The distinctive feature of our civilization today, one makes it different from all others, is the wide of use mechanical power. At one time, the primary source of power for the work of peace or war is mainly man`s muscles. Later animals were trained to do this job. But the great step was taken in this direction when man learned the art of energy conversion from one form to another. The machine which does this job is called an engine.
Definition of 'Engine'
An Engine is a device which transforms one form of energy to another form. However, while transforming energy from One form to another, the efficiency of conversion plays an important role. Normally most of the engine convert thermal energy into mechanical work & therefore there are called 'heat engines'.
Definition of 'Heat Engine'
Heat Engine is a device which transforms the chemical energy of a fuel into thermal energy & utilizes this energy to perform useful work. Thus, thermal energy is converted to mechanical energy in heat engine.
Heat Engines are broadly classified into two categories:
- Internal Combustion Engines (IC Engine)
- External Combustion Engines (EC Engine)
Classification of Heat Engines
Engines whether Internal Combustion or External Combustion are of two types, viz.,
- Rotary Engines
- Reciprocating Engines
A detailed classification of heat engines is given in Fig.1.1. Of the various types of heat engines, the most widely used ones are the reciprocating internal combustion engine, the gas turbine & the steam turbine. The steam engines are rarely used. The reciprocating internal combustion engines are preferred over the steam turbine due to absence of heat exchangers in the passage of the working fluid. This results in a considerable mechanical simplicity & improved power plant efficiency of the internal combustion engine.
Another advantage of the reciprocating internal combustion engine over the other two types is that all its components work at an average temperature which is much below the maximum temperature of the working fluid in the cycle. This is because the high temperature of the working fluid in the cycle persists only for a very few fraction of the cycle time. Further in IC Engines, higher thermal efficiency can be obtained with moderate maximum working pressure of the fluid in the cycle, and therefore, the weight to power ratio is less than that of the steam turbine plant.
Basic Components & Nomenclature
Even though reciprocating internal combustion engines look quite simple, they are highly complex machines. There are hundreds of components which have to perform their functions satisfactorily to produce output power.
For a four-stroke engine, key parts of the engine include the crankshaft (purple), one or more camshafts (red and blue) and valves. For a two-stroke engine, there may simply be an exhaust outlet and fuel inlet instead of a valve system. In both types of engines, there are one or more cylinders (grey and green) and for each cylinder there is a spark plug (darker-grey), a piston (yellow) and a crank (purple). A single sweep of the cylinder by the piston in an upward or downward motion is known as a stroke. The downward stroke that occurs directly after the air/fuel mix passes from the carburetor to the cylinder where it is ignited is known as a power stroke.
Cylinder Bore (d): The nominal diameter of the working cylinder is called the cylinder bore. It is usually expressed in millimeter (mm).
Piston Area (A): The area of a circle of diameter equal to the cylinder bore is called piston area and is usually expressed in square centimeter (cm2).
Stroke (L): The nominal distance through which a working piston moves b/w two successive reversals of its direction of motion is called stroke and is usually expressed in millimeter (mm).
Stroke to Bore Ratio: L/d ratio is an important parameter in classifying the size of an engine.
If d<L: - engine is under-square.
If d=L: - square engine.
If d>L: - over-square engine.
Dead Centre: The position of the working piston and the moving parts which are mechanically connected to it, at the moment when the direction of the piston is reversed at either end of the stroke is called Dead Centre. There are two dead centres in the engine.
- Top Dead Centre (TDC): It is the dead centre when the piston is farthest from the crankshaft. It is also known as Inner Dead Centre for horizontal engines.
- Bottom Dead Centre (BDC): It is the dead centre when piston is nearest to the crankshaft. It is also known as Outer Dead Centre for horizontal engines.
Swept Volume (Vs): The nominal volume swept by the working piston when travelling from one dead centre to another. It is expressed in cubic centimeter (cc) & given by:
Engine Capacity: The displacement volume of a cylinder multiplied by number of cylinders in an engine will give the cubic capacity. If there are K cylinders in an engine, then
Engine Capacity =Vs*K
Clearance Volume (Vc): The nominal volume of the combustion chamber above the piston when it is at the top dead centre is the clearance volume. It is expressed in cubic centimeter (cc).
Compression Ratio (r): It is the ratio of total volume cylinder when the piston is at BDC, Vt , to the clearance volume, Vc.
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