The learning objectives of this chapter are to introduce the learner to the need
to cool a diesel engine that the learner will know which diesel engine components
and systems require cooling
diesel engines are machines designed to convert the chemical energy of fuel to
mechanical energy in order to produce power this power is then used to do work
the work may be done in turning a propeller to drive a ship or to drive an
electrical generator to provide electrical power the energy conversion
process involves combustion of the fuel within the engine cylinder during
combustion heat is released and pressure is generated forcing down the piston the
piston is connected to the crankshaft through the connecting rod the downward
movement of the piston causes the crankshaft and the driven unit it is
connected to two-term unfortunately not all of the chemical energy can be
converted into work and some of the released heat energy transfers to the
engine components and some is lost to exhaust and the other engine fluids heat
is also lost to the surroundings due to radiation the moving parts of the engine
also subject to frictional effects and the work done in overcoming this
friction transfers heat to these components for example bearings piston
rings and cams click on the button to see various stages
if the diesel engine was left uncooled then serious operational problems are
likely to occur the high operating temperatures would result in clearances
between components reducing due to thermal expansion leading to seizure
material properties of the various components deteriorating leading to
failure due to excessive mechanical than thermal stress damage to pump seals and
ceilings leading to leakage oxidation and reduced viscosity of the lubricating
oil resulting in metal to metal contact between components due to oil film
failure you should therefore realize that there is a need to cool a diesel
engine in order to avoid these failures if it is to operate continuously for
long periods some small diesel engines may be directly air-cooled with or
without forced air circulation you may have some small diesel engines onboard
your ship possibly lifeboat engines or emergency compressor engines which are
air-cooled you will recognize them by the presence
of extended fins around the cylinder block or by a cooling fan unit direct
air cooling is only suitable for the smallest low rated diesel engines for
the vast majority of marine diesel engines direct air cooling is
insufficient and other cooling mediums must be used such as water or oil these
are usually closed systems with the same coolant circulated continuously the
fluids normally used in a closed circuit can be considered as the primary coolant
with a secondary coolant being used to take heat from the primary to avoid it
overheating primary coolants are usually freshwater or lubricating oil and
secondary coolant may be fresh water or sea water or in some cases air click the
images to see the unlock before moving on you should click on the button for a
brief explanation of the role of lubricating oil as a coolant
the primary purpose of lubricating oil is to form an oil film between two
moving surfaces and so reduce friction in doing so the oil heats up partly due
to the friction of the oil molecules moving across each other under load and
partly due to heat transfer from hot engine components as the oil heats up
the viscosity and therefore the load carrying capacity will reduce and it can
no longer keep the moving surfaces apart this will lead to further heating and
deterioration of the oil it is therefore necessary to cool the lubricating oil to
maintain its performance and condition in small engines the hot oil transfers
heat to the atmosphere through finned coils or through the surface of the oil
pan in larger engines it is necessary to circulate the oil through external heat
exchangers where the heat is transferred to a secondary coolant this continuous
circulation and transfers to a secondary coolant means that the lubricating oil
is being used as the primary coolant for some engine components an ideal coolant
should be low most readily available non scale forming
and non corrosive it should also have a high specific heat capacity this is a
measure of the amount of heat required to raise the temperature of a unit mass
of the substance by one Kelvin K the units for specific heat capacity are
usually given as kilo joules per kilogram Kelvin the higher the specific
heat capacity the more heat can be taken away from the materials which require
cooling it is also desirable for the coolant to have a low viscosity to
minimize flow resistance in the cooling system this makes water the first choice
for most cooling systems usually with the addition of additives such as
corrosion inhibitors when selecting a coolant it is also necessary to consider
the risk of contamination due to leakage so for example cooling a piston in a
diesel engine with water has a high risk of contaminating the lubricating oil
system in this case lubricating oil may be chosen to avoid this risk even though
water may be the better coolant click on each of the images to know more about
various coolants that are available in the marine environment although
is plentiful it has a relatively low specific heat capacity approximately 1.0
kilo joules per kilogram Kelvin the low density of air would require that large
volumes would need to be supplied to take away the heat from the various
diesel engine systems AirCam however be used as a coolant irrespective the
process temperature range air-cooled radiators are used on some smaller
engines with the air as the secondary coolant being forced through the
external fins of the radiator by a firm
the moderate specific heat capacity of mineral oil two point zero to two point
five kilo joules per kilogram Kelvin means that using oil as a coolant is
generally restricted to those applications where lubrication is also
required as stated earlier if current leakage would be a problem for example
water in the engine crankcase then cooling with oil may be the preferred
choice rather than using complex sealing arrangements oil has a wider useful
temperature range than water although excess temperatures can lead to
oxidation and carbon deposits synthetic or thermal oils can be used to extend
the temperature range at low pressure although thermal oil is normally used
for heating purposes rather than as a coolant
see water would be the first choice coolant for marine applications if it
was not for the associated scale problems it is cheap plentiful has good
specific heat capacity approximately 4.0 kilojoules per kilogram Kelvin and low
viscosity scale formation increases with operating temperature which limits the
useful temperature range and means that using direct sea water cooling for
modern engines is not really practical some cooling systems still have a
facility allowing emergency operation and sea water cooling but usually at
restricted loads in most cases the use of sea water as a coolant is limited to
secondary cooling applications where temperature requirements are lower and
scale formation can be minimized
freshwater meets all of the requirements for a coolant however it is not readily
available and has to be stored or produced on board the specific heat
capacity is approximately 4.2 kilo joules per kilogram Kelvin which gives a
very good cooling effect when produced on board by a freshwater generating
plant the water is almost pure therefore scale formation is not really a problem
however the slightly acidic water that is produced by such a plant has to be
chemically treated as we will see later to prevent corrosion fresh water is
preferred as the primary coolant for diesel engine jacket water systems and
as the secondary coolant for the other engine systems the temperature range is
limited by the relatively low boiling point of 100 degrees Celsius although
this can be increased by having a closed pressurized system
the table shows the comparison among the various coolants that are available in a
marine environment it is clear that there is a need to cool the diesel
engine we must now decide on which components and which ancillary systems
will have to be cooled the simple answer is any component or system that will not
function properly atomic temperature the items which will
be affected most are those which are directly exposed to the combustion
process or a subject of friction this will include all components forming the
combustion chamber and all moving components the engine fluids such as the
lubricating oil jacket water and charged air also need to be cooled we also need
to consider ancillary equipment such as the turbocharger which is subject to
heating due to friction and also the flow of exhaust gas through the turbine
side and the heating effect of compressing the air on the compressor
side click on the buttons to see which fluids and components require cooling
when the engine is operating it is worth noting at this point that too much
cooling can be detrimental to the operation of a diesel engine leading to
low thermal efficiency high running costs and engine damage obviously a
compromise is necessary before moving on you should click on the buttons to see
the components and ancillary equipment that need to be cooled as the engine
operates
the diesel engine components which are heated directly by the combustion
process are the piston and cylinder liner including any firing ring the
cylinder cover and fittings including exhaust valves inlet valves and fuel
injector and the piston rings in particular the top ring the diesel
engine components which heat up due to friction as they move are the piston
piston rings and cylinder liner the piston rods connecting rods and
associated bearings the cylinder cover valves and actuate in gear the
crankshaft and bearings including thrust arrangement the cam shaft and bearings
the chains chain wheels and bearings the internal gear drives and fuel injection
equipment click on the button to see the
you
as has already been stated the turbocharger will run at high
temperature due to the working conditions of the exhaust gas the
compression of the air and frictional heat from the bearings traditionally
large turbocharger casings were water-cooled usually using a sub circuit
of the main jacket water cooling system smaller units were awful air cooled many
of the latest generation of turbochargers have uncooled turbine
casings which reduces the total heat losses of the engine and increases
overall thermal efficiency with this design lubrication is usually taken from
the main engine lubricating oil system and the circulating oil carries away the
heat from the bearings exhaust side casings have to be well insulated for
the protection of personnel the vast majority of water-cooled engines use
freshwater as the primary coolant for the static components forming the
combustion chamber this is achieved by circulating the water through the space
between the cylinder liner and the cylinder block and the liner and the
cylinder jacket the system is therefore referred to as the jacket cooling water
system the water enters at the bottom of the cylinder liner and pulses up the
jacket and through the cylinder cover as it exits the engine this also provides
cooling for the valves and the operating gear many designs incorporate ball
cooling of these main components in which drilled passageways for the
coolant are positioned close to the surface of the component to be cold this
allows intense cooling while maintaining the strength of the component the air
for combustion is usually cooled by seawater after delivery from the
turbocharger before entering the cylinder although as we will see later
fresh water can be used the Pistons are usually cooled by a separate piston
cooling system which uses either fresh water or lubricating oil as the coolant
if freshwater cooling is used it is normally separate to the main jacket
water system the piston rings are cooled by conduction with heat transferring to
either the cooled cylinder wall or the Piston body the crankshaft and
associated bearings and four large slow speed engines the cross head arrangement
are cooled by the main lubricating oil system the camshaft and associated
bearings are cooled by lubricating oil circulation which in many designs is
separate to the engine's main lubricating system this minimizes the
risk of contamination of the main system by fuel oil leakage the frictional heat
generated by the piston motion in the liner is removed by the jacket water and
piston cooling systems in modern uncooled fuel injection systems the
component temperatures are normally maintained at acceptable levels by the
flow of fuel oil recirculation of fuel in the injectors may be used to assist
this in heavy fuel systems older fuel injection systems either relied on heat
transfer to the cylinder cover wet injector pockets in the cylinder cover
or forced circulation of coolant through balls in the injector body to maintain
acceptable temperatures the coolants used for this full circulation varied
and freshwater thermal oil and diesel oil have all been used by different
engine manufacturers in this particular module we will be
looking at engine cooling water systems but as you will see in many modern
designs this system is also used as the secondary coolant for the other engine
systems in older designs sea water was used as the secondary coolant for each
of the individual systems we shall have a detailed look at a basic jacket water
cooling system and also a central cooling system which is a common
solution used to minimize the use of sea water cooling and the high maintenance
needs of such a system