Cross section showing one cylinder of a four-stroke internal-combustion engine. In the first stroke elipsis
Learn more about internal-combustion engine with a free trial on Britannica.com.
The internal combustion engine is an engine in which the combustion of fuel and an oxidizer (typically air) occurs in a confined space called a combustion chamber. This exothermic reaction creates gases at high temperature and pressure, which are permitted to expand. Internal combustion engines are defined by the useful work that is performed by the expanding hot gases acting directly to cause the movement of solid parts of the engine.
The term Internal Combustion Engine (ICE) is often used to refer to an engine in which combustion is intermittent, such as a Wankel engine or a reciprocating piston engine in which there is controlled movement of pistons, cranks, cams, or rods. However, continuous combustion engines such as jet engines, most rockets, and many gas turbines are also classified as types of internal combustion engines. This contrasts with external combustion engines such as steam engines and Stirling engines that use a separate combustion chamber to heat a separate working fluid—which then in turn does work, for example, by moving a piston or a turbine.
A huge number of different designs for internal combustion engines exist, each with different strengths and weaknesses. Although they're used for many different purposes, internal combustion engines particularly see use in mobile applications such as cars, aircraft, and even handheld applications: all where their ability to use an energy-dense fuel (especially fossil fuels) to deliver a high power-to-weight ratio.
Internal combustion engines are most commonly used for mobile propulsion in vehicles and portable machinery. In mobile equipment, internal combustion is advantageous since it can provide high power-to-weight ratios together with excellent fuel energy density. Generally using fossil fuel (mainly petroleum), these engines have appeared in transport in almost all vehicles (automobiles, trucks, motorcycles, boats, and in a wide variety of aircraft and locomotives). This vehicles, when are not hybrid, are called All-Petroleum Internal Combustion Engine Vehicles (APICEVs) or All Fossil Fuel Internal Combustion Vehicles (AFFICEVs).
Internal combustion engines appear in the form of gas turbines as well where a very high power is required, such as in jet aircraft, helicopters, and large ships. They are also frequently used for electric generators and by industry.
Combustible mixtures are emplaced in the combustion chamber
The mixtures are placed under pressure
The cooled combustion products are exhausted
Many engines overlap these steps in time, jet engines do all steps simultaneously at different parts of the engines. Some internal combustion engines have extra steps.
All internal combustion engines depend on the exothermic chemical process of combustion: the reaction of a fuel, typically with oxygen from the air—although other oxidizers such as nitrous oxide may be employed. The combustion process typically results in the production of a great quantity of heat, as well as the production of steam and carbon dioxide and other chemicals at very high temperature; the temperature reached is determined by the chemical make up of the fuel and oxidisers (see stoichiometry).
The most common modern fuels are made up of hydrocarbons and are derived mostly from petroleum. These include the fuels known as dieselfuel, gasoline and petroleum gas, and the rarer use of propane. Except for the fuel delivery components, most internal combustion engines that are designed for gasoline use can run on natural gas or liquefied petroleum gases without major modifications. Liquid and gaseous biofuels, such as ethanol and biodiesel (a form of diesel fuel that is produced from crops that yield triglycerides such as soybean oil), can also be used. Some engines with appropriate modifications can also run on hydrogen gas.
All internal combustion engines must achieve ignition in their cylinders to create combustion. Typically engines use either a spark ignition (SI) method or a compression ignition (CI) system. In the past, other methods using hot tubes or flames have been used.Gasoline Ignition Process Gasoline engine ignition systems generally rely on a combination of a lead-acid battery and an induction coil to provide a high-voltage electrical spark to ignite the air-fuel mix in the engine's cylinders. This battery is recharged during operation using an electricity-generating device such as an alternator or generator driven by the engine. Gasoline engines take in a mixture of air and gasoline and compress to less than 185 psi, then use a spark plug to ignite the mixture when it is compressed by the piston head in each cylinder.Diesel Ignition Process Diesel engines and HCCI engines, rely solely on heat and pressure created by the engine in its compression process for ignition. The compression level that occurs is usually twice or more than a gasoline engine. Diesel engines will take in air only, and shortly before peak compression, a small quantity of diesel fuel is sprayed into the cylinder via a fuel injector that allows the fuel to instantly ignite. HCCI type engines will take in both air and fuel but continue to rely on an unaided auto-combustion process, due to higher pressures and heat. This is also why diesel and HCCI engines are more susceptible to cold-starting issues, although they will run just as well in cold weather once started. Light duty diesel engines in automobiles and light trucks employ glow plugs that pre-heat the combustion chamber just before starting to reduce no-start conditions in cold weather. Most diesels also have a battery and charging system; nevertheless, this system is secondary and is added by manufacturers as a luxury for the ease of starting, turning fuel on and off (which can also be done via a switch or mechanical apparatus), and for running auxiliary electrical components and accessories. Most new engines rely on electrical and electronic control systems that also control the combustion process to increase efficiency and reduce emissions.
Engine types vary greatly in a number of different ways:
Once ignited and burnt, the combustion products—hot gases—have more available thermal energy than the original compressed fuel-air mixture (which had higher chemical energy). The available energy is manifested as high temperature and pressure that can be translated into work by the engine. In a reciprocating engine, the high-pressure gases inside the cylinders drive the engine's pistons.
Once the available energy has been removed, the remaining hot gases are vented (often by opening a valve or exposing the exhaust outlet) and this allows the piston to return to its previous position (top dead center, or TDC). The piston can then proceed to the next phase of its cycle, which varies between engines. Any heat that isn't translated into work is normally considered a waste product and is removed from the engine either by an air or liquid cooling system.
Engine efficiency can be discussed in a number of ways but it usually involves a comparison of the total chemical energy in the fuels, and the useful energy abstracted from the fuels in the form of kinetic energy. The most fundamental and abstract discussion of engine efficiency is the thermodynamic limit for abstracting energy from the fuel defined by a thermodynamic cycle. The most comprehensive is the empirical fuel economy of the total engine system for accomplishing a desired task; for example, the miles per gallon accumulated.
Internal combustion engines are primarily heat engines and as such the phenomenon that limits their efficiency is described by thermodynamic cycles. None of these cycles exceed the limit defined by the Carnot cycle which states that the overall efficiency is dictated by the difference between the lower and upper operating temperatures of the engine. A terrestrial engine is usually and fundamentally limited by the upper thermal stability derived from the material used to make up the engine. All metals and alloys eventually melt or decompose and there is significant researching into ceramic materials that can be made with higher thermal stabilities and desirable structural properties. Higher thermal stability allows for greater temperature difference between the lower and upper operating temperatures—thus greater thermodynamic efficiency.
The thermodynamic limits assume that the engine is operating in ideal conditions. A frictionless world, ideal gases, perfect insulators, and operation at infinite time. The real world is substantially more complex and all the complexities reduce the efficiency. In addition, real engines run best at specific loads and rates as described by their power curve. For example, a car cruising on a highway is usually operating significantly below its ideal load, because the engine is designed for the higher loads desired for rapid acceleration. The applications of engines are used as contributed drag on the total system reducing overall efficiency, such as wind resistance designs for vehicles. These and many other losses result in an engines' real-world fuel economy that is usually measured in the units of miles per gallon (or kilometers per liter) for automobiles. The miles in, "MPG" represents a meaningful amount of work and the volume of hydrocarbon implies a standard energy content.
Most steel engines have a thermodynamic limit of 37%. Even when aided with turbochargers and stock efficiency aids, most engines retain an average efficiency of about 18%-20%.
There are many inventions concerned with increasing the efficiency of IC-Engines. In general, practical engines are always compromised by trade-offs between different properties such as efficiency, weight, power, heat, response, exhaust emissions, or noise. Sometimes economy also plays a role in not only in the cost of manufacturing the engine itself, but also manufacturing and distributing the fuel. Increasing the engines' efficiency brings better fuel economy but only if the fuel cost per energy content is the same.
For internal combustion engines in the form of jet engines, the power output varies drastically with airspeed and a less variable measure is used: thrust specific fuel consumption (TSFC), which is the number of pounds of propellant that is needed to generate impulses that measure a pound an hour. In metric units, the number of grams of propellant needed to generate an impulse that measures one kilonewton per second.
Internal combustion engines such as reciprocating internal combustion engines produce air pollution emissions, due to incomplete combustion of carbonaceous fuel. The main derivatives of the process are carbon dioxide , water and some soot—also called particulate matter (PM). The effects of inhaling particulate matter have been studied in humans and animals and include asthma, lung cancer, cardiovascular issues, and premature death. There are however some additional products of the combustion process that include nitrogen oxides and sulfur and some uncombusted hydrocarbons, depending on the operating conditions and the fuel-air ratio.
Not all of the fuel will be completely consumed by the combustion process; a small amount of fuel will be present after combustion, some of which can react to form oxygenates, such as formaldehyde or acetaldehyde, or hydrocarbons not initially present in the fuel mixture. The primary causes of this is the need to operate near the stoichiometric ratio for gasoline engines in order to achieve combustion and the resulting "quench" of the flame by the relatively cool cylinder walls, otherwise the fuel would burn more completely in excess air. When running at lower speeds, quenching is commonly observed in diesel (compression ignition) engines that run on natural gas. It reduces the efficiency and increases knocking, sometimes causing the engine to stall. Increasing the amount of air in the engine reduces the amount of the first two pollutants, but tends to encourage the oxygen and nitrogen in the air to combine to produce Nitrogen Oxides (NOx) that has been demonstrated to be hazardous to both plant and animal health. Further chemicals released are Benzene and 1,3-Butadiene that are also particularly harmful; and not all of the fuel burns up completely, so Carbon Monoxide (CO) is also produced.
Carbon fuels contain sulfur and impurities that eventually lead to producing sulfur oxides (SO) and Sulphur Dioxide (SO2) in the exhaust which promotes acid rain. One final element in exhaust pollution is ozone (O3). This is not emitted directly but made in the air by the action of sunlight on other pollutants to form "ground level ozone", which, unlike the "Ozone Layer" in the high atmosphere, is regarded as a bad thing if the levels are too high. Ozone is broken down by nitrogen oxides, so one tends to be lower where the other is higher.
For the pollutants described above (nitrogen oxides, carbon monoxide, sulphur dioxide, and ozone) there are accepted levels that are set by legislation to which no harmful effects are observed—even in sensitive population groups. For the other three: benzene: 1:3 butadiene: particulates, there is no way of proving they are safe at any level so the experts set standards where the risk to health is, "exceedingly small".
Finally, significant contributions to noise pollution are made by internal combustion engines. Automobile and truck traffic operating on highways and street systems produce noise, as do aircraft flights due to jet noise, particularly supersonic-capable aircraft. Rocket engines create the most intense noise.
For a four-stroke engine, key parts of the engine include the crankshaft (purple), connecting rod (orange), one or more camshafts (red and blue), and valves. For a two-stroke engine, there may simply be an exhaust outlet and fuel inlet instead of a valve system. In both types of engines there are one or more cylinders (grey and green), and for each cylinder there is a spark plug (darker-grey, gasoline engines only), a piston (yellow), and a crankpin (purple). A single sweep of the cylinder by the piston in an upward or downward motion is known as a stroke. The downward stroke that occurs directly after the air-fuel mix passes from the carburetor or fuel injector to the cylinder, where it is ignited. This is also known as a power stroke.
A Wankel engine has a triangular rotor that orbits in an epitrochoidal (figure 8 shape) chamber around an eccentric shaft. The four phases of operation (intake, compression, power, and exhaust) take place in what is effectively a moving, variable-volume chamber.
A Bourke Engine uses a pair of pistons integrated to a Scotch Yoke that transmits reciprocating force through a specially designed bearing assembly to turn a crank mechanism. The intake, compression, power, and exhaust occur in each stroke.
At one time the word, "engine" (from Latin, via Old French, ingenium, "ability") meant any piece of machinery—a sense that persists in expressions such as siege engine. A "motor" (from Latin motor, "mover") is any machine that produces mechanical power. Traditionally, electric motors are not referred to as, "engines"; however, combustion engines are often referred to as, "motors." (An electric engine refers to a locomotive operated by electricity).
However, many people consider engines as things which generate their power from within, and motors as requiring an outside source of energy to perform work.
Engines can be classified in many different ways: by the principles of operation: by the source of energy: by the use of the engine, or by the cooling system employed.
Spark-ignition two-strokes are small and light for their power output and mechanically very simple; however, they are also generally less efficient and more polluting than their four-stroke counterparts. In terms of power per cubic centimetre, a single-cylinder small motor application like a two-stroke engine produces much more power than an equivalent four-stroke engine due to the enormous advantage of having one power stroke for every 360 degrees of crankshaft rotation (compared to 720 degrees in a 4 stroke motor).
Small displacement, crankcase-scavenged two-stroke engines have been less fuel-efficient than other types of engines when the fuel is mixed with the air prior to scavenging allowing some of it to escape out of the exhaust port. Modern designs (Sarich and Paggio) use air-assisted fuel injection which avoids this loss, and are more efficient than comparably sized four-stroke engines. Fuel injection is essential for a modern two-stroke engine in order to meet ever more stringent emission standards.
Research continues into improving many aspects of two-stroke motors including direct fuel injection, amongst other things. The initial results have produced motors that are much cleaner burning than their traditional counterparts. Two-stroke engines are widely used in snowmobiles, lawnmowers, weed-whackers, chain saws, jet skis, mopeds, outboard motors, and many motorcycles.
The largest compression-ignition engines are two-strokes and are used in some locomotives and large ships. These particular engines use forced induction to scavenge the cylinders; an example of this type of motor is the Wartsila-Sulzer turbocharged two-stroke diesel as used in large container ships. It is the most efficient and powerful engine in the world with over 50% thermal efficiency. For comparison, the most efficient small four-stroke motors are around 43% thermal efficiency (SAE 900648); size is an advantage for efficiency due to the increase in the ratio of volume to area.
Beare Head Technology combines a four-stroke engine bottom end with a ported cylinder which closely resembles that of a two-stroke: thus, 4+2 equals a six-stroke. It has an opposing piston that acts in unison with auxiliary low pressure reed and rotary valves, which allows variable compression and a range of tuning options.
The IRIS design replaces the piston and cylinder architecture of conventional engines with a mechanism called the Internally Radiating Impulse Structure or "IRIS". In an IRIS combustion chamber, a number of inverted segments of a circle or "chordons" interact to create a continuously sealed chamber of variable volume. Instead of elongating during combustion—as a traditional engine does—the IRIS engines' chamber expands in diameter. This innovation may significantly enhance fuel efficiency by reducing waste heat and increasing the amount of surface area the engine has available to produce torque. The IRIS design was recently patented and may eventually yield engines that are smaller, lighter, and more efficient than traditional piston-based systems.
In this engine, two opposed cylinders are linked to the crank by a Scotch yoke. The Scotch yoke mechanism prevents side thrust which in turn prevents any piston slap allowing the operation as a detonation or "explosion" engine. This also greatly reduces friction between pistons and cylinder walls. The Bourke engine uses fewer moving parts and has to overcome less friction than conventional crank and slider engines with poppet valves, however no independent testing of this engine has ever expanded any of these claims.
A gas turbine is a rotary machine similar in principle to a steam turbine and it consists of three main components: a compressor, a combustion chamber, and a turbine. The air after being compressed in the compressor is heated by burning fuel in it. About two-thirds of the heated air combined with the products of combustion is expanded in a turbine resulting in work output which is used to drive the compressor. The rest (about one-third) is available as useful work output.
Even fluidized metal powders and explosives have seen some use. Engines that use gases for fuel are called gas engines and those that use liquid hydrocarbons are called oil engines, however gasoline engines are also often colloquially referred to as, "gas engines" ("petrol engines" in the UK).
The main limitations on fuels are that it must be easily transportable through the fuel system to the combustion chamber, and that the fuel releases sufficient energy in the form of heat upon combustion to make practical use of the engine.
Diesel engines are generally heavier, noisier, and more powerful at lower speeds than gasoline engines. They are also more fuel-efficient in most circumstances and are used in heavy road vehicles, some automobiles (increasingly so for their increased fuel efficiency over gasoline engines), ships, railway locomotives, and light aircraft. Gasoline engines are used in most other road vehicles including most cars, motorcycles, and mopeds. Note that in Europe, sophisticated diesel-engined cars have taken over about 40% of the market since the 1990s. There are also engines that run on hydrogen, methanol, ethanol, liquefied petroleum gas (LPG), and biodiesel. Paraffin and tractor vaporizing oil (TVO) engines are no longer seen.
At present, hydrogen is mostly used as fuel for rocket engines. In the future, hydrogen might replace more conventional fuels in traditional internal combustion engines. If hydrogen fuel cell technology becomes widespread, then the use of internal combustion engines may be phased out.
Although there are multiple ways of producing free hydrogen, those methods require converting combustible molecules into hydrogen or consuming electric energy. Unless that electricity is produced from a renewable source—and is not required for other purposes—it seems hydrogen does not solve any energy crisis. The disadvantage of hydrogen in many situations is its storage. Liquid hydrogen has extremely low density (14 times lower than water) and requires extensive insulation—whilst gaseous hydrogen requires heavy tankage. Even when liquefied, hydrogen has a higher specific energy but the volumetric energetic storage is still roughly five times lower than petrol. The 'Hydrogen on Demand' process (see direct borohydride fuel cell) creates hydrogen as it is needed, but has other issues such as the high price of the sodium borohydride which is the raw material.
Multiple crankshaft configurations do not necessarily need a cylinder head at all because it can instead have a piston at each end of the cylinder called an opposed piston design. This design was used in the Junkers Jumo 205 diesel aircraft engine, using at either end of a single bank of cylinders with two crankshafts, and most remarkably in the Napier Deltic diesel engines. These used three crankshafts to serve three banks of double-ended cylinders arranged in an equilateral triangle with the crankshafts at the corners. It was also used in single-bank locomotive engines, and continues to be used for marine engines, both for propulsion and for auxiliary generators. The Gnome Rotary engine, used in several early aircraft, had a stationary crankshaft and a bank of radially arranged cylinders rotating around it.
The downside to having more pistons is that the engine will tend to weigh more and generate more internal friction as the greater number of pistons rub against the inside of their cylinders. This tends to decrease fuel efficiency and robs the engine of some of its power. For high-performance gasoline engines using current materials and technology—such as the engines found in modern automobiles, there seems to be a break-point around 10 or 12 cylinders after which the addition of cylinders becomes an overall detriment to performance and efficiency. Although, exceptions such as the W16 engine from Volkswagen exist.
So at least in gasoline-burning engines, ignition timing is largely a compromise between an earlier "advanced" spark—which gives greater efficiency with high octane fuel—and a later "retarded" spark that avoids detonation with the fuel used. For this reason, high-performance diesel automobile proponents such as, Gale Banks, believe that
Fuels burn faster and more completely when they have lots of surface area in contact with oxygen. In order for an engine to work efficiently the fuel must be vaporized into the incoming air in what is commonly referred to as a fuel-air mixture. There are two commonly used methods of vaporizing fuel into the air: one is the carburetor: the other is fuel injection.
Autogas (LPG) engines use either fuel injection systems or open- or closed-loop carburetors.
Turbochargers are another type of forced induction system which has its compressor powered by a gas turbine running off the exhaust gases from the engine.
Duct jet engines use the same basic system, but eschew the piston engine, and replace it with a burner instead.
With a few exceptions, all internal combustion engines employ valves to control the admittance of fuel and air into the combustion chamber.
For jet propulsion internal combustion engines, the 'exhaust system' takes the form of a high velocity nozzle, which generates thrust for the engine and forms a colimated jet of gas that gives the engine its name.
Cooling systems usually employ air or liquid cooling while some very hot engines using radiative cooling (especially some Rocket engines). Some high altitude rocket engines use ablative cooling where the walls gradually erode in a controlled fashion.
Very many reciprocating internal combustion engines end up turning a shaft. This means that the linear motion of a piston must be turned into a rotation. This is typically achieved by a crankshaft.
Several different types of lubrication systems are used. Simple two-stroke engines are lubricated by oil mixed into the fuel or injected into the induction stream as a spray. Early slow-speed stationary and marine engines were lubricated by gravity from small chambers similar to those used on steam engines at the time—with an engine tender refilling these as needed. As engines were adapted for automotive and aircraft use, the need for a high power-to-weight ratio led to increased speeds, higher temperatures, and greater pressure on bearings which in turn required pressure-lubrication for crank bearings and connecting-rod journals. This was provided either by a direct lubrication from a pump, or indirectly by a jet of oil directed at pickup cups on the connecting rod ends which had the advantage of providing higher pressures as the engine speed increased.
Very many systems today are digital control systems, and are frequently termed FADEC (Full Authority Digital Electronic Control) systems.