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Extract from Machinery Lubrication publication:
Quote:
Sulfated Ash
The term sulfated ash relates to the amount of metallic elements in engine oils, which are mostly derived from the engine oil's detergent and antiwear additive chemistry. These additive packages contain multiple components based on metals such as calcium, magnesium, zinc, etc. Because a 100 percent seal between the piston rings can never be achieved, a certain amount of engine oil will enter the combustion burn.
As the engine oil enters the combustion chamber and burns, its residue forms an ash-like material. This ash-like material contributes to deposits in the crown land above the piston ring as well as to deposits in the ring grooves. These deposits can lead to rubbing wear on the cylinder liner and cause the piston rings to not operate freely. Ultimately, as the cylinder liner-to-ring interface is compromised high oil consumption can occur. In addition to these deposits, inorganic compounds from the lubricating oil's additives can become oxidized during combustion and generate metal oxide particles. These particles can be carried downstream with the exhaust and collect on the diesel particulate filter. These ash particles cannot be removed by filter regeneration because they are not combustible. As the ash particles accumulate, they result in filter blockage that increases back pressure to the engine, increasing fuel consumption and decreasing power. Ash particle buildup also necessitates more frequent cleaning of the particulate filters by mechanical means such as compressed air or water-pulse methods.
An engine oil's sulfated ash content also directly relates to an engine oil's acid neutralization capabilities (BN), because most of an engine oil's BN comes from the metal-containing detergent additives. Generally, the higher an engine oil's BN, the higher its ash content and the greater its ability to prevent acidic corrosion in the engine. Fortunately, with the mandated use of ultra low sulfur diesel fuel in on-highway applications, corrosion from fuel sulfur will require less of a need for BN control and thus a lower ash content.

In the analysis of oils, fats and waxes, the 'ash' value is obtained by combusting a weighed sample in a furnace at around 800C. The resulting solid (the ash) is sometimes treated with sulphuric acid and recombusted to convert the metal oxides (the ash) into sulphates, hence sulphated ash. Either way, the result is expressed as a percent by weight.