Diesels NEVER run rich!

They smoke more due to the fact that you spray diesel into really hot air. The only point of combustiong is when air is mixed with the diesel...

So you inject very hard (high pressures and small nozzels) to make the spray as fine as possible. But you are still relieing on air movement to mix the air and fuel so it burns.

Some engines smoke less than others for one main reason - combustion chamber design!!

Alex