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{{Short description|Measurement indicator of fuel conversion}}
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 engine]]s, jet engines tend to not be particularly efficient (<50%); a lot of the fuel is "wasted".{{citation needed|date=February 2025}} In the 1970s, rconomic 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'<ref>Gas Turbine Performance, Second Edition, Walsh and Fletcher 2004, {{ISBN|0 632 06434-X}}, Preface</ref> and, as such, criteria include thrust, (specific) fuel consumption, [[time between overhauls]], [[power-to-weight ratio]]. Some major factors affecting efficiency include the engines [[overall pressure ratio]], it's [[bypass ratio]] and the turbine inlet temperture.
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.{{citation needed|date=February 2025}}
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). EPR and N1 are indicators for thrust, wheras EGT is vital for gauging the health of the engine<ref>"EGT margin indicates engine health"' pp. 5–11, Safety first The Airbus Safety magazine, February 2022</ref>, as it rises progressively with engine use over thousands of hours, as parts wear, until it the engines has the be overhauled.
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