Internal combustion engine
An internal combustion engine is an engine that is powered by the expansion of hot combustion products of fuel directly acting within an engine.
By way of contrast, an external combustion engine such as a steam engine, does work by the combustion process heating a separate working fluid, such as water/steam, which then in turn does work.
Jet engines, rockets and gas turbines are classed as internal combustion engines, but the term 'internal combustion engine' is often loosely and incorrectly used to refer to piston internal combustion engine in which combustion is intermittent and the products act on reciprocating machinery- the most common subtype of this kind of engine.
| Contents |
History
The earliest and simplest internal combustion engines seem to have been rockets, fireworks, developed by the Chinese as early as B.C. 300, using gunpowder, and were essentially toys, until the 11th century when they evolved into use for weaponry.
Francois Issac de Rivaz built the first reciprocating 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.
The first patent for an internal combustion engine was awarded by the U.S. Patent Office in 1826 to Samuel Morey. This was one of the X-Patents, lost in a fire in July 1836 and not recovered until 2004.
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.
In 1930 Sir Frank Whittle patented the use of a gas turbine for jet propulsion. The first successful use of this engine was in April, 1937.
1939 Ernst Heinkel Aircraft flew the first flight of a gas turbine jet, the HE178.
Applications
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, mostly in the form of gas turbines in jet aircrafts and large ships. They are 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 high cost and weight of batteries and the lack of affordable onboard electric generators restrict their use.
Parts
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 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.
Operation
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. 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 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.
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.
Classification
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 (LPG) and biodiesel.
Cylinders
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.
- Most car engines have four to eight cylinders, with some high performance cars having ten or twelve, 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 five to twenty-eight cylinders. A row contains an odd number of cylinders, so an even number indicates a two- or four-row engine.
- Motor cycles commonly have from one to four cylinders, with a few high performance models having six.
- Small appliances such as chainsaws 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. 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. 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. 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.
Performance
The chief measure of a reciprocating 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 ω is the engine's speed in radians per second and Ω is the engine's speed in revolutions per minute
The engine's torque and speed are then related by the equation:
where T is the engine's torque in Newton metres, I is the rotational inertia attached to the engine and ω 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:
P = Tω
where P is the engine's power in watts, T is the engine's torque in Newton metres and ω 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 effectively 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. 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. A broad, flat torque curve is ideal. 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 cylinder is largely responsible for the overall shape of the power curve.
cs:Spalovacà motor de:Verbrennungsmotor eo:Eksplodmotoro es:motor de combustión interna fr:Moteur à combustion interne ja:内燃機関 nl:Verbrandingsmotor pl:Silnik o spalaniu wewnÄ™trznym ru:Двигатель внутреннего сгорания zh:内燃机
Categories: Piston engines