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Monday, January 09, 2006

Aircraft Engine.

Tradeoffs

Because of the complexities of flight, aircraft engines need to satisfy several requirements to sustain prolonged flights. These 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.
=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.

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 circumstances, may result in a fatal accident. For this reason the design of aircraft engines tend to favor reliability over performance. Even with this mindset, it took many years before the reliability was established to fly over the Atlantic or the Pacific Ocean [1].

Engine failure at all stages in flight is an important part of flight lessons for student pilots. 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, regardless of the fact that the additional systems decrease the reliability, statistically speaking. Many 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. But by doubling the number of engines, the chance of one failing is at least doubled when statistics are considered. However, the chance of both engines failing at the same time is relatively small. Interestingly enough, if one engine DOES fail, the aircraft loses about 80% of its power, not just 50% (remember, one engine would then have to overcome the weight of the aircraft, the dead engine, and itself!) Frequently, the greater economy and simplicity of a single-engine aircraft is preferred over the extra power and speed, as well as finesse needed, to operate a twin engine, especially for private pilots.

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.

Aircraft operate at higher altitudes where significantly less air (oxygen) is available than at ground level. As engines need oxygen to burn fuel, an induction-assist mechanism—like a turbocharger or supercharger—is especially appropriate for aircraft use. This does bring along the usual drawbacks of additional cost, weight and complexity.


History

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 one drawback however, they both have very large frontal areas (see drag equation). 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 performance but some mature-design radials tended to be more reliable. After the war, in the USA, 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 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).

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. Almost every engine flying is based on a design from the 1960s, or before, 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 and having ignition and other systems features that did not exist then either. 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. Accordingly, 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 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 difficult.

Another promising design for aircraft use was the Wankel engine. The Wankel engine is about 1/2 the weight and size of a traditional four stroke cycle piston engine of equal power, and much lower in complexity. In this role the power to weight ratio is king, and the Wankel makes particularly good sense. Furthermore, due to the composition of the engine with an aluminium housing and a steel rotor, unlike a piston engine the engine will not seize when overheated, as the aluminium expands more than the steel; this adds a safety factor for aeronautical use. 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. 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, Centurion, 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.


See also

Hyper engine
Air safety
List of aircraft engines

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