Aircraft engine

Aircraft engines
A Rolls-Royce Merlin installed in a preserved Avro York at the Imperial War Museum Duxford
Part of a series on Aircraft propulsion
Shaft engines (to drive propellers, rotors, ducted fans, or propfans)
Internal combustion engines: Piston engine Wankel engine Turbines: Turboprop Turboshaft External combustion engines: Steam-powered Others: Human-powered Electric
Reaction engines
Turbines: Turbojet Turbofan Rocket-powered Motorjet Pulsejet Ramjet Scramjet

An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. 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.

History of aircraft engines

Wright vertical 4-cylinder engine

• 1848: John Stringfellow made a steam engine capable of powering a model, albeit with negligible payload
• 1903: Charlie Taylor built a inline aeroengine for the Wright Flyer (12 horsepower)
• 1903: Manly-Balzer engine sets standards for later radial engines
• 1906: Léon Levavasseur produces a successful water-cooled V8 engine for aircraft use
• 1908: René Lorin patents a design for the ramjet engine
• 1908: Gnome Omega, the world's first rotary engine produced in quantity; in 1909, fitted in Farman III aircraft, won the Grand Prix for the greatest non-stop distance flown - 180 kilometres (110 mi) - and set a world record for endurance
• 1910: Coandă-1910, an unsuccessful ducted fan powered aircraft
• 1914: Auguste Rateau suggests using exhaust-powered compressor – a turbocharger – to improve high-altitude performance; not accepted after the tests
• 1918: Sanford Moss picks Rateau's idea and creates the first successful turbocharger
• 1926: Armstrong Siddeley Jaguar IV (S), the first series-produced supercharged engine for aircraft use; two-row radial with a gear-driven centrifugal supercharger
• 1930: Frank Whittle submitted his first patent for turbojet engine
• 1938: Heinkel HeS 3 turbojet propels the German He 178 aircraft
• 1940: Jendrassik Cs-1, the world's first run of the turboprop engine
• 1944: Messerschmitt Me 163 Komet, the world's first rocket propelled aircraft deployed
• 1947: Bell X-1 rocket propelled aircraft exceeds the speed of sound
• 1948: 100 shp 782, the first turboshaft engine; in 1950 used to develop the larger 280 shp (210 kW) Turbomeca Artouste
• 1949: Leduc 010, the world's first ramjet-powered aircraft flight
• 1950: Rolls-Royce Conway, the world's first production turbofan, enters service
• 1960s: TF39 high bypass turbofan enters service delivering greater thrust and much better efficiency
• 2002: HyShot scramjet flew in dive
• 2004: Hyper-X, the first scramjet to maintain altitude

Fuel

All aviation fuel is produced to stringent quality standards to avoid fuel-related engine failures. Aviation standards 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 reciprocating (piston) engines are typically designed to run on aviation gasoline. Avgas has a higher octane rating as compared to automotive gasoline, allowing the use of higher compression ratios, increasing power output and efficiency at higher altitudes. Currently the most common Avgas is 100LL, which refers to the octane rating (100 octane) and the lead content (LL = low lead).

Avgas is blended with tetra-ethyl lead (TEL) to achieve these high octane ratings, a practice no longer permitted with road vehicle gasoline. 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.

Turbine engines and aircraft diesel engines burn various grades of jet fuel. Jet fuel is a relatively heavy and less volatile petroleum derivative based on kerosene, but certified to strict aviation standards, with additional additives.

Shaft engines

Ranger L-440 air-cooled, six-cylinder, inverted, in-line engine used in Fairchild PT-19

In-line engine

Main article: Straight 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 controlled 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

Le Rhone 9C rotary aircraft engine.

Main article: 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 met 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.

A Rolls-Royce Merlin V-12 Engine

V-type engine

Main article: V engine

Cylinders in this engine are arranged in two in-line banks, tilted 30-60 degrees apart from each other. The vast majority of V engines are water-cooled. The V design provides a higher power-to-weight ratio than an inline engine, while still providing a small frontal area. Perhaps the most famous example of this design is the legendary Rolls-Royce Merlin engine, a 27-litre (1649 in³) 60° V12 engine used in, among others, the Spitfires that played a major role in the Battle of Britain.

Radial engine

A Pratt & Whitney R-2800 Engine

Main article: Radial engine

This type of engine has one or more rows of cylinders arranged in a circle around a centrally-located crankcase. Each row must have an odd number of cylinders in order to produce smooth operation. A radial engine has only one crank throw per row and a relatively small crankcase, resulting in a favorable power to weight ratio. Because the cylinder arrangement exposes a large amount of the engine's heat radiating surfaces to the air and tends to cancel reciprocating forces, radials tend to cool evenly and run smoothly.

The lower cylinders, which are under the crankcase, may collect oil when the engine has been stopped for an extended period. If this oil is not cleared from the cylinders prior to starting the engine, serious damage due to hydrostatic lock may occur.

In military aircraft designs, the large frontal area of the engine acted as an extra layer of armor for the pilot. However, the large frontal area also resulted in an aircraft with a blunt and aerodynamically inefficient profile.

Horizontally-opposed engine

Main article: Flat engine

A ULPower UL260i horizontally-opposed air-cooled aero engine.

An horizontally-opposed engine, also called a flat or boxer engine, has two banks of cylinders on opposite sides of a centrally located crankcase. The engine is either air cooled or liquid cooled, but air cooled versions predominate. Opposed engines are mounted with the crankshaft horizontal in airplanes, but may be mounted with the crankshaft vertical in helicopters. Due to the cylinder layout, reciprocating forces tend to cancel, resulting in a smooth running engine. Unlike a radial engine, an opposed engine does not experience any problems with hydrostatic lock.^{[citation needed]}

Opposed, air-cooled four and six cylinder piston engines are by far the most common engines used in small general aviation aircraft requiring up to 400 horsepower (300 kW) per engine. Aircraft which require more than 400 horsepower (300 kW) per engine tend to be powered by turbine engines.

Turboprop

Cutaway view of a Garrett TPE-331 turboprop engine. Notice the gearbox at the front of the engine.

Main article: Turboprop

While military fighters require very high speeds, many civil airplanes do not. Yet, civil aircraft designers wanted to benefit from the high power and low maintenance that a gas turbine engine offered. Thus was born the idea to mate a turbine engine to a traditional propeller. Because gas turbines optimally spin at high speed, a turboprop features a gearbox to lower the speed of the shaft so that the propeller tips don't reach supersonic speeds. Often the turbines which drive the propeller are separate from the rest of the rotating components so that they are free to rotate at their own best speed (referred to as a free-turbine engine). A turboprop is very efficient when operated within the realm of cruise speeds it was designed for, which is typically 200 to 400 mi/h (320 to 640 km/h).

Turboshaft

A Rolls-Royce Model 250 turboshaft engine common to many types of helicopters.

Main article: Turboshaft

Turboshaft engines are used primarily for helicopters and auxiliary power units. A turboshaft engine is very similar to a turboprop, with a key difference: In a turboprop the propeller is supported by the engine, and the engine is bolted to the airframe. In a turboshaft, the engine does not provide any direct physical support to the helicopter's rotors. The rotor is connected to a transmission, which itself is bolted to the airframe, and the turboshaft engine simply feeds the transmission via a rotating shaft. The distinction is seen by some as a slim one, as in some cases aircraft companies make both turboprop and turboshaft engines based on the same design.

Jet engines

Main article: Jet engine

The key part of a jet engine is the exhaust nozzle. This is the part which produces thrust for the jet; the hot airflow from the engine is accelerated when exiting the nozzle, creating thrust, which, in conjunction with the pressures acting inside the engine which are maintained and increased by the constriction of the nozzle, pushes the aircraft forward.

The most common jet propulsion engines flown are turbojet, turbofan and rocket. Other types such as pulsejets, ramjets, scramjets and Pulse Detonation Engines have also flown.

Turbojet

A General Electric J85-GE-17A turbojet engine. This cutaway clearly shows the 8 stages of axial compressor at the front (left side of the picture), the combustion chambers in the middle, and the two stages of turbines at the rear of the engine.

Main article: Turbojet

A turbojet is a type of gas turbine engine that was originally developed for military fighters during World War II. A turbojet is the simplest of all aircraft gas turbines. It features a compressor to draw air in and compress it, a combustion section which adds fuel and ignites it, one or more turbines that extract power from the expanding exhaust gases to drive the compressor, and an exhaust nozzle which accelerates the exhaust out the back of the engine to create thrust. When turbojets were introduced, the top speed of fighter aircraft equipped with them was at least 100 miles per hour faster than competing piston-driven aircraft. The relative simplicity of turbojet designs lent them to wartime production.^{[citation needed]} In the years after the war, the drawbacks of the turbojet gradually became apparent. Below about Mach 2, turbojets are very fuel inefficient and create tremendous amounts of noise. The early designs also respond very slowly to power changes, a fact which killed many experienced pilots when they attempted the transition to jets. These drawbacks eventually led to the downfall of the pure turbojet, and only a handful of types are still in production. The last airliner that used turbojets was the Concorde, whose Mach 2 airspeed permitted the engine to be highly efficient.

Turbofan

A cutaway of a CFM56-3 turbofan engine

Main article: Turbofan

A turbofan engine is much the same as a turbojet, but with an enlarged fan at the front which provides thrust in much the same way as a ducted propeller, resulting in improved fuel-efficiency. Although the fan creates thrust like a propeller, the surrounding duct frees it from many of the restrictions which limit propeller performance. This operation is a more efficient way to provide thrust than simply using the jet nozzle alone and turbofans are more efficient than propellers in the trans-sonic range of aircraft speeds, and can operate in the supersonic realm. A turbofan typically has extra turbine stages to turn the fan. Turbofans were the first engines to use multiple spools; concentric shafts which are free to rotate at their own speed; in order to allow the engine to react more quickly to changing power requirements. Turbofans are coarsely split into low-bypass and high-bypass categories. Bypass air flows through the fan, but around the jet core, not mixing with fuel and burning. The ratio of this air to the amount of air flowing through the engine core is the bypass ratio. Low-bypass engines are preferred for military applications such as fighters due to high thrust-to-weight ratio, while high-bypass engines are preferred for civil use for good fuel efficiency and low noise. High-bypass turbofans are usually most efficient when the aircraft is traveling at 500 to 550 miles per hour (800 to 885 km/h), the cruise speed of most large airliners. Low-bypass turbofans can reach supersonic speeds, though normally only when fitted with afterburners.

Rocket

An XLR99

Main article: Rocket engine

A few aircraft have used rocket engines for main thrust or attitude control, notably the Bell X-1 and North American X-15.

Rocket engines are not used for most aircraft as the energy and propellant efficiency is very poor except at high speeds, but have been employed for short bursts of speed and takeoff.

Rocket engines are very efficient only at very high speeds, although they are useful because they produce very large amounts of thrust and weigh very little.

Newer engine types

Wankel engine

Main article: Wankel engine

Powerplant from a Schleicher ASH 26e self-launching motor glider, removed from the glider and mounted on a test stand for maintenance at the Alexander Schleicher GmbH & Co in Poppenhausen, Germany. Counter-clockwise from top left: propeller hub, mast with belt guide, radiator, Wankel engine, muffler shroud.

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.

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 (220 kW) 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 seven 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.

Diesel engine

Main article: Aircraft 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 but, for example, the Clerget 14F diesel radial engine (1939) has the same power to weight as a gasoline radial . 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 gasoline designs currently being used in light aircraft) and the high relative taxation of AVGAS compared to Jet A1 in Europe have all seen a revival of interest in the concept. Thielert Aircraft Engines converted Mercedes diesel automotive engines, certified them for aircraft use, and became an OEM provider to Diamond Aviation for their light twin. Financial problems have plagued Thielert, so Diamond's affiliate—Austro Engine—developed the new AE300 turbodiesel, also based on a Mercedes engine. Competing new diesel engines may bring fuel efficiency and lead-free emissions to small aircraft, representing the biggest change in light aircraft engines in decades. Wilksch Airmotive build 2 stroke diesel engine (same power to weight as a gasoline engine) for experimental aircraft: WAM 100 (100 hp), WAM 120 (120 hp) and WAM 160 (160 hp)

Precooled jet engines

Main article: Precooled jet engine

For very high supersonic/low hypersonic flight speeds inserting a cooling system into the air duct of a hydrogen jet engine permits greater fuel injection at high speed and obviates the need for the duct to be made of refractory or actively cooled materials. This greatly improves the thrust/weight ratio of the engine at high speed.

It is thought that this design of engine could permit sufficient performance for antipodal flight at Mach 5, or even permit a single stage to orbit vehicle to be practical.

Electric

About 60 electrically powered aircraft, such as the QinetiQ Zephyr, have been designed since the 1960s. Some are used as military drones. In France in late 2007, a conventional light aircraft powered by an 18 kW electric motor using lithium polymer batteries was flown, covering more than 50 kilometers (31 mi), the first electric airplane to receive a certificate of airworthiness.

Limited experiments with solar electric propulsion have been performed, notably the manned Solar Challenger and Solar Impulse and the unmanned NASA Pathfinder aircraft.

See also

• Air safety
• Aircraft engine position number
• Engine configuration
• Hyper engine
• List of aircraft engines
• Model engine
• United States military aero engine designations

External links

• Aircraft Engines and Aircraft Engine Theory (includes links to diagrams)
• The Aircraft Engine Historical Society
• Aircraft Engine Efficiency: Comparison of Counter-rotating and Axial Aircraft LP Turbines
• The History of Aircraft Poiver Plants Briefly Reviewed : From the " 7 lb. per h.p" Days to the " 1 lb. per h.p" of To-day