エンジン

Engine — Grokipedia Fact-checked by Grok 3 months ago Engine Ara Eve Leo Sal 1x An engine is a machine designed to convert one or more forms of energy, typically thermal or chemical from fuel combustion, into mechanical power or motion. [1] [2] Engines are broadly classified as heat engines, which transform heat into work via thermodynamic cycles, or as electric motors that convert electrical energy directly into motion, though the term "engine" conventionally emphasizes combustion-based systems. [3] [4] The practical development of engines originated with early steam engines in the late 17th century, such as Thomas Savery's 1698 design, which used steam pressure to pump water, marking the inception of devices harnessing thermal expansion for mechanical output. [5] James Watt's improvements in the 1760s and 1770s, including the separate condenser, dramatically increased efficiency and fueled the Industrial Revolution by enabling reliable power for factories, mills, and locomotives. [5] The 19th century saw the rise of internal combustion engines, with Nikolaus Otto's 1876 four-stroke cycle providing a foundational design for gasoline engines that powers most automobiles today, converting fuel combustion within cylinders into reciprocating motion via pistons and crankshafts. [6] Rudolf Diesel's 1892 compression-ignition engine offered higher thermal efficiency for heavy-duty applications, exploiting self-ignition under high pressure to achieve greater fuel economy than spark-ignition counterparts. [7] Fundamentally, heat engine performance is constrained by the second law of thermodynamics, with the Carnot efficiency—defined as 1 − T c T h 1 - \frac{T_c}{T_h} 1 − T h ​ T c ​ ​ where T h T_h T h ​ and T c T_c T c ​ are the absolute temperatures of the hot and cold reservoirs—representing the theoretical maximum, underscoring inherent irreversibilities like heat loss that limit real-world yields to below 50% in most designs. [8] [9] These innovations not only propelled transportation—from horseless carriages to jet propulsion in aviation—but also underpin modern energy systems, though ongoing challenges include optimizing combustion for reduced emissions while maximizing power density. [6] [10] Fundamentals Definition and Principles An engine is a mechanical device that converts energy from a source, such as fuel combustion or electrical input, into useful mechanical work or motion, serving as a prime mover to drive machinery or vehicles. [11] [12] This conversion adheres to the first law of thermodynamics, which states that energy is conserved and cannot be created or destroyed, only transformed from one form to another, with the engine's output work equaling the input energy minus losses due to friction, heat dissipation, and other inefficiencies. [11] [13] In practice, engines are distinguished from electric motors, where "engine" typically implies a heat-based system involving cyclic processes of energy addition and extraction, whereas motors directly exploit electromagnetic forces for rotation without thermal cycles. [1] The core principles governing engine operation stem from thermodynamics , particularly for heat engines , which absorb thermal energy from a high-temperature reservoir , perform work via expansion of working fluids or gases, and reject waste heat to a lower-temperature sink. [13] This process is constrained by the second law of thermodynamics, which prohibits perpetual motion machines of the second kind and establishes that no heat engine can achieve 100% efficiency, with the maximum theoretical efficiency given by the Carnot limit: η = 1 - (T_L / T_H), where T_L and T_H are the absolute temperatures of the low- and high-temperature reservoirs , respectively. [11] [14] Real engines, such as those using the Otto or Diesel cycles, achieve far lower efficiencies—typically 20-40% for internal combustion types—due to irreversibilities like incomplete combustion, heat losses, and fluid friction. [6] [13] Fundamentally, engine design optimizes power density , torque , and efficiency through mechanisms like piston-cylinder assemblies in reciprocating engines or turbine blades in rotary types, where controlled expansion of high-pressure gases or fluids generates linear or rotational force transmitted via crankshafts or shafts. [6] [15] Auxiliary principles include mechanical advantage from leverage and gearing to match output to load requirements, as well as feedback controls like governors to regulate speed and prevent runaway operation, ensuring stable energy conversion under varying conditions. [16] These principles apply across engine classifications, though specifics vary: chemical energy release in combustion drives most traditional engines, while emerging designs incorporate electrical or hybrid inputs for improved controllability and reduced emissions. [11] Terminology and Classifications In mechanical engineering , an engine is defined as a device that converts thermal , chemical, or other forms of energy into mechanical work, typically through the expansion of a working fluid or direct combustion process. [11] This contrasts with a motor, which generally refers to an electric device that produces motion from electrical energy without internal combustion or heat transfer cycles, though colloquial usage often blurs the distinction in contexts like electric vehicles. [1] [17] Key terminology includes the bore , the internal diameter of the engine cylinder, measured in millimeters or inches; the stroke , the linear distance traveled by the piston from top dead center (TDC, the position farthest from the crankshaft) to bottom dead center (BDC, closest to the crankshaft); and displacement or swept volume, calculated as the product of bore area and stroke length multiplied by the number of cylinders, representing the total volume of air-fuel mixture processed per cycle. [18] Additional terms encompass compression ratio , the ratio of cylinder volume at BDC to TDC, influencing efficiency and power output; mean effective pressure (MEP) , the average pressure during the power stroke that yields net work; and brake mean effective pressure (BMEP) , an adjusted measure accounting for mechanical losses to reflect actual engine performance. [19] These parameters derive from first-principles kinematics and thermodynamics, where piston motion follows $ s = r(1 - \cos\theta) + (l - \sqrt{l^2 - r^2\sin^2\theta}) $, with $ r $ as crank radius, $ l $ as connecting rod length, and $ \theta $ as crank angle, enabling precise volumetric computations. [20] Engines are classified across multiple criteria to delineate design , operation, and application, rooted in thermodynamic cycles and mechanical configuration rather than arbitrary groupings. Classification Basis Categories Description Combustion Location Internal Combustion Engine (ICE); External Combustion Engine (ECE) In ICE, combustion occurs within the working fluid confines, as in gasoline engines; ECE separates combustion in an external chamber, heating a secondary fluid like steam . [4] [21] Ignition Method Spark Ignition (SI); Compression Ignition (CI) SI uses an electric spark for fuel-air mixture ignition, suited to volatile fuels like gasoline ; CI relies on high compression heat for auto-ignition of diesel, yielding higher efficiency but requiring robust components. [22] [18] Thermodynamic Cycle Two-Stroke; Four-Stroke Two-stroke completes intake , compression, power, and exhaust in two piston strokes via ports; four-stroke uses valves over four strokes, enabling better scavenging but doubling mechanical losses. [23] Fuel Type Gasoline /Petrol; Diesel; Gas (e.g., natural gas ); Dual-Fuel Determined by combustion characteristics; diesel offers superior thermal efficiency (up to 40-50% in large units) due to higher compression ratios (14:1 to 25:1). [21] Cylinder Arrangement Inline; V-Type; Opposed-Piston; Radial Inline aligns cylinders in a row for simplicity; V-type folds for compactness in high-power applications; radial suits aviation for even cooling. [24] Cooling Method Air-Cooled; Liquid-Cooled Air-cooled uses fins and airflow for lightweight designs; liquid-cooled employs coolant circuits for consistent temperature control in high-load scenarios. [25] These classifications reflect causal trade-offs: for instance, internal combustion enables higher power density (up to 100 kW/L in modern turbocharged units) via direct heat release but increases emissions, while external types prioritize steady-state efficiency at the cost of slower response. [26] Empirical data from standardized tests, such as indicated power versus brake power (accounting for friction via $ \eta_m = BP/IP $), validate these distinctions without reliance on biased institutional narratives. [19] Historical Development Ancient and Medieval Origins The earliest engines, understood as devices converting energy into mechanical motion, appeared in antiquity through hydraulic mechanisms. Water wheels, documented in Hellenistic Greece by the 3rd century BC , harnessed the potential energy of falling or flowing water to rotate wooden or metal wheels geared to grind grain or pump water , with archaeological evidence from sites like the Barbegal aqueduct complex in Roman Gaul featuring 16 overshot wheels operational by the 2nd century AD. These represented practical prime movers but were site-bound and dependent on geography. A pivotal ancient innovation was the aeolipile , developed by Hero of Alexandria around 10–70 AD. This steam-powered reaction turbine consisted of a closed boiler heating water to generate steam , which exited through L-shaped nozzles on a pivoted hollow sphere, imparting rotational torque via Newton's third law. Detailed in Hero's Pneumatica , the device spun at observable speeds but extracted no useful work, functioning as a demonstration of pneumatics rather than an efficient engine, limited by material frailties and lack of gearing for load-bearing tasks. [27] [28