Cold Flow Performance of Diesel Fuel and Winter Operability of Diesel Passenger Cars
Due to the paraffinic nature of diesel fuels, paraffinic wax crystals can form at low temperatures. This could in practice lead to a blocking in the fuel line system of diesel vehicles in harsh winter conditions therefore, in such conditions vehicle operability problems might occur. Through the application of different technical configurations/designs in the vehicle and the use of cold flow additives in diesel fuels, the cold operability temperature limit can be reduced despite the crystallisation of paraffin waxes in the fuel. After a sudden cold period during winter 2011/2012, a significant number of broken-down vehicles in the market were reported, resulting in the German BMUB initiating a working group within the DIN FAM for the investigation into the issue. The DIN FAM proposed cross-functional group to conduct a vehicle test programme in order to study the link between diesel fuel property parameters (analysed in the laboratory) and cold operability performance of modern diesel vehicles. The industry groups involved were vehicle manufacturers, fuel producers and fuel additive producers. For the programme two market diesel fuels were specifically selected due to their different cold flow properties (in particular CFPP values). One fuel was a typical winter diesel fuel and the other for the autumn period. Both diesel test fuels were analysed to confirm compliance to the DIN EN 590 diesel specification. In addition, the two fuels showed different paraffinic wax sedimentation performance and different wax crystal pattern at freezing temperatures. The two selected fuels were tested on cold operability test rigs already utilised by the participating companies (rigs differ in their technical configuration). These results showed a varied reproducibility when compared against each other, and also did not result in a clear difference in cold operability rating between the two fuels. There were differing opinions expressed within the project group regarding the need to develop an optimized cold operability test rig and associated test method. In the course of the vehicle programme, six diesel vehicles of current marketed technology were tested at a chassis dynamometer in a climate chamber (as per the test method procedure developed in the project). Overall the results of this project showed that the differing technical configurations/designs in the vehicles likely caused a wider spread of the cold operability test results when compared to the impact of the two investigated test fuels DK-1 and DK-2. The investigation of the suitability of any of the vehicles from the test pool to be assigned as a reference vehicle should be investigated in an additional test programme with a greater variety of winter diesel fuels.