INTERNAL-COMBUSTION ENGINE

any engine in which a fuel-air mixture is burned in the engine proper so that the hot gaseous products of combustion act directly on the surface of its moving parts, such as that of a piston or turbine rotor blade. Internal-combustion engines are the most widely used of all present-day power-generating systems. They include gasoline engines, diesel engines, gas-turbine engines, pure jet engines, and rocket engines and motors. Internal-combustion engines constitute one class of what are known as heat engines, devices that derive their power either directly or indirectly from an exothermic reaction (e.g., combustion). The other principal class of heat engines consists of external-combustion engines, of which the steam engine and the Stirling engine are notable examples. Unlike internal-combustion systems, these engines employ a secondary working fluid that is interposed between the combustion chamber and the power-producing elements. In the steam engine, for instance, fuel is burned in a furnace external to the engine, and a portion of the energy liberated is transferred through boiler or heat-exchanger walls to the secondary working fluid consisting of water. The water is converted into steam, which is then conveyed through pipes to the engine in which the pressure of the steam acts on pistons or turbine runners. In effect, the steam engine and other external-combustion systems operate with a high-pressure working medium produced by utilizing the expansion that accompanies the vaporization of a liquid. Internal-combustion engines, on the other hand, make use of the large volume of hot combustion products that become a high-pressure gaseous medium when confined. Internal-combustion engines are commonly divided into two major subclasses: continuous-combustion engines and intermittent-combustion engines. The first type, exemplified by the jet engine, is characterized by a steady flow of fuel and air into the engine where a stable flame is maintained for continuous combustion. In the second variety, typified by the gasoline-reciprocating piston engine, discrete quantities of fuel and air are processed in a cyclical manner, with a periodic ignition of the fuelair mixture. any of a group of devices in which the reactants of combustion (oxidizer and fuel) and the products of combustion serve as the working fluids of the engine. Such an engine gains its energy from heat released during the combustion of the nonreacted working fluids, the oxidizerfuel mixture. This process occurs within the engine and is part of the thermodynamic cycle of the device. Useful work generated by an internal-combustion (IC) engine results from the hot, gaseous products of combustion acting on moving surfaces of the engine, such as the face of a piston, a turbine blade, or a nozzle. Internal-combustion engines are divided into two groups: continuous-combustion engines and intermittent-combustion engines. The continuous-combustion engine is characterized by a steady flow of fuel and oxidizer into the engine. A stable flame is maintained within the engine (e.g., jet engine). The intermittent-combustion engine is characterized by periodic ignition of air and fuel and is commonly referred to as a reciprocating engine. Discrete volumes of air and fuel are processed in a cyclic manner. Gasoline piston engines and diesel engines are examples of this second group. Internal-combustion engines can be delineated in terms of a series of thermodynamic events. In the continuous-combustion engine, the thermodynamic events occur simultaneously as the oxidizer and fuel, and the products of combustion flow steadily through the engine. In the intermittent-combustion engine, by contrast, the events occur in succession and are repeated for each full cycle. With the exception of rockets (both solid-rocket motors and liquid-propellant rocket engines), internal-combustion engines ingest air, then either compress the air and introduce fuel into the air or introduce fuel and compress the airfuel mixture, burn the airfuel mixture, extract work from the hot, gaseous products of combustion by expansion, and ultimately exhaust the products of combustion. Their operation can be contrasted with that of external-combustion engines (e.g., steam engines), in which the working fluid does not chemically react and energy gain is achieved solely through heat transfer to the working fluid by way of a heat exchanger. Internal-combustion engines are the most broadly applied and widely used power-generating devices currently in existence. Examples include gasoline (or spark-ignition ) engines, diesel engines (sometimes referred to as compression-ignition engines), gas-turbine engines, and rocket propulsion systems. The most common internal-combustion engine is the four-stroke gasoline-powered, homogeneous-charge, spark-ignition engine. This is because of its outstanding performance as a prime mover in the ground-transportation industry. Spark-ignition engines also are used in the aeronautics industry; however, aircraft gas turbines have become the prime movers in this sector due to the emphasis of the aeronautics industry on range, speed, and passenger comfort. The domain of internal-combustion engines also includes such exotic devices as supersonic combustion ramjet engines (scramjets), as typified by the space plane, and sophisticated rocket engines and motors, as those used on the U.S. Space Shuttle and other space vehicles. It is the versatility and costboth capital and operationalof conventional internal-combustion engines that have led to their widespread use in contemporary energy production. Additional reading A historical treatment of the invention of the internal-combustion engine is provided in two articles in Technology and Culture by Lynwood Bryant, The Silent Otto, 7(2):184200 (Spring 1966), and The Origin of the Four-Stroke Cycle, 8(2):178198 (April 1967). Overviews include Lester C. Lichty, Combustion Engine Processes (1967); Edward F. Obert, Internal Combustion Engines and Air Pollution (1973); Ashley S. Campbell, Thermodynamic Analysis of Combustion Engines (1979, reprinted 1985); Charles Fayette Taylor, The Internal-Combustion Engine in Theory and Practice, 2nd ed. rev., 2 vol. (1985); Colin R. Ferguson, Internal Combustion Engines (1986); and John B. Heywood ,Internal Combustion Engine Fundamentals (1988). Charles Lafayette Proctor II