World Wide Aero-Engines - Aviation Propulsion Systems
The power plant is simply the propulsion system and consists of the engines. The sole purpose of the engines is to provide thrust for the airplane. There are many different types of aircraft engines including: piston, turboprop, turbojet and turbofan. Turbojet and turbofan are jet engines. Some aircraft, notably gliders, do not have an engine. To take off they must have another source of thrust - that is, the tow-plane which pulls them into the air.
Within a piston engine, the pistons can be arranged in four ways: radial, in-line, oppositional and "V." The radial engine has pistons arranged in a circle with the spinning shaft in the middle. These engines were once the most widely used aircraft engine. They never found much favor outside of aviation and are not used in modern aviation. The pistons on an in-line engine are lined up one behind the other along the length of the shaft that turns the propeller. These have been used in many applications including cars. They are not used a great deal in aircraft, as they tend to be long and heavy. Aircraft engines must be as lightweight and compact as possible. The oppositional piston engine is much like the in-line, except that the pistons are mounted in pairs on opposite sides of the shaft. This makes for a much shorter and lighter engine. In-line engines have become very popular in the small airplane market. The "V" engine is much like the oppositional engine, except that the pistons are not parallel to each other. Instead they are slanted in a "V" arrangement. The V8 engine is perhaps the most well known engine as it has been used to power millions of automobiles. The V8 is rarely used in airplanes as they tend to be heavier than the oppositional engines.
Piston engines drive a spinning shaft. The propeller is attached to that shaft. At least two (but usually three or four) blades make up the propeller. The more blades, the more air that can be moved by the propeller. A blade has an airfoil shape which generates lift as the blade slices through the air. Because the propeller is pointed forward the force generated is in a forward direction - that is, it thrusts the airplane forward.
Jet propulsion is similar to the release of an inflated balloon. The pressure inside the balloon is pushing in all directions. It is also "jetting" out from the mouth of the balloon. The end of the balloon opposite to the mouth is not open. This creates an imbalance and causes the balloon to move in the direction away from the open mouth. Jet engines work in a similar fashion. There are several types of jet engine: ramjet, pulsejet, turbojet, turbofan. The last two are the most widely used.
The ramjet is as simple a jet engine as can be found. Air enters the inlet and is compressed. This raises the pressure of the air. As the air arrives at the combustion chamber, fuel is added and an electric spark is generated. This causes a controlled explosion that raises the temperature and the pressure of the air tremendously. The hot, high-pressure air "jets" out the nozzle of the engine providing the forward thrust. This seems so simple, why would anyone want a more complex engine? The weakness of this engine is that the air coming in the inlet must be traveling at a very high speed (supersonic) for good efficiency. A ramjet does not work well at low speeds. This is simply not practical for most flying situations.
The pulse jet solves the problem of requiring supersonic speeds. It works well at a lower speed and with a little help, can get started when it is standing still. It is much like the ramjet, except that it has doors that close the inlet. When the doors are open, the air flows in and is compressed. The doors then close, forming a chamber in which the fuel is ignited. The hot, high-pressure gas then "jets" out the exhaust nozzle. The cycle of air in, doors closed, air out, then repeat, is where this engine gets its name. Pulse jets are not widely used for two reasons. They are very noisy and inefficient. They are the gas guzzlers of the aviation world.
The turbojet was the first really useful jet engine to be built. The air flows into the engine through the inlet. The design of the inlet makes the air slow down and also raises the pressure. The air then goes through the compressor where sets of blades compress the air even more, greatly raising the pressure. The air then enters the combustion chamber where the fuel is added and ignited. The very hot, high-pressure air rushes past the turbine blades making them spin very fast. The turbine blades are connected back to the compressor blades by a shaft. The turbine blades take some of the energy from the air and returns it to the compressor. The hot, high pressure air that gets past the turbine, "jets" out the exhaust nozzle thrusting the engine forward.
To increase the thrust available, a device called an afterburner is sometimes built into the engine. Fuel is dumped into the hot exhaust gas exiting the nozzle causing another controlled explosion. This makes the air even hotter which adds more energy to it, thereby increasing the thrust. This is not an efficient thing to do however, and is only done for brief periods when extra thrust is needed, for example, on takeoff or when a burst of speed is needed during a dog fight, or when an extra push is needed to reach supersonic speed. You may have seen movies with high performance jets, like the F-14. If you watch one of these jets from the back, and the pilot turns on the afterburners you will hear a burst of noise and see an orange glow around the outlet of the engines. The airplane will then shoot up into the sky.
The turbofan is a refinement to the turbojet that results in a more efficient engine. A large set of fan blades is set right in the front of the inlet. The fan works much like a propeller, thrusting the engine forward, pushing a large amount of air backwards. As the air is pushed back by the fan some of it goes into the engine and some bypasses the engine. The engine that sits behind the fan is basically a turbojet. The air that goes into this engine receives the same treatment as air that goes through the turbojet. The turbine in a turbofan drives the fan as well as the compressor. The air that "jets" out the back of this engine has less thrust than air that exits a turbojet, but that decrease in thrust is made up for by the thrust generated by the fan. A turbofan engine actually is more efficient than a turbojet and is quieter as well. Afterburners can be fitted to a turbofan if required.
The turboprop engine is essentially a turbofan engine where the fan is replaced by a propeller. The propeller is placed outside of the inlet. A gearbox is introduced which controls the spinning of the shaft, enabling speed control for the propeller. This is the most efficient means of propulsion, however it is limited in forward speed. Because the propeller is out in the free stream air, not mounted in the inlet (where the air speed is reduced) the propeller has to rotate at faster speeds. The speed of the propeller approaches the speed of sound well before the airplane itself. As the airplane approaches the speed of sound, drag greatly increases. So the speed of the airplane must be kept well below the speed of sound to prevent the tips of the propeller from going too fast.
For the turbojet engine, a key objective of the last half century has been to increase combustion temperatures for better efficiency and reduced fuel consumption, without burning up the turbine blades. This is done by better materials such as the ceramics mentioned elsewhere, better cooling approaches, and by better computational analysis methods. Reduced emissions and reduced noise are also becoming extremely important for the civil sector. Performance improvements in core engine technology have historically been driven by military programs, and commercial engine development will continue to benefit from military research efforts. Hot section advances are most directly linked to materials development and innovation in cooling techniques. Improved transmissions would be instrumental in moving forward with advanced rotocraft designs, such as the next generation of the tilt-rotor.
Efficiency of aircraft turbines significantly affects purchase price and operating costs of aircraft. Improved engines contribute to job creation in the aerospace sector, as well as to the competitiveness of the U.S. aerospace industry, because the U.S. is a major player in international aerospace markets. They also contribute to improvements in environmental quality by reducing emissions from aircraft engines and reduced energy consumption. Greater efficiency of aircraft engines also contributes to the warfighting capability of rapid global power projection.
The United States has the overall lead in aircraft turbine engine technology, based on its superior military technology, but shares the lead in commercial propulsion systems technology with the UK's Rolls Royce. Europe has an edge in facilities for propulsion/airframe integration testing of large models at high Reynolds Numbers. They may also have an edge in technologies for noise reduction. Second-tier manufacturers in France, Germany, and Japan have seen their capabilities increase through international joint ventures and European military development programs. This general pattern is likely to continue with France (likely to become more competitive with the United States over the next five years as they assume responsibilities for a greater percentage of engine designs and component manufacturing) leading the way and Japan bringing up the rear. While Japan will likely become a more attractive joint venture partner-- almost certainly the country's prime goal--technology transfer is unlikely to be enough to significantly alter capabilities relative to the world leaders over the next several years.
Because of the trend towards higher thrust engines, in concert with the trend towards larger aircraft, deliveries of engines with greater than 45,000 lb thrust are more than 50 percent of the market by value by the year 2000. The trend towards higher-thrust engines has two general consequences for the competitiveness of engine manufacturers. The higher costs associated with development of these large engines have led to more joint international programs and transfer of technology to second-tier manufacturers. Secondly, with the attention of the technology leaders focused at the high-thrust end of the market, second-tier manufacturers are increasing their roles in the development of smaller engines.
Performance improvements in propulsion systems are driven by core technology advances in engine component technologies, manufacturing capabilities, and systems integration. Europe continues to lag in turbine-blade technologies, but is keeping pace with U.S. developments. The United Kingdom and France are slightly behind the U.S., but have introduced some technologies first. For example, the United Kingdom was the first to introduce hollow fan blades, instrumental for reducing engine weight. France is a leader in advanced composite materials and has introduced silicon carbide nozzle flaps for their M88 engine.
Wide-chord fans--which do not require part-span shrouds and are therefore more efficient--are quickly being adopted industry-wide. Some operators of engines with wide-chord fans have also claimed that the fan blades' ability to flex makes them more resistant to birdstrike damage. Next generation engines will likely also incorporate swept aerodynamics in both fans and compressors to provide additional increases in efficiency. High strength-to-weight compressor materials are being developed to permit increased rotational speeds that--along with the adoption of low aspect ratio blade shapes--will enable reductions in the number of airfoils required.