Jet engine performance

A jet engine performs by converting fuel into thrust. How well it performs is an indication of what proportion of its fuel goes to waste. It transfers heat from burning fuel to air passing through the engine. In doing so it produces thrust work when propelling a vehicle but a lot of the fuel is wasted and only appears as heat. Propulsion engineers aim to minimize the degradation of fuel energy into unusable thermal energy. Increased emphasis on performance improvements for commercial airliners came in the 1970s from the rising cost of fuel.

The meaning of jet engine performance has been phrased as 'the end product that a jet engine company sells' and, as such, criteria include thrust and fuel consumption, life, weight, emissions, diameter and cost. 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. Categories of performance include performance improvement, performance deterioration, performance retention, bare engine performance (uninstalled) and performance when part of an aircraft powerplant (installed).

Jet engine performance (thrust and fuel consumption) for a pilot is displayed in the cockpit as engine pressure ratio (EPR) and exhaust gas temperature (EGT) or fan speed (N1) and EGT. EPR and N1 are indicators for thrust. EGT is an indicator for fuel flow but more importantly is a health monitor as it rises progressively with engine use over thousands of hours, as parts wear, until it reaches a limiting value.

The performance of an engine is calculated using a thermodynamic analysis of the engine cycle. It works out what happens inside the engine. The conditions inside the engine, together with the fuel used and thrust produced, may 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.

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. 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 $$\eta_{th}$$) and the ship's propeller (propulsive or Froude efficiency $$\eta_{pr}$$).

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 ( $$\eta_o = \eta_{th} \eta_{pr}$$).

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.

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. 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: "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". 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.

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 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.

The engine does work on the air going through it and this work is in the form of an increase in kinetic energy. The increase in kinetic energy comes from burning fuel and the ratio of the two is the thermal efficiency which equals increase in kinetic energy divided by the thermal energy from the fuel (fuel mass flow rate x lower calorific value). 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. The remainder of the T~s diagram expansion work is available for propulsion, but not all of which produces thrust work since it includes the residual kinetic energy or RVL.

The losses in the three areas for performance improvement, which are the gas generator, the parts transferring power to the bypass and the wake power, are each combined in their own efficiencies, core, transfer and propulsive. Also, all three are combined in an overall efficiency which is obtained by multiplying together the core thermal efficiency, the transfer efficiency and the propulsive efficiency, $$\eta_o = \eta_{th} \eta_{tr} \eta_{pr}$$

Jet engine configurations
Each of the jet engines, ramjet, turbojet, afterburning turbojet, turbofan and afterburning turbofan has a different set of components which compress, heat and expand the air passing through. The compression part of the cycle may come from just a compressor with no moving parts (the ramjet inlet/diffuser) or an aircraft inlet and engine compressor. Afterburning takes place in an additional combustor. The expansion part takes place in a nozzle, usually preceded by turbines. For turbofans energy transfer using a turbine and fan takes place from the core to bypass air.

Thrust and fuel consumption
Thrust and fuel consumption are key performance indicators for a jet engine. Improvements in thrust and fuel consumption are widely quoted for a new engine design compared to a previous to show that new technology has been incorporated which reduces fuel consumption. As an example the Pearl 10X turbofan has been reported as producing 8% more thrust and using 5% less fuel than the BR725. Thrust and fuel consumption are combined in a single measure, specific fuel consumption (SFC), which reflects the level of technology used in the engine since it is fuel needed to produce one pound or Newton of thrust regardless of engine size. Two engines separated by about fifty years of gaining knowledge in jet engine design, the Pratt & Whitney JT3C and the Pratt & Whitney 1100G, illustrate a 50% reduction in SFC from 26 to 13 mg/Ns.

Thrust is developed inside the engine as the components energize the gas stream. The same thrust value manifests itself without consideration of what is happening inside the engine. Treating the engine as a black box thrust is calculated knowing the mass flow rate and velocity of the air entering the engine and the increased velocity of the exhaust leaving the engine. Observing this increase implies a rearward accelerating force has been applied to the gas inside the engine. Thrust is the equal and opposite reaction on the engine internal parts which is transferred to the aircraft through the engine mounts.

Engine pressure ratio (EPR), low-pressure compressor speed (N1) and exhaust gas temperature (EGT)
EPR or N1 are used as cockpit indicators for thrust because one or the other, depending on the preference of the engine maker, is a valid alternative for thrust which is not measured in an aircraft. As such they are known as thrust setting parameters. N1 is preferred by General Electric Aviation and CFM International and EPR is preferred by Pratt & Whitney and Rolls-Royce. The meaning of EPR for a turbojet, compares the pressure in the jetpipe to the pressure outside the engine and the rise in pressure is the result of the pumping action of the engine. The combined action of the engine and an added nozzle is to produce thrust. The function of the basic engine (compressor, combustor and turbine) is to pump air to a pressure higher than that of the surrounding air. It is then accelerated by passing it through a constricted area known as a nozzle. For a bypass engine with two separate nozzles the pressures in each are weighted relative to the nozzle areas. As such the Rolls-Royce RB211 thrust indicator is known as integrated EPR (IEPR). Thrust is easily controlled by regulating airflow and since all of the airflow is pumped by the fan N1 is used for setting thrust by General Electric Aviation.

The EGT is a cockpit indicator for fuel flow since the fuel burned in the combustor determines the turbine entry temperature, which cannot be reliably measured, and EGT is a suitable alternative. Any deterioration from the engine as-new condition will require more fuel, resulting in higher temperature gas, to produce the thrust. 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. It progressively uses more fuel, until parts have to be replaced to restore the original lower operating temperature and reduce the cost of buying fuel.

Cockpit performance indicators may be misleading
Although EPR is directly related to thrust over the flight envelope American Airlines experience with their first jet engines, Pratt & Whitney JT3C, was marred by instrumentation problems so the cockpit reading was questioned and other parameters, FF and N1, were used by flight personnel in desperation.

EPR is based on pressure measurements with the sampling tubes vulnerable to getting blocked. Air Florida Flight 90 crashed on take-off in snow and icing conditions. The required take-off thrust was 14,500 lb which would normally be set by advancing the thrust levers to give an EPR reading of 2.04. Due to EPR probe icing the value set, i.e. 2.04, was erroneous and actually equivalent to 1.70 which gave an actual thrust of only 10,750 lb. The slower acceleration took 15 seconds longer than normal to reach lift off speed and contributed to the crash.

EGT readings can also be misleading. The temperature of the gas leaving the turbine increases with engine use as parts become worn but the Strategic Air Command approved J57 and TF33 engines for flight without knowing they had bent and broken turbine parts. They were misled by low-reading EGT which indicated, when taken at face value, that the engines were in acceptable condition. It was found that the EGT probes were not positioned correctly to sample a representative gas temperature for the true condition of the engine.

Performance improvement
Performance from an SFC viewpoint, rather than weight or size say, is the overall energy conversion efficiency of the whole powerplant, or the degree to which waste is minimized. The overall efficiency of the whole powerplant depends on the efficiencies of the constituent parts which all produce waste.

Performance improvement of the jet engine, first as a turbojet and then as a turbofan, has come from continuous increases in pressure ratio (PR) and component efficiencies, reduced pressure losses and from materials development which, together with cooling technologies, has allowed higher turbine inlet temperatures (TIT). It has also come from reduced leakage from the gas path because only the gas flow over the airfoil surfaces contributes to thrust. Increases in TIT mean a higher power output which for a turbojet leads to too high exhaust velocities for subsonic flight. For subsonic aircraft the high core power available from increased TIT is used to drive a large fan which adds less kinetic energy to a large amount of air. Kinetic energy is the unwanted byproduct, known as residual velocity loss, of increasing momentum which produces thrust. The aim of the propulsion engineer is to minimize the conversion or degradation of energy into heat rather than thrust work. Piston engines used some of their waste heat with turbocharging and turbo-compounding. Some was used for thrust from rearwards facing exhaust stubs. The waste heat from a jet engine cannot be used so performance is improved by reducing the amount produced while the air is passing through the engine. This includes loss in total pressure from entropy production in ducts as explained by Sullivan: Irreversibility or entropy production is a measure of the destruction in the conversion of energy from a high quality form to a low quality form. Fluid flow in a duct with high kinetic energy is a high quality energy datum and the boundary layer converts some of the kinetic energy to a lower quality form thermal energy.

A reason for increasing bypass when core power has been increased is given by Hartmann: Higher specific output, ie greater conversion of heat from fuel to KE of a jet, is poor exploitation of the KE needed for the production of thrust due to high energy losses at the outlet.

Increased overall pressure ratio
Increased pressure ratio is an improvement to the thermodynamic cycle because combustion at a higher pressure has a reduced entropy rise which is the basic reason for pursuing higher pressure ratios in the jet engine cycle which is known as the Brayton cycle. Increased pressure ratio can be achieved by using more stages or increasing the stage pressure ratio. The significance of higher pressure ratio to fuel consumption was demonstrated in 1948 when the J57 (12:1) was selected for the Boeing B-52 Stratofortress in place of a turboprop. Boeing previous experience with turbojet specific fuel consumptions up to that time was the General Electric J47 (5.4:1), used in the Boeing B-47 Stratojet, which initially led to the turboprop decision.

The radial flow compressor was widely used for early turbojet engines but advantages in performance that came with the axial compressor in terms of pressure ratio, SFC, specific weight and thrust for each square foot of frontal area were presented in 1950 by Hayne Constant However, a radial flow compressor is still the best choice for small turbofans as the last high pressure stage because the alternative very small axial stages would be too easily damaged and inefficient with tip clearance being significant compared to the blade height.

Enabling technologies for high overall pressure ratio
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. As soon as conditions change from the design point the blade incidence angle will change away from a low-loss value and ultimately the compressor will no longer operate in a stable manner. The deviations from design are acceptable if the compressor doesn't have to raise the air pressure too much, say to 5 atmospheres. For greater values variable features have to be incorporated which change the compressor geometry below the design speed. Engines that came after the J47 with its 5.4:1 PR had compressors with higher PRs that needed some form of variable feature which operated at low speeds to prevent front stage stall and flutter failure and rear stage choking. These were valves which opened to release air when all the stages could not pass the same flow and variable angle vanes to maintain acceptable velocity triangles made up from the velocity of the approaching air, blade velocity and the relative velocity of air to blade. Alternatively the compressor was split into two separately rotating compressors each with a low pressure ratio such as the J57 with 3.75 LP x 3.2 HP = 12:1 overall. Bleed valves, variable blade angles and split compressors are used together on modern engines to achieve high pressure ratios. The Rolls-Royce Trent 700 from the 1990s, with a pressure ratio of 36:1 and 3 separate compressor rotors, needs 3 rows of variable blades and 7 bleed valves.

In the beginning higher pressure ratios had to be obtained with many stages because stage pressure ratios were low, about 1.16 for the J79 compressor which needed 17 stages. Modern compressors have a higher PR per stage and still require the same variable features. The CFM International LEAP engine HP compressor with a PR 22:1 from 10 stages needs variable inlet guide vanes and 4 stages of variable stator vanes. The overall pressure ratio for an engine is limited by the temperature that goes with it. A compressor outlet temperature of about 900 K is the limit which is determined by material suitability in terms of weight and cost.

Increased stage pressure ratio
Air compression in a gas turbine is achieved by converting a proportion of the kinetic energy (compressor rotor generated, either by a centrifugal impeller or an axial row) of the air into static pressure one stage at a time. Most early jet engines used a single-stage centrifugal compressor with pressure ratios such as 3.3:1 (de Havilland Goblin). Higher pressure ratios came with the axial compressor because although stage pressure ratios were very low in comparison (1.17:1 BMW 003) more stages could be used as required for a higher overall pressure ratio. More advanced centrifugal stages are used in small turbofans as the last high-pressure stage behind axial stages (Pratt & Whitney Canada PW300 and others). The same technology level produces 8:1 when used as the only stage in Pratt & Whitney PW200 helicopter engines. A centrifugal stage consists of an impeller and diffuser vanes, or alternatively diffuser pipes which are considered to give less blockage as the static pressure rises with diffusion.

An axial compressor consists of alternating rows of rotating and stationary diffusers, each pair being a stage. These diffusers are diverging as necessary for subsonic flow. The channel formed by adjacent blades, amount of diffusion, is adjusted by varying their angle relative to tangential, known as stagger angle. More diffusion gives a higher pressure ratio but flow in compressors is very susceptible to flow separation because it is going against a rising pressure (gas naturally flows from high to low pressure). Stage pressure ratio had increased by 2016 such that 11 stages could achieve 27:1 (GE9X high pressure compressor).

Low aspect ratio compressor blades, with their better efficiency both aerodynamically and structurally, were introduced in the 1950s turbojet the Tumansky R-11, and subsequently examples of wide chord fan blades introduced in 1983 in the Garrett TFE731-5 and in 1984 in the RB211-535E4 and Pratt & Whitney Canada JT15D-5.

Fan efficiency
Fan blades on modern engines have a wide chord which replaced conventional narrow chord blades which needed snubbers, or shrouds, to prevent them vibrating to an unacceptable degree. Increasing the length of the chord by an amount which made the blade stiff enough to not require snubbers also made the blade more resistant to damage caused by bird, hail and ice ingestion, and brought several unrelated benefits of improved efficiency, surge margin and noise reductions. There is also a greater axial distance for centrifuging debris away from the compressor inlet to prevent erosion of the airfoil surfaces which lowers compressor efficiency.

Combustion
The effects of heat transfer and friction in a combustor, both engine and afterburner, cause a loss of stagnation pressure and an increase in entropy. The loss in pressure is shown on a T~s diagram where it can be seen to reduce the area of the work part of the diagram. The pressure loss through a combustor has two contributions. One due to bringing the air from the compressor into the combustion area including through all the cooling holes (friction pressure loss), that is with air flowing but no combustion taking place. The addition of heat to the flowing gas adds another type of pressure loss (momentum pressure loss).

In addition to stagnation pressure loss the other measure of combustion performance is incomplete combustion. 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. In the 1990s reduction of nitrogen oxides (NOx) became the focus due to its contribution to smog and acid rain for example. Combustor technology for reducing NOx is the Rich burn, Quick mix, Lean burn (RQL) introduced by Pratt & Whitney with the TALON (Technology for Advanced Low NOx) PW4098 combustor. RQL technology is also used in the Rolls-Royce Phase 5 Trent 1000 combustor and the General Electric LEC (Low Emissions Combustor).

Engine combustor configurations are reverse-flow separate, straight-through separate, can-annular (all three historic because the annular flow chamber gives more area and more even flow to the turbine), and modern annular and reverse-flow annular. 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).

Examples of early jet engines with centrifugal compressors, the Rolls-Royce Welland and General Electric J31, used reverse-flow combustors. More modern small jet engines incorporating a centrifugal final compressor stage also use reverse-flow combustors and range from the 1,000 lbf thrust Pratt & Whitney Canada PW600 in the 6,000 lb Eclipse 500 very light jet to the 7,000 lbf thrust Lycoming ALF 502 in the 97,000 lb British Aerospace 146 airliner.

Reduced pressure loss in ducts
Air passing through the engine goes through two components where velocities need to be high, of the order of the speed of sound. They are the components in which work is done, the compressor and turbine. In all the remaining components no work is done and the need to reduce pressure losses requires lower Mach numbers. These components are the engine combustor and afterburner, and the connecting ducting between components such as the tailpipe between the turbine and propelling nozzle.

The first duct in the powerplant is the inlet and loss in total pressure in front of the engine is particularly important because it appears twice in the production of thrust. Thrust is proportional to mass flow which is proportional to total pressure. Jet nozzle pressure and therefore thrust is also proportional to the total pressure at engine entry. In subsonic inlets the only total pressure losses are those due to friction along the duct passage walls. For supersonic inlets shockwave losses are also present and shockwave systems are required to minimize pressure loss with increasing supersonic Mn. Additional losses in total pressure come with boundary layer growth as the flow slows down. Boundary layers have to be removed before the location of the terminal shock to prevent shock-induced separation and excessive loss.

Leakage control
The jet engine has many sealing locations, more than fifty in a large engine. The cumulative effect of leakage on fuel consumption can be significant. Gas path sealing affects engine efficiency and became increasingly more important as higher pressure compressors were introduced.

There are unwanted leaks from the primary gas path and necessary bleeds from the compressor which enter the secondary or internal flow system. They are all controlled by seals with design clearances. When seals rub and wear, opening up clearances, there is performance deterioration (increased fuel consumption).

Sealing of the stators was initially accomplished using knife-edge fins on the rotating part and a smooth surface for the stator shroud. Examples are the Avon and Tumansky R-11. With the invention of the honeycomb seal the labyrinth seal has an abrazive honeycomb shroud which is easily cut by the rotating seal teeth without overheating and damaging them. Labyrinth seals are also used in the secondary air system between rotating and stationary parts. Example locations for these are shown by Bobo. Tip clearance between compressor and turbine blades and their cases is a significant source of performance loss. Much of the loss in compressors is associated with tip clearance flow. For a CFM56 engine an increase in high pressure turbine tip clearance of 0.25 mm causes the engine to run 10 °C hotter (reduced efficiency) to attain take off thrust. Tip clearances have to be big enough to prevent rubbing when they tend to close up during carcase bending, case distortion from thrust transfer, centre-line closure when the compressor case shrinks onto the rotor diameter( rapid reduction in temperature of air entering the engine), thrust setting changes (controlled by Active Clearance Control using compressor rotor cooling and turbine case cooling).

Tip clearance changes with thrust changes
An engine is designed to run steady state at design points such as take-off, climb, and cruise with running clearances which minimize fuel use. 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. During this time clearances between parts may close up to rubbing contact and wear to give larger clearances, and fuel consumption, at the important stabilized condition. This scenario inside the engine is prevented by internal compressor bore cooling and external turbine casing cooling on big fan engines (active clearance control).

Sealing at blade tips and stator shrouds
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. P&W considered 12:1 could be achieved but during pre-J57 development testing a compressor with 8:1 was tested and the leakage was so high that no useful work would have been produced. One benefit of the subsequent wasp waist was reduced leakage from the reduced sealing diameter. In 1954 a GE engineer invented a very effective sealing scheme, the honeycomb seal which reduces substantially the rubbing contact area and temperatures generated. The rotating part cuts into the cellular structure without being permanently damaged. It is widely used today. The primary gas flow through the compressor and turbine has to follow the airfoil surfaces to exchange energy with the turbomachinery. Any flow leaking past the blade tips generates entropy and reduces the efficiency of the compressor and turbine. Interlocking shrouds are present on the tips of low pressure turbine blades to provide an outer band to the flowpath which reduces tip leakage. Leakage is further reduced with the addition of seal teeth on the outer periphery of the shrouds which rub into open cell honeycomb shrouds.

Tip clearance with backbone bending and case out-of-roundness
The advent of the high bypass civil engines, JT9D and CF6, showed the importance of thrust take off locations on the engine cases. Also, large engines have relatively flexible cases inherent in large diameter flight-weight structures giving relatively large relative displacements between heavy stiff rotors and the flexible cases. Case distortion with subsequent blade tip rubbing and performance loss appeared on the JT9D installation in the Boeing 747 as a result of thrust being taken from a single point on top of the engine exhaust case. Thrust from the rear mount plane was a Boeing requirement. Compared to the 15,000 lb thrust JT3D with its four structural cases the 40,000 lb thrust JT9D made economical use of supporting structure with only three structural cases making a compact lightweight design. During flight testing the engines suffered violent surges and loss in performance which were traced to bending of the engine backbone by 0.043 in. at the combustor case and the turbine case going out-of-round which in turn caused blade tip rubs and increased tip clearance.

The three big fan engines introduced in the 1960s for wide-body airliners, Boeing 747, Lockheed Tristar, DC-10, had much higher thrust and size compared to the engines powering the previous generation of airliners. The JT9D and CF6 showed that rotor tip clearances were sensitive to the way the engines were mounted and performance was lost through rotor tip rubs due to backbone bending and local distortion of casings at the point of thrust transfer to the aircraft pylon. At the same time the RB211 performance didn't deteriorate so fast due to its shorter, more rigid, three-shaft configuration. For the Boeing 777 the Trent 800 and GE90 would incorporate two-point mounting for ovalization reduction.

The first high bypass fan engine, the TF39, transferred its thrust to the C5 pylon from the rear mount. It was a single point thrust location on the turbine mid-frame which locally distorted the casings, causing out of roundness of the turbine stators, increased clearances and a performance loss. The CF6-6, derived from the TF39 had thrust taken for the DC-10 from the front mount plane but also from a single point. This also caused single point distortion and unacceptable performance loss for the airliner. The distortion was reduced by taking thrust from two points which allowed smaller compressor running clearances and better SFC.

Internal or secondary air system
The use of air for internal systems increases fuel consumption so there is a need to minimize the airflow required. The internal air system uses secondary air for cooling, keeping oil in bearing chambers, to control bearing thrust load for bearing life, and preventing hot gas ingestion from turbine gas flow into disc cavities. It is a cooling system which uses airflow to transfer heat away from hot parts and maintain them at a temperature which ensures the life of parts such as turbine discs and blades. 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. It is used to cool or heat parts to control radial clearances (clearance control system). Early radial compressor engines used supplementary means for cooling air, for example a dedicated impeller or a fan machined integral with the turbine disc. The air sources for axial engines are different stages along the compressor depending on the different air system pressure requirements. Use of a single stage impeller as the last high pressure stage on small turbofan engines gives the flexibility of three different source pressures from the single stage, impeller entry, halfway through the stage (impeller tip) and diffuser exit (at combustor pressure). The air system sinks are the primary gas path where turbine cooling air is returned, for example, and the oil system vent overboard.

Performance deterioration
Gas path deterioration and increasing EGT coexist. As the gas path deteriorates the EGT limit ultimately prevents the take-off thrust from being achieved and the engine has to be repaired. The engine performance deteriorates with use as parts wear, meaning the engine has to use more fuel to get the required thrust. A new engine starts with a reserve of performance which is gradually eroded. The reserve is known as its temperature margin and is seen by a pilot as the EGT margin. For a new CFM International CFM56-3 the margin is 53 °C. Kraus gives the effect on increased fuel consumption of typical component degradation during service.

American Airlines experience with the JT3C turbojet included cracking and bowing of the turbine nozzle guide vanes which adversely affected the gas flow to the rotating turbine blades causing increased fuel consumption. More significant was erosion of turbine parts by hard carbon lumps which formed around the fuel nozzles and periodically breaking away and striking and eroding turbine blades and nozzle guide vanes causing loss of EGT margin.

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. The rising cost of fuel and a new awareness on conservation of energy led to a need to understand which type and amount of component degradation caused how much of an increase in fuel consumption. 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. Gas path surfaces in the fan and compressor were found to be coated with deposits of dirt, salt and oil which increased surface roughness and caused performance loss. A compressor wash on a particular Pratt & Whitney JT8D bypass engine reduced the fuel consumption by 110 pounds of fuel for every hour run.

Clearances between rotating and stationary parts are required to prevent contact. Increasing clearances, which occur in service as a result of rubbing, reduce the thermal efficiency which shows up when the engine uses more fuel than before. An American Airlines test on a Pratt & Whitney JT3D engine found that increasing the HP turbine tip clearance by 0.031 inch caused a 0.9% increase in fuel used.

The advent of the high bypass engines introduced new structural requirements necessary to prevent blade rubs and performance deterioration. 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. The JT8D had good performance retention with its moderate turbine temperature and stiff structure. Rigid case construction installed engine not adversely affected by axial bending loads from inlet on TO rotation. The engine had relatively large clearances between rotating and stationary components so compressor and turbine blade tip rubs were not significant and performance degradation came from distress to the hot section and compressor blade increasing roughness and erosion.

Emissions
The connection between emissions and fuel consumption is the combustion inefficiency which wastes fuel. Fuel should be completely burned so all chemical energy is liberated as heat. The formation of pollutants signifies that fuel has been wasted and more fuel is required to produce a particular thrust than would otherwise be.

Noise
Noise influences the social acceptability of aircraft and maximum levels measured during takeoff and approach flyover are legislated around airports. Military aircraft noise is the subject of complaints from people living near military airfields and in remote areas under the flight paths of low level training routes. Prior to the introduction into service of the first jet airliners noise was already the subject of citizen actions around airports due to unacceptable noise from the last generation of piston-engined airliners such as xxx. Forewarned early operators of jet airliners introduced their services with noise abatement takeoff procedures, Comet Caravelle,

Passenger cabin and cockpit noise in civil aircraft and cockpit noise in military aircraft has a contribution from jet engines both as engine noise and structure-borne noise originating from engine rotor out of balance.

Starting time
Starting time is the time taken from initiating the starting sequence to reaching idle speed. A CFM-56 typical start time is 45–60 seconds. Starting time is a flight safety issue for airstarts because starting has to be completed before too much altitude has been lost.

Weight
The weight of an engine is reflected in the weight of the aircraft and introduces some drag penalty. Extra engine weight means a heavier structure and reduces aircraft payload.

Size
The size of an engine has to be established within the engine installation envelope agreed during the design of the aircraft. The thrust governs the flow area hence size of the engine. A criterion of pounds of thrust per square foot of compressor inlet is a figure of merit. The first operational turbojets in Germany had axial compressors to meet a 1939 request from the German Air Ministry to develop engines producing 410 lb/sq ft.

Cost
A lower fuel consumption engine reduces airline expenditure on buying fuel for a given fuel cost. Deterioration of performance(increased fuel consumption) in service has a cumulative effect on fuel costs as the deterioration and rise in consumption is progressive. The cost of parts replacement has to be considered relative to the saving in fuel.

Clarifying momentum, work, energy, power
A basic explanation for the way burning fuel results in engine thrust uses terminology like momentum, work, energy, power and rate. Correct use of the terminology may be confirmed by using the idea of fundamental units which are mass M, length L and time T, together with the idea of a dimension, i.e. power, of the fundamental unit, say L1 for distance, and in a derived unit, say speed which is distance over time, with dimensions  L1 T −1 The object of the jet engine is to produce thrust which it does by increasing the momentum of the air passing through it. But thrust isn't caused by the change in momentum. It's caused by the rate of change in momentum. So thrust, which is a force, has to have the same dimensions as rate of change of momentum, not momentum. Efficiences may be expressed as ratios of energy rate or power which has the same dimensions.

Force dimensions are M1 L1 T−2, momentum has dimensions M1L1 T−1 and rate of change of momentum has dimensions M1 L1T−2, ie the same as force. Work and energy are similar quantities with dimensions M1 L2T−2. Power has dimensions M1 L2T−3.