Mechanical efficiency

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

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.

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2009)

In physics, mechanical efficiency is the effectiveness of a machine and is defined as

Mechanical Efficiency is the ratio of work input to work output. It is often expressed as a percentage. The efficiency of an ideal machine is 100 percent but an actual machine's efficiency will always be less than 100% because of the Second law of thermodynamics which states that the quality of energy will decay, eventually becoming heat. This means that some of the work put into the system is transformed (lost) into thermal energy (heat). In a mechanical system, friction is the most common cause of the work lost to heat.

The actual Mechanical advantage of a system is always less than the ideal mechanical advantage due to these losses. Another way to express mechanical efficiency is it is the ratio of actual mechanical advantage to ideal mechanical advantage.

100 percent Mechanical Efficiency is also the core principal in creating a perpetual motion machine of the third kind. By "re-using" the Work Output to conserve the Work Input, a perpetual motion machine could maintain its movement forever. In controlled environments, low friction mechanisms can come close to the ideal efficiency. However, to maintain a perfectly ideal mechanism, the temperature output must be the absolute zero, which is impossible to reach due to the Third law of thermodynamics. Therefore, a perfect mechanical efficiency can never be achieved.

See also

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

This classical mechanics-related article is a stub. You can help Wikipedia by expanding it. v • d • e

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)