Ash in Gas Engine Oils

WHAT IS SULPHATED ASH?

Sulphated Ash (Sulfated Ash) is the percentage of oil that remains after burning it, treating the initial residue with sulphuric acids, then burning the treated residue. It’s a measure of non-combustible (mostly metallic) components in an oil.

The majority of new industrial, mineral lubricating oils have few ash-forming substances. It is typically a feature of engine oils, and other lubricants containing metallo-organic ingredients, detergents, and other additives – these will leave a residue in the sulphated ash test. These metals are usually barium, calcium and magnesium, potassium, sodium, zinc, and/or tin, but non-metallic compounds containing sulphur, phosphorous, or chlorine can also contribute. 

WHAT IS THE PURPOSE OF ASH IN A GAS ENGINE OIL?

The primary distinguishing feature of a gas engine (as opposed to gasoline or diesel) is that the fuel is introduced into the combustion chamber during the gaseous phase. This has two significant impacts – a lack of cooling provided by liquid fuel, and the lack of lubrication in the form of liquid droplets or soot; the latter fact has a significant impact on the intake and exit valves.
A demonstration of the geometry of a recessed valve compared with an idealised valve in a gas engine
An example of a new, seated valve (left) vs a recessed valve (right)
Ash acts as a cushion to reduce metal-to-metal contact between the hot valve faces and their mating seats. Gas engines are dependent solely on lubricant ash to provide this protection. Too little ash can result in valve and seat wear; this will ultimately reduce the life expectancy of the cylinder head. Too much ash can lead to valve guttering and valve torching. Excessive ash in the combustion chamber also increases the risk of detonation. 
 
Gas engine original equipment manufacturers (OEMs) typically specify a narrow range of ash levels that provides the best performance. As the combustion products of most gases have few acidic components, excess ash does not need to be added to meet alkalinity requirements, and ash levels are optimized around the valve requirements. There are exception in untreated biogas and landfill gas applications in which more TBN is required to neutralize acid formation.
 

WHAT IS THE RIGHT AMOUNT OF ASH?

The “right amount” is dependent upon both oil composition and engine operation.

Ash accumulation can be affected by engine load. At full load, the butterfly valve (throttle) is wide open and creates positive air pressure in the intake manifold. Conversely at lower loads the throttle may be closed, creating a vacuum which pulls oil past the valve guides and into the combustion chamber. Counterintuitively, low loads will therefore lead to more ash buildup around the valve seat and face. This can reduce the life expectancy of the cylinder heads; if there is a requirement to run at low loads for operational reasons, an ashless oil may be a more appropriate choice that extends the life of engine components.
Leak path for oil past valve guide in a low-loading condition for a gas engine.
Leak path for oil past valve guide in a low-loading condition for a gas engine. The net force is created by the vacuum condition below the valve cause by a closed throttle.
Excessive ash can cause damage to the exhaust and aftertreatment systems. High ash is usually indicative of high levels of additives containing sulphur and phosphorous which (as in automotive applications) poison catalysts. In the combustion chamber, hot ash deposits can also act as ignition sources that ignite the air/fuel mixture independently of the spark plug and cause detonation.
 
As a special case – two-stroke engines usually require an ashless oil because even the smallest amount of ash can clog up the intake and exhaust ports.
 
As general guidance – select an oil with an ash level in line with OEM guidance. If operating the engine in a unique condition (such as low load) consult with both your OEM and lubricant supplier to determine the correct ash level in the oil.