The stress on gas engine oils in operation is essentially characterized by:

• high thermal and oxidative stress
• reactions of the combustion products with the lubricating oil, such as nitration (nitrogen oxides), sulfation (sulfur-containing reaction products) and oxidation
• the action of acidic reaction products and impurities
• the respective operating mode.

As a rule, these influences lead to a change in the lubricating oil, which can be assessed by determining some suitable characteristic values. Economical operation is achieved with the large oil filling quantity of the engines, among other things, by ensuring that the operating oil filling lasts as long as possible while basically avoiding damage and achieving the expected service life of important engine and system components. In order to achieve this objective, the leaflet describes the necessary parameters that must not be missing from an oil analysis. Limits and reference values are proposed, and when these are reached, appropriate measures become necessary. The data sheet was developed for stationary gas engines operating on natural gas. If special fuels are used, additional characteristic values must be determined in the oil, which are considered separately.

Proper sampling is a prerequisite for evaluable analytical results for every test. The sample must be representative of the operating oil charge and should - if possible - be taken during operation. The dead volume standing in the area of the drain cock must be drained off beforehand and disposed of properly. If no sampling device is available, the sample should be drawn directly into the sample bottle immediately after the motor stops, preferably with the aid of a sample pump.

Read also: Tips for correct sampling

Oil-resistant, transparent and clean bottles must be used as sampling bottles. OELCHECK uses only optimally suited bottles – they are included in the scope of every oil analysis.

The samples must be clearly labeled. OELCHECK's Sample Information Form includes the most important information:

• Operator or customer
• Manufacturer and designation of the aggregate (type)
• Engine or serial number
• Name of the lubricant
• Sampling date
• Engine operating hours
• Oil operating hours
• Refill quantity / oil consumption
• Total oil volume

The inspection intervals depend on the aggregate, the fuel and the mode of operation and are to be determined individually for each aggregate.

At the same time, however, the operating instructions of the individual plantmanufacturers must be observed.

For the purposes of the leaflet, special gases include all gases except natural gas and liquefied petroleum gas. The following parameters and warning values apply to them additionally or are evaluated differently:

• i pH value > 4
• If special gases contain chlorine and/or silicon compounds, an investigation of the chlorine and/or silicon content in used gas engine oils is required.

The presence of chlorine can lead to corrosion. However, this is only the case if chlorine is present in the form of water-soluble chlorides. Nevertheless, since the determination of the chlorine content with the currently common analysis practice results in the total chlorine content, the value determined in this way does not directly indicate a risk of corrosion in the plant. The risk of corrosion can only be carried out by elaborate special tests. Therefore, the specification of a limit value according to the current state of analytical practice is not possible.

On the one hand, silicon indicates the presence of organosilicon compounds in the special gases. On the other hand, silicon is also introduced via combustion air that is heavily contaminated with dust. The origin of the silicon cannot be differentiated in the analysis methodology usually applied.

For this reason, special attention should be paid to wear elements such as iron, chromium, copper and aluminum when silicon levels are high, and the engine manufacturer or maintenance company should also be consulted if necessary.

It is recommended (and it is also OELCHECK standard, but not oil manufacturer standard) to additionally determine the following elements, which essentially give an indication of the change in oil composition, by means of a lubricant analysis: Potassium, boron, sodium, calcium, magnesium, zinc, phosphorus, sulfur.

Potassium and sodium, in conjunction with elevated boron content, provide evidence of cooling water input. The other elements, including boron, can be components of the lubricating oil. They can therefore only be seen as an indicator of oil change intervals to a limited extent.

The values listed in the table below are for conventional mineral oils and mineral oil-based lubricants based on hydro-cracking or similar hydro-treatment (Group I and II oils). Synthetic oils are to be evaluated differently.

The values for wear, impurities and corrosion elements continue to apply only to natural gas engines with oil fillings between 30 and 300 l. The special limit values are reduced for larger oil volumes or oil sump contents that are increased by design. When using subsequently installed bypass filters, the warning values are not applicable.

Decisive for the informative value of the elements is a trend analysis, whereby trend is understood to mean both the increase of the individual elements within 1,000 operating hours and the regular attainment of these limit values after 1,000 operating hours. Irrespective of this, progressive wear processes can take place in the engine even at low concentrations of wear elements.

If oil analyses are always carried out after the same running time, e.g. on the occasion of the oil change, then even in the case of relatively small deviations from the trend or when the values mentioned here are reached in each case:

• a new sample is to be analyzed and
• the manufacture / maintenance company is to be consulted.

If only a single element or a single value is outside the trend, its plausibility must be checked.

Note:

OELCHECK determines the BN (base number) for engine oils and the AN (neutralization number) for all other oils using analysis kit no. 2. These two values offer a complementary statement for an oil change extension for the respective oil type. The BN describes the absorption capacity of the oil for the acidic components from the combustion gases which result from the combustion process. The AN indicates the degree of acidification. An engine oil is worse the lower the BN has become compared to the fresh oil. A hydraulic oil or an industrial lubricant is worse the higher the AN or NZ or neutralization number has become compared to the fresh oil.