Complete combustion: when fuel or combustible material reacts fully with oxygen and burns completely (technical/chemical meaning).
燃料が不十分だと完全燃焼が起きず、排気ガスが汚れる。
If the fuel is insufficient, complete combustion does not occur and exhaust becomes polluted.
エンジンの設計を改善して完全燃焼を促すことが目標だ。
The goal is to improve the engine design to promote complete combustion.
Figuratively, to give everything one has — to use all one's energy/passion and finish without regrets (often used as 完全燃焼する).
試合で完全燃焼して、負けても悔いはない。
I gave my all in the match, so even if I lose I have no regrets.
プロジェクトに完全燃焼するため、一時的に他の仕事を減らした。
To throw myself completely into the project, I temporarily reduced other work.
最後まで完全燃焼できるように体調を整えた。
I got myself in shape so I could give my all until the end.