Power-to-weight ratio

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The power-to-weight ratio (or specific power or power-to-mass ratio ) is a calculation generally applied to engines and mobile resources to allow comparison of one unit or design with another. The power-to-weight ratio is a measure of the actual performance of each machine or resource. It is also used as an overall vehicle performance measurement, with engine power output divided by the weight (or mass) of the vehicle, to provide a metric independent of the size of the vehicle. Power-to-weight is often quoted by the manufacturer at peak value, but the actual value may vary in use and the variation will affect performance.

The opposite of power-to-weight, weight-to-power ratio is a calculation commonly applied to aircraft, automobiles, and vehicles in general, to allow comparison of the performance of one vehicle with another. The power-to-weight ratio is equal to the thrust per unit mass multiplied by the speed of any vehicle.

Video Power-to-weight ratio

Power-to-weight (daya spesifik)

The power-to-weight formula (Specific Power) for machines (power plants) is the power generated by machines divided by mass. ("Weight" in this context is the day-to-day term for "mass." To see this, notice that what engineers mean by the "power-to-weight ratio" of the electric motor is not limited in zero-gravity environments.)

The typical turbocharged V8 diesel engine may have a 250 kW (340 hp) engine and 380 kg (840 lb) of mass, giving 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 is because of their ability to operate at a very high speed. For example, the Space Shuttle main engine uses a turbopump (a machine consisting of turbine driven pumps) to feed propellants (liquid oxygen and liquid hydrogen) into the engine combustion chamber. The original liquid hydrogen turbine has the same size as a car engine (weighing about 352 kilograms (775 pounds)) and generating 72,000 hp (53.6 MW) for a power-to-weight ratio of 153 kW/kg (93 hp/lb).

Physical Interpretation

Dalam mekanika klasik, daya sesaat adalah nilai pembatas dari rata-rata pekerjaan yang dilakukan per satuan waktu sebagai interval waktu? t mendekati nol.

${\ displaystyle P = \ lim _ {\ Delta t \ rightarrow 0} {\ tfrac {\ Delta W (t)} {\ Delta t}} = \ lim _ { \ Delta t \ rightarrow 0} P _ {\ mathrm {avg}} \,}  ÂÂ$

Satuan metrik yang biasanya digunakan dari rasio daya-ke-berat adalah ${\ displaystyle {\ tfrac {W} {kg}} \;}  ÂÂ$ yang sama dengan ${\ displaystyle {\ tfrac {m ^ {2}} {s ^ {3}}} \;}  ÂÂ$ . Fakta ini memungkinkan seseorang untuk mengekspresikan rasio power-to-weight murni oleh unit dasar SI.

Daya propulif

Jika pekerjaan yang harus dilakukan adalah gerakan garis lurus sebuah benda dengan massa konstan ${\ displaystyle m \;}  ÂÂ$ , yang pusat massanya akan dipercepat sepanjang garis lurus ke kecepatan ${\ displaystyle | \ mathbf {v} (t) | \;}  ÂÂ$ dan sudut ${\ displaystyle \ phi \;}  ÂÂ$ sehubungan dengan pusat dan radial bidang gravitasi oleh powerplant onboard, maka energi kinetik yang terkait untuk dikirimkan ke tubuh sama dengan

${\ displaystyle E_ {K} = {\ tfrac {1} {2}} m | \ mathbf {v} (t) | ^ {2}}  ÂÂ$

dimana:

${\ displaystyle m \;}  ÂÂ$ adalah massa tubuh

${\ displaystyle | \ mathbf {v} (t) | \;}  ÂÂ$ adalah kecepatan pusat massa tubuh, berubah seiring waktu.

Seketika mendorong mekanik/daya menarik dikirim ke tubuh dari powerplant kemudian

${\ displaystyle P_ {K} = {\ tfrac {1} {2}} m2 | \ mathbf {v} (t) | \ lim _ {\ Delta t \ rightarrow 0} {\ tfrac {\ Delta | \ mathbf {v} (t) |} {\ Delta t}} = m \ mathbf {a} (t) \ cdot \ mathbf {v} (t) = \ mathbf {F } (t) \ cdot \ mathbf {v} (t) = \ mathbf {\ tau} (t) \ cdot \ mathbf {\ omega} (t)}  ÂÂ$

dimana:

${\ displaystyle \ mathbf {a} (t) \;}  ÂÂ$ adalah akselerasi pusat massa tubuh, berubah seiring waktu.

${\ displaystyle \ mathbf {F} (t) \;}  ÂÂ$ adalah gaya linear - atau dorongan - diterapkan pada pusat massa tubuh, berubah seiring waktu.

${\ displaystyle \ mathbf {v} (t) \;}  ÂÂ$ adalah kecepatan pusat massa tubuh, berubah seiring waktu.

${\ displaystyle \ mathbf {\ tau} (t) \;}  ÂÂ$ adalah torsi yang diterapkan pada pusat massa tubuh, berubah seiring waktu.

${\ displaystyle \ mathbf {\ omega} (t) \;}  ÂÂ$ adalah kecepatan sudut dari pusat massa tubuh, berubah seiring waktu.

In propulsion, power is only delivered when the powerplant is in motion, and transmitted to cause the body to move. It is usually assumed here that mechanical transmission allows the powerplant to operate at peak output power. This assumption allows the setting of machines to trade the width of electrical band and machine mass for transmission and mass complexity. Electric motors do not suffer from this tradeoff, instead of swapping their high torque for low-speed traction. Thus, power advantage or power-to-weight ratio

${\ displaystyle {\ mbox {P-to-W}} = {\ frac {| \ mathbf {a} (t) || \ mathbf {v} (t) |} {| \ mathbf {g} |}} \;}$

dimana: xmlns

${\ displaystyle | \ mathbf {v} (t) | \;}  ÂÂ$ adalah kecepatan linear dari pusat massa tubuh.

Tenaga mesin

The actual useful strength of the traction engine can be calculated using a dynamometer to measure torque and rotational speed, with continuous peak power when the transmission and/or operator makes the torque product and rotation speed maximized. For a jet engine there is a handy e calculated there, for a rocket there is usually no speed of voyage, so that is less meaningful.

Peak power of the traction engine occurs at a higher rotational speed than the speed at which torsi is maximized and at or below the maximum rated rotation speed - Max RPM. Fast falling torque curves will be associated with sharp torque and peak power curves around their maxima at the same rotational speed, for example a small, lightweight engine with a large turbocharger. The slowly falling or near-flat torque curve will correspond to the power curve rising slowly to the maximum at a rotational speed approaching Max RPM, for example large, heavy multi-cylinder engines suitable for cargo/hauling. Falling torque curves can correspond to flat power curves near rotation speed for fine handling at different vehicle speeds, such as traction electric motors.

Maps Power-to-weight ratio

Example

Machine

Heat and heat pump

Thermal energy consists of molecular kinetic energy and latent phase energy. The heat engine can convert heat energy in the form of a temperature gradient between a heat source and a cold sink into another desired mechanical work. The heat pump takes the mechanical work to regenerate the heat energy in the temperature gradient. Care should be taken when interpreting the driving force, especially for jet engines and rockets, which can be sent from a heat engine to a vehicle.

Electric motors/Electromotive Generators

Electric motors use electrical energy to provide mechanical work, usually through the interaction of magnetic fields and conductors carrying currents. With the interaction of mechanical work on the electrical conductor in the magnetic field, electrical energy can be generated.

Machine fluid and fluid pump

Liquids (liquids and gases) can be used to send and/or store energy using pressure and other fluid properties. Hydraulic (liquid) and pneumatic (gas) engines convert fluid pressure into other desired mechanical or electrical work. The fluid pump converts mechanical or electrical work into movement or changes in fluid pressure, or storage in a pressure vessel.

Thermoelectric generators and electrothermal actuators

Various effects can be utilized to produce thermoelectricity, thermionic emission, pyroelectricity and piezoelectricity. Electrical resistance and ferromagnetism of materials can be utilized to produce thermo-magnetic energy from electric currents.

Electrochemical (galvanic) and electrostatic cell system

Battery

(closed cell)

All electrochemical cell batteries provide changing voltages due to their chemical changes from "charged" to "discharged". The nominal output voltage and cutoff voltage are usually specified for batteries by the manufacturer. The output voltage falls into the cutoff voltage when the battery becomes "discharged". The nominal output voltage is always less than the open circuit voltage generated when the battery is "charged". Battery temperature can affect the power it can produce, where lower temperatures reduce power. The total energy sent from one charging cycle is affected by the temperature of the battery and the power it produces. If the temperature drops or demand for power increases, the total energy delivered at the "discharge" point is also reduced.

The battery release profile is often explained in relation to the battery capacity factor. For example, a battery with the nominal capacity quoted in the ampere-hour (Ah) at current rated current C/10 (lowered in amperes) can safely provide a higher output current - and therefore a higher power to weight ratio - but only with a lower energy capacity. Therefore the power-to-weight ratio for batteries is less meaningful without reference to the appropriate energy-to-weight and temperature ratio of the cell. This relationship is known as Peukert's law.

Fuel cell stock and flow cell battery

Fuel cells and flow cells, although they may use chemically similar batteries, have no distinction to contain energy storage media or fuel. With continuous fuel and oxidant flow, available fuel cells and flow cells continue to convert energy storage media into electrical energy and waste products. Fuel cells clearly contain a fixed electrolyte while flow cells also require continuous electrolyte flow. The flow cells usually have dissolved fuels in the electrolyte.

Photovoltaics

Vehicles

The power-to-weight ratio for vehicles is usually calculated using pavement weight (for cars) or wet weight (for motorcycles), ie, excluding the weight of the driver and any cargo. This can be a bit misleading, especially with regard to motorcycles, where the driver may weigh 1/3 to 1/2 as much as the vehicle itself. In sports the performance of competitive cycling athletes is increasingly expressed in VAM and thus as a strength-to-weight ratio in W/kg. This can be measured through the use of bicycle powermeter or calculated from measuring the slope of the climbing path and the rider's time to ascend.

Utilities and practical vehicles

Most vehicles are designed to meet passenger comfort and cargo carrying requirements. Different designs offer power-to-weight ratios for increased comfort, cargo space, fuel savings, emissions control, energy security and robustness. Reducing drag resistance and lower rolling resistance in vehicle design can facilitate increased cargo space without increasing the power-to-weight ratio (zero charge). This increases the flexibility of the role of the vehicle. Energy security considerations can trade power (usually decreased) and weight (usually increased), and therefore power to weight ratio, for fuel versatility or drive-train hybridization. Some practical and practical vehicle variants such as hot hatches and sports-reconfigure power vehicles (typically increased) and heavy to deliver sports car perceptions such as performance or for other psychological benefits.

Locomotives in general must be very heavy to develop enough adhesion on the tracks to start the train. Because the coefficient of friction between the steel and rail wheel rarely exceeds 0.25 in many cases, increasing the locomotive's power-to-weight ratio is often counterproductive. However, the choice of power transmission systems, such as variable frequency drives versus direct current drivers, can support a higher power-to-weight ratio by managing better propulsion power.

Important low ratios

Public power

Fancy performance, roadster and light sport

Improved machine performance is a consideration, but also other features associated with luxury vehicles. Longitudinal machines are common. Bodies vary from hot hatches, sedans (saloons), coupà ©, convertibles and roadster. Dual-sport motors and mid-class cruisers tend to have similar weight-to-weight ratios.

Vehicles and airplanes

The power-to-weight ratio is an important vehicle characteristic that affects acceleration and handling - and therefore the driving pleasure - of any sports vehicle. Aircraft also rely on a high power-to-weight ratio to achieve sufficient lift.

Man

Power to weight ratio is important in cycling, as it determines acceleration and speed when climbing a hill. Since the cyclists' power of weight loss decreases with fatigue, it is usually discussed with regard to the length of time it retains that power. Professional cyclists can produce more than 20 W/kg as a maximum of 5 seconds.

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See also

• Energy density
• Machine power
• Supporting efficiency
• Specific output
• Thrust to weight ratio
• Vehicle metrics
• von KÃÆ'¡rmÃÆ'¡n-Gabrielli diagram

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References

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External links

• Measurespeed.com - Power Ratio Calculator on Weight

Source of the article : Wikipedia