Early diesel engines when driving would have a cloud of black smoke coming from the exhaust .
Due to modern emission laws we can no longer put the soot particulates ( black smoke ) into the air.
A device called a Diesel Particulate Filter ( DPF ) was invented.
A DPF catches the soot particulates and when it gets to a certain level the DPF will go through a REGENERATION phase, the exhaust temperatures can increase upto 700 degress whilst cleaning the DPF.
Once the DPF is clean the REGENERATION cycle can take on average 400 miles or a full tank of fuel for the process to start all over again .