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 expanding in the cylinder. In order to minimize leakage around the piston, water was introduced above it during the downstroke. Any of this water that flowed into the cylinder aided the condensation of the steam and flowed out of the cylinder with the condensate at the end of the power stroke. Newcomen’s engine was also used to power the pumps required to prevent the flooding of mineshafts and also used for mills and watersupply systems. James Watt set the basic pattern of all subsequent reciprocating steam engines by using the expanding steam to push the piston and by devising the valve system that permitted double-acting operation. His invention of the throttling governor to regulate the engine speed automatically is considered to be one of the first contributions to the science of automatic control. Watt did not originally undertake to create a new engine. His first intention was to improve the efficiency of Newcomen’s engine by eliminating the alternate heating and chilling of the cylinder. He accomplished this by adding a second separate chamber into which the steam passed and in which it was condensed after the piston was raised. This chamber was kept cold and the cylinder was kept hot. Watt maintained the temperature of the cylinder constant by covering it at the top, allowing steam to pass through a hollow jacket around the cylinder, and by insulating the entire cylinder from the air. In order to maintain a vacuum in the condenser and to remove the condensate, the condenser was equipped with a small pump run from the engine itself. As an additional improvement Watt introduced the use of oil to achieve a good vacuum seal around the piston. These improvements, as well as the idea of using the power of expanding rather than of contracting steam, were contained in a patent issued to Watt in 1769. At that time all steam engines still provided only straight-line motion, but in 1781 Watt patented a system for converting this to rotary motion. He subsequently employed a crankshaft, after an earlier patent on this basic device issued to James Pickard had expired. In 1782, Watt patented his double-acting engine and throttling governor. In this engine, a separate steam chest attached to the cylinder contains a sliding valve which alternately admits fresh steam to opposite sides of the piston at the same time that spent steam is being expelled. Both ends of the cylinder are closed, the piston rod must move in a straight line; outside the cylinder, it is pivoted at the crosshead so that the straight-line motion can be converted to rotary motion by the crankshaft. The rod that actuates the sliding valve has a similar crosshead so that the rotary motion of the crankshaft can be converted back to straight-line motion. Watt’s throttling governor made it possible for the engine to run at a steady speed without constant manual control of the main steam-inlet valve. This device consisted of a pair of weights spun by means of a belt drive from the flywheel. When the engine was running, centrifugal force pushed the weights apart, closing the valve and cutting down on the amount of steam supplied to the engine. This caused the engine to slow down, which, in turn, lessened the centrifugal force on the weights, permitting them to come closer to each other, causing the valve to reopen. The efficiency of an engine of this type depends entirely on the amount of the steam’s energy that the engine can extract. This energy may be thought of as consisting of two parts, the steam’s pressure and its temperature. The lower the temperature and pressure of the spent steam exhausted by the engine compared to the temperature and pressure of the steam admitted to the cylinder, the more efficient the engine will be. By condensing the exhausted steam, Watt made it possible to achieve low exhaust temperatures and pressures even with relatively short strokes, thereby improving engine efficiency without sacrificing speed.
In 1781 Jonathan Hornblower solved a difficulty by allowing the spent steam from one cylinder to pass into a second, larger cylinder for further expansion against a second piston. The temperature difference for each cylinder was made smaller and efficiency was improved. This technique is called compounding and has been used in engines containing as many as four stages, so that each charge of steam expands in four different cylinders of increasing size before it is completely spent. Strumpf’s engine, patented in 1906 by Johann Strumpf, eliminates the necessity for compounding to reduce the loss of heat to the valve system. In this engine there are no exhaust valves. Instead, the spent steam passes through a ring of holes in the middle of the cylinder so that it never comes into contact with the intake valves. These exhaust parts are uncovered by the thick piston as it reaches the end of its stroke. In the Stirling engine, the expansion of hot air or other gas, rather than steam, is used for power. Work is produced, as in an internal-combustion engine, when gas is compressed while cold and allowed to expand when hot. In this engine the gas is heated by an external burner, as in the steam engine. The Stirling engine was invented in Scotland in 1816 by the Rev. Robert Stirling, and was used in industry for a time. There are two basic methods of controlling the pressure and temperature of the gas. In the displacer system the hot and cold gases in the cylinder are separated by a second piston, called the displacer. The two chambers within the cylinder are connected through tubes running outside the cylinder. These tubes pass through a cooler, a regenerator, and a heater. At the beginning of the cycle, the two cylinders are separated. The cold gas is compressed between the displacer and the power piston, transferred through the tubes and the heater to the upper part of the cylinder, and there the hot gases are allowed to expand. At the end of the cycle, the displacer moves upward, transferring the gas downward through the cooler system to be compressed again. The regenerator is used to minimize waste of heat. It receives heat from the hot gas entering the cooler and gives it up to the cool gas entering the heater. In the double-acting system there is no displacer. Four cylinders are arranged in a square array, connected to one another through four heater-regenerator-cooled units. The pistons operate one quarter of a cycle out of phase with their neighbors. In this way the total volume in any two connected chambers is decreasing when the air is cool and increasing when it is hot. In internal-combustion engines, the chemical energy of the fuel is converted into heat inside the engine itself. Internal-combustion engines do not require a boiler or some other heating device as external-combustion engines do. Although it is possible to run an internal-combustion engine on any fuel in combination with air, virtually all commercial engines use petroleum fuels; gasoline or Diesel oil. In the operation of an internal-combustion engine, there are four steps that must take place: fuel and air must be introduced into the cylinder, the mixture must be compressed, it must then be burned, and the exhaust gases must be removed from the cylinder before introducing a fresh charge of fuel and air. In Diesel engines, the fuel and air do not enter the cylinder at the same time, but the cycle of operation is the same. There are two basic systems of accomplishing these operations, the four-stroke cycle and the two-stroke cycle. In the four-stroke cycle an intake valve is opened as the piston is at the top of the cylinder, and the charge of fuel and air is drawn into the cylinder by the partial vacuum created by the piston’s traveling down the cylinder. When the piston reaches the bottom of its stroke, the intake valve is closed and the piston compresses the charge. When the piston reaches the top again, the mixture is ignited, and the resulting expansion of the hot gases forces the piston down. When the piston is at the bottom of its stroke again, the exhaust valve opens and the rising piston forces out the exhaust gases, leaving the cylinder clear for the introduction of the next charge of fuel and air. The entire process requires two complete down-and-up motions of the piston or two revolutions of the crankshaft. A flywheel stores up enough energy from the power stroke to carry the piston through the other three trips before the next power stroke. The first practical engine employing this cycle of operation was made by Karl Otto in Germany in 1876. In the two-stroke cycle, the fresh charge of fuel and air is compressed below the piston or by some other means and forced into the cylinder when the piston is at the bottom of its stroke; the charge is then compressed by the piston’s moving upward, and ignited at the end of this compression stroke as before. At the end of the power stroke, a fresh charge sweeps the exhaust gases out of the cylinder. In this cycle there is one power stroke for each revolution of the crankshaft. As the piston rises during the compression stroke, a partial vacuum is created in the crankcase that draws in a charge of fuel. During the power stroke the downward motion of the piston compresses this charge in the crankcase until the outlet valve opens, permitting the charge to pass to the cylinder and sweep out the exhaust gases. In order to support combustion, air and fuel must be mixed together in the proper proportion. Air-fuel ratios, measured in pounds of air to pounds of fuel, range from 8:1 to 20:1, and may be rich or lean. For maximum power a fairly rich mixture of 10:1 or 12:1 is required. A leaner mixture of 14.5:1 or 15:1 is more commonly used and represents a compromise between power and economy. Although the purpose of an engine is to convert heat to mechanical energy, internal-combustion engines produce more heat than can safely be converted. In order to prevent this excess heat from destroying the engine, it is necessary to provide some means of cooling the cylinders. In small engines and in aircraft engines the cylinders are cooled by a stream of air moving past them. To facilitate cooling, the cylinders are equipped with fins, which increases the surface area that can be in contact with the cooling air system. In large engines the cylinders are cooled by pumping a liquid through a hollow jacket surrounding them. This liquid is usually water, or a relatively nonvolatile substance like ethylene glycol that does not freeze at low temperatures when the engine is not running. This liquid is cooled by passing it through a radiator that is exposed to a stream of air. Gasoline engines may work on either the two- or four-stroke cycle. In addition to the basic pistons, cylinders, and crankshaft, they require a system for mixing the air and fuel together in the proper ratio and a system for igniting the mixture at the proper moment; four-stroke engines also require a system of inlet and exhaust valves. The air and fuel are mixed in a device called the carburetor. Usually the fuel-air ratio is controlled by varying the amount of fuel allowed to mix with the air, if very rich mixtures are required the air is reduced, a process that is called choking. Ignition is accomplished by means of an electric spark created between the points of a spark plug placed in the head of the cylinder. The required electricity is produced by a battery or a small electric generator connected to the crankshaft and is stepped up to a high voltage by a transformer called a spark coil. The ratio of the total cylinder and combustion chamber volume to that of the combustion chamber alone, is known as the compression ratio. In general, the higher the ratio, the greater is the force pushing down on the piston. Automotive engine compression ratios range from about 7:1 to 11:1. In Diesel engines, the heat developed by the compression stroke is used to ignite the fuel. Instead of a fuel-air mixture being introduced into the cylinder, air alone is compressed and the fuel is injected into the cylinder at high pressure at the compression stroke. Diesel engines require no ignition system, are not subject to preignition, and can use relatively cheap fuel oils instead of expensive highly refined gasoline. Since the air and fuel are not mixed before entering the cylinder, no carburetor is required. The high compression ratios required to ensure ignition demand heavier construction, and the fuel must be compressed before it is sprayed into the cylinder.