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Someone in the comments was saying those things have so much power that they burn a crazy amount of fuel and it’s not physically possible to suck enough air into the engine to match the fuelling so a lot comes out the stacks.
I don’t know why they don’t turn down the fuelling in that case, I don’t know much about engines, let alone diesel truck engines.
PhilDAS-
there’s a few examples in this vid - craziest one from about 6:18
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If you're genuinely interested in learning a bit more, I recommend watching some videos with Gale Banks talking about how to make the most power from diesel engines.
He has some great stories about all the "cool" things he's done in his life (hot-rodding, power boat racing, land speed records). He's got the kind of voice I could listen to all day long, but also he says categorically a diesel engine belching out black smoke is inefficient, it's not making power, and it's generally because it's been designed and tuned by people who don't know what they're doing.
His company designed and built this racing truck for Pikes Peak, note the lack of black smoke.
https://www.youtube.com/watch?v=fkCnpqNU-hM&ab_channel=BanksPower
You can also look up his Sidewinder diesel drag pickup which held the diesel 1/4 mile record for a long time, almost zero black smoke.
hvsds
NurseHolliday
Surely that smoke is unburnt fuel? that doesnt look good