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:
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. With 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
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..
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.
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.
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 in) 60° V12 engine used in, among others, the Spitfires that drove the Luftwaffe from the skies over England during the Battle of Britain.
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.
An opposed-type 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.
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.
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 mph (640 km/h).
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.
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.
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 themselves to massive wartime production, but the war ended before any turbojets could be mass-produced. 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 to 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 flight crossed the threshold into efficient turbojet operation.
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 propeller. A turbofan has extra turbine stages to turn the fan. Thus, more power is extracted from the exhaust gases before they leave the engine. This operation is a more efficient way to provide thrust than the jet nozzle alone, resulting in improved fuel-efficiency. 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. Although the fan creates thrust like a propeller, the surrounding duct frees it from many of the restrictions which limit propeller performance. Turbofans are more efficient than propellers in the trans-sonic range of aircraft speeds, and can operate in the supersonic realm. 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 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 are useful because they produce very large thrust and weigh very little.
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 and multiple valves per cylinder were introduced, while automobile engines continued to use pushrods and didn't widely use more than two valves per cylinder until the 1990s.
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.
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.
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 gasoline 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. 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. However, 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.
Precooled jet engines
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.
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 for a record 48 minutes, covering more than 50 kilometers (31 miles), the first electric airplane to receive a certificate of airworthiness.
Published - July 2009
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