29-03-2014, 12:06 PM
Steam engine
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INTRODUCTION
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.
Using boiling water to produce mechanical motion goes back about 2,000 years, but early devices were not practical. Since the late 1700s steam engines have become a major source of mechanical power. The first applications were removing water from mines. In 1781 James Watt patented a steam engine that produced continuous rotative motion. These 10 hp engines enabled a wide range of manufacturing machinery to be powered. The engines could be sited anywhere that water and coal or wood fuel could be obtained. By 1883, engines that could provide 10,000 hp had become feasible. Steam engines could also be applied to vehicles such as traction engines and the railway locomotives which are commonly just called steam engines outside America. The stationary steam engine was a key component of the Industrial Revolution, allowing factories to locate where water power was unavailable.
Components and accessories of steam engines
There are two fundamental components of a steam plant: the boiler or steam generator, and the "motor unit", referred to itself as a "steam engine". Stationary steam engines in fixed buildings may have the boiler and engine in separate buildings some distance apart. For portable or mobile use, such as steam locomotives, the two are mounted together.
The widely used reciprocating engine typically consisted of a cast iron cylinder, piston, connecting rod and beam or a crank and flywheel, and miscellaneous linkages. Steam was alternately supplied and exhausted by one or more valves. Speed control was either automatic, using a governor, or by a manual valve. The cylinder casting contained steam supply and exhaust ports.
Monitoring and control
For safety reasons, nearly all steam engines are equipped with mechanisms to monitor the boiler, such as a pressure gauge and a sight glass to monitor the water level.
Many engines, stationary and mobile, are also fitted with a governor to regulate the speed of the engine without the need for human interference (similar to cruise control in some cars).
The most useful instrument for analyzing the performance of steam engines is the steam engine indicator. Early versions were in use by 1851, but the most successful indicator was developed for the high speed engine inventor and manufacturer Charles Porter by Charles Richard and exhibited at London Exhibition in 1862.The steam engine indicator traces on paper the pressure in the cylinder throughout the cycle, which can be used to spot various problems and calculate developed horsepower. It was routinely used by engineers, mechanics and insurance inspectors. The engine indicator can also be used on internal combustion engines. See image of indicator diagram below.
Simple engine
In a simple engine the charge of steam works only once in a cylinder.[47] It is then exhausted directly into the atmosphere or into a condenser.
Compound engines
As steam expands in a high pressure engine its temperature drops because no heat is added to the system; this is known as adiabatic expansion and results in steam entering the cylinder at high temperature and leaving at low temperature. This causes a cycle of heating and cooling of the cylinder with every stroke which is a source of inefficiency. A method to lessen the magnitude of this heating and cooling was invented in 1804 by British engineer Arthur Woolf, who patented his Woolf high pressure compound engine in 1805. In the compound engine, high pressure steam from the boiler expands in a high pressure (HP) cylinder and then enters one or more subsequent lower pressure (LP) cylinders. The complete expansion of the steam now occurs across multiple cylinders and as less expansion now occurs in each cylinder less heat is lost by the steam in each. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine. By staging the expansion in multiple cylinders, torque variability can be reduced. To derive equal work from lower pressure steam requires a larger cylinder volume as this steam occupies a greater volume. Therefore the bore, and often the stroke, are increased in low pressure cylinders resulting in larger cylinders.
The Idealized Diesel Cycle
The image on the left shows a p-V diagram for the ideal Diesel cycle; where is pressure and is specific volume. The ideal Diesel cycle follows the following four distinct processes (The color references refer to the color of the line on the diagram.):
Process 1 to 2 is isentropic compression of the fluid (blue colour)
Process 2 to 3 is reversible constant pressure heating (red)
Process 3 to 4 is isentropic expansion (yellow)
Process 4 to 1 is reversible constant volume cooling (green)