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A Motor Car is, broadly, any self-powered vehicle with more than two wheels and an enclosed passenger compartment, capable of being steered by the operator and designed for use on a road or street. The term is used more specifically to denote any such vehicle designed to carry two to seven people. A synonym is automobile (Grk. autos, "self"; Lat., mobilis, "movable"). Larger vehicles designed for more passengers are called buses, and those designed to carry freight are called lorries. The term automotive vehicle includes all of the above, as well as certain specialised industrial and military vehicles.
The primary components of a car are the power plant, the power transmission, the running gear, and the control system. These constitute the chassis, on which the body is mounted.
The Power Plant
The power plant includes the engine and its fuel, the carburettor, ignition, lubrication, and cooling systems, and the starter motor.
By far the greatest number of cars use piston engines, but from the early 1970s a significant number of rotary-engine vehicles came into use.
The four-cycle piston engine requires four strokes of the piston per cycle. The first downstroke draws in the petrol mixture. The first upstroke compresses it. The second downstroke - the power stroke - following the combustion of the fuel, supplies the power, and the second upstroke evacuates the burned gases. Intake and exhaust valves in the cylinder control the intake of fuel and the release of burned gases. At the end of the power stroke the pressure of the burned gases in the cylinder is 2.8 to 3.5 kg/sq cm. These gases escape with almost explosive violence with the sudden opening of the exhaust valve. They rush through an exhaust manifold to a silencer, an enlarged section of piping containing expanding ducts and perforated plates through which the gases expand and are released into the atmosphere.
Continuous availability of power and greater smoothness of operation of the four-cycle engine were provided by the development of the four-cylinder engine, which supplies power from one or another of the cylinders on each stroke of the cycle. A further increase in power and smoothness is obtained in engines of 6, 8, 12, and 16 cylinders, which are arranged in either a straight line or two banks assembled in the form of a V.
In the early 1970s one Japanese carmaker began to manufacture cars powered by the rotary-combustion, or Wankel, engine, invented by the German engineer Felix Wankel in the early 1950s. The Wankel engine, in which the combustion movement employs rotors instead of vertical pistons, can be up to one-third lighter than conventional vehicle engines because it needs fewer spark plugs, piston rings, and moving parts.
Air is mixed with the vapour of the petrol in the carburettor. To prevent the air and the carburettor from becoming too cold for successful evaporation of the fuel, the air for the carburettor is usually taken from a point close to a heated part of the engine. Modern carburettors are fitted with a so-called float-feed chamber and a mixing or spraying chamber. The first is a receptacle in which a small supply of petrol is maintained at a constant level. The petrol in most makes of cars is pumped from the main tank to this chamber, the float rising as the petrol flows in until the desired level is reached, when the inlet closes. The carburettor is equipped with such devices as accelerating pumps and economiser valves, which automatically control the mixture ratio for efficient operation under varying conditions. Level-road driving at constant speed requires a lower ratio of petrol to air than that needed for climbing hills, for acceleration, or for starting the engine in cold weather. When a mixture extremely rich in petrol is necessary, a valve known as the choke cuts down the air intake drastically, permitting large quantities of unvaporised fuel to enter the cylinder.
The mixture of air and petrol vapour delivered to the cylinder from the carburettor is compressed by the first upstroke of the piston. This heats the gas, and the higher temperature and pressure favour ready ignition and quick combustion. The next operation is that of igniting the charge by causing a spark to jump the gap between the electrodes of a spark plug, which projects through the walls of the cylinder. One electrode is insulated by porcelain or mica; the other is grounded through the metal of the plug, and both form part of the secondary circuit of an induction system.
The principal type of high-tension ignition now commonly used is the battery-and-coil system. The current from the battery flows through the low-tension coil and magnetises the iron core. When this circuit is opened at the distributor points by the interrupter cam, a transient high-frequency current is produced in the primary coil with the assistance of the condenser. This induces a transient, high-frequency, high-voltage current in the secondary winding. This secondary high voltage is needed to cause the spark to jump the gap in the spark plug. The spark is directed to the proper cylinder by the distributor, which connects the secondary coil to the spark plugs in the several cylinders in their proper firing sequence. The interrupter cam and distributor are driven from the same shaft, the number of breaking points on the interrupter cam being the same as the number of cylinders.
The electrical equipment controls the starting of the engine, its ignition system, and the lighting of the car. It consists of the storage battery, a generator for charging it when the engine is running, a motor for starting the engine, and the necessary wiring. Electricity also operates various automatic devices and accessories, including windscreen wipers, directional signals, heating and air conditioning, cigarette lighters, powered windows, and audio equipment.
In the force-feed system, a pump forces the oil to the main crankshaft bearings and then through drilled holes in the crankpins. In the full-force system, oil is also forced to the connecting rod and then out to the walls of the cylinder at the piston pin.
At the moment of explosion, the temperature within the cylinder is much higher than the melting point of cast iron. Since the explosions take place as often as 2,000 times per minute in each cylinder, the cylinder would soon become so hot that the piston, through expansion, would "freeze" in the cylinder. The cylinders are therefore provided with jackets, through which water is rapidly circulated by a small pump driven by a gear on the crankshaft or camshaft. During cold weather, the water is generally mixed with a suitable antifreeze, such as alcohol, wood alcohol, or ethylene glycol.
To keep the water from boiling away, a radiator forms part of the engine-cooling system. Radiators vary in shape and style. They all have the same function, however, of allowing the water to pass through tubing with a large area, the outer surface of which can be cooled by the atmosphere. In air cooling of engine cylinders, various means are used to give the heat an outlet and carry it off by a forced draught of air.
The Starter Motor
Unlike the steam engine, the petrol engine must usually be set in motion before an explosion can take place and power can be developed; moreover, it cannot develop much power at low speeds. These difficulties have been overcome by the use of gears and clutches, which permit the engine to travel at a speed higher than that of the wheels, and to work when the vehicle is at rest. Ease of starting and steadiness of operation are secured in the highest degree in a multicylinder engine. An electric starter motor, receiving its current from the storage battery, turns the crankshaft, thus starting the petrol engine. The starter motor is of a special type that operates under a heavy overload, producing high power for very short periods. In modern cars, the starter motor is automatically actuated when the ignition switch is closed.
The Power Transmission
The engine power is delivered first to the flywheel and then to the clutch. From the clutch, which is the means of coupling the engine with the power-transmission units, the power flows through the transmission and is delivered into the rear-axle drive gears, or differential, by means of the drive shaft and universal joints. The differential delivers the power to each of the rear wheels through the rear-axle drive shafts.
Some type of clutch is found in every car. The clutch may be operated by means of a foot pedal, or it may be automatic or semiautomatic. The friction clutch and the fluid coupling are the two basic varieties. The friction clutch, which depends on solid contact between engine and transmission, consists of: the rear face of the flywheel; the driving plate, mounted to rotate with the flywheel; and the driven plate, between the other two. When the clutch is engaged, the driving plate presses the driven plate against the rear face of the flywheel. Engine power is then delivered through the contacting surfaces to the transmission by means of a splined (keyed) shaft.
Fluid coupling may be used either with or without the friction clutch. When it is the sole means of engaging the engine to the transmission, power is delivered exclusively through an oil medium without any contact of solid parts. In this type, known as a fluid drive, an engine-driven, fan-bladed disc, known as the fluid flywheel, agitates the oil with sufficient force to rotate a second disc that is connected to the transmission. As the rotation of the second disc directly depends on the amount of engine power delivered, the prime result of fluid coupling is an automatic clutch action, which greatly simplifies the requirements for gear shifting.
Manual and Automatic Transmissions
The transmission is a mechanism that changes speed and power ratios between the engine and the driving wheels. Three general types of transmission are in current use: conventional or sliding-gear, Hydra-Matic, and torque-converter systems.
The conventional transmission provides for three or four forward speeds and one reverse speed. It consists of two shafts, each with gears of varying diameters. One shaft drives the other at a preselected speed by meshing the appropriate set of gears. For reverse speed, an extra gear, known as the idler gear, is required to turn the driven shaft in the opposite direction from normal rotation. In high gear, the two shafts usually turn at the same speed. In low, second, and reverse gears, the driven shaft turns more slowly than the driving shaft. When a pair of gears permits the driven shaft to turn more rapidly than the driving shaft, the transmission is said to have overdrive. Overdrive is designed to increase the speed of an automobile without taxing the engine beyond what is considered its normal operating limit.
The Hydra-Matic type of transmission combines the automatic clutch provided by fluid coupling with a semiautomatic transmission. A mechanical governor, controlled by the pressure exerted on the accelerator pedal, regulates gear selection through a system of hydraulically controlled shift valves. Hydra-Matic transmission provides for several forward gears.
The torque-converter type of transmission provides an unlimited number of gear ratios with no shifting of gears. The torque converter is a hydraulic mechanism using engine power to drive a pump, which impels streams of oil against the blades of a turbine. The turbine is connected to the drive shaft and causes it to rotate.
Both Hydra-Matic and torque-converter systems are controlled by a selector lever on the steering column, which provides also for reverse and sometimes for emergency-low gears.
The Running Gear
The running gear of the car includes the wheel-suspension system, the stabilisers, and the wheels and tyres. The frame of the car may be considered the integrating member of the running gear. It is attached to the rear axle and to the front wheels by springs. These springs, along with the axles, the control and support arms, and the shock absorbers, constitute the wheel-suspension system. In modern cars the front wheels are independently suspended from the frame in a manner that permits either wheel to change its plane without appreciably affecting the other. This type of front-wheel suspension is known popularly as knee-action suspension. The stabilisers consist of spring-steel bars, connected between the shock-absorber arms by levers, to decrease body roll and improve steerability.
The Control System
Steering is controlled by a hand wheel, mounted on an inclined column and attached to a steering tube inside the column. The other end of the tube is connected to the steering gear, which is designed to provide maximum ease of operation. Power steering, adapted for passenger cars in the early 1950s, is generally a hydraulic mechanism used as a booster to reduce the effort of steering.
A car has two sets of brakes: the hand or emergency brake and the foot brake. The emergency brake generally operates on the rear wheels only, but it may operate on the drive shaft. The foot brake in modern cars is always of the four-wheel type, operating on all wheels. Hydraulic brakes on cars and hydraulic vacuum, air, or power brakes on lorries apply the braking force to the wheels with much less exertion of force on the brake pedal than is required with ordinary mechanical brakes. The wheel brakes are generally of the internally expanding type, in which a convex strip of material is forced against a concave steel brake drum.
Oil shortages and rising fuel prices during the 1970s encouraged car engineers to develop new technologies for improving the fuel economy of existing petrol engines and to accelerate work on alternative engines. Large V-8 petrol engines became less common from the early 1980s, being replaced by 6-, 5-, 4-, and 3-cylinder V-engines, using microprocessors for improved fuel-air control and thus better fuel economy. During the early 1980s research and development began on automatic transmissions controlled electronically for maximum efficiency and having infinitely variable gear ratios. At the same time, digital speedometers, trip-information devices, and electronic devices to cue owners regarding maintenance and other chores were appearing in increasing numbers of cars.
Among alternatives to petrol engines, diesel and electric engines appeared the most promising. The turbine engine continued to be held back by high manufacturing costs and other problems; technical hurdles remained for the revived Stirling engine; the steam engine, which was the object of experiment in passenger cars during the 1960s and 1970s, proved impractical; and the Wankel rotary engine, inherently less fuel-efficient, remained a low-production, high-performance power plant.
Diesel V-8 engines appeared in the late 1970s in cars made by the United States manufacturer General Motors, and V-6, V-5, and V-4 diesels were used increasingly during the early 1980s because of the engine's superior fuel economy, which is up to 25 per cent better than that of a comparable petrol engine. Concern that diesel exhaust may contain carcinogens continues to retard diesel development. The advent of turbocharged diesels overcame one inherent problem of the engine: slow acceleration.
Important advances in battery technology have led to electric cars capable of speeds up to 80 km/h and a range of 160 km or more. Such cars might become popular because they can be recharged overnight when the power demand on electric generating stations is low. Mass use of electric vehicles would lower the demand for crude oil.
By using lightweight steel, aluminium, plastics, and magnesium, car manufacturers drastically reduced the size and weight of their models in the late 1970s and early 1980s in an effort to improve fuel efficiency. Front-wheel drive technology, which allows more passenger and cargo space inside smaller cars, has been adopted by carmakers worldwide, replacing the rear-drive arrangement commonly used since the motor industry's earliest days.
Now try the exercises: Exercise a, Exercise b, Exercise c, Exercise d.
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