Gas generators of this type are used to power turbopumps in rocket engines, and are used by some auxiliary power units to power electrical generators and hydraulic pumps. Other types include the gas generator in an automobile airbag, which is designed to rapidly produce a specific quantity of inert gas, and the chemical oxygen generator, which delivers breathable oxygen at a controlled rate over a prolonged period. During World War II, portable gas generators that converted coke to producer gas were used to power vehicles as a way of alleviating petrol shortages.