Mechanical efficiency

(mechanical engineering) In an engine, the ratio of brake horsepower to indicated horsepower.

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The ratio of the work output to work input. In studies of human movement, there are three main ways of describing mechanical efficiency during exercise: gross efficiency, net efficiency and mechanical efficiency. Gross efficiency (GE) is expressed as the percentage ratio of external work performed to the total production of energy (i.e. total energy expenditure) during the exercise: GE = W × 100/E, where W is the external work performed and E is total energy expenditure Net efficiency is expressed as the percentage ratio of work performed to the extra energy expenditure during the exercise: NE = W × 100/E − e, where e is energy expenditure at rest. Delta efficiency considers mechanical efficiency when work loads change (see delta efficiency). Net mechanical efficiency for muscle movements is generally low because of the loss of free energy as heat. Values vary for different muscles and for the different types of muscle action. The general opinion that mechanical efficiencies for muscular work are less than 25% has been challenged in recent years. A mechanical efficiency of up to 40% has been claimed for some runners. This level of efficiency was unexpected and is thought to be due to part of the energy of descent being absorbed by elastic components of joints, providing a store of free energy that can housed in the next stride (see stretch-shortening cycle). Training has a marked effect on efficiency. For example, the net efficiency of a novice swimmer maybe as low as 1%, while that of an elite swimmer may be more than four times as great.

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Mechanical efficiency measures the effectiveness of a machine in transforming the energy and power that is input to the device into an output force and movement. Efficiency is measured as a ratio of the measured performance to the performance of an ideal machine,

The efficiency of energy conversion of the power plant of a machine is often considered separately from the efficiency of the mechanism that transmits this power to achieve a particular force and movement.

Because the power transmission system or mechanism does not generate power, its ideal performance occurs when the output power equals the input power, that is when there are no losses. Real devices dissipate power through friction, part deformation and wear.

The ideal transmission or mechanism has an efficiency of 100%, because there is no power loss. Real devices will have efficiencies less than 100% because rigid and frictionless systems do not exist. The power losses in a transmission or mechanism are eventually dissipated as heat.

See also

• Mechanical advantage
• Thermal efficiency
• Electrical efficiency
• Internal combustion engine
• Electric motor

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