The Changing Role of Sulfur in Today's Engine Oils
Historically sulfur when used in lubricating oil formulation chemistry it has been used as an additive for an antioxidant, part of antiwear (AW) compounding, extreme pressure (EP), and enhanced lubricity properties (the oil's ability to lubricate under boundary metal-to-metal contact conditions).
Emission regulations in engine oils represents the area of most change in limiting the amount of sulfur allowed in today’s lubricants. Reduction in fuels and engine oil has impacted lubricating properties required for fuel injectors and other engine components by reducing the lubricity properties in these products. This requires new additive chemistry to fill the performance requirements in these products. Also, since sulfur acts as an effective antioxidant alternatives are more in demand.
These days most sulfur contained in formulated lubricating oils comes from the additives used. Her is not necessarily being eliminated completely, but the types of sulfur compounds used in engine oils are changing and the levels decreasing. Reduction of sulfur in engine oils that contribute to metal ash forming emissions is required to meet regulatory limits, but this reductions removes many of the positive properties sulfur contributed to the lubricant.
A reduction in the sulfur content in engine oils is also tied to reductions in phosphorus content. Changes in the allowed level of phosphorus in engines oils are tied into lower emission regulations. Phosphorus reduces the efficiency of active catalyst sites used in exhaust after-treatment devices. This has affected the sulfur content in lubricating oils that use zinc/phosphorus additive compounding for antiwear and antioxidant properties. This is because the zinc/phosphorus chemical compound contains significant sulfur. This has been a key additive for antiwear properties in lubricating oils over the years. Reduction of the zinc/phosphorus chemical compound to lower phosphorus limits has also lowered the sulfur.
Base oils contribute very little sulfur to engine oils these days. Groups II or III and IV base oils used to formulate engine oils have very low or no sulfur. Most base oils used to formulate lubricants for engine oils use Groups II or III base oils. Higher sulfur Group I base oils are more commonly used in industrial lubricant applications, though there are still some engine oil applications. Group IV is a fully synthesized base stock that is sulfur free and is used in combination with other base oil groups or 100% for extreme operating conditions.
The reduction in sulfur using Groups II or III base oil, which are in wide use, does have the advantage of allowing formulators to control the level and type of sulfur additive compounding in the finished lubricant product. This also reduces the presence of active sulfur in the base oils which can form acids with the presence of moisture and other chemicals. In some Group II base oil a little bit of sulfur is actually added back in as an antioxidant.
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