Heat engines :-
A device which converts the chemical energy of a fuel into thermal energy and uses this energy to produce mechanical work is known as heat engine.
Types of heat engine :-
Heat engines are of two types
1. External combustion Engines :-
In such engines the products of combustion of air and fuel transfer heat to a second fluid which is the working fluid of the cycle as in case of a steam engine or a steam turbine plant where the heat of combustion is employed to generate steam which is used in a piston engine or a turbine.
A close cycle gas turbine plant is an other example of external engine in which heat of combustion in an external furnace is transferred to gas, usually air which is used in a gas turbine. A Stirling engine is also an external engine.
2. Internal combustion engines :-
In an internal combustion engine the product of combustion are directly the motive fluid . Petrol, gas, diesel engine, wankel engine and open cycle gas turbine are examples of internal combustion engines.
The main advantages of internal combustion engines over external combustion engines like steam plants are greater mechanical simplicity, lower ratio of weight and bulk to output higher overall efficiency and lesser requirement of water for dissipation of energy through cooling system.
Four-stroke cycle Engines :-
(a) intake or suction stroke
Suppose that the piston is very near to the top dead centre piston (T. D. C) . During suction stroke, the inlet valve is opened moves down due to rotation of the crankshaft either getting energy from the flywheel or a motor starter.
(b) Compression stroke :-
During a compression stroke, both the valves are closed and the piston moves from bottom to top dead centre position. The charge or air is compressed upto a compression ratio which is depends upon the type and need of the engine. In petrol engines, the compression ratio varies from 5 to 10.5, and pressure and temperature at the end of compression are 7 to 14 bar and 250 to 300°C respectively.
(c) Working or Expansion or Power stroke :-
During power stroke both the valves are closed. The power stroke includes combustion of fuel and expansion of the products of combustion.
In C. I. Engine (mechanical air less injection), one or more jets of fuel, compressed to a pressure of 105 - 210 bar by an injector pump are injected into the combustion chamber. When the fuel vapor is raised to self ignition temperature, the combustion then starts automatically and there is a sudden rise of pressure at approximately constant volume.
(d) Exhaust stroke :-
When 7/8th of the power stroke is completed, the exhaust valve begins to open and the inlet valve remains constant.
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Technology