ignition vs smouldering

ignition

noun
  • The act of igniting. 

  • The phenomenon of a thermonuclear fusion reaction becoming self-sustaining and no longer requiring external heating. 

  • The initiation of combustion. 

  • A system for activating combustion in a combustion engine. 

  • Receptacle for a key used to start the engine in a vehicle. 

smouldering

noun
  • The act by which something smoulders; residual heat. 

adj
  • Burning slowly, producing smoke but no flame. 

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