A test procedure was set up in our laboratories to evaluate the propensity of fuels and lubricating oils towards the soot accumulation in Diesel Particulate Filters. The experimental work was carried out with the use of a passenger car diesel engine, retrofitted with an aftertreatment system composed by an oxidation catalyst and a DPF.
The soot propensity was evaluated by means of repeated measurements of differential exhaust backpressure gradient, during a running period at mid load and speed. The specific fuel consumption gradient was also measured to find a correlation between both the variables. After each soot loading period, a burning off period at full load was operated for the purpose of filter regeneration.
A two-phase experiment was undertaken to assess repeatability and discrimination capability of the test procedure.
During the first experimental phase, repeated tests were conducted on a fuel matrix containing some surrogate fuels. The results supported the conclusion that the soot loading test is characterized by good repeatability and very good discrimination capacity towards the fuel variable. Among the parameters that characterize the fuel matrix, the effects of Cetane Number and aromatic components were clearly pointed out.
During the second experimental phase, the soot propensity of different base oils was evaluated by means of repeated tests with use of different oils and a single fuel. Due to a moderate oil consumption level, the discriminating capacity of the soot loading test towards the oil variable revealed to be remarkably lower than the fuel variable, even in presence of good repeatability. In any case, the positive effect of synthetic basestocks emerged and was statistically validated.
During the whole experimentation, corresponding to an accumulated running period of 172 hours, the oil consumption remained quite limited and constant, which, along with the use of oils characterized by low saps technology, provided limited DPF weight increase but evident exhaust backpressure increase due to ash accumulation, probably enhanced by a slight downsizing of DPF.