Aircraft engine
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The term aircraft engine, for the purposes of this article, refers to reciprocating and rotary internal combustion engines used in aircraft. Jet engines and turboprops are the other common aviation powerplants; while operation differs substantially, the basics here apply to all types.
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[edit] Engine design
Engines must be:
- lightweight, as a heavy engine decreases the amount of excess power available.
- small and easily streamlined; large engines with substantial surface area, when installed, create too much drag, wasting fuel and reducing power output.
- powerful, to overcome the weight and drag of the aircraft.
- reliable, as losing power in an airplane is a substantially greater problem than an automobile engine seizing. Aircraft engines operate at temperature, pressure, and speed extremes, and therefore need to operate reliably and safely under all these conditions.
- repairable, to keep the cost of replacement down. Minor repairs are relatively inexpensive.
[edit] Power
Unlike automobile engines, aircraft engines run at high power settings for extended periods of time. In general, the engine runs at maximum power for a few minutes during taking off, then power is slightly reduced for climb, and then spends the 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 accelerating, followed by 80% of its time at 20% power while cruising.
[edit] Reliability
The design of aircraft engines tends to favor reliability over performance. It took many years before the reliability was established to fly over the Atlantic or the Pacific Ocean. Engine failure at all stages in flight is a part of flight lessons for student pilots.[1] Forced landings without power are practiced extensively over rural areas until the new pilot is proficient enough to handle such situation during a solo flight.
Long engine operation times and high power settings, combined with the requirement for high-reliability means that engines must have large engine displacement to minimize over-stressing the engine. The engine, as well as the aircraft, needs to be lifted into the air, meaning it has to overcome lots of weight. The thrust to weight ratio is one of the most important characteristics for an aircraft engine. A typical 250 hp engine weighs just 15% of the total aircraft weight when installed into a 3000 lb (1,400 kg) aircraft.
Aircraft engines also tend to use the simplest parts and include two sets of anything needed for reliability, including ignition system (spark plugs and magnetos) and fuel pumps. Independence of function lessens the likelihood of a single malfunction causing an entire engine to fail. Thus magnetos are used because they do not rely on a battery. Two magnetos were originally installed so a pilot can switch off a faulty magneto and continue the flight on the other—but, later, dual ignition was found to offer some detonation protection too. Similarly, a mechanical engine-driven fuel pump is often backed-up by an electric one.
Two engines are more attractive from a reliability angle than a single one. It is true that by doubling the number of engines, the chance of one failing is at least doubled, but the chance of both engines failing at the same time is quite small. This is why most countries require twin-engined airplanes for commercial passenger transport, with minor exceptions. Many light twin-engined aircraft are designed to be capable of at least a marginal climb on one engine, even carrying the maximum load at take-off. Loss of one engine on a twin-engine aircraft results in a loss of 50% of power available, but 80% of the performance (as measured by climb rate) due to aerodynamic factors.
Another difference between cars and aircraft is that the aircraft spend the vast majority of their time travelling at high speed. This allows aircraft engines to be air cooled, as opposed to requiring a radiator. In the absence of a radiator aircraft engines can boast lower weight and less complexity. The amount of air flow an engine receives is usually carefully designed according to expected speed and altitude of the aircraft in order to maintain the engine at the optimal temperature. Just like overheating, too much cooling can be a bad thing for an engine as well. Some aircraft employ controls that allow a pilot to manually adjust the airflow into the engine compartment.
Aircraft operate at higher altitudes where the air is less dense than at ground level. As engines need oxygen to burn fuel, a forced induction system such as turbocharger or supercharger is especially appropriate for aircraft use. This does bring along the usual drawbacks of additional cost, weight and complexity.
[edit] Size
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 World War I, however, when a particular class of air-cooled rotary engines became popular. These had a short lifespan, but by the 1920s a large number of engine designs were moving to the similar radial engine design. This combined air-cooled simplicity with large displacements and they were among the most powerful small engines in the world.
Both the rotary and radial engine have the drawback of a very large frontal area (see drag equation). As aircraft 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 performance but some mature-design radials tended to be more reliable. After the war, in the USA, the water-cooled designs rapidly disappeared.
[edit] Repairability
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 effectively cooled, notably the rearmost cylinders. To make repairs practical, each cylinder is individually replaceable, as are each of the accessories (pumps, generator and magnetos).
[edit] New Designs
[edit] Economics of new designs
Throughout most of the history of aircraft engine design, they tended to be more advanced than their automobile counterparts. High-strength aluminum alloys were used in these engines decades before they became common in cars. Likewise, those engines adopted fuel injection instead of carburetion quite early. Similarly, overhead cams were introduced, while automobile engines continued to use pushrods.
Today the piston-engine aviation market is so small that there is essentially no commercial money for new design work. Most aviation engines flying are based on a design from the 1960s, or before, using original materials, tooling and parts. Meanwhile the financial power of the automobile industry has continued improvement. A new car design is likely to use an engine designed no more than a few years ago, built with the latest alloys and advanced electronic engine controls. Modern car engines require no maintenance at all (other than adding fuel and oil) for over 100,000 km, aircraft engines are now, in comparison and paradoxically, rather heavy, dirty and unreliable.
Much of the innovation (and most newly constructed planes flying) in the past two decades in private aviation has been in ultralights and homebuilt aircraft, and so has innovation in powerplants. Rotax, amongst others, has introduced a number of new small production engine designs for this type of craft. The smallest of these mostly use two-stroke designs, but the larger models are four-strokes. For the reasons discussed above, some hobbyists and experimenters prefer to adapt automotive engines for their home-built aircraft, instead of using certified aircraft engines.
Over the history of the development of aircraft engines, the Otto cycle, that is, conventional gasoline powered, reciprocating-piston engines have been by far the most common type. That is not because they are the best but simply because they were there first and type-certification of new designs is an expensive, time-consuming process.
[edit] Wankel engine
Another promising design for aircraft use was the Wankel rotary engine. The Wankel engine is about one half the weight and size of a traditional four stroke cycle piston engine of equal power output, and much lower in complexity. In an aircraft application, the power to weight ratio is very important, making the Wankel engine a good choice. Because the engine is typically constructed with an aluminium housing and a steel rotor, and aluminium expands more than steel when heated, unlike a piston engine, a Wankel engine will not seize when overheated. This is an important safety factor for aeronautical use. Considerable development of these designs started after World War II, but at the time the aircraft industry favored the use of turbine engines. It was believed that turbojet or turboprop engines, could power all aircraft, from the largest to smallest designs. The Wankel engine did not find many applications in aircraft, but was used by Mazda in a popular line of sports cars. Recently, the Wankel engine has been developed for use in motor gliders where the small size, light weight, and low vibration are especially important.[2]
Wankel engines are becoming increasingly popular in homebuilt experimental aircraft, due to a number of factors. Most are Mazda 12A and 13B engines, removed from automobiles and converted to aviation use. This is a very cost-effective alternative to certified aircraft engines, providing engines ranging from 100 to 300 horsepower at a fraction of the cost of traditional engines. These conversions first took place in the early 1970s, and with hundreds or even thousands of these engines mounted on aircraft, as of 10 December 2006 the National Transportation Safety Board has only 7 reports of incidents involving aircraft with Mazda engines, and none of these is of a failure due to design or manufacturing flaws. During the same time frame, they have reports of several thousand reports of broken crankshafts and connecting rods, failed pistons and incidents caused by other components which are not found in the Wankel engines. Rotary engine enthusiasts refer to piston aircraft engines as "Reciprosaurs," and point out that their designs are essentially unchanged since the 1930s, with only minor differences in manufacturing processes and variation in engine displacement.
Peter Garrison, Contributing Editor for FLYING Magazine, has said that "the most promising engine for aviation use is the Mazda rotary." Garrison lost an airplane which he had designed and built (and missed death literally by inches), when a piston-powered plane had engine failure and crashed into Garrison's plane, which was waiting to take off.
[edit] Diesel engine
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. They generally had poor power-to-weight ratios and were uncommon for that reason. Improvements in diesel technology in automobiles (leading to much better power-weight ratios), the diesel's much better fuel efficiency (particularly compared to the old designs currently being used in light aircraft) and the high relative taxation of gasoline compared to diesel in Europe have all seen a revival of interest in the concept. As of May 2004 one manufacturer, Thielert Aircraft Engines, is already selling certified diesel aircraft engines for light aircraft, and other companies have alternative designs under development. It remains to be seen whether these new designs will succeed in the marketplace but they potentially represent the biggest change in light aircraft engines in decades.
[edit] References
- ^ Charles Edward Kingsford Smith. All Star Network. Retrieved on 2006-11-01.
- ^ Alexander Schleicher GmbH & Co., ASH 26 E Information. Retrieved on 2006-11-24.
[edit] See also
[edit] External links
- Aircraft Gas Turbine Engines and Spray Technology
- Aircraft Engines and Aircraft Engine Theory (includes links to diagrams)
