Marine vol 2 part 17

Combustion and the diesel cycle 4.1 Chemical composition of fuels 4.2 Chemistry of combustion 4.3 Combustion in the diesel engine 4.4 The diesel cycle 4.5 The idealised cycle 4.6 Compari

MARINE ENGINEERING PRACTICE

Published by The Institute of Marine Engineers

The Memorial Building

76 Mark Lane

London

EC3R 7JN

Copyright © 1978 The Institute of Marine Engineers

A Charity registered in England and Wales

All rights reserved No part of this publication may be reproduced, stored

in a retrieval system, or transmitted in any form of by any means, tronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher Enquiries should be addressed to The Insti- tute of Marine Engineers.

elec-ISBN: 0 900976 79 9

Printed in the United Kingdom by Hobbs the Printers Ltd,

Brunei Road, Totton, Hampshire 5040 3WX

3.3 Contamination and cleaning

3.4 Corrosion, erosion and other defects

3.5 Vibration

4 Combustion and the diesel cycle

4.1 Chemical composition of fuels

4.2 Chemistry of combustion

4.3 Combustion in the diesel engine

4.4 The diesel cycle

4.5 The idealised cycle

4.6 Comparison of idealised and actual diagrams 4.7 Out of phase diagrams

4.8 Exhaust smoke

5 Fuel injection systems and equipment

5.1 Types of fuel injection system

5.2 Jerk type fuel pumps

5.3 Common rail system fuel pump - Doxford 5.4 Fuel injection

5.5 Injection cycle and timing

5.6 Maintenance

6.4 Oilways and grooves

6.5 Overhauling and maintenance

6.6 Bearing failures

6.7 Bearing condition warning devices

7 Lubrication, lubricating oils, systems and treatments 7.1 Duties of a diesel lubricant

7.2 Cylinder lubrication

7.3 Crankcase lubrication

7.4 Maintenance of crank chamber oil

7.5 Treatment of lubricating oil

7.6 Testing of lubricating oil

7.7 Cleaning lubricating oil system

8 Fuel oil

8.1 Origin and refining

8.2 Properties of fuel oil

8.2.1 Specific gravity or density

8.2.11 Grade of fuel oil

8.3 Fuel oil system

8.4 Purification of fuel oils

8.5 Self-cleaning purifiers - automatic operation

9 Fresh and sea water cooling systems

9.1 The need for cooling

9.2 Open and closed fresh water systems 9.3 Sea water system

10.3 Starting air valve

10.4 Compressed air for starting

10.5 Speed regulation

10.6 Engine governors

11 Brief description of some engines

11.1 Doxford - Figs 35 and 36

11.2 Sulzer - Figs 37 and 38

11.3 Burmeister &Wain - Fig 39

11.4 Grandi Motori Trieste (G.M.T.)-Fig 40 11.5 M.A.N - Figs 41, 42, 43 and 44

11.6 Mitsubishi - Figs 45 and 46

12 Preparing a diesel engine for running

13 Watch keeping duties at sea

LIST OF FIGURES

Fig 1 Forms of scavenging 4Fig 2 Engine and turbocharger system 6Fig 3 Sectional arrangement of turbocharger 7Fig 4 Two-Stage Turbocharging system (Mitsubishi) 8Fig 5 Lubricating oil system for turbocharger with plain 11

bearings

Fig 6 Air side cleaning of intercooler 12Fig 7 Indicator diagram 17Fig 8 PV ideal diagram 20Fig 9 Pressure - crank angle diagram 22Fig 10 Out of phase diagram 23Fig 11 Interpretation of out of phase diagrams 24Fig 12 Essential elements of jerk pump fuel injection 27

Fig 21 Fuel injector valve 37Fig 22 Needle valve seat 38Fig 23 Fuel injector needle lift diagram 40Fig 24 Shell type bearing 44Fig 25 Bridge gauge (for wear down) 46Fig 26 A typical lubricating oil system 57Fig 27 A typical fuel oil system 63Fig 28 Purifier bowl operation 66Fig 29 Fresh water system open 71Fig 30 Fresh water system closed 71Fig 31 Sea water system 73

Fig 32 Heat exchanger 74Fig 33 Air starting system 77Fig 34 Diagram of an hydraulic governor 80Fig 35 Sectional views of Doxford 'J' engine 83Fig 36 Doxford engine running gear 85Fig 37 Section of Sulzer engine type RND M 87Fig 38 Combustion space in Sulzer type RND M engine 89Fig 39 Section of B & W engine type K90GF 91Fig 40 Section GMT engine type B1060 93Fig.41 M.AN engine frames and bedplate 94Fig 42 Section of M.A.N engine type KS2 90/160B 95Fig 43 M.AN cylinder liner 96Fig 44 M.AN cylinder cover and piston 97Fig 45 Section of Mitsubishi engine type 8UEC 85/180D 99Fig 46 Mitsubishi two-stage turbo charging system 101

With practical fuels, combustion is started by bringing them intocontact with the oxygen in air and raising their temperature locally to thepoint of ignition In the familiar automobile engine the passage of a sparkprovides ignition In the diesel engine the charge of air in the cylinder iscompressed to such a degree that its temperature is high enough to causethe fuel sprayed into the cylinder at the end of the compression stroke toignite spontaneously This distinctive feature of his engine was described

by Dr Rudolf Diesel in his patent of 1892

The essentials of the internal combustion cycle are compression of theworking fluid (air), heating by combustion of the fuel in it and expansionconverting heat energy into mechanical work The work done duringexpansion exceeds the work required for compression and the differenceconstitutes the power output of the engine

The foregoing applies to all internal combustion engines, including gasturbines In the case of reciprocating engines a charge is taken into thecylinder and is compressed, ignited, and expanded there each cycle At theend of the expansion the spent charge is exhausted and replaced by a freshcharge

If the processes of exchanging the spent charge for the fresh one arecarried out by means of piston movement and mechanically operatedvalves the result is the well known four stroke cycle This consists of thefollowing

Intake of fresh charge as the piston moves out

Compression as the piston moves in

Combustion at inner dead centre, followed by expansion as the

2 MARINE ENGINEERING PRACTICE

piston moves out

Exhaust as the piston moves in

If the gas exchange processes are carried out at the end of theexpansion stroke and the beginning of the compression stroke by means ofports uncovered by the piston, sometimes in combination withmechanically operated valves, the result is the equally well known twostroke cycle

Considerations of the gas exchange processes in most designs of stroke cycle engines tend to favour a relatively long stroke in proportion tothe bore Such a configuration also suits the low rotational speed which is

two-required of a direct-coupled engine if the propeller is to be efficient In

consequence, modern large marine diesel engines, almost withoutexception, operate on the two-stroke cycle principle

2 SCAVENGING, CHARGING, AND

SUPERCHARGING

2.1DEFINITION OF TERMS

Scavenging is the process by which the spent charge, or the remnants of

it, are displaced from the cylinder by fresh air blown through it.

Charging is the process by which the cylinder is filled with air at the beginning of the compression stroke.

Supercharging is a process or combination of processes whereby the density of the charge is increased to provide a greater mass of air in the cylinder in which a correspondingly larger amount of fuel can be burned,

so increasing the power output of the engine It should be noted that supercharging is not simply a matter of adding equipment to non- supercharged engines The supercharged engine must be designed to withstand the increased pressures and thermal loads which result.

2.2 PROVISION OF SCAVENGING AND CHARGING AIR

It is essential to the scavenging process that the air entering the cylinder

is at a higher pressure than the gas in the exhaust manifold Scavenge air is supplied to large diesel engine cylinders in a variety of ways The following methods have been used

(a) By direct driven reciprocating scavenge pumps, usually connected

by quadrants, or by links and levers to the crosshead bearing Positive displacement rotary blowers have also been used, usually gear driven and sometimes chain driven.

(b) By exhaust gas turbochargers in the case of supercharged engines.

These may be alone or assisted at low revolutions by an auxiliary fan.

(c) By turbochargers in series with single or multiple reciprocating

Opposed piston engines in which each cylinder has a piston at each

4 MARINE ENGINEERING PRACTICE

end, one of which controls the opening and closing of the exhaust portsand the other that of the scavenge ports, thereby creating a through oruniflow scavenging system

Loop, cross and transverse scavenge engines in which there is onepiston per cylinder which controls both the exhaust and scavenge ports.Uniflow scavenge engines having a single piston which controls thescavenge ports with cam-operated valves in the cylinder cover controllingthe exhaust

The representative forms of scavenging are shown diagrammatically

SCAVENGING, CHARGING AND SUPERCHARGING 5

2.3GAS EXCHANGE PROCESSES

When the piston nears the end of the expansion stroke the exhaust port

or valve is opened At this point in the cycle the pressure in the cylinder isconsiderably in excess of the pressure in the exhaust manifold with theresult that, asthe port or valve opens, exhaust gases from the cylinder flowrapidly out into the manifold whilst the pressure in the cylinder falls

As soon as it has fallen to a level less than the pressure in the airmanifold, the scavenging process can begin and the timing of the ports andvalves is designed so that this can take place effectively It is usual for thevolume of air used to scavenge and charge a cylinder, at the densityobtaining in the air manifold, to exceed slightly the volume of the cylinderitself, in order to ensure complete removal of combustion products

As the piston commences the compression stroke the exhaust andscavenge ports are closed, trapping the fresh charge of air in the cylinder.2.4TURBOCHARGING

The majority of large marine diesel engines in service at the presenttime are supercharged The supercharging is carried out by exhaust-driven turbochargers At the end of the expansion stroke, when theexhaust is released from the cylinder it still contains a considerable amount

of energy which can no longer be used on the piston, but can be used todrive a turbine wheel The turbine in turn drives a blower impeller toprovide air at increased pressure for the cylinder The combination ofturbine and compressor forms a free-running unit termed a turbochargerwhich is mechanically independent of the engine The arrangement isshown diagrammatically in Figs 2&3

When the exhaust gas is released from the cylinder it creates a pressurepulse in the exhaust system The energy in the pulse can amount to aconsiderable proportion of the total energy in the exhaust gas There aretwo ways of conserving this energy; one is to preserve the pulse by keepingthe cross sectional area of the exhaust system relatively small so that thepulse continues all the way to the turbine nozzles

Such a system has to be arranged so that the pulse from one cylinderdoes not interfere with the scavenging processes of the other cylinders This

is carried out by collecting the exhausts from the cylinders in groupsaccording to the firing order and intervals, and keeping them separateright up to the turbine nozzles Such a system is known as a pulse system.The other method is to allow the pressure pulses to enter a largeexhaust gas receiver in which their peak energy is dissipated in raising thegeneral energy in the whole system The turbine is then fed by exhaust gas

at a relatively uniform pressure This system is known as the constantpressure system

When the level of supercharging is not very high, that is, when theabsolute pressure of the scavenge air is less than about 2.5 times theatmospheric pressure, the pulses constitute a large proportion of theenergy in the exhaust gases and the pulse system of turbocharging offers

the highest efficiencies At higher levels of supercharging the pulsesrepresent a smaller proportion of the total energy in the exhaust gases andthe constant pressure system has more to offer as the turbine is moreefficient under these circumstances.

Under normal running conditions all the air required for scavengingand charging the cylinder is supplied by the turbocharger but formanoeuvring and slow running it is sometimes advantageous to have anelectrically driven blower or an engine driven positive displacementscavenge pump or rotary blower to supplement the turbocharger output.

2.5 INTERCOOLING

The action of compressing air also raises its temperature and insupercharged engines (unless the level of supercharging is very low) it isbeneficial and sometimes necessary to cool the air as it leaves thecompressor This cooling has two important effects Firstly it increases thedensity of the air so adding to the mass of air trapped in the cylinder atthe beginning of each compression stroke which is available for theproduction of power Secondly, it assists in keeping cool the internal parts

of the engine, rendering them better able to withstand the thermal loadsresulting from increased power

The air is cooled by passing it through an intercooler This consists of abank of tubes having closely spaced fins on their outside to provide a largesurface area The air passes between the fins whilst the cooling waterpasses through the tubes Sea water is the usual cooling medium as it givesthe greatest practical temperature difference, but fresh water is sometimesused in the case of vessels operating continually in waters which containcorrosive or contaminating elements

8 MARINE ENGINEERING PRACTICE

Two-stage turbocharging has frequently been used experimentally toachieve higher pressure ratios with better efficiency than is possible withstandard single-stage turbochargers offered by manufacturers but so far,only one company has used it for their production engines (See Section11.6) The reason for its comparative rarity in service is thatturbocharger development has kept pace with engine development to theextent that sufficiently high pressure ratios and efficiencies have always justbeen available from one single-stage machine, making it difficult tojustify the cost of an additional turbocharger, cooler and associatedducting

3 TURBOCHARGING, AIR FILTERS AND AIR

COOLERS

3.1GENERAL DESCRIPTION

The construction of a typical turbocharger can be seen in Fig 3 Themain body is built up from casings - a gas inlet casing, a gas outlet casingand the compressed air outlet casing The air inlet casing can be of the sideentry type To this ducting is attached which draws cool clean air through

a filter compartment which communicates with the outside atmosphere atdeck level This arrangement has the advantage of ensuring that thecompressors and air coolers are relatively free from contamination.However, it is expensive and it is more usual to have a combined airfilter/ silencer fitted to the turbocharger, drawing air directly from theengine room

The compressor wheel generally consists of two parts: the mainimpeller and the inducer, both made from a light alloy and keyed on to theshaft A labyrinth air seal on the rear face of the impeller rotor, togetherwith grooves in the heat shield form an effective seal between thecompressor and the exhaust side of the turbine The designed sealclearances should be maintained for', if they are excessive, the loss of airinto the exhaust can affect the efficiency of the turbocharger

The rotor shaft is supported at both ends by ball and roller, or plainbearings In the former case one end of the rotor bearing assembly consists

of a double-row ball-and-roller bearing, mounted in pulsation-dampingsprings, whilst the other consists of a single-row roller bearing in a similarresilient mounting

The ball-and-roller bearings have to be changed after prescribedperiods which depend upon the service speed of the turbocharger Theperiods recommended by the manufacturers are based upon empirical datawhich has been statistically interpreted Quick analysis by audio or visualmeans to assess the further use of old bearings is not recommended Only

a detailed examination at the maker's works is acceptable

3.2LUBR.lCATION SYSTEM

Lubricating oil sumps are situated immediately underneath the rollerbearing and are separate for the turbine and compressor ends Oil pumpsare mounted at each end and, driven by the shaft, spray oil on to theirrespective bearings This enclosed system is self-contained andindependent

\0 MARINE ENGINEERING PRACTICE

The lubricating system for a turbocharger with plain bearings can betaken from the main engine system or it can be independent

When the main engine system is used, adequate filtration and guards against interruption of supply must be provided

safe-For an independent lubricating system a drain tank is required,together with duplicate sets of lubricating oil pumps, a cooler, headerta~k, alarms, filter, shut-off valves, etc

The pressure gravity system is generally used in merchant vessels whereheight 'is not restricted It utilises an overhead tank about 5m above theturbochargers The head from the gravity tank is sufficient to produce acontinuous supply of oil to the turbocharger bearings and the capacityshould be such that the bearings will not be damaged, should bothlubricating oil pumps break down

The alarm in the gravity tank will alert the engineers on watch to slowdown or stop the main engine whilst an investigation takes place Thelubricating oil pump draws oil from the drain tank via the suction strainersand delivers the warm oil through fine-mesh filters to the oil coolers andthen to the turbochargers and gravity tank See Fig 5

The gravity tanks should have a capacity which will permit theturbochargers to run for approximately 15 minutes This reserve flow isextremely important to avoid damage to the bearings Another benefit isthat the gravity system can be isolated and the pressure system used forflushing

Contamination of the lubricating oil reduces the effective life of thebearings considerably and, when changing oil or replacing bearings,extreme cleanliness must be observed

3.3CONTAMINATION AND CLEANING

To ensure efficient functioning of the turbocharger it is essential notonly to protect the bearings but also the compressor and turbine fromcontamination At the intake to the compressor a filter/silencer is fitted.Between the compressor outlet and the engine an air-cooler is situated.Both these units are subject to fouling and should be cleaned regularly Inboth cases a constant check should be kept of the pressure drop acrossthese units as this will give an indication of the degree of fouling

Even with good filtration of air, deposits will build up after a period inthe air passages through the compressor and on the tubes of the air coolerwhich will eventually cause a decrease of the boost pressure andconsequent rise in exhaust gas temperatures When intercoolers have beenneglected and have become heavily contaminated the reduced air flowcould cause the compressor to surge The air flow through the compressorreverses in at least some of the passages This can be heard, depending onthe degree of contamination and the type of diffuser, as a type of buzzing

or as a series of explosions Surging of the compressor can also happenwith clean intercoolers during quick accelerations or shut-down of theengine This temporary surging of the compressor is harmless whereas

TURBOCHARGING, AIR FILTERS AND AIR COOLERS II

12 MARINE ENGINEERING PRACTICE

continuous surging can lead to breakdowns

A typical method of cleaning the water side of an intercooler is shown

in Fig 6

Surging is mainly caused by deposit build-up in the diffuser where theducts are narrowest and the velocities highest Deposits on the impeller orinducer have little influence on surging but can affect the balancing Thecompressor, and especially the diffuser, can be cleaned during service bythe injection of water into the air stream before the compressor The watershould be as pure as possible

The major cleaning effect is caused by the mechanical breakaway ofthe deposit when small drops of water strike the surface at high speed.Whilst the addition of a solvent can do no harm it is questionable whether

it makes a large contribution If the water is injected too slowly,evaporation or an excessively even distribution of the water droplets willresult with no effect on the deposit

This water washing technique has proved to be very effective and haslengthened the intervals between maintenance as long as themanufacturer's instructions were observed

The gas side of the turbocharger, that is, the nozzle ring and theturbine blades, are less sensitive to contamination than the compressor.Poor combustion, especially with engines that operate on heavy fuel,

TURBOCHARGING AIR FILTERS AND AIR COOLERS I3

sometimes causes a heavy build-up of deposits which reduce the efficiency

of the turbine The likelihood of this varies greatly from engine to engine.The type of fuel used, how it is treated prior to burning, and the operatingconditions of the engine, all have an effect on the fouling of the gas side ofthe turbocharger

Water-washing at recommended intervals is of advantage and is done

at slow speeds The method consists of injecting water before the pistonring traps, in larger amounts and over a longer period than those used forcompressor cleaning It is not only the force of the water striking andloosening the deposit that is advantageous but the fact that a greaterproportion of the gas-side deposits are solu ble in water

All the prescribed methods of cleaning turbochargers and piston ringtraps cannot replace the actual dismantling of the units by the maker'sexperts and the general maintenance carried out by them Needless to saythe intervals between such maintenance will depend upon the loading andoperation of the engine, the type of fuel used and the condition of theexhaust gases

3.4CORROSION AND OTHER DEFECTS

Some fuels have a sulphur content as high as 5% which, when burned,forms sulphur dioxide (S02) and sulphur trioxide (S03)' Thesecompounds combine with the steam which is always present in exhaustgases to form sulphurous and sulphuric acids The dew point of sulphuricacid is approximately 16(fC and as the wall temperature of water-cooledcasings may be lower, highly concentrated sulphuric acid may condenseand corrode the casing surface It is also possible for the casing to suffererosion from solid particles in areas of high exhaust gas velocity

The problem of corrosion is not as obvious as it was in the past formodern engines, with their higher specific output, are operating at higherexhaust gas temperatures The cooling water temperatures have also beenincreased in an attempt to overcome the corrosion problem and somemanufacturers of turbochargers have increased the gas casing wallthickness to try and prolong the life of their product It is recommendedthat all thicknesses should be checked during every overhaul

Should the gas outlet casing be cracked and no longer water tight, it ispossible, in some instances, to run at reduced power for short periodswithout cooling water but a close watch should be kept on the turbine-side lubricating oil sump where the oil temperature should not exceed

14 MARINE ENGINEERING PRACTICE

frequency than those which result from eccentricity of the rotating parts

If the frequency of the vibration corresponds with the speed of the rotor,the latter must be checked at the first opportunity and, if necessary,overhauled and re-balanced An uneven build-up of deposits or damage tothe turbine blades may be the cause Heavy eccentricity of the rotor couldlead to a breakdown of the bearings

Vibr.ation alarm instruments are available which can detecteccentricities They are especially useful in unattended engine-rooms andtheir use can prevent possible extensive damage to the rotor

4 COMBUSTION AND THE DIESEL CYCLE

4.1 CHEMICAL COMPOSITION OF FUELS:

The manner in which fuel oils are obtained from crude petroleum isdescribed in Section 8, together with some of their physical characteristicswhich are important to the engineer

From the chemical aspect all these fuel oils are composed of mixtures

of hydrocarbons, each of which has a formula of the general form Cx Hywhere both x & y can range from unity to several hundreds The lighterfractions, in which x& yare small, are gases; for example, methane, thechief constituent of natural gas, has the formula CHt At the other end ofthe range are the heavy fractions where x&yare large and the molecularstructure is complex

Hydrocarbons are classified into groups with similar molecularstructures and members of the same group exhibit largely similarproperties and patterns of behaviour during combustion

Although composed predominantly of carbon and hydrogen, liquidfuels may contain sulphur; up to about 10,10 in light diesel oils and up to3% or 4% in heavy residual fuels During combustion sulphur-oxides areformed which combine with moisture to form corrosive acids detrimental

to the engine The energy obtained from the combustion of sulphur isnegligible and it is preferable for a fuel to contain as little sulphur aspossible

4.2 CHEMISTRY OF COMBUSTION

In complete combustion a fixed amount of fuel combines with a fixedamount of oxygen and liberates a definite amount of energy as heat Theoverall process can be understood from the basic combustion chemistry ofthe elementary fuels, carbon and hydrogen, according to their proportions

in the fuel, regardless of their combinations into various hydrocarbons.Using the usual chemical symbols and equations

C+O2 CO2 +393.8 MJ/kmol

2H2+O2 2H20+261.06 MJ/kmol (With the liquid at2S'C)

The combined proportions by weight are obtained by considering themolecular weights

H2=2; C= 12; O2=32

Thus 12 parts by weight of carbon combine with 32 parts of oxygen to

COMBUSTION AND THE DIESEL CYCLE 17

CoMBUSTION IN THE DIESEL ENGINE

The distinguishing feature of the diesel engine is that the fuel is ignited

by bringing it into contact with the air in the cylinder at the end of thecompression stroke when the air is very hot

To achieve complete combustion of the fuel this process requires:onsiderable excess air At full load overall air/fuel ratios for the engineare in the region of 40 or 50: 1 and in the cylinder the ratio of air trapped tofuel is25:1 or even35:1 At lower loads air/fuel ratios are higher still Thecarbon in the fuel burns completely to form carbon dioxide CO2, There isvirtually no partial combustion of carbon to form carbon monoxide, CO,

isin the gasoline engine

~,4THE DIESEL CYCLE

Combustion converts the chemical energy in the fuel to heat Athermodynamic cycle is necessary to convert the heat energy intomechanical work The basic cycles, four stroke and two stroke, have been

~escribedin Section 1

In a slow speed diesel engine the changes in pressure and volume thatthe gases trapped in the cylinder actually undergo can be measured bymeansof an indicator This is an instrument which records graphically the

~vents in the cylinder in the form of a pressure-volume diagram An

~xampleof such a P V diagram (or indicator card) for a large marine two

;troke cycle engine is shown in Fig 7

The fresh charge of air is trapped in the cylinder by the closing ofvalves or ports at the point marked 1 From 1 to 2 it is compressed raisingits pressure as can be seen and also its temperature At this point fuel issprayed into the cylinder, the temperature of the hot compressed air ishigh enough to cause it to ignite and combustion takes place resulting in arapid rise in pressure to point 3 By now the piston has commenced tomove outward, allowing the burning gases to expand so that the pressurefalls, although combustion continues until point 4 is reached Expansionwithout combustion takes place from point 4 to point 5 when the exhaust

20 MARINE ENGINEERING PRACTICE

Values for BSFC, CV and J being in corresponding units as in 4.4.From the point of view of the engine user, who is concerned about thepower to be obtained from the fuel for which he has paid, BSFC and brakethermal efficiency are more useful than their indicated counterparts, butfrom the point of view of thermodynamic analysis of the performance ofthe engine ISFC and indicated thermal efficiency are of basic importance.4.5THE IDEA LIS ED CYCLE

To understand what is taking place in the cylinder it is useful to relatethe events to an idealised theoretical cycle and '-~ one usually chosen forcomparison with the diesel engine cycle is the dual combustion cycle This

is so named because the addition of heat (which corresponds tocombustion in the actual engine) is assumed to take place partly atconstant volume and partly at constant pressure

The working fluid for the idealised cycle is taken as air This is notunreasonable in the case of the diesel engine as by far the greaterproportion of the gases which pass through the diesel engine is thenitrogen in the air and of the oxygen in the air only half of it, or less, takespart in the combustion process; so that the actual working fluid is verylargely air

In the idealised cycle the compression and expansion processes areassumed to be adiabatic, i.e they take place without gain or loss of heatenergy This means that the relationship between pressure and volumefollows the natural law PV~ = C where ~ is the ratio of specific heats atconstant pressure and constant volume For air at normal temperature a-may

be taken as 1.4

A pressure volume diagram for the idealised cycle is shown in Fig 8

COMBUSTION AND THE DIESEL CYCLE 21

The numbered points correspond approximately to those in Fig 7 bearing the same numbers From point 1 to point 2 compression of the air takes place, raising both its pressure and its temperature From 2 to 3 heat is added at constant volume and from 3 to 4 more heat is added at constant pressure FrOm 4 to 5 expansion takes place and from 5 to 1 rejection of heat, restoring conditions to their original values In a practical engine this rejection of the heat is performed by exhausting the spent charge and taking in a fresh one.

The mechanical work done during one cycle is equivalent to the difference between the energy supplied as heat in the processes 2 to 3 and 3

to 4 and that rejected in the process 5 to 1 It is also the difference between the work done by the gas during expansion 3-4-5, and the work done on the gas during compression 1-2 The ratio of the mechanical work done to the heat energy supplied is the efficiency of the idealised cycle The ratio of the work done per cycle to the swept volume is the same effective pressure Methods of calculating the mean effective pressure and the efficiency

of idealised cycles, although quite simple, are outside the scope of this volume but will be found in text books on thermodynamics of internal combustion engines From a study of different forms of the dual combustion cycles it can be shown that efficiency is improved by raising the compression ratio and using high maximum pressure but there are obvious limits to doing this in the design of a practical engine.

In turbocharged engines the pressure of the trapped charge will be higher as the level of turbocharging is increased If the compression ratio were maintained at the same value all pressures throughout the cycle would increase correspondingly including the maximum pressure To design for very high maximum pressure would result in a heavy and expensive engine and it is usual for highly turbocharged engines to have reduced compression ratios with some sacrifice in cycle efficiency Fortunately, the brake thermal efficiency is not affected to the same extent

as the mechanical losses become a smaller proportion of the increased output from the highly boosted engine, which thus has an improved mechanical efficiency.

It is the aim of the designer to strike a compromise between the output (b.m.e.p.), maximum pressure and brake thermal efficiency which will result in an engine which is reliable and economic in both first cost and running costs.

4.6 COMPARISON OF IDEALlSED AND ACTUAL DIAGRAMS

If the idealised diagram is compared with the actual diagram it will be seen that the latter has a smaller area for the same limits of pressure and volume At the combustion end the corners become rounded and at the gas exchange end some of the expansion line is lost as the cylinder is blown down when the ports open The actual cycle thus has a lower output and therefore a lower efficiency than the idealised one.

The following are the major factors which bring this

about:-22 MARINE ENGINEERING PRACTICE

Combustion is obviously a continuous process as the pistonreaches the end of the stroke and starts to return The two stages

of constant volume and constant pressure are onlyapproximations to the actual state of affairs

The working fluid is regarded in the idealised cycle as a perfectgas; the specific heats at constant volume and constant pressureare looked upon as constant In practice they increase withtemperature

After combustion has commenced the working fluid contains CO2and H02 which are subject to dissociation at high temperatureswith consequent absorption of heat

Heat is lost during most of the cycle through the cylinder andcombustion chamber walls

This heat loss also modifies the compression and expansionprocesses so that they can no longer be treated as adiabatic Areasonable approximation to the real processes can be made byassuming they follow laws of the form PVn= C, where n has avalue of about 1.35 for compression and about 1.27 forexpansion These values vary a little with different types of engineand individual manufacturers have their own values which apply

to their own engines

Using reasonable practical values idealised cycle efficiencies rangefrom about 50% to about 65%, the indicated thermal efficiencies lie in theregion of 45% to 50070and brake thermal efficiencies in the region of 38 to43070for modern diesel engines

4.7OUT-Of-PHASE DIAGRAMS

The PV diagram is not very convenient for examining the combustionprocess as there is little change in volume from the moment that the fuel isfirst introduced into the cylinder to the time that it is completely burnt.When engine manufacturers wish to study combustion events they use

COMBUSTION AND THE DIESEL CYCLE 23

instruments which display changes in pressure with respect to time Theusual form is a P.8 diagram as shown in Fig 9,8 is the crank angle and isequivalent to time for any given rev/min

In Fig 9 the dotted line shows the pressures which result if fuel is notinjected and the full line shows the normal cycle Injection of fuelcommences at point 1 There is a delay period whilst this fuel warms upand some evaporates until a condition is reached at 2 when it ignites Thefuel which has entered the cylinder during the period 1-2 is well dispersedthroughout the air and once it has ignited the flame spreads rapidlythrough the cylinder causing the rapid rise in pressure from 2-3 Afterpoint 3 the rise in pressure is slower as the free fuel has been burnt andfurther combustion can only take place at a rate corresponding to the rate

at which fuel enters the cylinder through the fuel valve By the time the end

of injection is reached at 4 the pressure has ceased to rise as the piston hascommenced its outward stroke Burning of the remnants of fuel continuesbeyond this point, the process being slower because the oxygen is nolonger so abundant as it was at the beginning of combustion In an enginemaintained in good condition the fuel will be completely burnt by point 5and the gases are then expanded until the exhaust ports or valve opens atpoint 6 On board ship instruments for producing P.ediagrams will not

be available but it is possible to use an ordinary indicator to produce avery useful substitute This is done by using a cam set to operate the drummotion9ff out of phase with the crank A typical 'out of phase' diagramwhich results is shown in Fig 10 from which it will be seen that the various

salient features of the P.8 diagram can readily be recognised

It is often convenient to take both PV and out of phase diagrams onthe same card whilst the indicator is mounted on a particular cylinder.They then appear as in Fig II(c) As well as being most useful in detectingfaults in injection and combustion the out of phase diagram will oftenreveal faults in the indicator itself which may otherwise go undetected.Fig 11 shows some of the points for which a watch should be kept

4.8EXHAUST SMOKE

Malfunctioning of the fuel injection equipment resulting in such faults

as dribble from the fuel valves at the end of injection instead of a sharp cutoff, poor atomisation, or distortion of jets by carbon plugging the holes;causes a falling off in the quality of combustion These faults give rise tolocal regions of over rich mixture where combustion is incomplete leavingunburnt carbon particles in the exhaust These particles colour the exhaust

COMBUSTION AND THE DIESEL CYCLE 25

arey or brown and in extreme cases black smoke can be emitted Shortage

of charge air as a consequence of badly functioning turbochargers orfouled filters or silencers in the air inlet or exhaust systems will also bringabout similar conditions

Any attempt to overload a diesel engine by increasing the fuel willresult in shading of the exhaust

White smoke is occasionally produced, sometimes quite densely This

is usually the result of faulty fuel injection causing fuel to impinge oncomparatively cool surfaces in the combustion chamber from which it isvapourised but not burnt; the subsequent condensation into droplets as itleavesthe exhaust stack gives it its white appearance

Blue colouring in the exhaust is usually a sign of overlubrication of thecylinders Excess of lubricating oil can also contribute to white and darksmoke

5 FUEL INJECTION SYSTEMS AND

EQUIPMENT

5.1TYPES OF FuEL INJECTION SYSTEM

Fuel injection systems for diesel engines must be able to supply aImetered amount of fuel to each cylinder for each power stroke accordingIto the load on the engine, and must include a timing mechanism to ensurethat delivery0f this fuel commences at the correct moment, There are twotypes of injection system in use, both are basically hydro-mechanical Theone most commonly used is the jerk pump system, its essential elementsare shown in Fig 12, the other is the common rail system and its essentialelements are shown in Fig 13

In the jerk pump system a separate injection pump is provided for eachcylinder which operates once every cycle The barrel and plunger, togetherwith the cam, are dimensioned to displace fuel at the rate it is required inthe combustion chamber Ports in the barrel in combination with slots inthe plunger, or separate mechanically operated spill valves, determine theamount of fuel delivered and the timing of its entry to the cylinder Eachpump is connected to the injector, or injectors, serving one cylinder Theseinjectors have spring loaded differential needle valves which are set toensure that the fuel is raised to a sufficiently high pressure to causeatomisation when they automatically open to admit it to the cylinders Themetering and timing requirements demand that this pressurisation iscarried out over a short portion of the pumping stroke on the high velocitypart of the pump cam lift, this results in characteristically robust camshaftsand drives to cater for the high peak torques and in pipes between pumpsand injectors being kept as short as possible to avoid problems fromhydraulic wave action

The common rail system uses a multi-plunger fuel pump operatingcontinuously and discharging into a manifold which is maintained at ahigh pressure Fuel is supplied to the injectors (usually termed fuel valves)

in the cylinders from this manifold, the connection being controlled by atiming valve for each cylinder which determines the timing and duration(and hence the amount of fuel supplied) of each injection The pressure ofthe fuel in the manifold is controlled by spill valves and is suitably high toopen the differential needle valves in the injectors and cause atomisation

as it passes through into the cylinders The injectors are of the same design

in both systems Accumulator bottles connected to the manifold provide

26

FUEL INJECTION SYSTEMS AND EQUIPMENT 29

additional volume and help to damp out pressure pulses The fuel pumpshave their full stroke in which to pressurise the fuel so the peak torque isnot high in relation to the power required They are usually sited at oneconvenient point on the engine The timing valves are located close to thecylinders they serve but they require only a light driving shaft

5.2JERK TYPE FuEL PUMPS

The purpose of the fuel pump is to deliver fuel at a pressure which issufficiently high to cause the spring loaded needle valve of the injector tolift and so force fuel through an orifice drilled in the end of a nozzle Thepressure of this fuel is such that atomisation is caused by the velocity ofthe discharging jet relative to that of the dense air in the combustion space.Fuel pumps as generally fitted to marine diesel engines are of thespring-return ram plunger type, the plunger is lapped into a sleeve which ismade of hardened steel or of special cast iron The quantity of fuelrequired can be regulated in various ways The manner in which this iscarried out in the Bryce fuel pump is shown in Fig 14 The plunger is

actuated by a cam and roller tappet follower When the follower is on thebase circle of the cam the pump plunger is at the bottom of its stroke andthe inlet port in the barrel is uncovered allowing fuel in the gallery whichsurrounds the barrel to flow into and fill that portion of the barrel above

30 MARINE ENGINEERING PRACTICE

the plunger The plunger is a close fit within the barrel As the cam rotatesthe plunger rises and seals off the inlet ports, and at this point of the strokethe pumping action starts By means of adjustment in the tappet followerthis event can be timed in precise relationship to the crank angle Furtherupward movement of the plunger causes the fuel to be raised in pressureand expelled through the delivery valve to the injector A helical grooveextends from the top of the plunger part way down its cylindrical surface.Angular positioning of the plunger about its axis so locates this groove inrelation to the inlet port to give a greater or lesser stroke with the helical

groove In some designs a separate spill port is provided but as this is also

made to communicate with the inlet gallery the action is precisely thesame When the port is opened to the groove the high pressure in the fuelabove the plunger is released to the lower pressure in the gallery andpumping ceases although the plunger continues to move upwards Theamount of fuel delivered will vary in accordance with the effective length

of stroke thus determined by the angular position of the plunger Thisangular position is controlled by a rack and pinion, the latter forming asleeve having an axial slot or slots to receive a projection on the plunger.The rack position therefore determines the quantity of fuel supplied.The plungers are a close fit within the barrels to ensure sealing at highpressures, but some leakage of fuel between the plunger and barrel isinevitable and, in fact, leakage is necessary to provide lubrication for therelative movement

The timing of the injection is controlled by the instant that the pumpplunger closes the inlet port This instant can be adjusted by raising orlowering the plunger with reference to the cam Raising the level of theplunger will advance the point of injection

Direct reversing engines require a means of adjustment which willpermit alteration of the instant of timing for both senses of rotation Itwill be appreciated that altering the height of the tappet screw adjusts thepoint of injection in the same sense for both directions of rotation Byproviding a means of adjustment in a horizontal direction of the centre ofthe lever follower, shown in Fig 15, the timing can be altered in a waywhich will advance it in the ahead direction of rotation whilst retarding it

in the astern or vice versa This facility coupled with adjustment of thetappet screw allows timing to be adjusted precisely for both senses ofrotation

On the Sulzer engine each fuel pump is provided with a suction valve, adelivery valve, a spill valve and a relief valve The effective delivery stroke

is adjusted by altering the length of the spill valve or the suction valve pushrod Shortening the spill valve push rod means an increase in the effectivedelivery and lengthening the suction· valve push rod means a reducedeffective delivery

The upward stroke of the plunger begins when the driving roller isforced off the base circle of the cam If the suction valve is open no fuelcan be delivered as the pressure chamber of the pump communicates with

the suction chamber Fuel is delivered to the injector when the controls are

so arranged that the suction valve push rod is lowered and the suctionvalve closed The delivery valve operates as a non-return valve andremains open only as long as the pressure before the valve is greater thanthe pressure after, as soon as the spill valve opens the delivery valve closesautomatically

A fuel pump design by a.M.T is shown in Fig 16 On the suctionstroke of the pump fuel is forced by the booster pump through the suctionvalve into the chamber of the pump On the discharge stroke fuel is forcedout of the chamber and through a pipe to the injector When the injectoroperating pressure is reached the needle valve lifts and fuel is injected intothe cylinder The end of the injection phase is determined by the opening

of the spill valve which is mechanically actuated through a lever and tappetmechanism The moment the spill valve opens the pressure in the systemdrops and the injector needle seats During further lift of the pumpplunger fuel is discharged back to the suction side of the pump as long asthe spill valve remains open

The delivery valve of the jerk type fuel pump plays an important part

FUEL INJECTION SYSTEMS AND EQUIPMENT 33

in ensuring a clean end to the injection of fuel As well as acting as a return valve to prevent total back flow of fuel from the high pressure pipeconnecting the pump to the injector, it is designed to withdraw a smallamount of fuel from this pipe as it closes This causes the pressure at theinjector to fall more steeply than would otherwise be the case, thusproviding a sharp end to the injection and helping the injector needle toreseat without causing dribble at the nozzle holes Delivery valves aredesigned on one of two basic principles; volume unloading or pressureunloading Fig 18 In the volume unloading design the action is broughtabout by a piston portion formed on the valve as illustrated in Fig 17 The

non-L