incentive vs tiger in one's tank

incentive

noun
  • Something that motivates, rouses, or encourages. 

  • A bonus or reward, often monetary, to work harder. 

adj
  • Inciting; encouraging or moving; rousing to action; stimulating. 

  • Serving to kindle or set on fire. 

tiger in one's tank

noun
  • enthusiasm, energy, fervor 

How often have the words incentive and tiger in one's tank occurred in a corpus of books? (source: Google Ngram Viewer )