By Stuart Brown
Key concepts
- Because natural gas burns hotter than diesel fuel or gasoline, lubricants for natural gas engines can operate under severe heat stress.
- Natural gas is a ‘dry’ fuel that needs to be used with lubricating oil that produces ‘oil ash’ as a byproduct of combustion.
- The classification system used for automobile oils doesn’t apply to natural gas-burning engines.
In circumstances where natural gas is available as a fuel for piston engines, it can be burned under workloads ranging from very low to very high while engine rotational speed is often kept constant. Typical uses of such engines are in the oil and gas industry and in electric power generation where such tasks as compressing natural gas are common.
Because natural gas burns hotter than diesel fuel or gasoline, the lubricating oil for natural gas engines can operate under conditions of severe heat stress when temperatures exceed 90 C, particularly in certain areas within the engine such as the cylinder walls, piston under-crowns and valve guides. This is a little too hot for an engine’s long-term health.
“In the lubes industry we have a rule of thumb, which says for every 10 degrees above 80 C, the oxidation rate doubles,” says Kevin McDermott, senior technical services advisor at Petro-Canada Lubricants, Inc. Oxidation, along with the related process of nitration, must be kept in check to assure that oil can fend off chemical degradation. McDermott presented his views at an STLE Education Webinar titled "Natural Gas Engine Oils."
Compared to diesel fuel or gasoline, which are liquids, natural gas is a dry fuel that needs to be used with lubricating oil that produces oil ash as a byproduct of combustion. In the right concentration, ash will form a deposit on the seating surfaces of exhaust valves that are needed to prevent what’s known as valve recession. When the proper amount of ash is present—not too much and not too little—the faces of the exhaust valves and their hard metal seats located in the cylinder head are protected against the demon of recession. Too much ash, on the other hand, can permit excessive deposits to form and cause hot combustion gases to leak around the valve seats, burning them. Overly thick ash deposits also can behave as unwanted insulators that impede the natural flow of heat away from the valves. Bathed in a flow of fresh incoming fuel and air, intake valves run much cooler than exhaust valves and thus don’t need any ash to sustain their health (see Figure 1).
What lubrication oil formulation is the best one for a particular natural gas-fueled engine? It all depends.
Looking for an American Petroleum Institute service indicator label on an oil bottle won’t get you anywhere; the classification system used for automobile oils doesn’t apply to natural gas-burning engines. The user must proceed with lessons learned from field experience, along with specifications issued by engine manufacturers. Several engine OEMs list oils that are suitable for use in their equipment (see Figure 2).
Over the years natural gas engines have advanced immensely thanks to evolved mechanical designs and higher combustion temperatures that contribute to better fuel efficiency and power output for a given engine size. Along with these improvements, oils have been formulated with superior resistance to oxidation and nitration. Ash is derived from metallic elements in the additive package such as calcium, magnesium, zinc and phosphorous (see Getting the Ash Content Right).
Getting the ash content right
- Factors affecting ash deposit accumulations
- Engine make and model
- Oil consumption rate
- Engine load
- Fuel gas composition
- Engine setup
- Air/fuel ratio
- Exhaust gas temperature
- Ignition timing
- Foreign chemistry
- Aftermarket additives
- Coolant leaks
Dig deeper
This article features highlights of a Webinar originally presented by Kevin McDermott for STLE Education on Feb. 3, 2016. You can read the recap and purchase the full webinar recording of “Natural Gas Engine Oils” at www.stle.org.
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About the author
You can reach freelance writer Stuart F. Brown at www.stuartfbrown.com.
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It’s interesting that there are different kinds of lubricants for different kinds of engines. It makes sense that getting the right stuff would be important for the health of your vehicle! I can see why choosing the right stuff would be vital.
Thanks for this very interresting informations.
Now retired,but still involved in this field.
Best regards
Daniel
Hello i have a quick question four someone who can give me an idea, is it ok to put delo400 15w40 ng oil in a detroit series 60 engine?i came across some good deal oil so i both it for my truck please shere your opinion thank you
Quite an Informative Blogpost. Thanks for sharing this inforamtion
Great share! This post is very useful.
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Your service was excellent and very FAST. Many thanks for you kind and efficient service.
Your service was excellent and very FAST. Many thanks for you kind and efficient service.
1) What are the most important constituents to evaluate Fresh oil for Gas engine effective ness?
2)What effects Ash constants in Gas Engine Oil.What is the best Ash constants in Fresh Gas engine Oil?
3)How VI effects Gas engine oil efficiency?