Lubrication System:
Parts require lubrications Crankshaft bearing Piston pin
Timing gears Valve mechanism Piston ring and cylinder walls Camshaft and
bearings.
Purpose of lubrication:
·
Reduce friction & wear - by creating a thin
film (Clearance) between moving parts
·
Seal power - The oil helps form a gastight seal
between piston rings and cylinder walls
·
Cleaning - Cleans As it circulates through the
engine, the oil picks up metal particles and carbon, and brings them back down
to the pan.
·
Absorb shock - When heavy loads are imposed on the
bearings, the oil helps to cushion the load
·
Cooling. - Cools Picks up heat when moving through
the engine and then drops into the cooler oil pan, giving up some of this heat.
Types Lubrication System:
·
Petroil system
·
Splash system
·
Pressure system
·
Dry-sump system
Oil change:
·
Every 5000Km for four wheeler , Every 2000 Km in
two wheeler Ignoring regular oil change intervals will shorten engine life and
performance.
All internal combustion engines are equipped with an internal
lubricating system. Without lubrication, an engine quickly overheats and its
working parts seize due to excessive friction. All moving parts must be
adequately lubricated to assure maximum wear and long engine life.
Purpose of Lubrication;
The functions of an engine lubrication system are as follows:
Reduces friction and wear between moving parts. Helps transfer heat and cool
engine parts. Cleans the inside of the engine by removing contaminants (metal,
dirt, plastic, rubber, and other particles).
Absorbs shocks between moving parts to quiet engine operation
and increase engine life. The properties of engine oil and the design of modern
engines allow the lubrication system to accomplish these functions.
Types of Lubrication Systems;
Now that you are familiar with the lubricating system
components, you are ready to study the different systems that circulate oil
through the engine. The systems used to circulate oil are known as splash,
combination splash force feed, force feed, and full force-feed.
Splash
Systems
The splash system is no longer used in automotive engines. It
is widely used in small four-cycle engines for lawn mowers, outboard marine
operation, and so on. In the splash lubricating system, oil is splashed up from
the oil pan or oil trays in the lower part of the crankcase.
The oil is thrown upward as droplets or fine mist and provides
adequate lubrication to valve mechanisms, piston pins, cylinder walls, and
piston rings. In the engine, dippers on the connecting-rod bearing caps enter
the oil pan with each crankshaft revolution to produce the oil splash.
A passage is drilled in each connecting rod from the dipper to
the bearing to ensure lubrication. This system is too uncertain for automotive
applications. One reason is that the level of oil in the crankcase will vary
greatly the amount of lubrication received by the engine. A high level results
in excess lubrication and oil consumption and a slightly low level results in
inadequate lubrication and failure of the engine.
Combination Splash and Force Feed
In a combination splash and force feed, oil is delivered to
some parts by means of splashing and other parts through oil passages under
pressure from the oil pump. The oil from the pump enters the oil galleries.
From the oil galleries, it flows to the main bearings and camshaft bearings.
The main bearings have oil-feed holes or grooves that feed oil
into drilled passages in the crankshaft. The oil flows through these passages
to the connecting rod bearings. From there, on some engines, it flows through
holes drilled in the connecting rods to the piston-pin bearings. Cylinder walls
are lubricated by splashing oil thrown off from the connecting-rod bearings.
Some engines use small troughs under each connecting rod that
are kept full by small nozzles which deliver oil under pressure from the oil
pump. These oil nozzles deliver an increasingly heavy stream as speed
increases. At very high speeds these oil streams are powerful enough to strike
the dippers directly. This causes a much heavier splash so that adequate
lubrication of the pistons and the connecting-rod bearings is provided at
higher speeds. If a combination system is used on an overhead valve engine, the
upper valve train is lubricated by pressure from the pump.
Force Feed
A somewhat more complete pressurization of lubrication is
achieved in the force-feed lubrication system. Oil is forced by the oil pump
from the crankcase to the main bearings and the camshaft bearings. Unlike the
combination system the connecting-rod bearings are also fed oil under pressure
from the pump. Oil passages are drilled in the crankshaft to lead oil to the
connecting-rodbearings.
The passages deliver oil from the main bearing journals to the
rod bearing journals. In some engines, these opening are holes that line up
once for every crankshaft revolution. In other engines, there are annular
grooves in the main bearings through which oil can feed constantly into the
hole in the crankshaft. The pressurized oil that lubricates the connecting- rod
bearings goes on to lubricate the pistons and walls by squirting out through
strategically drilled holes. This lubrication system is used in virtually all engines
that are equipped with semi floating piston pins.
Full Force Feed
In a full force-feed lubrication
system, the main bearings, rod bearings, camshaft bearings, and the complete
valve mechanism are lubricated by oil under pressure. In addition, the full
force-feed lubrication system provides lubrication under pressure to the
pistons and the piston pins.
This is accomplished by holes drilled the length of the
connecting rod, creating an oil passage from the connecting rod bearing to the
piston pin bearing. This passage not only feeds the piston pin bearings but
also provides lubrication for the pistons and cylinder walls. This system is
used in virtually all engines that are equipped with full-floating piston pins.
Four-stroke Spark-ignition Engine
In a four-stroke engine, the cycle of operations is completed
in four strokes of the piston or two revolutions of the crankshaft. During the
four strokes, there are five events to be completed, viz, suction, compression,
combustion, expansion and exhaust. Each stroke consists of 180° of crankshaft
rotation and hence a four-stroke cycle is completed through 720° of crank
rotation. The cycle of operation for an ideal four-stroke SI engine consists of
the following four strokes:
i.
Suction or intake stroke;
ii.
Compression stroke;
iii. Expansion
or power stroke and
iv. Exhaust
stroke.
Working principle of a Four
Stroke SI Engine
i. Suction
or Intake Stroke: Suction stroke starts when the piston is at the top dead
centre and about to move downwards. The inlet valve is open at this time and
the exhaust valve is closed. Due to the suction created by the motion of the
piston towards the bottom dead centre, the charge consisting of fuel-air
mixture is drawn into the cylinder. When the piston reaches the bottom dead
centre the suction stroke ends and the inlet valve closes.
Compression
Stroke: The charge taken into the cylinder during the suction stroke is
compressed by the return stroke of the piston. During this stroke both inlet
and exhaust valves are in closed position. The mixture that fills the entire
cylinder volume is now compressed into the clearance volume. At the end of the
compression stroke the mixture is ignited with the help of a spark plug located
on the cylinder head. In ideal engines it is assumed that burning takes place
instantaneously when the piston is at the top dead centre and hence the burning
process can be approximated as heat addition at constant volume.
During the burning process the chemical energy of the fuel is
converted into heat energy producing a temperature rise of about 2000 °C. The
pressure at the end of the combustion process is considerably increased due to
the heat release from the fuel.
iii. Exhaust Stroke: At the end of the expansion stroke the
exhaust valve opens and the inlet valve remains closed. The pressure falls to
atmospheric level a part of the burnt gases escape. The piston starts moving
from the bottom dead centre to top dead centre and sweeps the burnt gases out
from the cylinder almost at atmospheric pressure.
The exhaust valve closes when the piston reaches T.D.C. at the
end of the exhaust stroke and some residual gases trapped in the clearance
volume remain in the cylinder. Residual gases mix with the fresh charge coming
in during the following cycle, forming its working fluid.
Each
cylinder of a four stroke engine completes the above four operations in two
engine
revolutions,
one revolution of the crankshaft occurs during the suction and compression
strokes and the second revolution during the power and exhaust strokes. Thus
for one complete cycle there’s
only one
power stroke while the crankshaft turns by two revolutions.
Consumption of lubricating oil is high in two-stroke engines due to higher temperature.
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