The present work investigates the impact of lubricating oil formulations on efficiency and durability of Diesel particulate filters (DPF) by means of tests on a passenger car diesel engine retrofitted with a continuously regenerating DPF. An accelerated test procedure, characterized by oil injection into the intake air manifold to simulate a higher oil consumption, was designed and used to enhance the oil effect on trap weight increase and exhaust gas backpressure.
Different prototype oils, designed to evaluate the influence of key additives characterized by a different elemental content, were used.
The accelerated tests revealed to be a good tool to discriminate, in a short time, oils having a different impact on DPF performances. The main role of oil consumption and oil ash content emerged from the tests. In particular, the DPF weight increase exhibited a linear trend as a function of oil consumption, so that the ratio between the two parameters could be assumed as an indicator of the oil contribution to DPF deposits at constant oil consumption for each test oil. The same ratio showed a less than linear relationship as a function of oil sulfated ash: the higher the oil ash content, the higher the percent difference between DPF ash deposit and “ash exposure” calculated on the basis of oil sulfated ash. The different oil additives gave a contribution to DPF deposits mostly depending on their ash content.
The accelerated test conditions emphasized the effect of oil formulation on exhaust backpressure: a marked increase of backpressure occurred as a consequence of filter deposit accumulation.
The filters used in the engine tests were subjected to a post mortem analysis in order to characterize the deposits in terms of composition (oil and engine related elements), chemical species formed, distribution of deposits on the filter. For this purpose different analytical methods (ICP, XRD, SEM/EDS) were used.