Some Factors Which Affect Octane Requirement Increase 750933

The influence of selected engine oil, fuel, and driving schedule variables on combustion chamber deposits and octane requirement increase (ORI) has been investigated during vehicle fleet tests with late model cars.
Greater octane requirement increases occurred with: (a) an engine oil containing bright stock compared to an engine oil without bright stock, (b) an unleaded fuel which contained a polymeric detergent-dispersant additive with a large amount of carrier oil, compared to other unleaded fuels with conventional additive packages, and (c) customer-type driving compared to rapid mileage accumulation on a chassis dynamometer. Octane requirement increase was not affected by: (a) lead content of the fuel; or (b) ashless engine oil compared to a conventional ash-containing oil.
All of the ORI with unleaded fuel can be eliminated by removing the combustion chamber deposits, and about two-thirds of the ORI is caused by deposit accumulation in the end-gas region of the chamber. In late model cars using unleaded fuel, about ten percent of the ORI is due to the volume effect of deposits and the other 90 percent is probably due to thermal effects.


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