Engines Essay Research Paper Engine is a

Engines Essay, Research Paper

Engine is a machine that converts heat energy to mechanical energy. In all conventional engines a hot gas expands within a cylinder, pushing a piston and that causes mechanical work. This straight-line motion is changed to rotary motion by a crankshaft connected to the piston. In external-combustion engines like the steam engine, the gas is heated by burning fuel outside the cylinder and enters the cylinder at a high temperature and high pressure. In expanding and moving the piston, the gas grows cooler and its pressure falls to a level not far above that of the outside atmosphere. This spent gas then leaves the cylinder, and a new charge is introduced either after the piston has been returned to its original position or on the opposite side of the piston from the spent charge. In the more recent type the expansion of the second charge restores the piston to its original position; in single-acting engines, the piston is returned by the action of a flywheel attached to the crankshaft. Double-acting engines provide two power strokes for each full rotation of the crankshaft and single-acting engines only one, so double-acting engines can develop twice as much power as single-acting engines of the same size and speed. Both internal- and external-combustion engines require a system of valves to allow the introduction of fresh charges of gas and the expulsion of spent gas. The gas must be introduced and expelled when the piston is in the correct position, and this timing is accomplished by controlling the mechanism that operates the valves from the crankshaft. In theory, any working substance can be heated and used to drive an external-combustion engine, but only steam has been used. The reason for this is that, at any given temperature, steam has more energy than any other readily obtainable working substance. If one were to run an engine on hot air, one could only get the same power output by heating the air to a higher temperature. This would not only involve a more elaborate system for heating the gas than a steam boiler, it would also mean that greater care would have to be taken to see that heat was not lost by conduction through the walls of connecting pipes, valve chests, and cylinders, since the rate of conduction of heat increases with increasing temperature. The fact that steam could be used to do work in a properly designed machine was recognized as early as the second century a.d. by Hero of Alexandria; however, the first systematic investigation of this use of steam was made by Giambattista della Porta in a book on pneumatics published in 1601. He proposed that water could be pumped by using the pressure exerted by expanding steam or the vacuum created when steam condensed on being cooled. An engine that probably employed these principles was built by Edward Somerset, Marquis of Worcester, in 1663, but no details of its operation survived. In 1698, Thomas Savery patented a pistonless steam engine used for pumping out accumulated water in mine shafts, and for providing water for driving water wheels as well as for drinking. In Savery’s engine a large airtight vessel was filled with steam from a boiler. The boiler valve was then closed, and the vessel was cooled. Opening a valve connected to a pipe leading into the mine shaft, permitted the vacuum in the vessel to draw the water up from the shaft. This engine, like its successor the Newcomen engine, was extremely inefficient because much of the heat energy of the steam was used to heat the vessel, which had to be cooled during each cycle. In 1680, Christiaan Huygens set up an engine in which a piston would be raised by the explosion of a small charge of gunpowder. Cooling the cylinder would create a partial vacuum, causing the piston to drop and do work in the process. In 1690, Denis Papin suggested that steam be used in an engine of this type, but his plan proved to be unworkable because he proposed heating the steam in the same cylinder in which it was to expand and condense. This engine, thus, would have been even less efficient than Savery’s; instead of merely wasting a large part of the energy of each charge of steam introduced into the cylinder from a continuously heated boiler, far more heat would have been required in alternate heating and cooling of the entire engine. This failure, and word of Savery’s success, led Papin to build a pumping engine rather like Savery’s, except that it used a piston in a cylinder to keep the steam and water separate. In 1705, Thomas Newcomen made the single improvement required in Papin’s original engine to make the idea workable. He used a separate boiler as a source of steam. The piston in a vertical cylinder was raised by the counterweight attached to an oscillating beam, while steam was admitted behind the rising piston. When the piston had reached the top of the cylinder, the steam valve was closed and a valve was opened to introduce a spray of cold water into the cylinder. This caused the steam to condense, and the resulting partial vacuum in the cylinder permitted the atmospheric pressure to push the piston back to its original position. The water from the condensed steam and the spray was drained from the cylinder by opening a third valve. When this valve was closed and the steam valve reopened the cycle repeated itself. Newcomen’s engine was single-acting, the actual power stroke taking place while the steam was contracting rather than when it was exp