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50-506: In a conventional turbofan, a single shaft (the "low-pressure" or LP shaft) connects the fan, the low-pressure compressor and the low-pressure turbine (a second concentric shaft connects the high-pressure compressor and high-pressure turbine). In this configuration, the maximum tip speed for the larger radius fan limits the rotational speed for the LP shaft and thus the LP compressor and turbine. At high bypass ratios (and thus also high radius ratios)

100-423: A bypass stream introduces extra losses which are more than made up by the improved propulsive efficiency. The turboprop at its best flight speed gives significant fuel savings over a turbojet even though an extra turbine, a gearbox and a propeller were added to the turbojet's low-loss propelling nozzle. The turbofan has additional losses from its extra turbines, fan, bypass duct and extra propelling nozzle compared to

150-403: A continuous flow of electrolyte. Flow cells typically have the fuel dissolved in the electrolyte. Power-to-weight ratios for vehicles are usually calculated using curb weight (for cars) or wet weight (for motorcycles), that is, excluding weight of the driver and any cargo. This could be slightly misleading, especially with regard to motorcycles, where the driver might weigh 1/3 to 1/2 as much as

200-406: A cutoff voltage are typically specified for a battery by its manufacturer. The output voltage falls to the cutoff voltage when the battery becomes "discharged". The nominal output voltage is always less than the open-circuit voltage produced when the battery is "charged". The temperature of a battery can affect the power it can deliver, where lower temperatures reduce power. Total energy delivered from

250-467: A fluid, or storage in a pressure vessel . A variety of effects can be harnessed to produce thermoelectricity , thermionic emission , pyroelectricity and piezoelectricity . Electrical resistance and ferromagnetism of materials can be harnessed to generate thermoacoustic energy from an electric current. All electrochemical cell batteries deliver a changing voltage as their chemistry changes from "charged" to "discharged". A nominal output voltage and

300-563: A higher discharge current – and therefore higher power-to-weight ratio – but only with a lower energy capacity. Power-to-weight ratio for batteries is therefore less meaningful without reference to corresponding energy-to-weight ratio and cell temperature. This relationship is known as Peukert's law . Capacitors store electric charge onto two electrodes separated by an electric field semi-insulating ( dielectric ) medium. Electrostatic capacitors feature planar electrodes onto which electric charge accumulates. Electrolytic capacitors use

350-406: A liquid electrolyte as one of the electrodes and the electric double layer effect upon the surface of the dielectric-electrolyte boundary to increase the amount of charge stored per unit volume. Electric double-layer capacitors extend both electrodes with a nanoporous material such as activated carbon to significantly increase the surface area upon which electric charge can accumulate, reducing

400-446: A misnomer, as it colloquially refers to mass. In a zero-gravity (weightless) environment, the power-to-weight ratio would not be considered infinite. A typical turbocharged V8 diesel engine might have an engine power of 250 kW (340 hp) and a mass of 380 kg (840 lb), giving it a power-to-weight ratio of 0.65 kW/kg (0.40 hp/lb). Examples of high power-to-weight ratios can often be found in turbines. This

450-647: A period of time is equal to the difference in its total energy over that period of time, so the rate at which work is done is equal to the rate of change of the kinetic energy (in the absence of potential energy changes). The work done from time t to time t + Δ t along the path C is defined as the line integral ∫ C F ⋅ d x = ∫ t t + Δ t F ⋅ v ( t ) d t {\displaystyle \int _{C}\mathbf {F} \cdot d\mathbf {x} =\int _{t}^{t+\Delta t}\mathbf {F} \cdot \mathbf {v} (t)dt} , so

500-628: A power-to-weight ratio of 153 kW/kg (93 hp/lb). In classical mechanics , instantaneous power is the limiting value of the average work done per unit time as the time interval Δ t approaches zero (i.e. the derivative with respect to time of the work done). The typically used metric unit of the power-to-weight ratio is W kg {\displaystyle {\tfrac {\text{W}}{\text{kg}}}\;} which equals m 2 s 3 {\displaystyle {\tfrac {{\text{m}}^{2}}{{\text{s}}^{3}}}\;} . This fact allows one to express

550-481: A requirement for an afterburning engine where the sole requirement for bypass is to provide cooling air. This sets the lower limit for BPR and these engines have been called "leaky" or continuous bleed turbojets (General Electric YJ-101 BPR 0.25) and low BPR turbojets (Pratt & Whitney PW1120). Low BPR (0.2) has also been used to provide surge margin as well as afterburner cooling for the Pratt & Whitney J58 . In

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600-463: A single charge cycle is affected by both the battery temperature and the power it delivers. If the temperature lowers or the power demand increases, the total energy delivered at the point of "discharge" is also reduced. Battery discharge profiles are often described in terms of a factor of battery capacity . For example, a battery with a nominal capacity quoted in ampere-hours (Ah) at a C/10 rated discharge current (derived in amperes) may safely provide

650-401: A speed | v ( t ) | {\displaystyle |\mathbf {v} (t)|\;} and angle ϕ {\displaystyle \phi \;} with respect to the centre and radial of a gravitational field by an onboard powerplant , then the associated kinetic energy is where: The work–energy principle states that the work done to the object over

700-667: A train. As the coefficient of friction between steel wheels and rails seldom exceeds 0.25 in most cases, improving a locomotive's power-to-weight ratio is often counterproductive. However, the choice of power transmission system, such as variable-frequency drive versus direct-current drive , may support a higher power-to-weight ratio by better managing propulsion power. Most vehicles are designed to meet passenger comfort and cargo carrying requirements. Vehicle designs trade off power-to-weight ratio to increase comfort, cargo space, fuel economy , emissions control , energy security and endurance. Reduced drag and lower rolling resistance in

750-522: A vehicle design can facilitate increased cargo space without increase in the (zero cargo) power-to-weight ratio. This increases the role flexibility of the vehicle. Energy security considerations can trade off power (typically decreased) and weight (typically increased), and therefore power-to-weight ratio, for fuel flexibility or drive-train hybridisation . Some utility and practical vehicle variants such as hot hatches and sports-utility vehicles reconfigure power (typically increased) and weight to provide

800-425: A zero-bypass (turbojet) engine the high temperature and high pressure exhaust gas is accelerated by expansion through a propelling nozzle and produces all the thrust. The compressor absorbs all the mechanical power produced by the turbine. In a bypass design, extra turbines drive a ducted fan that accelerates air rearward from the front of the engine. In a high-bypass design, the ducted fan and nozzle produce most of

850-413: Is a calculation commonly applied to aircraft, cars, and vehicles in general, to enable the comparison of one vehicle's performance to another. Power-to-weight ratio is equal to thrust per unit mass multiplied by the velocity of any vehicle. The power-to-weight ratio (specific power) is defined as the power generated by the engine(s) divided by the mass. In this context, the term "weight" can be considered

900-510: Is a measurement of actual performance of any engine or power source. It is also used as a measurement of performance of a vehicle as a whole, with the engine's power output being divided by the weight (or mass ) of the vehicle, to give a metric that is independent of the vehicle's size. Power-to-weight is often quoted by manufacturers at the peak value, but the actual value may vary in use and variations will affect performance. The inverse of power-to-weight, weight-to-power ratio (power loading)

950-462: Is also seen with propellers and helicopter rotors by comparing disc loading and power loading. For example, the same helicopter weight can be supported by a high power engine and small diameter rotor or, for less fuel, a lower power engine and bigger rotor with lower velocity through the rotor. Bypass usually refers to transferring gas power from a gas turbine to a bypass stream of air to reduce fuel consumption and jet noise. Alternatively, there may be

1000-402: Is an important vehicle characteristic that affects the acceleration of sports vehicles. Propeller aircraft depend on high power-to-weight ratios to generate sufficient thrust to achieve sustained flight, and then for speed. Jet aircraft produce thrust directly . Power-to-weight ratio is important in cycling, since it determines acceleration and the speed during hill climbs . Since

1050-527: Is because of their ability to operate at very high speeds. For example, the Space Shuttle 's main engines used turbopumps (machines consisting of a pump driven by a turbine engine) to feed the propellants (liquid oxygen and liquid hydrogen ) into the engine's combustion chamber. The original liquid hydrogen turbopump is similar in size to an automobile engine (weighing approximately 352 kilograms (775 lb)) and produces 72,000 hp (54 MW) for

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1100-417: Is conversely usually lower. Fuel cells and flow cells , although perhaps using similar chemistry to batteries, do not contain the energy storage medium or fuel . With a continuous flow of fuel and oxidant, available fuel cells and flow cells continue to convert the energy storage medium into electric energy and waste products. Fuel cells distinctly contain a fixed electrolyte whereas flow cells also require

1150-418: Is fitted with geared turbofans and is still one of the quietest commercial aircraft. A large part of the noise reduction is due to reduced fan tip speeds. In conventional turbofans the fan tips exceed the speed of sound causing a characteristic drone, requiring sound deadening. Geared turbofans operate the fan at sufficiently low rotational speed to avoid supersonic tip speeds. The first geared turbofan engine

1200-499: Is only delivered if the powerplant is in motion, and is transmitted to cause the body to be in motion. It is typically assumed here that mechanical transmission allows the powerplant to operate at peak output power. This assumption allows engine tuning to trade power band width and engine mass for transmission complexity and mass. Electric motors do not suffer from this tradeoff, instead trading their high torque for traction at low speed. The power advantage or power-to-weight ratio

1250-408: Is quoted for turboprop and unducted fan installations because their high propulsive efficiency gives them the overall efficiency characteristics of very high bypass turbofans. This allows them to be shown together with turbofans on plots which show trends of reducing specific fuel consumption (SFC) with increasing BPR. BPR is also quoted for lift fan installations where the fan airflow is remote from

1300-405: Is then where: The useful power of an engine with shaft power output can be calculated using a dynamometer to measure torque and rotational speed , with maximum power reached when torque multiplied by rotational speed is a maximum. For jet engines the useful power is equal to the flight speed of the aircraft multiplied by the force, known as net thrust, required to make it go at that speed. It

1350-450: Is trading exhaust velocity for extra mass flow which still gives the required thrust but uses less fuel. Turbojet inventor Frank Whittle called it "gearing down the flow". Power is transferred from the gas generator to an extra mass of air, i.e. a larger diameter propelling jet, moving more slowly. The bypass spreads the available mechanical power across more air to reduce the velocity of the jet. The trade-off between mass flow and velocity

1400-579: Is used in the following engines: Related lists Bypass ratio The bypass ratio ( BPR ) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core. Turbofan engines are usually described in terms of BPR, which together with engine pressure ratio , turbine inlet temperature and fan pressure ratio are important design parameters. In addition, BPR

1450-455: Is used when calculating propulsive efficiency . Thermal energy is made up from molecular kinetic energy and latent phase energy. Heat engines are able to convert thermal energy in the form of a temperature gradient between a hot source and a cold sink into other desirable mechanical work . Heat pumps take mechanical work to regenerate thermal energy in a temperature gradient. Standard definitions should be used when interpreting how

1500-442: The compression ratio of the system by adding to the compressor stage to increase overall system efficiency increases temperatures at the turbine face. Nevertheless, high-bypass engines have a high propulsive efficiency because even slightly increasing the velocity of a very large volume and consequently mass of air produces a very large change in momentum and thrust: thrust is the engine's mass flow (the amount of air flowing through

1550-474: The fundamental theorem of calculus has that power is given by F ( t ) ⋅ v ( t ) = m a ( t ) ⋅ v ( t ) = τ ( t ) ⋅ ω ( t ) {\displaystyle \mathbf {F} (t)\cdot \mathbf {v} (t)=m\mathbf {a} (t)\cdot \mathbf {v} (t)=\mathbf {\tau } (t)\cdot \mathbf {\omega } (t)} . where: In propulsion , power

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1600-624: The Conway varied between 0.3 and 0.6 depending on the variant The growth of bypass ratios during the 1960s gave jetliners fuel efficiency that could compete with that of piston-powered planes. Today (2015), most jet engines have some bypass. Modern engines in slower aircraft, such as airliners, have bypass ratios up to 12:1; in higher-speed aircraft, such as fighters , bypass ratios are much lower, around 1.5; and craft designed for speeds up to Mach 2 and somewhat above have bypass ratios below 0.5. Turboprops have bypass ratios of 50-100, although

1650-426: The LP turbine and the LP compressor, increasing efficiency and reducing weight. However, some energy will be lost as heat in the gear mechanism and weight saved on turbine and compressor stages is partly offset by that of the gearbox. There are manufacturing cost and reliability implications as well. The lower fan speed allows higher bypass ratios, leading to reduced fuel consumption and much reduced noise. The BAe 146

1700-458: The dielectric medium to nanopores and a very thin high permittivity separator. While capacitors tend not to be as temperature sensitive as batteries, they are significantly capacity constrained and without the strength of chemical bonds suffer from self-discharge. Power-to-weight ratio of capacitors is usually higher than batteries because charge transport units within the cell are smaller (electrons rather than ions), however energy-to-weight ratio

1750-479: The durability of the geared turbofan engine of Pratt & Whitney PW1000G family has been an ongoing issue, no reliability issues are connected to the geared design. Rolls-Royce's latest engine design for large turbofans (25,000lb to 110,000lb thrust), the UltraFan includes a Powergear rated at a new high of 64MW (87,000hp) and has demonstrated this full power during testing in 2021. Geared turbofan technology

1800-402: The early 1950s, was an early example of a bypass engine. The configuration was similar to a 2-spool turbojet but to make it into a bypass engine it was equipped with an oversized low pressure compressor: the flow through the inner portion of the compressor blades went into the core while the outer portion of the blades blew air around the core to provide the rest of the thrust. The bypass ratio for

1850-621: The engine and doesn't physically touch the engine core. Bypass provides a lower fuel consumption for the same thrust, measured as thrust specific fuel consumption (grams/second fuel per unit of thrust in kN using SI units ). Lower fuel consumption that comes with high bypass ratios applies to turboprops , using a propeller rather than a ducted fan. High bypass designs are the dominant type for commercial passenger aircraft and both civilian and military jet transports. Business jets use medium BPR engines. Combat aircraft use engines with low bypass ratios to compromise between fuel economy and

1900-415: The engine) multiplied by the difference between the inlet and exhaust velocities in—a linear relationship—but the kinetic energy of the exhaust is the mass flow multiplied by one-half the square of the difference in velocities. A low disc loading (thrust per disc area) increases the aircraft's energy efficiency , and this reduces the fuel use. The Rolls–Royce Conway turbofan engine, developed in

1950-402: The gas power is shared between a separate airstream and the gas turbine's own nozzle flow in a proportion which gives the aircraft performance required. The first jet aircraft were subsonic and the poor suitability of the propelling nozzle for these speeds due to high fuel consumption was understood, and bypass proposed, as early as 1936 (U.K. Patent 471,368). The underlying principle behind bypass

2000-443: The influence of BPR. Only the limitations of weight and materials (e.g., the strengths and melting points of materials in the turbine) reduce the efficiency at which a turbofan gas turbine converts this thermal energy into mechanical energy, for while the exhaust gases may still have available energy to be extracted, each additional stator and turbine disk retrieves progressively less mechanical energy per unit of weight, and increasing

2050-442: The perception of sports car like performance or for other psychological benefit . Increased engine performance is a consideration, but also other features associated with luxury vehicles . Longitudinal engines are common. Bodies vary from hot hatches , sedans (saloons) , coupés , convertibles and roadsters . Mid-range dual-sport and cruiser motorcycles tend to have similar power-to-weight ratios. Power-to-weight ratio

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2100-431: The power-to-weight ratio purely by SI base units . A vehicle's power-to-weight ratio equals its acceleration times its velocity; so at twice the velocity, it experiences half the acceleration, all else being equal. If the work to be done is rectilinear motion of a body with constant mass m {\displaystyle m\;} , whose center of mass is to be accelerated along a (possibly non-straight) line to

2150-441: The propulsion airflow is less clearly defined for propellers than for fans and propeller airflow is slower than the airflow from turbofan nozzles. Klimov RD-33 Power-to-weight ratio Power-to-weight ratio ( PWR , also called specific power , or power-to-mass ratio ) is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio

2200-701: The propulsive power of a jet or rocket engine is transferred to its vehicle. An electric motor uses electrical energy to provide mechanical work , usually through the interaction of a magnetic field and current-carrying conductors . By the interaction of mechanical work on an electrical conductor in a magnetic field, electrical energy can be generated . Fluids (liquid and gas) can be used to transmit and/or store energy using pressure and other fluid properties. Hydraulic (liquid) and pneumatic (gas) engines convert fluid pressure into other desirable mechanical or electrical work . Fluid pumps convert mechanical or electrical work into movement or pressure changes of

2250-434: The requirements of combat: high power-to-weight ratios , supersonic performance, and the ability to use afterburners . If all the gas power from a gas turbine is converted to kinetic energy in a propelling nozzle, the aircraft is best suited to high supersonic speeds. If it is all transferred to a separate large mass of air with low kinetic energy, the aircraft is best suited to zero speed (hovering). For speeds in between,

2300-433: The rotational speed of the LP turbine and compressor must be relatively low, which means extra compressor and turbine stages are required to keep the average stage loadings and, therefore, overall component efficiencies to an acceptable level. In a geared turbofan, a planetary reduction gearbox between the fan and the LP shaft allows the latter to run at a higher rotational speed thus enabling fewer stages to be used in both

2350-481: The thrust. Turbofans are closely related to turboprops in principle because both transfer some of the gas turbine's gas power, using extra machinery, to a bypass stream leaving less for the hot nozzle to convert to kinetic energy. Turbofans represent an intermediate stage between turbojets , which derive all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases (typically 10% or less). Extracting shaft power and transferring it to

2400-683: The turbojet's single nozzle. To see the influence of increasing BPR alone on overall efficiency in the aircraft, i.e. SFC, a common gas generator has to be used, i.e. no change in Brayton cycle parameters or component efficiencies. Bennett shows in this case a relatively slow rise in losses transferring power to the bypass at the same time as a fast drop in exhaust losses with a significant improvement in SFC. In reality increases in BPR over time come along with rises in gas generator efficiency masking, to some extent,

2450-516: The vehicle itself. In the sport of competitive cycling athlete's performance is increasingly being expressed in VAMs and thus as a power-to-weight ratio in W/kg. This can be measured through the use of a bicycle powermeter or calculated from measuring incline of a road climb and the rider's time to ascend it. A locomotive generally must be heavy in order to develop enough adhesion on the rails to start

2500-440: Was created in 1970. However, economically scaling the idea from small engines to medium and large ones was not possible until the 21st century. After considering a geared design, General Electric and Safran decided against it for their CFM LEAP due to weight and reliability concerns, postponing its use for a future application, when Pratt & Whitney began development of the geared PW1000G . Since its inception in 2016, while

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