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- INTERNAL COMBUSTION ENGINES
Heat Engines
Any type of engine or machine which derives heat energy from the combustion of fuel or any other source and converts this energy into mechanical work is termed as a heat engine. Heat engines may be classified into two main classes as follows :
1. External Combustion Engine.
2. Internal Combustion Engine.
1. External Combustion Engines (E.C. Engines)
In this case, combustion of fuel takes place outside the cylinder as in case of steam engines where the heat of combustion is employed to generate steam which is used to move a piston in a cylinder. Other examples of external combustion engines are hot air engines, steam turbine and closed cycle gas turbine. These engines are generally needed for driving locomotives, ships, generation of electric power etc.
2. Internal Combustion Engines (I.C. Engines)
In this case combustion of the fuel with oxygen of the air occurs within the cylinder of the engine. The internal combustion engines group includes engines employing mixtures of combustible gases and air, known as gas engines, those using lighter liquid fuel or spirit known as petrol engines and those using heavier liquid fuels, known as oil compression ignition or diesel engines.
- Four stroke petrol engine
Illustrates the various strokes/series of operations which take place in a four stroke petrol (Otto cycle) engine.
Suction stroke - During suction stroke a mixture of air and fuel (petrol) is sucked through the inlet valve (I.V.). The exhaust valve remains closed during this operation.
Compression stroke - During compression stroke, both the valves remain closed, and the
pressure and temperature of the mixture increase. Near the end of compression stroke, the fuel is ignited by means of an electric spark in the spark plug, causing combustion of fuel at the instant of ignition.
Working stroke - Next is the working (also called power or expansion) stroke. During this
stroke, both the valves remain closed. Near the end of the expansion stroke, only the exhaust valve opens and the pressure in the cylinder at this stage forces most of the gases to leave the cylinder.
Exhaust stroke - Next follows the exhaust stroke, when all the remaining gases are driven away from the cylinder, while the inlet valve remains closed and the piston returns to the top dead
centre. The cycle is then repeated.
Four Stroke petrol engine
Two stroke petrol engine :
The engines using this cycle were called two stroke cycle engines. In this engine suction and exhaust strokes are eliminated. Here instead of valves, ports are used. The exhaust gases are driven out from engine cylinder by the fresh change of fuel entering the cylinder nearly at the end of the working stroke.
- THERMODYNAMIC SYSTEMS
System - A system is a finite quantity of matter or a prescribed region of space.
Closed System - If the boundary of the system is impervious to the flow of matter, it is called a closed system.
Open System - An open system is one in which matter flows into or out of the system.
Most of the engineering systems are open.
Isolated System - An isolated system is that system which exchanges neither energy nor matter with any other system or with environment.
Boundary - The actual or hypothetical envelope enclosing the system is the boundary of the system.
Surrounding - Everything around a system is called surroundings.
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