三峡大学机械专业英语--第5课.pptVIP

8 Motor Motors were formerly divided into two classes: prime movers, which will start on their own once a source of low-grade energy has been supplied, and secondary motors which requires to be started by a prime mover. In this sense a steam engine is a prime mover, but a motor-car engine is not; an electric motor is not considered as a prime mover because it requires the prior generation of an electric current by other means. The distinction, however, is not a rigorous one, and the term ‘prime mover’ is now often loosely use to mean any artificial source of power. 8 Motor Common usage has also tended to limit the term ‘motor’ to clockwork, electric, and rocket motors , almost all other forms being described as engines. Most of the traditional kinds of motor apply their energy through rotating parts, and even the jet-engine has rotating components; but solid-fuel rocket motors and linear electric-motor have no rotating parts. The motors themselves moving as free units under the direct action of the applied forces. 8 Motor Motors providing rotary motion are of two principal kinds：the reciprocating and the turbine. The reciprocating motor is driven by a piston moving to-and –fro in a cylinder, rotary motion being produced by means of a connecting-rod and a crank. In the turbine a wheel is turned directly and continuously by the action of a stream of high-pressure gas or liquid on vanes set round its periphery. In both types a fuel supplying the initial energy may be burnt separately, as in steam engines, or in contact with the moving parts of the motor , as in internal combustion engines(内燃机). 8 Motor It is not, however ,necessary to provide either steam or gas to drive either type of motor. In the hot air engine, invented by George Cayley in 1807 and revived in Switzerland in 1947 ,the difference in temperature between hot and cold air is exploited, and in the compressed air motor. Invented before 1799, high-pressure air is