Influence of Lubricating Oil Ash on the ORI of Engines Running on Unleaded Fuel 720945

The paper gives a survey on the influence of lubricant ashes on octane requirement increase (ORI) at steady-state high speed of a typical engine: the 125 B model, modified to run on unleaded 91 NORM fuel.
Through preliminary tests, a method was set up to evaluate knock by oscilloscope, and a test cycle of 216 h was run which permits reproduction of the stabilization of deposits equivalent to 40,000 km (25,000 miles) of operation. With this type of test, straight mineral oils, ashless oils, and oils containing traditional organometallic, calcium/zinc, barium/zinc, and magnesium/zinc base additives as well as calcium, sodium, and zinc base additives of new formulation were evaluated. The results revealed that ORI values are almost identical, that is, 3-4 octane numbers for both the straight mineral and ashless oils.
As regards the oils with organometallic additives, ORI is influenced by the nature of the metal, and its values are in the range of 8-5 octane numbers.
Also, the relation between ash content and ORI value was evaluated for the calcium/zinc and barium/zinc oils; it appears that ORI decreases only when the ash content is less than 0.3%.
The results obtained with primary reference fuels (PRF) were confirmed with two full boiling range fuels (FBRF).


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