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Thanks, Karen, but I was thinking more of two-stroke petrol engines. Two-stroke diesel engines do exist, river barges over here are usually equipped with them, and they do emit all kinds of smoke. Did your book say anything about two-stroke diesel engines ? They seem to be running much cleaner than the diesel engines in trucks !Due to emission laws, two stroke petrol engines will probably disappear in the near future. But I have seen them emitting blue, white and black smoke - not at the same time of course. I remember that in the case of the black smoke, the mechanic first tried to solve the problem by changing spark plugs.
Because the smoke scatters the blue light and allows the redder end of the spectrum to pass through it, so the shadow lacks blue and has a predominance of red so it look sort of brownish.