What Are Engines?

Engines are machines that convert stored energy into mechanical work. In engineering, the term most commonly refers to heat engines, which extract work from the flow of thermal energy between a high-temperature source and a low-temperature sink, though the category also includes electric motors and hydraulic actuators. The thermodynamic behavior of heat engines is governed by the second law of thermodynamics, which places an absolute upper bound on efficiency determined by the temperatures of the hot and cold reservoirs involved.

Heat Engines and Thermodynamic Cycles

A heat engine operates by taking a working fluid through a cyclic thermodynamic process that includes heat addition, expansion, heat rejection, and compression. The Carnot cycle defines the theoretical maximum efficiency achievable between two thermal reservoirs: efficiency equals one minus the ratio of the cold reservoir temperature to the hot reservoir temperature, with both expressed in kelvin. Real engines deviate from this ideal because of irreversibilities including friction, heat transfer across finite temperature differences, and non-quasi-static processes.

The Rankine cycle is the basis for most steam power plants. Water is pumped to high pressure, heated in a boiler to produce steam, expanded through a turbine to generate shaft work, and condensed back to liquid. Variations such as reheat and regenerative Rankine cycles improve efficiency by reducing irreversibility in heat exchange. The Brayton cycle governs gas turbines used in power generation and aviation, involving compression of a gas working fluid, combustion at approximately constant pressure, and expansion through a turbine.

The NIST Chemistry WebBook provides thermodynamic property tables for common working fluids, enabling engineers to perform accurate cycle analyses across a wide range of pressures and temperatures.

Internal Combustion Engines

Internal combustion engines (ICEs) burn fuel inside the working fluid itself, typically within cylinders where pistons convert the pressure rise from combustion into reciprocating motion, which a crankshaft converts to rotation. The four-stroke Otto cycle (intake, compression, combustion and expansion, exhaust) underlies most gasoline passenger-vehicle engines, while the Diesel cycle, which relies on compression-ignition rather than a spark, is standard in heavy trucks, locomotives, and marine applications.

Engine efficiency in ICEs depends on compression ratio, combustion chamber geometry, fuel properties, and valve timing. Turbocharging recovers energy from exhaust gases to increase intake air density, raising power output without proportional increases in displacement. Advances in direct fuel injection, variable valve timing, and exhaust gas recirculation have improved efficiency and reduced criteria pollutant emissions in compliance with standards set by agencies such as the US Environmental Protection Agency.

Jet Engines

Jet engines are air-breathing engines that generate thrust by accelerating a mass of air rearward at high velocity. The turbojet, the simplest form, compresses inlet air, burns fuel in a combustor, expands the hot gas through a turbine that drives the compressor, and exhausts the remaining energy as a high-velocity jet. Turbofan engines add a large fan driven by the core turbine to accelerate a bypass stream of air around the core, greatly improving propulsive efficiency at subsonic speeds.

Engine pressure ratio and turbine inlet temperature are the principal design parameters controlling thermal efficiency. Modern high-bypass turbofan engines achieve thermal efficiencies approaching 55% and power plant overall efficiencies near 40% under cruise conditions. Research into ceramic matrix composite turbine blades and additive-manufactured cooling channels aims to push turbine inlet temperatures higher. The SAE International publishes standards and technical papers governing propulsion system performance testing and certification.

Applications

• Combined heat and power (cogeneration) plants using gas turbines to supply electricity and process heat to industrial facilities
• Hybrid electric vehicles pairing gasoline ICEs with electric motors and battery packs to reduce fuel consumption during low-load urban driving
• High-bypass turbofan engines powering long-haul commercial aircraft at cruise altitudes
• Organic Rankine cycle systems recovering waste heat from industrial exhaust streams
• Two-stroke compression-ignition engines in marine cargo vessels running on low-sulfur heavy fuel oil
• Experimental hydrogen-fueled ICEs and turbines targeting near-zero carbon combustion