Are Group II/III gas engine oils really better than Group I?

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 and will publish these on LinkedIn in the coming weeks. The second article deals with Group II/III base oils, and if they are really better than Group I. 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 2 of 3

Are Group II/III base oils really better than Group I?

Approximately 90% of stationary gas engine oil is base fluid, the remaining part is performance additives. Base oils are hydrocarbons produced from refining crude oil or through chemical synthesis. Base oil produced by refining crude oil is classified as mineral oil. The American Petroleum Institute (API) has categorised base oils into five groups. The first three groups are refined from petroleum crude oil and are mineral oils with an increasingly severe refining process. Group I is solvent-refined mineral oil, Group II is hydrotreated and Group III is hydrocracked material.

Group I is defined as base stock that contains less than 90% saturates and/or greater than 0.03% sulphur, while Group II and III base oils are categorised as fluids with more than 90% saturates and less than 0.03% sulphur.

Group II base oils are produced using hydrogen gas in a hydrotreating process, which converts unwanted unsaturated components, like aromatics, into saturated hydrocarbon structures. Saturated means that the hydrocarbon molecule consists of single bonds and is thus saturated with hydrogen. See the picture below.

Group II/III base oils
Group II/III base oils

The production process for Group III base oils is similar to that of Group II mineral oils, except the hydrogenation process is at high temperatures and pressures. Therefore, nearly all unwanted components within the oil are converted into saturated hydrocarbons, resulting in a purer base oil.

A very common urban legend in the industry is that Group II and III based lubricants are superior to Group I based lubricants. This assumption is based on the fact that the oxidation stability of Group II/III oils is better than for Group I.

The pro: oxidation stability means better performance

Oxidation is the process in which oil reacts with oxygen. Higher temperatures and the presence of catalysts, such as copper/iron wear particles, increase the speed of oxidation. Oxidation occurs to some degree in all lubricants; the effect of oxidation is increased oil viscosity and acid value, sludge and deposit formation and additive depletion.

Group II/III base oils are more stable than Group I in terms of oxidation. The reason for this improved oxidation stability can be related to the higher percentage of saturates. These saturates are less reactive with oxygen compared to unsaturates.

Owing to their improved oxidation stability, Group II/III based gas engine oils perform better in laboratory oxidation bench testing, but also in practice the oxidation value of condition monitoring samples shows improved performance with lower values. Based on this, the lubricant drain intervals of Group II/III based gas engines oils are longer than for Group I.

The con: oxidation stability lowers solubility

However, the problem is that improved oxidation stability comes at a cost. Group II/III based gas engine oils contain fewer aromatics/unsaturates than Group I. This higher saturation level, and therefore improved oxidation stability, is also the reason for lower solubility.

Solubility is the ability to keep soot and oxidation by-products dissolved in the oil. This is the reason why in many oxidatively stable Group II/III oils the soot separates from the oil and forms lacquer and deposits on critical engine parts such as liners, pistons and heads. A good example is the ring sticking, caused by deposits in the top ring, that occurs at high temperatures with some Group II/III based gas engine oils.

Also note the picture below which shows that improved oxidative stability of the base oil unfortunately also implies reduced solvency.


Group II/III base oils

Improved oxidative stability

Group II/III base oils
Linear and branched paraffinics

Proving the impact of good solubility

The effect of good solubility can be seen in the following laboratory experiment. Two oils are artificially aged, in a metal can inside an oven, in the laboratory. Oil A is developed with excellent oxidation stability, while oil B has poor oxidation stability but with better solubility.

Results are as shown below:

Group II/III gas engine oils
Oil A is developed with excellent oxidation stability, while oil B has poor oxidation stability but with better solubility.

The results confirm that oil A stays bright and clear, due to its excellent oxidation stability. Note that, after the test, the metal can is full of lacquer and varnish. Oil B darkens and ages faster; the oxidation by-products remain dispersed in the oil, indicating better solubility, but the metal can is clean after the test.

You can compare oil A with a gas engine oil where all the routine analyses are clear, but in time engine failure could occur due to extreme depositing in the engine.

Farewell to the urban legend

Although the trend towards Group II/III gas engine oils has resulted in more oxidatively stable gas engine oils, it is also important to acknowledge that solubility plays a major role in the overall performance. Group II/III oils are therefore not automatically better for the engine but, combined  with good solubility, their quality can be outstanding. Gas engine oils users should take care to distinguish between high- and poor-quality oils. Differentiating on base oil and oxidation stability alone is not sufficient for modern high BMEP type gas engine oils.

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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