An internal combustion engine is a heat engine in which fuel burns within the working fluid, and the resulting hot gases produce mechanical work through expansion. It converts chemical energy into thermal energy and then mechanical output. Unlike an externally heated steam engine, it does not require a separate boiler to transfer combustion heat to its working fluid. The term commonly refers to piston engines, although combustion-driven gas turbines also belong to the broader category. Applications include road vehicles, ships, aircraft, machinery, and electricity generation. (ocw.mit.edu)
Operating principles and construction
In a reciprocating engine, combustion raises gas temperature and pressure inside a cylinder. Gas pressure pushes a piston, which transmits force through a connecting rod to a crankshaft. This mechanism converts back-and-forth motion into rotation. Some work is required to compress the incoming charge; useful output depends on obtaining more work during expansion than is consumed by compression and other processes. (energy.gov)
The cylinder’s changing volume is central to engine operation. Its compression ratio is the maximum cylinder volume divided by the minimum volume. Engine displacement is the volume swept by the pistons, rather than the cylinders’ total volume at their largest size. Cylinder pressure, displacement, and operating speed together influence the output obtainable from an engine. (www1.grc.nasa.gov)
Fuel-delivery and ignition systems determine how combustion begins. Intake and exhaust passages manage gas exchange, while cooling and lubrication address heat transfer and mechanical friction. These supporting processes affect durability, efficiency, and the difference between work developed in the cylinders and useful shaft output. (ocw.mit.edu)
Ignition and operating cycles
The two principal ignition methods in conventional vehicle engines are spark ignition and compression ignition. In a spark-ignition engine, an electric spark starts combustion of a fuel–air mixture. Fuel may be introduced upstream of the cylinder or injected directly into it. In a diesel engine, air is compressed until sufficiently hot, and injected fuel then autoignites. Thus, ignition method is distinct from the location of fuel injection. (energy.gov)
A four-stroke engine completes its operating sequence in four piston strokes, corresponding to two crankshaft revolutions:
- Intake: Fresh air or a fuel–air mixture enters.
- Compression: The piston reduces the charge volume.
- Power: Combustion and subsequent expansion drive the piston.
- Exhaust: Burned gases leave the cylinder.
Actual ignition and valve events need not coincide exactly with the boundaries between strokes. The sequence describes mechanical operation, not four perfectly separated thermodynamic processes. (www1.grc.nasa.gov)
A two-stroke engine completes its sequence in one crankshaft revolution. Gas exchange occurs during a shorter portion of the cycle, making scavenging—removing exhaust while supplying fresh charge—particularly important. More frequent power strokes do not automatically yield twice the useful power, because gas exchange and other practical limitations affect performance. (ocw.mit.edu)
Thermodynamic models and performance
Engine analysis uses thermodynamics to relate fuel input, compression, expansion, and heat rejection. The ideal Otto cycle represents spark-ignition operation using constant-volume heat addition and idealized compression and expansion. The ideal Diesel cycle instead uses constant-pressure heat addition. These are analytical models: real combustion takes time, gas properties change, and heat passes through engine walls. (www1.grc.nasa.gov)
Indicated work is obtained from the cylinder’s pressure–volume cycle. Brake output is useful output delivered at the shaft after mechanical losses. Torque measures rotational effort, while power measures the rate of doing work. Efficiency therefore cannot be inferred from displacement or peak power alone; operating speed and load also matter. Fuel-consumption maps show that the same engine can operate at substantially different efficiencies under different conditions. (www1.grc.nasa.gov)
A turbocharger uses exhaust-driven turbine power to compress intake air, increasing the air available for combustion. Boosting changes cylinder filling and the balance of engine and exhaust-system performance. Its benefits depend on operating conditions and engine design, rather than representing a fixed efficiency improvement in every application. (ocw.mit.edu)
Other forms and historical development
A gas turbine uses continuous flow through a compressor, combustor, and turbine instead of repeated piston strokes. Its idealized thermodynamic model is the Brayton cycle. Turbine expansion supplies compressor power; depending on configuration, remaining output can provide shaft power or support thrust production through an exhaust nozzle. (grc.nasa.gov)
Commercial development proceeded through several nineteenth-century designs. Étienne Lenoir produced an early practical gas engine around 1860 without prior charge compression. Nikolaus Otto and Eugen Langen developed an atmospheric free-piston engine in 1867, and Otto’s four-stroke engine followed in 1876. Rudolf Diesel developed compression-ignition machinery in the 1890s; a functioning 1897 engine is preserved by the Deutsches Museum. These milestones involved different mechanisms rather than a single unchanged invention. (ocw.mit.edu)
Fuels, emissions, and applications
Engine fuels include gasoline, diesel fuel, alcohols, and natural gas. Conventional petroleum fuels are fossil fuels, but suitable engines can also use ethanol or biodiesel. Internal combustion engines power automobiles and can be combined with an electric motor in hybrid powertrains. Ignition, fuel properties, and application requirements determine which combinations are practical. (energy.gov)
Exhaust can contain carbon monoxide, unburned hydrocarbons, nitrogen oxides, and particulate matter. Incomplete oxidation produces carbon monoxide and unburned fuel emissions; high-temperature reactions involving nitrogen and oxygen produce nitrogen oxides. Particles can arise in locally fuel-rich regions and from lubricating oil. (ocw.mit.edu)
A catalytic converter promotes reactions that reduce selected gaseous pollutants. Diesel particulate filters capture particles, while selective catalytic reduction systems target nitrogen oxides. These technologies address different pollutants and may be combined; reducing particulate emissions is not equivalent to reducing nitrogen oxides. Their performance depends on the engine, exhaust conditions, and treatment-system configuration. (ocw.mit.edu)