The body of an automobile is categorizedaccording to the number of doors, the arrangement of seats, and the roof structure. Their roofs are conventionally supported by pillars on each side of the body in recent times, there are convertible models with retractable fabric tops that rely on the pillar at the side of the windshield for upper body strength, as convertible mechanisms and glass areas are essentially nonstructural. The glass areas have been increased for improved visibility and for aesthetic reasons. New designs are usually programmed on three- to six-year cycles with generally minor refinements appearing during the cycle.
Redesigning was a tough job in the past, when 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.
Sheet steel is generally used to make automotive bodies. 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. Other materials for certain other materials are also used. Other materials, such as aluminum, fiberglass, and carbon fiber reinforced plastic are used because of their special properties.
For more toughness and resistance to brittle deformation, Polyamide, polyester, polystyrene, polypropylene, and ethylene plastics have been formulated. Tooling for plastic components generally costs less and requires less time to develop than that for steel components.
Painting and priming processes are used to protect bodies from corrosive elements and to maintain their strength and appearance. Bodies are first dipped in cleaning baths to remove oil and other foreign matter and then they 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. To speed up the drying process in the factories, ovens with conveyer lines are used. In those body areas that are more susceptible to corrode, galvanized steel with a protective zinc coating and corrosion-resistant stainless steel are used.
CHASIS
THE chassis forms the main structure of the modern automobile. A large number of designs in pressed-steel frame form a skeleton on which the engine, wheels, axle assemblies, transmission, steering mechanism, brakes, and suspension members are mounted. During the manufacturing process the body is flexibly bolted to the chasis.
This combination of the body and frame performs a variety of functions. It absorbs the reactions from the movements of the engine and axle, receives the reaction forces of the wheels in acceleration and braking, absorbs aerodynamic wind forces and road shocks through the suspension, and absorbs the major energy of impact in the event of an accident.
There has been a gradual shift in modern small car designs. There has been a trend toward combining the chasis frame and the body into a single structural element. In this grouping, the steel body shell is reinforced with braces that make it rigid enough to resist the forces that are applied to it. To achieve better noise-isolation characteristics, separate frames are used for other cars. The presence of heavier-gauge steel components in modern separate frame designs also tends to limit intrusion in accidents.
Automotive production down the ages has required a wide range of energy-conversion systems. These include electric, steam, solar, turbine, rotary, and different types of piston-type internal combustion engines. The reciprocating-piston internal -combustion system, operating on a four-stroke cycle, has been the most successful for automobiles, while diesel engines are widely used for trucks and buses.
The gasoline engine was originally selected for the automobile due to its flexibility over a wide range of speeds. Also, 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 fuel--gasoline. Reliability, compact size, and range of operation later became important factors.
In today’s world, there has been a growing emphasis on the pollution producing features 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. However, the gasoline engine, with its new emission-control devices to improve emission performance, has not yet been challenged significantly.
The first half of the twentieth century saw a trend to increase engine horsepower, particularly in the 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).
Smaller cars brought about a return a to smaller engines, the 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.
The automobile engines from Europe had a bigger range, varying from 1to12 cylinders with corresponding differences in overall size, weight, piston displacement, and cylinder bores. Four cylinders and horsepower ratings from 19 to 120 was followed in a majority of the models. Several three-cylinder, two-stroke-cycle models were built while most engines had straight or in-line cylinders. There were several V-type models and horizontally opposed two- and four-cylinder makes too. 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. The 1970s and '80s saw an increased interest in improved fuel economy which brought in a return to smaller V-6 and four-cylinder layouts, with as many as five valves per cylinder to improve efficiency.
FUEL INJECTION
IN an internal combustion engine, the fuel injection system is that which delivers fuel or a fuel-air mixture to the cylinders by means of pressure from a pump. It was originally used in diesel engines because of diesel fuel's greater viscosity and the need to overcome the high pressure of the compressed air in the cylinders. A diesel fuel injector sprays an intermittent, timed, metered quantity of fuel into a cylinder, distributing the fuel throughout the air within. Fuel injection is also now used in gasoline engines in place of a carburetor. In gasoline engines the fuel is first mixed with air, and the resulting mixture is delivered to the cylinder. Computers are used in modern fuel injection systems to regulate the process. The positive effects of fuel injection are that there is more efficient fuel combustion, better fuel economy and engine performance and reduced polluting exhaust emissions.
No comments:
Post a Comment