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?


2 Answers 2


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.

  • $\begingroup$ mmmm EXTREMELY interesting what you found Chuck! $\endgroup$
    – ergon
    Commented Nov 8, 2016 at 14:21
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    $\begingroup$ @ergon and extremely LMWTFY too. There really are web sources for what should be obviously common questions. $\endgroup$ Commented Nov 8, 2016 at 14:24
  • $\begingroup$ @CarlWitthoft well, I read the wikipedia article of 'soot' and 'particulate matter' and didn't have anything! Don't accuse me that I didn't at least try! $\endgroup$
    – ergon
    Commented Nov 8, 2016 at 14:28
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    $\begingroup$ @CarlWitthoft - Yeah, it's a lame answer, but I'm not really sure what OP wants. Why do diesels make smoke? Locally rich combustion. I feel like a heel posting it, but I'm also not really sure how to expand it or to make it more substantial. $\endgroup$
    – Chuck
    Commented Nov 8, 2016 at 14:33
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    $\begingroup$ @Chuck Apologies - I wasn't dumping on you - I upvoted 'cause this is a nice clean answer. I was accusing :-) you of doing the LMWTFY for the OP. Sorry for the confusion. $\endgroup$ Commented Nov 8, 2016 at 19:18

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