Aircraft engine performance
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Aircraft engines are a mechanical component of the propulsion system on an airplane, helicopter, rocket or UAV which produces rotary energy to be transferred to a propeller or kinetic energy as a high pressure air exhaust stream. The most common aircraft engine types are turboprop, turbojet, turbofan and turboshaft, however electric engines and piston engines, both used more predominantly in recreational personal aircraft and older model aircraft also exist. Aircraft engine performance has improved dramatically since the advent of the first powered flight in 1848 by John Stringfellow. Aircraft engine manufacturers are constantly innovating to produce more efficient and more reliable engines for aircraft manufacturers. The first jet aircraft and subsequent powered flight occurred on the 27th of August 1939 by a Heinkel He 178. Today, most commercial aircraft are powered by jet engines for long-haul journeys. Efficient aircraft engines have significant positive economic impacts, as they reduce fuel costs for airline companies, enabling cheaper ticket prices and greater flight distances.
Factors affecting aircraft engine performance
Fuel variety plays a significant role in the dynamics and characteristics of engine performance, the operating cost of an aircraft and ultimately the safety and reliability of an engine. Conventional fuels consist of jet fuel and AVGAS (aviation gasoline), which differ from automotive engine fuels. These are fuels likely to be supplied at airports and are used for commercial aircraft because of their abundance, reliability and low cost. Aircraft fuels are not interchangeable and the fuel variety used by an engine must not be mixed. Different fuels are used for different applications due to their performance characteristics. Some fuels need to burn violently with great levels of thrust, such as rocket fuel, whereas in applications such as turbofan engines on airliners, fuel is designed to last at a slower burn rate for maximum range and economy.
Kerosene jet fuel, also known as aviation turbine fuel (ATF), is designed to be used in aircraft powered by gas turbine engines. Jet fuel used to power gas turbine engines has been the preferred propellant since the advent of this type of engine due to the fuel's favourable combustion characteristics and relatively high energy content. Jet fuel remains the most commonly used fuel in aviation due to the popularity of turbofan and turboprop engines. Turbofan engines power most large commercial passenger and cargo aircraft today. Civil jet fuel grades include A-1, A, B, TS-1. Military grades include JP-4, JP-8 and JP-5. Military varieties differ from civil jet fuels due to the addition of corrosion inhibitors and anti-icing additives. JP-8 jet fuel is the most common fuel among NATO aircraft fleets. An example of the properties of jet fuel A-1 can be seen in the table below.
|Property||Test method||Limits ASTM D1655-15D||Results|
|Density in 15 °C||kg/m^3||775.00||788.0|
|Viscosity in -20 °C||mm^2/s||Max 8.000||2.992|
|Heat of combustion||MJ/kg||Min 42.80||43.45|
AVGAS (aviation gasoline) is widely used in reciprocating engines (piston engines). Aviation gasoline is highly volatile and very flammable, with a low flash point, which makes it unsuitable for use in gas turbine engines. Volatility is how easily a substance will change from a liquid to a gaseous state. Highly volatile fuel is required to power reciprocating engines as the liquid gasoline pumped to the carburettor must readily vaporise in order to combust in the engine. There is however a balance of volatility needed. If AVGAS fuel is too volatile, it may cause vapour lock and early detonation in the engine cylinder. If the AVGAS is not volatile enough, there will be inconsistent engine acceleration and power throughout the revolution range. AVGAS is commonly supplemented with Tetraethyl-lead (TEL) to prevent engine knocking, which is a damaging build-up of pressure inside the engine caused by low octane rated fuel which may lead to engine failure in reciprocating engines. Antiknock additives allow for greater efficiency and peak power. Engine failure in an aircraft at altitude has the potential to be catastrophic, as witnessed with the death of 111 passengers onboard United Airlines Flight 232 in 1989. TEL has been banned by the European Union for automotive use due to environmental concerns, but remains approved for use in aircraft.
Rocket fuel consists of solid, liquid and gel state fuels for propulsion. In order to power rockets, a fuel and an oxidiser are mixed within the combustion chamber, producing a high energy propulsive exhaust as thrust. The main uses for rocket fuel are for space shuttle boosters in order to propel the craft out of the atmosphere, or for missiles. Solid rocket propellant does not degrade in long-term storage and remains reliable on combustion. This allows munitions to remain loaded and fired when needed, which is highly regarded for military use. Once ignited, solid rocket propellants cannot be shutdown. The fuel and the oxidiser are stored within a metal casing. Once ignited, the fuel burns from the centre of the solid compound towards the edges of the metal casing. Burn rates and intensity are manipulated by the changing of the shape of a channel between the fuel and the casing shell. Two varieties of solid rocket fuel propellants exist. These include homogeneous and composite solid rocket fuels. These fuels are characteristically dense, stable at ordinary temperatures and easily storable. Liquid fuels are more controllable than solid rocket fuels, and can be shutoff after ignition and restarted, as well as offering greater thrust control. Liquid propellants are stored in two parts in an engine, as the fuel in one tank and an oxidiser in another. These liquids are mixed in the combustion chamber and ignited. Hypergolic fuel is mixed and ignites spontaneously, requiring no separate ignition. Liquid fuel compounds include petroleum, hydrogen and oxygen.
Electricity may be transmitted to an aircraft's electric motors through batteries, ground power cables, solar cells, ultra-capacitors, fuel cells and power beaming. Electrically powered engines are currently only suitable for light aircraft and UAV's (unmanned aerial vehicles). Electrical engines are praised for being environmentally friendly and relatively quiet. There are a multitude of personal UAV's and drones available for purchase without a licence or age restriction globally, capable of high speed manoeuvres and agile flight characteristics. Typically aircraft with electric engines have significantly shorter flight durations than conventional fuel powered aircraft although battery technology developments and solar energy conversion has created potential for use in commercial aircraft. Jeffrey Engler, CEO of Wright Electric, estimates that commercially viable electric planes will reduce energy costs by 30%.
Hydrogen as a fuel, through the combustion of hydrogen in a jet engine or fuel cell, is a viable fuel source for aircraft engines. Currently, pressurised tanks to hold the hydrogen fuel with sufficient volume and a low enough weight are not available for large commercial aircraft, but have been successfully implemented on smaller personal aircraft such as the Boeing Fuel Cell Demonstrator by Boeing Phantom Works and on launch rockets for space shuttles when stored cryogenically. Hydrogen can be used to power a multitude of craft, via turbine engines, piston engines and rocket engines. Hydrogen fuel cells create electrical power through hydrolysis and are in various stages of research for applications in environmentally friendly engines as they emit no toxic exhaust. Hydrogen powered engines only emit water through the bonding of oxygen and hydrogen, aswell as any excess hydrogen as exhaust. This means that this is a highly environmentally friendly propulsion system.
Researchers from MIT (Massachusetts Institute of Technology) have developed an ion drive propulsion system with no moving parts. The 'engine' is propelled by ionic wind, also known as electro-aerodynamic thrust. This new form of aircraft propulsion would be completely silent and require far less maintenance than conventional fossil-fuel powered engines. This technology has the potential to be used in conjunction with conventional aircraft combustion engines as a hybrid system with further development or even as propulsion systems on spacecraft.
Climatic conditions are an important consideration in the analysis of the factors contributing to differing aircraft engine performance. Aircraft engine performance decreases as altitude and temperature increase. These factors include altitude, temperature and humidity. In the instance of high humidity, the volume of air available for combustion is reduced, causing loses in power in combustion engines. Aircraft engine performance is measured at baseline parameters of a standard atmosphere or 29.92” of mercury at 15°C. It is important to not operate aircraft engines outside of these parameters in the interest of safety. Weather may be a physical barrier to engine performance, as is in the case of hail or volcanic ash particulates entering the engine.
When altitude is increased, air density decreases. With lower air density, air molecules are further apart from each other, which will lead to declines in combustion engine performance. Electric powered aircraft will not see loses on power output at high altitude, but rather aerodynamic losses as propellers work harder to propel the same amount of air at ground level. However, cooling capacity will decline on both combustion and electric motors at high altitude due to the lower volume of air. This phenomenon is why the operating limit of helicopters is constrained, as propeller thrust returns to a value of 0 when the air becomes too thin at high altitude. This makes high altitude airports significantly more dangerous than airports at sea level.
Temperature has significant effects on the operational efficiency of an aircraft engine. This applies for combustion and electrical engines. Commercial pilots will account for the ambient temperature on the day of a flight in order to calculate takeoff angle of attack, ascent, range, descent, and landing angle of attack values. Extreme heat or cold temperatures are performance limitations for aircraft engines. An aircraft flying at a constant altitude with an ambient air temperature of 20°C would experience more favourable performance than flying with an ambient air temperature of 40°C. With cold, air is denser and a larger mass of air/fuel mixture is combusted, leading to higher efficiency and power.
Humidity affects the oxygen content of air in the atmosphere, reducing the burn rate and increasing the combustion time of fuel in a combustion engine which will reduce thermal efficiency.Minimal losses of power occur where the energy of the engine's combustion heat the moisture in the engine. For electrical components found within electric motors, excess moisture is capable of damaging circuits and electrical systems. In reality, air is never full dry, or without absence of moisture in the atmosphere. Even when air is dry, it retains a moisture content of around 5%.
Weather has significant impacts on both the performance of an engine, but also the propensity to cause engine malfunction or failure. Winds are both beneficial and unfavourable depending on the direction of the wind and the heading of the aircraft. In headwinds, engines must work harder to maintain airspeed, burning more fuel. This causes the engine to become less efficient. Conversely, a tailwind will allow the pilot to use less engine power in order to maintain the same speed, improving engine efficiency. A significant weakness of many aircraft is their use of propellers or turbines in their engines. This is because any particulates that enter the engine other than air may cause damage. An example of this is hail, when precipitation freezes. If the hail is severe enough, engine inlet guide vanes or compressor blades can bend or break under impact. Volcanic ash ejected into the atmosphere due to a volcanic eruption is another example of reduced engine performance due to weather. Particles of volcanic ash are abrasive at high speed, leading to abrasion on compressor fan blades. The glass-like silicate compound found in volcanic ash has a lower melting point than the combustion temperature of fuel and air in a jet engine. When ingested into the engine, the material melts and deposits in cooler areas of the engine, leading to compressor stall and thrust loss.
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