Biogas Digesters - Engine Lubricating Oil

Proper care and maintenance of the generator engine will ensure many years of trouble free use.
Biogas Digesters - Engine Lubricating Oil - Articles


Figure 1. A common generator setup for Pennsylvania Farms.


The most common electrical generator system used at farm biogas facilities today is a stationary internal combustion engine that has been modified to 1) run on biogas, 2) drive a generator, and 3) produce single or three phase electrical power. Lubricating oil is distributed throughout the engine's moving parts to keep the device running smoothly and to reduce wear. Proper care and maintenance of the generator engine will ensure many years of trouble free use. However, failure to take care of regular maintenance is a sure way to invite trouble. This is definitely true when it comes to the engine's lubricating oil.

Key Properties of Generator Lube Oil

While the lubricating oil for a biogas generator is very similar to normal diesel engine oil, there are some unique aspects of biogas fuel that makes it necessary for operators to use oil with special additives that accommodate the properties of biogas. The main issue at hand is the relatively high sulfur content of biogas. If the sulfur is not neutralized, sulfuric acid will accumulate in the engine oil, resulting in accelerated corrosion and early failure of engine components. The ability of oil to neutralize sulfur is measured by its "Total Base Number (TBN)". For digester engines, a rated TBN of 10 or higher in new oil is usually desirable.

In past years, "locomotive oil" was often used for biogas generators because train engines typically ran on high sulfur diesel fuel. However, locomotives now operate on low sulfur fuel, and as a result this specialized lubricating oil is not as widely available. Nonetheless, many manufacturers still provide oils that are suitable for digesters.

Selecting an Appropriate Oil

Cost, availability, and performance are key factors to consider when selecting lubricating oil for your biogas engine. The most important input regarding this choice is that of your engine's manufacturer. Different engines have different operating characteristics, which may affect the specific performance needed from the oil. Common oil properties and their typical desirable values for biogas engines are given in Table 1.

Table 1. Common lubricating oil properties.
PropertyDescriptionTypical Value
Viscosity GradeSelected to maintain an adequate lubricant film at engine operating temperatures both high and low.SAE 40 is common
Pour PointThis is the temperature below which the oil no longer flows. This is an important property in the case of cold startup of an engine−20°C or below
Alkalinity Reserve (TBN)Describes the ability of the fuel to absorb and neutralize acids.Usually 10 or higher
AdditivesA wide variety of other additives are available which improve the oil's properties, including detergents, corrosion inhibitors, and anti-oxidants.varies

Oil Maintenance and Oil Analysis

Oil analysis is an important tool for assessing the condition of your oil and of your engine, and is relatively inexpensive when compared to the cost of the repairs it can help prevent. Information from the oil analysis can tell you how quickly the oil is losing its quality, how often it should be changed, as well as telltale signs of engine problems. An oil analysis should be carried out every 100 hours of operation during the first 1000 hours of operation, then at every oil change thereafter or as directed by the engine manufacturer1. Standard oil analysis tests are carried out by many laboratories, lubricant suppliers, and engine dealers, and involve a wide range of diagnostic tests that can alert you to problems with your engine before they cause serious damage.

1 The 100 hour service interval for new engines is due to the higher rate of debris accumulation that occurs during the "wear in" of the pistons and other moving parts.

When requesting engine oil analysis, the lab will need to know information such as: lubricating oil (name, type, and grade), engine type, model, and engine make. They will then provide you with a report that summarizes their findings. While the format and details of the various labs are somewhat different, all laboratories can be expected to provide the following information:

RemarksExplains if wear levels are normal, viscosity as compared to new oil, TAN (total acid number) measured (indicating if oil needs replacement) other signs that engine is wearing abnormally
Wear Metals
(usually reported from Spectrochemical analysis)
Reports values of wear metals present; predictive measurements of engine component failure. Metals include Iron, Chromium, Nickel, Aluminum, Copper, Lead, Tin, Silver, Cadmium, Vanadium, and Titanium.
Additive MetalsValues of metals present from additives: Magnesium, Calcium, Barium, Phosphorus, Zinc. Indicates condition of TBN boosters, lubricating extenders, viscosity improvers, and antioxidants.
Contaminant MetalsValues of metal contaminants present, including Silicon, Sodium, and Potassium.
Multisource MetalsMetal that could be either from contaminants or additives in the oil: Molybdenum, Antimony, Manganese, Lithium, and Boron.
Physical Properties / ContaminantsFuel dilution, viscosity, water vol.%, coolant, soot, particle size analysis.

Most of us will not know the significance of all of the values that are reported in an engine oil analysis. However, it is important to be on the lookout for any changes from previous or typical values. Often, the oil analysis laboratory will pick up on these variations, and can sometimes point you to the problem that may be occurring. The results of the analysis will be able to tell you if the oil's acid neutralizing capacity is being used up or not, but it can also tip you off to other problems in the engine.

Some engine components, when they are nearing their failure point, will leave distinctive trace elements in the engine oil - this can be detected in the oil analysis, allowing you to order and install replacement parts on a scheduled basis rather than suffering an unexpected and potentially costly breakdown. Farmers are encouraged to develop a relationship with their engine manufacturer so as to obtain help interpreting the significance of their oil analysis results.

Figure 2. Typical results from an engine oil analysis.


A biogas internal combustion engine uses lubricating oil to cool, flush and lubricate the many moving parts inside the unit. Biogas, with its high hydrogen sulfide content, creates acids that contaminate the lube oil requiring frequent oil replacement. An engine oil analysis program can help predict and prevent engine component failures and extend oil drain intervals based on oil lubricating qualities instead of engine run hours. Contact your engine oil supplier to find oil analysis service providers in your area, and utilize this important tool to improve the performance and life of your biogas generator engine.

For more information on biogas, check out these websites.

Daniel Ciolkosz, Extension Associate, Agricultural and Biological Engineering, Pat Topper, Sr. Research Technologist, Agricultural and Biological Engineering, and Robert Graves, Professor, Agricultural and Biological Engineering.