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Internal combustion engine
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Internal combustion engine

An internal combustion engine is any engine that operates by burning its fuel inside the engine. This can be contrasted with external combustion engines such as steam engines and Stirling engines, which burn their fuel outside the engine. Jet engines and gas turbines use internal combustion, but the term 'internal combustion engine' normally refers to engines in which combustion is intermittent and there exists reciprocating machinery.

Table of contents
1 History
2 Applications
3 Parts
4 Operation
5 Classification
6 Performance


Francois Issac de Rivaz built the first internal combustion engine in 1807. However his engine was impractical for many uses because it lacked power and relied upon a mixture of hydrogen and oxygen for fuel.

In 1858, Jean Lenoir invented the first practical internal combustion engine. It relied upon coal gas that was sucked into the cylinder at the beginning of each stroke and then ignited to push the piston to the other end of the cylinder. This process was then repeated at the other end of the cylinder making the engine double-acting.

In 1867, Nikolaus Otto built the first four-stroke internal combustion engine. This engine proved more efficient than Lenoir's design and was successfully marketed for industrial purposes. The design was later improved by Gottlieb Daimler who focused on making the technology practical for use in automobiles most notably by incorporating a gasoline carburettor. In 1890, Wilhelm Maybach built the first four-cylinder internal-combustion engine. Both Maybach and Daimler were originally employees of Otto's company but left in 1882 to form their own company.

Over the same time period the two-stroke internal combustion engine was being perfected. In 1867, Sir Dougald Clerk invented the first two-stroke internal combustion engine. The design was later simplified by Joseph Day in 1891.


Internal combustion engines are most commonly used for mobile propulsion systems. They appear in most cars, motorbikes and boats and in a wide variety of aircraft and locomotives though are displaced by jet engines in jet aircrafts. They may also be used by industry.

For many non-mobile applications, an electric motor is a competitive alternative. In the future electric motors may also become competitive for most mobile applications. However, at the moment the cost of batteries and the lack of affordable onboard electric generatorss restrict their use.


(See picture below.) 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, one or more camshafts (magenta and blue) and valvess. 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.


All internal combustion engines depend on the chemical process of combustion and explosion, that is the reaction of a fuel with oxygen. See also: 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. Some have theorized that in the future hydrogen might replace such fuels. The advantage of hydrogen is that its combustion produces only water (the chief disadvantage of using hydrogen is that presently no method exists for efficiently producing it in quantities sufficient to power internal combustion engines on a large scale). This is unlike the combustion of hydrocarbons which also produces carbon dioxide - a major cause of global warming.

Whatever the choice of fuel, all internal combustion engines rely on the effects of a controlled explosion, where a gaseous fuel reacts with oxygen. The explosion products (hot gases) occupy a larger volume than the original compressed fuel/air mixture and the resulting high pressure in the cylinders drives the engine's pistons down. Relieving the pressure after this has occurred (either by opening a valve or exposing the exhaust outlet) allows the piston to return to its uppermost position (Top Dead Center - TDC). The piston can then proceed to the next phase of its cycle (which varies between engines). The resulting heat from the explosion is a waste product and is removed from the engine either by an air or liquid cooling system. (An ideal, 100% efficient engine would run and remain at ambient temperature and convert all the energy in the fuel to kinetic energy - not into heat).

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 rely on the heat already present in the compressed air in the engine's cylinders to ignite the fuel when it is injected.


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

Engine cycle

Engines based around the two-stroke cycle produce two strokes for every power stroke and are used in lawnmowers, mopeds, outboard motors and some motorcycles. They are generally louder, less efficient and smaller than their four-stroke counterparts. Engines based around the four-stroke cycle or Otto cycle have one power stroke for every four strokes and are used in cars, larger boats and larger aircrafts. 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. Diesel engines are often considered to be based around the four-stroke cycle but with a compression heating ignition system however it is possible to talk separately about a diesel cycle.

Fuel type

Diesel engines are generally heavier, noisier and more powerful at lower speeds than gasoline engines. They are also more fuel-efficient in some circumstances and are used in heavy road-vehicles, ships and some locomotives. Gasoline engines are used in most other road-vehicles including most cars, motorcycles and mopeds. Both gasoline and diesel engines produce significant emissions. There are also engines that run on hydrogen, liquefied petroleum gas and biodiesel.


Internal combustion engines can contain any number of cylinders with numbers between one and twenty being common. More cylinders result in greater torque but obviously larger engines and greater fuel consumption.

Ignition system

Internal combustion engines can be classified by their ignition system. Today most engines use an electrical or compression heating system for ignition. However outside flame and hot-tube systems have been used in the past.

Engine configuration

Internal combustion engines can be classified by their configuration which affects their physical size and smoothness (with smoother engines producing less vibrations). 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", "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.

Engine capacity

An engine's capacity is the displacement or swept volume by the pistons of the engine. It is generally measured in litres 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 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. 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.

Other classifications

All internal combustion engines are heat engines and thus have a physical upper bound on their efficiency achieved only by the theoretical Carnot heat engine. A few internal-combustion engines that use rotary instead of linear piston motion are known as Wankel engines, Orbital engines or Quasiturbines.


The chief measure of an internal combustion engine is the turning force or torque it provides for any given speed. Here the speed of an engine means the number of revolutions per minute the engine makes. The SI unit for angular velocity is radians per second and a conversion from revolutions per minute to radians per second is found using the formula:

where is the engine's speed in radians per second and is the engine's speed in revolutions per minute

The engine's torque and speed are then related by the equation:

where is the engine's torque in Newton metres, is the rotational inertia attached to the engine and is the speed of the engine in radians per second

If a solid cylinder were attached to the engine, the rotational inertia of that cylinder would simply be a function of its radius and mass. Using this fact and the above equation, it is possible to build a device to measure an engine's torque - such a device is called a dynamometer.

After measuring an engine's torque, the engine's power can be found using the equation:

where is the engine's power in watts, is the engine's torque in Newton metres and is the speed of the engine in radians per second.

Using a simple conversion this value for power may also expressed in horsepower.

Power is useful from an engineering perspective in that it provides the rate of mechanical work possible however motoring enthusiasts will tell you that torque is what the driver "feels." This is because under identical load conditions the torque is proportional to acceleration. It is possible to increase the performance of an engine through engine tuning and although engine performance is important in most systems engineers must balance it with economic considerations, physical weight limitations, vibrational requirements and fuel efficiency.

It is also important to note that the torque of an engine can be increased by sacrificing speed through the use of gears however in such situations the power output ideally remains the same (in reality some power is lost due to friction between the gears).

Some indication of an engine's performance is given by graphing the engine's torque against its speed - this is known as a dyno graph. A sample dyno graph for the 2.7 litre 6-cylinder engine in the April 2004 Porsche Boxster is included below. Engines of lesser quality would typically have the torque peak at a lower value and a lower engine speed, though it should be noted that an engine designer can place the peak of the torque curve more or less anywhere he chooses, depending on the engine's application. Most modern car engines are designed to give peak torque as high in the rev range as possible, with a generally linear increase of torque with speed. Other applications such as engines to drive aircraft, boats, pumps, generators, etc often have maximum torque at a much lower speed, or a sharper curve so that power is maximised over a narrower range of operating speeds. The profile of the camshaft is largely responsible for the overall shape of the power curve.