fossil fuel vs refuel

fossil fuel

noun
  • Any fuel derived from hydrocarbon deposits such as coal, petroleum, natural gas and, to some extent, peat; these fuels are non-renewable, and their burning moves carbon from underground into the atmosphere via the greenhouse gas carbon dioxide. 

refuel

noun
  • An act or instance of refilling with fuel. 

verb
  • To refill with fuel. 

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