Why is it necessary to cool the cylinder heads or covers, cylinder liners and pistons of diesel engines? What is used as the cooling medium?

The temperature inside the cylinders of diesel engines rises to approximately 2000°C during combustion of the fuel and drops to approximately 600°C at the end of expansion. With temperatures in this range the metal of the cylinder covers, cylinder liners and pistons would quickly heat up to the point where its strength would be insufficient to withstand the cylinder pressures; also, no oil film would be able to exist on the cylinder walls, and lubrication of the cylinder and piston rings would break down. Cooling is necessary to maintain sufficient strength in the parts and to preserve the oil film on the cylinder.
The cooling medium for cylinder liners and covers is a flow of distilled or fresh water: the medium for cooling pistons is also distilled or fresh water, or oil from the crankcase system. The amount of heat extracted from the various parts /must be such that they operate at temperatures well within the strength limits of the materials used. The coolant flow patterns must also be arranged so that the surfaces of all parts are as near uniform temperature as possible to prevent large thermal stresses being set up.
With modem highly rated engines the temperatures of the parts subjected to combustion temperature are much lower than in earlier engines. This has been made possible by the availability of better temperature measuring devices and the research carried out by engine builders. The temperature of the combustion chamber surfaces of cylinder covers, piston crowns and cylinder liners varies between 200°C and 350°C in modern highly rated engines. The variation in temperature of the different parts of the surface of cylinder covers will be within about 50°C to 100°C, and for piston crowns the temperature variation will be 75°C to 100°C. Cylinder liners show greater temperature variation throughout their length, but in the highly critical area at the top of the liner the variation is kept to within approximately 100°C.
Small diesel engines with pistons less than about ISO mm (6 in) diameter have ■ only the cylinders and covers cooled by water. The piston crown will be cooled by excess lubricant from the gudgeon bearing and by the heat transfer to the walls of the piston which are then cooled by the cylinder liner. Small high-speed diesel engines may also be cooled by forced air flow passing over fins fitted on the outside of cylinders and cover. It should be noted that air-cooled diesel engines have very low cylinder wear.
Note With pressure-charged engines the air flow during the scavenge period (in two-stroke and four-stroke engines) over the hot internal surfaces of the cylinders, covers and piston crowns helps to maintain low surface temperatures. It also reduces the temperature gradient across the material section and in turn lowers the thermal stresses.

What are the reasons for the deterioration in the quality of the fuel supplied for use In marine diesel engines?

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.