Engine Friction and
Lubrication (Part IV)
The main function of lubricating
system is to provide sufficient and cool filtered oil to all the moving
parts of an engine. The systems are classified as splash lubricating system
and pressure feed lubricating system.
We have discussed about
basic of Engine friction and lubrication in previous blogs. The link for
part I, II and III are provided below.
1. Splash
Lubrication System
The
arrangement of the system is shown in the figure below:
This
method generally used for vertical engines with a closed crankcase. The sump is
located at the bottom of the crankcase. When the engine crankshaft rotates, the
big end of the connecting rod splashes oil by centrifugal action. The
connecting rod big end has a hollow pipe called a scoop which is fitted to the
bearing cap and pointed towards the direction of the rotation of the crank
shaft. The lubricating oil passing through the scoop, lubricate the big end
bearing and gudgeon pin bearing. All other parts are lubricated by the splash.
Excess oil is collected in the troughs located as shown in the figure and is
provided with overflows and collected in the main sump. The level of the oil in
trough is maintained constant. The dripping from the cylinders is also
collected in the sump. The oil from the sump is recirculated with the help of
the pump as shown in figure.
The
inability to regulate the quantity of oil splashed against the cylinder wall or
inability to keep the oil from getting past the piston head into the combustion
chamber, burning with the fuel and passing out with exhaust gases are
limitations of this system.
2.
Pressure Feed Lubricating System:
All
modern car and bus engines are lubricated by high pressure feed system as shown
in the figure given below:
Such
a system supplies oil under pressure (2 to 5 bar) directly to the connecting
rod bearings, to the camshaft bearings, to the valve gear and to the camshaft
drive. Indirect supplies reach the cylinder walls, the gudgeon pin, the
distributor and pump drives.
Oil
is carried in the sump, a deep tray which closes the bottom of the crankcase
and is circulated by the gear pump which sucks from the sump through a strainer
as shown in the figure. The pump delivery pressure is controlled by a relief valve
and the oil passes through a very fine filter before it reaches the main
distributor gallery. From the various bearings, surfaces and gears, the oil
simply drains back into the sump.
After
lubricating the big end bearings, the oil is fed to the gudgeon pins through
the oil way in the connecting rod and further squirted into the cylinder wall.
3.
Charge Lubrication System
This
is the simplest method of lubrication and does not require oil filter and oil
pump. In this system, the lubricating oil is pre-mixed with the petrol
therefore the fuel carries the lubricating oil in the cylinder which helps for
lubricating the piston and cylinder. Most of the oil burns with the fuel due to
high temperature and burnt oil is carried with the exhaust gases. The lubricating
oil cannot be recovered in this system.
This
type of lubrication is generally used for two stroke spark ignition engines of
scooter and motorcycle. The quantity of lubricating oil mixed with the petrol
is 3 to 6% of petrol.
The
advantages of this system are listed below:
a. It
does not require separate lubricating system so it is most economical.
b. There
is no risk of failure of lubricating system.
c. The
lubricating oil supplied is regulated at various loads and speeds by the
increased fuel flow.
The
carbon deposits due to the burning of the oil on the spark plug and on other
parts and non-recovery of the oil used are the main disadvantages of this
system.
The
lubricating system is also classified as wet-sump lubrication and dry sump
lubrication system.
1. Wet
Sump Lubricating System:
The arrangement is shown in the figure
below:
This is called wet sump as sump
is always full of oil. The working is just similar to the pressure feed
lubricating system.
Oil is drawn from the sump by an oil
pump through an oil strainer. A pressure relief valve is provided which
automatically maintains the delivery pressure constant. If the pressure exceeds
than the predetermined pressure, the valves opens and allows some of the oil to
return to the sump and relives the oil pressure in the system. The oil from the
pump goes to the bearings and part of it passes through a filter which removes
solid particles from the oil. As all the oil is not passed through the filter,
the system is known as by-pass filtering system. Advantages of this
system are that a clogged filter will not restrict the flow of oil to the
engine.
2. Dry
Sump Lubricating System:
The dry sump lubricating system is shown
in the figure given below:
This is known as dry-sump as the
sump does not contain oil and all the oil required for lubrication remains in
the circulation only. High speed racing cars and military jeeps use this type
of lubricating system as the oil in the wet sump is subjected to large back and
forth acceleration.
An auxiliary tank is used to supply the
oil to the main bearings with the help of the pump. The oil returns back to
tray and then returned back to the auxiliary tank by scavenging pump, the
capacity of which is always 20 to 30% more than the pressure pump to avoid
flooding of the crankcase.
If the filter is clogged, the pressure relief valve
opens permitting oil to flow bypassing the filter and reaches the supply tank.
The oil is then circulated to the bearings from the supply tank. A separate oil
cooler is used to cool the oil to remove the heat from the oil as heating of
oil is rapid because of rapid circulation of oil and high speed of the engine.
