Scavenge fires were a regular feature on several of the Union Castle/Clan ships I sailed on and like Robin never really knew what it was all about!
I seem to remember a whoosh and a load of soot from the funnel all over the bridge then the engines would stop!
Scavange pumps were used before Turbo-Charging was developed and were to supply excess air to the cylinders before combustion and scavange fires took place in the scavange space(hence the name)below each cylinder due to liner lubrication catching fire (more preverlent in double acting engines because this is a combustion space eg.Union Castle ships) from either over heating due to lack of lubrication or combustion blow pass due to worn piston rings.
Many thanks for the information, Soot and sparks was something that I had seen at times blowing out of the funnel, but as 'water & oil' never mixed, I was on the deck side, never got around to finding out. 40 years on and I now know !