Effects of Engine Oil Formulation Variables on Exhaust Emissions in Taxi Fleet Service

The relationship between engine oil formulations and catalyst performance was investigated by comparatively testing five engine oils. In addition to one baseline production oil with a calcium plus magnesium detergent system, the remaining four oils were specifically formulated with different additive combinations including: one worst case with no detergent and production level zinc dialkyldithiophosphate (ZDTP), one with calcium-only detergent and two best cases with zero phosphorus.

Emissions performance, phosphorus loss from the engine oil, phosphorus-capture on the catalyst and engine wear were evaluated after accumulating 100,000 miles of taxi service in twenty vehicles. The intent of this comparative study was to identify relative trends. The study found that contamination of catalysts by phosphorus adversely affects emissions of hydrocarbons, carbon monoxide and nitrogen oxides and, for the observed levels of captured phosphorus, emissions degradation correlates directionally with the amount found on the close-coupled catalysts. The vehicles operating with an experimental oil containing ZDTP but without detergent resulted in the largest deterioration of emission system performance and the greatest amount of phosphorus captured onto the catalyst. Phosphorus transport by selective volatilization of phosphorus components from the oil was found to be a significant factor. There was no evidence that abnormal or excessive wear or corrosion occurred based on used oil wear metal data.