Jet engine performance
One key metric of performance is the thermal efficiency; how much of the chemical energy (fuel) is turned into useful work (thrust propelling the aircraft at high speeds).
[citation needed] In the 1970s, rconomic pressure due to the rising cost of fuel resulted in increased emphasis on efficiency improvements for commercial airliners.
A recent development are ceramic matrix composite turbine blades, resulting in lightweight parts that can withstand high temperatures, while being less susceptible to creep.
[citation needed] The following parameters that indicate how the egine is performing are displayed in the cockpit: engine pressure ratio (EPR), exhaust gas temperature (EGT) and fan speed (N1).
Jet engines perform in two basic ways, the combined effect of which determines how much waste they produce as a byproduct of burning fuel to do thrust work on an aircraft.
[5] First is an energy conversion as burning fuel speeds up the air passing through which at the same time produces waste heat from component losses (thermal efficiency).
As a result of burning fuel thrust is a forward-acting force on internal surfaces whether in the diffuser of a ramjet or compressor of a jet engine.
As such, Struchtrup et al.[15] show the benefit of the high bypass turbofan engine from an entropy-reducing perspective instead of the usual propulsive efficiency advantage.
The need for an additional diagram, as opposed to understanding difficult theories, recognized the value of graphically representing heat transfers to and from an engine.
[24] Fuel energy released in the combustor is accounted for in two main categories: acceleration of the mass flow through the engine and residual heat.
The expansion following combustion is used to drive the compressor-turbine and provide the ram work when in flight, both of which cause the initial rise in temperature in the T~s diagram.
At the take-off EPR, for example, the fuel flow and hence EGT rise with time in service as the engine deteriorates from its as-new condition.
Increases in TIT mean a higher power output which for a turbojet leads to too high exhaust velocities for subsonic flight.
[53] The axial compressor has a geometry applicable to its high speed design condition at which the airflow approaches all the blading with little or no incidence, a requirement to keep flow losses to a minimum.
Combustion efficiency had always been close to 100% at high thrust levels meaning only small amounts of HC and CO are present, but big improvements had to be made near idle operation.
Fuel preparation for combustion is either done by converting it into small drops (atomization) or heating it with air in tubes immersed in flame (vaporization).
Early tests on afterburning showed the pressure loss due to burning increased rapidly if the Mach number at entry to the combustion zone was more than 0.3.
This is lower than the Mn leaving the turbine so a diffusing section is required to slow the gas before the flameholders where combustion begins and is maintained in the recirculation zone.
Stabilization of the flame is achieved in the engine combustor using airflow only, obtaining flow reversal, for example, by using swirl vanes around the fuel injector combined with air entering through holes in the liner.
[98] Although there is no turbine to limit the temperature of an afterburner there is still a cooling air requirement for the duct liner and variable nozzle which is about 10% of the engine entry airflow.
Steady state means being at a constant rpm for long enough (several minutes) for all parts to have stopped moving relative to each other from transient thermal growths.
[121][122][123] In the late 1940s it was considered by most US engine manufacturers that the optimum pr was 6:1 in light of the amount of leakage flow expected with the then-current sealing knowledge.
It is also a purge system which uses air to pressurize cavities to prevent hot flowpath gas from entering and overheating disc rims where blades are attached.
Early radial compressor engines used supplementary means for cooling air, for example a dedicated impeller or a fan machined integral with the turbine disc.
[155] Prior to the doubling and tripling price of fuel in the early 1970s the regain of performance after deterioration was largely a by-product of maintaining engine reliability.
[156] Higher bypass ratio engines were shown to be more susceptible to structural deformations which caused blade tip and seal clearances to be opened up by rubs.
American Airlines conducted tests on early bypass engines to understand to what degree component wear and accumulation of atmospheric dirt affected fuel consumption.
Prior to this the JT8D, for example, had thrust bending deflections minimized with a long stiff one-piece fan duct which isolated the internal engine cases from aerodynamic loads.
Noise influences the social acceptability of aircraft and maximum levels measured during takeoff and approach flyover are legislated around airports.
[166] A basic explanation for the way burning fuel results in engine thrust uses terminology like momentum, work, energy, power and rate.
