Four-stroke cycle (or Otto cycle)
1. intake
2. compression
3. power
4. exhaust

The internal combustion engine is a heat engine in which the burning of a fuel occurs in a confined space called a combustion chamber. This exothermic reaction of a fuel with an oxidizer creates gases of high temperature and pressure, which are permitted to expand. The defining feature of an internal combustion engine is that useful work is performed by the expanding hot gases acting directly to cause movement, for example by acting on pistons, rotors, or even by pressing on and moving the entire engine itself.

This contrasts with external combustion engines such as steam engines which use the combustion process to heat a separate working fluid, typically water or steam, which then in turn does work, for example by pressing on a steam actuated piston.

The term Internal Combustion Engine (ICE) is almost always used to refer specifically to reciprocating engines, Wankel engines and similar designs in which combustion is intermittent. However, continuous combustion engines, such as Jet engines, most rockets and many gas turbines are also very definitely internal combustion engines.

A colorized automobile engine


The first internal combustion engines did not have compression, but ran on what air/fuel mixture could be sucked or blown in during the first part of the intake stroke. The most significant distinction between modern internal combustion engines and the early designs is the use of compression and in particular of in-cylinder compression.

  • 1509: Leonardo da Vinci described a compression-less engine. (His description may not imply that the idea was original with him or that it was actually built.)
  • 1673: Christiaan Huygens described a compression-less engine.
  • 1780's: Alessandro Volta built a toy electric pistol ([1]) in which an electric spark exploded a mixture of air and hydrogen, firing a cork from the end of the gun.

Early internal-combustion engines were used to power farm equipment similar to these models.

  • 17th century: English inventor Sir Samuel Morland used gunpowder to drive water pumps.
  • 1794:Robert Street built a compression-less engine whose principle of operation would dominate for nearly a century.
  • 1823: Samuel Brown patented the first internal combustion engine to be applied industrially. It was compression-less and based on what Hardenberg calls the "Leonardo cycle," which, as this name implies, was already out of date at that time. Just as today, early major funding, in an area where standards had not yet been established, went to the best showmen sooner than to the best workers.
  • 1824: Sadi Carnot established the thermodynamic theory of idealized heat engines in France in 1824. This scientifically established the need for compression to increase the difference between the upper and lower working temperatures, but it is not clear that engine designers were aware of this before compression was already commonly used. It may have misled designers who tried to emulate the Carnot cycle in ways that were not useful.
  • 1826 April 1: The American Samuel Morey received a patent for a compression-less "Gas Or Vapor Engine".
  • 1838: a patent was granted to William Barnet (English). This was the first recorded suggestion of in-cylinder compression. He apparently did not realize its advantages, but his cycle would have been a great advance if developed enough.
  • 1854: The Italians Eugenio Barsanti and Felice Matteucci patented the first working efficient internal combustion engine in London (pt. Num. 1072) but did not get into production with it. It was similar in concept to the successful Otto Langen indirect engine, but not so well worked out in detail.
  • 1860: Jean Joseph Etienne Lenoir (1822 - 1900) produced a gas-fired internal combustion engine closely similar in appearance to a horizontal double-acting steam beam engine, with cylinders, pistons, connecting rods, and flywheel in which the gas essentially took the place of the steam. This was the first internal combustion engine to be produced in numbers. His first engine with compression shocked itself apart.
  • 1862:Nikolaus Otto designed an indirect-acting free-piston compression-less engine whose greater efficiency won the support of Langen and then most of the market, which at that time, was mostly for small stationary engines fueled by lighting gas.
  • 1870: In Vienna Siegfried Marcus put the first mobile gasoline engine on a handcart.
  • 1876: Nikolaus Otto working with Gottlieb Daimler and Wilhelm Maybach developed a practical four-stroke cycle (Otto cycle) engine. The German courts, however, did not hold his patent to cover all in-cylinder compression engines or even the four stroke cycle, and after this decision in-cylinder compression became universal.

Karl Benz

  • 1879: Karl Benz, working independently, was granted a patent for his internal combustion engine, a reliable two-stroke gas engine, based on Nikolaus Otto's design of the four-stroke engine. Later Benz designed and built his own four-stroke engine that was used in his automobiles, which became the first automobiles in production.
  • 1892: Rudolf Diesel invented the diesel engine.
  • 1893 February 23: Rudolf Diesel received the patent for the diesel engine.
  • 1896: Karl Benz invented the boxer engine, also known as the horizontally opposed engine, in which the corresponding pistons reach top dead centre at the same time, thus balancing each other in momentum.
  • 1900: Rudolf Diesel demonstrated the diesel engine in the 1900 Exposition Universelle (World's Fair) using peanut oil (see biodiesel).


Internal combustion engines are most commonly used for mobile propulsion systems. In mobile scenarios internal combustion is advantageous, since it can provide high power to weight ratios together with excellent fuel energy-density. These engines have appeared in almost all automobiles, motorbikes, many boats, and in a wide variety of aircraft and locomotives. Where very high power is required, such as jet aircraft, helicopters and large ships, they appear mostly in the form of gas turbines. They are also used for electric generators and by industry.

Internal combustion mechanics

The potato cannon uses basic principles behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel a potato 500 feet. In this case, the energy is translated into potato motion. You can also use it for more interesting purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and if you can harness that energy in a useful way, what you have is the core of a car engine!

Almost all cars currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are illustrated in Figure 1. They are:

  1. Intake stroke
  2. Compression stroke
  3. Combustion stroke
  4. Exhaust stroke


All internal combustion engines depend on the exothermic chemical process of combustion: the reaction of a fuel, typically with air, although other oxidisers such as nitrous oxide may be employed. Also see stoichiometry.

The most common fuels in use today are made up of hydrocarbons and are derived from petroleum. These include the fuels known as diesel, gasoline and liquified petroleum gas. Most internal combustion engines designed for gasoline can run on natural gas or liquified petroleum gases without modifications except for the fuel delivery components. Liquid and gaseous biofuels, such as Ethanol can also be used. Some can run on Hydrogen, however this can be dangerous. Hydrogen burns with a colorless flame, and modifications to the cylinder block, cylinder head, and head gasket are required to seal in the flame front.

All internal combustion engines must have a means of ignition to promote combustion. Most engines use either an electrical or a compression heating ignition system. Electrical ignition systems generally rely on 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 can be recharged during operation using an alternator driven by the engine. Compression heating ignition systems, such as diesel engines and HCCI engines, rely on the heat created in the air by compression in the engine's cylinders to ignite the fuel.

Once successfully ignited and burnt, the combustion products, hot gases, have more available energy than the original compressed fuel/air mixture (which had higher chemical energy). The available energy is manifested as high temperature and pressure which can be translated into work by the engine. In a reciprocating engine, the high pressure product 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 - TDC). The piston can then proceed to the next phase of its cycle, which varies between engines. Any heat not translated into work is a waste product and is removed from the engine either by an air or liquid cooling system.


An illustration of several key components in a typical four-stroke engine

The parts of an engine vary depending on the engine's type. For a four-stroke engine, key parts of the engine include the crankshaft (purple), 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), a piston (yellow) and a crank (purple). A single sweep of the cylinder by the piston in an upward or downward motion is known as a stroke and the downward stroke that occurs directly after the air-fuel mix in the cylinder is ignited is known as a power stroke.

A Wankel engine has a triangular rotor that orbits in an epitroichoidal (figure 8 shape) chamber around an eccentric shaft. The four phases of operation (intake, compression, power, exhaust) take place in separate locations, instead of one single location as in a reciprocating engine.

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. Intake, compression, power, and exhaust all occur in each stroke of this yoke. To tell you the truth i have no idea of what im doing or saying.


There is a wide range of internal combustion engines corresponding to their many varied applications. Likewise there is a wide range of ways to classify internal-combustion engines, some of which are listed below.

Although the terms sometimes cause confusion, there is no real difference between an "engine" and a "motor." At one time, the word "engine" (from Latin, via Old French, ingenium, "ability") meant any piece of machinery. A "motor" (from Latin motor, "mover") is any machine that produces mechanical power. Traditionally, electric motors are not referred to as "engines," but combustion engines are often referred to as "motors." (An electric engine refers to locomotive operated by electricity).

Principles of operation

A 1906 gasoline engine



Continuous combustion:

  • Gas turbine
  • Jet engine
  • Rocket engine

Engine cycle


Engines based on the two-stroke cycle use two strokes (one up, one down) for every power stroke. Since there are no dedicated intake or exhaust strokes, alternative methods must be used to scavenge the cylinders. The most common method in spark-ignition two-strokes is to use the downward motion of the piston to pressurize fresh charge in the crankcase, which is then blown through the cylinder through ports in the cylinder walls. Spark-ignition two-strokes are small and light (for their power output), and mechanically very simple. Common applications include snowmobiles, lawnmowers, chain saws, jet skis, mopeds, outboard motors and some motorcycles. Unfortunately, they are also generally louder, less efficient, and far more polluting than their four-stroke counterparts, and they do not scale well to larger sizes. Interestingly, the largest compression-ignition engines are two-strokes, and are used in some locomotives and large ships. These engines use forced induction to scavenge the cylinders.


Engines based on the four-stroke cycle or Otto cycle have one power stroke for every four strokes (up-down-up-down) and are used in cars, larger boats and many light aircraft. They are generally quieter, more efficient and larger than their two-stroke counterparts. There are a number of variations of these cycles, most notably the Atkinson and Miller cycles. Most truck and automotive Diesel engines use a four-stroke cycle, but with a compression heating ignition system it is possible to talk separately about a diesel cycle.

Bourke Engine

In this engine, two diametrically opposed cylinders are linked to the crank by the crank pin that goes through the common scottish yoke. The cylinders and pistons are so constructed that there are, as in the usual two stroke cycle, two power strokes per revolution. However, unlike the common two stroke engine, the burnt gases and the incoming fresh air do not mix in the cylinders, contributing to a cleaner, more efficient operation. The scotch yoke mechanism also eliminates side thrust and thus greatly reduces friction between pistons and cylinder walls.

Controlled Combustion Engine

These are also cylinder based engines may be either single or two stroke but use, instead of a crankshaft and piston rods, two gear connected, counter rotating concentric cams to convert reciprocating motion into rotary movement. These cams practically cancel out sideward forces that would otherwise be exerted on the cylinders by the pistons, greatly improving mechanical efficiency. The profiles of the cam lobes(which are always odd and at least three in number) determine the piston travel versus the torque delivered. In this engine, there are two cylinders that are 180 degrees apart for each pair of counter rotating cams. For single stroke versions, there are the same number of cycles per cylinder pair as there are lobes on each cam, twice as much for two stroke units.


The Wankel engine operates with the same separation of phases as the four-stroke engine (but with no piston strokes, would more properly be called a four-phase engine), since the phases occur in separate locations in the engine; however like a two-stroke piston engine, it provides one power 'stroke' per revolution per rotor, giving it similar space and weight efficiency. The Bourke cycle's combustion phase more closely approximates constant volume combustion than either four stroke or two stroke cycles do. It also uses less moving parts, hence needs to overcome less friction than the other two reciprocating types have to. In addition, its greater expansion ratio also means more of the heat from its combustion phase is utilized than is used by either four stroke or two stroke cycles.


A new invention by Carmelo Scuderi, the Scuderi Split Cycle Engine claims to improve the efficiency of an engine from 33.2% to 42.6%. In addition, toxic emissions are claimed to be reduced by as much as 80%.

Disused methods

In some old non-compressing internal combustion engines: In the first part of the piston downstroke a fuel/air mixture was sucked or blown in. In the rest of the piston downstroke the inlet valve closed and the fuel/air mixture fired. In the piston upstroke the exhaust valve was open. This was an attempt at imitating the way a piston steam engine works.

Fuel and oxidizer types

Fuels used include gasoline (British term: petrol), liquified petroleum gas, vapourized petroleum gas, compressed natural gas, hydrogen, diesel fuel, JP18 (jet fuel), landfill gas, biodiesel, biobutanol, peanut oil and other vegoils, bioethanol, biomethanol (methyl or wood alcohol) and other biofuels. Even fluidised 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 unfortunately also often colloquially referred to as 'gas engines'.

The main limitations on fuels are that the fuel must be easily transportable through the fuel system to the combustion chamber, and that the fuel release sufficient energy in the form of heat upon combustion to make use of the engine practical.

The oxidiser is typically air, and has the advantage of not being stored within the vehicle, increasing the power-to-weight ratio. Air can, however, be compressed and carried aboard a vehicle. Some submarines are designed to carry pure oxygen or hydrogen peroxide to make them air-independent. Some race cars carry nitrous oxide as oxidizer. Other chemicals such as chlorine or fluorine have seen experimental use; but mostly are impractical.

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 more 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 become quite prevalent since the 1990s, representing around 40% of the market. Both gasoline and diesel engines produce significant emissions. There are also engines that run on hydrogen, methanol, ethanol, liquefied petroleum gas (LPG) and biodiesel. Paraffin and tractor vaporising oil (TVO) engines are no longer seen.

Some have theorized that in the future hydrogen might replace such fuels. Furthermore, with the introduction of hydrogen fuel cell technology, the use of internal combustion engines may be phased out. The advantage of hydrogen is that its combustion produces only water. This is unlike the combustion of hydrocarbons, which also produces carbon dioxide, a major cause of global warming, as well as carbon monoxide, resulting from incomplete combustion. The big 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 very heavy tankage. Although hydrogen has a higher specific energy, the volumetric energetic storage is still roughly five times lower than petrol, even when liquified. (The 'Hydrogen on Demand' process, designed by Steven Amendola, creates hydrogen as it is needed, but this has other issues, such as the raw materials being relatively expensive.)

One-cylinder gasoline engine (c. 1910).


Internal combustion engines can contain any number of cylinders with numbers between one and twelve being common, though as many as 36 (Lycoming R-7755) have been used. Having more cylinders in an engine yields two potential benefits: First. the engine can have a larger displacement with smaller individual reciprocating masses (that is, the mass of each piston can be less) thus making a smoother running engine (since the engine tends to vibrate as a result of the pistons moving up and down). Second, with a greater displacement and more pistons, more fuel can be combusted and there can be more combustion events (that is, more power strokes) in a given period of time, meaning that such an engine can generate more torque than a similar engine with fewer cylinders. The down side to having more pistons is that, over all, the engine will tend to weigh more and tend to generate more internal friction as the greater number of pistons rub against the inside of their cylinders. This tends to decrease fuel efficiency and rob 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 addition of cylinders becomes an overall detriment to performance and efficiency, although exceptions such as the W16 engine from Volkswagen exist.

  • Most car engines have four to eight cylinders, with some high performance cars having ten, twelve, or even sixteen, and some very small cars and trucks having two or three. In previous years some quite large cars, such as the DKW and Saab 92, had two cylinder, two stroke engines.
  • Radial aircraft engines, now obsolete, had from three to 28 cylinders, such as the Pratt & Whitney R-4360. A row contains an odd number of cylinders, so an even number indicates a two- or four-row engine. The largest of these was the Lycoming R-7755 with 36 cylinders (four rows of nine cylinders) but never entered production.
  • Motor cycles commonly have from one to four cylinders, with a few high performance models having six (though some 'novelties' exist with 8, 10 and 12).
  • Snowmobiles usually have two cylinders. Some larger (not necessarily high-performance, but also touring machines) have four.
  • Small portable appliances such as chainsaws, generators and domestic lawn mowers most commonly have one cylinder, although two-cylinder chainsaws exist.

Ignition system

Internal combustion engines can be classified by their ignition system. The point in the cycle at which the fuel/oxidiser mixture are ignited has a direct effect on the efficiency and output of the ICE. For a typical 4 stroke automobile engine, the burning mixture has to reach its maximum pressure when the crankshaft is 90 degrees after TDC. The speed of the flame front is directly affected by compression ratio, fuel mixture temperature and octane or cetane rating of the fuel. Modern ignition systems are designed to ignite the mixture at the right time to ensure the flame front doesn't contact the decending piston crown. If the flame front contacts the piston, pinking or knocking results. Leaner mixtures and lower mixture pressures burn more slowly requiring more advanced ignition timing. Today most engines use an electrical or compression heating system for ignition. However outside flame and hot-tube systems have been used historically. Nikola Tesla gained one of the first patents on the mechanical ignition system with Template:US patent, "Electrical Igniter for Gas Engines", on 16 August 1898.

Fuel systems

Main article: Fuel injection

Often for simpler reciprocating engines a carburetor is used to supply fuel into the cylinder. However, exact control of the correct amount of fuel supplied to the engine is impossible.

Larger gasoline engines such as used in cars have mostly moved to fuel injection systems (see Gasoline Direct Injection). Diesel engines always use fuel injection.

LPG engines use a mix of fuel injection systems and closed loop carburetors.

Other internal combustion engines like jet engines use burners, and rocket engines use various different ideas including impinging jets, gas/liquid shear, preburners and many other ideas.

Engine configuration

Internal combustion engines can be classified by their configuration which affects their physical size and smoothness (with smoother engines producing less vibration). Common configurations include the straight or inline configuration, the more compact V configuration and the wider but smoother flat or boxer configuration. Aircraft engines can also adopt a radial configuration which allows more effective cooling. More unusual configurations, such as "H", "U", "X", or "W" have also been used.

Multiple-crankshaft configurations do not necessarily need a cylinder head at all, but 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 two crankshafts, one at either end of a single bank of cylinders, and most remarkably in the Napier Deltic diesel engines, which 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.

Engine capacity

An engine's capacity is the displacement or swept volume by the pistons of the engine. It is generally measured in litres or cubic inches for larger engines and cubic centimetres (abbreviated to cc's) for smaller engines. Engines with greater capacities are usually more powerful and provide greater torque at lower rpms but also consume more fuel.

Apart from designing an engine with more cylinders, there are two ways to increase an engine's capacity. The first is to lengthen the stroke and the second is to increase the piston's diameter (See also: Stroke ratio). In either case, it may be necessary to make further adjustments to the fuel intake of the engine to ensure optimal performance.

An engine's quoted capacity can be more a matter of marketing than of engineering. The Morris Minor 1000, the Morris 1100, and the Austin-Healey Sprite Mark II all had engines of the same stroke and bore according to their specifications, and were from the same maker. However the engine capacities were quoted as 1000cc, 1100cc and 1098cc respectively in the sales literature and on the vehicle badges.

Engine pollution

Generally internal combustion engines, particularly reciprocating internal combustion engines, produce moderately high pollution levels, due to incomplete combustion of carbonaceous fuel, leading to carbon monoxide and some soot along with oxides of nitrogen & sulfur and some unburnt hydrocarbons depending on the operating conditions and the fuel/air ratio. The primary causes of this are the need to operate near the stoichiometric ratio for petrol engines in order to achieve combustion (the fuel would burn more completely in excess air) and the "quench" of the flame by the relatively cool cylinder walls.

Diesel engines produce a wide range of pollutants including aerosols of many small particles (PM10) that are believed to penetrate deeply into human lungs. Engines running on liquified petroleum gas (LPG) are very low in emissions as LPG burns very clean and complete and does not contain sulphur or lead.

  • Many fuels contain sulfur leading to sulfur oxides (SOx) in the exhaust, promoting acid rain.
  • The high temperature of combustion creates greater proportions of nitrogen oxides (NOx), demonstrated to be hazardous to both plant and animal health.
  • Net carbon dioxide production is not a necessary feature of engines, but since most engines are run from fossil fuels this usually occurs. If engines are run from biomass, then no net carbon dioxide is produced as the growing plants absorb as much, or more carbon dioxide while growing.
  • Hydrogen engines need only produce water, but when air is used as the oxidizer nitrogen oxides are also produced.

See also

  • William Barnett - an early patentee (1838)


  • Singer, Charles Joseph; Raper, Richard, A history of technology : The Internal Combustion Engine, edited by Charles Singer ... [et al.], Clarendon Press, 1954-1978. pp.157-176[2]
  • Hardenberg, Horst O., The Middle Ages of the Internal combustion Engine, Society of Automotive Engineers (SAE), 1999

External links