Oil for gas engines

One of the subjects of comparative analysis was, among others, SG class mineral oil (lower-class oil, currently the highest class is marked as SM) used in a car with the simplest first generation system. Working with a gas-powered engine, oil lost all its essential characteristics after 15 thousand kilometers. It was similar to oil wear in a petrol-powered car after 20 thousand kilometers. Another example is the analysis of semi-synthetic oil, also SG class, that worked in an engine powered by a second generation gas system. With mileage between 10 and 15 thousand kilometers, oil lost its resistance to oxidation. After 10 thousand kilometers, oil lost its ability to neutralize acidic materials. The loss of anticorrosive properties was also observed at similar point.

The abovementioned examples show that the loss of properties of traditional oils (specified based on the requirements for petrol-powered engines) in engines fueled with LPG with gas systems of older generations occurs after the time when oil should be replaced (with a relatively low SG quality class). With higher-quality oils, the loss of properties would surely be slower. It’s similar to the currently most popular fourth generation gas systems, characterized by much more precise dosage of fuel (there’s no uncontrolled temperature rise in combustion chambers, associated with blend depletion, and the oil does not oxidize). We can expect a much slower loss of properties of engine oil, especially since modern engines require oils with much higher quality class, containing more effective additives.

Observations conducted by the Oil and Gas Institute confirmed the need to develop specific requirements for lubricating oils used in gas-powered engines, which means using special additives that will extend mileage after which engine oil loses its essential properties.

When replacing oil in an LPG-powered engine, we might have the impression that it’s not entirely used. It’s clear, which is primarily due to the absence of particulate matter (soot) in the exhaust gas of LPG-powered engines. But the tests conducted by the Oil and Gas Institute prove that oil’s appearance doesn’t tell us much about its properties. However, when using engine oil that meets quality and viscosity requirements – one that additionally has the approval of a specific engine manufacturer (or even exceeds the requirements) – we can feel safer. Oil replaced often enough (considering the intensity of use) will surely fulfill the requirements of the manufacturers.

© MPK RzeszówVehicles powered by natural gas are mainly public transport buses that operate in especially difficult conditions. Proper selection of engine oil is particularly important, because oils for CNG-powered vehicles are completely different from lubricants for petrol and diesel engines. Choosing an appropriate formulation of such oils is very difficult and requires meeting often conflicting requirements

CNG-powered engines

Using CNG as fuel is especially popular in public transport and in other municipal services.

Work conditions for engines in public transport are particularly difficult. Constant acceleration and breaking, as well as traffic jams mean that the engine works in extreme conditions. In addition, using CNG as engine fuel results in much greater heat load. This is due to its properties similar to LPG (higher temperature and slower combustion process, and the valves are not cooled by the fuel blend). All this means that various components of gas-powered engines (head, valves, piston rings) are subject to high heat loads.

Leaks in the piston-crank system cause direct contact of gas combustion products with engine oil. Acidic substances, which are created as a result of combustion, leak to the oil and accelerate engine corrosion, especially on alloys used in the plain bearings of the engine.

Therefore, oil used in natural gas-powered engines must have all mentioned features. In addition, oil must be resistant to contamination in the form of ammonia and water from fuel.

When it comes to oils used in natural gas-powered engines, there are also no specific requirements for essential parameters. They are set by individual manufacturers of gas-powered engines. Their test procedures are quite demanding and include bench and road testing. Those tests are pretty rigorous and not all oils are able to meet these requirements, that’s why the number of commercially available oils for CNG-powered engines is not big.

© FuchsTimely replacement of engine oil in an LPG-powered engine is especially important. No grading for this type of engines means that in most cases oils for petrol engines are used. Because of slightly different LPG combustion conditions and higher content of sulfur related to the presence of odorants, degradation of such oil occurs slightly faster. Car manufacturers, while specifying mileage between oil changes, include a safety margin, so even when you use gas, this period will not be shorter

Usually, engine manufacturers specify the minimum quality parameters for such engine oils. They must meet several essential requirements, e.g. they need to pass the NOACK Volatility Test (parameter that informs us about potential loss of oil during operation), they have to have low sulfated ash levels (responsible for the durability of catalytic purification systems and the creation of carbon deposits – high-temperature deposits in the combustion chambers) and a correct TBN – Total Base Number (ability to neutralize acidic products of combustion).

Oils for natural gas-powered engines are based on formulations that are significantly different from those used in conventional engine oils. This is due to the aforementioned parameters, which are often contradictory. For example, reaching high TBN value and very low sulfated ash content is not possible in conventional engine oil even when it meets very stringent requirements for wear, cleanliness and reducing the formation of low- and high-temperature deposits.

That’s why when we drive a natural gas-powered vehicle, we should pay special attention to the type of oil that we use. It should be a lubricant adapted to use in natural gas-powered engines. These oils are completely different from those used in petrol and diesel engines. It’s not as important in the case of LPG – using high quality oil that meets the quality and viscosity requirements specified by the manufacturer should be enough. If we use oil dedicated to LPG-powered engines, we can further improve our engine’s working conditions, assuming that the information provided by the manufacturer is accurate.