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Tamil Nadu HSE Second Year Basic Automobile Engineering study materials
Tamil Nadu HSE Second Year Basic Automobile Engineering study resources

Tamil Nadu HSE Second Year Basic Automobile Engineering practice questions

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Question 1Automobile Engine Fundamentals and Operation

In a four-stroke petrol engine, during which stroke does the intake valve remain open while the piston moves from TDC to BDC?

  1. Power stroke
  2. Exhaust stroke
  3. Suction stroke Correct answer.
  4. Compression 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 inlet valve open. The other strokes deal with compressing, burning, or expelling gases, so the inlet valve is closed during them.

Question 2Automobile Engine Fundamentals and Operation

The total volume swept by the piston as it moves from TDC to BDC in one cylinder is known as the:

  1. Clearance volume
  2. Compression ratio
  3. Cylinder volume
  4. Swept volume Correct answer.

Explanation

Swept volume is the volume displaced by the piston during one full stroke between top and bottom dead centre. Clearance volume is only the small space left above the piston at TDC, and compression ratio is a ratio, not a volume.

Question 3Automobile Engine Fundamentals and Operation

Which component converts the reciprocating motion of the piston into rotary motion of the crankshaft?

  1. Push rod
  2. Flywheel
  3. Camshaft
  4. Connecting rod Correct answer.

Explanation

The connecting rod links the piston to the crankpin, so as the piston reciprocates the rod swings and turns the crankshaft. The camshaft operates the valves, the flywheel stores energy, and the push rod transfers cam motion in an OHV engine.

Question 4Automobile Engine Fundamentals and Operation

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

  1. A glow plug fires throughout the power stroke
  2. The fuel is pre-mixed with air in the carburettor
  3. The high compression temperature of air ignites the injected fuel Correct answer.
  4. A spark plug fires just before top dead centre

Explanation

A diesel draws only air, compresses it to a high ratio so its temperature rises above the fuel's self-ignition point, then injects fuel that ignites spontaneously. There is no spark plug or carburettor in the combustion process of a conventional diesel.

Question 5Automobile Engine Fundamentals and Operation

The function of the flywheel in an engine is to:

  1. Filter impurities from the lubricating oil
  2. Open and close the inlet and exhaust valves
  3. Increase the compression ratio of the engine
  4. Store energy and smooth out torque fluctuations between power strokes Correct answer.

Explanation

The heavy flywheel absorbs energy during the power stroke and releases it during the other strokes, giving smoother rotation and helping the engine run past idle. It has no role in compression ratio, oil filtration, or valve timing.

Question 6Automobile Engine Fundamentals and Operation

Which of the following is the correct order of strokes in a four-stroke SI engine?

  1. Suction, compression, power, exhaust Correct answer.
  2. Compression, suction, exhaust, power
  3. Power, suction, compression, exhaust
  4. Suction, power, compression, exhaust

Explanation

A four-stroke cycle proceeds as intake of the charge, compression of the charge, combustion producing the power stroke, and finally exhaust of the burnt gases. The crankshaft makes two revolutions for this complete sequence.

Question 7Automobile Engine Fundamentals and Operation

The device that mixes petrol with air in the correct proportion before it enters the cylinder of a conventional petrol engine is the:

  1. Alternator
  2. Fuel injector
  3. Radiator
  4. Carburettor Correct answer.

Explanation

A carburettor uses the venturi effect to draw fuel into the incoming air stream and deliver a combustible mixture to the manifold. Fuel injectors spray fuel under pressure, while the radiator and alternator serve cooling and charging duties.

Question 8Automobile Engine Fundamentals and Operation

In a two-stroke engine, one complete cycle of operation is completed in:

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

Explanation

A two-stroke engine performs suction, compression, power, and exhaust functions within one up-and-down movement of the piston, so the crankshaft turns once per cycle. A four-stroke engine needs two revolutions to complete its cycle.

Question 9Automobile Engine Fundamentals and Operation

The compression ratio of an engine is defined as the ratio of:

  1. Bore to stroke
  2. Total cylinder volume to clearance volume Correct answer.
  3. Swept volume to clearance volume
  4. Clearance volume to swept volume

Explanation

Compression ratio compares the cylinder volume with the piston at BDC (swept plus clearance volume) to the volume with the piston at TDC (clearance volume). It is therefore always greater than one and indicates how strongly the charge is squeezed.

Question 10Automobile Engine Fundamentals and Operation

A four-cylinder engine has a bore of 80 mm and a stroke of 90 mm. Using the swept volume formula, the approximate total swept volume of the engine is:

  1. 1810 cc Correct answer.
  2. 905 cc
  3. 452 cc
  4. 7240 cc

Explanation

Swept volume per cylinder is (pi/4) x bore squared x stroke, which gives about 452 cubic centimetres, and multiplying by four cylinders gives roughly 1810 cc. The other figures correspond to a single cylinder or to arithmetic errors such as using diameter instead of radius.

Question 11Automobile Engine Fundamentals and Operation

Valve overlap in an engine refers to the period when:

  1. The inlet valve remains closed after BDC of the suction stroke
  2. The exhaust valve opens before the power stroke ends
  3. Both valves are fully closed during compression
  4. Both inlet and exhaust valves are open near the end of the exhaust stroke Correct answer.

Explanation

Valve overlap occurs around top dead centre between exhaust and intake, when the exhaust valve has not yet closed and the inlet valve has already begun to open. This uses the scavenging effect of gas flow to improve filling at higher speeds.

Question 12Automobile Engine Fundamentals and Operation

Which statement correctly distinguishes the working of a spark-ignition engine from a compression-ignition engine?

  1. SI engines have higher compression ratios than CI engines
  2. SI engines inject fuel after compression, while CI engines use a carburettor
  3. SI engines use a spark to ignite a premixed charge, while CI engines ignite injected fuel by hot compressed air Correct answer.
  4. SI engines require no ignition system while CI engines need spark plugs

Explanation

In an SI engine a spark plug ignites a homogeneous air-fuel mixture, whereas in a CI engine only air is compressed and the resulting high temperature ignites the injected fuel. This is why diesel engines run at higher compression ratios and need no spark plug.

Question 13Automobile Engine Fundamentals and Operation

The main purpose of the cooling system in an automobile engine is to:

  1. Lubricate the moving parts of the engine
  2. Increase the compression pressure inside the cylinder
  3. Supply fuel to the combustion chamber
  4. Remove excess heat so the engine runs at an efficient operating temperature Correct answer.

Explanation

Cooling carries away surplus combustion heat to keep metal temperatures within safe limits and maintain an efficient operating range. Overcooling or overheating both harm efficiency, so the system regulates rather than simply chills the engine.

Question 14Automobile Engine Fundamentals and Operation

In a multi-cylinder engine, the firing order is arranged so that:

  1. All cylinders fire at the same instant for maximum power
  2. The camshaft rotates in the opposite direction to the crankshaft
  3. Power strokes occur at evenly spaced intervals to reduce vibration and load on the crankshaft Correct answer.
  4. Only one cylinder ever produces power

Explanation

A suitable firing order spaces the power impulses evenly around the cycle, giving smoother running and reducing torsional stress on the crankshaft. Simultaneous firing would create severe vibration and shock loading.

Question 15Automobile Engine Fundamentals and Operation

The clearance volume of a cylinder is the volume:

  1. Occupied by the connecting rod
  2. Above the piston crown when it is at top dead centre Correct answer.
  3. Between the valve stem and the guide
  4. Swept by the piston during one stroke

Explanation

Clearance volume is the small space remaining in the combustion chamber when the piston reaches TDC. Together with swept volume it determines the compression ratio of the engine.

Question 16Automobile Engine Fundamentals and Operation

A camshaft in a four-stroke engine is driven at:

  1. The same speed as the crankshaft
  2. One quarter of the crankshaft speed
  3. Half the crankshaft speed Correct answer.
  4. Twice the crankshaft speed

Explanation

Each valve must open once every two crankshaft revolutions, so the camshaft turns once for every two turns of the crank, that is, at half speed. This relationship is fixed by the timing gears or belt ratio.

Question 17Automobile Engine Fundamentals and Operation

Detonation (knocking) in a petrol engine is most likely to occur when:

  1. The spark timing is retarded excessively
  2. The fuel's octane rating is too low for the compression ratio used Correct answer.
  3. The air filter is clean and unrestricted
  4. The engine is running cold at idle speed

Explanation

Knock happens when the end gas ignites spontaneously before the flame front arrives, which is favoured by low-octane fuel, high compression, or over-advanced timing. A fuel with adequate octane resists this auto-ignition.

Question 18Automobile Engine Fundamentals and Operation

The valve clearance in a mechanical tappet system is provided to:

  1. Reduce the compression ratio slightly
  2. Allow for thermal expansion of the valve stem so the valve still seats when hot Correct answer.
  3. Keep the valve permanently open for better breathing
  4. Increase the lift of the valve

Explanation

Valve stems expand when hot, so a small clearance prevents the valve from being held off its seat when the engine warms up. Too little clearance causes leakage and burning, while too much causes noise and reduced lift.

Question 19Automobile Engine Fundamentals and Operation

In an engine, the term 'stroke' refers to:

  1. The number of cylinders in the block
  2. The distance travelled by the piston from one dead centre to the other Correct answer.
  3. The diameter of the cylinder bore
  4. The angle through which the valve rotates

Explanation

Stroke is the linear distance the piston moves between TDC and BDC, equal to twice the crank radius. Bore, by contrast, is the cylinder diameter, and the two together define the swept volume.

Question 20Automobile Engine Fundamentals and Operation

A four-stroke engine develops one power stroke per cylinder in every:

  1. Two revolutions of the crankshaft Correct answer.
  2. One revolution of the crankshaft
  3. Half revolution of the crankshaft
  4. Four revolutions of the crankshaft

Explanation

The cycle needs four piston strokes, which correspond to two complete turns of the crankshaft. Hence each cylinder fires once in every two revolutions, unlike a two-stroke that fires every revolution.

Question 21Automobile Engine Fundamentals and Operation

Which of these best explains why a diesel engine generally has higher thermal efficiency than a petrol engine of similar size?

  1. Its higher compression ratio raises the cycle efficiency and it is not throttled Correct answer.
  2. It uses a spark plug to control combustion precisely
  3. Its fuel has a higher calorific value than petrol
  4. It runs always at stoichiometric air-fuel ratio

Explanation

Higher compression ratio raises the ideal cycle efficiency, and diesel engines avoid pumping losses because they are load-controlled by fuel quantity rather than a throttle. Diesel fuel's calorific value per kilogram is in fact slightly lower than petrol's, so that is not the reason.

Question 22Automobile Engine Fundamentals and Operation

If the inlet valve of a four-stroke engine is set to close well after bottom dead centre, the likely effect at low engine speeds is:

  1. The valve overlap period is eliminated
  2. The exhaust gases are completely scavenged from the cylinder
  3. The compression ratio increases significantly
  4. Some fresh charge is pushed back into the manifold, reducing low-speed torque Correct answer.

Explanation

Late inlet closing uses intake inertia to improve filling at high speed, but at low speed the piston begins to rise before the valve shuts and part of the charge returns to the port. This lowers volumetric efficiency and torque at low speed.

Question 23Automobile Engine Fundamentals and Operation

An engine is described as having a compression ratio of 10:1. If the clearance volume is 50 cc, the swept volume of that cylinder is:

  1. 450 cc Correct answer.
  2. 550 cc
  3. 50 cc
  4. 500 cc

Explanation

Compression ratio equals (swept + clearance) divided by clearance, so 10 equals (swept + 50) divided by 50. Solving gives swept volume of 450 cc, and the total cylinder volume at BDC is 500 cc.

Question 24Automobile Engine Fundamentals and Operation

In a two-stroke engine using crankcase compression, the transfer port opens when the piston:

  1. Moves upward on compression with all ports covered
  2. Moves downward and uncovers the port, admitting the compressed mixture from the crankcase Correct answer.
  3. Closes the exhaust port after scavenging
  4. Reaches top dead centre and the spark occurs

Explanation

As the piston descends, it first uncovers the exhaust port and then the transfer port, letting the slightly compressed mixture in the crankcase flow into the cylinder and sweep out burnt gases. The crankcase pressure was built up during the upward stroke.

Question 25Automobile Engine Fundamentals and Operation

The main reason for providing an exhaust valve lead (opening before BDC) is to:

  1. Start releasing high-pressure burnt gases early so pumping losses during the exhaust stroke are reduced Correct answer.
  2. Increase the compression pressure in the cylinder
  3. Improve the volumetric efficiency of the intake stroke directly
  4. Prevent the inlet valve from opening at all

Explanation

Opening the exhaust valve slightly before BDC lets the still-high cylinder pressure blow down the burnt gases, so the piston has less resistance on the upward exhaust stroke. The small loss of expansion work is outweighed by reduced pumping loss and better scavenging.

Question 26Automobile Engine Fundamentals and Operation

In a four-stroke spark-ignition engine, during which stroke does the piston move from top dead centre to bottom dead centre with both valves closed, while the burning gases push it down?

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

Explanation

The power (expansion) stroke follows combustion, when the burnt charge expands and drives the piston from TDC towards BDC with both valves seated. This is the only stroke that delivers work to the crankshaft, the other three strokes being preparatory and consuming energy.

Question 27Automobile Engine Fundamentals and Operation

A technician finds that the engine oil in a petrol car has become thick, dark and gritty after many months of city driving. Which function of the lubrication system has been most directly affected by this condition?

  1. Operating the valves against spring pressure
  2. Sealing the combustion chamber during the power stroke
  3. Reducing friction and carrying away heat and wear particles Correct answer.
  4. Maintaining the correct air-fuel ratio in the carburettor

Explanation

Engine oil's main duty is to form a film that separates moving surfaces, thereby reducing friction, absorbing heat and flushing metallic wear particles to the filter and sump. When oil turns gritty and thick, this film breaks down and both friction and wear rise.

Question 28Automobile Engine Fundamentals and Operation

In a four-stroke engine, the inlet valve opens 10° before top dead centre and closes 40° after bottom dead centre. The inlet valve remains open for how many degrees of crankshaft rotation?

  1. 240°
  2. 180°
  3. 230° Correct answer.
  4. 220°

Explanation

The inlet valve is open through the whole 180° suction stroke, plus the 10° before TDC and the 40° after BDC. Adding these gives 180 + 10 + 40 = 230° of crankshaft rotation, which is the valve's total opening duration.

Question 29Automobile Engine Fundamentals and Operation

During a road test, an engine with a badly clogged air filter shows reduced power and black smoke at the exhaust. Which explanation best fits these symptoms?

  1. The mixture becomes too lean, so combustion temperature rises sharply
  2. The mixture becomes too rich, so combustion is incomplete and carbon forms Correct answer.
  3. The spark timing retards automatically, so the power stroke begins late
  4. The compression ratio increases, so the charge burns faster

Explanation

A choked air filter restricts the air entering the engine, so for the same fuel drawn in the air-fuel mixture becomes richer than required. Rich mixtures burn incompletely, wasting fuel and producing black smoke and soot, while power falls.

Question 30Automobile Engine Fundamentals and Operation

An engine has a bore of 100 mm, a stroke of 120 mm and four cylinders. Taking π as 3.14, the approximate total swept volume (engine capacity) is closest to:

  1. 3770 cc Correct answer.
  2. 942 cc
  3. 2260 cc
  4. 1508 cc

Explanation

Swept volume of one cylinder is π/4 × d² × L = 0.785 × 10² × 12 = 942 cc. Multiplying by four cylinders gives about 3768 cc, which rounds to roughly 3770 cc.

Question 31Fuel, Ignition, Cooling and Lubrication Systems

In a simple carburettor, which component controls the quantity of fuel drawn into the air stream at the venturi throat?

  1. The air cleaner element
  2. The choke valve
  3. The main jet Correct answer.
  4. The throttle valve

Explanation

The main jet is a calibrated orifice that meters fuel from the float chamber into the venturi, so its size fixes the fuel quantity for a given pressure drop. The choke and throttle act on air flow, and the air cleaner only filters incoming air.

Question 32Fuel, Ignition, Cooling and Lubrication Systems

The float and needle valve assembly in a carburettor float chamber serves to

  1. increase the velocity of air at the venturi
  2. atomise the fuel before it leaves the jet
  3. prevent backfiring into the intake manifold
  4. maintain a constant fuel level in the float chamber Correct answer.

Explanation

The float rises and falls with the fuel level and, through the needle valve, opens or closes the fuel inlet so that the level stays essentially constant. A constant level keeps the head of fuel over the jet steady, which keeps the mixture strength steady.

Question 33Fuel, Ignition, Cooling and Lubrication Systems

Which fuel-supply system delivers fuel under high pressure through a nozzle into the inlet port just upstream of the inlet valve?

  1. Gravity feed system
  2. Vacuum tank system
  3. Petrol injection system Correct answer.
  4. Carburettor system

Explanation

In a petrol injection system a pump raises fuel to high pressure and an injector sprays it into the inlet port or cylinder, giving precise metering. Carburettors rely on venturi depression, and gravity or vacuum tanks only store and deliver fuel to the carburettor.

Question 34Fuel, Ignition, Cooling and Lubrication Systems

In a diesel engine, fuel is ignited inside the cylinder because

  1. the high compression ratio raises air temperature above the fuel's ignition point Correct answer.
  2. a glow plug fires continuously during running
  3. the injector nozzle contains a heating coil
  4. a spark plug fires at the end of the compression stroke

Explanation

A diesel draws only air, compresses it to a high ratio so its temperature rises well above the fuel's self-ignition temperature, then injects fuel which ignites spontaneously. There is no spark during normal running.

Question 35Fuel, Ignition, Cooling and Lubrication Systems

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

  1. transfer heat from the coolant to the surrounding air Correct answer.
  2. regulate the coolant flow according to temperature
  3. pressurise the cooling system
  4. circulate the coolant through the engine jacket

Explanation

The radiator exposes a large coolant surface to air flow so that heat absorbed in the engine jacket is rejected to atmosphere. Circulation is done by the pump, pressure control by the radiator cap, and flow regulation by the thermostat.

Question 36Fuel, Ignition, Cooling and Lubrication Systems

The thermostat in an engine cooling system is normally fitted

  1. in the coolant passage between the engine outlet and the radiator inlet Correct answer.
  2. at the bottom of the radiator
  3. on the engine sump drain plug
  4. inside the water pump impeller housing

Explanation

The thermostat sits in the coolant outlet path from the engine so it senses the temperature of coolant leaving the jacket and opens to admit flow to the radiator. This position lets it warm the engine quickly and then regulate.

Question 37Fuel, Ignition, Cooling and Lubrication Systems

In a forced-feed lubrication system, the component that draws oil from the sump and delivers it under pressure to the oil gallery is the

  1. oil cooler
  2. breather
  3. oil pump Correct answer.
  4. oil filter

Explanation

The oil pump is driven from the engine and generates the pressure that pushes oil through the filter and galleries to the bearings. The filter cleans the oil, the cooler lowers its temperature, and the breather vents crankcase gases.

Question 38Fuel, Ignition, Cooling and Lubrication Systems

Which property of lubricating oil describes its resistance to flow at a given temperature?

  1. Cloud point
  2. Flash point
  3. Pour point
  4. Viscosity Correct answer.

Explanation

Viscosity is the internal friction of the oil, or its resistance to flow, and it falls as oil warms up. Flash point, pour point and cloud point describe ignition and cold-behaviour limits rather than flow resistance.

Question 39Fuel, Ignition, Cooling and Lubrication Systems

In a coil ignition system, the purpose of the condenser connected across the contact breaker is to

  1. limit the current flowing through the ignition coil primary winding
  2. distribute the high-tension voltage to each spark plug in turn
  3. step up the battery voltage to the firing voltage
  4. reduce sparking at the contacts and speed up the collapse of the primary current Correct answer.

Explanation

The condenser forms a tuned circuit with the primary coil so the primary current collapses rapidly and contact arcing is suppressed, which preserves contact life and improves the induced high voltage. Voltage step-up is done by coil turns ratio, not by the condenser.

Question 40Fuel, Ignition, Cooling and Lubrication Systems

The firing order of a four-cylinder engine is 1-3-4-2. If cylinder 1 is on its compression stroke, which cylinder is on the power stroke?

  1. Cylinder 2
  2. Cylinder 4 Correct answer.
  3. Cylinder 3
  4. Cylinder 1

Explanation

In the sequence 1-3-4-2 the cylinder that fired immediately before cylinder 1 is the last in the order, cylinder 2, so cylinder 2 is on its power stroke while cylinder 1 compresses. Cylinder 4 is one step ahead of cylinder 1 in the order and is therefore on its intake stroke.

Question 41Fuel, Ignition, Cooling and Lubrication Systems

A spark ignition engine shows a rich mixture at idle. Which carburettor circuit is most likely supplying fuel at this condition?

  1. The accelerator pump circuit
  2. The main metering circuit
  3. The idle and progression circuit Correct answer.
  4. The power enrichment circuit

Explanation

At closed or nearly closed throttle the air velocity at the venturi is too low to draw fuel from the main jet, so the idle and progression circuit, which opens downstream of the throttle plate, supplies the mixture. The main circuit takes over at higher throttle openings.

Question 42Fuel, Ignition, Cooling and Lubrication Systems

An engine with a pressurised cooling system is fitted with a radiator cap marked 0.9 bar. What is the main advantage of this pressure cap?

  1. It allows the coolant to expand into the radiator top tank
  2. It raises the boiling point of the coolant so the engine can run hotter without boiling Correct answer.
  3. It prevents the thermostat from opening too early
  4. It increases the coolant flow rate through the radiator

Explanation

Raising system pressure raises the boiling temperature of the coolant, so a higher coolant temperature can be tolerated without boiling and the radiator size can be reduced. Coolant flow is set by the pump, and expansion is handled by the overflow or recovery bottle.

Question 43Fuel, Ignition, Cooling and Lubrication Systems

A vehicle's engine oil pressure warning lamp glows at idling speed and goes out as engine speed increases. Which is the most probable cause?

  1. Incorrect ignition timing
  2. Worn oil pump or excessively thin oil at low speed Correct answer.
  3. A faulty radiator pressure cap
  4. A blocked oil filter bypass valve stuck open

Explanation

Oil pressure depends on pump delivery and oil viscosity; at idle a worn pump or diluted, thin oil cannot maintain pressure, but faster pump speed restores it. That pattern points to the pump or oil condition rather than to ignition or cooling parts.

Question 44Fuel, Ignition, Cooling and Lubrication Systems

The main reason for providing a water jacket around the cylinder and combustion chamber is to

  1. lubricate the piston rings during the stroke
  2. carry away heat that would otherwise overheat the metal and distort it Correct answer.
  3. increase the compression ratio of the engine
  4. improve the volumetric efficiency by cooling the charge only

Explanation

Only part of the heat released by combustion is converted to work; the surplus must be removed or the metal overheats, loses strength and distorts, and the oil film breaks down. The jacket circulates coolant to carry this heat to the radiator.

Question 45Fuel, Ignition, Cooling and Lubrication Systems

Which statement correctly compares the air-cooled and water-cooled systems used on small engines?

  1. An air-cooled engine uses fins on the cylinder to increase heat-transfer area and needs no radiator or pump Correct answer.
  2. A water-cooled engine cannot use a thermostat because coolant flow must remain constant
  3. An air-cooled engine needs a thermostat and radiator to control cylinder temperature
  4. A water-cooled engine needs no water pump because convection alone circulates the coolant

Explanation

Air cooling relies on extended surfaces, the fins, so that moving air can carry heat away, and it avoids the weight and plumbing of a radiator and pump. Water cooling needs a pump to circulate coolant and a thermostat to regulate its flow.

Question 46Fuel, Ignition, Cooling and Lubrication Systems

A distributor's vacuum advance unit is connected to the carburettor port just above the throttle plate. Its purpose is to

  1. retard the timing whenever engine speed increases
  2. advance the ignition timing when the throttle is partly open and manifold vacuum is high Correct answer.
  3. advance the timing at full throttle and low vacuum
  4. operate the contact breaker points mechanically

Explanation

At part throttle, lean mixture burns more slowly, so extra advance is needed; the port above the throttle plate supplies vacuum in that condition and the diaphragm advances the timing. At full throttle the port vacuum falls and the advance is removed.

Question 47Fuel, Ignition, Cooling and Lubrication Systems

The purpose of the accelerator pump in a carburettor is to

  1. enrich the mixture at steady high speed
  2. hold the float chamber fuel level constant
  3. prevent fuel from leaking when the engine is stopped
  4. supply a temporary extra charge of fuel when the throttle is suddenly opened Correct answer.

Explanation

When the throttle snaps open, air flow increases instantly but fuel inertia delays the enrichment, so the mixture leans out; the accelerator pump injects a measured extra quantity of fuel to cover this lag. Steady-speed enrichment is handled by the power circuit and level control by the float.

Question 48Fuel, Ignition, Cooling and Lubrication Systems

Diesel knock (combustion knock) is most likely to occur when

  1. ignition delay is long, so a large quantity of fuel accumulates before auto-ignition Correct answer.
  2. the compression ratio is raised
  3. the injection timing is too far retarded
  4. the fuel cetane number is increased

Explanation

If the fuel does not ignite quickly after injection, more fuel collects in the cylinder and then burns almost at once, producing a steep pressure rise and the characteristic knock. A higher cetane number shortens the delay and reduces knock.

Question 49Fuel, Ignition, Cooling and Lubrication Systems

Which part of a full-flow oil filtration system allows oil to reach the engine bearings if the filter element becomes blocked?

  1. The bypass valve in the filter or filter head Correct answer.
  2. The oil level dipstick tube
  3. The crankcase breather
  4. The oil pressure relief valve in the pump

Explanation

A full-flow filter carries all the pump delivery, so a blocked element would starve the bearings; the bypass valve opens under the resulting pressure difference and lets unfiltered oil pass. The relief valve instead protects the system from excessive pump pressure.

Question 50Fuel, Ignition, Cooling and Lubrication Systems

A petrol engine starts readily but misfires steadily at high speed. Which ignition fault best explains a misfire that appears only at high speed?

  1. A stuck-open thermostat
  2. A clogged fuel tank vent causing a vacuum in the tank
  3. An over-rich idle mixture
  4. A weak coil or excessive spark plug gap that cannot fire under high cylinder pressure Correct answer.

Explanation

At high speed and load, cylinder pressure and the demand for voltage rise, so a weak coil or a wide plug gap may fail to produce a spark, giving a steady high-speed miss. Mixture and fuel-supply faults usually show at other conditions and often affect all speeds.