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Heat Engines

Heat

Heat is a form of energy.It is the total kinetic energy of molecules of a matter .

Thermal expansion of the body.(Expansion of the concrete road or railway tracks during summer)Increase in temperature.(Temperature of body increases due to heat)Change of state of matter.(If we heat the iron it will become soft OR water changes to steam when it is heated)Chemical change of the substance.(Endothermic reactions-chemical reactions which absorb\use the heat, ex: Si + CSiC)EFFECTS OF Heat

HEAT ENGINES

Heat engine is a device which converts heat energy into useful mechanical energy.The high latent heat of steam (amount of heat given out by the unit mass of steam when it is converted into water without changing its temperature) is useful in working of a heat engine which converts heat energy into mechanical energy. If a small quantity of water is converted into steam,it occupies more volume by expansion and exerts pressure.Approximately the volume of the steam is 700 times more at STP than the volume of water from which it is obtained by heating.

External combustion engine (ECE)Steam engine.Internal combustion engine (ICE)Petrol engine. Diesel engine.

Types of HEAT ENGINES

STEAM ENGINE In 1698 a British inventor, Thomas Savery developed a pump engine without a piston. This engine could pump water.In 1705 Thomas Newcomen built an improved steam engine with a piston that operated in a cylinder. He used the engine to pump water.Later James Watt registered a patent on a steam engine with a crankshaft.This engine converted the up and down motion into a rotary motion.In 1782 Watt built his first double-action steam engine.The power was generated by steam and not by atmospheric pressure.

A typical steam engine consists of a cylinder fitted with an airtight piston.The piston is connected to crankshaft by the means of a connecting rod.A boiler is connected to the cylinder through a valve called inlet valve . Cylinder is also connected to a condenser through another valve called outlet valve. The condenser is connected to the boiler through a pump.

Construction

Working EXPANSION STROKE

The water heated in a boiler, is to generate steam of high pressure.The steam under high pressure gets into the cylinder through the inlet valve.The expanding steam pushes the piston forward.crankshaft connected to the piston also moves.Crankshaft converts linear movement into circular motion.The wheel of the crankshaft makes half rotation during expansion stroke.In expansion stroke the outlet valve remains closed. EXHAUST STROKE After expansion stroke and the forward movement of the piston, steam loses its kinetic energy and condenses into water.As the piston moves inwards due to low pressure, crankshaft makes another half cycle due to inertia.The spent steam in the cylinder is pushed into condenser through outlet valve.In exhaust stroke inlet valve remains closed.This stroke is called exhaust stroke.

As the steam is stored at a high pressure the risk of accidents due to bursting of boiler is very high.(In order to get the maximum work from heat energy, the steam has to be maintained at the high temperature and a very high pressure in the boiler. If by chance the steam pressure increases the boiler may burst, leading to loss of life and goods). Getting steam at high pressure is time consuming, hence the engine cannot be started instantaneously.A major portion of heat energy is wasted. (efficiency is very less of about 15% to 20% )Steam engines are bulky (since the boiler of the steam engine is very heavy) hence cannot be used to run small machines and vehicles.

Disadvantages of steam engine

FOUR STROKE ENGINEIn 1839 the American Isaac Babbitt invented a material for bearings.This material allowed engines to run at high speeds.On 9 May 1876 the public witnessed Nikolaus August Ottos new four-stroke engine.It produced 3 h.p. at 180 r.p.m. and was known as the Otto Cycle engine.

TWO STROKE ENGINEIn 1876 the American George Brayton built the first two-stroke engine.Sir Dugald Clerk also invented a two-stroke engine, an improvement on Braytons engine , in 1880.Clerk is named the father of the two-stroke engine.In 1885 Gottlieb Daimler built the first motor bike.Later in that year he built the first motor boat and the first motor coach.Still today two-stroke engines are widely used by motorcycles, lawn mowers and motorboats.

A petrol engine has 4 strokesIntake strokeCompression strokePower [Ignition and combustion] strokeExhaust stroke

Petrol engine

The intake strokeThe piston moves from top dead centre towards bottom dead centreWhen the piston descends from top dead centre, the intake valve starts opening.With the exhaust valve closed during the stroke, a partial vacuum or depression is created in the cylinder above the piston.Atmospheric pressure fills this partial vacuum and, in doing so, passes through the carburettor barrel.In the carburettor barrel petrol is mixed with air.This air-fuel mixture fills the cylinder via the intake manifold and past the open intake valve.Just after the piston reaches bottom dead centre, the intake valve is closed.

The compression strokeBoth the intake valve and the exhaust valve remain closed during this stroke and the piston moves from Bottom dead centre to top dead centre.The petrol mixture is compressed in the relatively small combustion chamber.Just before the piston reaches top dead centre, a high tension spark is introduced into the combustion chamber by means of the sparkplug and the petrol mixture is ignited.

The power strokeBoth valves still remain closed and as a result of ignition, combustion of the petrol mixture takes place rapidly.High temperatures are developed as a result of this combustion and cause the gases to expand.This expansion exerts considerable pressure on the piston and piston moves from top dead centre to bottom dead centre.This force is transmitted to the crankshaft thereby giving a rotatory motion to the crankshaft.

The exhaust strokeAs the piston reaches bottom dead centre, the exhaust valve is opened.The crankshaft rotates as a result of momentum in the crankshaft and the piston moves from bottom dead centre to top dead centre.The piston forces out the burnt gases past the open exhaust valve to the exhaust manifold from where it is sent to the atmosphere by pipes.When the piston reaches top dead centre, the engine is ready to start the next cycle.

CARBURETTOR

SPARK PLUG

NIKOLAUS OTTO

CYLINDER

PETROL ENGINEVALVES

It is similar to the petrol engine except for spark plug. In diesel engine spark plug is replaced by fuel injector.In diesel engine only air is let into the cylinder in intake stroke.At the end of compression stroke the fuel injector injects diesel. The temperature created by the compression of air is enough to ignite diesel. Diesel combusts and expands. This is power stroke.Diesel Engine

RUDOLF DIESEL4 STROKES DIESEL ENGINEDIESEL ENGINE FUEL INJECTOR CYLINDER

ADVANTAGES :Petrol are used in light weight vehicles like cars, scooters and motor cycles.It can be started instantaneously.Efficiency is more than steam engine.(efficiency is about 40%) There is no risk of accidents due to the storage of material at high pressure. DISAVANTAGES : The cost of petrol is very high when compared to fuels like diesel.Compression ratio of air into fuel in the petrol engine is 4:1 to 10:1. Compression ratio of air into fuel in the diesel engine is 4:1 to 25:1. Therefore the ignition temperature of diesel is more than the petrol. Petrol engine as to be ignited by spark plug.The continuous supply of the petrol also results in the wastage of small quantity of petrol, as a part of petrol does not undergo combustion completely.

Advantages and disadvantages of petrol engine

Diesel engines are used in generators to produce electricity.Diesel engines are used to crush the sugarcane by sugarcane vendors.The cost of diesel is less than the petrol.Diesel engines are used in concrete mixing and in road constructions to mix the Tar.The compression ratio of air is to fuel in diesel engine is 4:1 to 25:1.The ignition temperature of diesel is more than the petrol.Efficiency of the diesel engine is comparatively more.(Efficiency is about 45% to 60%)There is no risk of accidents due to the storage of material at high pressure.

Advantages of diesel engine

PETROL ENGINEDIESEL ENGINEa) During intake stroke the fuel mixture from the carburetor enters the cylindera)During intake filtered air is taken into the cylinder and compressed.b)At the end of compression stroke spark is released by the spark plug.b) At the end of compression stroke diesel is injected into cylinder in the form of micells.c)It is fitted with spark plug and carburettor.c)It is fitted with air filter and fuel injunction pump.d)Its efficiency is comparatively low.d)Its efficiency is comparatively high.e)Expansion against constant pressure.e)Expansion of gaseous products with pressure impulse.f)The cost of petrol is comparatively high.f)The cost of diesel is comparatively less.

DIFFERENCES BETWEEN PETROL ENGINE AND DIESEL ENGINE

EMISSION TEST In internal combustion engine the exhaust gases contains CO2, colloidal carbon particles, water vapour and CO.CO is formed due to the incomplete combustion of fuels.More CO is formed if engine is not maintained properly.It is a health hazard and causes air pollution.To control the pollution of air there is certain limit on the amount of CO released by vehicles.Periodically vehicles have to undergo Emission Test which is a test for CO.The exhaust gases released by the vehicles should not cross the limit fixed, during the test.

EFFICIENCY OF HEAT ENGINES The efficiency of a heat engine is defined as the fraction of heat converted into useful mechanical work. ORThe ratio of the actual work done to the heat energy consumed. Percentage of ratio is taken for calculation. Efficiency of an engine (Eta) = Work done X 100 Total heat supplied

EFFICIENCY OF HEAT ENGINES : Steam engine -- 15% Petrol engine -- 40% Diesel engine -- 45% to 60% No engine is 100% efficient because no work is possible in nature without an expenditure of certain amount of energy to heat the body in its environment before conversion of work.