Defined in the simplest terms a machine is a device that uses force to accomplish something. More technically, it is a device that transmits and changes force or motion into work. This definition implies that a machine must have moving parts. A machine can be very simple, like a block and tackle to raise a heavy weight, or very complex, like a railroad locomotive or the mechanical systems used for industrial processes.
A machine receives input from an energy source and transforms it into output in the form of mechanical or electrical energy. Machines whose input is a natural source of energy are called prime movers. Natural sources of energy include wind, water, steam, and petroleum. Windmills and waterwheels are prime movers; so are the great turbines driven dy water or steam that turn the generators that produce electricity; and so are internal combustion engines that use petroleum products as fuel. Electric motors are not prime movers, since an alternating current of electricity which supplies most electrical energy does not exist in nature.
Terms like work, force, and power are frequently used in mechanical engineering, so it is necessary to define them precisely. Force is an effort that results in motion or physical change. If you use your muscles to lift a box you are exerting force on that box. The water which strikes the blades of a turbine is exerting force on those blades, thereby setting them in motion.
In a technical sense work is the combination of the force and the distance through which it is exerted.
To produce work, a force must act through a distance. If you stand and hold a twenty-pound weight for any length of time, you may get very tired, but you are not doing work in an engineering sense because the force you exerted to hold up the weight was not acting through a distance. However, if you raised the weight, you would be doing work.
Power is another term used in special technical sense in speking of machines. It is the rate at which work is performed.
In the English-speaking countries, the rate of doing work is usually given in terms of horsepower, often abbreviated hp. You will remember that expression resulted from the desire of inventor James Watt to describe the work his steam engines performed in terms that his customers could easily understand.After much experimentation,he settled on rate of 33,000 footpounds per minute as one horsepower.
In the metrick system power in terms of watts and kilowatts.The kilowatt,a more widely used term, equals a thousand watts or approximately 1 1/3 horsepower in the English system.
Components of the Automobile
Automobiles are trackless, self-propelled vehicles for land transportation of people or goods, or for moving materials. There are three main types of automobiles. They are passenger cars, buses and lorries (trucks).The automobile consists of the following components: a)the engine; b)the framework: c)the mechanism that transmits the power-engine to the wheels; d)the body.
Passenger cars are, as a rule, propelled by an internal combustion engine. They are distinguished by the horse-power of the engine, the number of cylinders on the engine and the type of the body, the type of tpansmission, wheeelbase, weight and overall length.
There are engines of various designs. They differ in the number of cylinders, their position, their operating cycle, valve mechanism, ignition and cooling system.
Most automobile engines have six or eight cylinders, although some four-, twelve-, and sixteen-cylinder engines, are used. The activities that take place in the engine cylinder can be divided into four stages which are called strokes. The four strokes are: intake, compression, power and exhaust. «Stroke» refers to the piston movement. The upper limit of piston movement is called top dead centre, TDC. The lower limit of piston movement is called bottom dead centre, BDC.A stroke constitutes piston movement from TDC to BDC or from BDC to TDC. In other words, the piston completes a stroke each time it changes the direction of motion.