fire up vs light a fire under

fire up

verb
  • To excite; to infuse with energy. 

  • Of an engine or similar, to start. 

  • To ignite. 

  • To launch; to run. 

light a fire under

verb
  • To motivate or encourage (someone) to start sooner or move faster. 

How often have the words fire up and light a fire under occurred in a corpus of books? (source: Google Ngram Viewer )