diesel engines

Diesel Engines Diesel Engines THIRD EDITION A.J Wharton, CEng, FI MarE OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYNDEY TOKYO Contents Engine Types Cycles and Timing Gas Exchange Processes Engine Parts Operating Systems Control Safety and Operation Index Preface This third edition of an established book has become necessary to enable the marine engineer to keep abreast of new developments in design and manufacture of marine diesel engines As the most efficient means of producing power from low grade fossil fuels, the diesel engine is predominant as the propulsion machinery for ships It should be remembered that the majority of the world's produce is still transported by sea The book continues to meet the requirements of Marine Engineer Officers preparing for examinations The wider coverage is supplemented by additional diagrams to aid understanding and retention of important details and to facilitate easy reference Engine descriptions are limited by the size of the book but are sufficient to show present trends in current engines Some earlier large engines have been retained, as these illustrate some of the major changes in past development Modern designers have the benefits of computer aided design and finite element analysis together with improved materials and manufacturing processes, all of which have contributed to the highly efficient and reliable engines available today New developments in turbocharge systems improve fuel combustion efficiency over a wide range of engine speeds and allow the effective use of previously wasted energy to improve the total energy output It is not surprising that engines from different manufacturers appear to have many features in common It is the adding of new technology to well proven practice that ensures reliability and gives ship-owners the confidence to order new engines even before they have been proved in service Acknowledgements The author wishes to express his appreciation for the assistance given by the following manufacturing companies, which were most generous in providing information and allowing the use of their material ABB, Asea Brown Boveri Ltd; MAN-B & W, MAN GHH (Great Britain) Ltd; SEMT Pielstick, NEI Allen Ltd Crossley Engines; Stork-Wiirtsilii Diesel Ltd; New Sulzer Diesel UK Ltd; Wiirtsilii Diesel Oy He is also grateful for the help of the Information Centre, Institute of Marine Engineers vii SI UNITS Mass Force Length Pressure Temperature = kilogram = newton = metre = newton/sq metre = degrees celsius CONVERSIONS I inch=25.4mm=0.025m I foot=0.3048m I square foot=0.093m1 I cubic foot=0.028mJ I pound mass (lb)=0.453kg I UK ton (mass) = 1016kg I short ton (mass) = 907 kg I tonne mass = IOOOkg I pound force (lbf)=4.45N I ton force (tonf)=9.96kN I kgf=9.81 N (kg) (N) (m) (N/m1) ("C) 0.OOlin=0.025mm ("F-32)x;-=oC I Ibf/in1= 6895N/m1= 6.895kN/m1 I kgf/cm1= I kp/cm1= 102kN/m1 I atmos= 14.7Ibf/in1= 101.35kN/m1 I bar= 14.5Ibf/in1= IOOkN/m1 Note: For approximate conversion of pressure units lOOkN/m1= I bar = I kg/cm1= I atmos I tonf/in1= I5440kN/m1 = 15.44MN/m1 I HP=0.746kW CHAPTER Engine Types Marine engines are required to operate continuously, reliably and safely in unmanned engine rooms and with extended periods between planned overhauls They may be expected to operate for considerable periods at low power without ill effects, and to be tolerant oflow quality fuels while maintaining very high thermal efficiency Continuing development and improvement in design are necessary to meet these demands and new generations of engine models are produced to take advantage of advances in research and experience Considerable improvements in design and performance of turbochargers and charge air systems in recent years have contributed substantially to increases in engine power and economy The supply of charge air at reduced power is sufficient to maintain efficient combustion, arid at full power surplus exhaust gas energy can be diverted to operate 'power take off systems which will supplement the main or electrical power output Changes in world trade may lead to the emergence of different ship types with special demands upon their power systems Manufacturers produce a range of cylinder sizes and numbers in each model so that a wide selection of power or other parameters is available Further sizes are developed when a particular demand becomes evident Practically all new merchant ships are powered by diesel engines, and some existing large steamships have been re-engined with diesel power to improve their economy and extend their useful life Basically, marine diesel engines can be divided into two main types: large, slowrunning direct drive engines with limited numbers of cylinders, or medium to high speed engines driving through reduction gears Cylinder sizes not necessarily distinguish between these; slow-speed engines are available with cylinder bores down to 260 mm, while medium speed engines are produced with bores up to 620 mm Although there are distinct differences between both types, much of the subject matter in this book is relevant to all engines Where necessary their characteristics are dealt with separately SLOW-SPEED ENGINES This category refers to engines operating at between 55 to 150rpm Large, slow-speed, direct drive main engines operate exclusively on the two-stroke cycle and are of crosshead construction which allows complete isolation between the cylinders and the crankcase They are best able to tolerate low-quality fuels and burn these successfully to obtain the highest thermal efficiencies Slow piston speed and fewer working parts make them very economical in lubricating oil, and give low rates of wear and remarkable reliability Although there are now few manufacturers producing these engines, they dominate the market particularly in ocean-going ships A variety of size and numbers of cylinders are available, to suit all power requirements In addition to the standard models they are produced in long-stroke I ENGINE TYPES versions with stroke/bore ratio up to 3.8: I and at speeds down to 55 rpm, allowing the use oflarge, slow propellers for high efficiency in vessels such as bulk carriers and large tankers Slightly shorter stroke engines operating in the speed range of 100 rpm are designed for high speed container ships Short-stroke models cater for ships with limited draft, propeller size or engineroom height Many of the basic engine components are common to all versions Current models of two-stroke engines are illustrated here, together with some older models which are still in operation and have distinctive features of which the student must be aware SULZER RTA Engines Fig 1.1 shows a Sulzer RTA 84C engine which is a typical modern slow-speed, two-stroke, crosshead type, long stroke engine It has a boreof840mm, a stroke of2400mm and an operating speed of 100rpm It is available with between four and twelve cylinders and is particularly produced for the large, fast container ship market Its design and construction is similar to other engines in the RTA Series which offer a number of cylinder sizes down to 380mm ENGINE TYPES A deep single-wall, box type bedplate fabricated from welded steel plat.es and castings, and substantial welded 'A' frames surmounted by cast iron cylinder jackets boIted together to form a cylinder block, together give a rigid overall construction The structure is pre-stressed by long vertical tie bolts Cylinder liners are of alloy cast iron A stiff collar at the upper end resists the heavy gas load; this lands on the cylinder jacket It is bore cooled with water flow rates regulated to maintain correct temperatures throughout the liner The lower end is uncooled within the scavenge space Multi-level cylinder lubrication is used to reduce liner wear rates The solid forged steel cylinder cover is bore cooled to reduce thermal stress and to conduct heat from the fuel injector pockets A centrally positioned large exhaust valve cage has its valve seat intensively cooled, taking water from the cover The valve is manufactured of Nimonic 80 A alloy and is rotated by vanes fitted to its spindle It is opened by hydraulic pressure from a cam driven actuator and is closed under the action of an air spring The piston has an alloy steel crown and has five compression rings fitted in chromium plated grooves There is a short cast iron skirt The piston is oil cooled using ENGINE TYPES both the 'shaker' method and small jets to propel oil into bore holes close to the underside of the crown and behind the ring grooves Oil for cooling is supplied and returned through a bore in the piston rod from swinging links at the crosshead The single piece cross head has the piston rod bolted to its upper surface and a continuous full-length lower half, top end bearing, which is of white metal and lubricated with high pressure oil Guide shoes are attached to each end of the crosshead A semi-built up crankshaft has slim webs to allow large bearing areas; main bearing caps are secured by jackbolts from the engine frames The main camshaft is gear driven and is fitted with servomotors to re-time fuel pump cams and the air start distributor when operating the engine astern Fuel pumps are of the cam-driven valve-timed type, with variable ignition timing to adjust timing and maintain efficient combustion at low speed Each pump supplies three uncooled fuel injectors placed symmetrically in each cover Hot fuel oil circulates the valves when not injecting The engine has through-scavenge and constant pressure turbocharging with a high efficiency, uncooled turbocharger supplemented at very low speeds by two constant speed, electric driven auxiliary air blowers Fig 1.2 shows a Sulzer RT A Series I engine These have many similarities to the Series engines which were developed from it This engine has water cooled pistons Water is supplied and returned through telescopic glands These are entirely separated from the crankcase to avoid any possibility of contamination A separate piston cooling water circulation system is used The crosshead has split top end bearings The piston rod is attached to the centre block of the crosshead between the two top end journals Each bearing has a flexible support to limit load concentration Thin-wall aluminium-tin bearing shells are fitted MAN-B & W MC Engines Fig 1.3 is of a MAN-B & W K90 MC-·C engine This is a large crosshead type two-stroke engine with a bore of 900 mm, a 2300 mm stroke and an operating speed of 104 rpm It is constructed with between six and twelve cylinders Developed as one of the extensive range of the manufacturer's MC engines, it is of the power and speed best suited to large, fast container ships The increase in running speed is obtained by a slight decrease in engine stroke High thermal efficiency is maintained by an increase in mean effective pressure Construction can be considered generally as typical for the whole range The engine bedplate is of rigid box form, fabricated from steel plates with main bearing supports of cast steel Welded 'A' frames are assembled into a frame box which contains the crankcase, the crosshead guides and also supports the wheels for the chain drive of the camshaft A cast iron cylinder frame accommodates the scavenge space between the cylinder jacket and the diaphragm, both of which are water cooled Long pre-stressed tie bolts are fitted between the top of the frame and the underside of the bedplate girders The cylinder liner is of alloy cast iron, its upper flange lands on top of the frame and has bore cooling It is secured by a forged steel cylinder cover which is also bore cooled and is shaped internally to accommodate most of the combustion space Cylinder lubricating oil is injected at one level in the liner Pistons have a chrome-molybdenum alloy steel crown with hard chrome-surfaced ring grooves in which four compression rings are fitted In this particular model a protective layer oflnconel is welded to part of the crown surface to prevent high temperature corrosion The piston is oil cooled, oil The effect of fuel properties on performance The relationship between density and calorific value has been studied by a number of authorities and empirical expressions have been developed as a result For example, the formulae given in BS 2869: Part (Appendix C, Section C 5) enable sufficiently accurate calculations to be made for Qgv and Qnp using the density of the fuel and applying corrections for any sulphur, water and ash present The British Standard also offers charts (derived from these equations) which enable rapid estimates to be made of the values The equations and figures are identical with those published in ISO 8217: 1987 12.6 The effect of fuel properties on performance The effects on engine performance of the various fuel properties are summarized in the sub-sections which follow 12.6.1 Viscosity The lubricating property of a fuel oil reduces as its viscosity reduces The range of kinematic viscosity in commercial distillate fuels is 1.5 to 5.5 cSt at 40°C (BS2869) Injection pump wear may be accelerated if the kinematic viscosity falls below cSt at that temperature The fuel itself provides the lubrication for the plungers in fuel pumps Distillate fuels with viscosities between and cSt are 'dry' and lack sufficient lubricity to adequately lubricate the cams, tappets and bearings of the injection pumps This could result in scuffing and ultimate failure of these parts Fuels of this type are widely used in Canada and are referred to as 'arctic' fuels Special precautions also need to be taken when high-speed engines are required to use the 'dry' type aviation fuels obtained from kerosene-naphtha fractions These may include Avtur 40/50, Avtag lP-4 and Avcat lP-5 It becomes necessary to regularly drain the fuel injection pump and fill it with lubricating oil, or to employ a separate lubricating oil tank and pump for automatic lubrication of the injection pump Heavy residual fuels must be heated (to reduce their viscosity) before they are admitted into the fuel injection systems of medium-speed and slow-speed engines In practice, a composite system of heating, treating and filtering is used in order to ensure that the fuel arrives at the engine pumps and injectors in the correct condition for combustion The fuel's temperature is progressively increased to about 95°C before it is transferred to the daily service tank The general aim is to maintain a fuel viscosity between 10 and 20cSt during treatment Thereafter, the correct injection pressure, injection timing, and running temperatures must be sustained 441 It is normal practice to employ gravity feeds from day tanks to engines when operating on distillate and intermediate fuels This is not feasible with residual fuels and special steps have to be taken to avoid the oil viscosity being too high at the fuel injection pumps because of temperature drop due to low velocity in the delivery pipes One engine manufacturer [10] uses a bus-rail system around which a small pump circulates oil at a rate times greater than the engine's full-load consumption A bus-rail heater placed between this pump and the engine compensates for any heat losses from the lagged delivery pipes between daily tank and engine and from the bus rail Incidentally, when heavy residual fuels are used the engines must be started and stopped on light distillate oil, 12.6.2 Density We established in Sub-section 12.5.11 that the density of a fuel is related to its calorific value This means that density has a direct bearing on an engine's fuel consumption and on its power output Also, the 'burnability' (or the ignition quality) of a fuel is influenced by its cetane number, which also has a direct relationship with density Heavier residual fuels have lower heating values on a weight basis but increasing values on a volumetric basis Since fuels are bought on a volumetric basis there is an incentive to select those of high density Any economic advantages so gained may be offset by the operational problems that could arise These include engine knocking, after-burning, uneven burning, variation in ignition delay, and a steeper ignition pressure gradient (see Figure 1.4 of Chapter 1) As Wilbur and Wight point out [9] ' these factors contribute to increased fatigue of engine components, excessive thermal loading, increased exhaust emissions, and critical piston ring and liner wear The long-term effects on an engine are la significant increase in fuel consumption and component damage The greatest fouling and deposit build-up will occur when the engine is operated at reduced or very low loads ' Another important consideration with high density residual fuels is that with values approaching 1.0 glml (they can be as high as 0.995 for some lowgrade fuels) there will be separation problems during pre-treatment This is because above a density of 1.0 it is not possible to separate water from the fuel oil by normal centrifugal action [11] More sophisticated pre-treatment plant is required The performance figures for high-speed and medium-speed engines operating on distillate fuels are usually based on fuels complying with BS 2869 and ASTM D-975, i.e those with densities around 0.835 to 0.865 Since fuel pump settings are 'calibrated' on volume of fuel it becomes necessary to adjust 442 Fuels and lubricating oils the engine maker's declared performance figures if the density of the fuel to be used varies from the calibrated value This is of particular significance when intermediate grade fuels, produced by mixing the lighter residual fuel oils with distillates, are to be used 12.6.3 Low temperature characteristics The cold filter plugging point (CFPP) for distillate fuels should always be below the minimum starting temperature at which the engine is to operate This ensures satisfactory fuel flow through the filters and prevents wax crystals collecting on them and blocking them The nearer the operating temperature is to a fuel's CFPP the shorter the life of the filters In those areas of the world where high day-time temperatures are followed by low night temperatures, the CFPP should be below the night-time temperatures One way of using fuels with a high CFPP is to arrange for an auxiliary supply of low CFPP fuel to start and run the engine until it is thoroughly warmed, after which, the high CFPP fuel may be used Operation reverts to the low CFPP fuel just before the engine is stopped The engine is run on this fuel for a sufficient time to ensure that the filters, fuel injection pumps, and nozzles are saturated with the fuel Similar precautions are necessary with the distillates used for starting those engines which operate on the heavier residual fuels Problems are more likely to occur with mobile and transportable equipment operating in low ambient conditions Shipboard generators, those installed in power stations, and standby generators housed in heated plant rooms should experience no difficulties 12.7 Harmful constituents in fuel oils We shall now discuss the effects of those constituents most harmful to engine operation Problems may be magnified where combinations of several harmful constituents occur 12.7.1 Carbon residue Prolonged operation on a fuel with a high carbon content is likely to result in the formation of hard combustion residues These will be particularly evident at injector nozzles and in the piston crown, and will also lead to excessive top ring groove wear and sticking of rings It is important that the fuel injection characteristics of engines using residual fuels (which can have carbon residues between and 15 %) are satisfactorily maintained The main hindrance to clean injection will be the build-up of carbonaceous deposits on nozzles (trum- peting) due to nozzle temperature This causes intermittent sticking of the needle, resulting in a wet nozzle and carbon formation on the tip The trumpet formation has the effect of reducing combustion efficiency More fuel is required for a given load on the engine and cylinder temperature rises The conditions under which pistons, rings, and exhaust valves must operate then become progressively more arduous In practice, the carbon builds up on the nozzles to a point where the trumpets detach themselves from the nozzles, whereupon, injection conditions return to near normal, and the cycle recommences A periodic rise and fall of cylinder exhaust gas temperatures at steady load is evidence of this condition occurring In extreme cases the temperatures may reach such high values that the needle seat may begin to lose its hardness and the needle rapidly hammers its way into its seat until the nozzle is ruined [10] Various methods may be used to control nozzle temperature These include cooling by water or by a separate distillate fuel circuit Engine wear rate may be greatly increased where high carbon content coincides with high sulphur content 12.7.2 Sulphur content Residual fuels may contain up to % sulphur by weight Part of this sulphur (in the form of sulphur trioxide, S03) combines with water vapour, which is another combustion product, to form sulphuric acid (H2S04), If the temperature of the acid vapour falls below its dew point when in contact with the relatively cool surfaces of the the cylinder walls the vapour condenses on the cylinder walls (Note: the dew point is that temperature at which the vapour saturates and begins to condense - anywhere between 110°C and 1800C) This causes serious corrosion on liners, piston-ring grooves, and piston rings Careful contr61 of cylinder liner temperatures is therefore required to ensure that they are maintained above the acid dew point Corrosive wear may be minimized by using a highly-alkaline cylinder lubricating oil Problems are not confined to engines running on residual fuels They may also occur on higher speed engines using distillate fuels with a combined sulphur and carbon content exceeding % Again, wear rate is minimized by maintaining high jacket water temperatures and using heavy duty lubricating oils 12.7.3 Ash content The ash-forming constituents of most concern when operating on heavy fuels are vanadium, sodium, aluminium and silicon Gaseous fuels High vanadium content may lead to: • hot corrosion on parts such as exhaust valve seats, valve plates, and the turbine blades of turbochargers; and • the formation of hard, brittle deposits on these parts Partial breaking away of deposited layers may cause blow-by at the exhaust valves The turbine efficiency is reduced because its passageways are narrowed, and its blade contours are altered by the deposits [11] These problems are increased when appreciable quantities of sodium are also present in the fuel - because the melting point of the vanadium-sodium eutectic falls to about 550°C Methods of preventing, or delaying, deterioration of valve seats include the use of exhaust valve rotation and water-cooled valve cages The fuel may also be water-washed to remove the sodium compounds This is not a feasible solution in marine applications Problems are not confined to residual fuels - in which vanadium and sodium contents may be as much as 400 parts per million (ppm) and 150ppm, respectively They are also likely to occur on high speed engines using the lighter distillates Contents of 50-60ppm may cause trouble Limiting values of lOppm are usually recommended if frequent exhaust valve and turbocharger rotor renewals are to be avoided The aluminium-silicate compounds used by refineries in catalytic cracking processes may not be fully removed from end products The compounds are hard and can cause wear on injection pump plungers, injector nozzles, and piston rings 12.8 Gaseous fuels The gaseous fuels used in spark-ignition, dual-fuel and alternative fuel engines may be classified under two broad groupings: • natural gas (NG); and • by-product gases They differ in their properties and in their commercial availability 12.8.1 Natural gas This is defined in BS 1179 as one 'consisting mainly of methane, occurring naturally in underground accumulations' The terms associated and nonassociated are applied to reserves of natural gas Those in which the gas is in contact with crude oil in what is predominantly an oilfield are known as associated reserves The gas may be in a dissolved state within the crude oil (solution gas), or it may be 443 in contact with gas-saturated crude (gas cap) Gas production is very dependent upon the development programme for the crude oil and on the market potential for that gas which is surplus to oilfield requirements (e.g not re-injected or re-cycled) Those reserves from which it is only economical to produce gas, or where gas production is not significantly related to oil production, are known as nonassociated reserves The gas may be retained in its reservoir until suitable marketing outlets have been identified Although it is usually found in deeply buried bed formations the gas is fairly easy to collect and has the advantage of being available at high pressure [12, 13] The natural gas industry's activities may be broadly classified as: • • • • production; transmission; storage; and distribution Production This includes treatment and processing It is not within the scope of this chapter to describe these operations in any detail Very briefly, the field gas which is brought up to the well-head passes through a treatment station This may be at the well-head site or at a location central to several well-heads In the case of off-shore fields the station will be a shore terminal The operations at the treatment station may include the separation of any brine, condensed water, and liquid hydrocarbons present in so-called wet gases They may also include desulphurization; carbon dioxide and nitrogen removal, and dust and oil-fog removal (Note: the term dry gas is used to describe that gas which contains no (or very little) liquid hydrocarbons.) Where natural gas needs to be converted to its liquified form for transportation it must be purified The process consists of the removal of impurities such as water, carbon dioxide and hydrogen sulphide, and any heavy hydrocarbons such as propane and butane Purification may take place either before or during liquefaction In the latter case the impurities are frozen-out during the process The critical temperature of any gas is defined as that temperature above which it cannot be liquified by pressure alone Since methane, the major constituent of NG, has a critical temperature well below ambient, it needs to be cooled to about -160°C before it can be transformed into liquid at atmospheric pressure The major natural gas production areas are the USSR and North America - accounting for about 55 % of the world's output The rest comes mostly 444 Fuelsand lubricating oils from the Middle East, Africa and Western Europe (including the North Sea) A typical analysis [13] for North Sea gas is: % volume Methane Ethane Propane Butanes Other hydrocarbons (e.g pentanes) Carbon dioxide Nitrogen 94.5 3.3 0.7 0.3 0.2 0.7 0.3 -100.0 Transmission Long-distance transmission of NG is achieved in two ways: by passage as gas through pipelines; or by transportation as a liquid in ships, rail cars and road tankers An example of how both methods may be integrated into the one system is provided by the British Gas Council's transmission system for natural gas from the North Sea [13] The welI-head gas pressure is usualIy reduced to about 70 bar (7 MN/m2) before transmission along pipelines It may be necessary to employ intermediate gas compression stations, spaced along the length of the main transmission lines They serve to restore the gas pressure in the system to near its initial inlet level Liquefied natural gas (LNG) must be transported and stored at temperatures below its boiling point of -160°C The insulation and refrigeration problems this initialIy posed have now been overcome but the cost of transportation and storage is high when compared with that for other petroleum fuels Peebles and Clayton [12] point out that this cost differential is further exacerbated by the fact that the density of LNG is about half that of crude oil, and its calorific value (per unit weight) is not much more In the 1970s the capital cost of a large crude oil carrier was about one-sixth of that of an LNG tanker of the same 'heat-carrying' capacity There is no reason to believe that this cost ratio has changed since then Despite this, the movement of natural gas in liquified form has opened up the possiblities of developing gas reserves in locations distanced from large energy-consuming markets Storage Large quantities of natural gas may be stored in aquifers which are underground ieservoirs similar in structural form to NG fields but with no gas in the porous layer [13] Long-term storage under high pressure in underground steel reservoirs, and by solution in propane, are other possibilities Variations in weekly and seasonal demands are met in this way Pipeline storage is useful in meeting peak demands of shorter duration The technique is known as line-packing Gas is stored in the pipelines at a higher pressure than is theoreticalIy needed so that the various elements in the piping network act as high-pressure gas holders Very accurate and centralized control of the system is then required to cope with variations in demand from different parts of the total system The British Gas Council operates one such computerized system which controls the transmission of gas from the off-shore gas fields, through shore terminals, to the bulk distribution grids of the Area Boards [13] Six hundred cubic metres of NG reduce to m3 of LNG Storage of NG in its liquid form thus requires much less space at the receiving terminal than it does in its gaseous form The LNG must be regasified and the calorific value of the resultant gas adjusted to that of the gas distributed to consumers Distribution Gas is metered at the transmission line take-off points and its pressure is adjusted to about bar (0.5 NMlm2) for the medium pressure distribution feeders to major industrial consumers In areas of heavy demand there may be further low-pressure feed-in points It may be necessary to compress the gas at the inlet to a dual-fuel engine to compensate for the supply pressure fluctuations that would otherwise occur when the engine operates at varying load levels TypicalIy, the gas may be compressed to 2.75 bar (0.275 MN/m2) The simplified schematic diagram of Figure 12.2 [12] shows the activity phases for both NG and LNG schemes 12.8.2 By-product gases There are four natural sources of methane gas (CH4) The first is natural gas The second is marsh gas, associated with marshy areas in which the methane is produced by the decomposition of plants by bacteria in the absence of air [14] The third source is that produced by the digestion of animal, human and industrial waste materials (bio-mass) The recovery process makes use of digesters The most widely used large-scale digesters are those found in sewage works The fourth source, and one which is being increasingly exploited, is bio-gas from landfill sites which contain domestic and industrial wastes Here, the gas is piped from large waste tips Gaseous fuels It is usually dried, but it may also be scrubbed, before use The scrubber is designed to remove impurities (particularly those of a corrosive nature) and to increase the calorific value of the gas A large-scale method of scrubbing the gas is to bubble it through water at high pressure Aerobic digestion takes place in the presence of free oxygen while the anaerobic process occurs in the absence of free oxygen Both systems rely on bacteria to break down organic matter Sewage works may use a combination of them in a two-stage process in which the secondary digesters are open tanks The quantity and composition of the bio-gas produced depends upon the organic feed materialsthe major constituents of which are carbohydrates, fat, and protein Gas composition may be in the range 60 to 70 % methane and 30 to 40 % carbon dioxide Other gases, such as carbon monoxide, hydrogen, nitrogen, oxygen and hydrogen sulphide, may also be present Their proportions depend on the digested feedstock The methane content of landfill gas may be as low as 30 % This is reflected in the comparison of its lower calorific value with those typical of other gaseous fuels: 445 Lower calorific value (MJ/m3) Well head gas Piped natural gas Sewage gas Landfill gas 40.0 34.7 25.0 12.0 One of the problems with landfill gas is that it may contain trace elements based on fluorines and chlorides, which can be very corrosive on engine parts [15] Large sewage works are usually self-sufficient in terms of electrical energy They use dual-fuel engines which may run on diesel alone or on a mixture of diesel and bio-gas It is standard practice to use the waste heat energy derived from the generator to heat the plant's digesters Small scale digesters may be installed on farms The gas may be used to provide heat for intensive animal rearing or to power refrigerators and generate electricity for milking machinery on dairy farms Other types of by-product gas are those obtained from refinery processes These consist mainly of C1 446 Fuels and lubricating oils to C4 hydrocarbons with variable amounts of hydrogen, nitrogen and, possibly, hydrogen sulphide Typical examples are commercial butane (C4HIO) and propane (C3HS)' which are capable of being kept in the liquid state by bottling under pressure 'Sweeting plants' are used at the refinery to remove the hydrogen sulphide (HzS) from them They have high calorific values - typically 94 MJ/m3 for propane and 122MJ/m3 for butane Bottled gas (NG is also available in this form) is more appropriate to domestic and commercial consumers located in remote areas where it is used in burners for space heating and for air conditioning It is unlikely to be used for other than very small generators 12.8.3 Gas analysis Engine makers request complete analyses for the gases on which their engines are to operate They are particularly concerned with the presence of any inert gases which would have the effect of reducing the engine's power output on site Also, hydrogen sulphide contents in excess of 500ppm may be problematic The corrosive effects of HzS are similar to those resulting from excessive sulphur content in liquid fuels Jones [15] reports that much of the matching of lubricating oils to his company's (Dorman Diesels) range of gas engines was carried out in well-head gas applications, and it has been found that this oil technology has also been applicable to engines operating on 'sour' bio-gas produced in both digesters and landfill sites (Note: a sour gas is one containing acid gases, such as hydrogen sulphide and carbon dioxide.) Correct timing of the lubricating oil changes is critical when engines operate on bio-gases which contain acidic products as the oil loses its reserve alkalinity quicker The methods for the analysis of fuel gases are described in BS 3156 (see referenced standards in Section 12.11 of this chapter) Some fuels, particularly those of the bio-gas type, are wet gases This means that they contain moisture which needs to be removed before they are used This is particularly necessary when the gas has been cooled below its dew point Knock-out pots (vessels which act as receivers for condensate and remove water droplets) may be employed in the gas stream An alternative approach (used on larger installations) is to chill the gas and then allow its temperature to rise again to reduce the moisture content [15] Gas supplies should not be assumed to be clean Rust which may have accumulated in pipework can be scoured off with the passage of gas It is, therefore, desirable to have efficient filtration close to installed engines 12.8.4 Gas properties The significant properties of fuel gases are: • Calorific value • Relative density • Wobbe number (related to both of the above properties) The calorific value (cv) of a gaseous fuel is a measure of the heat liberated by the complete combustion under specified conditions of unit volume of the gas It is conveniently expressed in MJ/m3 The gross calorific value relates to the condition in which it is assumed that the latent heat of the water vapour produced by the combustion of the fuel is released The net calorific value, on the other hand, assumes that the water produced by combustion remains as a vapour The latter may also be referred to as the lower calorific value (see Sub-section 12.8.2) Methods for the determination of calorific value are specified in BS 3804 The descriptions rich gas and lean gas are applied to gases of relatively high calorific value (those containing mainly low boiling point hydrocarbons) and of relatively low cv, respectively The term lean gas should not be confused with 'lean burn', which refers to the running of a gas engine on a high air-to-fuel ratio See Section 1.8 of Chapter The relative density of a gas is the ratio of the mass of unit volume of dry gas to that of unit volume of dry air under the same conditions of temperature and pressure (BS 1179) It is therefore a pure number Typical figures for natural gas may vary from 0.59 to 0.64 The reader may meet the term Wobbe number in a gas supplier's specification It represents a measure of the heat released when a gas is burned at constant gas supply pressure and is given by the formula: Associated terms are gas modulus, which is given by the formula: and gas group, which is the classification of a gas according to its Wobbe number A supplier's specification may also include the octane number (or the motor octane number) of the gas It is a measure of the limit of detonation, and corresponds to the percentage of iso-octane in a mixture (of iso-octane and heptane) which is equal to the product gas in knock characteristics Knock- Lubricatingoil ing describes the detonation in a gas engine cylinder due to over-compression of the air-gas mixture before sparking The octane number is analogous to the cetane number of a distillate fuel oil Both relate to the ignition quality of a fuel Jones [15] advises that a 'mean octane number' calculated from those of the various constituents in a gas is not always a good guide to a fuel's ignition quality The heavy fractions in the gas can give rise to detonation where the mean number might suggest that no problem exists This particularly applies to turbocharged engines where compression pressures are high Also, because most engines of this type are charge cooled, the temperature of the air-gas mixture entering the cylinder is critical If heavy fractions are present in the gas the inlet manifold air temperature must be kept low 12.9 Lubricating oil The many tasks that a lubricating oil has to perform within an engine were discussed in Chapter (Subsection 1.7.1) When selecting oils for service requirements one needs to take account of the conditions that exist between engines of different speeds and piston connections For instance combined cylinder and bearing lubrication is used in high-speed and medium speed trunk piston engines, whereas separate lubrication of cylinders and bearings is customarily applied to low-speed, crosshead type engines Cylinder lubrication conditions can be particularly severe in those engines which use the combined system The period between engine overhauls will largely depend on the rate of cylinder wear and the condition of the cylinders themselves [16] Modern oils fur engines are of the mineral type, manufactured from crude oil (see Figure 12.1) Most are now refined using solvent extraction processes, in which undesirable compounds are selectively dissolved and removed [17] Further processes follow, such as refrigeration prior to dewaxing and clay filtering, to give a rationalized range of oils from the one refinery These oils are then blended and compounded with additives to give the many grades and special properties needed for engine duty 12.9.1 Properties of lubricating oils The chief properties of lubricating oils measurable by physical and chemical laboratory tests are listed below The first nine are common to those of liquid fuel specifications (see Section 12.5) and the tests used to determine them are similar We shall consider the significance of the properties in relation to engine lubrication 10 11 12 13 14 15 447 viscosity relative density carbon residue ash content sulphur content water content pour point cloud point flash point acidity/alkalinity demulsification number insolubles content diesel fuel diluent foaming oxidation stability Viscosity This is perhaps the most important of the oil properties since it determines the effectiveness of the oil film separating moving and rubbing surfaces It controls the load-carrying ability and affects the friction and wear of bearings, governs the oil's sealing effect and the rate at which the oil is consumed [6] While every attempt is made to standardize on the use of kinematic viscosity (cSt) in oil specifications, the arbitrary (flow time) units of Saybolt, Redwood and Engler still persist within the industry Also, the Society of Automotive Engineers (SAE) viscosity classifications continue to be widely applied Devised initially for the automotive industry, the SAE system uses numbers which represent a range of viscosity at given below-zero temperatures for the lower viscosity oils and at 100°C (212°F) for those of higher viscosities The oil grades are classified in the order of their viscosity (see Table 12.1) The viscosity of the six thinner grades (carrying the suffix W for 'winter') is the dynamic viscosity, at sub-zero temperaturesexpressed in centipoise (cP) That for the four Table 12.1 SAE Grades for engine oils SAE Number OW 5W lOW 15W 20W 25W 20 30 40 50 Max viscosity (cP) at °C 3250 at 3500 at 3500 at 35po at 4500 at 6000 at - - - - 30 -25 - 20 -15 -10 -5 Kinematic viscosity at ]()(f'C Max Min 3.8 3.8 4.1 5.6 5.6 9.3 5.6 9.3 12.5 16.3 -