Diesel engine bearings are lubricated by fluid Aims. The journal is always smaller than its surrounding bearing. When the shaft is static it will make contact with the bearing and this contact will be a line. On each side of this line the normal distance between the shaft and the bearing will increase gradually and will in effect be a curved wedge. When the shaft revolves in the presence of an adequate liquid supply (lubricating oil), the oil is pulled into the wedge and pressure is set up. If these liquid pressures were to be plotted on lines drawn -radially from the centre of the bearing.it would be seen that the plot of these pressures would form a bulge something like a cam profile. The pressure of liquid in the wedge-shaped space sets the shaft over to one side and lifts the shaft away from the bearing so that it is supported on an oil film. The position where the oil film thickness is least will be a small distance away from the static contact line in the direction of shaft rotation. For pressures to be built up to a value high enough to separate the shaft from the bearing, the oil must have sufficient viscosity and the speed of the shaft must be above a certain value. This form of lubrication is referred to as fluid film or hydrodynamic.
Boundary lubrication occurs when the rotational shaft speed falls and the oil wedge is lost. Metal to metal contact then occurs. To prevent metallic contact under boundary conditions greases may be used or additives may be added to the oils. The bearings of a diesel engine do not work under boundary conditions. Very highly loaded crosshead bearings in two-stroke engines may approach boundary conditions.
Diesel engine bearings are lubricated by oil films built up under the conditions described. The bearings are supplied with large amounts of oil which are used to maintain the oil Aim and remove the heat generated. Removal of the heat generated keeps the working parts at temperatures that will not reduce the oil viscosity to values low enough to allow breakdown of the oil Aim.
The air inlet and exhaust valyes of four-stroke engines and the exhaust valves of two-stroke engines are opened by cams, and closed by springs. In four-stroke engines the camshaft runs at half the crankshaft speed; in two-stroke engines the speed of camshaft and crankshaft are the same.
When a valve is opened the coil spring is compressed and loaded. When the cam rbller rides off the cam the resilience in the spring closes the valve. During the closing period the spring may set up a reverse torque on the camshaft by driving the cam. The force required to open an air inlet valve or an exhaust valve will be the sum of the following forces: the product of the valve lid area and pressure difference on the valve, the acceleration forces during the opening period, the force to overcome the spring and the force to overcome friction of the moving parts.
The torque on an engine camshaft may have wide variations, even to the extreme condition in which, during valve opening the crankshaft drives the camshaft, but during valve closing the camshaft feeds back work into the crankshaft.
The mechanism consists of a cam engaging with a cam roller. The roller may be Atted between the forked end of a valve lever which receives its motion from the action of the cam. Upward movement of the cam end of the lever causes downward movement at the end connected to the valve and the valve is opened. In other cases the cam roller may be connected to a push rod which is connected at its upper end to the valve lever. Where push rods are fitted in large engines a hydraulic loading device is fitted at the foot of the push rod; this permits smaller tappet clearance without fear of the valve being kept off the seat during the closed period. The camshaft is connected to the crankshaft through gearing or roller chains.
The two areas requiring maintenance are those associated with (a) combustion, and (b) bearing adjustment, and maintenance of correct alignment in all running parts. There is some overlap between the two areas of activity.
Maintenance work associated with combustion involves scavenge port and valve cleaning, piston ring replacement, air inlet and exhaust valve changes and overhaul, cleaning turboblower blading, compressor air inlet filters, scavenge and charge air cooler, and attending to instrumentation associated with combustion. The items mentioned cover all types of engines.
The other type of maintenance work covers all the moving and static parts of the engine, and includes bearing examination and adjustments, lubrication and cooling services, examination of bedplates, frames, cover, safety devices, etc.
The value of the maximum load on a cylinder cover and piston will be approximately the same, and will be the product of the area of the piston and the maximum gas pressure. In the case of the cover area it will be the projected area of the cylinder cover measured to the outer edge of the joint spigot. Ill some engines this may be considerably more than the piston area.
Cylinder diameter = 980mm Maximum pressure = 78.5 bars (80 kg/cm2)
Maximum load * 0.7854 x 982 x 80/1000 tonnes = 600 tonnes approx
The load on the cylinder cover is transmitted into the cylinder beam through the cover studs. The load on the cylinder beam is passed down to the bedplate through the tie-bolts and transverses supporting the crankshaft main bearings. The upward direction of the forces on the cylinder cover is balanced by the downward forces on the piston, which are transmitted through the piston-rod, crosshead block, bearings, connecting-rod, crankshaft and crankshaft main bearings.
The system of parts may be likened to a square flat plate tied to a square frame below it by four long tie-bolts at the corners. If a round shaft is placed across the frame, bearing on the two sides, and a jack is inserted between the shaft and the plate, and loaded, we may say that the system of engine parts is
simulated. The load on the jack, which is simulating the firing load on the piston and transmission of the load to the shaft, produces tension in the tie- bolts, a bending moment on the shaft and a bending moment on the two sides of the frame supporting the shaft. The fiat plate at the top will also be subjected to a bending moment. The actual parts of an engine will be subjected to the same loads as the simulation rig. The engine tie-rods will be in tension and the trans- verses supporting the main bearings will be subjected to a bending moment. The cylinder beam will also be subjected to bending moments.
Note The gas load coming on to the cylinder cover studs will be calculated on the area to the outer edge of the spigot. The tension on the tie-bolts from gas load will be calculated on the area of the piston.
The stresses coming about from the bending moment on the shaft in the simulated rig will be additive to the other stresses set up during engine operation.
Fossil fuels are the remains of prehistoric animals and plants and are found below the surface of the earth; they may be solid, liquid or gaseous.
S Solid fuels. Coal is the most important solid fuel used commercially.
Liquid fuels of a wide variety are obtained from distillation and other processes carried out on crude oil. The products obtained are essentially engine fuels, j boiler burner fuels, and lubricants.
Note—The oiHndustry is also a large supplier of chemicals used in other industries such as plastics, paints and compositions, synthetic rubbers and the | like.
1 Gaseous fuels may exist naturally in the ground or be produced from coal or
crude oil. Liquefied petroleum gases (LPG) are increasingly used.
! The fossil fuels are essentially carbon-hydrogen compounds. The energy is
derived from them by the exothermic action of converting the carbon to carbon dioxide and the hydrogen to water, which will be in the form of steam at the end – of the combustion process.
! The other types of fuel used are nuclear, which are fissile materials used in a
| reactor. One of the isotopes of uranium is commonly used. „
The fuels used in diesel engines are the gas oils and diesel oils which boil off from crude at temperatures between approximately 200°C and 400°C, or blends of diesel oil and residual fuel which have higher boiling points.
Note Liquefied petroleum gases must be stored under pressure or in refrigerated conditions, since their boiling points are low.
There are no universally accepted standards for classifying crude oil. For our purposes crude oil can be classified as paraffinic, as found in Pennsylvania, naphthenic as found in the Caucasus, and asphaltic as found in Texas. Many types of crude oil are found throughout the world but the majority will be within the groups paraffin base, naphthenic base, or some intermediate base.
Crude oil is a mixture of hydrocarbons, and, although there are considerable differences in the physical properties of the various hydrocarbons, the variation in chemical analyses is small. The carbon content varies from 83% to 87% and the hydrogen content from 14% to 11%. The balance is made up of sulphur, sodium, vanadium, water, etc., which may be classed as impurities.
The molecular structure of the fuel determines its physical properties. This structure can have numerous forms and may be such that the carbon atoms form either chains or chains with side chains, or have a ring structure. The hydrocarbons with the ring structure are more stable chemically.
The oil refinery processes are generally devised with a view to obtaining the highest yield of fuels in the range from the the liquefied petroleum gases through to the paraffins (kerosine) and gas oil. The crude oil is first stored in a settling tank to separate water, sand and earthy matter. After separation of the heavier impurities the crude oil is pumped into an oil- or gas-fired heat exchanger (pipe still) and’heated fo approximately 350°C, which brings a large part of the crude oil to above its boijing point. The heated crude oil is then passed to fractionating towers. The fractionating towers are in effect vertical condensers with horizontal partitions. The heated crude oil is passed in near the bottom and the major part flashes off and passes up through the fractionating tower. The rising vapour condenses at the various levels of the horizontal parti¬tions and is piped off from them as various grades according to their boiling points. The vapour leaving the top consists mainly of petroleum gases, part of which is condensed, while the remainder may be used as fuel for heating processes within the refinery. The condensed portion may be recirculated through the first tower. The bottoms from the first fractionating tower are passed through a second tower from which petrol is produced. The bottoms from the second tower are passed to a third tower from which a range of other products are obtained. The residuum from the third tower may be treated in various plants, in which the molecular formation of the residuum is reformed to increase the yield of the light constituents.
if the base of the crude is satisfactory the residuum may become the stock for production of lubricants. Lubricants can be produced from most types of crudes and their properties will vary according to the crudes from which they are produced. The residuum contains waxes, resins, asphalts and unstable hydrocarbons.
Lubricants are produced by solvent refining and acid refining. In solvent refining the solvent is pumped into the top of a tower and the stock is pumped in at the side. The solvent takes out the unwanted constituents in the stock, which pass to the bottom of the tower. The refined oil is passed out from the top of the tower. It is further treated to remove waxes, impurities and discoloration.
Modern oil refineries are highly automated and much of the equipment used has now found its place in ships’ engine rooms and other industrial plant.
The sale of energy in any form of the three types of fossil fuel is a highly competitive business. When the cost of crude oil rose sharply during 1973 the suppliers of refined crude oil products were forced to compete at a considerable disadvantage with the suppliers of fuels such as coal and natural gas, / Furthermore, at about this time some countries were bringing in legislation to reduce and eventually stop the supply of leaded fuels. This was done to reduce the very harmful atmospheric pollution resulting from increased use. of the automobile and the faulting increase in gaseous pollutants containing compounds of lead.
Oil refining techniques were updated to meet the increasing demand for unleaded petrol or lead-free gasoline having an acceptable octane rating, and to increase the yield of the more valuable fuels from the crude oil stock. This modification and updating gave a greater yield of the more valuable distillation products and reduced the amount of the, remaining less valuable residual products. –
Increased yields are obtained by subjecting the residue from the atmospheric distillation process to a vacuum distillation process. This increases the amount of distillate from that part of the residue having a higher boiling point. While under a reduced pressure the boiling point of the liquid is lowered and distilla¬tion then takes place without subjecting the residue to such high temperatures.
The distillate from the vacuum distillation process may then be reheated and treated in a catalytic cracking reactor.
Note There are many different forms of the catalytic cracking process.
The fluidized solid catalytic cracking process uses silicon oxide (silica) and aluminium oxide (alumina) as the catalyst. It is used in a powdered form so that it behaves like a fluid when in a stream of air or vapour. Some of the particles break up and catalyst dust is formed. The dust is referred to as catalyst fines or CC fines.
The cracked oil vapours or light hydrocarbons from the reactor create gases, petrol or gasoline, and light fuel oils. The residue left from the process often contains some of the catalyst carried over from the reactor.
The other cracking process used is known as thermal cracking. This may be used for altering the molecular structure of distillates and residues from the atmospheric distilling process. The thermal cracking process uses distillate to increase the yield of high octane petrol or gasoline, and the residue to increase the yield of light fuel oils.
A form of the thermal cracking process may also be used to reduce the viscosity of residual products. This is known as ’Visbreaking’.
These modifications in oil refinery practice result in a reduced amount of
residuum. The impurities such as sulphur, vanadium, sodium, barium, calcium, and ash, etc., while remaining the same in a unit amount of crude oil, become much more concentrated in the lesser amounts of residue.
Similarly carry-over of silica and alumina from the fluid catalytic cracking process also shows a greater concentration in the lesser residue amount.
The fuels supplied to diesel-propelled ships are obtained by blending a residual fuel having a relatively high viscosity with a distillate fuel having a lower viscosity. The resultant blend then has a viscosity complying with the viscosity stated in the order for the fuel. When the residual component of the blend has a viscosity lowered by the visbreaking process, the amount of distillate fuel (the ‘cutter stock’) required to bring the blend to the required viscosity is again reduced in amount. This leads to a further increase in the concentration of the impurities.
Another complication arising and leading to more problems with blended fuels is that in many cases the cracking processes increase the amount of the aromatic constituent. The increased aromatic constituent may then lead to problems with combustion and the cleaning of the fuel with centrifugal . separators and clarifiers.
The following changes in quality may be apparent. In some cases most of the mentioned changes may be present while in other cases only one or a combina-tion of two or more may be present.
Increase in aromatics giving a high density
Increase in ash content
Increase in asphaltic material content
Increase in carbon residue content
Increase in catalytic cracking fines content
Increase in sodium content
Increase in density
Increase in sulphur content
Increase in vanadium .content
Note The cracking or molecular reforming of liquid hydrocarbon fuels are not recent advances in oil refining techniques. The first forms of the thermal cracking process were begun at about the time of the First World War; catalytic cracking processes were started during the mid 1930s.
The flash-point is the lowest temperature at which an oil will give off suffi¬cient inflammable vapour to produce a flash when a small flame is brought to the surface of the oil. The flash-point may be measured as an open or closed
flash-point figure. Fuels for use aboard ships are tested in a Pensky-Martens instrument which measures the closed flash-point. The Department of Trade & Industry sets the lower limit of 65 °C for the flash-point of fuels used aboard merchant vessels and also stipulates that fuel in storage tanks must be kept at temperatures at least 14eC lower than its flash-point. The flash-point of an oil gives no indication of its suitability for use in a diesel engine. It only serves as a guide to the temperature below which it can be stored and handled with reasonable safety. A knowledge of the flash-point of the lubricating oil used in the crankcase of a diesel engine is useful, since lowering of the flash-point inducates that the lubricant may be contaminated with fuel.