Abstract
It is generally believed that lubricating Oil (LO) of heat engines blackens mainly due to soot.However, the authors' previous study shows that LO is blackened also in gas engine without soot. That may be due to cracking of LO molecules under high temperature but this is not proved yet. The purpose of this experimental study is to verify whether any LO is blackened without soot not only in heat engines but also in other fields subject to high temperature operating condition.
