Automobiles are four-wheeled vehicles designed to carry passengers and often propelled by an internal combustion engine that burns a volatile fuel. Thousands of individual parts make up modern automobiles, and the design of each car is a compromise between various factors. Air pollution laws and safety systems are constant drivers for automobile manufacturers, while consumer demand and marketing strategies influence many other aspects of car design.
The first automobiles were powered by steam, electricity and, later, gasoline. They were slow, heavy and expensive. Today the automobile is a key means of transportation, and there are an estimated 1.4 billion cars in use worldwide. The majority are passenger cars, although there are also special cars for hauling cargo, assisting police and fire fighting, or traversing harsh terrain.
Most automobiles have front-wheel drive, which allows them to turn more quickly than rear-wheel drives. Some have independent suspension, which improves handling and ride comfort. The engine, which powers the wheels of an automobile, has from four to eight cylinders. These cylinders work together to turn the crankshaft, and each of them is fired when a spark plug ignites the gas in the combustion chamber.
While automobiles allow people to move around much more rapidly than they could by foot or bicycle, the combination of cars in urban areas causes traffic congestion and produces excessive air pollution. In addition, there are few places that can’t be reached by public transportation buses, trains and trams, which are slower than automobiles but much less expensive and environmentally harmful.