How is the ash content for a gas engine oil determined?

Two key parameters define an oil for stationary gas engines:

• The TBN: Total Base Number (mg KOH/g):

It neutralises the acid products of combustion and reduces oxidation of the oil which encourages corrosion and deposits in engines. 

• Ash content (% by mass):

This varies in line with the TBN and reflects the quantity of calcium, zinc and other compounds originating from oil additives.
It should be noted that a minimum ash content is required to provide good protection against engine wear, but that an excess gives rise to spark plug fouling and creates deposits in the combustion chamber which encourages the risk of knocking.

Four types of oils are therefore defined:

• Ashless oils: ash content < 0.01%m

The closer an oil gets to this property, the cleaner the engine will be, but the risk of wear will be high, and oil changes will be frequent.

• Low ash oils: ash content < 0.5%m
• Medium ash oils: ash content < 1%m
• High ash oils: ash content > 1%m

The closer an oil gets to this property, the longer the drain interval will be, but the risk of deposits and fouling is high.