I'll give you my explanation: A diesel particulate filter gathers the black smoke you used to see when starting up a diesel, then burns it off at very high temperature later in the car's journey. It cannot do that if there is no 'later' and the exhaust system of the car does not get hot enough. Generally you need to do more than 10 - 15 miles and more than 20 miles in the winter for a DPF to start to regenerate. The vehicle cannot be used for a succession of short runs from cold. Diesel now only makes sense for very high mileage drivers doing 20,000 miles a year or more because over six years the cost of replacement DPFs, EGRs, DMFs and turbos otherwise outweighs any savings in fuel over a petrol engine.