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Automotive body designs are frequently categorized according to the number of doors, the arrangement of seats, and the roof structure. Automobile roofs are conventionally supported by pillars on each side of the body. Convertible models with retractable fabric tops rely on the pillar at the side of the windshield for upper body strength, as convertible mechanisms and glass areas are essentially nonstructural. Glass areas have been increased for improved visibility and for aesthetic reasons.



The high cost of new factory tools makes it impractical for manufacturers to produce totally new designs every year. New designs usually have been programmed on three- to six-year cycles with generally minor refinements appearing during the cycle. In the past, as much as four years of planning and new tool purchasing was needed for a completely new design. Computer-aided design (CAD) and computer-aided manufacturing (CAM) techniques may now be used to reduce this time requirement by 50 percent or more.



Automotive bodies are generally formed out of sheet steel. Elements are added to the alloy to improve its ability to be formed into deeper depressions without wrinkling or tearing in manufacturing presses. Steel is used because of its general availability, low cost, and good workability. For certain applications, however, other materials, such as aluminum, fibreglass, and carbon-fibre reinforced plastic, are used because of their special properties. Polyamide, polyester, polystyrene, polypropylene, and ethylene plastics have been formulated for greater toughness and resistance to brittle deformation. This material has been designed successfully for some body panels. Tooling for plastic components generally costs less and requires less time to develop than that for steel components and therefore may be changed by designers at a lower cost.



To protect bodies from corrosive elements and to maintain their strength and appearance, special priming and painting processes are used. Bodies are first dipped in cleaning baths to remove oil and other foreign matter. They then go through a succession of dip and spray cycles. Enamel and acrylic lacquer are both in common use. Electrodeposition of the sprayed paint, a process in which the paint spray is given an electrostatic charge and then attracted to the surface by a high voltage, helps assure that an even coat is applied and that hard-to-reach areas are covered. Ovens with conveyor lines are used to speed the drying process in the factory. Galvanized steel with a protective zinc coating and corrosion-resistant stainless steel are used in body areas that are more likely to corrode.



Engine

A wide range of energy-conversion systems has been used experimentally and in automotive production. These include electric, steam, solar, turbine, rotary, and a variety of piston-type internal-combustion engines. The most successful for automobiles has been the reciprocating-piston internal-combustion engine, operating on a four-stroke cycle, while diesel engines are widely used for trucks and buses. The gasoline engine was originally selected for automobiles because it could operate more flexibly over a wide range of speeds, and the power developed for a given weight engine was reasonable; it could be produced by economical mass-production methods; and it used a readily available, moderately priced fuelgasoline. Reliability, compact size, and range of operation later became important factors.





There has been an ongoing reassessment of these priorities with new emphasis on the pollution-producing characteristics of automotive power systems. This has created new interest in alternate power sources and internal-combustion engine refinements that were not economically feasible in prior years. Although a few limited-production battery-powered electric vehicles have appeared from time to time, they have not proved to be competitive owing to costs and operating characteristics. The gasoline engine, with new emission-control devices to improve emission performance, has not yet been challenged significantly.



In the late 1940s a trend began to increase engine horsepower, particularly in American models. Design changes incorporated all known methods of raising engine capacity, including increasing the pressure in the cylinders to improve efficiency, increasing the size of the engine, and increasing the speed at which power is generated. The higher forces and pressures created by these changes created engine vibration and size problems that led to stiffer, more compact engines with V and opposed cylinder layouts replacing longer straight-line arrangements. In passenger cars, V-8 layouts were adopted for all piston displacements greater than 250 cubic inches (4 litres).



The advent of smaller cars brought a return to smaller engines, four- and six-cylinder designs rated as low as 80 horsepower, compared with the standard-size V-8 of large cylinder bore and relatively short piston stroke with horsepower ratings in the range from 250 to 350.



European automobile engines were of a much wider variety, ranging from 1 to 12 cylinders, with corresponding differences in overall size, weight, piston displacement, and cylinder bores. A majority of the models had four cylinders and horsepower ratings from 19 to 120. Several three-cylinder, two-stroke-cycle models were built. Most engines had straight or in-line cylinders. There were, however, several V-type models and horizontally opposed two- and four-cylinder makes. Overhead camshafts were frequently employed. The smaller engines were commonly air-cooled and located at the rear of the vehicle; compression ratios were relatively low. Increased interest in improved fuel economy during the 1970s and '80s brought a return to smaller V-6 and four-cylinder layouts, with as many as five valves per cylinder to improve efficiency.








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