unit –iii(b) power plants

Introduction to EC and IC enginesHydro ThermalNuclear Power plants and their layoutsRelated numerical s

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EC engines

Power is produced in a unit other than where heat is generated.

Working fluid is heated by external heat source.

Requires heat exchangers Most of the components

are subjected to maximum temperature of working fluid.

Moderately higher operating temperature.

More weight for same power.

Occupies more space

IC engines

Heat is generated within the power producing unit.

Products of combustion itself is the working fluid.

No need of heat exchangers Most of the components are

subjected to lesser temperature.

So higher operating temperature can be used

Lighter weight for same power

Compact.

IC ENGINESIC ENGINES

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Classification of IC Classification of IC engines engines

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Based on number of strokesTwo strokeFour stroke

Based on Ignition Spark Ignition Compressed ignition

Based on coolingWater cooledOil cooledAir cooled

Based on Fuel usedVolatile liquid and gaseous fuel Low volatile liquid fuel

Based on orientation of cylinderInline engineRadial engineHorizontalV shaped engines

Components and Components and nomenclature nomenclature

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ComponentsComponents Cylinder block is the main part which is cast as a

single unit. It carries cavities of large diameter called cylinders.

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Cylinder head is mounted on top of block which has provisions to accommodate valves, spark plug, fuel injector etc. it is held tight to the block by studs.

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Crank case is generally integral with the cylinder block. It accommodates crank shaft.

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Piston fits in to the cylinder. It moves between TDC and BDC within cylinder. Its function is to transmit the force created by combustion to connecting rod. Generally made of light weight metals like aluminium.

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Cam controls the opening and closing of valves and introduction of spark. The cam shaft will be coupled to crank shaft by gears or chain.

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Crank shaft is the component where we are getting useful mechanical power. It will be coupled to the mechanism to be driven by the engine.

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Oil pan is pressed steel or alloy cast iron component assembled to bottom of cylinder block and serves as reservoir for lubricating oil.

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Piston rings : upper ring is compression ring and lower ring is oil control ring which remove surplus oil from cylinder wall.

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Connecting rod connects piston to crank shaft. This helps in converting reciprocating motion of piston to rotation of crank shaft.

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Inlet valve allows induction of fresh charge during suction stroke and

Exhaust valve allows removal of combustion products during compression stroke.

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Swept volume and Swept volume and compression ratiocompression ratioThe volume swept by the piston

movement in between the TDC and BDC

The ratio of max volume formed in the cylinder to the clearance volume

Given by r = V(max) /V(min) =V(tdc) /V(bdc)

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Mean effective pressure Mean effective pressure (MEP)(MEP)It is a fictitious force acting on the

cylinder head during the entire power stroke.

Equals to the work done during the operation during the complete actual cycle

Wnet = MEP * piston area *Stroke = MEP*Displacement volume MEP= Wnet /(Vmax-Vmin)

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Four stroke Petrol Four stroke Petrol engineengine

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Four stroke Diesel Four stroke Diesel engineengine

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Cycle of Four stroke Cycle of Four stroke engineengine1. Suction stroke2. Compression stroke3. Expansion/power stroke4. Exhaust stroke

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1. SUCTION STROKE Piston at TDC and move towards BDC IV is open and EV is closed Suction is created which draws fresh

charge in to the cylinder ( air in CI engine and A/F mixture in SI engine

Ends when piston reaches BDC. IV gets closed at the end of suction

stroke

2. COMPRESSION STROKEThe charge is compressed by the return of pistonBoth valves remain closed during this strokeThe volume of charge which was VT is compressed to Vc. At the end of compression stroke, combustion takes place.In SI engine the mixture is ignited by using spark plug and burning takes place instantaneously so that volume remains constant during combustion. Pressure and temperature increases.

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In CI engine, fuel injection is started nearly at the end of compression stroke. Combustion will take place in such a way that the increase in pressure due to combustion is compensated by the increase in area during the expansion stroke.

3. EXPANSION STROKE (POWER STROKE) starts with piston at TDC The high pressure gas pushes the piston

towards BDC and forces the crank to rotate Power is generated

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4. Exhaust strokeAfter expansion, piston reaches BDCEV is opened and upward movement of the piston pushes the exhaust out of the cylinder.Piston reaches TDC again and some gases are trapped in clearance volume.

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27

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Two stroke engineTwo stroke engine

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Two stroke engine cyclesTwo stroke engine cycles

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1. Compression stroke In the compression stroke, piston moves

upwards from BDC. Initially there is no compression as both TP and EP are open.

As the piston moves upwards, TP is closed first, then EP is closed and compression is begun.

Due to the upward movement of piston, the pressure in the crank case is reduced, the spring loaded inlet valve is opened and fresh charge enters to the crank case through inlet port.

Ignition happens at end of compression stroke.

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2. Expansion stroke The increase in pressure caused by the combustion pushes the piston downwards.This compresses the charge in the crank case and it is moved towards TP. Towards the end of expansion stroke, piston uncovers EP and the exhaust gas starts leaving the cylinder.Further movement of piston uncovers TP and slightly compressed charge enters the cylinder. This further pushes the exhaust out.

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ADVANTAGES Two unproductive strokes are eliminated. Valves are replaced by ports Suction and exhaust gas emission are

achieved by some alternate arrangement One power stroke for each revolution of

crank shaft The power output is theoretically doubled Filling process is accomplished by charge

compressed in the crank case. The induction of fresh charge moves out

products of combustion.

Lubrication systemLubrication systemNeed:Reduces the frictional forces that acts

on the system parts.Prevents corrosion by forming a layer on

all the parts where its been circulated.

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WorkingWorkingOil from the sump is pumped

through tubing arrangements where ever required.

Due to the property of the type of selected oil forms layers on the area where ever its is sprinkled , causes sliding of layers. Intern causes reduction of friction.

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Cooling system Cooling system Need: Compact size makes the engines

to under go more heat.If dissipation doesn't occur

causes engine break down.Comfort or Ease while using the

equipment.

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TypesTypesBased on the Heat dissipation

cooling systems are classified into

Air cooled systemsLiquid cooled systems

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Air cooled systems Air cooled systems Air plays a major role in heat

transferContact area of the engine outer

surface will be subjected to airSpecial modification of outer

surface area of the engine block is called ‘Fins’.

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Air cooled systems (fins)Air cooled systems (fins)

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Fins structure (sectional Fins structure (sectional view)view)

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Liquid cooled systemsLiquid cooled systemsNeed :When ever there exist larger area that need to be cooled at regular intervals.Multi cylinder engines where heat needs to be dissipated quickly and uniformly.Where complex structure of the engine needs a cooling.

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Illustration of Liquid cooled Illustration of Liquid cooled system system

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Types of liquid coolingTypes of liquid coolingMost liquid-cooled engines use a

mixture of water and chemicals such as antifreeze and rust inhibitors.

De-mineralized water Heavy water Mineral oil or synthetic oilLiquid Nitro coolant

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