Automobiles are vehicles that carry passengers. Vehicles that carry cargo are called trucks or vans. There are an estimated 1.4 billion automobiles in operation around the world. In recent years, passenger cars have emerged as the primary means of family transportation in many countries. Some 1.3 billion of these are in the United States, where people travel more than three trillion miles (five billion kilometres) each year.
Most modern automobiles use an internal combustion engine to produce power that turns the wheels of the car. The engine can run on gasoline (petrol in American English), diesel fuel, kerosene, or any other kind of oil. The fuel is burned in a cylinder and the explosion of that burning releases heat and pressure to turn the wheels. An engine can also generate electricity to run lights and other systems in the automobile.
Several systems are required for a car to work, such as an engine, transmission, chassis, bodywork, controls, electrical equipment, and service devices. An automobile must be safe to drive, so it needs safety features like seatbelts and airbags. It must be economical to operate, so it needs fuel-efficient engines and electrical systems. It must be able to handle high speeds, so it needs good handling and aerodynamic design.
Millions of people worldwide rely on automobiles to get to work and school and to shop and play. However, accidents and traffic congestion cause millions of deaths and injuries each year. Automobiles pollute the atmosphere with gases that contribute to climate change. Public transportation, such as buses, trains (steam-powered, diesel-powered, monorail or light rail), trams and subways, can help reduce the number of automobiles on the road.