All new diesel vehicles are now fitted with a diesel particulate filter to help trap and burn off harmful diesel smoke emissions. These filters often fail to reliably work as they were intended. Diesel is a very dirty fuel which is slow burning, and causes excessive soot (unburned fuel), even under normal conditions, to be expelled out of the exhaust. The DPF captures and stores all these dirty diesel particulates and, all being well, rids itself of them periodically when the ECU initiates a controlled regeneration process. Regeneration is where the captured soot is re-ignited and burned in the exhaust, creating even more carbon dioxide than the car manufacturers publish in their emissions figures. Most vehicles fitted with a DPF will eventually suffer from a blockage leading to power loss, excessive fuel consumption or total failure, rendering the vehicle undriveable.
Running a diesel vehicle on a mixture of gas and diesel can greatly reduce these particulate emissions and, any soot captured inside the DPF will be drier and the particulates smaller, than it would with diesel alone, making any regeneration much more likely to succeed.