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I have often noticed that on the road, whenever a heavy diesel accelerates, it emits a huge cloud of black smoke. If this happens in front of me, I put the air recirculation mode on. This typically happens with heavy trucks, and the smoke clouds from heavy trucks are clearly the largest. For some reason, I don't see it typically happening with buses. Passenger cars and vans are in severity and probability between buses and heavy trucks.

Why does this happen? What is the process that produces these huge clouds of black smoke? I understand that the huge cloud of black smoke is unburnt fuel, so is this related to worn injectors?

Is the cause lack of maintenance? Or just using worn equipment as long as possible?

Does a diesel particulate filter filter too these huge clouds of black some away, or is the DPF there only for the smallest particles?

  • Wow, that's a lot of questions there ... care to dial it back a few and maybe ask it as a separate question(s)? Link back to this question to give full understanding of where you're going from. Also, it's a bit of an assumption to think it's mainly "old" diesel trucks which this happens to ... it can be caused from a multitude of things, whether the truck is new or old. – Pᴀᴜʟsᴛᴇʀ2 Jul 19 at 19:21
  • Ok, I removed the "how to fix it?" part. – juhist Jul 19 at 19:22
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Why does this happen?

It happens when there's too rich of an air/fuel mixture. You usually see it happen during hard acceleration, when the engine is trying to produce more power so it can speed up.

What is the process that produces these huge clouds of black smoke? I understand that the huge cloud of black smoke is unburnt fuel, so is this related to worn injectors?

It's usually not worn injectors which causes this. There are two main reasons:

  1. Too much fuel - This can be caused from someone monkeying with the tune to allow more fuel to enter during the combustion process. It's not usually caused by an issue with the injectors because most injectors are either electro-mechanical or purely mechanical ... they inject only so much fuel. They don't stick open, but rather won't inject enough fuel when there's an issue.
  2. Not enough air - If there's any problem with the intake system, such as the owner hasn't done maintenance on the air cleaner, or if the turbo charger isn't working well enough (for whatever reason).

Is the cause lack of maintenance? Or just using worn equipment as long as possible?

See above.

Does a diesel particulate filter filter too these huge clouds of black some away, or is the DPF there only for the smallest particles?

As far as the DPF goes, it depends on what the filter is designed to do. As per this article on Wiki - Diesel Particulate Filter on Wikipedia.:

Unlike a catalytic converter which is a flow-through device, a DPF retains bigger exhaust gas particles by forcing the gas to flow through the filter; however, the DPF does not retain small particles and maintenance-free DPFs break larger particles into smaller ones. There are a variety of diesel particulate filter technologies on the market. Each is designed around similar requirements:

  1. Fine filtration
  2. Minimum pressure drop
  3. Low cost
  4. Mass production suitability
  5. Product durability

It really depends is the answer to this one.

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Many diesel cars and trucks have the issue that the turbo "clogs" up with particles when under low load ie coping with city traffic. Once they can accelerate the turbo spins up and cleans itself emitting the black smoke or particles.

I had an Audi that was classic for that, if you cruised at just under 2000rpm it was happy but the turbo was not doing anything, accelerate and it would spin up releasing lots of smoke... But then would run clean.

It's not the only issue, worn injectors are also a common cause. I worked on a friend's tractor and he complained that it smoked too much. So, I had the injectors refurbished and it made so little smoke he was stunned - said it was like new !!

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One additional cause of black smoke is a faulty air flow meter or map sensor. At full ‘throttle’, to avoid over fuelling, the ecu will monitor the amount of air being ingested. If the air flow is not being monitored due to a faulty sensor, the ecu can inject too much fuel and cause black smoke.

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A diesel will run with more things not working properly compared to a gasoline engine. So while a gasoline engine will not start and the operator is forced get maintenance done, a diesel will chug away ; making black smoke, even when components like injectors are malfunctioning.

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