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BD Marine Engineer Class 1 (Chief Engineer) study materials
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BD Marine Engineer Class 1 (Chief Engineer) practice questions

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Question 1Marine Propulsion Machinery and Engine Room Systems

In a four-stroke diesel engine, what is the purpose of the valve overlap period around top dead centre?

  1. To use the inertia of the exhaust gas and the incoming air to improve scavenging and reduce residual gas Correct answer.
  2. To permit the fuel injector to cool between injections
  3. To reduce the compression pressure at low load
  4. To allow the incoming scavenge air to cool the exhaust valve seat

Explanation

During overlap, both inlet and exhaust valves are briefly open near top dead centre. The momentum of the outgoing exhaust gas plus the incoming charge helps sweep residual gas from the clearance volume, improving volumetric efficiency and reducing thermal loading.

Question 2Marine Propulsion Machinery and Engine Room Systems

Which component converts the reciprocating motion of the piston into rotary motion in a large marine diesel engine?

  1. The entablature
  2. The camshaft
  3. The crosshead and connecting rod assembly Correct answer.
  4. The thrust block

Explanation

The crosshead guides the piston rod in a straight line while the connecting rod transmits the gas force to the crankpin, converting linear motion into rotation. The thrust block only transfers axial propeller thrust to the hull.

Question 3Marine Propulsion Machinery and Engine Room Systems

What is the primary function of the scavenge air receiver in a uniflow-scavenged two-stroke engine?

  1. To damp torsional vibration of the crankshaft
  2. To provide a stable reservoir of pressurised air for the scavenge ports Correct answer.
  3. To store fuel for injection
  4. To separate water from the exhaust gas

Explanation

The scavenge air receiver buffers the pulsating delivery from the turbocharger, giving a steady pressure at the scavenge ports. This ensures consistent scavenging and charging across all cylinders.

Question 4Marine Propulsion Machinery and Engine Room Systems

Why is a diesel engine generally more thermally efficient than a spark-ignition petrol engine of similar size?

  1. Because it has no exhaust stroke
  2. Because it runs at lower peak pressures
  3. Because it operates at a higher compression ratio Correct answer.
  4. Because it uses a lighter fuel

Explanation

Compression ignition permits much higher compression ratios than spark ignition without pre-ignition, raising the theoretical Carnot-limited efficiency. The higher expansion ratio extracts more work from the burnt gas.

Question 5Marine Propulsion Machinery and Engine Room Systems

What is the main reason for fitting a spring-loaded bypass valve across a positive displacement lubricating oil pump?

  1. To preheat the oil before it reaches the bearings
  2. To increase the pump delivery at high speed
  3. To act as a non-return valve for reverse flow
  4. To protect the discharge line from overpressure if the flow is restricted Correct answer.

Explanation

A relief valve opens when discharge pressure exceeds the spring setting, returning oil to suction or sump. This protects the pump and piping from overpressure when filters block or valves are shut.

Question 6Marine Propulsion Machinery and Engine Room Systems

Which of the following best describes the role of the air cooler after the turbocharger compressor?

  1. To increase the charge density by reducing its temperature Correct answer.
  2. To remove water vapour from the scavenge air
  3. To increase the exhaust gas temperature
  4. To reduce the turbocharger speed

Explanation

Cooling the compressed charge lowers its temperature and therefore its specific volume, so a greater mass of air enters the cylinder for the same swept volume. This raises the trapped air mass and hence the achievable fuel quantity.

Question 7Marine Propulsion Machinery and Engine Room Systems

In a marine diesel engine, what does the term 'brake mean effective pressure' represent?

  1. The pressure drop across the air filter
  2. The pressure in the brake air reservoir
  3. The average pressure that, acting over the piston stroke, would produce the measured shaft output Correct answer.
  4. The maximum cylinder pressure recorded by the indicator

Explanation

BMEP is a calculated mean pressure derived from measured brake power, engine speed and swept volume. It allows engines of different sizes and speeds to be compared on a common basis of work output.

Question 8Marine Propulsion Machinery and Engine Room Systems

What is the primary purpose of the stuffing box around the piston rod in a crosshead engine?

  1. To lubricate the crosshead pin
  2. To measure the piston rod position
  3. To support the weight of the piston
  4. To prevent scavenge air and combustion products entering the crankcase Correct answer.

Explanation

The stuffing box seals the space between the scavenge space and the crankcase around the piston rod. It keeps contaminated air and combustion blow-by out of the crankcase oil, which protects lubricant quality.

Question 9Marine Propulsion Machinery and Engine Room Systems

A main engine suddenly shows a rise in exhaust temperature on one cylinder with a corresponding fall in firing pressure. Which fault is most consistent with these readings?

  1. A blocked scavenge air cooler on all cylinders
  2. An incorrect fuel viscosity at the settling tank
  3. A partially seized exhaust valve remaining open Correct answer.
  4. A leaking fuel injector with excessive delivery

Explanation

A valve held open by seizure or deposit leaks compression and combustion gas, lowering firing pressure and allowing unburnt fuel to burn late, which raises exhaust temperature. The symptom is isolated to one cylinder, which points to a cylinder-specific fault.

Question 10Marine Propulsion Machinery and Engine Room Systems

During a manoeuvring period, why must the main engine be operated with reduced maximum fuel index?

  1. Because the cooling water pressure is too high at low speed
  2. Because the governor is disconnected during manoeuvring
  3. Because the turbocharger cannot supply sufficient air at low speed, causing incomplete combustion Correct answer.
  4. Because the fuel pumps cannot deliver fuel below rated speed

Explanation

At low speed the turbocharger delivers little air, so the air-fuel ratio is already poor. Limiting the fuel index prevents over-fuelling, smoky combustion, thermal overload and fouling during rapid manoeuvres.

Question 11Marine Propulsion Machinery and Engine Room Systems

What is the effect of increasing the opening pressure of a fuel injector nozzle on the injection process?

  1. It retards the start of injection and may increase the injection duration Correct answer.
  2. It has no effect on injection timing
  3. It advances the start of injection and shortens the injection duration
  4. It reduces the fuel quantity delivered regardless of index

Explanation

A higher spring setting requires greater fuel pressure to lift the needle, so opening occurs later. Because the cam-driven pump still delivers its full stroke, the same quantity is injected over a shorter available period, often lengthening and steepening the injection.

Question 12Marine Propulsion Machinery and Engine Room Systems

Why is a thrust block essential between the main engine and the propeller shafting?

  1. To reduce torsional vibration to acceptable limits
  2. To allow the shafting to rotate freely without alignment
  3. To support the weight of the intermediate shafting only
  4. To absorb the axial thrust generated by the propeller and transmit it to the hull Correct answer.

Explanation

The rotating propeller produces axial thrust, which the thrust bearing picks up through its pads and transmits to the engine seating and hull. Without this, the engine crankshaft would carry the thrust and be damaged.

Question 13Marine Propulsion Machinery and Engine Room Systems

Auxiliary engine jacket cooling water temperature is found to be falling below normal with the engine at steady load. Which is the most likely cause?

  1. A partially closed sea water valve on the cooler
  2. A worn fresh water pump impeller
  3. A stuck-open thermostat or a failed cooler bypass Correct answer.
  4. A fouled heat exchanger on the fresh water side

Explanation

If the thermostat remains open or the bypass is stuck, full flow passes through the cooler even when the engine needs less cooling, so the temperature drops. The engine is otherwise at steady load, so the cause lies in temperature regulation.

Question 14Marine Propulsion Machinery and Engine Room Systems

What is the function of the labyrinth seal in the exhaust gas turbocharger?

  1. To filter the exhaust gas before the turbine
  2. To prevent oil from the bearing housing entering the gas or air spaces Correct answer.
  3. To cool the turbine blades
  4. To increase the pressure ratio of the compressor

Explanation

Labyrinth seals create a tortuous path that restricts leakage between the rotating shaft and the stationary casing. They keep lubricating oil out of the gas and air passages and limit gas leakage into the bearing spaces.

Question 15Marine Propulsion Machinery and Engine Room Systems

In a central fresh water cooling system, why is a chemical treatment regime maintained in the primary circuit?

  1. To allow the use of sea water in the primary circuit
  2. To raise the specific heat capacity of the water
  3. To control corrosion, scale and biological growth in the closed circuit Correct answer.
  4. To increase the boiling point above the operating temperature

Explanation

Inhibitors maintain a protective film on metal surfaces, prevent scale deposition and control biological activity in the closed loop. This preserves heat transfer and extends the life of coolers, pipes and engine passages.

Question 16Marine Propulsion Machinery and Engine Room Systems

An engine room watchkeeper notices the main engine fuel oil viscosity at the injector pumps has fallen below the specified range. What immediate risk does this present?

  1. Over-speeding of the fuel booster pump
  2. Excessive cooling of the fuel in the heater
  3. Blockage of the fuel filters by wax formation
  4. Increased wear of pump plungers and reduced injection quality Correct answer.

Explanation

Too low a viscosity reduces the lubricating film between plunger and barrel and can cause leakage past the plunger, lowering injection pressure and degrading atomisation. The correct viscosity must be restored by adjusting the heater.

Question 17Marine Propulsion Machinery and Engine Room Systems

What is the purpose of the turning gear interlock on a large marine diesel engine?

  1. To prevent the engine being started while the turning gear is engaged Correct answer.
  2. To lock the fuel rack in the stop position
  3. To keep the indicator cocks open during starting
  4. To limit the engine speed during manoeuvring

Explanation

The interlock ensures the starting air system cannot be admitted while the turning gear is engaged, which would otherwise cause catastrophic damage. It may also prevent the turning gear being engaged while the engine is running.

Question 18Marine Propulsion Machinery and Engine Room Systems

Why is a two-stage air compressor with intercooling used for starting air rather than a single stage?

  1. To reduce the power required to drive the compressor to zero
  2. To reduce the final delivery temperature and improve volumetric efficiency Correct answer.
  3. To eliminate the need for a relief valve
  4. To allow the use of a smaller receiver

Explanation

Intercooling between stages lowers the air temperature before the second stage, reducing the work of compression and the final discharge temperature. Cooler, denser air also improves volumetric efficiency and avoids lubricant degradation.

Question 19Marine Propulsion Machinery and Engine Room Systems

What does an increase in the compression pressure reading of one cylinder, with unchanged scavenge air pressure and fuel index, most probably indicate?

  1. A leaking exhaust valve
  2. A faulty indicator or pressure transducer Correct answer.
  3. A worn piston ring set
  4. A restricted scavenge port on that cylinder

Explanation

Compression pressure is set by the trapped air mass and the clearance volume, which are stable at constant scavenge pressure and load. A single anomalous high reading with no other change most likely reflects an instrumentation error rather than a genuine mechanical change.

Question 20Marine Propulsion Machinery and Engine Room Systems

Why must the lubricating oil for a crosshead engine's cylinder liners be kept separate from the crankcase oil?

  1. Because cylinder oil is consumed in the cylinder and must resist high alkalinity from combustion products Correct answer.
  2. Because crankcase oil has a higher viscosity index than cylinder oil
  3. Because cylinder oil is a synthetic product that attacks steel
  4. Because crankcase oil cannot be filtered

Explanation

Cylinder oil is a total-loss lubricant that must neutralise acidic combustion products and burn cleanly, so it is formulated with high base number and is not returned to the sump. Crankcase oil has a different duty and must not be contaminated by combustion residues.

Question 21Marine Propulsion Machinery and Engine Room Systems

A main engine cylinder liner shows heavy scuffing on the exhaust side at mid-stroke. Which combination of factors is most likely responsible?

  1. Inadequate lubrication combined with thermal distortion of the liner Correct answer.
  2. Excessive cylinder oil feed and low jacket water temperature
  3. Too high a scavenge air pressure at full load
  4. Incorrect fuel injection timing on the adjacent cylinder

Explanation

Scuffing arises when the oil film breaks down and metal-to-metal contact occurs, often worsened by bore distortion from thermal or mechanical loading. The exhaust side runs hottest, so it is the first area affected.

Question 22Marine Propulsion Machinery and Engine Room Systems

During a crash-stop manoeuvre, what is the main reason for limiting the number of consecutive starting air applications?

  1. To prevent excessive cooling of the starting air
  2. To prevent the governor from overspeeding the engine
  3. To protect the turning gear from engagement
  4. To avoid over-heating of the starting air valve and depletion of receiver pressure Correct answer.

Explanation

Repeated starting air admissions without firing heat the air valves and can cause leakage or seizure, while the receiver pressure falls rapidly. The bridge and engine room must allow for replenishment between attempts.

Question 23Marine Propulsion Machinery and Engine Room Systems

In a constant-pressure turbocharging system with a single large exhaust receiver, why does the turbine see a nearly steady gas pressure rather than individual pulses?

  1. Because the receiver volume dampens the pressure pulses from all cylinders Correct answer.
  2. Because the exhaust gas is cooled before the turbine
  3. Because the exhaust valves are held open continuously
  4. Because the turbine rotates faster than the engine

Explanation

The large receiver acts as an accumulator, smoothing the discrete blowdown pulses from individual cylinders into an almost constant pressure. The turbine then operates at high efficiency on a steady flow, though some pulse energy is lost.

Question 24Marine Propulsion Machinery and Engine Room Systems

A main engine governor is found to have a droop characteristic. What is the practical consequence for parallel operation with a shaft generator?

  1. The engine cannot be synchronised to the busbar
  2. The engine will share load in proportion to its droop setting Correct answer.
  3. The fuel index will remain constant regardless of load
  4. The engine speed will rise indefinitely as load increases

Explanation

Droop gives a defined relationship between load and speed, allowing machines to share load in proportion to their droop settings. Without droop, small speed differences would cause one machine to take all the load.

Question 25Marine Propulsion Machinery and Engine Room Systems

Why is it important to maintain the correct clearance volume in a diesel engine cylinder?

  1. It sets the maximum fuel injection pressure
  2. It determines the compression ratio and therefore ignition quality and thermal efficiency Correct answer.
  3. It controls the cooling water flow through the liner
  4. It fixes the valve overlap period

Explanation

Clearance volume, together with swept volume, fixes the compression ratio. Too large a clearance lowers compression temperature and impairs ignition, while too small a clearance risks valve-to-piston contact.

Question 26Marine Propulsion Machinery and Engine Room Systems

In a large two-stroke marine diesel engine, what is the primary function of the scavenge air receiver?

  1. To separate water and oil mist from the exhaust gas before the turbocharger
  2. To store compressed air so that each cylinder is supplied with a steady charge of fresh air at the start of scavenging Correct answer.
  3. To dampen torsional vibration in the crankshaft during firing
  4. To act as a reservoir of high-pressure air for starting the engine

Explanation

The scavenge air receiver is a large volume vessel downstream of the air cooler that evens out the pulsations from the turbocharger and delivers an approximately constant pressure of fresh air to the scavenge ports. This steady supply is needed so each cylinder receives an adequate air charge for combustion regardless of the intermittent demand of individual cylinders. It is not a starting-air reservoir, which operates at much higher pressure and is a separate system.

Question 27Marine Propulsion Machinery and Engine Room Systems

Which component converts the reciprocating motion of the piston into rotation of the crankshaft in a large marine diesel engine?

  1. The governor
  2. The connecting rod and crank mechanism Correct answer.
  3. The thrust block
  4. The camshaft

Explanation

The connecting rod transmits the gas force from the piston to the crankpin, and the crank offset converts linear piston motion into rotation of the crankshaft. The complete slider-crank mechanism is the fundamental means of power transmission in a reciprocating engine. The other components serve different purposes such as valve actuation, thrust transmission, and speed control.

Question 28Marine Propulsion Machinery and Engine Room Systems

What is the primary purpose of the cylinder lubricating oil in a large crosshead two-stroke engine?

  1. To actuate the exhaust valve hydraulically
  2. To provide a hydrodynamic film between the liner and piston rings and neutralise acidic combustion products Correct answer.
  3. To seal the crankcase from the scavenge space
  4. To cool the piston crown directly

Explanation

Cylinder oil is injected at the liner wall to form a lubricating film between the rings and liner and to neutralise the acidic products of combustion, particularly sulphur acids. It is formulated with a high base number for this neutralising duty. Piston cooling, valve actuation, and crankcase sealing are handled by other systems.

Question 29Marine Propulsion Machinery and Engine Room Systems

On a four-stroke auxiliary engine, what does the term 'valve overlap' refer to?

  1. The period during which both inlet and exhaust valves are open around top dead centre Correct answer.
  2. The clearance between the valve stem and the rocker arm
  3. The difference in opening pressure between two injectors
  4. The amount by which the valve lift exceeds the port diameter

Explanation

Valve overlap is the crank angle period near the end of the exhaust stroke and the beginning of the induction stroke when both the inlet and exhaust valves are open simultaneously. It uses the momentum of the exhaust gas to help scavenge the clearance volume and draw in fresh charge. Overlap is a timing characteristic, not a mechanical clearance or lift dimension.

Question 30Marine Propulsion Machinery and Engine Room Systems

A main engine cylinder unit shows a gradually rising exhaust gas temperature and a falling compression pressure over several days at constant load, with the fuel pump index unchanged. Which fault is most consistent with these readings?

  1. A leaking exhaust valve seat Correct answer.
  2. A worn fuel injector nozzle
  3. A faulty tachometer on the turbocharger
  4. A blocked air cooler on the water side

Explanation

A leaking exhaust valve allows high-pressure combustion gas to escape past the seat, which reduces the effective compression pressure and increases the temperature of the gas leaving the cylinder. The fuel index is unchanged, so the fault lies downstream of the combustion process in the gas path. A worn nozzle or blocked cooler would affect all cylinders or show different symptoms.

Question 31Marine Propulsion Machinery and Engine Room Systems

Why is a spring-loaded bypass valve fitted across the lubricating oil filter in a main engine system?

  1. To permit sampling of oil without stopping the engine
  2. To regulate the oil pressure delivered to the bearings at all times
  3. To drain the filter housing during maintenance
  4. To allow oil to bypass the filter if the pressure drop becomes excessive and so maintain lubrication Correct answer.

Explanation

The bypass valve opens when the differential pressure across the filter element exceeds a set value, usually because the element is clogged. This ensures that lubricating oil continues to reach the bearings rather than starving the engine, at the cost of temporarily unfiltered oil. Normal pressure regulation is done by a separate pressure control valve.

Question 32Marine Propulsion Machinery and Engine Room Systems

During manoeuvring, a main engine is required to operate at reduced maximum fuel index. Which condition is the principal reason for this limitation?

  1. The turbocharger cannot accelerate quickly enough to supply sufficient scavenge air for full fuel delivery at low speed Correct answer.
  2. The governor is unable to control the engine below half load
  3. The starting air compressor cannot keep up with demand
  4. The fuel oil viscosity is too high at low engine speeds

Explanation

At low engine speeds the turbocharger delivers less air, so the air-to-fuel ratio is low and combustion would become smoky and thermally overstressed if full fuel index were applied. Limiting the fuel index during manoeuvring ensures adequate excess air and protects the engine from thermal overload. Viscosity, governor capability, and starting air are managed by separate means.

Question 33Marine Propulsion Machinery and Engine Room Systems

A main engine governor is set with a small amount of speed droop. What is the practical reason for retaining this droop in a propulsion engine?

  1. It increases the maximum continuous rating of the engine
  2. It allows stable load sharing and prevents hunting when the engine is coupled to a fixed-pitch propeller Correct answer.
  3. It reduces the specific fuel consumption at part load
  4. It eliminates the need for a fuel index limiter

Explanation

A small droop gives the governor a stable, slightly speed-dependent characteristic that prevents hunting and allows predictable load sharing when engines run in parallel or when the engine drives a fixed-pitch propeller. Without droop, the governor would tend to oscillate around the set point. Droop does not change the engine rating or eliminate the need for a fuel limiter.

Question 34Marine Propulsion Machinery and Engine Room Systems

In a central fresh water cooling system, what is the function of the temperature control valve on the fresh water circuit?

  1. To bypass fresh water around the sea water cooler so that the engine outlet temperature is held at the design value Correct answer.
  2. To regulate the pressure of fresh water in the circuit
  3. To dose chemical treatment into the fresh water
  4. To isolate the sea water side during maintenance

Explanation

The temperature control valve senses the fresh water temperature leaving the engine and modulates a bypass around the sea water cooler. This maintains the jacket water at the design temperature regardless of load and sea water temperature. It does not control pressure, dosing, or isolation.

Question 35Marine Propulsion Machinery and Engine Room Systems

Why is the thrust block of a direct-drive diesel installation located as close to the engine as practicable?

  1. To improve the hydrodynamic lubrication of the stern tube bearing
  2. To allow the propeller shaft to be withdrawn without removing the engine
  3. To keep the thrust load path short and avoid excessive bending and vibration in the intermediate shafting Correct answer.
  4. To reduce the weight of the propeller shaft

Explanation

The propeller thrust must be transmitted to the hull through the thrust block, and placing it near the engine shortens the load path through the intermediate shafting. This reduces the bending and torsional effects on the shafting and lowers the risk of vibration and misalignment. The position does not change propeller shaft weight or stern tube lubrication.

Question 36Marine Propulsion Machinery and Engine Room Systems

A turbocharger is found to have a fouled turbine nozzle ring. What effect will this have on the engine at constant load?

  1. Both scavenge air pressure and exhaust gas temperature will remain unchanged
  2. Scavenge air pressure will fall and exhaust gas temperature will rise Correct answer.
  3. Scavenge air pressure will rise and exhaust gas temperature will fall
  4. Scavenge air pressure will rise and the fuel index will fall

Explanation

A fouled nozzle ring restricts the gas flow through the turbine, so less energy is extracted and the turbocharger speed falls, reducing the scavenge air pressure. With less air available the combustion becomes less efficient and the exhaust gas temperature rises at constant load. This is a classic indication of turbine-side fouling.

Question 37Marine Propulsion Machinery and Engine Room Systems

What is the purpose of the water separator and drain in a starting air system?

  1. To lubricate the starting air valves
  2. To increase the pressure of the starting air
  3. To cool the air after compression
  4. To remove condensed water and oil from the air before it reaches the starting valves Correct answer.

Explanation

Compressed starting air cools as it passes through the system, and moisture and oil carry-over condense out. The separator and drain remove this condensate so that it does not enter the starting valves or cylinders, where it could cause corrosion or hydraulic lock. The separator does not change pressure, cool the air, or lubricate the valves.

Question 38Marine Propulsion Machinery and Engine Room Systems

A main engine fuel pump is found to have excessive wear on its plunger and barrel. Which symptom would you expect at the affected cylinder?

  1. A fall in firing pressure and a rise in exhaust temperature with unchanged fuel index Correct answer.
  2. No change in firing pressure but a rise in scavenge air pressure
  3. A rise in firing pressure and a fall in exhaust temperature
  4. A fall in both firing pressure and exhaust temperature

Explanation

Worn plunger and barrel clearances allow fuel to leak past during injection, so less fuel is delivered and the combustion is later and less complete. This lowers the firing pressure and raises the exhaust temperature at constant fuel index. The other combinations do not match the effect of reduced fuel delivery.

Question 39Marine Propulsion Machinery and Engine Room Systems

In a large two-stroke engine fitted with exhaust valve actuation by hydraulic servo oil, what is the function of the 'air spring' in the exhaust valve assembly?

  1. To cool the valve spindle during operation
  2. To seal the valve guide against exhaust gas leakage
  3. To assist in closing the valve and to cushion the valve against the seat Correct answer.
  4. To open the valve against the hydraulic servo pressure

Explanation

The air spring stores energy as the valve opens and returns it to help close the valve rapidly, while also cushioning the final seating impact. This reduces the load on the hydraulic actuator and improves valve life. It does not open the valve, cool the spindle, or seal the guide.

Question 40Marine Propulsion Machinery and Engine Room Systems

During a slow-speed manoeuvre, the main engine is observed to have a high scavenge air temperature and a smoky exhaust despite normal cooling water temperatures. Which combination of causes is most likely?

  1. A fouled air cooler on the air side together with a restricted scavenge air receiver drain Correct answer.
  2. A worn fuel injector nozzle together with a faulty tachometer
  3. A faulty governor together with a low starting air pressure
  4. A leaking exhaust valve together with a high fuel oil viscosity

Explanation

A fouled air cooler on the air side reduces heat transfer, so the scavenge air temperature rises even when cooling water temperatures appear normal. If the receiver drain is restricted, condensate and oil accumulate and are carried into the cylinders, producing smoky exhaust. Together these explain the symptoms at low load. The other combinations do not account for both the high scavenge temperature and the smoke.

Question 41Shipboard Electrical, Electronic and Control Engineering

On a 60 Hz three-phase alternator feeding a balanced load, what is the synchronous speed of a machine with four poles?

  1. 900 rpm
  2. 1200 rpm
  3. 1800 rpm Correct answer.
  4. 3600 rpm

Explanation

Synchronous speed is found from N = 120f/p, where f is frequency in hertz and p is the number of poles. Substituting 120 x 60 / 4 gives 1800 rpm for a four-pole machine on a 60 Hz supply.

Question 42Shipboard Electrical, Electronic and Control Engineering

Which device is installed to protect a shipboard electrical distribution system against the damaging effects of lightning surges and switching transients?

  1. Buchholz relay
  2. Surge arrester Correct answer.
  3. Current transformer
  4. Earth fault relay

Explanation

A surge arrester limits transient overvoltages by diverting the surge current to earth and then restoring its normal high impedance. This protects insulation on cables and equipment from lightning and switching transients.

Question 43Shipboard Electrical, Electronic and Control Engineering

In a ship's insulated (IT) distribution system, what is the principal purpose of the earth fault monitoring equipment?

  1. To automatically disconnect the faulty circuit within 0.4 seconds
  2. To give an alarm on the first earth fault so it can be traced and cleared before a second fault causes a short circuit Correct answer.
  3. To limit the prospective short-circuit current to a safe value
  4. To trip the main generator breaker immediately on any earth fault

Explanation

An insulated system can tolerate one earth fault without interrupting supply, but a second fault on another phase produces a short circuit through earth. The monitor therefore alarms on the first fault so it can be located and cleared while the system remains operational.

Question 44Shipboard Electrical, Electronic and Control Engineering

A three-phase induction motor is started direct-on-line on a 440 V supply. Which statement correctly describes the starting current compared with full-load current?

  1. It is limited to twice full-load current by the motor's slip
  2. It is typically four to eight times full-load current and falls as the motor accelerates Correct answer.
  3. It is lower than full-load current until the motor reaches rated speed
  4. It is approximately equal to full-load current throughout the start

Explanation

At standstill the rotor appears as a short-circuited secondary with maximum slip, so the motor draws a large locked-rotor current. This is typically four to eight times full-load current and decreases as the rotor accelerates and slip falls.

Question 45Shipboard Electrical, Electronic and Control Engineering

What is the function of the automatic voltage regulator on a ship's diesel-driven alternator?

  1. To control excitation current so terminal voltage stays constant as load changes Correct answer.
  2. To keep the prime mover at constant speed regardless of load
  3. To synchronise the generator before paralleling
  4. To limit the generator output current during overload

Explanation

The automatic voltage regulator senses terminal voltage and adjusts the field excitation current to hold voltage at the set value as load and power factor vary. It acts on the exciter field, not on engine speed or synchronising.

Question 46Shipboard Electrical, Electronic and Control Engineering

Two identical alternators are operating in parallel. If the excitation of one machine is increased while the prime mover inputs are unchanged, what happens?

  1. Its speed rises and it takes more active load
  2. Its active power output increases while reactive output stays constant
  3. Its terminal voltage falls and it takes less reactive load
  4. Its reactive power output increases and its power factor becomes more lagging Correct answer.

Explanation

With governor settings unchanged, active power sharing stays the same, but raising excitation raises the internal emf so the machine delivers more reactive power. The machine therefore operates at a more lagging power factor while the other absorbs the difference.

Question 47Shipboard Electrical, Electronic and Control Engineering

A transformer has 500 primary turns and 100 secondary turns. If the primary is connected to a 440 V alternating supply, what is the secondary voltage on open circuit?

  1. 2200 V
  2. 88 V Correct answer.
  3. 110 V
  4. 44 V

Explanation

The voltage ratio equals the turns ratio, so Vs = Vp x Ns/Np. Substituting 440 x 100/500 gives 88 V on open circuit.

Question 48Shipboard Electrical, Electronic and Control Engineering

Which type of semiconductor device is used as the controlled switch in the inverter section of a modern variable-frequency drive for a propulsion motor?

  1. Silicon controlled rectifier
  2. Zener diode
  3. Insulated gate bipolar transistor Correct answer.
  4. Thermistor

Explanation

The insulated gate bipolar transistor combines a high-impedance gate with low on-state losses, making it the standard switch for pulse-width-modulated inverters in motor drives. Silicon controlled rectifiers are line-commutated and less suited to high-frequency PWM inverter operation.

Question 49Shipboard Electrical, Electronic and Control Engineering

A pulse-width-modulated inverter feeding an induction motor is controlled to maintain a constant volts-per-hertz ratio as frequency is reduced. What is the main reason for this?

  1. To reduce the harmonic content of the output waveform
  2. To keep the inverter switching frequency constant
  3. To keep the magnetic flux, and therefore torque capability, approximately constant Correct answer.
  4. To ensure the motor runs at synchronous speed

Explanation

Flux is proportional to voltage divided by frequency, so reducing frequency without reducing voltage would saturate the magnetic circuit. Holding the ratio constant keeps flux and available torque approximately constant over the speed range.

Question 50Shipboard Electrical, Electronic and Control Engineering

In a marine automation system, what distinguishes a closed-loop control system from an open-loop system?

  1. It requires no sensor on the process
  2. It always operates without a controller
  3. It uses only digital rather than analogue signals
  4. It measures the controlled variable and feeds it back to correct the output Correct answer.

Explanation

A closed-loop system compares the measured process value with the setpoint and uses the error to adjust the actuator. This feedback allows it to compensate for disturbances and changes in process conditions.