Stacks and oil burners

Operating under the theory of the only dumb question is the one that goes unasked, "why do oil burners always have straight stacks?"

I realize that locomotives that burned wood or coal had a wide variety of stacks that were really spark arresting devices, such as a Rushton, or cabbage, stack as used by the Argent Lumber Company.

Surly oil burners produced sparks when the flues were sanded? Even if burning atomized fuel oil there had to be a carbon build up that from time to time blew out the stack due to the draft and somehow got ignited in the process.

Was there an internal spark arrestor? Was the volume of sparks produced with an oil burner so low that it did not matter?

Thanks!