Marine Propulsion Oral Exam Questions — Engine, Fuel Systems & Troubles

Otto vs Diesel vs Dual vs Carnot on a p-V sketch?

Otto adds heat vertically (constant volume), Diesel horizontally (constant pressure), Dual first vertical then horizontal — the real marine cycle. Carnot pairs two isotherms with two adiabatics: the unreachable ideal all are judged against.

Two-stroke or four-stroke without opening anything?

Count fires per revolution and camshaft speed: every-rev firing with a same-speed camshaft is two-stroke; alternate-rev with a half-speed cam is four-stroke. Exhaust note and scavenge belt back it up.

Name the strokes of each engine.

Two-stroke: compression (up, with scavenge and exhaust events inside it) and power/exhaust (down). Four-stroke: suction, compression, power, exhaust — gas exchange owns two full strokes of its own.

Under-piston scavenging — series or parallel?

Each unit scavenges its own cylinder from a common manifold in parallel — one choked unit never starves its neighbours.

Scavenge types? Pulse vs constant pressure — why is constant more efficient?

Cross, loop, uniflow in rising efficiency — uniflow ports plus exhaust valve wins. Constant pressure steadies turbine flow at high load with no pulse losses and simple manifolds; pulse wins response and low-load scavenging.

What is supercharging? Miller supercharging, what for?

Supercharging force-feeds charge air above atmospheric so more fuel burns per stroke. Miller closes the inlet early or late to shorten effective compression while keeping full expansion — lower peak temperatures and NOx with better part-load economy.

Compression ratio meaning?

Swept plus clearance volume over clearance volume — how hard the charge is squeezed before firing. Quote your engine's book figure with its peak pressure.

Indicated, max and mean effective pressures apart?

Indicated pressure is the card's mean height; max (peak) pressure is its highest point; MEP/MIP is the rectangle equal to the card area — the single number that, times L, A and N, becomes power.

MCR, CSR, NCR? SFOC with units?

MCR is maximum continuous rating, CSR the derated economical service rating, NCR the nameplate figure. SFOC is specific fuel oil consumption in g/kWh — fuel price per unit of work.

Power from a card? IHP vs BHP? PS to kW?

PmLAN — n is rev/s for two-stroke, half that for four-stroke. IHP is power developed in the cylinder, BHP what survives to the shaft after friction. 1 PS equals 0.7355 kW.

Power vs rpm vs ship speed? Aground under power? One unit cut out?

Power follows revs cubed and ship speed follows power near-cubed — 20% slower burns roughly half power. Aground, power makes wash not thrust: stop the engine and let tugs and tide work. With a unit cut out, run derated by about that unit's share.

CPP — what and how?

Controllable-pitch propeller: blades pivot in the hub so thrust reverses without stopping or reversing the engine — the fixed-rpm answer for ferries, tugs and manoeuvring-heavy ships.

Critical speed? Firing order significance — which device decides it?

Critical rpm is where firing pulses excite shaft resonance — cross it quickly, never dwell. Firing order spaces power strokes evenly for smooth torque and balanced bearings; the camshaft and fuel cams enforce it, so any cam or pump swap must respect it.

BHP, rpm, scavenge pressure, exhaust temperature of your engine?

There is no viva answer except your ship's book figures — state BHP, rpm, scavenge pressure, MCR, SFOC and cylinder-oil consumption from the manual, or say you would read them before answering.

Tie rods — how many, function, and removal if broken from the bottom?

One or two per frame line from cylinder block to bedplate, pre-tensioned so firing loads travel in the rod, never the frame. Broken at the bottom thread: slacken its partner, support the block, extract downward through the bedplate, renew and re-tension to book stretch in sequence.

Stuffing box — role, components, overhaul?

Seals the piston rod where it passes from scavenge space to crankcase: scraper rings strip oil downward, sealing rings stop scavenge air and dirt upward, all in a bolted housing. Overhaul means rings renewed, clearances to book, drains proved clear — because a failed box feeds crankcase contamination and fires.

Stuffing-box drain oil vs scavenge-space drain oil?

Stuffing-box drain is dirty system oil scraped off the rod — its quantity and smell diagnose ring wear and blow-by. Scavenge drain is unburnt fuel, cylinder oil and water condensed from charge air — analysed for fire risk and combustion health.

Why telescopic pipes for oil lubrication?

Crosshead motion would fatigue any hose — sliding telescopic legs feed oil across the reciprocating gap with metal-to-metal joints that extend and collapse every stroke.

Crosshead parts? Lubrication B&W vs Sulzer?

Pin, shoes, guides, rod foot and palm. B&W feeds down rod drillings; Sulzer boosts 10–16 bar through a swinging arm — both timed to low-load crank angles with grooved shells spreading it. Human side of the same circuit: shoe and guide clearances to book, feeler-checked, poker-gauge readings trended in place.

Under-slung crankshaft — what and why? Vee engines — how many shafts, rods how fitted?

Throws hung below the main-bearing line in a deep rigid bedplate for maximum alignment stiffness under firing loads. A vee has one crankshaft; master-and-articulated (slave) rods share each crankpin with pairs phased to it.

Thrust block role — and why aft, close to the engine?

Hands propeller thrust into the hull through tilting white-metal pads. Aft end because the shaft line starts there; hard against the engine and ahead of the stern tube so thermal growth runs one predictable way and alignment stays readable.

Hunting gear mechanism?

In steering follow-up control, the hunting gear drives the control lever back toward neutral as the rudder reaches the ordered angle — pump stroke dies exactly on helm, so the rudder never hunts past it.

How does a roto-cap work? Why two valve springs?

Roto-cap is a valve rotator: balls and springs in the cap turn the valve a few degrees each lift for even seating and wear. Two nested springs fight surge at speed and still close the valve if one breaks — check free length, squareness and cracks at overhaul.

Exhaust valve operation in brief? Why three injectors? Quills — what, where?

Hydraulic opening against air-spring closing gives smooth equal wear with only axial spindle load. Three injectors spread spray evenly across a large bore with redundancy. Quills are the non-return lubricating points in the liner wall feeding cylinder oil into its grooves.

Chain drive lubrication? Piston palm connected where?

Timed spray jets from the LO system keep chain and wheels wet — dry running stretches the chain and drifts timing. The piston palm (rod foot flange) bolts to the crosshead with fitted bolts.

Sulzer hydraulic jack bolts — why? Do two-strokes need tappet clearance?

Jacks stretch the bolt hydraulically, the nut follows finger-tight, pressure release leaves exact enormous preload — even and measurable, no hammering. Two-strokes have no tappets: exhaust valves are hydraulic or electronic and ports are piston-timed, so clearance questions belong to four-stroke rocker gear.

Crankcase inspection and deflection in one breath?

Look for wiped metal, slack locks, slipped punch marks, blue hot spots, breathing relief doors — and read web breathing on the gauge at even keel, five positions a throw, a kinked curve convicting its bearing.

Why measure butt clearance? How?

Butt gap absorbs ring heat growth: closed gaps seize, open gaps blow by. Measured with feelers in the unworn bore against maker-table limits — maximum and minimum both quoted from the book, never guessed.

Fuel valve opening vs injection pressure? Valve lift?

Opening pressure lifts the needle off its seat; injection pressure is the 200–400 bar line pressure driving atomisation through the holes. Lift is a few tenths of a millimetre to book figure — too little starves spray, too much hammers seats.

Fuel pump lead? Effective vs idle stroke?

Lead is how early the plunger seals the suction ports before the cam's delivery stroke — set by plunger height on shims, it fixes start of injection. Idle stroke is the dead lift before port closure; effective stroke is the lift that actually delivers fuel.

Why is fuel timing important? How checked and adjusted?

Timing places peak pressure just after TDC: early hammers bearings, late sends heat down the exhaust. Checked against flywheel marks with the pump's spill or sensor reading, adjusted by plunger shims or VIT setting per engine type.

Viscotherm working? Differential-pressure transmitter? Viscosity before injection?

The viscotherm throttles the fuel heater to hold injection viscosity near 10–15 cSt at the engine inlet; its capillary or DP transmitter senses viscosity change and trims steam. Wrong viscosity means bad spray — too thick dribles, too thin leaks and loses timing.

Mixing column purpose? Fuel index?

The mixing column deaerates return oil, blends HFO with diesel during changeover, and steadies inlet temperature so the viscotherm sees no shocks — venting air back to the service tank. Fuel index is the rack position per pump: equal indices mean balanced load.

Common-rail advantages? Fuel pump types?

Rail pressure independent of rpm, flexible timing with pre- and post-injection, no per-unit cam pumps. Pump families: jerk (helix or port-controlled), common rail with electronic injectors, and VIT variants advancing timing with load.

Overhaul a fuel injector — order of work?

Clean, strip, inspect needle and nozzle under magnification, lap mating faces, renew seals, reassemble, pressure-test spray pattern and set opening pressure before refit — atomisation is proved on the test rig, not on the engine.

Diesel knock vs afterburning?

Knock is a violent premixed pressure spike hammering bearings — cut fuel per cylinder to find it. Afterburning is late lazy burn raising exhaust temperature with smoky lagging power: timing, atomisation and air at fault.

One exhaust up, all up, one down?

One up: its injector, valve, cam, ports, rings, fire. All up: shared air, fuel grade, timing, overload. One down: starved cylinder — leaking pump valves, seating, pipes, mistimed pump, dead exhaust valve.

SFOC calculation — work it with density correction?

Measure fuel consumed (litres × corrected density = kg) over one hour at steady load, divide by power developed (BHP/kW) for g per unit — e.g. 115 L/h × 0.9787 = 112.55 kg/h ÷ 1400 kW = 80.4 g/kWh. Density at 15 °C corrects by −0.00064 per °C above 15 (e.g. 0.9931 at 38 °C → 0.9787); volume × corrected density is mass, and mass ÷ power is the answer the surveyor checks.

Power balancing? TBN split between cylinder and crankcase oils?

Balance by equal peak pressures and exhaust temperatures per unit, trimming pump indices — equal fuel index means equal load. TBN is alkaline reserve in mg KOH/g against sulphuric acid: cylinder oil faces firing-zone acids (high, near 70), crankcase system oil milder duty (low, near 5–10).

TAN vs TBN? Water in crankcase oil — where from? Cylinder-oil properties?

TAN counts acids present, TBN counts neutralising reserve. Water enters via cooler leaks, condensation, piston-cooling or jacket breaches, failed purification — find the entry, batch-treat the charge. Cylinder oil needs alkalinity, detergency, film strength and spreadability metered per cylinder on the upstroke.

Scraper ring function — why the holes? Hydrodynamic lubrication?

Scraper rings strip surplus oil off the liner on the downstroke; holes drain it back instead of letting it burn. Hydrodynamic lubrication is the wedge of oil the moving journal drags under itself — load rides on liquid, metal never touches.

One jacket-water outlet high — reasons and actions?

Its cooling scaled or air-locked, its load high from over-fuelling, its sensor lying. Prove the reading, check flow and venting, balance load, inspect at the next stop — a climbing trend beats a single number.

Main vs auxiliary engine governors?

Main governors hold propeller-law speed with droop across the load range; auxiliary governors hold fixed-frequency isochronous speed for the alternator — different duties, different settings.

Full reversing story in one minute? Lost motion?

Stop, shift timing by axial cams, roller shift or lost-motion linkage, re-time the distributor with it, verify interlocks, restart astern — or dodge it all with gearbox, CPP or electric drive. Rules behind it: astern power and timed crash-stop proven, with 12-start air capacity. Lost motion is the built-in slack (near 98° of crank angle) that lets the servo re-time cams while the crankshaft stands still.

Starting interlocks? Slow turning — how?

Turning gear out and locked, control air proven, no stop or shutdown active, direction agreed, telegraph answered. Slow turning bars the engine with indicator cocks open to prove freedom and expel fluid — two revolutions minimum before air is admitted.

Why blow through? Starting-air overlap? Air distributor?

Blow-through with cocks open, inside 30 minutes of starting, expels water, oil and fuel that would hydraulic-lock or fire a cylinder. Overlap keeps one cylinder always on air through dead centres for reliable rolling. The distributor sequences air to each start valve in firing order — and re-times with the engine on reversal.

Engine turns on air, dies on fuel — first three?

Fuel path shut, air-locked pumps, dead compression from rings or scavenge pressure — then filters, valves, timing and tank level in order.

Air starting line safeties? Line explosion — cause, checks, leaking valve?

Relief valves, flame arresters, non-return protection and drains guard the line; oil mist plus a leaking start valve lets combustion flash back into it. Prove a suspect valve by feeling its branch heat with the engine stopped, or by cocks-open blow-through showing one cylinder breathing air it should not.

OMD principle? Mist-alarm script? Relief doors tested how?

Sampled crankcase air scatters a light beam — obscuration alarms. On alarm: confirm the unit, inform seniors, dead-slow toward stop, evacuate, cool sealed for 30 minutes, inspect with safe lamps. Relief doors lift at slight overpressure and reseat through flame arresters — proved by gentle pressure test per manual, never hammered.

Scavenge-fire signs and actions? Uptake fire?

Trunk hot, revs sagging, drains sparking, turbocharger hunting, smoke rising: cut fuel to the unit, slow down, fight from outside, cool sealed before opening. Uptake (EGB/funnel) fire: stop soot-blowing, keep water circulating, starve air at the casing, boundary-cool — never open hot casing to air.

Funnel sparks? Black smoke? Piston running hot?

Sparks mean burning soot breaking loose — soot-blow load-dependent and watch the uptake. Black smoke is incomplete combustion: air first (filter, cooler, turbocharger), then fuel (timing, atomisation), then load. A hot piston shows in rising exhaust, hot drains and scuffing risk — cut load, prove cooling and lubrication, inspect at the stop.

Auxiliary overspeed trip — function and cut-off? Flywheel vs governor?

Flyweights trip the fuel rack above rated speed and latch until reset — proved by manual test rig per schedule. Flywheel stores rotation energy to smooth pulses; governor meters fuel to hold speed.

Telegraph use? Starting an auxiliary — and decarbonising?

The telegraph transmits bridge orders and its answer-back proves compliance — every movement logged. Start an auxiliary on air with prelube and warming per book, synchronise before loading. Decarbonise by scheduled head, ports and scavenge cleaning — carbon is read, not guessed, at each opening.