Stationary gas engines - Assessment and limit values

The working group "Gas Engines" has developed a leaflet for the monitoring of used oils for stationary gas engine plants. It has been approved by lubricant and gas engine manufacturers, well-known machinery insurers and by OELCHECK, the leading independent laboratory for lubricant analysis. The following article contains the most important information of the leaflet.

Table of contents

  1. Sampling
  2. Sample bottles and labeling
  3. Inspection intervals
  4. Special fuels
  5. Optional tests
  6. Limit values and validity

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

Sample bottles and labeling

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

Inspection intervals

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.

Special fuels

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.

Optional tests

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.

Limit values and validity

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.

Test parameters Limit values Causes / conclusions
Wear Symbol Unit Limit value  
Iron Fe mg/kg 21
  • from cylinder liners, cams/lifters, shaft journals, piston rings, gears
Chrome Cr mg/kg 5
  • from piston rings, valve stems, cams/lifters, chrome-plated piston ring grooves, other high-alloy engine components
Tin Sn mg/kg 5
  • from plain bearings
Aluminum Al mg/kg 10
  • from pistons and plain bearings
  • also a component of contaminated intake air
Nickel Ni mg/kg 3
  • of exhaust valves, high-strength steels (gears)
Copper Cu mg/kg 15
  • from bearings
  • Corrosion product from oil coolers and lubricating oil lines
  • lComponent of assembly pastes
Lead Pb mg/kg 20
  • from plain bearings
Molybdenum Mo mg/kg 5
  • Lubricating oil active ingredient element
  • Component of engine base oils and fuel gases
Silicon/dust Si mg/kg 4-7
  • Antifoaming agent and component of gas engine oils
  • from combustion air
  • Organic compounds from special gases
Potassium K mg/kg 25
  • Elements of cooling water corrosion protection agents,
  • Possible indication of cooling water contamination
  • Intake air containing salt
Sodium Na mg/kg Fresh oil +25
  • typical elements of cooling water corrosion protection agents
  • Intake air containing salt
Water H2O m % 0.2
  • Leaks in the cooling water system
  • Condensation processes in the lubricating oil system or during storage
  • Insufficient ventilation of the crankcase/active lubricating oil tank
  • Incorrect setting of the lubricating oil separator in large oil circulation systems
Glycol   ppm 500 (FTIR positive)
  • Leaks in the cooling water system,
  • Contamination with a foreign polyglycol-based product
Oil condition

Viscosity at 40° C


Viscosity 100° C

Viscosity index



SAE 40:

min. 12, max 18

SAE 30:

min. 9, max 15

Increase max 3 times

Characterizes the fluidity of the lubricating oil (resistance to displacement of two adjacent
layers, internal friction) and is temperature-dependent.

Increase in viscosity occurs due to:

Aging/nitration, soot/solid foreign matter Decrease in viscosity occurs due to evaporation of light boiling components, fuel contamination (diesel gas/pilot injection engines only), shear of VI improvers if present in the lubricating oil.

Oxidation   A/cm 20
  • through reaction of the base oil and active ingredient molecules with oxygen
Nitration   A/cm 20
  • through reaction of the base oil and active ingredient molecules with nitrogen oxides
Sulfation   A/cm 36
  • through reaction of the base oil and active ingredient molecules with sulfur-containing gas or fuel



















in comparison

to fresh oil

  • Elements of lubricating oil active ingredients
  • Elements of lubricating oil active ingredients
  • Elements of lubricating oil active ingredients, coolant additive
  • Elements of lubricating oil active ingredients
  • Elements of lubricating oil active ingredients
  • Elements of lubricating oil active ingredients
BN   mgKOH/g

> 50% of the fresh oil
but >2

The BN indicates the alkaline reserve of the oil and characterizes the chemical neutralization capacity. Depends on the fresh oil additive. When operating with contaminated fuel gases
(especially landfill, sewage and biogases), rapid degradation of BN is to be expected.

AN   mgKOH/g Fresh oil value +2.5 Captures the strong and weak acids, lubricating oil active ingredients affect the value for gas engine fresh oils, which can range from 0.5 to over 2 mgKOH/g. There is a rough correlation between AN increase and oil aging and oil nitration.
i-pH     > 4.5 Also covers acids that are not represented by a BN. Especially important for landfill gases.
SAN   mgKOH/g not detectable The method only detects strong acids, such as sulfuric acid. If a SAN is detected, there is a risk of corrosion.
Soot   M. % 1.5 Soot is formed during the combustion of hydrocarbons in the absence of air. In particular, diesel gas/pilot injection engines may have larger amounts of soot in the used oil. Soot in the lubricating oil leads to an increase in viscosity and, at high concentrations, to an increase in component wear.



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.