combust vs burn up

combust and burn up both are verbs.

Word	Noun	Adjective	Verb	Adverb
combust	No	No	Yes	No
burn up	No	No	Yes	No

As verbs, burn up is a hyponym of combust; that is, burn up is a word with a more specific, narrower meaning than combust:

Other hyponyms of combust include burn down, go up, smolder, smoulder, scorch, sear, singe, deflagrate, flame, blaze up, flame up, flare, blaze.

combust (verb)	burn up (verb)
undergo combustion	burn completely; be consumed or destroyed by fire
get very angry and fly into a rage	use up (energy)
cause to become violent or angry	burn brightly
start to burn or burst into flames
cause to burn or combust