Since diesel runs lean, why is soot produced in diesel exhaust?
Soot is carbon particles due to incomplete combustion. But, since diesel runs lean with plenty of oxygen, why does this happen?
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One important reason is that diesel fuel had a high molecular weight compared to gasoline this means that is is more difficult to disperse as it forms liquid droplets as opposed to vapour and even more importantly there are many more intermediate reactions involved in complete combustion.
For example Hydrogen, H2 burns very easily in oxygen as combustion is essentially one reaction process, H2 and O2 are separated and recombine as H20, simple.
However if you have a complex hydrocarbon fuel with long chain branched molecules there are lots of intermediate steps to go through to fully break it down into Carbon dioxide and water.
In an IC engine this combustion process has a very limited time to take place as an ignition cycle may take a fraction of a second
From the Wikipedia article:
All diesel engines can be considered to be lean-burning with respect to the total volume, however the fuel and air is not well mixed before the combustion. Most of the combustion occurs in rich zones around small droplets of fuel. Locally rich combustion like this is a source of NOx and particles.