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Jet Engine Basics, Metrics, and Technological Trends 13 Therefore, a high-BPR turbofan engine has a relatively large diameter fan, which handles much more air than the high-pressure compres- sor it precedes. These high-BPR turbofans are significantly more fuel-efficient than turbojets or low-BPR turbofans. This increased ef- ficiency makes the added size and complexity of a large fan and cor- responding low-pressure turbine cost effective for many applica- tions.4 On the other hand, high-BPR turbofans have large diameters and relatively low thrust-to-weight ratios, requiring large nacelles on wings or large ducts through fuselages. This is incompatible with air- craft designed for supersonic flight due to the high drag and weight implications. Instead, fighter engines are typically designed with low BPRs (typically 0.3 to 0.8) to strike a balance between engine effi- ciency, diameter, and weight. Turboprop and turboshaft engines also operate on variations of the Brayton cycle. These engines have cores similar to turbojet and tur- bofan cores. In addition, they typically have a low-pressure turbine that extracts most of the remaining available energy from the com- bustion products after they leave the core. This low-pressure turbine turns a shaft, which is not connected to a fan or compressor. Instead, this shaft is used to drive a propeller (turboprop) or a helicopter rotor (turboshaft).5 Intuitively, it may be helpful to think of a turboprop as a turbofan with an extraordinarily large bypass ratio but without a nacelle around the propeller to form the bypass duct. At times, the visible presence of a propeller or rotor leads some to incorrectly as- ______________ 4It is instructive to understand why a turbofan (especially a high-BPR turbofan) im- proves fuel efficiency. This is best understood by considering the definitions of kinetic energy (kinetic energy = mV2) and momentum (momentum = mV) in the light of the thrust equation presented earlier (Equation 1). In these definitions, m is the mass of a moving object and V is its velocity. When fuel is burned to heat the air flowing through a jet engine, it increases the flow’s internal energy, which is partially converted to ki- netic energy in the engine’s nozzle. Depending upon the bypass ratio of an engine de- sign, a given change in kinetic energy can take the form of a small mass of air undergo- ing a large increase in V2, or a large mass undergoing a small increase in V2. However, as Equation 1 reveals, thrust is produced in proportion to the change in velocity through the engine, not the change in velocity squared (in other words, thrust increases in pro- portion to the increase in momentum [mV] rather than the increase in kinetic energy [mV2]). When the fuel’s energy is used to create a very large V2, the thrust increases only by the square root of this increase (V). Therefore, it is most efficient to accelerate a large amount of air by a small increase in velocity, leading engine manufacturers to design turbofans with a high BPR, if practical for the aircraft’s mission. 5Turboshafts are also used to drive other devices, such as the M-1 tank, Navy ships, and Brayton cycle power plants.PDF Image | Military Jet Engine Acquisition Technology Basics and Cost-Estimating Methodology
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