DIESEL ENGINE

any internal-combustion engine in which air is compressed to a temperature sufficiently high to ignite fuel injected into the cylinder where combustion and expansion actuate a piston. It converts the chemical energy stored in the fuel into mechanical energy, which can be used to power freight trucks, large tractors, locomotives, and vessels. A limited number of automobiles also are diesel-powered, as are small electric-power generators. The diesel engine differs from other internal-combustion engines such as gasoline engines that induct and moderately compress an inflammable mixture of air and vaporized or gaseous fuel and then ignite it by an electric spark. It employs no ignition devices, and so is often called a compression-ignition engine. The possibility of compression ignition appears to have been first mentioned by the French physicist Sadi Carnot in 1824. The principle on which the modern automobile engine operates was conceived in 1862 by Alphonse Beau de Rochas, also of France; however, it was not applied to a practical engine until 1876, when the German engineer Nikolaus Otto built the first engine in which the charge (fuelair mixture) was compressed in the cylinder before burning. This was the engine that Rudolf Diesel, a German engineer, undertook to improve when he started the experiments that led to the diesel engine. After several years of studying the problems involved, Diesel applied for and was granted patents in 1892 and 1893. The processes described in these patents represented a cycle that differed from the theoretical or ideal cycle followed by existing engines only in the rate of combustion of the fuelair charge. He proposed to burn the fuel during the first portion of the power stroke of the piston so slowly that no pressure rise would occur. As a means of slowing down combustion to avoid the almost instantaneous explosion of the spark-ignition engine, he suggested inducting air into the cylinder and compressing it so highly that it would attain a temperature above the ignition point of an appropriate fuel, which would be gradually sprayed into the combustion chamber during the descent of the piston. This method of igniting fuel requires that the air be compressed to a pressure of at least 3,450 kilopascals (500 pounds per square inch), which produces a temperature of approximately 540 C (1,000 F). Diesel presumably thought that any fuel would be suitable for an engine operating in the manner he described. He attempted to build an engine that would burn powdered coal, the cheapest conceivable fuel. The coal-burning project, however, was soon abandoned, and an oil-burning version of the engine was completed in 1897. The fuel economy of Diesel's engine proved to be better than that of any other existing power plant. The engine, however, was not rapidly adopted. Until his death in 1913, Diesel insisted that all engines manufactured under his licenses be made to operate with combustion at practically constant pressure, as described in his 1893 patent. This restriction meant that the engines had to run at a very low speed, and thus possible design improvements were delayed. The early diesel engines were so large and heavy in proportion to their power output that they had no application other than as stationary power plants. The first marine installation of a diesel engine was completed in 1910; the diesel engine became the primary power plant for submarines during World War I. The first diesel engine that was small and light enough for use in automobiles was built in 1922 in Germany, and it opened up numerous fields of application that had previously been closed to diesel engines because of their low specific power output. The higher-speed diesel engines do not follow the slow-burning cycle originated by Diesel. Fuel is injected into the cylinder near the end of the compression stroke and burned rapidly, with sharply rising pressure, while the piston is near its dead-centre position. Only the compression-ignition and fuel injection of Diesel's original engine are retained in the modern high-speed version. Since the mid-1940s the diesel engine has become the predominant source of industrial power throughout the world for units up to roughly 5,000 horsepower, principally because it is capable of burning a low-grade fuel at a comparatively low rate of consumption per horsepower per hour. Relatively unrefined fuels can be burned by a diesel engine because of the nature of its fuel-injection system and combustion process. Low fuel consumption results primarily from the higher compression ratio used. A greater fuel saving is effected at partial load than at full load since it is not necessary to throttle the inlet air, as in the case with spark ignition, to maintain an inflammable fuelair mixture. (Only about two-thirds as much fuel is required.) Diesel engines do have some disadvantages, however. They are, for example, handicapped somewhat by their higher initial cost and greater weight per horsepower, by their emission of high levels of air pollutants (e.g., nitric oxide and soot) and odour, and by their greater operating noise and vibration. any internal-combustion engine in which air is compressed to a temperature sufficiently high to ignite fuel injected into the cylinder, where combustion and expansion actuate a piston. It converts the chemical energy stored in the fuel into mechanical energy, which can be used to power freight trucks, large tractors, locomotives, and vessels. A limited number of automobiles also are diesel powered, as are small electric-power generators. Additional reading Diesel engines are discussed in S.D. Haddad and N. Watson (eds.), Principles and Performance in Diesel Engineering (1984); and Frank J. Thiessen and Davis N. Dales, Diesel Fundamentals, 2nd ed. (1986). Charles Lafayette Proctor II