Steam engine is also a heat engine like a gasolene engine, but it is categorised as external combustion engine. It is because combustion of fuel takes place outside the engine in a boiler. Heat of combustion is utilised to evaporate water into steam. Steam contains a lot of heat energy. Steam engine converts the heat energy in steam to mechanical energy. Thus steam engine is a prime mover and was extensively used in automobiles, locomotives, marine applications, pumping stations, power generation etc., in the past. The advent of I.C. engines gave a set back to steam engines on account of higher thermal efficiency and better performance of the former. However, in the recent times, steam engines are once again appearing in the field because of their reduced noise level and lower atmospheric pollution.
James Watt (1736-1819), a Scottish engineer is the inventor of the steam engine. It consists of a cylinder in which a double-acting piston operates. By double-acting piston, it is meant that both the faces of piston are working faces. Steam pressure acts on these faces alternatively. When one face is working, the other face is exhausting. Working pressures and temperatures inside the cylinder are comparatively lower than those in I.C. engines and for that reason, ordinary alloys of C.I. are used in cylinder materials.
CONSTRUCTIONAL DETAILS OF A DOUBLE - ACTING RECIPROCATING STEAM ENGINE :
Figure shows the basic elements or principal components of a simple double-acting, horizontal reciprocating steam engine.
MAIN PARTS OR COMPONENTS OF STEAM ENGINE AND THEIR FUNCTIONS :
1. Cylinder : It is made of cast iron. It houses the piston in it. Both ends of the cylinder are made steam tight. The end of the cylinder nearer to the crank is called crank end and that remote from the crank is called cover end.
2. Steam Chest : It is integral with cylinder. It receives steam-from the boiler and supplies it to the cylinder.
3. D-slide Valve : It is a valve that closes and opens the inlet ports. It reciprocates in the steam chest with a simple harmonic motion:. It resembles the letter D and hence the name. It exhausts steam from the cylinder. It is actuated by an eccentric rod.
4. Ports : These are the holes or passages provided in the cylinder body. Steam is admitted into the cylinder from the steam chest through inlet ports alternatively on to either side of the piston. After doing work steam is exhausted out through the exhaust port.
5. Piston : This is a reciprocating member in the form of a circular disc inside the cylinder. Steam pressure acting on the piston its motion indicating the work done by steam. C.I. piston rings are mounded on the piston. The piston with piston rings provide almost a steam-tight sliding joint in the cylinder.
6. Piston Rod : It connects the piston to the cross head. It transmits load on the piston and transfers motion from the piston. This is made of mild steel and has to withstand rapid alternations of tensile and compressive stresses.
7. Cross Head : This is a link between piston rod and connect-ing rod. It reciprocates between the fixed guides. It prevents piston rod from buckling.
8. Connecting Rod : It is a link between crosshead and crankshaft. It acts both as a tie and a strut because of tensile and compressive forces coming over it. It is also subjected to buckling and bending between its ends. It is made of forged steel and may have circular, rectangular or I-section. Its function is to convert reciprocating motion of crosshead into rotary motion of the crankshaft. It oscillates between its ends.
9. Eccentric : It is a cast iron member fitted to the crankshaft. It provides reciprocating motion to the D-slide valve, through the eccentric and valve rods.
10. Stuffing Box : It is located at the crank end of cylinder and another one at the end of steam chest. It prevents steam escaping into atmosphere thus provides a fluid tight seal.
WORKING PRINCIPLE OF A STEAM ENGINE :
The working of a steam engine can be well explained with reference to the various events that occur during its operation.
Figure shows the indicator diagram (a plot of pressure Vs volume variation along the length of the stroke). It is helpful in explaining different operations as follows.
1. Inlet port is opened by the movement of slide valve. Steam is admitted into the cylinder from the steam chest. It exerts thrust or force on the piston facing cover and of the cylinder.
2. Piston is at the extreme position towards cover end. Inlet port is closed.
3. Supply of steam is completely cut off. Steam expands doing work on the piston. This work is transmitted from piston to the crank (shaft) through piston rod, cross head and connecting rod in order.
4. Slide valve opens the cylinder to the exhaust port. Pressure at this point is slightly more than exhaust pressure. Pressure falls at constant volume (4) — (5) until it is reduced to exhaust pre., Spent up steam finds its way through the exhaust port and exhaust continues upto (6).
From (6) to (1) steam which is left in the cylinder is slightly compressed to admission pressure corresponding to (1).
In double acting engine, by the time piston approaches crank end of cylinder, the second port opens and fresh steam is admitted. It pushes the piston towards cover end facilitating effective exhaust of previous charge.
COMPOUND STEAM ENGINE:
In modern steam engine practice, high pressure steam is used since it gives greater efficiency and plant requires less room per kW power developed. But it is found undesirable if steam is expanded from high pressure to atmospheric or condenser pressure in a single cylinder. In such case a robust engine with long stroke length and heavy flywheel are required.
Say for example, steam at 16 bar pressure is to be expanded down to 0.2 bar,, instead of carrying out this process in a single cylinder, engine may have two or more stages or cylinders. In this example, expansion from 16 bar to 2 bar pressure may be carried over in high pressure (H.P.) cylinder and from 2 to 0.2 bar in another cylinder called low pressure (L.P.) cylinder. Such operation is called compounding.
Methods of Compounding:
In Tandem compounding two pistons have in common piston rod crank and connecting rod. Steam after expanding m H.P. cylinder enters L.P. cylinder for further expansion. This type of engine requires relatively a heavy fly wheel.
In Woolf compounding the cranks are placed at 1800. In this type also steam after expanding in H.P. cylinder enters L.P. cylinder for further expansion.
In Receiver Type Compound engines cranks the two cranks are placed at 90° to each other. The H.P. cylinder exhausts the steam into receiver at the end of expansion, while L.P. cylinder draws steam from receiver at the same time where it further expands.
Advantages of Compounding:
• lighter construction of engine
• reduced cylinder condensation
• low leakage losses
• uniform torque on crank shaft
• better mechanical balance and less vibration
• smaller size flywheel is required
• reduced cost
• engine can be started at any position of piston.