Knowledge Base

Online resource for product help and technical support, as well as informative articles about ester based lubricating oil technology.

Synthetic Oil - Ester based or PAO?

The word ‘synthetic’ in the lubricants industry has historically been synonymous with polymerized base oils, such as poly-alpha olefins (PAO’s), which are made from small molecules. As the PAO market grew, some base oil manufacturers began using higher VI Group III feedstocks (usually by-products from wax manufacturing) to make mineral oils with Viscosity Improvers that matched the PAO’s.

 

The trend toward globalized lubricant specifications and worldwide OEM specifications is now creating more demand for Group III base oils. This is particularly true in North America due to the 1999 ruling by the National Advertising Department of the Better Business Bureau that allows Group III base oils to be considered synthetic.

 

In many ways esters are very similar to the PAO’s. Like PAO’s, esters are synthesized from relatively pure and simple starting materials to produce predetermined molecular structures designed specifically for high performance lubrication.

 

Both types of synthetic base stocks are primarily branched hydrocarbons which are thermally stable, have high viscosity indices, and lack the undesirable and unstable impurities found in conventional petroleum based oils (Group I,II & III).

 

The primary structural difference between esters and PAO’s is the presence of oxygen in the hydrocarbon molecules in the form of multiple ester linkages  (C-O-O-R) which impart polarity to the molecules.

 

This polarity affects the way esters behave as lubricants and gives the ester base oil its unique performance:

 

Volatility: The polarity of the ester molecules causes them to be attracted to one another and this intermolecular attraction requires more energy (heat) for the esters to transfer from a liquid to a gaseous state.Therefore, at a given molecular weight or viscosity, the esters will exhibit a lower vapor pressure which translates into a higher flash point and a lower rate of evaporation for the lubricant. Generally speaking, the more ester linkages in a specific ester, the higher its flash point and the lower its volatility (improved VI).

 

Lubricity: Polarity also causes the ester molecules to be attracted to positively charged metal surfaces. As a result, the molecules tend to line up on the metal surface creating a film which requires additional energy (load) to wipe them off. The result is a stronger oil film which translates into higher lubricity and lower energy consumption in lubricant applications.

 

Detergency/Dispersency: The polar nature of esters also makes them good solvents and dispersants. This allows the esters to solubilize or disperse oil degradation by-products which might otherwise be deposited as varnish or sludge, and translates into cleaner operation and improved additive solubility in the final lubricant.

 

Another important difference between esters and PAO’s is the incredible versatility in the design of ester molecules due to the high number of commercially available acids and alcohols from which to choose.

 

With esters, literally dozens of certain viscosity products can be designed each with a different chemical structure selected for the specific desired property. The performance properties that can be varied in ester design include viscosity, viscosity index, and volatility, high temperature coking tendencies, biodegradability, lubricity, hydrolytic stability, additive solubility, and seal compatibility.

 

The most common concern when formulating with ester base stocks is compatibility with the elastomer material used in seals. All esters will tend to swell and soften most elastomer seals, however the degree to which they do so can be controlled through proper selection. Seals should be changed to those types which are more compatible with esters.

 

Esters have been now been used successfully in lubrication for more than 60 years and are the preferred stock in many severe applications where their benefits solve problems or bring operational value.

 

For example, esters have been used exclusively in jet engine lubricants worldwide due to their unique combination of low temperature flowability with clean high temperature operation.

 

The new frontier for esters are the stationary gas engines where the number of products, applications, and operating conditions is enormous.

 

In many gas engine cases, the very same equipment which operates satisfactorily on mineral oil in one location, could benefit greatly from the use of an ester lubricant in another location where the equipment is operated under more severe conditions or uses different fuel.

 

The high performance properties and custom design versatility of esters is ideally suited to lubricate today’s modern high performance gas engines.

When one focuses on temperature extremes and their telltale signs, such as shortened oil life and deposits, the potential applications for the problem solving ester lubricants are virtually endless.

 

Fouling of Heat Exchangers greatly reduced by using ECOSYN

Cogeneration systems, also called combined heat and power (CHP) systems, are designed to generate both heat and power. CHP systems use waste heat accrued during an engine’s operation to generate overall plant efficiencies of more than 90 percent. This efficient and economical method of energy conversion achieves primary energy savings of roughly 40 percent by using a gas engine cogeneration system instead of separate power and heat generation equipment.

One of the basic structures of CHP systems includes an engine/generator unit and a heat exchanger or heat recovery unit that make use of waste heat in the exhaust gases. Engine exhaust heat represents from 30 to 50 percent of the available waste heat. Exhaust temperatures of 850 to 1200°F are typical.

Exhaust heat is typically used to generate hot water to about 230°F or low-pressure steam (up to 150 psig). Only a portion of the exhaust heat can be recovered since exhaust gas temperatures are generally kept above temperature thresholds to prevent the corrosive effects of condensation in the exhaust piping. For this reason, most heat recovery units are designed for a 250 to 350°F exhaust outlet temperature.

Accumulation of unwanted deposits on the exchangers’ surfaces is known as fouling in exchangers. Fouling includes crystalline, biological materials, products of chemical reactions, involving corrosion, or special materials.

The amount of fouling deposited depends on the type of fuel (natural gas, landfill gas etc.) and the type of lubricating oil residue transmitting through the exchanger.

The detrimental effects of fouling are as follows:
•    Reducing the heat transfer rate
•    Increasing thickness of the deposited layer, metal walls has been destroyed by further heat; fire hazards
•    Increasing operating costs for cleaning
•    Corrosion caused by electrolysis between fouling layers and the metal body
•    Increasing the rate of environmental pollution
•    Pressure drop and reducing fluid flow
•    Bursting Pipes due to too much pressure arising from clogging on them
•    Shortening the tube life
•    Reducing the exchanger and consequently engine houses efficiency

De-fouling of the heat exchanger is the most expensive thing in the maintenance of the exchangers which results in a waste of capital and time. Fouling on heat transfer surfaces reduces heat transfer rate and increases fuel consumption resulting in increased production of carbon dioxide which lead to global warming. De-fouling is therefore inevitable causing loss of production time and enormous cost.

Engines running ECOSYN Gas Engine Oils consume up to 60% less lube oil and ECOSYN burns cleaner because of the 100% fully synthetic ester based base oil, therefore reducing the amount of deposits in the heat exchanger due to lube oil consumption.


ECOSYN gas engine oils also have up to 20% less ash content than common used mineral based oils. The use of higher ash oils cause more deposits to accumulate in the engine and heat exchanger.

Test have been performed and CHP installations running ECOSYN have recorded maintenance interval extensions of heat exchangers of up to 5 times their regular intervals running mineral oil.

Siloxanes and ECOSYN Gas Engine Oils

ECOSYN GE series gas engine oils are especially formulated for gas engines running on all types of biogas, including gas produced from biomass or manure, sewage gas and landfill gas.

Contaminants like siloxanes have a direct influence on deposit formation in the engine’s combustion chamber. In order to control the total level of deposits, the lubricant’s contribution to deposit formation should be minimized.

It is therefore crucial that the lubricant is able to prevent formation of ash deposits as much as possible.

A siloxane is a (gaseous) hydrocarbon molecule with a silicon (Si) atom in it. Siloxanes are typically found in sewage gas and in landfill gas. When combusted, the silicon atom is joined with oxygen atoms to form silicon dioxide or silica (SiO2), the chemical formula of sand and glass. The silica is being formed in the combustion chamber, and deposited on the surfaces in the combustion chamber, such as the piston crown, the cylinder head flame bottom and the valve discs.

Potential consequences are:


•    The deposits on piston crown and on the valve disc reduce the clearance between these components, and there is the risk that valve and piston crown touch each other.


•    As a result of the deposits, the compression ratio increases, which can promote detonation (also called knocking).


•    Because of the chemical composition, the deposits are very hard and abrasive. They also have a different coefficient of thermal expansion from metal. As a result of temperature changes, parts of the deposit layer will break off from the surface of piston and cylinder head. These parts may get trapped between ring and liner where they are ground and contribute to high wear rate of these components.

Accumulation of Si in the lube oil is always seen at plants where siloxanes are present in the fuel gas. Sometimes customers use Si as a criterion for oil drain. Several customers have seen a correlation between Si in oil and engine wear rate.

This is correct: the correlation is there, but it is not a causal correlation. Both are a consequence of high siloxanes in the fuel.

If the oil filtering arrangements in the plant are correct, so that particles are being removed, any Si reported in the lube oil is in principle harmless.

Although some OEMs limit Si in used oil to 300 ppm, others explicitly state that the Si content of used oil has no limit.

The direct influence of ECOSYN on the Si count in the oil is limited, since the formation of hard deposits as a result of siloxanes in the fuel are formed directly during the combustion of the fuel with minimal interference of the lubricating oil.

One aspect however where ECOSYN contributes, because of its lower consumption, is by minimizing the formation of lubricating oil related deposits, e.g. ash and oil coke (calcium).

ECOSYN will offer better engine protection than a mineral oil used in engines burning gaseous fuels with high levels of siloxanes, because:


•    It has a low consumption rate and low ash content, significantly reducing the contribution of lube oil to combustion chamber deposits, which in turn reduces the total number of harmful deposits in the combustion chamber and therefore reducing the amount of deposit layers potentially breaking off and causing damage. On top of that, ECOSYN has outstanding anti-wear and anti-scuff properties because of its ester base fluid and additive package.


•    It has an advanced additive package on top of the ester base oil, which helps to further minimize ash and carbon deposits.


•    Being an ester it has excellent oxidative and thermal stability, excellent film strength and an extremely low tendency to leave deposits on machine surfaces. The low deposit-forming tendency is really due to two properties – ECOSYN’s ability to dissolve deposits and the fact that the oil burns very clean. So when exposed to a very hot surface, ECOSYN is less likely to leave residue that will form silica deposits and liner lacquering: deposits stay dissolved!


•    ECOSYN has excellent detergency: keeping the engine, especially the combustion chamber, clean means minimizing deposits.

Piston running 11,000 hours on ECOSYN GE 4004 and landfill gas with high siloxane
Piston running 11,000 hours on ECOSYN GE 4004 with small amount of silica build up

Factors limiting oil life in gas engines

A lubricating oil consists of a base oil and selection of additives. Base oils exist in a
large variety of qualities, ranging from refinery streams that have received basic treatment, to severely hydro treated streams to fully synthetic fluids. With the help of additives, the lubricating oil manufacturer tries to enhance, suppress or complement certain properties of the base oil.

 

ECOSYN consists of a 100% fully synthetic ester base fluid, in combination with carefully selected additives. Each product line is custom designed to meet specific physical and performance properties.


ECOSYN lubricants are therefore unique gas engine oils (GE line), designed to provide specific performance attributes that should help the end user achieve best value for money.

When the oil is in service, its properties will change as a result of degradation processes.


These are:

- Oxidation: this is the chemical reaction of the hydrocarbon molecules from the base oil with oxygen. As a result of this reaction weak acids are formed as well as
polymerization products.

- Nitration: this is the chemical reaction of the hydrocarbon molecules from the base oil with nitrogen oxides (NOx). As a result of this reaction weak acids are formed as well as polymerization products. (only mineral oils, not the ester based ones)

- Reduction of alkalinity reserve, as indicated by a reduction of the base number (BN) of the oil. BN depletion is normally caused by the neutralization of the acidic products of the oxidation and nitration reactions. However if the fuel gas contains acidic species, which is often the case with biogas, then this will accelerate the depletion of BN.

- Increase of the concentration of acids in the oil. Although most acids formed by oxidation and nitration reactions are neutralized by the alkaline additives in the oil, some acids are so weak that they do not react with these additives. Because of their weakness, and as long as their concentration is not too high, these acids are not harmful to engine (bearing) metals either.

The total acid number (TAN) can be used as an indicator of acids in the oil, where one should realize that even a fresh oil will yield a TAN value when tested, even if there are no acids present at all. The concentration of acids is therefore best represented by the difference between the TAN of the used oil and the TAN of the fresh oil. An additional measure is the ipH number that describes the strength of the acids accumulated in the lubricating oil.

- Increase of the viscosity of the oil. This is mainly caused by the polymerization products formed during the oxidation and nitration reactions.

- Increased level of contaminants in the oil, such as Si, water, soot, other insolubles, etc.

It is good practice to take regular oil samples. Analysis of these samples will then indicate oil condition, the rate of oil deterioration and will help to determine safe oil drain intervals.

 

This is even more important in biogas applications as fuel quality can vary significantly over time.

 

Also oil analysis can detect premature wear processes in the engine, notably bearing wear, and cooling water leakage, and can therefore provide additional safety and peace of mind for the operator. 

How to select the right lubrication oil if my engine runs with biogas and with a high sulfur (H2S) content?

Biogas from anaerobic digestion consists of methane (CH4) and carbon dioxide (CO2) and a number of trace compounds.

Biogas may contain acid producing species such as hydrogen sulphide (H2S), hydrogen fluoride (HF) and hydrogen chloride (HCl). The H2S is found in all types of biogas, but especially in biogas produced from agricultural material, manure and sewage, whereas HF and HCl are typically found in landfill gas.

After combustion and in combination with water these species can form sulphuric acid (H2SO4), hydrofluoric acid (HF) and hydrochloric acid (HCl). These are highly corrosive to engine components such as liners, piston rings, piston ring grooves and bearings, and must be neutralized by the lubricating oil before doing any harm.

For this reason, the lubricating oil contains alkaline additives that will react with the acids when they get into contact with the oil film before they can reach metal surfaces.

 

ECOSYN possesses a high natural solvency for deposits and is enhanced with detergent type additives. This results in cleaner engines and less wear.


Because of this neutralization reaction, the alkaline additives in the oil are being consumed whilst the oil is in service, and the oil needs to be changed when the alkaline additives have been depleted.

The alkalinity reserve of a lubricating oil is represented by its base number (BN).

Since every engine burns a small amount of lubricating oil, and since many types of alkaline additives are ash producing when burnt, they contribute to the formation of ash deposits in the combustion chamber. For this reason, the engine manufacturers limit the amount of ash producing additives in the lubricating oil.

Most OEMs limit the ash to 0.6%, such oils are called low ash oils. Some OEMs allow oils with up to 1.0% ash, such oils are called medium ash oils.

 

ECOSYN has a very low oil consumption, which results in less build up of silicates.


Because of the limited amount of alkaline additives in the fresh lubricating oil, the achievable life time of a given oil is highly dependent on the amount of acidic species in the fuel gas.

Regular oil samples should be examined until a base line has been established. Oil change intervals highly depend on service intervals as well - you don't want a service crew to go to an engine just to change the oil, that's bad economy.

 

ECOSYN's Lubrication Specialists will set up a custome made lubrication plan, optimizing the oil drain intervals with the regular service intervals of the engine, so no unnecessary service calls are being made.

Chlorine containing compounds in landfill gas

Chlorine containing compounds from solvent and thinners, and fluorine containing chloro-fluoro-carbons (CFCs), mostly from aerosol cans and refrigeration and air conditioning units, are the major concern when landfill gas is used as an engine fuel. 

 

When burned, these compounds release chlorinated and fluorinated compounds which may, in the presence of moisture, form hydrochloric and hydrofluoric acids. 

 

Additive components and ECOSYN's 100% ester base fluid solubilize the acids in the oil and neutralize them before they attack engine components.  To minimize acid condensation, engines in landfill gas service tend to be operated at higher water jacket temperatures.  ECOSYN is perfectly capable of handling this added thermal stress because of its base fluid design.

Fuel pre-treatment is generally necessary to make landfill gas suitable for reciprocating engines. Used oil analysis (UOA) will permit oil drain intervals to be adjusted as required to accommodate variances in fuel composition. 

 

Additionally, used oil analysis is necessary for new engines to ensure results are kept within the warranty protection limits set by the equipment manufacturer.

As with all gas fuelled, spark ignition engine applications, the choice of lubricant ash level will be a trade-off between prevention of exhaust valve recession, prevention of exhaust valve distress and preignition, and provision of adequate alkalinity reserve. In our experience an ash level of 0.4% has proven to be adequate for most applications, which positions it lower in ash content than its competitors.

 

Added benefits of lower ash percentages in the lubrication oil are: less spark plug fouling, less port plugging, less combustion chamber deposits, less valve torching - a better running and more fuel efficient engine.

Esters in Synthetic Lubricants

By T. G. Schaefer

In the simplest terms, esters can be defined as the reaction products of acids and alcohols.

 

Thousands of different kinds of esters are commercially produced for a broad range of applications. Within the realm of synthetic lubrication, a relatively small but still substantial family of esters have been found to be very useful in severe environment applications.

 

This paper shall provide a general overview of the more common esters used in synthetic lubricants and discuss their important benefits and utilities.

Esters have been used successfully in lubrication for more than 60 years and are the preferred stock in many severe applications where their benefits solve problems or bring value.

 

For example, esters have been used exclusively in jet engine lubricants worldwide for over 50 years due to their unique combination of low temperature flowability with clean high temperature operation. Esters are also the preferred stock in the new synthetic refrigeration lubricants used with CFC replacement refrigerants. Here the combination of branching and polarity make the esters miscible with the HFC refrigerants and improves both low and high temperature performance characteristics.

 

The new frontier for esters is the industrial marketplace where the number of products, applications, and operating conditions is enormous. In many cases, the very same equipment which operates satisfactorily on mineral oil in one plant could benefit greatly from the use of an ester lubricant in another plant where the equipment is operated under more severe conditions.

 

This is a marketplace where old problems or new challenges can arise at any time or any location. The high performance properties and custom design versatility of esters is ideally suited to solve these problems.

 

Ester lubricants have already captured certain niches in the industrial market such as reciprocating air compressors and high performance gas engines or gas engines running on aggressive gases. When one focuses on temperature extremes and their telltale signs such as smoking and deposits, the potential applications for the problem solving ester lubricants are virtually endless.

Ester Chemistry

In many ways esters are very similar to the more commonly known and used synthetic hydrocarbons or PAOs. Like PAOs, esters are synthesized from relatively pure and simple starting materials to produce predetermined molecular structures designed specifically for high performance lubrication. Both types of synthetic basestocks are primarily branched hydrocarbons which are thermally stable, have high viscosity indices, and lack the undesirable and unstable impurities found in conventional petroleum based oils.

 

The primary structural difference between esters and PAOs is the presence of oxygen in the hydrocarbon molecules in the form of multiple ester linkages (COOR) which impart polarity to the molecules. This polarity affects the way esters behave as lubricants in the following ways:

1) Volatility: The polarity of the ester molecules causes them to be attracted to one another and this intermolecular attraction requires more energy (heat) for the esters to transfer from a liquid to a gaseous state. Therefore, at a given molecular weight or viscosity, the esters will exhibit a lower vapor pressure which translates into a higher flash point and a lower rate of evaporation for the lubricant. Generally speaking, the more ester linkages in a specific ester, the higher its flash point and the lower its volatility.

2) Lubricity: Polarity also causes the ester molecules to be attracted to positively charged metal surfaces. As a result, the molecules tend to line up on the metal surface creating a film which requires additional energy (load) to wipe them off. The result is a stronger film which translates into higher lubricity and lower energy consumption in lubricant applications.

3) Detergency/Dispersency: The polar nature of esters also makes them good solvents and dispersants. This allows the esters to solubilize or disperse oil degradation by-products which might otherwise be deposited as varnish or sludge, and translates into cleaner operation and improved additive solubility in the final lubricant.

4) Biodegradability: While stable against oxidative and thermal breakdown, the ester linkage provides a vulnerable site for microbes to begin their work of biodegrading the ester molecule. This translates into very high biodegradability rates for ester lubricants and allows more environmentally friendly products to be formulated.

Another important difference between esters and PAOs is the incredible versatility in the design of ester molecules due to the high number of commercially available acids and alcohols from which to choose.

 

For example, if one is seeking a 6 cSt synthetic basestock, the choices available with PAOs are a straight cut 6 cSt or a “dumbbell” blend of a lighter and heavier PAO. In either case, the properties of the resulting basestock are essentially the same. With esters, literally dozens of 6 cSt products can be designed each with a different chemical structure selected for the specific desired property.

 

This allows the “ester engineer” to custom design the structure of the ester molecules to an optimized set of properties determined by the end customer or application. The performance properties that can be varied in ester design include viscosity, viscosity index, volatility, high temperature coking tendencies, biodegradability, lubricity, hydrolytic stability, additive solubility, and seal compatibility.

As with any product, there are also downsides to esters. The most common concern when formulating with ester basestocks is compatibility with the elastomer material used in the seals. All esters will tend to swell and soften most elastomer seals however, the degree to which they do so can be controlled through proper selection. When seal swell is desirable, such as in balancing the seal shrinkage and hardening characteristics of PAOs, more polar esters should be used such as those with lower molecular weight and/or higher number of ester linkages. When used as the exclusive basestock, the ester should be designed for compatibility with seals or the seals should be changed to those types which are more compatible with esters.

Another potential disadvantage with esters is their ability to react with water or hydrolyze under certain conditions. Generally this hydrolysis reaction requires the presence of water and heat with a relatively strong acid or base to catalyze the reaction. Since esters are usually used in very high temperature applications, high amounts of water are usually not present and hydrolysis is rarely a problem in actual use. Where the application environment may lead to hydrolysis, the ester structure can be altered to greatly improve its hydrolytic stability and additives can be selected to minimize any effects.

Summary

Esters are a broad and diverse family of synthetic lubricant basestocks which can be custom designed to meet specific physical and performance properties.

 

The inherent polarity of esters improves their performance in lubrication by reducing volatility, increasing lubricity, providing cleaner operation, and making the products biodegradable.

 

A wide range of available raw materials allow an ester designer the ability to optimize a product over a wide range of variables in order to maximize the performance and value to the client.

 

Esters have been used in synthetic lubricants for more than 60 years and continue to grow as the drive for efficiency make operating environments more severe. Because of the complexity involved in the designing, selecting, and blending of an ester basestock, the choice of the optimum ester should be left to a qualified ester engineer who can better balance the desired properties.