add fuel to the fire vs set afire

add fuel to the fire

verb
  • Used other than figuratively or idiomatically: see add, fuel, fire. 

  • To worsen a conflict between people; to inflame an already tense situation. 

set afire

verb
  • Synonym of set on fire 

How often have the words add fuel to the fire and set afire occurred in a corpus of books? (source: Google Ngram Viewer )