An automobile, also known as a car, is a self-propelled motor vehicle designed primarily for passenger transportation on land. It usually has four wheels and is commonly propelled by an internal-combustion engine using a volatile fuel. The modern automobile is a complex technical system employing subsystems with specific design functions, consisting of thousands of component parts that have evolved from breakthroughs in existing technology or from new technologies such as electronic computers, high-strength plastics, and new alloys of steel and nonferrous metals. Passenger cars have emerged as the primary means of family transportation, with an estimated 1.4 billion in operation worldwide.