Back in the 1970s my old man worked at a large factory/works in Shrewsbury which had its own behemoth of a diesel engine on site to run the place during power cuts.
The problem was, this vast diesel mill wasn't in its first flush of youth, so when it was run every now and again to test a little oil and unburnt diesel would make its way through the cylinders to the exhaust.
Then, one day, the big beast was fired up in anger, and ran up to full temperature. This warmed up the oily crud in the exhaust to the point where the diesel/oil mixture self-ignited.
The blowtorch like flames were apparently over 10ft long!