Natural Gas Engine Oils
In essence, a lubricant of any machinery lubrication is as important as the design of the components of the machinery. Equipment manufacturers perform extensive research and development in trying to optimize each and every component of the machine to deliver the best possible performance.
The last 40 years of reciprocating engines have seen many changes, whether for the purpose of mobility or captive power generation. While the 1960s was the beginning of building up machines' continuous improvements in power-to-weight ratio, the 1970s raised serious fuel conservation concerns and led to the creation of engines and vehicles with better fuel efficiency. The 1980s and '90s saw two prime objectives driving automobile manufacturers: emission control and safety. Auto emission control assumed greater importance in India in the latter half of the 1990s, with compressed natural gas (CNG) becoming the mandatory fuel for commercial vehicles in the National Capital Region (NCR). New exploration license policy has led to natural gas fi nds and liquefied natural gas (LNG) terminals in Dahej, Hazira. In the coming years, the possibilities for increases in natural gas-driven automobiles and industrial engines are very bright.
This article will share some of the interesting and unique aspects of natural gas engine oils.
Principles of Gas Engine Operation
Gas engines are generally categorized as two stroke (SI), four stroke (OSI) and four stroke (duel fuel). Gas engines operate on similar principles as petrol or diesel engines. However, there are some differences in the construction and operation of gas engines compared with other types of internal combustion engines. The differences include:
- Gas engines usually operate at a lower compression ratio than diesel engines.
- The volumetric efficiency of gas engines is generally lower than petrol engines due to the addition of gas, which reduces the volume of air intake.
- The combustion temperature in gas engines generally tends to be higher than that of diesel engines, especially the stoichiometric gas engines.
- Gas engines normally have more advanced ignition timing than petrol engines, which vary with speed and load.
- Due to the high temperature and soot-free nature of gas combustion, valve and seat wear may be a problem in some four-stroke gas engines. To combat this problem, gas engine manufacturers often use more sophisticated valve and seat materials and wider valve seats.
Advantages of Gas Engines
The main advantage of gas engines is their low emission characteristics and hence improved environmental safety. The most important advantages are:
- Lower carbon-dioxide (NOx) emissions, especially lean-burn engines types, compared with diesel and gasoline engines.
- Lower carbon-dioxide (CO2) emissions, compared with a comparable diesel engine.
- Very low particulate emissions.
- Ability to use lower cost gaseous fuel.
Disadvantages of Gas Engines
- Generally lower power output than the equivalent sized diesel engines.
- Requires pipeline fuel supply or pressurized storage tanks.
Natural Gas Engine Operation, Lubrication and Performance Requirements
Natural gas engines (NGE) are commonly used to power natural gas compressors, standby electric generators, fire water and irrigation pumps, and are increasingly being used to power primary cogeneration electrical power plants. The main advantages of a natural gas engine over a diesel engine are the lower exhaust emissions of nitrogen oxides (NOx), carbon monoxide (CO), particulates and in some cases, lower fuel costs. Natural gas engines used for industrial application, namely as the prime movers for gas pumping and captive power generation, normally operate constantly on high loads, high temperatures, for longer periods of time and often in remote locations. Therefore, the demand on the reliability of the engine and engine oil is very high.
In terms of lubrication requirements, engine oils are divided into categories based on whether the engine is four stroke or two stroke along with the type of fuel used and hence the combustion mode. The lubrication methods and system of natural gas engines are similar to those of petrol and diesel engines, although the functional requirement and hence the quality of natural gas engine oils are a little different from the conventional engine oils.
Selecting natural gas engine oil will depend on:
- Engine type
- Type of usage
- Operating parameters such as load factors, ambient conditions, etc.
- Specific engine designs, including fuel intake design, ignition methods, combustion chambers design, valve metallurgy, after an exhaust treatment device, etc.
- Original equipment manufacturer's (OEM) engine oil specification.
Natural gas is obtained as a by-product when drilling for petroleum, from gas fields or as coal bed methane. The composition of natural gas can vary considerably from one location to another. Dry natural gas consists almost entirely of methane. Natural gas can be further classified as sweet or sour depending on sulfur compounds present in the gas.
The primary fuel used in gas engines is methane. Its four carbon-hydrogen bonds give it a higher specific heat content than liquid fuels like gasoline or diesel, which contain some lower energy carbon bonds. Consequently, it burns hotter than other fuels under typical conditions.
In addition, since it is already a gas, methane does not cool the intake air by evaporation as liquid fuel droplets do. Methane burns hot and can cause severe oxidation and nitration of the engine oil in gas engines.
Gas engines do not produce soot, and there is no liquid fuel to help lubricate the intake and exhaust valves. The oil's ash is relied upon to lubricate the hot valve face/seat interface. Consequently, ash content and composition can have a significant effect on head life. Many gas engine installations also run full load operation continuously, placing the engine under extended exposure to severe operation conditions. Shortened oil and filter life as well as higher piston and engine sludge deposits can occur if the oil has inadequate oxidation and nitration resistance for gas engines. Increased valve wear and detonation can result from using an oil with an inappropriate ash content.
Further more many gas engine models are run either at or near stoichiometric conditions, where less excess air is available to dilute and cool combustion gases. As a result, gas engines can generate higher combustion gas temperatures than engines burning liquid hydrocarbon fuels.
Since the rate of formation of NOx increases exponentially with temperature, gas engines can generate NOx concentrations high enough to cause severe nitration of the engine oil. If the engine oil is not formulated to handle this environment, it will deteriorate in a manner similar to severe oxidative degradation, causing engine sludge, piston deposits, oil filter plugging and in severe cases accelerated ring and liner wear. So a product with good nitration resistance is required in most gas engine installations.
As stated previously, gas engines are solely dependent on the lubricant ash to provide lubrication between the hot valve face and its mating seat. Too little ash or the wrong type can accelerate valve and sheet wear, while too much ash may lead to valve guttering and subsequent valve torching. Too much ash can also result in detonation from combustion chamber deposits. Therefore gas engine builders frequently specify a narrow ash range that provides the optimum performance.
Classification of Gas Engine Oil
There are no industry standards for gas engine oils and hence no standardized test for evaluating performance of gas engine oils. Gas engine manufacturers generally have widely varying requirements, and oil approvals are only granted following field trials. During the trial, fuel oil and engine oil condition monitoring with engine inspection is required. However, SAE and ASTM together with engine manufacturers and oil and additive companies have formed a taskforce to develop natural gas engine oil performance categories.
The proposed categories are:
- NG1: Stoichiometric engines
- NG2: Lean-burn engines
- NG3: Automotive gas engines
| Type | S. Ash (% Wt.) | TBN (mg KOH/g) |
| Ashless | < 0.1 | 1 - 3 |
| Low Ash | 0.1 - 0.5 | 3 - 6 |
| Medium Ash | 0.6 - 1.4 | 6 - 12 |
| High Ash | > 1.4 | 13 |
Selection of Gas Engine Oil
At the time of formulating a gas engine oil for a four-stroke engine, the following factors are to be considered:
- High resistance to oxidation and nitration
- Good valve wear performance
- Low combustion chamber deposits
- No spark plug fouling
- Good piston deposits control
- Good anti-wear and anti-scuff properties
- Optimized ash content
At the time of formulating the additive package, care should be taken to minimize combustion chamber deposits and spark plug fouling by way of setting the ash content. Since the ash levels are limited, use caution in the detergent selection to reduce piston deposits and ring sticking. Good wear protection is also required to prevent scuffing and corrosion. The anti-wear components used in the formulation must be selected carefully for gas engine application to ensure that nitration-resistant additives are used.
In practical terms, this will depend on
- the quality of natural gas,
- the oil sump capacity/oil consumption,
- the operating parameters and the metallurgy.
Natural gas in terms of hydrogen sulfide content and other impurities like halides (landfi ll gas) will primarily determine the base number of the natural gas engine oil.
A small sump with a low oil consumption would put enormous stress on the oil and reduce its life, whereas higher oil consumption would extend its life but can lead to the formation of deposits and particulate emission. A natural gas engine oil should have the sufficient oxidation stability and low volatility to fulfill these conditions.
Engines with high operating temperatures would require an engine oil with high oxidation/thermal stability and viscosity retention even at high temperature and high shear conditions. An oil with very good inherent viscosity index and a shear, stable viscosity-index improver is a prerequisite. The viscosity required of an engine oil is also influenced by the load, speed and bearing geometry. In India, prevailing tropical conditions call for natural gas engine oil of SAE 40 in monograde and SAE 15W40 in multigrade, though SAE 20W50 is also not uncommon.
Keep in mind that all of these conditions, i.e., required base number, oxidation inhibition, anti-wear and detergency, are ensured with the limitation of ash content prescribed by the OEM.
Requirements of Natural Gas Engine Oils
Neutralisation Value
One of the primary functions of engine oils is to neutralize acids formed during the process of combustion and pockets of condensation. This property in lubricant terminology is known as base number (BN) expressed in milligrams of potassium hydroxide per gram (mg KOH/g). This is built into engine oils through detergent additives. It is understandable that BN is not significant criteria for natural gas engine oils compared to conventional engine
oils. However, the basic requirement always exists and is important as even the natural gas engine oil has to combat acids resulting from hydrogen sulfide
in natural gas and nitration that was discussed previously. Normal BN in natural gas engine oils are in the range of 5 to 8 mg KOH/g.
Oxidation and Nitration Control
Higher temperatures encountered in natural gas engines trigger the formation of nitrogen oxides (NOx) by combining nitrogen and oxygen from the combustion air. Oxidation thus promoted leads to:
- Viscosity increase in engine oil due to polymerization
- Corrosive wear through nitration
- Engine deposits in the form of sludge and varnish
- Filter blockage due to sludge.
The above calls for formulations that should include lube base oil with high inherent oxidation stability fortified with additives to resist oxidation and
nitration. The additives thus added should not unduly increase the ash content in the natural gas engine oil.
Reducing Friction and Wear
In reciprocating engines, a hydrodynamic lubrication condition exists (normally full-film lubrication) in most of the rotating and reciprocating areas like main bearings, cam shaft bearings, piston rings, cylinder liner interfaces, etc., but there are also spots that experience a boundary lubrication condition (possibility of metal-to-metal contact).
Boundary lubrication conditions exist in cams and cam followers at the top dead center (piston thrust side), bottom dead center (piston thrust side) and the cylinder liner's portion of the top dead center and bottom dead center. Hydrodynamic conditions can be managed with viscosity and copious supplies of engine oil.
However, boundary conditions that can occur comparatively more frequently in stop/start conditions of fleet operation can be managed only by fortifying the engine oils with anti-wear additives but again with an eye on the ultimate ash content in the oil.
Engine Cleanliness
One of the very unique and important attributes of any engine oil is the ability to keep the engine parts clean. Natural gas engine oil is relatively less loaded in this area because of the clean-burning natural gas. However, natural gas engine oil should contain adequate detergent/dispersant additives to clean the dirt and oxidation products and keep them in suspension. This prevents ring sticking, bore polishing and the buildup of undercrown deposits while also keeping the crankshaft space, camshaft space, cylinder cover and rocker-arm space clean.
Gas Engine Oil Change Intervals
These are the days of core competency, cost-cutting and conservation. The differentiating quality of engine oil along with the reduced cost demanded by the customer is the challenge posed to the oil formulator and marketer.
OEMs and users alike look for natural gas engine oils with extended drain periods along with optimum-rated specific oil consumption (oil extends its period itself when there is excessive consumption, which is not desirable).
While OEMs normally adopt a cautious approach on extending the drain intervals for obvious reasons, sometimes customers and oil suppliers can take a proactive step in this direction by condition monitoring the engine oil system.
The use of low-quality fuels as well as many other mechanical factors, such as coolant leaks, poor filtration and high loads, can significantly reduce these drain intervals. They can also be extended but require good maintenance and operational practices along with oil monitoring through used oil analysis. The procedure involves well-documented steps to draw representative engine oil samples in a predetermined way at prescribed intervals. The used oil samples are tested for critical parameters, and the results are compared with fresh oil values and condemning limits (normally prescribed by the OEM).
Trend analysis can be done by plotting the values against running hours. The useful life of the engine oil can then be established. This can be a one-time exercise for relatively smaller sumps (less than 1,000 liters).
For bigger sumps, this programme can be part of overall condition monitoring of the engine, and the oil can be discarded based on analysis. Broad guidelines to oil condemning limits are given in Table 1, although it is always better to follow condemning limits as stipulated by the OEM.
| Parameter | Value |
| Kinematic viscosity at 100 °C |
+25% of upperlimit of fresh oil |
| BN | Max 50% reduction |
| Oxidation | 25 abs/cm in FTIR |
| Nitration | 25 abs/cm in FTIR |
| Total Insolubles | 1.5% w max. |
| Wear metals max | 150 ppm |
| Iron | 90 ppm |
| Copper | 40 ppm |
| Aluminum | 40 ppm |
| Lead | 40 ppm |
| Chromium | 50 ppm |
| Silicon | 40 ppm |
