Thermodynamics and Marine Engineering II: Marine Diesel Engines, Turbines, Boilers, Engine Dynamics and Auxiliary Systems

Most merchant ships are driven by diesel engines. Low-speed engines (below about 300 rpm) are large two-stroke crosshead engines coupled directly to a fixed-pitch propeller, with no gearbox; the crosshead separates the cylinder from the crankcase, so the cylinder is lubricated separately by an alkaline cylinder oil matched to the fuel’s sulphur content and the crankcase runs on system oil. Medium-speed engines (about 300–1000 rpm) are four-stroke trunk-piston engines, whose crankcase oil lubricates the liner too; they drive the propeller through a reduction gearbox, often with a controllable-pitch propeller, and they drive the generators. A modern low-speed engine has the lowest specific fuel consumption of any prime mover in service, with a thermal efficiency approaching 50%.

Scavenging and turbocharging. A two-stroke engine clears its cylinder with the incoming air: cross scavenging (air ports on one side, exhaust ports opposite), loop scavenging (both sets of ports on one side, the air looping up and back) and uniflow scavenging (air ports round the liner bottom, exhaust through a valve in the cover), the last giving the cleanest sweep and used in modern low-speed engines. Turbocharging uses the exhaust gas to drive a turbine on a common shaft with a compressor that raises the density of the charge air, which is then cooled in a charge-air cooler; the turbine may be fed at constant pressure from a large exhaust receiver, which suits steady high loads, or by pulses from short pipes, which responds better at low load; auxiliary electric blowers take over at low load when the exhaust energy is too small.

Torque and power measurement. The indicated power comes from the indicator diagram, P_i = p_m L A n k (mean indicated pressure, stroke, piston area, power strokes per second per cylinder, cylinders). The brake (shaft) power is P = 2πNT/60: on the test bed a dynamometer absorbs and measures the torque, and at sea a torsionmeter — strain gauges on the shaft reading its shear strain — gives T, so that an engine turning at 90 rpm with a torque of 500 kN·m delivers 2π × 1.5 × 500 = 4712 kW. The specific fuel consumption is fuel mass flow over power, in g/kWh: 2000 kg/h at 10 000 kW is 200 g/kWh, and with a calorific value of 42 700 kJ/kg the brake thermal efficiency is 3600/(0.2 × 42 700) = 42.2%. Waste-heat recovery takes the heat the engine rejects — exhaust gas through an exhaust-gas boiler (economiser) raising steam for heating or a turbo-generator, a power turbine on surplus exhaust, jacket water to a fresh-water generator — and returns part of it as useful energy.

MARPOL, EEDI and SEEMP. The International Convention for the Prevention of Pollution from Ships (MARPOL) has six annexes: I oil, II noxious liquid substances in bulk, III harmful substances in packaged form, IV sewage, V garbage and VI air pollution — sulphur oxides (a global limit of 0.50% sulphur in fuel since 2020 and 0.10% in emission control areas), nitrogen oxides (tiered engine limits), ozone-depleting substances and energy efficiency. Under Annex VI the Energy Efficiency Design Index (EEDI) is a design-stage figure for a new ship — its CO₂ emission per unit of transport work, in grams of CO₂ per tonne-nautical mile — whose attained value must not exceed a required value that tightens in phases; the Ship Energy Efficiency Management Plan (SEEMP) is an operational, ship-specific plan for improving efficiency in service — voyage and speed optimisation, hull and propeller cleaning, trim, and monitoring — kept on board by existing ships as well as new ones.

A steam plant works on the Rankine cycle: the feed pump raises the condensate to boiler pressure (3→4), the boiler and superheater add heat at constant pressure (4→1), the turbine expands the steam (1→2) and the condenser rejects heat (2→3). Its efficiency is η = [(h₁ − h₂) − (h₄ − h₃)]/(h₁ − h₄). With h₁ = 3200, h₂ = 2200, h₃ = 190 and a pump work of 5 kJ/kg (h₄ = 195), η = (1000 − 5)/(3200 − 195) = 33.1%; the pump work is small because it compresses a liquid. Reheat returns partly expanded steam to the boiler to be heated again, which raises the mean temperature of heat addition and, above all, keeps the steam drier at the turbine exhaust, where wet steam erodes the last blades; regenerative feed heating with bled steam raises the feed temperature and the efficiency further.

Types and compounding. In an impulse turbine the whole pressure drop of a stage takes place in fixed nozzles, and the moving blades only turn the jet, changing its momentum at constant pressure; in a reaction turbine the pressure falls in both the fixed and the moving blades, which act as nozzles too (the Parsons turbine has 50% reaction). A single impulse stage would need an impossibly high blade speed, so the energy is divided — compounded: velocity compounding (the Curtis stage) expands the steam in one set of nozzles and absorbs its velocity in two or more rows of moving blades with guide blades between; pressure compounding (Rateau) divides the pressure drop among a series of nozzle-and-blade stages, the nozzles being carried in diaphragms between the wheels; pressure–velocity compounding combines them. Construction: rotors are solid forgings or built-up discs; blades are held by fir-tree or similar roots and tied by shrouds; the casing is split horizontally, the HP and LP turbines usually in separate casings; glands where the shaft leaves the casing are labyrinths fed with gland steam and exhausted to a gland condenser, so that steam does not leak out at the HP end and air does not leak in at the LP end; journal bearings are white-metal lined and a tilting-pad thrust bearing locates the rotor. A forced lubrication system — pumps, gravity tank, coolers and filters — serves the bearings and gears; expansion arrangements (sliding feet and keys from a fixed point) let the hot casing grow without distorting; and double-reduction double-helical gearing brings some thousands of rpm down to propeller speed.

Marine gas turbines run on the Brayton cycle: an axial compressor, a combustor (can, can-annular or annular) and a turbine — in propulsion units a gas-generator turbine that drives the compressor and a separate free power turbine that drives the load through a reduction gearbox. Their operational features are a very high power-to-weight ratio, starting and loading in minutes, low manning and low vibration, set against a high fuel consumption at part load, the need for clean distillate fuel, large air intakes and uptakes, and no reversing — so a controllable-pitch propeller or reversing gear is fitted. Controls govern fuel flow, speed and temperature and protect against overspeed, over-temperature and compressor surge. Combined cycles pair prime movers — CODOG, CODAG and COGAG arrangements of diesels and gas turbines for cruise and boost, and COGES/COGAS, in which the gas turbine’s exhaust raises steam for a steam turbine; ideally η = η_GT + η_ST(1 − η_GT), so 35% and 30% combine to 54.5%.

Nuclear propulsion. In a reactor, a neutron splits a nucleus of uranium-235, releasing heat and two or three further neutrons that sustain a chain reaction. A moderator (in marine reactors, water) slows the neutrons so that they cause fission efficiently; control rods of neutron-absorbing material (boron, cadmium or hafnium) are moved in and out to hold the reactor exactly critical and to shut it down; a coolant carries the heat away; and heavy shielding protects the crew. Seagoing reactors are mostly pressurised-water reactors: the primary water, kept liquid by high pressure, carries the heat to a steam generator, where a secondary circuit raises steam for the turbines. Nuclear propulsion gives years of endurance without refuelling and needs no air, which is why submarines, large aircraft carriers and icebreakers use it; its cost, shielding weight, regulation and restricted port access keep it out of merchant ships. Electrical propulsion separates power generation from propulsion: diesel or turbine generators feed a common switchboard, and electric motors — on the shaft or in azimuthing pods — drive the propellers through variable-frequency converters. It suits ships with large hotel loads and varied operating profiles, such as cruise ships, dynamic-positioning vessels and icebreakers, because the right number of generators can run near their best load at every speed.

Boiler mountings are the fittings on the pressure vessel that make it safe and controllable: at least two safety valves set to lift at the design pressure; the main stop valve; the feed check valve, a non-return valve on the feed inlet; two independent water-level gauges; blow-down valves (bottom and surface/scum); a pressure gauge, air vent and sampling connection; and, on automated boilers, low-water and flame-failure cut-outs. Combustion: the burner atomises the fuel (pressure-jet, steam- or air-assisted, or rotary cup) and the air register swirls in the right quantity of air — too little leaves unburnt fuel and smoke, too much carries heat up the funnel — and soot blowers keep the gas-side surfaces clean. The feed system runs from the condenser through the extraction pump, a deaerator that removes dissolved oxygen, the feed pump and feed heaters to the boiler. Feed-water treatment prevents scale (hardness salts deposited on hot surfaces, which insulate tubes until they overheat), corrosion (by dissolved oxygen and low pH) and foaming and carry-over: phosphate treatment turns hardness into a removable sludge, oxygen scavengers such as hydrazine or sodium sulphite remove the last oxygen, alkalinity is kept up, and blow-down limits the dissolved solids, all checked by routine tests of chloride, pH and alkalinity.

A boiler’s efficiency is the heat taken up by the steam over the heat released by the fuel, η = ṁ_s(h_s − h_f)/(ṁ_f × CV). A boiler making 10 000 kg/h of steam at 2800 kJ/kg from feed at 420 kJ/kg while burning 800 kg/h of fuel of 42 000 kJ/kg has η = 10 000 × 2380/(800 × 42 000) = 70.8%; the losses are chiefly the heat in the flue gas, which an economiser and an air preheater recover in part.

Torsional vibration. The crankshaft, flywheel, line shaft and propeller form a chain of rotating inertias joined by torsionally elastic shafts. Two rotors of inertias I₁ and I₂ joined by a shaft of torsional stiffness k vibrate in opposition about a node at the natural frequency ω = √[k(I₁ + I₂)/(I₁I₂)]: with k = 2 × 10⁶ N·m/rad, I₁ = 100 and I₂ = 400 kg·m², ω = √25 000 = 158.1 rad/s, f = 25.2 Hz. Multi-rotor systems are solved by the Holzer tabulation or by eigenvalue analysis. The engine’s gas and inertia torques contain harmonics at integer and, for four-stroke engines, half-integer orders of the rotational speed, so a natural frequency f_n is excited at the critical speed N_c = 60 f_n/order rpm — a 25 Hz mode is excited by the 6th order at 1500/6 = 250 rpm. Where a critical cannot be moved out of the running range by changing stiffness or inertia, it is controlled by a viscous or spring damper, a tuned flexible coupling, or a barred speed range through which the engine is taken quickly. Axial vibration of the crankshaft and shafting is excited by the propeller’s thrust fluctuations and the crank throws’ deflection and is controlled by an axial damper; whirling of the shafting at its lateral critical speed is avoided by bearing spacing.

Engine rating states the power an engine may give: the maximum continuous rating (MCR) it may deliver indefinitely, and the lower continuous service rating at which the ship is designed to run, leaving a margin for weather and fouling. Ratings apply at stated reference ambient conditions of air temperature, pressure and cooling-water temperature; rating corrections reduce the permissible power in hotter air or warmer sea water, because the charge air is less dense and the engine hotter. Trials: shop (test-bed) trials of the engine, dock trials of the installation, and sea trials — speed on a measured course, endurance, astern running, crash stop and steering — prove the ship. Engine and propeller: a fixed-pitch propeller absorbs power as the cube of its speed, P ∝ N³ (torque ∝ N²) — running at 80% speed needs 0.8³ = 51.2% of the power — and the engine’s load diagram must contain this propeller curve with a light-running margin, because fouling and bad weather make the propeller “heavy” and move the operating point towards the engine’s torque limit. Classification society rules govern engine construction — approval of drawings, crankshaft dimensions, materials and their testing, safety devices and surveys through the ship’s life.

The engine-room arrangement places the main engine on the centre line low and aft, with the shafting in line to the stern tube, and groups around it the generators, boilers, purifiers, pumps, compressors and heat exchangers so that piping is short, every item can be reached and lifted out for overhaul, ventilation is adequate and there are two escape routes. Automation lets the machinery space run unattended for periods: alarm and monitoring systems, bridge control of the main engine, automatic start of standby pumps and generators, and a power-management system that starts, synchronises and sheds generator load. Maintenance is breakdown (run to failure), planned (at fixed running hours) or condition-based — trending vibration, lubricating-oil analysis, thermography and performance data to overhaul when the condition demands. Reliability is the probability of running without failure for a stated time, often summarised by the mean time between failures (MTBF), and the availability A = MTBF/(MTBF + MTTR) — 950 h between failures and 50 h to repair give A = 0.95 — is improved by redundancy of the critical systems.

Pumps and piping. Centrifugal pumps move large volumes of low-viscosity liquid — sea water for cooling, ballast and fire — and need priming; positive-displacement pumps (gear, screw and reciprocating) move viscous fuel and lubricating oil and are self-priming, which makes the reciprocating pump the classic bilge pump; ejectors use a driving jet. The ship’s piping systems include sea-water cooling from sea chests through strainers; the fresh-water cooling circuits; drinking water, made on board by a fresh-water generator that boils sea water under vacuum using the engine’s jacket-water heat, or by reverse osmosis, then sterilised and remineralised; hot water from calorifiers; and the fuel-oil and lubricating-oil transfer and service systems with their filters (often automatic back-flushing) and coolers (plate or shell-and-tube).

Centrifuges clean fuel and lubricating oil by spinning them in a bowl: a purifier separates both water and solids from the oil, with a water outlet and a gravity disc chosen to suit the oil’s density so that the oil–water interface sits in the right place; a clarifier separates solids only, has no water outlet, and is often run in series after a purifier. The bilge system collects drainage from the machinery spaces and holds through suctions with non-return valves; oily bilge water is processed by the oily-water separator, which uses gravity and coalescing elements and may discharge overboard only when the oil content is below 15 ppm, an oil-content monitor stopping discharge automatically above it. The ballast system fills and empties ballast tanks to control draught, trim and stability, and ballast water must now be treated to kill organisms before discharge. Sewage is treated biologically before discharge or held in a tank. Air compressors — multi-stage reciprocating machines with intercoolers — supply starting air at about 30 bar and service and control air. Heat exchangers, waste-heat recovery, and heating, ventilation and air conditioning (the plant of the previous chapter) complete the engine-room services.

Deck machinery and cargo handling. The windlass heaves the anchor cable over a gypsy (cable lifter); mooring winches and capstans handle the mooring lines, some with automatic tensioning; cranes and derricks handle cargo, and tankers discharge through cargo pumps — steam-turbine, electric or hydraulic, or submerged deep-well pumps in each tank. Propulsion and steering gear: the steering gear turns the rudder stock through an electro-hydraulic actuator — a ram type, in which hydraulic cylinders push a tiller, or a rotary-vane type, in which vanes on the stock turn inside a housing — powered by at least two pump units and controlled from the bridge by a telemotor or electrical follow-up system, with emergency steering available from the steering-gear compartment.