Jamieson, Fausset and Brown on Genesis 8:21

21. And the Lord smelled a sweet savour--The sacrifice offered by a righteous man like Noah in faith was acceptable as the most fragrant incense.

Lord said in his heart--same as "I have sworn that the waters of Noah should no more go over the earth" (Is 54:9).

for--that is, "though the imagination is evil"; instead of inflicting another destructive flood, I shall spare them--to enjoy the blessings of grace, through a Saviour.