Here is some more exhaust fodder for consideration, specific mostly to US locos:
Prior to the 1990's, exhaust smoke was pretty much ignored in engine design--the emphasis was on performance, serviceability, longevity, and cost. Fuel economy was, for decades, also a secondary consideration. The Alco shown above was an example. Alcos were notorious for turbo lag and overfueling coming off of idle. They would often smoke nearly pure black until the turbo spooled up. (The slow-spooling non-trubo Alcos would do a lot of the same thing.) Before the advent of electronic fuel injection, mechanical fuel injectors were less precise and also would contribute to smoking prime movers, regardless of manufacturer.
The early electronically fuel-injected prime movers had their own teething problems. The mid-1990's electronically fuel-injected GE FDL engines could smoke pure black when coming off of idle, with the added "feature" of flaming at the stack as the turbo spooled up and the exhaust temperature increased. Here is a photo that I took in 1996 of some GE AC swing helpers throttling up off idle to couple up with the head end of their train at Minturn, Colorado. Plenty of smoke. Not shown in the photo, as the locos passed me, they also flamed about 3 feet out of the stack until the partially unburned fuel in the stack finished "cooking off." Engineers in those days remarked that throttling up a GE AC or Dash 9 under a pedestrian overpass might possibly barbecue a pedestrian. Those fuel injection systems were later modified to eliminate at least some of the problem.
http://www.elvastowe...26627_thumb.jpg
As I noted earlier, however, quite often these GE's, so dirty coming off idle, would smoke less and less as they approached fuel throttle, with throttle-up in the higher notches eliciting far less throttle-up smoke. That pretty much remained true for GE's until the "smoke-free" Tier 4 GE's appeared.
Newer locomotives have had considerable more technology applied to fuel injection, turbo, combustion, exhaust systems, and the diesel fuel itself to minimize NOx and particulate emissions. The culmination of that is the Tier 4 locomotives that, under normal conditions, should emit no visible particulates at all, under any loading conditions.