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.
The de Havilland Gypsy Queen engine, powering Dove and Heron propeller aircraft
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, Wilhem 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.
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, camshaft 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.
All internal-combustion engines depend on the chemical process of combustion, that is the reaction of a fuel with oxygen.
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 this is that the combustion of hydrogen produces only water. This is unlike the combustion of hydrocarbons which also produce carbon dioxide - a major cause of global warming.
Whatever the choice of fuel, all internal-combustion engines rely on the heat produced when such fuels participate in combustion. The more heat produced, the better. This is because the heat causes a rapid rise in temperature. As predicted by the universal gas law, this rise in temperature then causes a rise in pressure. It is this pressure that 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. The piston can then proceed to the next phase of its cycle (which varies between engines).
All internal-combustion engines must have a means of ignition to promote combustion. Most engines use either an electrical or a compression heating system for ignition. Electrical ignition systems generally rely on a battery to provide an electrical spark to ignite the air-fuel mix in the engine's cylinders. This battery can be recharged by the engine during operation. 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.
Engines based around the two-stroke cycle produce two strokes for every power stroke and are used in lawnmowers, mopeds, outboard motors and most 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.
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. Most cars contain four to eight cylinders with some high performance cars containing ten or twelve.
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.
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. Aircrafts can also adopt a radial configuration which allows more effective cooling.
An engine's capacity is the volume swept 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.
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.
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.
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 kilowatts, 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, vibrartional requirements and fuel efficiency.
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.
History
Applications
Parts
Operation
Classification
Engine cycle
Fuel type
Cylinders
Ignition system
Engine configuration
Engine capacity
Other classifications
Performance
