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Unlike an automobile engine, aircraft run at high power settings for very long times. In general the engine is run at maximum power for a few minutes while taking off, then at a slightly reduced power for climb, and then spends the vast majority of its time at a cruise setting, typically 65% to 75% of full power. In contrast a car engine might spend 20% of its time at 65% power while accelerating away from a red light, followed by 80% of its time at 20% power while cruising.

If a car engine fails, you simply pull over to the side of the road. If the same occurs in an single engine aircraft, it will glide but, depending on the terrain, may crash at landing. Aircraft engines tend to spend more effort on reliability than performance for this reason, and even then it was many years before they had the reliability needed to fly over the Atlantic for instance.

Long duration at high power, combined with the requirement for high reliability, means the engine must have high engine displacement and be built very tough. Aircraft engines tend to be somewhere between 50% and 100% larger than a similar powered car engine for a given power output. They also tend to use the most simple parts and include two sets of anything they can afford to carry, including ignition system ( spark plugs, magnetos), fuel pumps and electrical systems. Independence of function lessens the likelihood of a catastrophic failure. Thus magnetos are used because they do not rely on a battery. This concept is used throughout aircraft design. Redundancy in the ignition system allows the pilot to switch off a faulty magneto and thus continue the flight.

In the aircraft application the engine needs to be lifted, powering itself into the air. Light weight for any given power, the so called power to weight ratio is one of the most important features for an aircraft engine. Two smaller engines are more attractive from a redundancy angle than one larger engine. However the aicraft must then be capable of a marginal climb on one engine, carrying its load at take off, in the event of a failure. Two engines with this requirement add to the weight, complexity and cost of the aircraft. By doubling the number of engines, the risk of one failing is doubled. Added to this is the fact that in a twin application the engines are allowed to be stressed at higher levels to achieve the single engine climb requirement. This further increases the risk of a failure. While a twin is usually seen as worthwhile in aircraft used for flight in cloud, many people prefer the savings in cost of a single if they will be flying visually.

Another difference between cars and aircraft is that the aircraft spends the vast majority of its time travelling at high speed. This allows aircraft engines to be air cooled, as opposed to requiring a radiatorA radiator is common term for a heat exchanger of some sort. The term normally applies to one of two uses: #in automobiles with an internal combustion engine: a radiator is connected to channels running through the engine and cylinder head, through which, which can lead to lower weight and complexity.

At one time all engine designs were new, and there was no particular difference in design between aircraft and automobile engines. This changed by the start of WWI however, when a particular class of air-cooled rotary engineAlternative meaning: Wankel engine The rotary engine was a common type of internal combustion aircraft engine in the early years of the 20th century. It was also used in a few motorcycles and cars. In concept, a rotary engine is simple. It is a standard Os became popular. These had a short lifespan, but by the 1920sCenturies: 19th century 20th century 21st century Decades: 1870s 1880s 1890s 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s Years: 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 Events and trends Technology John Logie Baird invents the first working t the vast majority of aircraft engines were moving to the similar radial enginebiplane The radial engine is a configuration of internal combustion engine, commonly used in aircraft, in which the cylinders are arranged pointing out from a central crankshaft like the spokes on a wheel. The cylinders are connected to the crankshaft wit design. This combined air-cooled simplicity with large displacements, and were generally the most powerful small engines in the world.

Both the rotary and radial engine have one drawback however, they both have very large frontal areas (see drag equationIn physics, the drag equation gives the drag experienced by an object moving through a fluid. where :D is the force of drag, :C is the drag coefficient (a dimensionless constant, e. 45 for a car, 1 for a cylinder) :ρ is the density of the fluid , :v i). As planes increased in speed and demanded better streamlining, designers turned to water-cooled inline engines. Throughout WWII the two designs were generally similar in terms of power and overall performace, but the radials tended to be more reliable. After the war the water-cooled designs rapidly disappeared.

For the smaller application, notably in general aviation, a hybrid design in the form an air-cooled inline, almost always 4 or 6 cylinders horizontally opposed, is most common. These combine small frontal area with air-cooled simplicity, although they required careful installation in order to be effective cooled, notably for the rearmost cylinders.

Throughout most of the history of aircraft engine design, the engines tended to be more advanced than their automobile counterparts. High-strength aluminum alloys were used very early in these engines, decades before it became common in cars. Likewise these engines adopted fuel injection vs. carbureted quite early, overhead camshafts, and a host of other features now common in car engines as well.

Today the piston-engine aviation market is so small that there is essentially no money for new design work. Almost every engine flying is based on a design from the 1960s, using original materials, tooling, and parts. Meanwhile the relentless financial power of the automobile industry has continued improvement, a new car design is likely to use an engine designed in the last three years, build of alloys that did not exist more than five years ago. Whereas modern car engines require no maintenance at all (other than adding fuel and oil) for over 100,000 km, aircraft engines are now, paradoxically, rather heavy, dirty and unreliable in comparison.

Over the history of the development of aircraft engines, the Otto cycle, that is, conventional gasoline powered engines, have been by far the most common type. That is not because they are the best, but simply because they were there first.

Another promising design for aircraft use was the Wankel engine. The Wankel engine produces less power for any given size of engine, about 2/3rds that of a conventional design, but does so for about 1/2 the weight and complexity. In this role the power to weight ratio is king, and the Wankel makes particularly good sense. Considerable thinking on such designs started in the post-war era, but at the same time the entire industry felt that jets, often in the form of turboprops, would power everything from the biggest to smallest designs. In the end little work was actually carried out, much to the chagrin of many.

The diesel engine is another engine design that has been examined for aviation use. In general diesel engines are more reliable and much better suited to running for long periods of time at medium power settings - this is why they are widely used in trucks for instance. Several attempts to produce diesel aircraft engines were made in the 1930s, but at the time the alloys were not up to the task of handling the much higher compression ratios used in these designs, and they generally had poor power-to-weight ratios and were uncommon for that reason. However, improvements in diesel technology in automobiles, leading to much better power-weight ratios, the diesel's much better fuel efficiency (particularly compared to the inefficient old designs currently being used in light aircraft), and the high relative taxation of gasoline compared to diesel in Europe, has seen a revival of interest in the concept. As of May 2004, one manufacturer, Centurion, is already selling certified diesel aircraft engines for light aircraft based on an automotive design, and other companies have alternative designs under development. It remains to be seen whether these new designs will succeed in the marketplace, but they represent potentially the biggest change in light aircraft engines in decades.