Do higher ash gas engine oils always cause dirty engines?

In the stationary gas engine industry there are many ‘urban legends’ and ‘rules of thumb’ about lubricants. Often these ‘rules’ are very important because they steer the direction of gas engine oil development but they also contribute to misinterpreting the performance of stationary gas engine oils. A few examples of these urban legends are ‘replenish at 50% TBN’, ‘Group II is better than Group I’ and ‘more ash, more deposits’. Challenging these accepted rules is very important to make the next step towards better and more innovative stationary gas engine oils. I have written a series of articles about these urban legends. The third article is about higher ash gas engine oils and their relation to deposits. Please feel free to share, comment or to contact me.

Joris Van der List – Technology Manager

Challenging the urban legends of stationary gas engine oils: Article 3 of 3

Do higher ash gas engine oils always mean greater deposits?

Probably the most persistent rule of thumb in the stationary gas engine industry is the rule that gas engine oils with a higher ash content cause greater deposits in the engine. This is also the reason why, historically, the sulphated ash limit for natural gas applications has been a maximum of 0.5 %mass. This value is considered to be low to medium ash oil.

A closer look at sulphated ash

First it is good to clarify what sulphated ash actually is. Sulphated ash is a measure of the metal content of a lubricant; basically the amount of residue that is left after combustion of the gas engine oil. Note that different metals provide varying contributions to the sulphated ash content.

Element Factor (= S-Ash contribution)
Barium (Ba) 1.7
Calcium (Ca) 3.4
Magnesium (Mg) 5.0
Zinc (Zn) 1.5
Copper (Cu) 1.3
Sodium (Na) 3.1
Lead (Pb) 1.5
Boron (B) 3.2
Molybdenum (Mo) 1.5

Note that typical condition oil monitoring reports show the additive and wear metal content of the oil. Based on this data and the ash contribution factor the sulphated ash content of a gas engine oil can be calculated. To give an example, consider an oil with the following additive metals:

Element: Additive content: Ash contribution (= Treat rate x Factor)
Calcium (Ca) 0.12 %-mass 0.408 (= 0.12 · 3.4)
Zinc (Zn) 0.04 %-mass 0.06 (= 0.04 · 1.5)
Boron (B) 0.01 %-mass 0.032 (= 0.01 · 3.2) +
0.5 %-mass

Apart from the calculation, the sulphated ash content can also be measured according to ASTM D 874.

It is also important to understand that detergents are one of the largest contributors of ash, but multiple additive components also contribute to the sulphated ash level. The major sources for sulphated ash are: dispersants, detergents, overbased detergents and anti-wear additives.

Higher ash oils can mean cleaner engines

Coming back to the ‘higher ash results in more deposits’ rule, this has some logic  because when metallic detergent additives burn, they form ash, which can deposit on engine components such as pistons, heads and valve stems. Although a small amount of deposit on the valves can help to prevent valve recession, too much will create unwanted and harmful deposits.

However, in gas engine operations the oil can lead to various types of deposit such as sludge, varnish, lacquering and ash residue. The first three are controlled by the detergent/dispersant additives. This means that, with the right gas engine oil formulation, the deposit tendency can be improved significantly over traditional type gas engine oils. Consequently, there are very good high ash ‘clean technology’ products on the market with outstanding low deposit tendency.

This statement can be confirmed in laboratory bench testing. The example below shows the result of a Panel Coker Test (detergency capacity tests), in which the oil is splashed by a spindle on an aluminium plate at a temperature comparable with a combustion chamber temperature (>300°C). The pictures of the aluminium plates are from the end of the test.

higher ash gas engine oils
Group III based GEO 0.8% Sulphated Ash ‘clean technology’
higher ash gas engine oils
Group II based GEO 0.5 % Sulphated Ash
higher ash gas engine oils
Group III based GEO 0.5 % Sulphated Ash

Results confirm that the clean technology product performs better than the other products, even though the ash level is higher (0.8 % ash) than the traditional 0.5 %-mass ash products.

But what about oil consumption?

Oil consumption also plays a major role in deposit tendency; higher oil consumption leads to more oil being burnt in the combustion chamber. In modern gas engines oil consumption can be very low, even as little as 0.05 gr/kWh in some engines.

In the oil consumption debate it is important to include the aspect of used oil performance. When the oil is fresh, or the engine has just had a major overhaul, oil consumption is typically under control and therefore it is essential to evaluate the oil consumption trend over a longer period.

Oils that prevent liner lacquering, bore polishing and liner scuffing have a lower oil consumption over time.

Note the example in the graph below where the gas engine was lubricated by a traditional product for up to 45 months and at this point the engine was overhauled and the owner switched to a modern gas engine oil.  The difference between the two products is not the oil consumption just after the overhaul, or when fresh, the difference is that the oil consumption stays low over time.

higher ash gas engine oils
The reason why oil consumption remains at a low level is the increased performance of the oil over time to prevent liner lacquering and bore polishing.

The reason why oil consumption remains at a low level is the increased performance of the oil over time to prevent liner lacquering and bore polishing. In fact, modern higher ash (≈ 0.8 %-mass) / clean technology stationary gas engine oils can prevent the formation of deposits in the combustion chamber, for example on the head, piston top, liners and valves. This is one of the key elements in preventing pre-ignition, knocking and ring-sticking, but eventually also achieves better performance when controlling oil consumption over time.

Farewell to the urban legend

There are still many traditional gas engine oils on the market to which the ‘higher ash results in more deposits’ rule can be applied. But, with good gas engine oil technology, higher ash products deliver additional performance in terms of keeping the engine cleaner, controlling oil consumption and increasing the drain interval. Some OEMs have already approved higher ash oils such as Q8 Mahler G8 and Q8 Mahler GR8 for natural gas operations, even in high BMEP steel piston type engines.  These oils are outperforming the traditional 0.5 %-mass ash type oils and also keep the engine cleaner.  OEMs and lubricant companies should re-evaluate this urban legend in order to improve the SGEO recommendations to support the use of better lubricants.

Please feel free to share, comment or to contact me to discuss in more detail.

Joris van der List

From our expert Joris van der List

After working 8 years in the Q8Research institute in Rotterdam, Joris van der List joined Q8Oils in 2011. Next to being Technology Manager, he is expert in the Energy segment and has a background in mechanical engineering.

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