Ship Power Plant

8/2/2019 Ship Power Plant

1/141

Faculty of Engineering

Naval Architecture and Marine Engineering Department

MR351 - Ship Propulsion SystemsThird Year

Prepared By:Dr. Mohamed Morsy El-GoharyEng. Hossam Ahmed El-Sherif


2/141

Course Contents

SubjectPage

No.1. Introduction 1

2. Review of main machinery 6

3. Transmission system 43

4. Propulsors 60

5. Fuel types in marine field 71

6. Marine diesel engines 87

7. Unit conversion factors 137

References1. Marine Engineering, SNAME, 19922. Introduction to marine engineering 2nd ed., Taylor, 19963. Pounders marine diesel engines and gas turbines 8th ed., 20044. Design of propulsion and electric power generation systems, IMarEST, 2002


3/141

1Ship propulsion systems

Introduction

Ship propulsion system is that part of marine engineeringconcerned by the design and/or selection of main propulsion plantequipments and machineries. The main role of this plant is to

produce enough power to overcome the ship resistance and togenerate the needed electric power for the various applicationsonboard the ship (lighting, control systems, pumps, navigationequipments, HVAC, etc).

The above figure shows the main two forces considered inpropulsion system; the resistance of the water to the ship motion(R) and the thrust developed by the propeller (T). Whenconsidering only the engine room area, the various powers fromthe engine to the propeller are showed as follows.


4/141


Where:BHP = brake horsepowerDHP = developed horsepowerEHP = effective horsepowerGB = gearbox efficiency (1% ~ 3%)

shaft = shafting efficiency (1% ~ 2%)P = propeller open water efficiency (30% ~ 60%)

The following relations link these terms together:

EHP =R V

c

V = ship speed

c = units conversion constant

QPC =EHP

DHP

QPC = Quasi-Propulsive Coefficient

QPC = H RR P

H = hull efficiency = EHP/THP (THP = Thrust horsepower)RR = Relative Rotative efficiency

BHP =DHP

shaft GB

The thrust developed by the propeller is linked to the shipresistance by the following formula:

R = T . (1 t)

t = thrust deduction fraction


5/141


The thrust deduction fraction is a parameter related to the shipdesign and it is related to another parameter which is the wakefraction.

Example

Given the following ship particulars, find the required engine brakepower.

Ship speed V=20 knotsThrust=40 tonnesWake fraction =0.3Thrust deduction fraction t=0.6*Quasi-propulsive coefficient QPC=0.68Transmission efficiency t=0.95

Solution

t = 0.6 x 0.3 = 0.18R = T x (1 - t)=40 x (1 - 0.18) = 32.8 tonnes

EHP =R Vs

c

EHP =32800 20 0.514

75 EHP = 4496DHP=EHP/QPC=4496/0.68=6611 HPBHP=DHP/t=6611/0.95=6960 HP

BHP=6960


6/141


The main components of a propulsion system are shown on thenext diagram:

Prime mover:The function of the prime mover is to deliver mechanical energy tothe propulsor. The prime mover may be one of the following:

Diesel engine

Gas turbine

Steam turbine

Electric motor

The diesel engine is the most common prime mover in themerchant marine, mainly due to its low fuel consumption incomparison with other prime movers.

Gas turbines find their application in fast and advanced ship typesand naval vessels. The power to weight ratio of gas turbines ishigher than that of diesel engines.

Some ship types, such as naval vessels and LNG carriers may

have a steam turbine as propulsion engine. Two kinds of steamplants can be distinguished in marine applications: fossil-firedsteam plants and nuclear steam plants. Fossil-fired steam plantsare frequently found on board naval vessels and LNG carriers.Submarines and aircraft carriers may be equipped with nuclearsteam plants. Some commercial ice-breaking vessels especially inRussian arctic areas were provided with nuclear power plantssince these vessels may stay for months in sea.

Electric motors found their way as prime mover in the 90s; they

are used with electric generation plant combined of an engine (oneof the above types) and an electric generator. They are mainlyfound in advanced passenger ships, some new designs of offshoresupport vessels (OSV) are intended to use electric motorsespecially for dynamic positioning applications.

Prime Mover

(Power Plant)

Transmission Propulsor


7/141


Transmission:Transmission is a sub-system of the propulsion system. It is asystem itself built up from components such as shafts, gearboxesand bearings. The transmissions functions are:

1. To transfer the mechanical energy generated from the prime

mover to the propulsor2. To transfer the thrust generated by the propulsor to the

ships hull

The latter is done by means of a thrust bearing; a component thatis found in every transmission system.

Tow types of transmission are used:

Direct: the prime mover is coupled directly, through a shaft to

the propulsor (this is the case with low speed diesel engines) Geared: the prime mover delivers its energy through a

gearbox and a shaft to the propulsor. The function of thegearbox is to reduce the rotational speed of the engine tomatch the desired rotational speed of the propulsor.

Propulsor:The propulsor converts the rotating mechanical power delivered bythe engine into translating mechanical power to propel the ship.The most common propulsor is the propeller. In general, two types

of propeller are distinguished, fixed pitch and controllable pitchpropellers. Other types of propulsors are for example, waterjetsand Voith-Schneider propulsors (vertical axis propeller).


8/141


Review of main machinery

In this chapter we will make a brief review of the main types ofprime movers stated before; diesel engines, gas turbines, steamplants, electric plants. From now on, these types will be named

power plants, i.e. diesel power plant, gas power plant, etc, sincethe whole engine room arrangement is affected by the type ofprime mover installed.

Power plant concepts

The ships engine room may contain more than one type of primemovers, in this case the power plant will be called combined, andthis makes the basic types of power plants as follows:

Diesel power plant

Gas turbine power plant Steam power plant

Nuclear power plant

Combined power plant

1. Diesel power plant

1.1 Overview

The diesel engine is reciprocating internal combustion engine.

Diesel engines are used to drive cars, trains ships and othermarine structures, electric generators, pumps, compressors, etc.The diesel engine is still the most frequently used prime mover inthe merchant marine field. Power ranges between 0.25 MW for thesmallest high speed engines to 90 MW for the for the biggest low-speed engines.The main advantages of diesel engines are:

It is relatively insensitive to fuel quality; it can be operated bylight fuel as well as the heaviest residual fuels.

High reliability

High maintainability due to simple technology

High efficiency, can reach more than 50%

Low cost, in terms of initial and operational costs.


9/141


While the main disadvantages of diesel engines are:

Pollutant emissions

Low power to weight ratio if compared with gas turbine

Vibration and noise

From the application viewpoint, three main types of diesel enginesare available:

Low speed diesel engines (rpm


10/141


The engines shown in the previous figure are in-line engines, i.e.all cylinders are positioned on one line. 4-stroke engines can bebuilt with V-configuration or star configuration.

The diesel engines aspiration, i.e. the method of air admission into

the engine, can be done in two ways; natural aspiration orsupercharging.Naturally aspirated engines suck the atmospheric air in the suctionstroke without any additional assistance, while the superchargedengines are supplied by the air at higher pressure thanatmospheric by the aid of a compressor, when this compressor iscoupled to an exhaust driven turbine, the process of superchargingis called turbocharging.

Principle diagram of exhaust-driven turbocharging of a diesel engine.

Also, the diesel engine can be categorized by the method ofcooling; either by air or by water.

As a summary, diesel engines can be categorized by:

Speed

Construction

Configuration

Aspiration

Cooling


11/141


1.2 Arrangement options

When the diesel engine used is of the low speed type it is directlycoupled to the propeller without gearboxes since the engine speedcan be in the range suitable for the efficient running of the

propeller. In this case the propeller can be of the fixed pith (FPP)or the controllable pitch propeller (CPP). When FPP is used, theengine chosen has to be able to reverse its rotation direction forthe astern operation of the ship, but when the engine is not able todo so, the propeller has to be of the CPP type for performing theastern operation by changing the pitch angle of the propellerblades.

Low and medium speed diesel-based propulsion machinery options for a91 000 dwt tanker and associated auxiliary power generating (G) source

(Numbers over the engines are engines names from MAN enginemanufacturer)


12/141


Low speed diesel machinery arrangement

Ship Ship typeMain

engineShaftgen.

Prop.Aux.

engines

Tarquin LochLPG carrier

6270 m33590 kW170 rpm

400 kW CPP3x450 kW1800 rpm

Kanata SpiritOil tanker128993 m3

14700 kW105 rpm

- FPP3x720 kW720 rpm

Black Marlin

Heavy liftvessel

98500dwt9550 kW127 rpm

- CPP4x990 kW720 rpm

P&ONedlloyds

Southampton

Containership

6690TEU

66845 kW

100 rpm

3500 kW FPP4x3600

kW

600 rpm

Examples of ships with low speed diesel engines


13/141


Medium speed diesel machinery arrangement

Ship Ship typeMain

engineGearbox

Shaftgen.

Prop.Aux.

engines

MakiriGreen

Generalcargo

12000 dwt

7800 kW500 rpm

134 rpm 800 kW CPP3x485 kW1200 rpm

IsolaGialla

Chemical

tanker43157 dwt

8775 kW500 rpm 112.5 rpm 1800 kW CPP 3x880 kW900 rpm

MrsSonja

Generalcargo

4930 dwt

1800 kW750 rpm

175 rpm - CPP 2x210 kVA

Examples of ships with medium speed diesel engines


14/141


Diesel electric power plant is used today for many advantages likethe ability to change the position of the engine room to whereverpossible in the ship since no direct link between the engine and thepropulsor. The main components of a diesel electric power plantare:

Diesel engine Electric generator

Power cables for power transmission

Electric motor

Machinery arrangement in a diesel electric tanker1. Diesel genset 2. Switchboard 3. Propulsion motor

4. Stern thrusters 5. Cargo pumps 6. Engine control room


15/141


1.3 Applications

The diesel engine is much older than the gas turbine and in thedays of steam found its application in ships where the requiredpower was modest. Since 1950, the development of turbocharging

has resulted in a power increase in the order of 2 to 3 for a givencylinder volume. As a result, it is now possible to power even thelargest ships with diesel engines.Low speed engines are dominant in the mainstream deep seatanker, bulk carrier and containership sectors while medium speedengines are favored for smaller cargo ships, ferries, cruise liners,RORO freight carriers and diverse specialist tonnage such asicebreakers, offshore support and research vessels. The powerdensity of medium speed engines is higher than that of low speedengines, this results in lower weight and volume for a required

power. While high speed engines may be found in smaller units astugs, pilot vessels, fishing vessels, fast ferries, patrol boats etc,and as gensets in small and medium sized vessels.

Specific dataDiesel engines

Low speed Medium speed High speed

Cycle 2-stroke 4-stroke 4-stroke

construction crosshead Trunk piston Trunk piston

Output range [kW] 8000-90000 500-35000 500-9000

Fuel type HFO HFO or MDF MDF

SFC [g/kWh] 160-180 170-210 200-220

Spec. NOx emissions[g/kWh]

14-22 10-18 7-13

Specific mass [kg/kW] 17-60 5-20 2.3-6

Specific cost [Euro/kW] 400-420Line 220-330

V 170-280 V 180-240


16/141


Example of data from Rolls-Royce Bergen propulsion engine brochure


17/141



18/141



19/141


2. Gas turbine power plant

2.1 Overview

The first instance of naval propulsion using gas turbines was in

1947 in the UK using a Metrovick Gatric engine in a modified gunboat. This was based on the F2 jet engine but with a free powerturbine in the tail pipe and burning diesel. Sea trials lasted fouryears and convinced doubters that operation of a simple cyclelightweight engine at sea was practical. The major client for marinegas turbines these days is the naval forces worldwide.

Unlike the diesel engine, the gas turbine consists of rotatingcomponents only, so it can be categorized as a rotating machine.In marine applications, gas turbines are currently used in a range

of 4 to 30 MW.

A gas turbine module for marine applications GE LM2500

The gas turbine first superseded the steam installation as thepropulsion power for naval ships for the following advantages:

- Better efficiency- Fast starting-up time- Modular construction- Easy automation

- High reliability and maintainability

When compared to the diesel engine, the gas turbine has a highpower density, so it is a light compact piece of machinery. Thismajor advantage for vessels where space and weight are precious,has to be weighed against the following disadvantages:

- It has a low efficiency and high fuel consumption


20/141


- It needs higher quality fuel than diesel engines (recentdevelopments have started to solve this issue)

- It is more difficult to repair in situ because it has beendesigned for repair by replacements

2.2 Working principle

The above picture shows the basic components of a marine gasturbine. The energy conversion process in a basic gas turbine is asimple Brayton cycle: compression, combustion (heat addition),expansion and exhaust (heat rejection). In the rotatingcompressor, air is compressed, in one or two compressor sections,from atmospheric pressure to the combustion pressure, which is in

order of 10 to 30 bar. Fuel is injected in the combustion chamberand, after combustion at almost constant pressure, the hot gasesexpands to atmospheric pressure in the turbine. The turbinedelivers power to drive the compressor and load. The output speedis high; between 3000 and 7000 rpm, so if a gas turbine is used todrive a propeller, a reduction gearbox is required.

In basic applications the turbine that drives the compressor is alsoconnected to the load, the gas turbine is of the single shaft type.This type is used for generator drive in which case the shaft speed

is kept constant.

For direct mechanical drive of a propeller shaft, a gas turbine has aseparate turbine for the load: the power turbine. The compressor,the corresponding compressor turbine and the combustionchamber form a separate unit: the gas generator.


21/141


Specific data Gas turbines

Process Simple cycle Advanced cycle

Construction 2-shaft 2-shaft

Output [kW] 6000-26000 24000

Speed [rpm] 3600-7000 3600

Fuel MDF MDF

SFC [g/kWh] 240-280 200

Spec. NOx emissions [g/kWh] 2-5 3

Specific mass [kg/kW] 1.0-1.4 1.8Specific cost [Euro/kW] 180-280 470


22/141


2.3 Installation on board

Gas turbine installation of a fast ferry with water jet propulsion

Gas turbine installation of a navy frigate


23/141


Up-takes and down-takes of a gas turbine driven vessel

2.4 Applications

Many types of marine vehicles can benefit from the developmentsof gas turbines, as stated before the navy ships are the main clientof this type of power plants, however and due to the level oftechnology gas turbines reached, many commercial vessels havenow gas turbines installed onboard, one of the examples is thehuge cruise liner Queen Mary II as gensets and many other luxury

passenger ships adopted gas turbine gensets in the past fewyears. Beside the electric generation applications, gas turbinesfound their way to the propulsion of commercial vessels especiallyin the fast ferries market.


24/141


Two major companies produce marine gas turbines, GeneralElectric and Rolls-Royce.

Rolls-Royce General Electric

Spey SM1C WR 21 LM 1600 LM 2500

Power [kW] 19500 25240 14900 24000Speed [rpm] 5500 3600 7000 3600

SFC [g/kWh] 230 200 233 238

Weight [kg] 25700 46000 15440 22000

Dimensions [m] 7.5x2.3x3.1 8.0x2.7x4.8 6.5x2.3x3.0 8.3x2.7x3.0


25/141


3. Steam and nuclear power plant

3.1 Overview

The steam turbine has lost ground in the propulsion power

applications because it has low power density, lower fuel economythan diesel engines and high initial costs. Currently, a steamturbine plant may be used in naval vessels (aircraft carriers andsubmarines) and LNG carriers.

3.2 Components of the plant

A steam turbine plant consists of one or two boilers and a numberof turbines. Steam is generated in the boilers and then it expandsin the turbines. The boilers can be powered by any of a variety of

fuels (poor quality oil, coal, LNG) or by a nuclear reactor (navalapplications). Steam turbines rotate at high speed (~6000 rpm), soit cannot drive a propeller directly, only geared.

The steam turbine plant consists of:- Boiler(s) in which steam is generated by burning fuel or

nuclear reactor- Turbine(s) in which steam expands delivering power to an

output shaft, they may be connected to an alternator forelectric power generation (turbo-generator) or to a gearbox

for propulsion- A sea water-cooled condenser in which steam condenses towater that can re-enter the power cycle

- A pump which feeds water into the boiler

In large installations the boilers are water tube units. The walls ofthese boilers consist of tubes in which water is vaporized. Theboiler also contains oil-burners, so it requires inlet and exhaustducts for the air and exhaust gases. Marine boilers have apressure of about 40 bar, which corresponds to a vapor

temperature of 250C, and in case of superheating thetemperature reaches 450C.

The expansion may take place in several stages: a high, a mediumand a low pressure turbine, each connected to a gearbox. Also aseparate turbine for astern operation may be provided.


26/141


Diagram of a basic fossil-fuelled steam turbine plant

Schematic layout of steam turbine plant including an economizer, a reheaterand a superheater in the boiler (l=liquid, g=steam)


27/141


Ship(LNGCarrier)

Steamturbine

Boilers Turbo-alt Prop.Aux.

engines

HanjinMuscat

138366 m328610 kW 2x68 t/hr 2x3450 kW

FPP83 rpm

3450 kW

AmanSendai

18928 m3 5516 kW 2x17 t/hr 1450 kW FPP125 rpm

1450 kW

Two examples of vessel with steam plants

A steam plant may also be powered by a nuclear reactor instead ofoil-fired boiler. In merchant shipping nuclear options are notcommercially feasible, however, some icebreakers working in thefrozen waters in northern Russia have been designed with nuclearpower plants. For submarines the main military advantage is that

the nuclear reactor does not need air as does a boiler or acombustion engine, so they can stay below sea level for months.

In a nuclear installation the reactor adds heat to the primary watercircuit. The primary circuit is radioactive. The water/steam systemas found in conventional steam turbine plants is found here as thesecondary water circuit. The secondary circuit obtains heat fromthe primary circuit in a heat exchanger. Temperatures in thesecondary system are lower than those in a conventional steamsystem, so the thermodynamic efficiency of this system is even

lower.

Diagram of a basic steam turbine plant powered by a nuclear reactor(l=liquid, g=steam)


28/141


3.4 Installation


29/141


Steam power plant


30/141


Nuclear power plant


31/141


4. Combined power plants

Sometimes due to the ship functions a single prime mover may notbe suitable for the ship services, and the combined plant may be apreferred choice. In combined plants, two or more prime movers

are usually connected to the propulsor through a commontransmission system to take advantage of the desirable features ofeach prime mover.

Many combined plants configurations have been in use in severalapplications, e.g. CODAG, CODOG, COGAS, CODLAG, etc.

COmbined Diesel And Gas (CODAG)

The figure shows the CODAG concept for a fast ship. In thisconcept, the diesel engines and gas turbines each drive a waterjet.The jets driven by the diesel engines may be steerable and will beused for manoeuvring and low speed sailing. The jets driven by thegas turbines may be fixed and will be used to boost the ship tomaximum speed together with the diesel driven jets.


32/141


COmbined Diesel Or Gas (CODOG)

The above figure is a typical power plant for a navy ship. The two

diesel engines are usually high speed engines and they are usedfor the low speed operation. The gas turbines are the mainmachinery and they are used for full speed operation. Gas turbinesand high speed diesel engines are not reversible, thus CPPs areused.

COmbined Gas And Steam (COGAS)

This system is suggested to be used as an upgrade for shipspowered by gas turbines, the steam is generated by using the heat

in the exhaust of the gas turbine thus recovering some of the lostheat. This recovered heat can provide the plant by up to 25% of itstotal power with overall efficiency up to 55%.


33/141


COmbined Diesel eLectric And Gas (CODLAG)

This system is considered a hybrid system since it consists of bothmechanical and electrical drives. Mechanical propulsion power isdeveloped by two gas turbines, each through a gearbox connectedto FPP. Additionally electric motors, fed by diesel gensets, aredelivering propulsion power.


34/141


Engine room layout

Principal alternatives in the selection of the propulsion arrangement

The arrangement of machinery in machinery spaces is stronglyaffected by the selection and specification of the machinery and bythe overall ship design.

Number of machinery spaces

The number of machinery spaces mainly depends on thecomplexity and the extent of the machinery plant, the overall shipdesign and the required or desired level of separation ofmachinery. Identical machinery might need to be located inseparate spaces to provide redundancy, other equipment may beseparated to isolate fire or other casualties.The ships hull normally is divided to a number of watertightcompartments. If the space required by machinery exceeds the


35/141


space available in one compartment, another space or part of it willbe required to accommodate the machinery.

Redundancy and other safety considerations may also lead toseparate machinery spaces. For instance, the use of two

propulsion machinery spaces may ensure that even in case ofserious hull damage, resulting in one of the machinery spacesflooded or otherwise unavailable, part of the propulsion power maystill be delivered.

Machinery may also be located in separate spaces in order tocontrol damage by fire or explosion from adjacent machinery, or inorder to contain dirt, noise or heat. Also, classification societies,national authorities and international organizations may requiremachinery to be located in separate spaces. Emergency generator

room and the steering gear room are two examples of machineryspaces required to be separate by regulations.

For a small cargo vessel, machinery spaces include:- Main machinery space (propulsion engine, gearbox,

transmission, gensets, auxiliaries)- Steering gear room- Workshop- Control room (usually contains also the main switchboard)

For a cruise vessel or a naval vessel, machinery plant is moreextensive:- One or two propulsion engine rooms- One or two diesel generator rooms- One or two main switchboard rooms- One or two chiller rooms- Separate room for separators- Pump rooms for FiFi (Fire Fighting) equipment- Machinery control room- Workshops

Location of machinery spaces

The location of machinery spaces also depends strongly on thecomplexity and the extent of the plant and the overall ship design.For a simple machinery plant of a small cargo vessel or a large


36/141


tanker the main machinery space is usually located in theaftermost hull compartment. The transmission system then is shortand compact, auxiliary systems are also compact and themachinery does not interfere with cargo spaces.

The shape of the aftship, and consequently the width available forthe m/c (machinery) space may not be suitable for multiple engineor configurations. Multiple engine configurations with mechanicaltransmission require that the m/c space is low and wider than theaftermost compartment. Consequently, the engine room is usuallylocated at 1/3 or the ships length from aft.

In case of electrical transmission only the electric motors need tobe close to the propulsors. Gensets may be located at anyconvenient place in the ship; sometimes low in the ship to avoid

noise and sometimes high enabling access for maintenance andreplacement.

Dimensions of machinery spaces

The dimensions of the m/c spaces are dictated by the volume ofthe m/c in the space and clearances necessary for maintenanceand overhaul.The length of the engine room is limited by the damage stability

criteria and also affected by the main engines length.The main engine room may occupy the full width of the ship at thatlocation, other spaces like generators room, chiller room mayoccupy only a part of the width.The height of the machinery spaces is also determined by thedimensions of the machinery. But usually the full height, frombottom to main deck, is occupied. On the other hand, the design ofa RORO ferry may limit the height of the engine room.Consequently, the ship layout dictates the use of smaller enginesto accommodate more car decks above the machinery spaces.

Arrangement of the equipment

The arrangement of the equipment inside the machinery spacefollows a limited number of considerations:


37/141


- Mechanical drive propulsion plant is located in such a waythat it can be connected to the propulsors.

- Auxiliary equipment is located in the vicinity of the mainequipment it has to support. This reduces piping and cabling.

- Some equipment, like the sea water cooling pumps, bilgepumps and fuel and lube oil pumps need to be located lowin the machinery space.

- Some equipment, like cooling water expansion tank, sterntube lubrication tank, ventilation fans and exhaust gas boilersneed to be located high in the machinery space.

- Many equipment, like chillers, hydraulic equipment,

compressors, boilers and switchboards do not have strictlocation requirement. Considerations may be location ofweight and centre of gravity and vicinity of consumers.

- Addition spaces should be available for access, control,monitoring and maintenance

Ship typePropulsionpower [MW]

Auxiliary power [MW]

Hotelservices

Operationalpower

Total aux.power

Container ship( 7000 TEU) 50.0 70.0 2.5 5.0 3.5 6.5 6.0 11.5

Container ship( 300 TEU)

2.0 5.0 0.3 1.0 0.3 1.0 0.6 2.0

General cargo 5.0 10.0 0.5 1.0 0.5 1.5 1.0 2.5

Stern trawler 5.0 10.0 0.5 1.5 2.5 5.0 3.0 6.5

Beam trawler 0.5 1.5 0.1 0.3 0.1 3.0 0.2 3.3

DP Semi-Sub 7.5 12.5 1.0 3.0 1.5 3.0 2.5 6.0

Dredger 15.0 25.0 1.0 3.0 12.0 14.0 13.0 17.0

Cruise ship 15.0 40.0 0.6 2.0 3.0 8.0 3.6 10.0


38/141


Noise in machinery spaces

Noise is unwanted sound. It is also a pervading nuisance and ahazard to hearing, if not to health itself. Noises are calibrated withreference to a sound pressure level of 0.0002 dynes/cm2 for a pure

1000 Hz sine wave. A dyne is the force that gives 1 g anacceleration of 1 cm/s2. It is 105 newtons. A linear scale givesmisleading comparisons, so noise levels are measured on alogarithmic scale of bels. (1 bel means 10 times the referencelevel, 3 bel means 1000 times the reference level, i.e. 103.) Forconvenience the bel is divided into 10 parts, hence the decibel ordb. This means that 80 db (8 bel) is 108 times the referencepressure level. For practical purposes, sounds are measuredaccording to a frequency scale weighted to correspond to theresponse of the human ear. This is the A scale and the readings

are quoted in dBA.

Any prolonged exposure to levels of 85 db or above is likely to leadto hearing loss in the absence of ear protection; 140 db or above islikely to be physically painful.

Lining as much as possible with sound-absorbing materials doestwo things:

1. It reduces the echo, so that moving away from the enginegives a greater reduction in perceived noise.

2. It tends to reduce resonant vibration of parts of the shipsstructure, which, by drumming, add to the vibration and noisewhich is transmitted into the rest of the ship.

Anti-vibration mounts help in the latter case also but are notalways practical. The only other measure which can successfullyreduce noise is to put weight, particularly if a suitable cavity can beincorporated (or a vacuum), between the source and the observer.A screen, of almost any material, weighing 5 kg/m2 will effect areduction of 10 db in perceived noise.


39/141


Typical noise sources in an engine room


40/141


IMO Noise Limits (Sound Pressure Level) in dB(A)

Workspaces

Machinery spaces (continuously unmanned)* 90

Machinery spaces (not continuously manned)* 110

Machinery control rooms 75

Workshops 85

Unspecified workspaces* 90

Navigation spaces

Navigating bridge and chartrooms 65

Listening posts (including bridge wings and windows) 70

Radio rooms (with radio equipment operating but not producing audio

signals) 60

Radar rooms 65

Accommodation spaces

Cabins and hospital 60

Messrooms 65

Recreation rooms 65

Open recreation areas 75

Offices 65

* Ear protectors should be worn when the noise level is above 85 dB(A), andno individuals daily exposure duration should exceed four hours continuouslyor eight hours in total.


41/141


Simple torsional vibration calculations

The shafting arrangement of the ships propulsion system can beregarded, from the vibration point of view, as a two rotorarrangement. This is the simplest method to consider the system

and facilitate the calculations.

The purpose of the calculations is to find the natural frequency oftorsional vibration of the shafting system and also to find theposition of the node in this system. The natural frequency of thesystem must be avoided, if the system rotates with the samefrequency resonance occurs and shaft failure become a risk. Thenode of the system is the point which can be considered fixed, theamplitude of vibration changes sign at this point.

Consider the above system, two rotors having moment of inertia I1and I2 [kg.m

2] respectively. The two rotors are connected with ashaft having length L [m] and stiffness K [N.m/rad]. This system isthe simplest for modeling the propulsion system where one of therotors is the engine, including the cylinders and the flywheel, and

the other is the propeller.

The natural frequency n [rad/s] of this system is given by:

n2 = K

I1 + I2

I1 I2


42/141


The critical shaft speed Ncr is then:

Ncr = 30

To find the node position, we have to consider the fixed point as aseparator of two single shaft system, since the amplitude at thispoint is zero. Each single shaft system has a natural frequency of:

n2 =

Ki

I i The shaft connecting the two rotors is now split to two shafts withL1 and L2 as length and K1 and K2 as stiffness. If we find the ratiobetween the two lengths and knowing that the total length L is the

sum of the two shafts lengths, then we can determine the nodeposition.

The shaft stiffness K1 is given by:

K1 =G J

L1 Where G is the torsional modulus of rigidity [GPa] and J is thepolar moment of inertia [m4] of the shaft. The product G.J is thesame for the two parts of the shaft, the same shaft. Thus the

following can be derived:

K1

K2=

L2

L1


43/141


The following empirical formulas are given for the propulsionsystem:

Diesel engine moment of inertia IE:

IE = 5160

BHP

rpm

Propeller moment of Inertia IP:

IP =1.25 Weight 1000

980

0.41 Diameter

2

2

The internal natural frequency of diesel engines:

n = Z rpmi 60


44/141


Example 1

A free rotating shaft carries a flywheel with I1 = 2 kg.m2 at one end

and I2 = 4 kg.m2 at the other.

The shaft connecting them has a stiffness of 4 MN.m/rad.

Calculate the natural frequency and the position of the node.

Solution

n2 = K

I1 + I2

I1 I2 n = 1732 rad/sNcr= 16542 rpm

n2 =

Ki

I i K1 = 6x10

6

K2 = 12x106

L2 / L1 = 0.5

Then the node is located at 2/3 the shaft length from the first rotor.


45/141


Transmission system

The transmission system is located between the prime mover andthe propulsor. Its main function is to convert or transmitmechanical energy. The transmission system transmits (1) thetorque generated by the prime mover to the propulsor, and (2) thethrust generated by the propulsor to the hull.

Transmission components in a direct drive

The following components can be distinguished from the abovefigure:

- One or multiple line shaftstransmit the torque generated bythe engine, and they transmit thrust if located behind thethrust bearing. The shaft sections are connected to each

other with flange couplings.

- The thrust bearing and the thrust shaft (with thrust collar)transmit the thrust, generated by the propeller, to the hull.The thrust bearing may be independent of the engine, butmostly is integrated in the engine.

- The shaft bearingssupport the weight of the shafts.

- The propeller shaftconnects the shafting system inside theship with the propeller.

- The stern tubeguides the propeller shaft through the hull. Inthe stern tube, the shaft is supported by one or two oil-lubricated bearings: the aft and forward bearing. Thesebearings carry the shaft and propeller weight, and also thetransverse hydrodynamic load acting on the propeller.


46/141


- The forward stern tube seal assures that the lubrication oilstays within the stern tube.

- The aft stern tube seal has two functions: to keep thelubrication oil in and to keep sea water out.

- Where the shaft line passes through a bulkhead, a bulkheadstuffing boxassures that the bulkhead stays watertight.

In more complex power plant configuration such as in geared drivewith one or more prime mover, some additional components maybe encountered.

- The gearbox is installed in order to reduce the speed of theengine to the speed required for efficient operation of the

propeller. Reduction can be achieved in one or two steps: inone step for medium and high speed diesel engines (1:2 to1:6) and in two steps for gas turbines and high speed dieselengines (1:10 to 1:35). The thrust bearing is usuallyintegrated with the gearbox or installed close to the gearbox.

- A clutchis used to connect or disconnect the engines to theshaft line. It is often included in the gearbox, but sometimes itis integrated with an elastic coupling.

- The elastic coupling has two functions: (1) it improves thetorsional behavior of the installation, and (2) itaccommodates inaccuracies of shaft alignment andmovements of the engine relative to the gearbox.

- The stern tube bearingmay be water lubricated instead of oillubricated. In that case, only one stern tube seal will benecessary to prevent sea water from entering the ship.

- The propeller shaft is situated behind the ship in the water. It

is supported by the strutand water lubricated strut bearing just before the propeller. Due to its shape this strut is oftenreferred to as an A-bracket.

- A muff coupling connects the propeller shaft and the sterntube shaft. This coupling does not require flanges at the end


47/141


of the shaft, so it enables removal of the shafts through thestrut bearing or the stern tube.

Transmission components of a twin screw geared drivewith two diesel engines per propeller shaft


48/141


Transmission components

Propeller shaft

In general shaft are made of forged (mild steel). Sometimes high

tensile steel, or alloys such as stainless steel are used and may beof composite materials. Most often shafts are solid, but they mayalso be hollow for example when light shafts are required inpassenger vessels or naval vessels or when CPPs are used.

Approximate composition Material properties

0.2 - 0.5 % C

0.4 0.9 % Mn


49/141


- Higher maintenance costs- Applicable for shaft diameters up to 600 mm

Self aligning sleeve bearing

Roller bearing in a line shaft bearing (left) and in a thrust bearing (right)


50/141


Thrust bearing

The thrust bearing converts the mechanical energy in the rotatingshaft into translating mechanical energy to propel the ship. Thethrust bearing has to transfer thrust to the hull while sailing both

forward and astern.

Michell type thrust bearing

In a Michell type thrust bearing, the thrust is transferred through

the thrust collar on the thrust shaft to tilting pads that aresupported by an oil film. The bearing capacity of this type lies inthe range of 2 to 3.5 N/mm2 on the pads.

Stern tube

In general, two types of stern tube can be distinguished:- Stern tube with oil lubricated bearings- Stern tube with water lubricated bearings

Water lubricated bearings are rarely applied in merchant vessels.In naval vessels the stern tube bearings and the A-bracket bearingare sometimes water lubricated. In that case, the shaft is fitted witha bronze sleeve for protection against corrosion by the sea water.The bearings will consist of a bronze bearing bush on which thebearing material, rubber or synthetic material, will be mounted.In case of oil lubricated stern tube bearings, the shaft does notneed to be protected against corrosion because the stern tube is


51/141


filled with oil from a tank. This tank is located 3 to 5 m above thewaterline and ensures a slight overpressure relative to the seawater pressure. The bearing bush is often of cast iron and theinner surface of the bush is centrifugally cast with white metal.

Oil lubricated stern tube seals and bearings

The stern tube will require two seals: the aft seal and the forwardseal. The aft seal shown in the next figure includes three lip seals:

two water repellent lip seals to keep water out and one oil repellentlip seal to keep oil in. The forward seal has two lip seals, both tokeep oil in the stern tube.


52/141


Standard SUPREME aft (left) and forward (right) seal

Flexible coupling

To reduce vibration in a system to an acceptable level, flexiblecouplings need to be fitted. In a geared drive, these couplings arefitted between the engine and gearbox to allow some misalignmentand to control the torque variations within the system. An elastic

coupling introduces a low stiffness, thus reducing the naturalfrequencies of the system. Also, they may have good dampingquality thus reducing the amplitude of the torsional vibrations.

Rubber elements are not the only solution to effectively damptorsional vibrations. Instead of rubber elements a coupling mayalso use elastic leaf springs combined with oil displacementdamping (hydrodynamic damping). The springs themselves have astiffness, and the oil, while moving from one oil chamber toanother, is subjected to resistance, which retards the movement of

the outer part relative to the inner part of the coupling.


53/141


Vulkan RATO-S coupling with rubber elements and a membrane

Geislinger Elastic Damping Coupling with leaf springsand hydrodynamic damping by oil displacement


54/141


Low speed engines have a rigid foundation, but it is commonpractice to mount medium and high speed engines resiliently.Vibration absorbing mounts, usually of rubber material, reduce thetransmission of hull borne noise originating from the engine to thehull. If resilient mounting is applied, the elastic coupling should be

able to absorb the displacements of the engine that result from thisconfiguration. The engine will be moving in reaction to the enginetorque and the ships motions. To accommodate the enginemotions the above mentioned couplings are often not sufficient, sospecial arrangements need to be made, for example:

- Two elastic couplings in series with an intermediate shaft- A coupling with flexible elements in series with an elastic

coupling.With these solutions radial displacements up to 50 mm may beabsorbed.

Highly flexible RATO-S coupling with 2-row element for articulated driveshafts


55/141


Geislinger Flexible Link in series with a leaf spring elastic coupling:the flexible links are shown in section A

Clutches

If a ship is equipped with one shaft line and two or more engines,the need arises to connect and disconnect engines to the shaft linein order to sail with one or more engines. This is the task of aclutch. They are either pneumatically or hydraulically actuated.

The next figure shows an air actuated clutch integrated with anelastic coupling. The connection between input and output shaft isestablished by compressed air forcing the inner ring of the drum tomove into contact with the drive.

In a plate type clutch the input shaft has a hub with steel pressureplates at its extreme end. When the input shaft has to beconnected to the drive, the pressure plates and the clutch platesare moved into contact. The clutch plates are connected to theclutch spider and the pinion.


56/141


Combination of an elastic coupling and an air actuated clutch

Diagram of a plate clutch integrated in a gearbox


57/141


In configurations with a steam or gas turbine a self-shifting-synchronous (SSS) clutch will often be used. This a teethed clutchwhich engages automatically when input and output speeds aresynchronized.

Hydraulic or fluid couplings combine the clutch function and thevibration attenuation function of a flexible coupling. In a hydrauliccoupling the input shaft delivers kinetic energy to oil, and the oilwill transfer the kinetic energy to the output shaft. The clutchoperates smoothly and no wear will take place between the shafts.

Gearboxes

Basically, marine gearboxes consist of meshing teeth on pinionsand wheels, which transfer power from a drive shaft (primary) to a

driven shaft (secondary) and reduce speed: [i = nengine/npropeller].Three configurations will be discussed; parallel, locked train andepicyclic.

Parallel configurations consist of pinions and wheels with teeth onthe periphery. Single and double stage reduction systems areused. In single gears, the diesel engine drives a pinion with a smallnumber of teeth. This pinion drives the main wheel that is directlycoupled to the propeller shaft. The double reduction systems aremore usual for turbine drives. In a double gear, the prime mover

would drive the primary pinion, which drives the primary wheel.The primary wheel is connected by a shaft to the secondary pinion,which drives the main wheel.

A special type of the double gear has a quill shaft with a PTO(power take-off) for the drive of, for instance, a generator. Thecombination of multiple disc couplings and quill shafts makes itpossible to use the engine to drive only the PTO shaft or only thepropeller shaft or both shafts. A quill shaft consists of a hollowshaft through which another shaft is led.

Marine gears are often of the double helical type which meansthey have two sets of helical teeth in opposite direction on thesame wheel or pinion. A single set would produce a resulting axialforce, the double set balances out the axial force.


58/141


Single input, single output gear

Double input, single output gear


59/141


Double input, single output and PTOShown are one of the input shafts and the PTO

Schematic layout of the gear transmission system (starboard side)of a CODOG propulsion plant

The above figure shows the schematic layout of a geartransmission system for a CODOG propulsion plant of a navalvessel. It shows the input line for the diesel engine, which drives a

pinion with double helical teeth through two clutches. The dieselengine is provided with a single stage reduction. The two clutchesare installed in series. The first is a fluid coupling with oil. Thesecond is self-shifting-synchronous (SSS) clutch.


60/141


The gas turbine input line is also provided with an SSS clutch. Thegas turbine needs a higher reduction ratio and consequentlyprovided with two reduction stages.Because of the high torque to be transmitted, the gas turbinepower is split over two parallel gear trains. The gas turbine input

pinion meshes with two with two intermediate gear wheels, whichshould transmit 50% of the torque each. The intermediate gearwheels are connected by intermediate shafts to secondary pinions,which mesh with the main gear wheel. This type of geartransmission is called a locked train.

Planetary gear (epicyclic transmission gear)


61/141


In an epicyclic system, one or more wheels travel around theoutside or inside of another wheel whose axis is fixed. They arereferred to as planetary, solar and star gears. The next figureshows an example of this type of gears. Note that the input andoutput shafts are in-line. The wheel on the principal axis is called

the sun wheel. The wheels whose axis revolves around theprincipal axis are the planet wheels. The internal teeth-gear thatmeshes with the planet is called the annulus. The differentarrangements of fixed arms and the sizing of sun and planetwheels provide a variety of different reduction ratios.


62/141


Propulsors

The screw propeller is the most common propulsor, but there areother types used like the waterjet and the Voith Schneider.

The screw propeller

A propeller generates thrust by means of lift on the blades thatrotate at an angle of attack relative to a flow. The geometry ofblades is very important in light of efficiency and cavitation.

The propeller consists of blades and a hub or boss. Theconnection between hub and blades is the fillet area or the bladeroot. If a ship is viewed from aft, the side of the blades facing aft isthe face or the pressure side, whereas the side facing the ship isthe back or the suction side.

A propeller is said to be right-handed if viewed from aft thepropeller rotates clockwise during sailing ahead. The edge of theblade facing the flow of the water when rotating is the leadingedge. The flow leaves the blade at the trailing edge.

The propeller pitch is the distance that a propeller theoretically(without slip) advances during one revolution. The pitch anglevaries with increasing radius. For calculation purposes, a nominalpitch is defined; it is the pitch at 0.7 of the radius.

Sketch of a screw propeller


63/141


Two other properties of the propeller blade are rake and skew.Rake is the distance between the blade and the propeller plane ata certain angle. A backward rake, increasing the tip clearancewhen fitted behind a ship, is a positive rake. A propeller is skewedwhen the tip of the blades is shifted in relation to the blade

reference line.

Propeller terminology

Ship propellers may have from three to six similar blades, the

number being consistent with the design requirements. It isimportant that the propeller is adequately immersed at the servicedrafts and that there are good clearances between its workingdiameter and the surrounding hull structure.

The screw propeller may be either fixed pitch or controllable pitchpropeller.The pitch of the FPP, although not constant along the radius of theblades, is fixed in any point, since the blades are rigidly attached tothe hub. The amount of thrust developed by the propeller is

controlled by the rotational speed of the propeller. Stopping andreversing the ship require special measures: it must be possible tochange the direction of rotation of the propeller in either thegearbox or the engine.


64/141


7.15m diameter, 6-blade, highly skewed FPP, nickel-aluminum-bronzepropeller loaded for transport

A CPP consists of a hub with the blades mounted on separately,so that they can rotate, thus changing their pitch. The shaft ishollow and contains a control system, mainly hydraulic, that canadjust the pitch angle of the blades. Adjusting the position of theblades changes the angle of attack in the flow, thus changing thethrust without changing the rotational speed. This has majoradvantages with respect to manoeuvrability of the ship. On theother hand, the disadvantages are a larger hub, restrictions to theblade design, slightly lower efficiency, more complicated andexpensive.

CPP is used instead of FPP for one of the following reasons:- To improve the low speed manoeuvrability of the ship- To adapt the load characteristics to the drive characteristic- To generate constant frequency electric power with a shaft

generator

The next figure shows how the propeller blades are attached to thehub and the controlling mechanism of the blades. The propellerblade (1) is connected to the crank ring (2) by means of bolts (3)

through the blade root and the crank ring thus enclosing a part ofthe hub body (4). All hydrodynamic forces on the blade aretransmitted to the hub through the bearing between this system offlanges and the hub. The moving cylinder (5) is sealed off at thefront by the propeller shaft (6) and at the rear by a cylinder, whichis part of the hub cap (7). To rotate the blades, there is a three,four, or five sided crosshead, depending on the number of theblades on the outside of the cylinder (5). At each side of this


65/141


crosshead there is a so called Scotch-yoke that is connected to thecrank ring by means of an alignment pin, thus transforming thelongitudinal motion of the cylinder into a rotating movement of theblades.

Cross section of a CPP hub

Controllable pitch propellerinstalled on a ship


66/141


Screw propellers can be fitted in several configurations accordingto the type of the ship and the area in which she is operated.

Ducted propeller (kort nozzle)

Complete propulsion arrangement consisting of the engine, the transmissionand a steerable ducted propeller

Side view of the steerableducted propeller


67/141


Twin screw tug boat with kort nozzles (non-steerable)

Contra-rotating propeller

Azimuthing contra-rotating

propeller


68/141


Contra-rotating propellers one on each side of the strut

Contra-rotating propellers each on a shaft

Contra-rotating propellers with two concentric shafts


69/141


Podded propeller

This configuration incorporated an electric motor in a cylindermounted outboard, it steerable and save lot of space required bythe motor, only the genset and the steering device is inboard

Podded propeller

Cross section showing the principle of podded propellers


70/141


The waterjet

For high speed crafts the waterjet is often attractive, light andefficient solution. A waterjet mainly consists of a water inlet

channel, a pump that accelerated the water and a nozzle. In anideal waterjet, the thrust developed is equal to the change invelocity over the pump times the mass flow:

T m V= & The water inlet is located in the bottom of the ship and the outletnozzle in the ships stern, either just under or just above waterlevel. Behind the nozzle, in the stream of the water at the outlet, asteering and reversing bucket is mounted, which is controlled byhydraulic rams.

Advantages of the waterjet are:- No underwater appendages, so suitable for draft restricted

units- Low weight- No reverse gear required- No long and complex transmission line

A typical cross section of a waterjet


71/141


Voith Schneider propeller

A Voith Schneider propeller consists of number of vertically placedfoils underneath the ship. It offers excellent manoeuvrability andlow noise and vibration. These propellers are found in ships in

which accurate propulsion and steering are the main functions,such as tugs, mine hunters, ferries and floating cranes. Thepropeller blades rotate along a circle and around their own verticalaxis in such a manner that thrust is generated. Thrust can beproduced in any direction.

An example of a tug with Voith Schneider propeller

Voith Schneiderpropeller


72/141


73/141


Fuel types in marine field

The fuels used in marine combustion engines, gas turbines andoil-fired boilers are fossil fuels. The properties of fossil fuels aremainly determined by their chemical structure. These properties fixthe ratio of carbon to hydrogen atoms (C/H ratio) which isimportant for many properties such as density, viscosity,stoichiometric ratio and heating value.

For many years the British Standard Specifications were usedwhen buying fuels, also CIMAC (Conseils International desMachines Combustion) has been publishing recommendationsregarding fuel requirements for marine and stationary dieselengines since 1982.In 1987, International Standard ISO 8217 was issued concerningmarine fuels and has replaced the national standards. For gasturbines marine gas oil (MGO) or jet fuel (JP-5) are used.

Products ISODistillate products

- Gaseous fuelsMethanePropaneButane

- Light fuelsGasoline (petrol)KeroseneGas oil (GO), bunker gas oil, marine gas oil

- Diesel fuelsMarine gas oil (MGO)Light diesel fuel oil (LDF or LDO)Marine diesel fuel oil (MDF or MDO)Blended marine diesel fuel oil (BMDF)

(light distillate oil blended with up to 20%residual oil)

- Lubricating oil

DMXDMADMBDMC


74/141


Residual products

- Intermediate fuel oils (IFO)Also referred to as Light Marine Fuel Oil or Thin Fuel Oil(TFO)

(residual oil blended with up to 40% distillate oil)

- Heavy fuel oil (HFO)Also referred to as Marine Fuel Oil (MFO), Bunker FuelOil (BFO) or Bunker C

RMA toRMH

RMH toRML

Marine fuels and their ISO designation

The ISO standard uses the DM (Distillate Marine) and RM(Residual Marine) type designation. Additionally, the standardspecifies for every fuel type the density at 15C, the kinematicviscosity at 100C, the flash point, the pour point, the carbon

residue and the ash, water, sulphur, vanadium and aluminiumcontent.

Fuel properties definitions

Density

The density of fuel oils is normally less than that of water, althoughfor heavy fuel the difference may be very small. It is an importantparameter for transport and storage, and for the selection of the

method of purification. According to the ISO standard the densityshould be determined at a reference temperature of 15C. Thelimits of density for fuels used in marine sector are min. 840 kg/m3,max. 1010 kg/ m3.

Viscosity

The viscosity of a fuel is a measure of resistance of the fuel to flowat a quoted temperature. The viscosity used to be given inRedwood, Sayblot and Engler units at degrees Fahrenheit (100F)

but these units are now obsolete. With metrification it becamekinematic viscosity in centistokes (cST=10-6 m2/s). for distillatefuels the reference temperature is 40C and for residuals 50C.According to the ISO standard the kinematic viscosity of heavyfuels should be specified in cSt at 100C.For pumping the viscosity should not exceed 500 cSt, forseparating in a centrifuge 40 cSt and for injection 15 cSt.


75/141



76/141


Heating value (calorific value)

The heating value, defined as the amount of heat that is releasedduring combustion of 1 kg of fuel. Assumed that after combustion

the water content in the fuel is present as vapor, the condensationheat is not included in the heating value and it is referred to as thelower heating value

Fuel Lower heating value (kJ/kg)Gasoline 44000

MDF 42000

HFO 40500

Ignition properties

One of the older methods to measure ignition properties is theCetane number. The ignition properties of the fuel underconsideration are compared with a blend of:

- Cetane (CT) which has very good ignition properties- Heptamethylnonane (HMN) which has very low ignition

quality)CN = %CT + 0.15 . %HMN

Cetane index (CI) which is calculated from the density and the

mid-boiling oint (temp. at which 50% of the fuel is evaporated) is acalculated Cetane Number used to indicate the ignition propertiesof distillate fuels.

For the ignition quality of residual fuels two empirical measureswere developed: the calculated ignition index (CII) and thecalculated carbon aromaticity index (CCAI).

Carbon residue

This is a measure of the tendency to form carbon deposits in thecylinder, particularly near the exhaust valve. It is the amount ofcarbon that remains after heating a fuel in a Ramsbottom orConradson test apparatus. The carbon residue for distillate fuels islow (0.25%) on a mass basis, but for residual fuels it can reach22%.


77/141


Ash content

The ash content is the amount of inorganic materials such asmetals and metal oxides.

Aluminium content

It is the remnant from the catalysts added during the refiningprocess and thus a measure of the number of catfines (smallparticles that can cause abrasive wear. The limit is 30 mg/kg.

Vanadium content

Vanadium will form vanadium pentoxide V2O5, which is highlycorrosive and below 675C can form deposits in the cylinder and

on the exhaust valves.

Cloud point

The temperature at which paraffin (wax) crystals will begin to form.This is not wanted particularly during storage.

Cold filter plugging point (CFPP)

The paraffin crystals can obstruct the flow through filters and

narrow flow areas. The CFPP is the temperature below which it isnot possible to pump the fuel through a 45 micron filter.

Pour point

The temperature at which so many paraffin crystals are formedthat the fuel is hardly liquid. Together with viscosity, it is a measurefor pumping.

Flash point

The temperature at which it is possible to ignite the fuel vaporabove the fuel with a small lighter. For safety reasons, theflashpoint of fuel stored onboard ships must be higher than 60C.


78/141


Brome number

A measure for the mixing capability and storage stability of thefuel.

Sulphur content

The sulphur content is highly dependent on the source of the crudeoil. It lowers the heating value and after combustion forms sulphuroxides, which are a major exhaust emission pollutant. The sulphurcontent of heavy residuals is normally around 3%, IMO limit is4.5%. For lighter distillate fuels, European legislation limit is 0.2%.At low temperature, H2SO4 may be formed in the exhaust gases, itis highly corrosive so the exhaust temperature should not dropunder 120C.

Water content

If water emulsions are proposed as a measure for exhaustemission reduction (NOx), the water in the mixture should bedistilled. Any foul water, from the storage tanks, must be avoidedin a diesel engine.

Calculating the air/fuel ratio

The air/fuel ratio is the mass of air needed for the completecombustion of 1 kg of fuel, it is called the stoichiometric air fuelratio.

For a specific hydrocarbon fuel, two methods can be adopted torepresent the fuel contents: the chemical formula or constituentspercentages. The chemical formula is the fuel chemical symbol,e.g. C8H8, while for constituents percentages, the percentages ofcarbon, hydrogen, sulphur, nitrogen, ash and water.

In the first method, the following chemical formula is used tocalculate the amount of oxygen for burning 1 kg of fuel:

CmHn + (m + n/2)O2 (m)CO2 + (n/2)H2O


79/141


The molecular weight of the main elements must be known

Element Molecular weightCarbon (C) 12

Hydrogen (H) 1Oxygen (O) 16

Sulphur (S) 32

When constituents percentage method is used the oxygen neededfor combustion of each constituent is calculated and then summedup to get the oxygen required for the fuel.

The final step is to calculate how much air is needed for thecalculated amount of oxygen, by mass air contains 23% oxygen,so the amount of oxygen is divided by 0.23.

Example 1

Find the stoichiometric air/fuel ratio for C8H8

Solution

C8H8 + 10 O2 8 CO2 + 4 H2O

C8H8 = 8(12) + 8(1) = 104

10 O2 = 10 x 2(16) = 320

For 1 kg of C8H8, 320/104 = 3.077 kg of oxygen are needed

The mass of air then is equal to 3.077/0.23 = 13.37

Then the air/fuel ratio for C8H8 is 13.37


80/141


Example 2

Find the stoichiometric air/fuel ratio for a fuel with the followingcomposition

Fuel contentsCarbon 87 %

Hydrogen 10 %

Oxygen 1 %

Sulphur 1 %

Water 1 %

Solution

C + O2 CO212 + 32 44

1 + 2.67 3.67

H2 + 1/2 O2 H2O2 + 16 181 + 8 9

S + O2SO232 + 32 641 + 1 2

Weight of O2 = (2.67)(0.87) + (8)(0.1) + (1)(0.01) - 0.01 = 3.1229Kg

Carbon hydrogen sulphur oxygen

The oxygen content in the fuel must be subtracted from theamount of oxygen needed as the amount in the fuel will be used incombustion

Weight of air = 3.1229/0.23 = 13.58

Then the air/fuel ratio for the given fuel is 13.58


81/141


Fuel treatment

Before the fuel is burnt in a diesel engine, a gas turbine or a boiler,the fuel needs to be treated after bunkering.

Bunker tanks for storage of heavy fuel oils onboard ships must beheated since otherwise pumping to the settling tanks will not bepossible. A temperature of 5C above the pour point is usuallysufficient. In general the temperature is kept at about 35C.

The settling tank is the first step in the fuel cleaning process.Water and sediments can be segregated by gravity. The tank mustbe sufficiently high and preferably tapered to the bottom. Formodern heavy fuels the settling tank must be heated to 50 to100C to increase the rate of separation.

After settling, fuel treatment of distillate fuels may only consist of afilter if virtually no water is present. If water is expected acentrifuge and a filter will be fitted to remove any water. In case ofmore stringent requirements a centrifuge and a coalescer filtermight have to be installed.For residual fuels, the treatment is more complex: in addition to thesettling tanks and filters, centrifuges will be installed to separateparticles (clarifier) and water (purifier) from the fuel.


82/141


Clarifying centrifuge arrangement

Purifying centrifuge arrangement


83/141


Emissions

The emissions found in the exhaust gases are directly linked to thecombustion of fossil fuels. The more important is the pollutantemissions. They may be gaseous (SOx, CO2 NOx) or solid

(particulate matter PM, soot C).Two parameters of measuring the emissions amount areintroduced:Specific pollutant emission (g/kWh) and the pollutant emissionratio (g/ kg fuel).

Emission Pollutant emissionration (g/kg)

Specific pollutantemission (g/kWh)

CO2 (86% C in fuel) 3200 500-700

Sox per % S in fuel 20 3.2-4.4

NOx 40-100 6-22HC (hydrocarbons) 0.5-4 0.1-0.9

CO 2-20 0.3-4.4

Particulates 0.5-2 0.1-0.4

Order of magnitude of diesel engine exhaust emissions

Specific pollutant emission = pollutants mass flow rate / engineoutputPollutant emission ratio = pollutants mass flow rate / fuel mass flow

rate

NOx emission ratio for prime movers


84/141


Specific fuel consumption of prime movers

CO2 carbon dioxide: This gas is naturally present in theatmosphere at low concentration (approximately 0.035%). Itabsorbs infrared energy and is thus a greenhouse gas (acontributor to global warming). The internal combustion enginecontributes to the increased concentrations of CO2 in theatmosphere. It does not have a great impact on the immediateurban environment.

CO carbon monoxide: The main source of CO is the internalcombustion engine, where it is produced by incomplete

combustion.. CO is highly toxic: it binds to hemoglobin morestrongly than oxygen does, thus reducing the capacity of thehemoglobin to carry oxygen to the cells of the body. CO can alsobe oxidized to CO2 in a catalytic converter.


85/141


* At CO levels in air of just 10 ppm, impairment of judgment and visualperception occur; exposure to 100 ppm causes dizziness, headache,and weariness; loss of consciousness occurs at 250 ppm; andinhalation of 1,000 ppm results in rapid death. Chronic long-termexposures to low levels of carbon monoxide are suspected of causingdisorders of the respiratory system and the heart.

NOx oxides of nitrogen:While some nitrogen may be present inthe fuel, most oxides of nitrogen are produced when elementalnitrogen (N2) in the air is broken down and oxidized at hightemperatures (approx. 1000 K or greater) and pressures within theinternal combustion engine. Nitrogen monoxide (NO) is producedin higher concentrations than nitrogen dioxide (NO2) but the twospecies are in any case interconvertible by means ofphotochemical interactions. Other oxides of nitrogen, such asN2O4, may occur; but are rarer. They react with the oxygen in the

air to produce ozone, which is also an irritant and eventually formnitric acid when dissolved in water. When dissolved in atmosphericmoisture the result can be acid rain which can damage both treesand entire forest ecosystems.

HC hydrocarbons: Fuel close to the wall of the combustionchamber may be quenched by the relative coolness of that areaand not be burned. Hydrocarbons are also released to theatmosphere by evaporation from fuel tanks. Hydrocarbons can bedangerous to human health.

SO2 Sulphur dioxide: Fossil fuels are derived from once-livingorganisms. Some sulphur occurs in protein and will still be presentin the fuel. Under combustion this sulphur reacts with oxygen toform sulphur dioxide. Sulphur is more prevalent in solid fuel than inliquid, but some sulphur dioxide emission does occur fromengines. SO2 is an acidic pollutant which dissolves in moisture inthe atmosphere to form sulphurous and sulphuric acids(components of acid rain). These corrode metal surfaces andweather limestone buildings. In humans, sulphur dioxide irritates

the eyes, the mucous membranes, and the respiratory tract, alongwith the skin in general. SO2 also has the effect of slowing downthe movements of the cilia the hairs in the trachea which act toprevent dusts entering the lungs , thus exacerbating theirritation caused by allowing more pollutant to access therespiratory system.


86/141


Future fuels

In order to overcome the problem of fossil fuel emissions, theworld has started to search for new kinds of fuels to comply withthe emission control regulations. Many technologies have been

developed in order to make the internal combustion enginescleaner, some of these technologies are related with the enginedesign and control and they will be discussed in the diesel engineschapter, but here the solutions related to the type of fuel will bediscussed.

Gaseous fuels are anticipated to be the future clean fuels for alltypes of transportation, two of them are discussed here; naturalgas and hydrogen.

Property Gasoline Methane Hydrogen

Density (Kg/m3) 4.40 0.65 0.084

Ignition Temperature (oC) 228-471 540 585

Flame Temperature In Air (oC) 2197 1875 2045

Higher calorific value (MJ/Kg) 45 55 150


87/141


The above table shows the levels of emissions from three types offuels, we are concerned with the first two; natural gas and oil. It isclear that most of pollutants are far low in the case of NG.

Many engine manufacturers have developed marine engines

working with natural gas to suit the increasing market demand ongreen energy. The principles of using NG in marine engines will bediscussed in the diesel engines chapter.

Hydrogen is used these days only for land transportation as manyproblems related to its storage have not been solved yet. BMWproduced several designs for hydrogen internal combustionengines since the 1970s and many car manufacturers in the USAand Japan followed. Many researches have been made in this fieldto try to adopt the hydrogen fuel in the marine field and overcome

the problems associated with it.

The main problem with hydrogen storage is its low densityrequiring large volumes to store it. Hydrogen becomes liquid at -253C and even at this temperature the volume needed to store anamount of hydrogen with the same energy output as diesel isabout 3.5 times that of diesel. That means bigger spacesdedicated for fuel onboard the ship or reduced sailing range withthe same fuel capacity.


88/141


Regarding the emissions, Hydrogen is the cleaner fuel availablesince it contains no carbon, thus no carbon monoxide or dioxidewill be available in the combustion gases, only amounts of NOx willbe available due to the high combustion temperature of hydrogen,but this problem can be dealt with easily since the NOx elimination

technologies have reached its maturity level for existent internalcombustion engines.

Hydrogen combustion produces, theoretically, nothing but watervapor which is not harmful to the human health or the environment,although it is considered one of the green house gases, thosegases leading to global warming, its nature and properties makes itnot dangerous with the anticipated level of use.

Another method to use hydrogen instead of using it in internal

combustion engines or gas turbines is the use of fuel cells.Although the fuel cell is not a generator of mechanical power itmay be a future alternative for prime movers. It converts chemicalenergy directly into electric energy without combustion enginesand generators.

A fuel cell consists of two electrodes with an electrolyte in themiddle. A fuel, hydrogen, is continuously fed to one electrode (theanode) and oxygen to the other (the cathode). Chemical reactionsat the electrodes form ions that will pass through the electrolyte,

and electrons create a current that can be utilized to energiseelectric users before the electrons are returned to the cathode.Advantages of fuel cells are numerous: high efficiency, cleanemissions (water) and silent operation.

Working principle of one type of fuel cells


89/141


Marine diesel engines

Working principles and cycles

Working principle

Diesel engines transform chemical energy stored in fuels intomechanical energy at the output shaft. This conversion processtakes place in two steps: first, chemical energy is converted intothermal energy by means of combustion reactions of the fuel andsecond, the thermal energy is converted into mechanical energy.

Theoretical cycle

The basic diesel cycle consists of air inlet, compression,combustion and expansion and finally exhaust.

In the theoretical cycle of the diesel engine the following isassumed:

- The physical and chemical properties of the working fluidremain unchanged within the cycle

- The quantity of the working fluid remains constant during thecycle, therefore the process of filling the cylinder with a fresh


90/141


charge of gas and removing the exhaust gases are non-existent

- The processes of compression and expansion of the gasesare isentropic

- After compression the working fluid receives heat from an

external source of heat and after expansion it rejects heat toa cold source

Actual cycle

The actual cycle inside the engine can be done either in fourstrokes (two crank revolutions) or in two strokes (one crankrevolution).

A stroke (L) is defined as the distance travelled by the pistonbetween the top dead centre (TDC) and the bottom dead centre(BDC). The inside diameter of the cylinder is the bore (D). thecylinder volume that corresponds with the stroke is the sweptvolume or stroke volume (Vs)

Vs =

4 L D 2

The volume above the piston at TDC is the clearance volume (Vc)

1. Four stroke cycle


91/141


A. compression strokeThe piston moves upward from BDC to TDC. Inlet and exhaustvalves are closed and the combustion air is compressed. Thecompression of air causes an increase in temperature. Fuel isinjected several crank degrees before TDC and ignited by the high

temperature of the compressed charge. At the end of thecompression stroke combustion has started.

B. Power strokeThe combustion is continued over a considerable crank angle afterTDC, while the combustion gases expand and perform work on thepiston forcing it down. Towards the end of the stroke the exhaustvalve opens, thereby releasing the gas into the exhaust manifold.

C. Exhaust stroke

The piston moves from BDC to TDC. The exhaust valve is openand the rest of the combustion gases are forced out of the cylinderby the upward stroke of the piston. The gases that remain in theclearance volume are dispelled by a scavenging process; the inletvalve is opened early whereas the exhaust valve is closed late, sothat both are open at the same time (overlap period).

D. Intake strokeThe piston moves downward from TDC to BDC. The inlet valve isopen and the exhaust valve closed, while the cylinder fills with a

charge of fresh air and will be ready for the compression stroke.

A complete cycle takes four strokes, only one of them is usefulwhich is the power stroke, this means useful stroke every twocrank revolutions.


92/141


Valve timing diagram of a 4 stroke engine

2. Two stroke cycle


93/141


The main difference with the 4 stroke cycle is that charging andexhaust take place without the piston enforcing the process.

A. Compression strokeThe inlet ports and exhaust valve are closed and a volume of air is

trapped in the cylinder. The piston moves upward to TDC thuscompressing this combustion air and causing a temperature risethat is sufficient to ignite the fuel that has been injected severaldegrees before TDC. At the end of the compression strokecombustion has started.

B. Power strokeCombustion is continued. The combustion gases expand andperform work on the piston forcing it down from TDC to BDC.Towards the end of expansion exhaust valve opens.

C. ExhaustThe combustion gases blow down to manifold pressure. By thetime the inlet ports are open, the cylinder pressure will havereached a pressure lower than that of the scavenging air, soscavenging starts.

D. ScavengingScavenging, which started in C while the piston moved downward,is completed while the piston moves upward. Both the inlet ports

and exhaust valve are open: fresh air (scavenging air) enters thecylinder forcing the exhaust gases out. In order to scavenge thecylinder it is necessary to pre-compress the scavenging air with ascavenging air compressor or with the compressor of theturbocharging system.

Processes B and C take place in one stroke, A and D in another.Here one power stroke occurs every two strokes, or every onecrank revolution.

The process described here is foruniflowscavenging. This type ofengine has inlet ports low in the cylinder wall and an exhaust valvein the cylinder head. This is the common type of 2 stroke enginesnowadays. Another kind of scavenging was common until the1980s is the loopscavenging. The engine in this type has inlet aswell as exhaust ports in the cylinder wall causing the flow to loop.


94/141


Scavenging in a 2 stroke engine

A) Uniflow scavenging and B) Loop scavenging

Timing diagram of a 2 stroke engine


95/141


Two stroke or four stroke

The main difference between the two cycles is the powerdeveloped. The two-stroke cycle engine, with one working orpower stroke every revolution, will, theoretically, develop twice the

power of a four-stroke engine of the same swept volume.Inefficient scavenging however and other losses, reduce the poweradvantage to about 1.8. For a particular engine power the two-stroke engine will be considerably lighter, an importantconsideration for ships. Nor does the two-stroke engine require thecomplicated valve operating mechanism of the four-stroke. Thefour-stroke engine however can operate efficiently at high speedswhich offsets its power disadvantage; it also consumes lesslubricating oil.

Each type of engine has its applications which on board ship haveresulted in the slow speed (i.e. 80 100 rev/min) main propulsiondiesel operating on the two-stroke cycle. At this low speed theengine requires no reduction gearbox between it and the propeller.The four-stroke engine (usually rotating at medium speed,between 250 and 750 rev/ min) is used for auxiliaries such asalternators and sometimes for main propulsion with a gearbox toprovide a propeller speed of between 80 and 100 rev/min.

Pressure indicator


96/141


Indicator diagram

The indicator diagram shows the relation between the volume andpressure in a cylinder. It can be obtained from a cylinder with asensor that measures gas pressure during the cycle. This pressure

sensor is a mechanical device called a pressure indicator. Thearea in the diagram represents the work developed within thecylinder; the indicated work.

Cycle process of a diesel engine in indicator diagram

The following table describes the processes that can bedistinguished in the diagram:

Process Description

1 2 The air in the cylinder is compressed by the upward moving piston

2 3Combustion of the injected fuel takes place at almost constantvolume (pressure increases)

3 4Combustion continues at almost constant pressure (volumeincreases)

4 5Expansion of the combustion gases until the exhaust valve isopened before BDC


97/141


5 6The combustion gases blow down to exhaust manifold pressurebefore the piston reaches BDC

6 7The combustion gases are forced out of the cylinder. In 4 strokeengine, this is done by the piston. In 2 stroke engine, the inletports are open and scavenging starts

7 8Through the opened inlet valve (4 stroke) the cylinder is chargedwith air. In 4 stroke engine, the piston moves down

8 1In a 4 stroke engine, the inlet valve is often closed after BDCwhereby some charge air may be lost. In a 2 stroke engineprocess 8 1 is available for scavenging

Engine performance definitions

Standard air diesel cycle

The area enclosed by the P-V diagram is the specific work done inthe engine:

W = Qin Qout [kJ/kg]Vs = stroke/swept volume (= V1 V2) [m

3/kg]W/Vs = mean effective pressure [kJ/m

3 kPa]

IHP = the power developed inside the engine cylindersBHP = the power at the flywheelFHP = power lost in friction (= IHP BHP)

BHP =pb Vs N Z

i c

pb = brake mean effective pressureVs = /4 x D

2 x LN = engine speed [rpm]Z= number of cylinders


98/141


C = unit conversion constanti = factor for accounting for engine number of strokes (1 for 2-stroke, 2 for 4-stroke)

m = mechanical efficiency = BHP / IHP (~ 95 98 %)

pi = indicated mean effective pressure

A/F = stoichiometric air/fuel ratio = excess air factor, the ratio of the actual mass of air to thestoichiometric

value

wf= fuel mass flow rate [kg/hr]bi = indicated specific fuel consumption (= wf / IHP) [kg/HP.hr]be = brake specific fuel consumption (= wf / BHP) [kg/HP.hr]

wa = air mass flow rate (= wfx A/F x ) [kg/hr]wexh = exhaust mass flow rate (= wa + wf= wfx (1+ A/F x ) )

vol = volumetric efficiency (= wa / Vs N Z ) (~ 98%) = mixture density 1.2 kg/m3

ith = indicated thermal efficiency (= IHP / Qa)bth = brake thermal efficiency (= BHP / Qa)Qa = heat added = wfx CVCV = fuel calorific value

Qa = BHP x c + Qcw + Qexh + QradQcw = energy lost in cooling water (= mcw x Cw x Tcw)Qexh = energy lost in exhaust (= wexh x Cpexh x Texh)Qrad = energy lost due to radiation (~ 2 5%)

NOTE: TAKE CARE OF UNIT CONVERSIONS


99/141


Example

Given the following engine data:BHP=7790 @ 127 rpmStroke bore ratio L/D=1.5

Indicated specific fuel consumption bi=150 g/HP hr8 cylinder 2-strokeAmbient air temperature=20oCExhaust temperature=580oCMechanical efficiency m=0.95Cooling water temperature

Ship Power Plant

Documents

Transcript of Ship Power Plant