exhaust valve vs safety valve

exhaust valve

noun
  • a valve through which the products of combustion escape from the cylinder to the manifold 

safety valve

noun
  • A relief valve set to open at a pressure below that at which a container (such as a boiler) would burst. 

  • Any mechanism offering relief from pressure or tension, e.g. emotionally. 

  • A United States law, codified at 18 USC §3553(f), authorizing a judge to disregard mandatory minimum sentences for some criminals with few or no prior offenses. 

How often have the words exhaust valve and safety valve occurred in a corpus of books? (source: Google Ngram Viewer )