An aircraft engine is a propulsion system for an aircraft. Aircraft engines are almost always either lightweight piston engines or gas turbines. This article is an overview of the basic types of aircraft engines and the design concepts employed in engine development for aircraft.
Engine design considerations
The process of developing an engine is one of compromises. Engineers design specific attributes into engines to achieve specific goals. Aircraft are one of the most demanding applications for an engine, presenting multiple design requirements, many of which conflict with each other. An aircraft engine must be:
* 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.
* lightweight, as a heavy engine increases the empty weight of the aircraft & reduces its payload.
* powerful, to overcome the weight and drag of the aircraft.
* small and easily streamlined; large engines with substantial surface area, when installed, create too much drag, wasting fuel and reducing power output.
* repairable, to keep the cost of replacement down. Minor repairs should be relatively inexpensive.
* fuel efficient to give the aircraft the range the design requires.
* capable of operating at sufficient altitude for the aircraft
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. The power of an internal combustion reciprocating or turbine aircraft engine is rated in units of power delivered to the propeller (typically horsepower) which is torque multiplied by crankshaft revolutions per minute (RPM). The propeller converts the engine power to thrust horsepower or thp in which the thrust is a function of the blade pitch of the propeller relative to the velocity of the aircraft. Jet engines are rated in terms of thrust, usually the maximum amount achieved during takeoff.
The design of aircraft engines tends to favor reliability over performance. Long engine operation times and high power settings, combined with the requirement for high-reliability means that engines must be constructed to support this type of operation with ease. Aircraft engines tend to use the simplest parts possible and include two sets of anything needed for reliability. Independence of function lessens the likelihood of a single malfunction causing an entire engine to fail. For example, reciprocating engines have two independent magneto ignition systems, and the engine's mechanical engine-driven fuel pump is always backed-up by an electric pump.
Aircraft spend the vast majority of their time travelling at high speed. This allows an aircraft engine 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.
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.
History of aircraft engines
* 1633: Lagari Hasan Çelebi took off with what was described to be a cone shaped rocket and then glided with wings into a successful landing
* 1848: John Stringfellow made a steam engine capable of powering a model, albeit with negligible payload
* 1903: The Wright brothers commissioned Charlie Taylor to build an inline aeroengine (12 horsepower) for the Wright Flyer
* 1906:Traian Vuia flew his first airplane "Vuia I" at Montesson on 18th of March, achieving the first ever "only by on-board means" flight, without any "outside assistance", be it an incline, rails, a catapult, etc.
* 1908: René Lorin patents a design for the ramjet engine
* 1909: Roger Ravaud' Gnôme rotary engine in Henry Farman's aircraft won the Grand Prix for the greatest non-stop distance flown - 180 kilometres (110 mi) - and created a world record for endurance flight
* 1910: Henri Coanda displays the first jet powered aircraft at the second International Aeronautic Salon in Paris; he also tries to pilot the jet aircraft however he crashlands.
* 1911: Adams-Farwell's rotary engines powered fixed-wing aircraft in the US
* 1916: Auguste Rateau suggests using exhaust-powered compressors to improve high-altitude performance, the first example of the turbocharger.
* 1930: in Frank Whittle submitted his first patent
* 1938: The German Heinkel HeS 3 turbojet propels the Heinkel He 118 into the air
* 1939-1942: The world's first turboprop-the Jendrassik Cs-1 is designed by the Hungarian mechanical engineer György Jendrassik
* 1944: Messerschmitt Me 163 Komet, the worlds first rocket propelled aircraft deployed
* 1947: Bell X-1 rocket propelled aircraft exceeds the sound barrier
* 1948: the first turboshaft engine, the 100 shp 782. In 1950 this work was used to develop the larger 280 shp (210 kW) Artouste
* 1949: The Leduc 010 the world's first ramjet powered aircraft flies
* 1950(late): Rolls-Royce Conway the worlds first production turbofan enters service
* 1960s: TF39 high bypass turbofan enters service delivering greater thrust and much better efficiency
* 1960s: X-15 rocket plane flys at more than 50 miles (80 km) altitude at more than 3,000 mph (4,800 km/h).
* 2002: HyShot scramjet flew in dive
* 2004: Hyper-X first scramjet to maintain altitude
Propellant
Fuel
All aviation fuel is produced to stringent quality standards to avoid fuel-related engine failures. Aviation standards for octane ratings and vapor pressure are much more strict than those for road vehicle fuel, because an aircraft engine must meet a strictly defined level of performance under known conditions. These high standards mean that aviation fuel costs much more than fuel used for road vehicles.
Aircraft piston engines are typically designed to run on Avgas. Avgas has a higher octane rating compared to automotive gasoline, allowing a higher compression ratio and thus more power out of an engine with the same engine displacement. Currently the most common Avgas is 100LL, which refers to the octane rating (100 octane) and the lead content (LL = Low Lead). Avgas uses tetraethyl lead (TEL) to achieve these high octane ratings, a practice banned in automobile fuel. The shrinking supply of TEL, and the possibility of environmental legislation banning its use, has made a search for replacement fuels for general aviation aircraft a priority for pilot's organizations.[1].
Turbine engines burn various grades of jet fuel, a relatively heavy and less volatile petroleum derivative similar to diesel fuel.
Shaft engines
In-line engine
This type of engine has cylinders lined up in one row. It typically has an even number of cylinders, but there are instances of three- and five- cylinder engines. The biggest advantage of an inline engine is that it allows the aircraft to be designed with a narrow frontal area for low drag. If the engine crankshaft is located above the cylinders, it is called an inverted inline engine, which allows the propeller to be mounted up high for ground clearance even with short landing gear. The disadvantages of an inline engine include a poor power-to-weight ratio, because the crankcase and crankshaft are long and thus heavy. An in-line engine may be either air cooled or liquid cooled, but liquid-cooling is more common because it is difficult to get enough air-flow to cool the rear cylinders directly. Inline engines were common in early aircraft, including the Wright Flyer, the aircraft that made the first powered flight. However, the inherent disadvantages of the design soon became apparent, and the inline design was abandoned, becoming a rarity in modern aviation.
Rotary engine
Early in World War I, when aircraft were first being used for military purposes, it became apparent that existing inline engines were too heavy for the amount of power needed. Aircraft designers needed an engine that was lightweight, powerful, cheap, and easy to manufacture in large quantities. The rotary engine filled these goals. Rotary engines have all the cylinders in a circle around the crankcase like a radial engine (see below), but the difference is that the crankshaft is bolted to the airframe, and the propeller is bolted to the engine case. The entire engine rotates with the propeller, providing plenty of airflow for cooling regardless of the aircraft's forward speed. Some of these engines were a two-stroke design, giving them a high specific power and power-to-weight ratio. Unfortunately, the severe gyroscopic effects from the heavy rotating engine made the aircraft very difficult to fly. The engines also consumed large amounts of castor oil, spreading it all over the airframe and creating fumes which were nauseating to the pilots. Engine designers had always been aware of the many limitations of the rotary engine. When the static style engines became more reliable, gave better specific weights and fuel consumption, the days of the rotary engine were numbered.
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