ENGINE
SUPPORT SYSTEMS:
·
Cooling system
·
Lubrication system
·
Fuel and ignition/injection system
·
Intake system Exhaust system
1.
Cooling system:
The
cooling system removes excess heat to keep the inside of the engine at an
efficient temperature.
·
Air Cooling
·
Liquid Cooling
·
Water cooling Coolant. Water Jackets:
Water Jackets Surrounds the
cylinders with water passage. Absorbs heat from the cylinder wall. Pump move
water to radiator where heat is exchanged to the air. 66
Coolant Flow:
Coolant flows through the water
jackets where it absorbs heat. It then flows through the radiator where heat is
transferred to the air passing through. The amount of flow is determined by the
water pump. The flow direction is controlled by the thermostat.
Warm Engine:
The thermostat opens when the
engine warms up. This allows coolant to circulate through the radiator and the
water jackets.
Cold Engine:
When an engine is cold, the
thermostat is cold. Coolant flow is through the bypass hose and the water
jackets. This allows the engine to warm up evenly.
Coolant
:
·
Coolant Water (Boiling Point 100° C)
·
Glycerin (Boiling Point 290 ° C)
·
Ethylene glycol (Boiling Point 197 ° C)
·
Antifreeze (methyl alcohol, ethyl alcohol )
Cooling
System:
·
Water pump is driven by the crankshaft through
Timing Belt ( Keeps Cam and Crank shafts in time)
·
Drive/accessory Belt (Runs alternator,
power-steering pump, AC, etc.) Serpentine Belt V-Belt
·
Electric fan is mounted on the radiator and is
operated by battery power. It is controlled by the thermostat switch.
Need for cooling system
The
cooling system has
four primary functions. These functions are as follows:
1.
Remove excess heat from the engine.
2. Maintain a constant
engine operating temperature.
3.
Increase the temperature of a cold engine as
quickly as possible.
4.
Provide a means for heater operation (warming
the passenger compartment).
Types of cooling system:
The
different Types of cooling system are
1.
Air cooling system
2.
Liquid cooling system
3.
Forced circulation system
4.
Pressure cooling system
Air-Cooled System :
The simplest type of cooling is the air-cooled, or direct,
method in which the heat is drawn off by moving air in direct contact with the
engine Several fundamental principles of cooling are embodied in this type of
engine cooling. The rate of the cooling is dependent upon the following:
1.
The area exposed to the cooling medium.
2.
The heat conductivity of the metal used &
the volume of the metal or its size in cross
section .
3.
The amount of air flowing over the heated
surfaces.
4.
The difference in temperature between the exposed
metal surfaces and the cooling air.
Liquid-cooled system;
Nearly all multi cylinder engines used in automotive,
construction, and material-handling equipment use a liquid-cooled system. Any
liquid used in this type of system is called a COOLANT.
A simple liquid-cooled system consists of a radiator, coolant
pump, piping, fan, thermostat, and a system of water jackets and passages in
the cylinder head and block through which the coolant circulates. Some vehicles
are equipped with a coolant distribution tube inside the cooling passages that
directs additional coolant to the points where temperatures are highest.
Cooling of the engine parts is accomplished by keeping the
coolant circulating and in contact with the metal surfaces to be cooled. The
operation of a liquid- cooled system is as follows:
The pump draws the coolant from the bottom of the radiator,
forcing the coolant through the water jackets and passages, and ejects it into
the upper radiator tank. The coolant then passes through a set of tubes to the
bottom of the radiator from which the cooling cycle begins.
The radiator is situated in front of a fan that is driven
either by the water pump or an electric motor. The fan ensures airflow through
the radiator at times when there is no vehicle motion. The downward flow of
coolant through the radiator creates what is known as a thermosiphon action.
This simply means that as the coolant is heated in the jackets of the engine,
it expands. As it expands, it becomes less dense and therefore lighter. This
causes it to flow out of the top outlet of the engine and into the top tank of
the radiator. As the coolant is cooled in the radiator, it again becomes more
dense and heavier. This causes the coolant to settle to the bottom tank of the
radiator.
The heating in the engine and the cooling in the radiator
therefore create a natural circulation that aids the water pump. The amount of
engine heat that must be removed by the cooling system is much greater than is
generally realized. To handle this heat load, it may be necessary for the
cooling system in some engine to circulate 4,000 to 10,000 gallons of coolant
per hour. The water passages, the size of the pump and radiator, and other
details are so designed as to maintain the working parts of the engine at the
most efficient temperature within the limitation imposed by the coolant.
Pressure cooling system
Radiator Pressure Cap
The radiator pressure cap is used on nearly all of the modern
engines. The radiator cap locks onto the radiator tank filler neck Rubber or
metal seals make the cap-to-neck joint airtight. The functions of the pressure
cap are as follows:
1.
Seals the top of the radiator tiller neck to
prevent leakage.
2. Pressurizes
system to raise boiling point of coolant.
3. Relieves
excess pressure to protect against system damage.
4. In a
closed system, it allows coolant flow into and from the coolant reservoir.
The radiator cap pressure valve consists of a spring- loaded
disc that contacts the filler neck. The spring pushes the valve into the neck
to form a seal. Under pressure, the boiling point of water increases. Normally
water boils at 212°F.
However, for every pound of pressure increase, the boiling
point goes up 3°F. Typical radiator cap pressure is 12 to 16 psi. This raises
the boiling point of the engine coolant to about 250°F to 260°F. Many surfaces
inside the water jackets can be above 212°F. If the engine overheats and the
pressure exceeds the cap rating, the pressure valve opens. Excess pressure
forces coolant out of the overflow tube and into the reservoir or onto the
ground.
This prevents high pressure from rupturing the radiator,
gaskets, seals, or hoses. The radiator cap vacuum valve opens to allow reverse
flow back into the radiator when the coolant temperature drops after engine
operation. It is a smaller valve located in the center, bottom of the cap.
The cooling and contraction of the coolant and air in the
system could decrease coolant volume and pressure. Outside atmospheric pressure
could then crush inward on the hoses and radiator. Without a cap vacuum or vent
valve, the radiator hose and radiator could collapse.
2. 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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