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Pancyprian TS Marine Mech. II study materials
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Pancyprian TS Marine Mech. II practice questions

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Question 1Marine propulsion engines: diesel engine principles, construction, operation and performance

In a four-stroke marine diesel engine, during which stroke does the intake air enter the cylinder of a naturally aspirated unit?

  1. The stroke in which the piston moves downward with the inlet valve open Correct answer.
  2. The stroke in which fuel is injected near top dead centre
  3. The stroke in which the piston moves upward with both valves closed
  4. The stroke in which the piston moves downward with the exhaust valve open

Explanation

In the four-stroke cycle the piston descending with the inlet valve open creates the depression that draws fresh air into the cylinder; this is the induction stroke. Fuel injection occurs near the end of compression, and the power and exhaust strokes follow.

Question 2Marine propulsion engines: diesel engine principles, construction, operation and performance

Which component of a marine diesel engine directly converts the reciprocating motion of the piston into rotary motion of the crankshaft?

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

Explanation

The connecting rod links the gudgeon pin to the crankpin, so its angular swing converts piston reciprocation into crankshaft rotation. The camshaft and pushrods operate valves, and the liner is the cylinder bore surface.

Question 3Marine propulsion engines: diesel engine principles, construction, operation and performance

The compression ratio of a diesel engine is defined as the ratio of:

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

Explanation

Compression ratio compares the cylinder volume with the piston at bottom dead centre to the volume remaining at top dead centre, that is total volume divided by clearance volume. Diesel engines use high ratios so compression alone raises air temperature above fuel ignition point.

Question 4Marine propulsion engines: diesel engine principles, construction, operation and performance

In a two-stroke crosshead marine diesel engine, scavenging air is normally supplied by:

  1. A mechanically driven Roots blower or turbocharger providing air above atmospheric pressure Correct answer.
  2. Natural aspiration through the piston underside alone
  3. A hand-operated priming pump
  4. The exhaust gas receiver acting as a reservoir

Explanation

Large two-stroke engines need a positive pressure difference to sweep burnt gas out through the exhaust ports, so scavenge air is delivered by Roots blowers or by turbochargers. Without this pressure the cylinder cannot be cleared effectively during the short scavenge period.

Question 5Marine propulsion engines: diesel engine principles, construction, operation and performance

Which fuel property primarily determines the ignition delay of diesel fuel in a marine engine cylinder?

  1. Colour of the fuel
  2. Sulphur content
  3. Viscosity at 50 degrees Celsius
  4. Cetane number Correct answer.

Explanation

Cetane number rates how readily a fuel self-ignites under compression, so a higher cetane number shortens ignition delay. Viscosity, sulphur and colour affect handling, emissions or appearance but do not directly set the delay period.

Question 6Marine propulsion engines: diesel engine principles, construction, operation and performance

A marine diesel engine indicator diagram shows the pressure inside the cylinder against crank angle. A rise in peak firing pressure with unchanged compression pressure most likely indicates:

  1. A worn fuel injection nozzle with reduced delivery
  2. Increased ignition delay with more fuel burnt after top dead centre Correct answer.
  3. A blocked scavenge air port
  4. A leaking exhaust valve

Explanation

If compression pressure is normal but firing pressure rises, more fuel is burning after top dead centre, which is characteristic of longer ignition delay or over-advanced timing with excess fuel. A leaking exhaust valve or blocked port would lower compression pressure, and reduced delivery would reduce firing pressure.

Question 7Marine propulsion engines: diesel engine principles, construction, operation and performance

Why are exhaust valves in large slow-speed diesel engines often fitted with a valve rotator?

  1. To allow the valve to open twice per cycle
  2. To increase valve lift at high load
  3. To distribute wear and thermal load evenly around the valve seat and face Correct answer.
  4. To reduce the spring force needed to close the valve

Explanation

Rotating the valve on each operation spreads contact and temperature around the seat and face, reducing local burning, fouling and uneven wear. It does not change lift, spring requirement or the timing sequence.

Question 8Marine propulsion engines: diesel engine principles, construction, operation and performance

A main engine turbocharger is fitted with an air cooler after the compressor. The main purpose of this cooler is to:

  1. Reduce the pressure of the scavenge air
  2. Cool the exhaust gas before the turbine
  3. Remove water from the fuel oil
  4. Increase the mass of air trapped in the cylinder by raising its density Correct answer.

Explanation

Cooling the compressed charge increases its density, so a greater mass of air is trapped for the same swept volume and more fuel can be burnt. The cooler works on the air side after the compressor, not on fuel or turbine exhaust.

Question 9Marine propulsion engines: diesel engine principles, construction, operation and performance

During a load increase on a running marine diesel engine, the governor responds by:

  1. Opening the starting air valve
  2. Increasing fuel delivery to maintain the set engine speed Correct answer.
  3. Advancing the exhaust valve timing
  4. Reducing scavenge air pressure to match the load

Explanation

When load rises the engine tends to slow, and the governor senses the speed drop and increases fuel rack position to restore the set speed. Valve timing and starting air are not modulated for load control.

Question 10Marine propulsion engines: diesel engine principles, construction, operation and performance

Which statement correctly describes the function of the crosshead in a large two-stroke marine diesel engine?

  1. It supports the main bearings of the crankshaft
  2. It seals combustion gas from the crankcase
  3. It guides the piston rod and absorbs the side thrust so the piston does not bear against the liner Correct answer.
  4. It drives the fuel injection pump

Explanation

The crosshead runs in guides and takes the transverse reaction of the connecting rod, keeping the piston rod in line and the piston free of side thrust. Piston rings and stuffing boxes handle gas sealing, and the injection pump and main bearings are separate items.

Question 11Marine propulsion engines: diesel engine principles, construction, operation and performance

A marine diesel engine shows a gradual fall in compression pressure across all cylinders together with increased lube oil consumption. The most probable cause is:

  1. Air in the fuel system
  2. Incorrect valve clearance on one unit
  3. Wear of piston rings and cylinder liners Correct answer.
  4. A single faulty fuel injector

Explanation

Uniform loss of compression with rising oil consumption points to general wear of rings and liners, allowing gas to blow by and oil to enter the combustion space. Single-cylinder faults or fuel-side problems would not affect all units uniformly.

Question 12Marine propulsion engines: diesel engine principles, construction, operation and performance

The firing order of a six-cylinder four-stroke engine is arranged so that power strokes are evenly spaced. For a typical order such as 1-5-3-6-2-4, the interval between successive power strokes is:

  1. 180 degrees of crankshaft rotation
  2. 360 degrees of crankshaft rotation
  3. 120 degrees of crankshaft rotation Correct answer.
  4. 720 degrees of crankshaft rotation

Explanation

In a four-stroke cycle 720 degrees is shared between six cylinders, giving 120 degrees between successive firing events. Even spacing smooths torque and torsional loading on the shafting.

Question 13Marine propulsion engines: diesel engine principles, construction, operation and performance

In a diesel engine, the function of the fuel injection pump plunger and barrel assembly is to:

  1. Filter solid particles from the fuel
  2. Cool the injector nozzle
  3. Regulate the lube oil pressure
  4. Meter the fuel quantity and generate the high pressure needed for atomisation Correct answer.

Explanation

The plunger traps fuel in the barrel and pressurises it to injection pressure while the effective stroke, set by the helix or control sleeve, meters the quantity delivered. Filtration, nozzle cooling and lube oil regulation are handled by other components.

Question 14Marine propulsion engines: diesel engine principles, construction, operation and performance

A marine diesel engine is rated at a brake mean effective pressure of 18 bar. If the indicated mean effective pressure is 20 bar, the mechanical efficiency is approximately:

  1. 80 percent
  2. 10 percent
  3. 90 percent Correct answer.
  4. 111 percent

Explanation

Mechanical efficiency is the ratio of brake to indicated mean effective pressure, so 18 divided by 20 gives 0.90, or 90 percent. The difference represents friction and auxiliary losses within the engine.

Question 15Marine propulsion engines: diesel engine principles, construction, operation and performance

Which of the following best explains why diesel engines are more thermally efficient than spark-ignition petrol engines of similar output?

  1. They operate at higher compression ratios and are unthrottled, reducing pumping losses Correct answer.
  2. They burn fuel more slowly at constant pressure throughout the cycle
  3. They require no cooling system
  4. They use a spark to control combustion timing precisely

Explanation

High compression ratio raises the ideal cycle efficiency, and the absence of a throttle avoids intake pumping losses at part load. Diesel combustion is not a slow constant-pressure process throughout, and diesel engines still need cooling.

Question 16Marine propulsion engines: diesel engine principles, construction, operation and performance

A ship's main engine suddenly loses power and black smoke appears from the funnel after a fuel changeover. The most likely explanation is:

  1. Fuel viscosity is too high because the changeover temperature is too low Correct answer.
  2. The cooling water temperature is too high
  3. The starting air pressure is too low
  4. The governor is set to a lower speed

Explanation

Heavy fuel needs heating to reach the correct viscosity at the injectors; if it is too cold and viscous, atomisation is poor, combustion is incomplete and black smoke with power loss results. Cooling water, starting air and governor settings produce different symptoms.

Question 17Marine propulsion engines: diesel engine principles, construction, operation and performance

In a four-stroke engine, valve overlap is the period during which:

  1. The piston dwells at bottom dead centre
  2. Both inlet and exhaust valves are open around top dead centre Correct answer.
  3. Both valves are closed at the end of compression
  4. The injector is open before the inlet valve closes

Explanation

Overlap occurs near the end of the exhaust stroke and start of induction, when the exhaust valve has not yet closed and the inlet valve has begun to open. It uses scavenge air flow to clear residual gas from the clearance volume.

Question 18Marine propulsion engines: diesel engine principles, construction, operation and performance

A two-stroke uniflow-scavenged engine uses exhaust valves in the cylinder head and scavenge ports at the lower end of the liner. Scavenge air enters when the piston:

  1. Is at mid-stroke on the power stroke
  2. Passes the exhaust valve opening point on the upstroke
  3. Reaches top dead centre on compression
  4. Uncovers the inlet ports near bottom dead centre Correct answer.

Explanation

In uniflow scavenging the piston acts as the inlet port valve, uncovering the liner ports as it approaches bottom dead centre while the exhaust valve is open. Air then flows upward and out through the head valve.

Question 19Marine propulsion engines: diesel engine principles, construction, operation and performance

A marine diesel engine cylinder unit shows a high exhaust temperature and a low firing pressure compared with its neighbours. Which combination best fits these readings?

  1. Correctly set injection timing with a clean nozzle
  2. Over-advanced injection with normal nozzle condition
  3. Late injection with poor atomisation causing afterburning and incomplete combustion Correct answer.
  4. A slight increase in scavenge air temperature

Explanation

Late injection and poor atomisation delay and prolong combustion, so much of the fuel burns late, giving high exhaust temperature while peak pressure stays low. Over-advance would instead raise peak pressure, and correct settings would match the other units.

Question 20Marine propulsion engines: diesel engine principles, construction, operation and performance

In a turbocharged diesel engine, the turbine and compressor are matched so that at steady load the compressor absorbs the power produced by the turbine. If the air filter becomes heavily fouled, the immediate effect on the engine is:

  1. Increased scavenge pressure and lower exhaust temperature
  2. Reduced compressor mass flow, lower scavenge pressure and higher exhaust temperature Correct answer.
  3. Higher compression ratio with unchanged air mass
  4. No change because the turbocharger is self-regulating

Explanation

A restricted inlet lowers compressor mass flow and pressure ratio, so less air is trapped and combustion becomes fuel-rich, raising exhaust temperature. The turbocharger cannot compensate because the mass flow limitation is upstream.

Question 21Marine propulsion engines: diesel engine principles, construction, operation and performance

A marine diesel engine is developing its rated power at a brake specific fuel consumption of 180 g/kWh. If the fuel has a lower calorific value of 42 700 kJ/kg, the brake thermal efficiency is closest to:

  1. 47 percent Correct answer.
  2. 30 percent
  3. 55 percent
  4. 38 percent

Explanation

Brake thermal efficiency equals 3600 divided by the product of specific fuel consumption in kg/kWh and calorific value in kJ/kg, so 3600 divided by (0.180 times 42 700) is about 0.468, or 47 percent. This reflects how much fuel energy appears as shaft work.

Question 22Marine propulsion engines: diesel engine principles, construction, operation and performance

The purpose of a piston rod stuffing box in a two-stroke crosshead engine is to:

  1. Prevent crankcase oil from entering the scavenge space and combustion gas from reaching the crankcase Correct answer.
  2. Guide the crosshead in its slides
  3. Seal the exhaust valve spindle
  4. Support the weight of the piston assembly

Explanation

The stuffing box around the piston rod separates the crankcase from the scavenge and combustion spaces, keeping lube oil out of the cylinder and gas out of the crankcase. Support and guidance are duties of the crosshead and guides.

Question 23Marine propulsion engines: diesel engine principles, construction, operation and performance

Which operating parameter is most directly used to monitor the mechanical loading of a marine diesel engine cylinder for comparison with the shop trial value?

  1. Scavenge air box temperature
  2. Lube oil outlet temperature
  3. Fuel oil supply pressure
  4. Peak firing pressure from the indicator diagram Correct answer.

Explanation

Peak firing pressure read from the indicator card reflects the maximum mechanical load on the piston, rod and bearings and is compared with trial values. Temperatures and supply pressures give useful condition information but do not directly indicate cylinder mechanical loading.

Question 24Marine propulsion engines: diesel engine principles, construction, operation and performance

The main reason for preheating heavy fuel oil before it reaches the injection system is to:

  1. Lower its viscosity so it can be pumped, filtered and atomised correctly Correct answer.
  2. Reduce the compression ratio needed
  3. Increase its sulphur content
  4. Raise its cetane number

Explanation

Heating reduces viscosity to the range the fuel system is designed for, allowing proper pumping, filtration and atomisation at the nozzle. It does not change cetane number, sulphur level or the engine's compression ratio.

Question 25Marine propulsion engines: diesel engine principles, construction, operation and performance

A marine diesel engine's governor is described as isochronous. This means that, within its capacity, the governor:

  1. Only limits maximum speed by fuel rack stop
  2. Allows speed to fall proportionally as load increases
  3. Permits speed to rise with load
  4. Maintains a constant engine speed regardless of load Correct answer.

Explanation

An isochronous governor adjusts fuel to hold the set speed constant as load varies, which is required for alternator drives. Droop governors, by contrast, allow a speed decrease with increasing load.

Question 26Marine propulsion engines: diesel engine principles, construction, operation and performance

In a large two-stroke marine diesel engine, which component transmits the combustion load from the piston to the connecting rod while keeping the piston aligned in the liner?

  1. The piston rod and crosshead assembly Correct answer.
  2. The gudgeon pin and piston skirt
  3. The camshaft and roller followers
  4. The thrust block and line shafting

Explanation

In a crosshead engine the piston rod is bolted to the piston and passes down to the crosshead, which carries the piston rod and is joined to the connecting rod by the crosshead pin. The crosshead slippers run against guide rails, so the crosshead takes the side thrust and keeps the piston centred in the liner. This separates the piston's guiding function from the connecting rod, which is why the piston skirt can be short.

Question 27Marine propulsion engines: diesel engine principles, construction, operation and performance

A four-stroke auxiliary diesel engine has its inlet valve opening 20 degrees before top dead centre and closing 40 degrees after bottom dead centre. The inlet valve remains open for approximately how many degrees of crankshaft rotation?

  1. 240 degrees Correct answer.
  2. 200 degrees
  3. 180 degrees
  4. 220 degrees

Explanation

The inlet valve opens 20 degrees before top dead centre and stays open through the whole intake stroke of 180 degrees, then closes 40 degrees after bottom dead centre. Adding these gives 20 + 180 + 40 = 240 degrees of crankshaft rotation. Late closing uses intake inertia to improve cylinder filling at higher speeds.

Question 28Marine propulsion engines: diesel engine principles, construction, operation and performance

Which quantity is obtained by subtracting friction and pumping losses from the indicated power of a marine diesel engine?

  1. The indicated mean effective pressure
  2. The fuel rack position
  3. The scavenge air pressure
  4. The brake power at the output coupling Correct answer.

Explanation

Indicated power is the power developed inside the cylinders, while friction in bearings, piston rings and auxiliaries plus pumping losses consume part of it. The remainder delivered at the flywheel or output coupling is the brake power, which is what a dynamometer or torsionmeter measures. The difference between indicated and brake power is the mechanical loss.

Question 29Marine propulsion engines: diesel engine principles, construction, operation and performance

On a marine diesel engine, what is the primary function of the fuel injector nozzle spring and thrust piece arrangement?

  1. To hold the needle valve closed until the set opening pressure is reached Correct answer.
  2. To atomise the fuel by spinning the needle during injection
  3. To cool the nozzle tip during combustion
  4. To meter the total quantity of fuel delivered per stroke

Explanation

The nozzle spring loads the needle onto its seat and keeps it closed until fuel pressure from the injection pump exceeds the set opening pressure. Once that pressure is reached the needle lifts and fuel is sprayed through the holes; when pressure falls the spring reseats the needle sharply to cut off injection. This gives a defined opening pressure and clean end of injection.

Question 30Marine propulsion engines: diesel engine principles, construction, operation and performance

A main engine is operating at constant load when the fuel injection timing is accidentally advanced by several degrees. Which change in the indicator diagram would you expect?

  1. Peak cylinder pressure is unchanged but exhaust temperature falls sharply
  2. Peak cylinder pressure rises because more fuel is injected per stroke
  3. Peak cylinder pressure rises and the expansion line starts earlier Correct answer.
  4. Peak cylinder pressure falls and combustion continues into the expansion stroke

Explanation

Advancing injection starts combustion earlier, closer to top dead centre, so more heat is released near constant volume and the peak pressure rises. The whole pressure trace shifts earlier, so the expansion line begins at a higher pressure and earlier crank angle. This raises mechanical loading on the bearings and can cause knocking if taken too far.

Question 31Marine propulsion engines: diesel engine principles, construction, operation and performance

A turbocharged two-stroke engine is fitted with a scavenge air receiver. Which statement best describes the receiver's role during scavenging?

  1. It stores air at near-constant pressure so each cylinder receives an even scavenge supply despite pulsations Correct answer.
  2. It increases the pressure of the air above the compressor delivery pressure
  3. It removes water and oil mist from the charge before the cylinders
  4. It cools the scavenge air before it enters the cylinder liner ports

Explanation

The scavenge receiver is a large volume between the turbocharger air cooler and the cylinder ports. Its volume damps the pressure pulses caused by successive cylinders opening their scavenge ports, so each cylinder sees a steady supply pressure. This helps give consistent scavenging and trapping of fresh air across all units.

Question 32Marine propulsion engines: diesel engine principles, construction, operation and performance

During a routine check, a cylinder unit shows a normal compression pressure but a firing pressure noticeably lower than the other units, with a higher exhaust temperature. Which cause is most consistent with these readings?

  1. Late or faulty fuel injection at that unit Correct answer.
  2. Excessive piston ring blow-by at that unit
  3. A leaking exhaust valve at that unit
  4. A restricted scavenge air supply to that unit

Explanation

Normal compression pressure shows the piston, rings and valves seal adequately, so the fault lies in combustion. Late or defective injection delays heat release, lowering peak firing pressure while the fuel burns later and raises exhaust temperature. The compression reading effectively rules out sealing faults as the primary cause.

Question 33Marine propulsion engines: diesel engine principles, construction, operation and performance

Which statement correctly describes the purpose of the engine-driven lubricating oil pump's relief valve in a marine diesel engine?

  1. It maintains a minimum oil level in the sump
  2. It regulates oil temperature by diverting oil through the cooler
  3. It limits oil system pressure by bypassing excess oil back to suction or sump Correct answer.
  4. It opens automatically to admit oil to the bearings only when pressure is low

Explanation

The relief valve is a safety device set to open at a defined pressure and bypass excess oil, protecting the system from overpressure if the pump output exceeds demand or a filter blocks. It does not control level or temperature. Correct setting ensures bearings receive adequate pressure without risking pipe or cooler damage.

Question 34Marine propulsion engines: diesel engine principles, construction, operation and performance

A ship's main engine is running steadily when the exhaust gas boiler is suddenly bypassed, raising turbocharger turbine outlet back-pressure. What is the most likely immediate effect on the engine?

  1. Engine speed increases because less heat is lost to the boiler
  2. Scavenge air pressure falls and exhaust temperatures rise as the turbocharger slows Correct answer.
  3. Scavenge air pressure rises because the turbine extracts more energy
  4. Fuel consumption falls because the turbocharger does less work

Explanation

Higher back-pressure after the turbine reduces the pressure ratio across the turbine, so the turbocharger delivers less air for the same exhaust energy. With less scavenge air the trapped air quantity falls, combustion becomes less complete and exhaust temperatures rise. The engine may need to be de-rated until the restriction is removed.

Question 35Marine propulsion engines: diesel engine principles, construction, operation and performance

Which of the following best describes the function of the air starting valve on a large marine diesel engine cylinder?

  1. It admits compressed air into the cylinder at the correct crank angle to turn the engine during starting Correct answer.
  2. It releases excess cylinder pressure during the compression stroke
  3. It provides combustion air when the turbocharger is not running
  4. It supplies compressed air to the fuel injector for atomisation

Explanation

Air starting valves are mounted on each cylinder head and are opened by the starting air distributor at the appropriate point in the expansion stroke. Compressed air admitted then pushes the piston down and rotates the crankshaft until the engine fires on fuel. Once the engine runs, the starting air is isolated and the valves remain closed.

Question 36Marine propulsion engines: diesel engine principles, construction, operation and performance

A four-stroke engine is running at 720 rpm. What is the approximate firing frequency of a single cylinder?

  1. 6 firings per second Correct answer.
  2. 3 firings per second
  3. 24 firings per second
  4. 12 firings per second

Explanation

In a four-stroke engine each cylinder fires once every two crankshaft revolutions. At 720 rpm there are 720/60 = 12 revolutions per second, so one cylinder fires 12/2 = 6 times per second. This firing frequency is relevant when matching turbochargers and diagnosing vibration.

Question 37Marine propulsion engines: diesel engine principles, construction, operation and performance

During manoeuvring, a main engine is repeatedly started, stopped and reversed. Which practice best protects the engine from thermal and mechanical distress?

  1. Keeping the fuel rack at maximum during astern running to maintain speed
  2. Running at full load immediately after each reversal to clear deposits
  3. Isolating cylinder lubrication during short manoeuvres to save oil
  4. Following the manufacturer's load programme, allowing adequate warming and cooling periods between manoeuvres Correct answer.

Explanation

Rapid load changes cause uneven thermal expansion and high stresses in pistons, liners and valves. The load programme limits rate of change and gives time for temperatures to equalise, particularly after reversal when the liner and piston temperatures must settle. Following it reduces the risk of cracking, scuffing and seizure.

Question 38Marine propulsion engines: diesel engine principles, construction, operation and performance

A two-stroke engine is fitted with a variable injection timing (VIT) system. What is the principal benefit of advancing injection at part load?

  1. It improves fuel economy and reduces peak pressure at high load by matching timing to load Correct answer.
  2. It eliminates the need for a governor at part load
  3. It increases the compression ratio at low load
  4. It allows the engine to run without scavenge air at low load

Explanation

VIT adjusts the injection point according to load, advancing it at part load to keep combustion near top dead centre and improve fuel consumption. At high load the timing is retarded to limit peak cylinder pressure and protect the bearings. The system therefore optimises the compromise between economy and mechanical loading across the load range.

Question 39Marine propulsion engines: diesel engine principles, construction, operation and performance

Which measurement is most directly used to assess the mechanical loading on a cylinder head stud and bearing shell during combustion?

  1. Fuel oil viscosity at the injection pump inlet
  2. Scavenge air receiver pressure
  3. Exhaust gas temperature after the cylinder
  4. Peak cylinder firing pressure from the indicator diagram Correct answer.

Explanation

Peak firing pressure reflects the maximum gas force acting on the piston, cylinder head and bearings, so it is the direct indicator of mechanical loading. Exhaust temperature indicates thermal load and combustion quality, not peak force. Comparing firing pressures across units reveals overloaded cylinders.

Question 40Marine propulsion engines: diesel engine principles, construction, operation and performance

A marine diesel engine's crankshaft is fitted with a torsional vibration damper. What is the damper designed to control?

  1. Longitudinal vibration transmitted from the propeller
  2. Angular oscillations of the shaft about its axis caused by periodic cylinder firing Correct answer.
  3. Radial deflection of the shaft due to bearing wear
  4. Axial movement of the crankshaft caused by propeller thrust

Explanation

Each firing impulse twists the crankshaft slightly, and if an excitation frequency coincides with a natural frequency of the shafting the torsional oscillation can become damaging. The damper absorbs energy at these frequencies and limits the amplitude of angular twist. This protects the shaft, coupling and gearing from fatigue.

Question 41Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

On a typical medium-speed marine diesel engine, what is the primary function of the fuel injection pump's delivery valve (snubber valve)?

  1. To filter abrasive particles from the fuel before it reaches the injector
  2. To increase the maximum injection pressure to the nozzle opening pressure
  3. To sharpen the end of injection and prevent secondary injection by rapidly dropping nozzle pressure Correct answer.
  4. To circulate excess fuel back to the service tank during the injection stroke

Explanation

The delivery valve closes sharply at the end of pumping and, through its unloading (retraction) action, drops the pressure in the high-pressure line quickly. This gives a clean end of injection and prevents dribble or secondary injection at the nozzle. It does not filter fuel, raise injection pressure, or return fuel to the tank.

Question 42Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

A jerk-type fuel injection pump is described as having 'constant stroke'. What does this mean in practice?

  1. The plunger stroke is fixed by the cam, so delivery quantity is varied by spill timing rather than stroke length Correct answer.
  2. The delivery valve stroke is adjusted to control the quantity injected
  3. The camshaft speed varies so that injection duration stays constant at all loads
  4. The plunger stroke changes with engine load to match the fuel demand

Explanation

In a constant-stroke (jerk) pump the plunger travel is set by the cam profile and does not change with load. Output is regulated by a spill port or helix that ends effective delivery earlier or later, so the injected quantity varies while the full stroke stays the same.

Question 43Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

In a common-rail fuel injection system on a marine engine, what component determines the injection pressure?

  1. The high-pressure pump, which maintains pressure in the accumulator rail independent of injection timing Correct answer.
  2. The injector needle spring, which sets pressure at the moment of opening
  3. The camshaft profile acting directly on each injector plunger
  4. The transfer pump, which pressurises fuel in the low-pressure circuit

Explanation

In common-rail systems the high-pressure pump charges an accumulator rail to the required pressure, which is controlled independently of engine timing. Each injector is then opened electronically to meter timing and quantity while rail pressure is held by the pump and pressure control valve.

Question 44Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

A marine engine fuel injector is found with a carbon-clogged nozzle hole while the other holes are clear. Which effect on combustion is most likely?

  1. Uneven spray pattern causing poor atomisation and local overheating of the piston crown Correct answer.
  2. Automatic advance of injection timing at that cylinder
  3. Complete loss of injection pressure at that cylinder
  4. Excessive fuel delivery because the remaining holes enlarge

Explanation

A blocked hole distorts the spray pattern, so fuel is poorly atomised and unevenly distributed in the combustion chamber. This leads to smoky combustion and local hot spots that can damage the piston crown and liner. It does not change injection pressure, increase delivery, or alter timing.

Question 45Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

Why must marine fuel be heated before it reaches the injectors when burning heavy fuel oil?

  1. To remove sulphur compounds before combustion
  2. To increase its density so the injection pump can meter it
  3. To lower its viscosity so it can be atomised and pumped effectively Correct answer.
  4. To raise its cetane number for easier ignition

Explanation

Heavy fuel oil is very viscous at ambient temperature and must be heated to a controlled viscosity (typically around 10-15 cSt at the injector) so it can be pumped and atomised. Heating does not change cetane number, density for metering, or sulphur content.

Question 46Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

On a lubricating oil system for a medium-speed marine diesel, what is the main purpose of the oil cooler bypass (temperature control) valve?

  1. To relieve excessive pressure in the oil system
  2. To supply oil directly to the turbocharger bearings during starting
  3. To regulate oil viscosity by controlling the temperature of oil supplied to the bearings Correct answer.
  4. To bypass the filter when it becomes blocked

Explanation

The temperature control valve diverts more or less oil around the cooler so the oil reaches the bearings at the designed temperature and thus the correct viscosity for a stable oil film. Filter bypass and pressure relief are separate functions.

Question 47Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

A main engine shows slowly falling lube oil pressure with normal temperature and no external leaks. Which cause is most consistent with these symptoms?

  1. A progressively clogged suction strainer or worn main bearings increasing clearance Correct answer.
  2. A stuck-open oil cooler bypass valve
  3. A failed oil temperature sensor
  4. A blocked crankcase breather

Explanation

A clogged suction strainer reduces pump intake and worn bearings increase clearance, both lowering system pressure while temperature stays normal. These fit the gradual trend described. A stuck-open bypass would affect temperature, a failed sensor is an indication fault, and a breather affects crankcase pressure rather than oil pressure.

Question 48Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

What is the function of the crankcase breather on a marine diesel engine?

  1. To allow oil mist to escape so the sump level can be checked
  2. To supply fresh air to the turbocharger intake
  3. To vent fuel vapours from the high-pressure pump
  4. To release blow-by gases and maintain near-atmospheric crankcase pressure Correct answer.

Explanation

Blow-by gases leaking past the piston rings must be vented to prevent a pressure build-up that could force oil past seals. The breather releases these gases while keeping the crankcase close to atmospheric pressure. It is not an air supply, an oil-level device, or a fuel vent.

Question 49Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

In a closed-circuit (freshwater) engine cooling system, what is the role of the expansion tank?

  1. To absorb the volume change of the coolant as it heats and to allow air separation Correct answer.
  2. To act as a heat exchanger between fresh water and sea water
  3. To store reserve coolant for the sea water circuit
  4. To increase the cooling capacity of the freshwater pump

Explanation

The expansion tank gives the coolant a place to expand into as it warms and allows entrained air to separate out, keeping the circuit full. It is not a pump, a sea water store, or a heat exchanger.

Question 50Engine systems: fuel injection, lubrication, cooling, starting, reversing and control systems

A marine engine's fresh water cooling system is overheating while the sea water outlet flow appears normal. Which fault is most plausible?

  1. A blocked sea water suction strainer
  2. An over-open sea water temperature control valve
  3. A failed sea water pump impeller
  4. A fouled fresh water side of the cooler or a partially seized fresh water pump Correct answer.

Explanation

If sea water flow is normal, the problem lies on the fresh water side, such as fouling in the cooler or reduced circulation from a faulty pump. Blocked sea water strainer, failed sea water impeller, or an over-open valve would all show abnormal sea water flow or temperature.