<P> An engine - generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of self - contained equipment . The engines used are usually piston engines, but gas turbines can also be used . And there are even hybrid diesel - gas units, called dual - fuel units . Many different versions of engine - generators are available - ranging from very small portable petrol powered sets to large turbine installations . The primary advantage of engine - generators is the ability to independently supply electricity, allowing the units to serve as backup power solutions . </P> <P> A generator can also be driven by human muscle power (for instance, in field radio station equipment). </P> <P> Human powered direct current generators are commercially available, and have been the project of some DIY enthusiasts . Typically operated by means of pedal power, a converted bicycle trainer, or a foot pump, such generators can be practically used to charge batteries, and in some cases are designed with an integral inverter . An average "healthy human" can produce a steady 75 Watts (0.1 horsepower) for a full eight hour period, while a "first class athlete" can produce approximately 298 Watts (0.4 horsepower) for a similar period . At the end of which an undetermined period of rest and recovery will be required . At 298 Watts the average "healthy human" becomes exhausted within 10 minutes . The net electrical power that can be produced will be less, due to the efficiency of the generator . Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio . During the mid 20th century, pedal powered radios were used throughout the Australian outback, to provide schooling (School of the Air), medical and other needs in remote stations and towns . </P> <P> Designed to measure shaft speed, a tachogenerator is a device which produces an output voltage proportional to that speed . Tachogenerators are frequently used to power tachometers to measure the speeds of electric motors, engines, and the equipment they power . speed . With precise construction and design, generators can be built to produce very precise voltages for certain ranges of shaft speeds . </P>

What type of voltage does a generator produce