Specific power

Specific power

In engineering, the term specific power can refer to power either per unit of mass, volume or area, although power per unit of volume is more formally known as power density, and power per unit area as surface power density. [ [http://physics.nist.gov/Pubs/SP811/sec04.html] ] The following sections give several examples of where these terms are used.

Power per unit of volume

*Heat engines can be characterized by their specific power, which is typically given in kilowatts per litre of engine displacement (in the U.S. also horsepower per cubic inch). The result offers an approximation of the peak-power output of an engine. This is not to be confused with fuel efficiency, since high-efficiency often requires a lean fuel-air ratio, and thus lower power density. A modern high-performance car engine makes in excess of 75 kW/L (1.65 hp/in³).

Power per unit of mass

*With batteries, specific power usually refers to the power-to-weight ratio, measured in kilowatts per kilogram (generally, kW/kg). Fuels are usually measured in terms of energy density, since the available power is usually not a limitation. A high-octane fuel actually burns more slowly, and due to engine dynamics this can actually allow an engine to produce more power. In high performance vehicles, the specific power determines the vehicle's acceleration.
*The specific power on a mass basis is also used to characterize solar panels for use in aerospace applications where this parameter is critical for an efficient launch.

Power per unit of area

* The intensity of electromagnetic waves can be expressed in W/m². An example of such a quantity is the solar constant.
* Wind turbines are often compared using a specific power measuring watts per square meter of turbine disk area, which is pi times the length of a blade squared. This measure is also commonly used for solar panels, at least for typical applications.

ee also

*Energy density
*Specific absorption rate (SAR)
*Power-to-weight ratio


External links

* [http://www.nytimes.com/2008/01/13/automobiles/13ULTRA.html?_r=2&pagewanted=1&ref=automobiles&oref=slogin&oref=slogin Difference between energy density and power density] .

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