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:
- 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.
- 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?
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:
- Fine filtration
- Minimum pressure drop
- Low cost
- Mass production suitability
- Product durability
It really depends is the answer to this one.