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NSSCO Motor Mechanics 6189 study materials
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NSSCO Motor Mechanics 6189 practice questions

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Question 1Workshop safety, health and environmental practice

In a motor mechanics workshop, which type of fire extinguisher is the correct choice for a fire involving petrol that has spilled and ignited near a vehicle?

  1. Dry powder extinguisher Correct answer.
  2. Foam (AFFF) extinguisher
  3. Carbon dioxide extinguisher only
  4. Water (stored pressure) extinguisher

Explanation

Petrol is a Class B flammable liquid fire, and dry powder (ABC or BC) is the standard workshop choice because it blankets the fuel and interrupts the chain reaction. It is also safe on live electrical equipment nearby, which suits a workshop environment. Water must never be applied because it spreads the burning fuel.

Question 2Workshop safety, health and environmental practice

A technician must raise a vehicle to work underneath it. Which practice meets accepted workshop safety requirements?

  1. Leave the engine running so the vehicle settles evenly on the lift points
  2. Support the vehicle on purpose-made axle stands placed under strong chassis or suspension points Correct answer.
  3. Stack wooden blocks or bricks under the sills at each corner
  4. Lower the hydraulic jack slowly until the vehicle rests on the jack with the handle locked

Explanation

A hydraulic jack is a lifting device only; once raised, the vehicle must be supported by rated axle stands positioned under manufacturer-recommended strong points. This prevents the vehicle dropping if the jack seal fails or the load shifts. Improvised supports and relying on the jack alone are common causes of fatal crush injuries.

Question 3Workshop safety, health and environmental practice

Before starting work on a vehicle fitted with a supplementary restraint system (airbag), what is the essential first safety step?

  1. Warm the engine to operating temperature so the system deactivates
  2. Remove the steering wheel immediately to access the airbag connector
  3. Test the airbag circuit with a 12 V test lamp while the ignition is on
  4. Disconnect the battery and wait the manufacturer-specified time for the capacitor to discharge Correct answer.

Explanation

An airbag module stores enough energy to deploy for a period after the battery is disconnected, so the battery must be isolated and the specified waiting time observed. Only then is it safe to handle connectors or remove components. Probing the circuit with power on can trigger deployment.

Question 4Workshop safety, health and environmental practice

A technician notices that a two-post vehicle lift's safety locking pawls do not engage when the arms are raised. What is the correct action?

  1. Take the lift out of service, tag it, and report the fault for repair Correct answer.
  2. Continue working but place a jack under the vehicle as a backup
  3. Raise the vehicle fully and rely on the hydraulic pressure to hold it
  4. Operate the lift only for light vehicles until the next service

Explanation

A lift with inoperative safety locks is defective and must be isolated from use, clearly tagged, and reported so a competent person can repair it. Mechanical locks are the last line of defence against hydraulic failure. Working around a known defect transfers the risk to everyone in the bay.

Question 5Workshop safety, health and environmental practice

Which statement correctly describes the purpose of a local exhaust ventilation (LEV) system in a workshop spray booth?

  1. It cools the booth to prevent the paint from curing too quickly
  2. It supplies compressed air for the spray gun
  3. It captures airborne solvent vapour and overspray at source and removes them from the breathing zone Correct answer.
  4. It recirculates filtered air to keep the booth warm in cold weather

Explanation

LEV works by capturing contaminants such as solvent vapour, isocyanate mist and overspray close to where they are generated, before they enter the operative's breathing zone. It is a primary engineering control under COSHH-type regulations. Air recirculation and temperature control are separate functions.

Question 6Workshop safety, health and environmental practice

A used engine oil drum is being emptied into the workshop waste oil tank. Which precaution best prevents environmental contamination?

  1. Store the drum open so any water content can evaporate
  2. Use a funnel over a bunded tank and keep a spill kit nearby Correct answer.
  3. Flush the drum into the general refuse skip
  4. Pour the oil onto the yard surface so it drains away with rainwater

Explanation

Used oil must be transferred into a bunded container using a funnel so that any spill is contained, with absorbent material available to deal with drips. Oil reaching drains or soil is a polluting discharge and is an offence in most jurisdictions. Used oil is also a controlled waste requiring authorised disposal.

Question 7Workshop safety, health and environmental practice

Why is compressed air never used to clean brake dust from wheel assemblies and hubs?

  1. It is not powerful enough to remove the dust effectively
  2. It cools the brake components too quickly and causes warping
  3. It removes the protective coating from the brake discs
  4. It can force fine dust into the air where it may be inhaled and can damage eyes and skin Correct answer.

Explanation

Brake dust may contain respirable particles and, in older linings, asbestos fibres; compressed air converts settled dust into an airborne cloud that can be inhaled deep into the lungs. The safe method is a dedicated brake vacuum or wet cleaning with appropriate PPE. Cooling and coating effects are not the reason for the prohibition.

Question 8Workshop safety, health and environmental practice

Which PPE combination is appropriate when grinding a steel component on a pedestal grinder?

  1. Ear defenders alone
  2. Cotton gloves and a welding helmet
  3. Ordinary spectacles and an apron
  4. Safety goggles, face shield, hearing protection and a dust mask Correct answer.

Explanation

Grinding generates flying particles, sparks, noise and fine metal dust, so eye and face protection, hearing protection and respiratory protection are all needed. Gloves must not be worn near rotating machinery because they can be caught. Ordinary spectacles do not provide impact protection to the sides.

Question 9Workshop safety, health and environmental practice

A vehicle is being run in the workshop with the engine idling while a fault is diagnosed. What is the main health risk if the exhaust extraction hose is not connected?

  1. Excessive noise exceeding the legal limit
  2. Build-up of carbon monoxide, an odourless gas that reduces the blood's oxygen-carrying capacity Correct answer.
  3. Corrosion of the workshop floor by condensate
  4. Increased fuel consumption causing a fire risk

Explanation

Engine exhaust contains carbon monoxide, which binds to haemoglobin far more readily than oxygen and can cause unconsciousness or death in a poorly ventilated space. Because it is colourless and odourless, it gives no warning. Exhaust extraction hoses or adequate mechanical ventilation are required whenever engines run indoors.

Question 10Workshop safety, health and environmental practice

When storing acids, solvents and other chemicals in a workshop store, which arrangement is correct?

  1. Store oxidisers next to flammable solvents to save space
  2. Keep incompatible substances segregated and store flammables in a fire-resistant cabinet Correct answer.
  3. Keep containers open so fumes do not build up
  4. Store all chemicals together alphabetically on one shelf for easy location

Explanation

Chemical storage must keep incompatible substances apart so that a leak or breakage cannot cause a violent reaction, and flammable liquids belong in a fire-resisting cabinet with a spill-retaining sump. Containers must be closed and labelled. Alphabetical storage ignores compatibility.

Question 11Workshop safety, health and environmental practice

An apprentice is about to use a two-part epoxy adhesive. Where should the safety data sheet (SDS) be consulted before opening the containers?

  1. Only if a spill occurs
  2. After the job is finished, when reporting any reaction
  3. Before starting, to identify hazards, required PPE and first-aid measures Correct answer.
  4. Only when requested by a customer

Explanation

The safety data sheet must be read before use so that the hazards, exposure controls, PPE requirements and emergency procedures are known in advance. It is also the reference for spill response and first aid. Consulting it afterwards defeats its purpose as a preventive control.

Question 12Workshop safety, health and environmental practice

A workshop uses a degreasing solvent in an open parts washer. Which control measure most directly reduces a technician's inhalation exposure?

  1. Adding more solvent to the washer to dilute the contamination
  2. Increasing the number of air changes by opening a fire door
  3. Wearing a high-visibility vest while using the washer
  4. Replacing the open washer with an enclosed unit fitted with extraction Correct answer.

Explanation

Engineering controls act at source and are more reliable than administrative or PPE measures, so enclosing the process and extracting vapour is the most effective way to reduce exposure. Substitution and enclosure rank above procedural controls in the hierarchy of control. Adding solvent increases the vapour source.

Question 13Workshop safety, health and environmental practice

A hydraulic hose on a vehicle lift bursts and oil sprays across the workshop floor. Which sequence of actions is most appropriate?

  1. Mop the oil into the drain, then continue lifting the vehicle
  2. Isolate the lift, clear the area, contain and absorb the spill, and report the defect Correct answer.
  3. Cover the oil with cardboard and carry on working
  4. Wash the floor with water and restart the lift immediately

Explanation

The energy source must be isolated first to stop further release, then the area is made safe, the spill is contained with absorbent material and the equipment defect is reported. Oil must not enter drains. Restarting equipment after a failure without inspection risks a repeat incident.

Question 14Workshop safety, health and environmental practice

Which of the following best explains why a residual current device (RCD) is required on portable electrical tools used in a workshop?

  1. It prevents the tool's motor from overheating under load
  2. It converts alternating current to direct current for the tool
  3. It disconnects the supply quickly if current leaks to earth, reducing the risk of fatal electric shock Correct answer.
  4. It limits the current drawn by the tool to reduce electricity bills

Explanation

An RCD continuously compares current in the live and neutral conductors and trips when a small imbalance indicates leakage to earth, typically within milliseconds at 30 mA. This protects the user if a cable is damaged or a tool becomes live. It does not regulate power consumption or motor temperature.

Question 15Workshop safety, health and environmental practice

A workshop stores oxy-acetylene cylinders for welding. Which practice is correct?

  1. Lay acetylene cylinders horizontally under a bench to save floor space
  2. Store oxygen and acetylene cylinders together in a locked cupboard with no ventilation
  3. Store cylinders upright, secured against falling, with oxygen and fuel gas separated and away from ignition sources Correct answer.
  4. Keep cylinders near the workshop heater so regulators do not freeze

Explanation

Cylinders must be upright and restrained so they cannot fall and shear the valve, and fuel gas must be segregated from oxygen by distance or a fire-resisting barrier. They must be kept away from heat and ignition sources and the store must be ventilated. Acetylene must never be stored or used lying down.

Question 16Workshop safety, health and environmental practice

A technician must remove a component that may contain asbestos friction material from an old vehicle. What is the most appropriate control?

  1. Burn the old lining in the workshop stove
  2. Break the component apart outdoors to disperse the fibres
  3. Wet the component, avoid abrasion, and place it in a sealed labelled bag for authorised disposal Correct answer.
  4. Dry-brush the dust into a bin and continue working

Explanation

Keeping the material wet suppresses fibre release, and avoiding cutting, grinding or brushing prevents asbestos fibres becoming airborne. The waste must be double-bagged, labelled and consigned to a licensed disposal route. Burning or dry-brushing releases fibres into the air.

Question 17Hand tools, measuring instruments and workshop equipment

A micrometer screw gauge has a main scale reading of 6.5 mm and the thimble, which has 50 divisions, reads 24 on the sleeve datum line. What is the total reading?

  1. 6.524 mm
  2. 6.98 mm
  3. 6.74 mm Correct answer.
  4. 7.24 mm

Explanation

The thimble has 50 divisions on a 0.5 mm pitch, so each division equals 0.01 mm. The thimble reading is therefore 24 × 0.01 mm = 0.24 mm, which is added to the main scale reading of 6.5 mm to give 6.74 mm. The thimble value must be added, not appended as extra digits.

Question 18Hand tools, measuring instruments and workshop equipment

Which instrument is most suitable for measuring the depth of a blind hole in a cylinder head to an accuracy of 0.02 mm?

  1. Outside micrometer
  2. Telescopic gauge
  3. Steel rule
  4. Vernier caliper with depth blade Correct answer.

Explanation

A vernier caliper fitted with a depth blade reaches into a hole and reads to 0.02 mm on its vernier scale. A steel rule is too coarse and cannot reach squarely to the bottom of a blind hole. An outside micrometer measures external dimensions only, and a telescopic gauge transfers a bore size but does not itself give a depth reading.

Question 19Hand tools, measuring instruments and workshop equipment

What is the correct procedure for zeroing a vernier caliper before use?

  1. Open the jaws to 10 mm and set the vernier to zero with the locking screw
  2. Slide the jaw until the vernier reads zero and tighten the clamp screw
  3. Close the jaws and adjust the vernier plate until the reading shows zero
  4. Close the jaws fully and check that the zero lines on both scales coincide Correct answer.

Explanation

With the measuring faces clean and fully closed, the zero of the vernier scale must line up exactly with the zero of the main scale. If the zeros do not coincide, the instrument has a zero error that must be recorded and applied to every reading. Adjusting scales on a closed caliper would mask or create error rather than detect it.

Question 20Hand tools, measuring instruments and workshop equipment

A torque wrench is stored with its scale set at 40 N·m after use. Why is this incorrect practice?

  1. The scale markings will fade under load
  2. The socket will corrode in the stored position
  3. The spring will lose its calibration if left compressed Correct answer.
  4. The ratchet will reverse direction over time

Explanation

A spring-type torque wrench relies on the elastic behaviour of its internal spring. Leaving the spring compressed between uses causes it to take a permanent set, so the wrench under-reads and loses calibration. The correct practice is to wind the scale back to its lowest setting before storage.

Question 21Hand tools, measuring instruments and workshop equipment

Which file cut is best suited to removing a large amount of mild steel quickly during rough shaping?

  1. Single-cut second-cut file
  2. Double-cut smooth file
  3. Single-cut smooth file
  4. Double-cut bastard file Correct answer.

Explanation

A double-cut file has two sets of crossing teeth that break up the chips and cut fast, and the bastard grade is coarse enough for heavy stock removal. Smooth files are for finishing, and single-cut files are generally used for sharpening or fine work. Rough shaping therefore calls for a coarse double-cut file.

Question 22Hand tools, measuring instruments and workshop equipment

When using a hand hacksaw to cut a 25 mm mild steel bar, which blade specification and action are correct?

  1. 14 teeth per inch with light pressure and full strokes
  2. 18 teeth per inch with heavy pressure and short strokes
  3. 32 teeth per inch with heavy pressure and full strokes
  4. 24 teeth per inch with light pressure and short strokes Correct answer.

Explanation

A 24 TPI blade gives at least three teeth in contact with 25 mm section steel, which prevents tooth snagging, and light pressure with full steady strokes keeps the cut straight. Coarse blades catch on thin sections, while very fine blades clog. Pressure should be applied on the forward stroke only.

Question 23Hand tools, measuring instruments and workshop equipment

A cold chisel is being used to cut a keyway in a mild steel shaft. What is the correct workshop practice?

  1. Grind a long sharp point and strike with a light hammer
  2. Lubricate the chisel tip to reduce friction and heat
  3. Hold the chisel at 60 degrees and strike with a club hammer
  4. Wear eye protection and cut towards a supported edge Correct answer.

Explanation

Chipping and cutting with a cold chisel produces flying chips, so eye protection is essential and the work must be securely supported with the cut directed away from the body. The chisel should be held firmly at about 45 degrees with a suitable hammer. Lubricant on a chisel tip does not help and can make the tool slip.

Question 24Hand tools, measuring instruments and workshop equipment

Which spanner should be used to loosen a tightly corroded hexagonal pipe union in a confined space?

  1. Adjustable wrench
  2. Box spanner
  3. Flare nut (crow-foot) spanner Correct answer.
  4. Open-ended spanner

Explanation

A flare nut spanner grips all six flats while its slot allows it to pass over the pipe, so it can apply full torque to a corroded union without rounding it. An open-ended spanner contacts only two flats and slips under high load. Adjustable and box spanners cannot be fitted over a pipe already connected.

Question 25Hand tools, measuring instruments and workshop equipment

A bench vice is used for filing a component. Which practice protects both the vice and the workpiece?

  1. Fit soft jaws and clamp the work low in the jaws Correct answer.
  2. Grip the work as far from the jaws as possible
  3. Clamp the work at the top of the jaws and lean on it
  4. Tighten the vice with a hammer on the handle

Explanation

Soft jaws (copper or aluminium caps) prevent the hardened jaw serrations from marking a finished surface, and clamping the work low in the jaws reduces leverage and vibration. The work should be held as close to the jaws as possible. The vice handle should be tightened by hand, never struck with a hammer.

Question 26Hand tools, measuring instruments and workshop equipment

What is the purpose of a centre punch when preparing a hole for drilling?

  1. To remove burrs from the drilled hole
  2. To enlarge the hole after drilling
  3. To make an indentation that stops the drill wandering Correct answer.
  4. To cut the thread before tapping

Explanation

A centre punch is struck to form a small conical indentation at the marked centre, which guides the drill point and prevents it from wandering on the surface. A centre drill may then be used for accuracy. Reamers enlarge holes, taps cut threads, and deburring tools remove burrs.

Question 27Hand tools, measuring instruments and workshop equipment

A dial test indicator is mounted on a magnetic base to check the runout of a flywheel. What does a total indicated reading of 0.08 mm mean?

  1. The indicator has a zero error of 0.08 mm
  2. The maximum and minimum readings differ by 0.08 mm Correct answer.
  3. The flywheel is 0.08 mm out of parallel
  4. The flywheel diameter is 0.08 mm oversize

Explanation

A dial test indicator shows variation from a datum as the workpiece rotates. The total indicator reading is the difference between the highest and lowest needle positions, so 0.08 mm is the total runout. It does not directly measure diameter, parallelism, or instrument zero error.

Question 28Hand tools, measuring instruments and workshop equipment

Which tool is used with a torque wrench to tighten a cylinder head bolt to a specified angle after the initial torque stage?

  1. A protractor or angle gauge Correct answer.
  2. A depth micrometer
  3. A spring balance
  4. A feeler gauge

Explanation

Torque-to-yield head bolts are tightened to a set torque and then rotated through a specified angle, which is measured with a protractor or angle gauge fitted to the wrench. Depth micrometers, feeler gauges and spring balances measure other quantities and cannot measure rotation angle.

Question 29Hand tools, measuring instruments and workshop equipment

A feeler gauge is being used to check valve clearance. What does a correct result look like?

  1. A blade two sizes smaller fits loosely
  2. The specified blade slides in with slight drag at the specified clearance Correct answer.
  3. The specified blade cannot enter the gap at all
  4. The next larger blade enters the gap easily

Explanation

The correct clearance is present when the blade of the specified thickness passes between the valve stem and rocker with a slight drag. If the blade will not enter, the gap is too small; if a larger blade enters easily, the gap is too large. Loose fitting of smaller blades indicates excessive clearance.

Question 30Hand tools, measuring instruments and workshop equipment

A compression tester gives a low reading on one cylinder of a four-cylinder engine. Which follow-up test best distinguishes worn piston rings from leaking valves?

  1. Repeat the test with the throttle closed
  2. Add a small quantity of oil to the cylinder and repeat the test Correct answer.
  3. Test with all spark plugs removed
  4. Run the engine to operating temperature and retest

Explanation

Adding a few millilitres of engine oil temporarily seals worn rings, so if the reading rises noticeably the rings are worn. If the reading stays low, the leakage is past the valves or head gasket. The other options change test conditions but do not separate the two fault sources.

Question 31Hand tools, measuring instruments and workshop equipment

A bench grinder is fitted with a new grinding wheel. Which check must be made before running it up to speed?

  1. Remove the wheel guard to inspect for cracks
  2. Ring test the wheel and check the rated speed exceeds the grinder speed Correct answer.
  3. Apply light pressure to the wheel face by hand
  4. Adjust the tool rest to 5 mm from the wheel

Explanation

A bonded abrasive wheel can burst if it is cracked or run above its rated speed, so it must be ring tested and its maximum speed checked against the machine. The tool rest should be set close to the wheel (about 2 mm) and the guard must stay in place. Spinning a damaged wheel by hand is dangerous.

Question 32Hand tools, measuring instruments and workshop equipment

Which equipment is used to press a bearing onto a shaft without damaging the bearing race?

  1. A copper mallet and a drift
  2. A vice closed on the outer race
  3. A hydraulic press with a suitable sleeve on the inner race Correct answer.
  4. A slide hammer

Explanation

Force must be applied to the race that is being fitted, so pressing on the inner race through a sleeve transmits load through the correct path. Striking with a mallet or pressing on the outer race when fitting to a shaft brinells the balls and races. A slide hammer is a removal tool.

Question 33Hand tools, measuring instruments and workshop equipment

A hydraulic floor jack is used to raise a vehicle for wheel removal. What is the correct sequence of support?

  1. Raise the vehicle, place axle stands, then lower it onto the stands Correct answer.
  2. Place axle stands first, then raise the vehicle on the jack
  3. Raise the vehicle and work under it while the jack holds it
  4. Use two jacks and leave both in place while working

Explanation

A hydraulic jack is a lifting device, not a support, and can creep down under load. The vehicle must be raised at the correct jacking point, axle stands placed under a strong structural point, and the load lowered onto them before anyone works beneath it. Leaving a jack as the only support is a fatal-risk practice.

Question 34Hand tools, measuring instruments and workshop equipment

A bench drill is to be used to drill a 10 mm hole in a steel plate. Which practice gives an accurate hole and protects the operator?

  1. Clamp the work to the table and use the correct speed with coolant Correct answer.
  2. Wear gloves and feed slowly without clamping
  3. Hold the plate by hand and feed quickly
  4. Remove the chuck guard to see the drill clearly

Explanation

Clamping the workpiece prevents it spinning or being snatched by the drill, and correct spindle speed with coolant controls heat and prolongs drill life. Gloves must never be worn near rotating machinery, and the chuck guard must stay in place. Hand holding a plate under a bench drill is a serious injury risk.

Question 35Hand tools, measuring instruments and workshop equipment

A vernier caliper reads 12.00 mm when its jaws are closed. How should this be handled when measuring a component?

  1. Subtract 12.00 mm from every reading taken Correct answer.
  2. Divide every reading by 12.00
  3. Add 12.00 mm to every reading taken
  4. Replace the caliper because it cannot be used

Explanation

A caliper that reads 12.00 mm with the jaws closed has a positive zero error of 12.00 mm. Every measurement therefore includes this extra amount, so the error must be subtracted to obtain the true size. The instrument can still be used if the error is recorded and applied consistently.

Question 36Hand tools, measuring instruments and workshop equipment

Which measuring instrument is appropriate for checking the diameter of a cylinder bore to 0.01 mm?

  1. Steel rule
  2. Vernier height gauge
  3. Inside micrometer or bore gauge Correct answer.
  4. Outside micrometer

Explanation

An inside micrometer or a bore gauge set from an outside micrometer measures internal diameter directly and reads to 0.01 mm. An outside micrometer measures external sizes only, a steel rule is far too coarse, and a height gauge measures heights and marking-out positions.

Question 37Engine principles and four-stroke / two-stroke operation

In a four-stroke petrol engine, during which stroke does the piston move downward with the inlet valve open?

  1. The power stroke
  2. The exhaust stroke
  3. The induction stroke Correct answer.
  4. The compression stroke

Explanation

The induction (intake) stroke draws the air-fuel mixture into the cylinder, so the inlet valve is open and the piston travels down the bore. The other strokes involve closed valves (compression, power) or the exhaust valve open with the piston rising.

Question 38Engine principles and four-stroke / two-stroke operation

Which component opens and closes the valves of a four-stroke engine at the correct time relative to crankshaft rotation?

  1. The oil pump
  2. The flywheel
  3. The starter motor
  4. The camshaft Correct answer.

Explanation

The camshaft carries lobes that actuate the valves, and it is driven at half crankshaft speed so each valve operates once per two crank revolutions. The flywheel smooths rotation, the oil pump circulates lubricant, and the starter motor cranks the engine.

Question 39Engine principles and four-stroke / two-stroke operation

A two-stroke engine completes one full cycle of operations in how many crankshaft revolutions?

  1. One Correct answer.
  2. Half
  3. Two
  4. Four

Explanation

In a two-stroke engine every revolution of the crankshaft produces one power stroke because the ports are uncovered by the piston as it moves. A four-stroke engine needs two revolutions, giving one power stroke per two turns.

Question 40Engine principles and four-stroke / two-stroke operation

Which statement correctly describes the purpose of the compression stroke in a four-stroke engine?

  1. It expels burnt gases from the cylinder
  2. It draws fresh charge into the cylinder
  3. It raises the pressure and temperature of the charge to aid combustion Correct answer.
  4. It transfers power from the piston to the crankshaft

Explanation

Compressing the charge raises both its pressure and its temperature, which improves combustion and increases the energy released. Expelling gases is the exhaust stroke, drawing charge is induction, and power transfer happens on the power stroke.

Question 41Engine principles and four-stroke / two-stroke operation

In a simple two-stroke engine, what is the function of the transfer port?

  1. To admit air directly from the atmosphere into the cylinder
  2. To supply lubricating oil to the big-end bearing
  3. To allow the fresh mixture to flow from the crankcase into the cylinder Correct answer.
  4. To release exhaust gases from the cylinder

Explanation

The transfer port connects the crankcase to the cylinder and lets the slightly pressurised fresh mixture enter when the piston uncovers it. The inlet port admits mixture to the crankcase, the exhaust port releases burnt gases, and lubrication comes from oil mixed with the fuel.

Question 42Engine principles and four-stroke / two-stroke operation

Which term describes the volume swept by the piston as it moves from top dead centre to bottom dead centre?

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

Explanation

Swept volume is the volume displaced by the piston during one full stroke, calculated from the bore and stroke. Clearance volume is the space above the piston at top dead centre, compression ratio compares total volume to clearance volume, and bore is the cylinder diameter.

Question 43Engine principles and four-stroke / two-stroke operation

Why does a two-stroke engine generally produce a power stroke on every crankshaft revolution?

  1. Because the piston uncovers the ports during each revolution, so the cycle repeats every turn Correct answer.
  2. Because it has twice as many cylinders as a four-stroke
  3. Because it uses a higher compression ratio than a four-stroke
  4. Because its camshaft rotates at crankshaft speed

Explanation

In a two-stroke engine the piston itself acts as the valve, uncovering inlet, transfer and exhaust ports as it moves, so all phases occur within one up-and-down movement. This gives a power stroke each revolution rather than every second revolution.

Question 44Engine principles and four-stroke / two-stroke operation

A four-stroke engine has a bore of 80 mm and a stroke of 90 mm. Using the swept volume formula, which value is closest to the swept volume of one cylinder?

  1. 905 cm³
  2. 452 cm³ Correct answer.
  3. 181 cm³
  4. 226 cm³

Explanation

Swept volume equals π/4 × bore² × stroke. With bore 8 cm and stroke 9 cm: 0.7854 × 64 × 9 ≈ 452 cm³. Care must be taken to convert millimetres to centimetres before calculating.

Question 45Engine principles and four-stroke / two-stroke operation

During the power stroke of a four-stroke engine, which valves are open?

  1. Both the inlet and exhaust valves
  2. The inlet valve only
  3. Neither the inlet nor the exhaust valve Correct answer.
  4. The exhaust valve only

Explanation

On the power stroke the burning gases expand and push the piston down, so both valves remain closed to contain the pressure. The inlet opens on induction and the exhaust opens on the exhaust stroke.

Question 46Engine principles and four-stroke / two-stroke operation

What is valve overlap in a four-stroke engine?

  1. The gap between the camshaft lobe and the tappet
  2. The distance the valve lifts off its seat
  3. The clearance between the valve stem and the rocker arm
  4. A period near top dead centre when both inlet and exhaust valves are slightly open Correct answer.

Explanation

Valve overlap is the short interval around top dead centre at the end of exhaust and start of induction when both valves are open. It helps scavenge burnt gases and improve cylinder filling. The other options describe clearances or lift, not timing overlap.

Question 47Engine principles and four-stroke / two-stroke operation

Why is the crankcase of a simple two-stroke engine sealed and used to store the incoming mixture?

  1. So that the descending piston can pressurise the mixture for transfer into the cylinder Correct answer.
  2. So that lubricating oil can be stored separately from the fuel
  3. So that the crankshaft can run without any bearings
  4. So that exhaust gases can be recirculated

Explanation

In a crankcase-scavenged two-stroke the descending piston compresses the mixture in the sealed crankcase, and this pressure drives it through the transfer port when the piston uncovers it. The crankcase therefore acts as a pump.

Question 48Engine principles and four-stroke / two-stroke operation

A technician finds that a four-stroke engine has very low compression and the pressure escapes into the inlet manifold. Which fault best explains this?

  1. A slipping alternator belt
  2. A worn crankshaft main bearing
  3. A blocked fuel filter
  4. A burnt or badly seating inlet valve Correct answer.

Explanation

Compression leaking into the inlet tract points to the inlet valve not sealing, so pressure escapes past it during compression. A burnt valve face or incorrect tappet clearance would produce this symptom. The other faults do not affect cylinder sealing.

Question 49Engine principles and four-stroke / two-stroke operation

In a two-stroke engine, what is scavenging?

  1. The process of igniting the mixture with the spark plug
  2. The process of driving out burnt gases and filling the cylinder with fresh mixture Correct answer.
  3. The process of lubricating the cylinder bore
  4. The process of cooling the engine with air

Explanation

Scavenging is the flow of fresh mixture through the cylinder that pushes the remaining burnt gases out of the exhaust port. Good scavenging improves power and reduces emissions, while poor scavenging leaves burnt gas behind.

Question 50Engine principles and four-stroke / two-stroke operation

How many degrees of crankshaft rotation does one complete four-stroke cycle occupy?

  1. 540 degrees
  2. 360 degrees
  3. 180 degrees
  4. 720 degrees Correct answer.

Explanation

A four-stroke cycle takes two complete crankshaft revolutions, and each revolution is 360 degrees, giving 720 degrees in total. Each individual stroke therefore occupies 180 degrees of crank rotation.