TWO-STROKE AND DIESEL ENGINES

Most diesels are also four-stroke engines. The first or suction stroke draws air, but no fuel, into the combustion chamber through an intake valve. On the second or com­pression stroke the air is compressed to a small fraction of its former volume and is heated to approximately 440°C by this compression. At the end of the compression stroke vaporised fuel is injected into the combustion chamber and burns instantly because of the high temperature of the air in the chamber. Some diesels have auxiliary electri­cal ignition systems to ignite the fuel when the engine starts and until it warms up. This combustion drives the piston back on the third or power stroke of the cycle. The fourth stroke is an exhaust stroke.

The efficiency of the diesel engine is greater than that of any petrol engine and in actual engines today is slightly over 40 per cent. Diesels are in general slow-speed engines with crankshaft speeds of 100 to 750 revo­lutions per minute (rpm) as compared to 2, 500 to 5, 000 rpm for typical petrol engines. Some types of diesel, how­ever, have speeds up to 2, 000 rpm. Because diesels use compression ratios of 14 or more, they are generally more heavily built than petrol engines, but this disadvantage is counterbalanced by their greater efficiency and the fact that they can be operated on less expensive fuel.

Two-Stroke Engines

By suitable design it is possible to operate a diesel as a two-stroke or two-cycle engine with a power stroke every other stroke of the piston instead of once every four strokes. The efficiency of such engines is less than that of four-stroke engines, and therefore the power of a two-stroke engine is always less then half that of a four-stroke engine of comparable size.

The general principle of the two-stroke engine is to shorten the periods in which fuel is introduced to the combustion chamber and in which the spent gases are exhausted to a small fraction of the duration of a stroke instead of allowing each of these operations to occupy a full stroke.

In the simplest type of two-stroke engine, the valves are the openings in the cylinder wall that are uncovered by the piston at the end of its outward travel. In the two-stroke cycle the fuel mixture or air is introduced through the intake port when the piston is fully withdrawn from the cylinder. The compression stroke follows and the charge is ignited when the piston reaches the end of this stroke. The piston then moves outward on the power stroke, uncovering the exhaust port and permitting the gases to escape from the combustion chamber.

 

DIRECT-CURRENT (DC) GENERATORS

If an armature revolves between two stationary field poles, the current in the armature moves in one direc­tion during half of each revolution and in the other di­rection during the other half. To produce a steady flow of unidirectional, or direct, current from such a device, it is necessary to provide a means of reversing the cur­rent flow outside the generator once during each revolu­tion. In older machines this reversal is accomplished by means of a commutator (коллектор) — a split metal ring mounted on the shaft of the armature. The two halves of the ring are insulated from each other and serve as the terminals of the armature coil. Fixed brushes of metal or carbon are held against the commutator as it revolves, connecting the coil electrically to external wires. As the armature turns, each brush is in contact alternately with the halves of the commutator, changing position at the moment when the current in the armature coil reverses its direction. Thus there is a flow of unidirectional cur­rent in the outside circuit to which the generator is con­nected. DC generators are usually operated at fairly low voltages to avoid the sparking between brushes and com­mutator that occurs at high voltage. The highest poten­tial commonly developed by such generators is 1500 V. In some newer machines this reversal is accomplished using power electronic devices, for example, diode recti­fiers.

Modern DC generators use drum armatures that usu­ally consist of a large number of windings set in longitudinal slits in the armature core and connected to appro­priate segments of a multiple commutator. In an arma­ture having only one loop of wire, the current produced will rise and fall depending on the part of the magnetic field through which the loop is moving. A commutator of many segments used with a drum armature always connects the external circuit to one loop of wire moving through the high-intensity area of the field, and as a re­sult the current delivered by the armature windings is virtually constant. Fields of modern generators are usu­ally equipped with four or more electromagnetic poles to increase the size and strength of the magnetic field. Sometimes smaller interpoles are added to compensate for distortions in the magnetic flux of the field caused by the magnetic effect of the armature.

DC generators are commonly classified according to the method used to provide field current for energizing the field magnets. A series-wound generator has its field in series with the armature, and a shunt-wound genera­tor has the field connected in parallel with the armature. Compound-wound generators have part of their fields in series and part in parallel. Both shunt-wound and com­pound-wound generators have the advantage of deliver­ing comparatively constant voltage under varying elec­trical loads. The series-wound generator is used princi­pally to supply a constant current at variable voltage. A magneto is a small DC generator with a permanent-mag­net field

 

AC MOTORS

Two basic types of motors are designed to operate on alternating current: synchronous motors and induction motors. The synchronous motor is essentially a three-phase alternator operated in reverse. The field magnets are mounted on the rotor and are excited by di­rect current, and the armature winding is divided into three parts and fed with three-phase alternating current. The variation of the three waves of current in the arma­ture causes a varying magnetic reaction with the poles of the field magnets, and makes the field rotate at a con­stant speed that is determined by the frequency of the current in the AC power line.

The constant speed of a synchronous motor is advan­tageous in certain devices. However, in applications where the mechanical load on the motor becomes very great, synchronous motors cannot be used, because if the motor slows down under load it will «fall out of step» with the frequency of the current and come to a stop. Synchro­nous motors can be made to operate from a single-phase power source by the inclusion of suitable circuit elements that cause a rotating magnetic field.

The simplest of all electric motors is the squirrel-cage type of induction motor used with a three-phase supply. The armature of the squirrel-cage motor con­sists of three fixed coils similar to the armature of the synchronous motor. The rotating member consists of a core in which are imbedded a series of heavy conduc­tors arranged in a circle around the shaft and parallel to it. With the core removed, the rotor conductors resemble in form the cylindrical cages once used to ex­ercise pet squirrels. The three-phase current flowing in the stationary armature windings generates a rotating magnetic field, and this field induces a current in the conductors of the cage. The magnetic reaction between the rotating field and the current-carrying conductors of the rotor makes the rotor turn. If the rotor is re­volving at exactly the same speed as the magnetic field no currents will be induced in it, and hence the rotor should not turn at a synchronous speed. In operation the speeds of rotation of the rotor and the field differ by about 2 to 5 per cent. This speed difference is known as slip.

Motors with squirrel-cage rotors can be used on sin­gle-phase alternating current by means of various ar­rangements of inductance and capacitance that alter the characteristics of the single-phase voltage and make it resemble a two-phase voltage. Such motors are called split-phase motors or condenser motors (or capacitor motors), depending on the arrangement used. Single-phase squirrel-cage motors do not have a large starting torque, and for applications where such torque is required, repulsion-induction motors are used. A repulsion-induction motor may be of the split-phase or condenser type, but has a manual or au­tomatic switch that allows current to flow between brushes on the commutator when the motor is start­ing, and short-circuits all commutator segments after the motor reaches a critical speed. Repulsion-induc­tion motors are so named because their starting torque depends on the repulsion between the rotor and the stator, and their torque while running depends on in­duction. Series-wound motors with commutators, which will operate on direct or alternating current, are called universal motors. They are usually made only in small sizes and are commonly used in household ap­pliances.

 

ENGINEERING AS A PROFESSION

 

Electrical and Electronics Engineering

Electrical and electronics engineering is the largest and most diverse field of engineering. It is concerned with the development and design, application, and manu­facture of systems and devices that use electric power and signals. Among the most important subjects in the field are electric power and machinery, electronic cir­cuits, control systems, computer design, superconduc­tors, solid-state electronics, medical imaging systems, robotics, lasers, radar, consumer electronics, and fibre optics.

Despite its diversity, electrical engineering can be di­vided into four main branches: electric power and ma­chinery, electronics, communications and control, and computers.

Electric Power and Machinery

The field of electric power is concerned with the de­sign and operation of systems for generating, transmit­ting, and distributing electric power Engineers in this field have brought about several important developments since the late 1970s. One of these is the ability to trans­mit power at extremely high voltages in both the direct current (DC) and alternating current (AC) modes, reduc­ing power losses proportionately. Another is the real time control of power generation, transmission, and dis­tribution, using computers to analyze the data fed back from the power system to a central station and thereby optimizing the efficiency of the system while it is in op­eration.

A significant advance in the engineering of electric machinery has been the introduction of electronic con­trols that enable AC motors to run at variable speeds by adjusting the frequency of the current fed into them. DC motors have also been made to run more efficiently this way.

Electronics

Electronic engineering deals with the research, de­sign, integration, and application of circuits and devices used in the transmission and processing of information. Information is now generated, transmitted, received, and stored electronically on a scale unprecedented in history, and there is every indication that the explosive rate of growth in this field will continue unabated.

Electronic engineers design circuits to perform spe­cific tasks, such as amplifying electronic signals, add­ing binary numbers, and demodulating radio signals to recover the information they carry. Circuits are also used to generate waveforms useful for synchronization and timing, as in television, and for correcting errors in dig­ital information, as in telecommunications.

Prior to the 1960s, circuits consisted of separate elec­tronic devices — resistors, capacitors, inductors, and vacuum tubes — assembled on a chassis and connected by wires to form a bulky package. The electronics revo­lution of the 1970s and 1980s set the trend towards inte­grating electronic devices on a single tiny chip of silicon or some other semiconductive material. The complex task of manufacturing these chips uses the most advanced technology, including computers, electron-beam lithog­raphy, micro-manipulators, ion-beam implantation, and ultraclean environments. Much of the research in elec­tronics is directed towards creating even smaller chips, faster switching of components, and three-dimensional integrated circuits.

Communications and Control

Engineers work on control systems ranging from the everyday, passenger-actuated, such as those that run a lift, to the exotic, such as systems for keeping spacecraft on course. Control systems are used extensively in air­craft and ships, in military fire-control systems, in power transmission and distribution, in automated manufac­turing, and in robotics.

Computers

Computer engineering is now the most rapidly grow­ing field. The electronics of computers involve engineers in design and manufacture of memory systems, of cen­tral processing units, and of peripheral devices. The field of computer science is closely related to computer engi­neering; however, the task of making computers more «intelligent» (artificial intelligence), through creation of sophisticated programs or development of higher level machine languages or other means, is generally regarded as the aim of computer science.

One current trend in computer engineering is micro­miniaturization. Engineers try to place greater and greater numbers of circuit elements onto smaller and smaller chips. Another trend is towards increasing the speed of computer operations through the use of parallel processors and superconducting materials.

Mechanical Engineering

Engineers in this field design, test, build, and oper­ate machinery of all types; they also work on a variety of manufactured goods and certain kinds of structures. The field is divided into (1) machinery, mechanisms, materials, hydraulics, and pneumatics; and (2) heat as applied to engines, work and energy, heating, ventilat­ing, and air conditioning. The mechanical engineer, therefore, must be trained in mechanics, hydraulics, and thermodynamics and must know such subjects as metallurgy and machine design. Some mechanical en­gineers specialise in particular types of machines such as pumps or steam turbines. A mechanical engineer de­signs not only the machines that make products but the products themselves, and must design for both economy and efficiency. A typical example of modern mechani­cal engineering is the design of a car or an agricultural machine.

Safety Engineering

This field of engineering has as its object the preven­tion of accidents. In recent years safety engineering has become a specialty adopted by individuals trained in other branches of engineering. Safety engineers develop methods and procedures to safeguard workers in hazard­ous occupations. They also assist in designing machin­ery, factories, ships and roads, suggesting alterations and improvements to reduce the possibility of accident.

In the design of machinery, for example, the safety en­gineer try to cover all moving parts or keep them from accidental contact with the operator, to put cutoff switches within reach of the operator and to eliminate dangerous sharp parts. In designing roads the safety engineer seeks to avoid such hazards as sharp turns and blind intersections that lead to traffic accidents

AUTOMATION IN INDUSTRY.

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