impel vs light a fire under

impel

verb
  • To urge a person; to press on; to incite to action or motion via intrinsic motivation. 

  • To drive forward; to propel an object, to provide an impetus for motion or action. 

light a fire under

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

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