Introduction:
In all steam engines, the steam is used as a working substance. These engines operate on the principle of first law of thermodynamics, i.e. heat and work are mutually convertible. In a steam engine, as the heat energy of steam is converted into mechanical work by the reciprocating (to and fro) motion of the piston, it is called reciprocating steam engine. Moreover, as the combustion of the fuel takes place outside the engine cylinder, then it is called external combustion engine.
Classification of steam engines:
The steam engines are classified in different different ways which are discussed below-
1.) ACCORDING TO NUMBER OF WORKING STROKES
a. Single acting steam engine
b. Double acting steam engine
When steam is admitted on one side of the piston and one working stroke is produced during each revolution of crankshaft, then it is said to be a single acting steam engine. But when the steam is admitted on both sides of the piston and two working strokes are produced during each revolution of the crankshaft, then it is said to be a double acting steam engine. A double acting steam engine produces double the power than that produced by a single acting steam engine.
2.) ACCORDING TO POSITION OF THE CYLINDER
a. Horizontal steam engine
b. Vertical steam engine
When the axis of the cylinder is horizontal, it is said to be a horizontal steam engine. But when the axis of the cylinder is vertical then it is said to be a vertical steam engine. A vertical steam engine requires less floor area then the horizontal steam engine.
3.) ACCORDING TO THE SPEED OF THE CRANKSHAFT
a. Slow speed steam engine
b. Medium speed steam engine
c. High speed steam engine
When the speed of the crankshaft is less than 100 revolution per minute, it is called a slow speed steam engine. When the speed of the crankshaft is between 100 r.p.m. to 250 r.p.m. then it is called a medium speed steam engine. Similarly when the speed of the crankshaft is above 250 r.p.m. then it is known as a high speed steam engine.
4.) ACCORDING TO THE TYPE OF EXHAUST
a. Condensing steam engine
b. Non – condensing steam engine
When steam after doing work in the cylinder passes into a condenser, which condenses the steam into water at a pressure less than the atmospheric pressure, it is said to be a condensing steam engine. But when the steam after doing work in the cylinder is exhausted into atmosphere, it is said to be a non condensing steam engine. The steam pressure in the cylinder is, therefore, not allowed to fall below the atmospheric pressure.
5.) ACCORDING TO EXPANSION OF THE STEAM IN THE ENGINE CYLINDER
a. Simple steam engine
b. Compound steam engine
When expansion of the steam is carried out in a single cylinder and then exhausted into the atmosphere or a condenser, it is said to be a simple steam engine. But when the expansion of the steam is completed in two or more cylinders, the engine is called a compound steam engine. The compound steam engines are generally condensing engines. But some of them may be non condensing also.
6.) ACCORDING TO THE METHOD OF GOVERNING EMPLOYED
a. Throttling steam engine
b. Automatic cut off steam engine.
When the engine speed is controlled by means of a throttle valve in the steam pipe, which regulates the pressure of the steam to the engine, then it is called a throttling steam engine. But when the speed is controlled by controlling the steam pressure with an automatic cut off governor, it is called an automatic cut off steam engine.
Important parts of a Steam Engine:
Schematic diagram of single cylinder, double acting steam engine.
Above figure shows a single cylinder, double acting, horizontal reciprocating steam engine. The important parts of a Steam engine are discussed as below.
1.) FRAME- It is a heavy cast iron part which supports all the stationary as well as moving parts and hold them in proper position. It generally rests on engine foundations.
2.) CYLINDER- It is also a cast iron cylindrical hollow vessel, in which the piston moves to fro under the steam pressure. Both ends of the cylinder are closed and made steam tight. In small steam engines, the cylinder is made an integral part of the frame.
3.) STEAM CHEST- It is casted as an integral part of the cylinder. The superheated steam at a high pressure (above 20 ATM) from the boiler is fed into the steam chest and it then supplies to the cylinder with the movement of D- slide valve.
4.) D- SLIDE VALVE- It moves in the steam chest with simple harmonic motion. Its function is to exhaust steam from the cylinder at proper moment.
5.) INLET AND EXHAUST PORTS- These are holes provided in the body of the cylinder for the movement of steam. The steam is admitted from the steam chest alternately to either sides of the cylinder through the inlet ports P1 and P2. The steam after doing its work in the cylinder, is exhausted through the exhaust port.
6.) PISTON- It is a cylindrical disc, moving to and fro, in the cylinder because of the steam pressure. Its function is to convert heat energy of the steam into mechanical work. Piston rings, made from cast iron, are fitted in the grooves in the piston. Their purpose is to prevent the leakage of steam.
7.) PISTON ROD- It is a circular rod, which is connected to the piston on one side and cross head to the other. Its main function is to transfer motion from the piston to the cross head.
8.) CROSS- HEAD- It is a link between the piston rod and connecting rod. Its function is to guide motion of the piston rod and to prevent it from bending.
9.) CONNECTING ROD- It is made of the forged steel, whose one end is connected to the cross head and other to the crank. Its function is to convert reciprocating motion of the piston (or cross head) into rotary motion of the crank.
10.) CRANKSHAFT- It is the main shaft of the engine having a crank. The crank works on the lever principle and produces rotary motion of the shaft. The crankshaft is supported on main bearings of the engine.
11.) ECCENTRIC- It is generally made of cast iron and is fitted to the crankshaft. Its function is to provide reciprocating motion to the slide valve.
12.) ECCENTRIC ROD AND VALVE ROD- The eccentric rod is made of forged steel, whose one end is fixed to the eccentric and other to the valve rod. Its function is to convert rotary motion of the crankshaft into reciprocating motion of the valve rod. The valve rod connects the eccentric rod and the D- slide valve. Its function is to provide simple harmonic motion to the D- slide valve.
13.) FLYWHEEL- It is a heavy cast iron wheel mounted on the crankshaft. Its function is to prevent the fluctuations of engine. It also prevents the jerks to the crankshaft.
14.) GOVERNOR- It is a device to keep the engine speed, more or less, uniform at all load conditions. It is done either by controlling the quantity or pressure of the steam supplied to the engine.
That’s all about steam engine, their classification and different parts of steam engine.
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