Why is soot produced in diesel exhaust, since diesel runs lean?

Question

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?

score 5 · Answer 1 · answered Nov 08 '16 at 14:02

5

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.

answered Nov 08 '16 at 14:02

Chuck

3,565
12
28

mmmm EXTREMELY interesting what you found Chuck! – ergon Nov 08 '16 at 14:21
1

@ergon and extremely LMWTFY too. There really are web sources for what should be obviously common questions. – Carl Witthoft Nov 08 '16 at 14:24
@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! – ergon Nov 08 '16 at 14:28
1

@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. – Chuck Nov 08 '16 at 14:33
2

@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. – Carl Witthoft Nov 08 '16 at 19:18
@CarlWitthoft - Ah, no worries :) – Chuck Nov 08 '16 at 19:35
As an aside, gasoline engines with direct injection have similar emission problems as diesels, because there too the mixing of the fuel and air is not very effective. The term in the literature for cylinder injection engines is 'stratified charge', as opposite to 'homogenous charge' which describes regular gasoline engines. – JanKanis Aug 15 '19 at 19:47

score 2 · Accepted Answer · answered Nov 08 '16 at 17:55

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

Why is soot produced in diesel exhaust, since diesel runs lean?

2 Answers2