotive Engine Fundamentals

The device that mixes petrol with air in the correct proportion in a carburetted engine is the

  1. carburettor Correct answer.
  2. fuel pump
  3. fuel injector
  4. air filter

Explanation

The carburettor uses the venturi effect to draw fuel into the incoming air stream and so prepares the combustible mixture. The fuel pump only supplies fuel, and the air filter only cleans the air.

Question 37Automotive Engine Fundamentals

The main advantage of a multi-point fuel injection system over a carburettor is that it

  1. needs no fuel pump
  2. uses no electronic sensors
  3. requires no air filter
  4. meters fuel more precisely for each cylinder Correct answer.

Explanation

Multi-point injection delivers a precisely metered spray to each cylinder, so mixture strength is controlled much more accurately than by a carburettor. This gives better economy, lower emissions and smoother running.

Question 38Automotive Engine Fundamentals

The clearance between the piston and the cylinder bore is necessary because

  1. pistons expand when heated and would seize without clearance Correct answer.
  2. the clearance helps pump oil to the sump
  3. the rings need space to rotate around the piston
  4. the piston must rock freely to open the valves

Explanation

Pistons run hotter than the cylinder wall and expand more, so a small cold clearance is provided to prevent seizure at operating temperature. The piston rings then seal the gap.

Question 39Automotive Engine Fundamentals

In a four-stroke engine running at 3000 rpm, the number of power strokes produced by a single cylinder in one minute is

  1. 750
  2. 3000
  3. 1500 Correct answer.
  4. 6000

Explanation

A four-stroke cycle needs two crankshaft revolutions to produce one power stroke, so one power stroke occurs every two revolutions. At 3000 rpm the cylinder therefore fires 3000 divided by 2, equal to 1500 times per minute.

Question 40Automotive Engine Fundamentals

The turbocharger fitted to a diesel engine is driven by

  1. the camshaft gear
  2. an electric motor
  3. the exhaust gas turbine Correct answer.
  4. the engine crankshaft through a belt

Explanation

Exhaust gas spins a turbine on one end of the turbocharger shaft, and the compressor on the other end forces more air into the cylinder. The energy comes from the exhaust stream, not from the engine's mechanical drive.

Question 41Automotive Engine Fundamentals

Black smoke from the exhaust of a diesel engine most commonly indicates

  1. a weak battery
  2. coolant entering the combustion chamber
  3. engine oil burning in the cylinder
  4. excessive fuel or insufficient air for combustion Correct answer.

Explanation

Black smoke is unburnt carbon from an over-rich mixture, typically caused by too much fuel, a clogged air filter or a faulty injector. White smoke points to coolant or unburnt fuel, and blue smoke points to oil burning.

Question 42Automotive Engine Fundamentals

The firing order of an engine is arranged mainly to

  1. distribute power strokes evenly and balance the crankshaft Correct answer.
  2. reduce the size of the flywheel
  3. increase the compression ratio
  4. simplify the wiring of the spark plugs

Explanation

By spacing the power strokes evenly along the crankshaft rotation, the firing order smooths torque delivery and reduces vibration and bearing loads. It also helps keep the engine balanced.

Question 43Automotive Engine Fundamentals

The main purpose of the flywheel on an engine is to

  1. open and close the valves
  2. cool the exhaust gases
  3. store energy and smooth out the power impulses Correct answer.
  4. filter the engine oil

Explanation

The heavy flywheel absorbs energy during the power stroke and releases it during the other strokes, so the crankshaft turns more evenly. It also provides a mounting face for the clutch.

Question 44Automotive Engine Fundamentals

Tappet noise in an engine with mechanical valve lifters is usually corrected by

  1. replacing the camshaft
  2. replacing the spark plugs
  3. adjusting the valve clearance Correct answer.
  4. increasing the oil pressure

Explanation

A tappet noise is a ticking caused by excessive clearance between the valve stem and rocker arm or tappet. Setting the clearance to the manufacturer's specification with a feeler gauge removes the noise.

Question 45Automotive Engine Fundamentals

Which statement about the compression ratio of a petrol engine is correct?

  1. Lowering the compression ratio always increases power output
  2. Raising the compression ratio generally improves thermal efficiency but can cause knocking Correct answer.
  3. Compression ratio is the ratio of oil pressure to fuel pressure
  4. Compression ratio is measured in litres

Explanation

A higher compression ratio extracts more work from the burning charge and raises thermal efficiency, but it also raises peak pressure and temperature, which can lead to detonation or knocking. Fuel octane rating and chamber design limit how high the ratio can safely go.

Question 46Automotive Engine Fundamentals

The exhaust valve of an engine is usually made of a heat-resistant alloy because it

  1. is exposed to hot exhaust gases Correct answer.
  2. carries the spark current
  3. runs cooler than the intake valve
  4. supports the valve spring

Explanation

The exhaust valve is bathed in hot combustion gases and must withstand much higher temperatures than the intake valve. Special heat-resisting steels are therefore used for it.

Question 47Automotive Engine Fundamentals

The component that maintains system pressure and prevents boiling of coolant in a sealed cooling system is the

  1. water pump
  2. thermostat
  3. radiator cap Correct answer.
  4. fan belt

Explanation

The radiator cap contains a pressure valve that holds the system above atmospheric pressure, which raises the boiling point of the coolant. It also has a vacuum valve to allow coolant back from the expansion tank as the engine cools.

Question 48Automotive Engine Fundamentals

During the compression stroke of a four-stroke petrol engine, both valves are closed and the piston moves from

  1. BDC to TDC Correct answer.
  2. TDC to BDC
  3. TDC to TDC
  4. BDC to BDC

Explanation

After the intake stroke the piston is at bottom dead centre, and during compression it rises to top dead centre with both valves shut, squeezing the trapped mixture. Ignition then occurs near the end of this stroke.

Question 49Automotive Engine Fundamentals

An engine shows low oil pressure at idle and a warning lamp that stays on. Which check should be made first?

  1. Engine oil level and condition on the dipstick Correct answer.
  2. Tyre inflation pressure
  3. Spark plug gap
  4. Radiator coolant concentration

Explanation

Low oil level or diluted, degraded oil is the most common and simplest cause of low pressure, so the level and condition should be checked before suspecting the pump or bearings. If the level is correct, the pressure gauge and relief valve are checked next.

Question 50Automotive Engine Fundamentals

In a four-stroke diesel engine, the fuel injector sprays diesel into the cylinder at the end of the compression stroke because

  1. the air charge has been heated to a high temperature by compression and ignites the fuel Correct answer.
  2. the injector adds fuel early so that it can mix during the whole intake stroke
  3. a spark plug fires at that instant to ignite the air-fuel mixture
  4. the exhaust valve is open at that moment to help the fuel vapour escape

Explanation

A diesel engine has no spark plug; it relies on compression ignition. Compressing the air charge to a high pressure raises its temperature well above the ignition point of diesel, so fuel sprayed in at the end of compression self-ignites. This is why the injector timing and the compression ratio are so critical on a diesel.