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Originally Posted by instarx
We've all seen old diesels pouring black smoke from their exhausts. That smoke is unburned fuel because a non-turbocharged diesel simply can't get enough O2 when it is under load. Turbocharging the exact same engine will supply enough O2 to completely burn the fuel, improving FE. However, a diesel not under heavy load will not have its FE improved by a turbo.
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You don't smoke because you don't have a turbo, you smoke because you are dumping too much fuel in for the air you can get. I had a 1990 NA diesel and it wasn't smoking at all at full load. I can get my 1999 TDI to smoke like an old freight train if I want by offsetting the base inj qty. Most of the time, if your stock diesel is smoking it just means your intake is restricted for the power you're trying to make. On TDIs it's often filling with gunk from the mix of EGR soot and oil from the crankcase so it'll smoke at high loads.
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I'm not convinced by the claims of automakers of new turbocharged gas engines that increase FE. I suspect the increased FE comes from use of a smaller engine rather than from the turbos per se.
AH! as I was writing this I see that lunarhighway posted something that supports this thinking... a 0.9L turbo engine replacing a 1.2L NA engine.
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There are certain parts of the map where bsfc is improved with a turbo for a given engine size, but downsizing is really where the major benefit reside.
http://ecomodder.com/forum/15012-post5.html