The main source of Sulfated Ash in engine oils is actually from the detergents. Detergents are used for a variety of purposes - everything from preventing and removing deposits, neutralizing acidic byproducts and acting as dispersants to prevent engine sludge. The metal compounds used to create detergents is typically the what becomes sulfated ash, however it can also come from other metal containing additives used in the oil formula.
Sulfated ash is measured by treating the oil with sulfuric acid and then igniting the mixture. The residue is then measured. Oils that produce ash of 0.1% or lower are considered ashless; up to 0.6% are classed as low ash, 0.7-1% medium ash and over 1% is a high ash oil.
Engine designers have begun to look at SA as a factor in combustion chamber deposits and the precursors of emission problems. The presence of more than the optimal amount can cause excessive deposits on various parts, which can result in reduced heat transfer, pre-ignition or detonation, or both, ring sticking or breaking, plug fouling, and valve burning. This became a focus during the development of CJ-4 oils for diesels which requires low ash content to assist with the efficiency of after treatment emission systems (DPF AND SCR systems). These systems rely on clean burning in combustion chambers to minimize harmful emissions. It is not surprising that this will affect gasoline and direct injection engines in a similar way, with medium - low ash oils being preferred by engine manufacturers in these cases.
It will be interesting to see what develops in terms of ashless detergents and how they are applied. Various types of calcium sulfonate is one of the most commonly used detergents in commercial additive packages, and removing them could cause many formulators big head aches. The difficulty is that detergents prevent sludge and varnish in the crankcase, but when burned in the cylinders or when experiencing blow-by it can cause deposits. The presence of these detergents is usually identified by the calcium number found on your UOA.
