A jet engine converts fuel into thrust. 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). Like a lot of heat engines, jet engines tend to not be particularly efficient (<50%); a lot of the fuel is "wasted". In the 1970s, economic pressure due to the rising cost of fuel resulted in increased emphasis on efficiency improvements for commercial airliners.
Jet engine performance has been phrased as 'the end product that a jet engine company sells'Gas Turbine Performance, Second Edition, Walsh and Fletcher 2004, , Preface and, as such, criteria include thrust, (specific) fuel consumption, time between overhauls, power-to-weight ratio. Some major factors affecting efficiency include the engine's overall pressure ratio, its bypass ratio and the turbine inlet temperature. Performance criteria reflect the level of technology used in the design of an engine, and the technology has been advancing continuously since the jet engine entered service in the 1940s. It is important to not just look at how the engine performs when it's brand new, but also how much the performance degrades after thousands of hours of operation. One example playing a major role is the creep in/of the rotor blades, resulting in the aeronautics industry utilizing directional solidification to manufacture turbine blades, and even making them out of a single crystal, ensuring creep stays below permissible values longer. A recent development are ceramic matrix composite turbine blades, resulting in lightweight parts that can withstand high temperatures, while being less susceptible to creep.
The following parameters that indicate how the engine is performing are displayed in the cockpit: engine pressure ratio (EPR), exhaust gas temperature (EGT) and fan speed (N1). EPR and N1 are indicators for thrust, whereas EGT is vital for gauging the health of the engine,"EGT margin indicates engine health"' pp. 5–11, Safety first The Airbus Safety magazine, February 2022 as it rises progressively with engine use over thousands of hours, as parts wear, until the engine has to be overhauled.
The performance of an engine can calculated using thermodynamic analysis of the engine cycle. It calculates what would take place inside the engine. This, together with the fuel used and thrust produced, can be shown in a convenient tabular form summarising the analysis.
Introduction An introductory look at jet engine performance may be had in a cursory but intuitive way with the aid of diagrams and photographs which show features that influence the performance. An example of a diagram is the velocity triangle which in everyday life tells cyclists why they struggle against wind from certain quarters (and where head-on is worst) and in the engine context shows the angle air is approaching compressor blades (head-on is best for low losses). The use of velocity triangles in compressors and turbines to show the all-important angle at which air approaches the blading goes back to early steam turbines.https://arc.aiaa.org/doi/abs/10.2514/1.9176?journalCode=jpp,"Ideas and Methods of Turbomachinery Aerodynamics: A Historical View", Cumpsty and Greitzer, Fig. 1
Photographs show performance-enhancing features such as the existence of bypass airflow (increased propulsive efficiency) only visually obvious on engines with a separate exit nozzle for the bypass air. They are also used to show rarely seen internal details such as honeycomb seals which reduce leakage and save fuel (increased thermal efficiency), and degrading details such as the rub marks on centrifugal impeller blades which indicate loss of material, increased air leakage and fuel consumption.
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.An engine applies a thrust force to a stationary aircraft and thrust work is done on the aircraft when it moves under the influence. 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). Second, part of the power which has been given to the air by the engine is transferred to the aircraft as thrust work with the remaining part being kinetic energy waste in the wake (propulsive efficiency). The two efficiencies were first formulated in the 19th century for the steam engine (thermal efficiency ) and the ship's propeller (propulsive or Froude efficiency ).
A visual introduction to jet engine performance, from the fuel efficiency point of view, is the Temperature~entropy (T~s) diagram. The diagram originated in the 1890s for evaluating the thermal efficiency of steam engines. At that time entropy was introduced in graphical form in the T~s diagram which gives thermal efficiency as a ratio of areas of the diagram. The diagram also applies to air-breathing jet engines with an area representing kinetic energy added to the air flowing through the engine. A propulsion device, a nozzle, has to be added to a gas turbine engine to convert its energy into thrust. The efficiency of this conversion (Froude or propulsive efficiency) reflects work done in the 1800s on ship propellers. The relevance for gas turbine-powered aircraft is the use of a secondary jet of air with a propeller or, for jet engine performance, the introduction of the bypass engine. The overall efficiency of the jet engine is thermal efficiency multiplied by propulsive efficiency ( ).
There have been rapid advances in aero-engine technology since jet engines entered service in the 1940s. For example, in the first 20 years of commercial jet transport from the Comet 1 Ghost engine to the 747 JT9D Hawthorne scales up the Ghost to give JT9D take-off thrust and it is four and a half times as heavy. Gaffin and Lewis make an assessment using one company's design knowledge. Using JT3D-level technology (1958) to produce a JT9D cycle (1966), with its higher bypass ratio and pressure ratio, an hypothetical engine came out 70% heavier, 90% longer and with a 9% bigger diameter than the JT9D engine.
Conversion of fuel into thrust The type of jet engine used to explain the conversion of fuel into thrust is the ramjet. It is simpler than the turbojet which is, in turn, simpler than the turbofan. It is valid to use the ramjet example because the ramjet, turbojet and turbofan core all use the same principle to produce thrust which is to accelerate the air passing through them. All jet propulsion devices develop thrust by increasing the velocity of the working fluid.
Conversion of fuel into thrust may be shown on a sketch which illustrates, in principle, the location of the thrust force in a much simplified internal shape representing a ramjet. 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. Although the momentum of the flow leaving the nozzle is used to calculate thrust the momentum is only the reaction to the static pressure forces inside the engine and these forces are what produce the thrust.The Aerothermodynamics of Aircraft Gas Turbine Engines, Oates, Editor, AFAPL-TR-78-=52, WP AFB, Ohio, pp. 1–41
Conversion of fuel into thrust and waste
The waste leaving a jet engine is in the form of a wake which has two constituents, one mechanical, called the residual velocity loss (RVL) due to its kinetic energy, and the other thermodynamic, due to its high temperature. The waste heat in the exhaust of a jet engine can only be reduced at source by addressing the loss-making processes and entropy generated as the air flows through the engine. For example, a more efficient compressor has lower losses, generates less entropy and contributes less to the temperature of the exhaust leaving the engine. Another example is the transfer of energy from an engine to air bypassing the engine. In the case of a high bypass engine there is a large proportion (~90%) of barely-warm (~60 °F warmer than ambient) thrust-producing air with only a 10% contribution from the much hotter exhaust from the power-producing core engine. As such, Struchtrup et al. show the benefit of the high bypass turbofan engine from an entropy-reducing perspective instead of the usual propulsive efficiency advantage.
The power expenditure to produce thrust consists of two parts, thrust power from the rate of change of momentum and aircraft speed, and the power represented by the wake kinetic energy.
Entropy, identified as 's', is introduced here because, although its mathematical meaning is acknowledged as difficult, its common representation on a Temperature~entropy (T~s) diagram for a jet engine cycle is graphical and intuitive since its influence is shown as areas of the diagram. The T~s diagram was invented to help engineers responsible for the operation of steam engines to understand the efficiency of their engines. It supplemented the already-existing p~v diagram which only gave half the heat engine efficiency story in only showing the cylinder work done with no reference to the heat supplied and wasted in doing so. 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.Transactions The Manchester Association of Engineers 1904, The Temperature-Entropy Diagram, Mr. G. James Wells, p. 237 It would show areas representative of heat converted to work compared to heat supplied (thermal efficiency).
The mathematical meaning of entropy, as applicable to the gas turbine jet engine, may be circumvented to allow use of the term in connection with the T~s diagram: Quoting Frank Whittle:"Gas Turbine Aero-thermodynamics", Sir Frank Whittle, , p. 2 "Entropy is a concept which many students have a difficulty in assimilating. It is a somewhat intangible quantity...". Entropy is generated when energy is converted into an unusable form analogous to the loss of energy in a waterfall where the original potential energy is converted to unusable energy of turbulence.
Cumpsty says "... a rise in entropy is a loss in the capability of turning thermal energy into work".
Denton compares it with aircraft drag, which is intuitive, "For an aircraft the ultimate measure of lost performance is the drag of its components....entropy creation reflects loss of efficiency in jet engines".Entropy Generation In Turbomachinery Flows"'Denton, SAE 902011, p. 2251 He uses an analogy which imagines any inefficiency mechanism, such as the creation of whirls in the airflow, as producing smoke. Once created it cannot be destroyed and the concentration at the exit of the engine includes contributions from all loss-producing sources in the engine. The loss of efficiency is proportional to the concentration of the smoke at the exit."Loss mechanisms in Turbomachines" Denton, ASME 93-GT-435, p. 4
Thrust is generated inside a jet engine by internal components as they energize a gas stream. Fuel energy released in the combustor is accounted for in two main categories: acceleration of the mass flow through the engine and residual heat. Acceleration of the flow through the engine causes simultaneous production of kinetic energy accompanying the thrust-producing backward momentum. The kinetic energy is left behind the engine without contributing to the thrust power'Jet Propulsion For Airplanes', Buckingham, NACA report 159, p. 85 and is known as residual velocity loss. The thrust force from a stationary engine becomes thrust power when an aircraft is moving under its influence.
Zhemchuzhin et al. show an energy balance for a turbojet engine in flight in the form of a Sankey diagram. Component losses leave the engine as waste heat and add to the heat rejected area on a T~s diagram reducing the work area by the same amount.