liquefied petroleum gas vs refuel

liquefied petroleum gas

noun
  • A mixture of propane and butane with traces of some other gases, which is kept in a liquid state at high pressure in metal bottles, and used as a source of gas for cooking, heating and as a fuel in internal combustion engines in vehicles. 

refuel

noun
  • An act or instance of refilling with fuel. 

verb
  • To refill with fuel. 

How often have the words liquefied petroleum gas and refuel occurred in a corpus of books? (source: Google Ngram Viewer )