Internalcombustion

Internal combustion refers to a process in which fuel is burned inside a confined space to release energy that directly drives a mechanical system. In the most common form, the internal combustion engine, combustion occurs within cylinders in which a piston converts the expanding gases into rotational motion. This contrasts with external combustion, where heat from combustion of fuel occurs outside the engine, such as in a steam engine.

Most internal combustion engines operate on a cycle that includes intake of air and fuel, compression, combustion and expansion, and exhaust. In spark-ignition engines, a spark plug ignites a premixed air–fuel charge; in compression-ignition engines, compression raises the charge to a temperature at which fuel ignites spontaneously. The sequence is typically arranged as four strokes (intake, compression, power, exhaust) or, in some designs, two overlapping strokes. The thermodynamic cycles are commonly referred to as the Otto cycle for gasoline engines and the Diesel cycle for compression-ignition engines.

Fuels used include gasoline, diesel, natural gas, propane, ethanol, biodiesel, and hydrogen in specialized engines. Efficiency and emissions vary by design; modern gasoline engines can be more efficient through direct injection and turbocharging, while diesels generally offer higher thermal efficiency but emit more NOx and particulates unless treated with exhaust aftertreatment. Emission-control technologies include catalytic converters, exhaust gas recirculation, and particulate filters.

Internal combustion engines power most road vehicles, many aircraft, ships, and stationary generators. In recent years, efficiency improvements, strict emissions standards, and the growth of electrification have reduced the share of energy from internal combustion propulsion in some sectors, but ICEs remain a major component of global transport and industry. Research continues in areas such as alternative fuels, advanced combustion modes, and hybrid architectures to reduce environmental impact.