Seeing that accident and wondering what could have started the fire made me wonder if perhaps the truck was fueled with natural gas. While #1 diesel fuel is classified as combustible, with a flash point between 100 degrees Fahrenheit and 200 F, and #2 diesel fuel has a flash point of about 200 F, gasoline and natural gas are flammable, with a flash point at less than 100 F. (For convenience, Im going to use natural gas to refer to CNG, LNG or any other gaseous fuel.)

By now, youre probably wondering what this could possibly have to do with universal oils. As you may know, universal oils are designed for both gasoline and diesel engines. The logic behind this move is to minimize inventories in fleet maintenance facilities. The concept worked well for many years, but has been more challenging recently.

While gasoline engine oil requirements began to reduce phosphorus content to minimize exhaust catalyst poisoning, the phosphorus needs of diesel engines remained high due to very heavy loads on valve trains, etc. Until the introduction of API CK-4, universal oil labeling allowed phosphorus content to be waived for gasoline engines to allow higher phosphorus levels. When CK-4 was introduced, The API S categories for gasoline engines required lower phosphorus levels for SAE grades 0W-20, 0W-30, 5W-20, 5W-30 and 10W-30. For these grades, the only way a universal oil can be so labeled is to meet the lower phosphorus levels.

For the non-automotive grades (e.g. SAE 15W-40), the phosphorus level is still waived, and its business as usual.

Now were throwing in natural gas fueled engines. Gas engine oils have been around for a long time. They are a mainstay of remote operations such as pumping stations and oil platform auxiliary engines. One of the more important uses for gas engines is for emergency generators in critical applications such as hospitals, where maintaining electricity is vital. Of course, these are stationary engines. The mobile applications are more recent and are the subject of my musings. The chemistry of these oils is different from either gasoline or diesel fueled engines (more on that later).

The Pros of a Multi-fuel Engine Oil

Obviously, the fleet maintenance people would love to have one oil that does it all. The only issue for them would be which viscosity grade is best. The heavy-duty engine oil market has been moving toward SAE 10W-30, since it offers some fuel economy advantages over the classic SAE 15W-40. CK-4 and FA-4 are available in SAE 10W-30 grades, with the only difference being high-temperature, high-shear viscosity.

In the meantime, natural gas fueled engines have been growing in the commercial transportation market. They offer an easily available, alternative fuel for the heavy-duty market in the United States. There are thousands of natural gas delivery trucks, transit buses, waste collection trucks and other vehicles now using the fuel. The market is expanding in those areas where long range is not a major criterion.

Natural gas vehicle fuel provides an excellent means to reduce emissions. Due to its clean-burning nature, natural gas powered heavy-duty vehicles can achieve low emission rates without major emission control equipment. Other advantages of natural gas fueled vehicles include favorable economics over diesel and abundant supplies with an existing distribution system.

In areas where emissions are an important factor, such as California, natural gas fueled vehicles are rapidly taking over the distribution of products. The Los Angeles basin is a prime spot for these vehicles, and companies such as Ryder have natural gas terminals throughout the area.

There are safety issues that favor natural gas, as well. Fuel tank design is such that natural gas tanks are much stronger than diesel or gasoline tanks. However, if a natural gas tank should rupture, the gas escapes straight into the atmosphere and doesnt pool around the vehicle. (Obviously, my truck on I-10 wasnt natural gas fueled).

The Cons of a Multi-fuel Engine Oil

What is there about natural gas fueled engines that makes them a candidate for their own unique chemistry, and how do those oils differ from API CK-4?

First, gas engines run at higher temperatures than diesel fueled engines. As Ive mentioned before, the rate of oil oxidation doubles for every 10 degrees Celsius (18 F) increase in temperature. API Group II and Group III base stocks, as well as synthetics, are one way to improve the oxidation resistance of engine oils. In addition, there are a number of antioxidant additives that are used to enhance this very important property. Of course, oxidation resistance is needed for diesel engine oils, as well. Suffice it to say that the higher operating temperatures in gas engines makes it extremely important.

Deposit control is another area of concern, but it is not as large a problem in gas engines as it is in diesel engines. Thats because diesel combustion produces a lot of contaminants that get into the crankcase, fouling piston ring grooves and forming deposits. Varnish also forms on piston skirts. The source of all of this nastiness is diesel fuel. Its much better now that 15 parts per million sulfur is the norm. Heavy-duty engine oils typically have about twice the amount of metallic ash as natural gas engine oils, and some are lower or essentially ashless.

An interesting sidelight to this issue is that many oil platforms use gas produced on the platform as fuel for auxiliary engines. They typically use HDEO to lubricate the engines, and it is often a monograde SAE 30 or 40.

The use of biodiesel (blended into regular diesel at 20 percent or less) also shows that there are some deposit concerns. Early tests with single-cylinder Caterpillar test engines showed higher levels of deposits. Biodiesel also may have low temperature pumpability concerns. Gas, on the other hand, is less prone to deposit formation, since there is nothing in the fuel to cause problems.

Wear protection is an issue for both natural gas and diesel fueled engines. It is addressed with classic zinc dithiophosphate chemistry at similar levels. Soot formation is often the result of mechanical issues within the engine fueling system and does not represent a problem for natural gas engines. Diesel engines are prone to injector problems, which are clearly visible in the exhaust.

The Verdict

Even though the chemistry of natural gas vs. diesel fueled engine oils can be markedly different, modern additive technology is in play, with products developed that meet not only API CK-4 and SN but also the newest specifications for natural gas fueled engines, such as Cummins CES20092.

Formulators have found ways to cover not only diesel fueled engines that require deposit control, oxidation resistance and wear protection, but also to do this with just enough ash content to provide the necessary protection while keeping phosphorus levels at API SN required levels.

In the near future, there will need to be some adjustments to meet the new API SP category, which will be available in May 2020. These truly multi-use engine oils can effectively provide the type of lubrication and protection required by modern engines, no matter what fuel is being used.

When I first decided to write about this, I expected to find that universal oils would be impossible to create, but I should have known better. The major additive companies are technically so astute that Im beginning to doubt anything is beyond their abilities. Good job!

Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society, ASTM International and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg@cox.net.

Thom Smith

Thomas Thom Ray Smith, 63, died on June 3, 2019, at the Mayo Clinic in Rochester, Minnesota. Thom earned a Bachelor of Science with honors in chemistry from the University of Waterloo, Canada, and had a successful career. Thoms activities in the lubricant industry spanned over forty years, including roles with Shell, Gulf Canada (now Petro-Canada), Ultramar and Lubrizol. He moved to Lexington, Kentucky, in 1997, when he began his career with Ashland Oil, now Valvoline. He was vice president of branded lubricant technology at Valvoline.

Josh Frederick and Fran Lockwood, both of Valvoline, said that Thom was one of the most affable, optimistic, humble and fair gentlemen they had ever known. He touched many lives and will be greatly missed.

Angela Willis, who followed Thom as chair of the ASTM Passenger Car Engine Oil Classification Panel, commented that Thom brought humanism to the role. He always incorporated ways to help lighten the mood with his trivia tidbits, while still orchestrating a very professional, well-facilitated meeting.

The industry knew him as a vigorous participant in American Petroleum Institute and ASTM International forums, who could always be counted on to be at once outspoken, open-minded and fair. In addition to his 13-year leadership of PCEOCP, Thom served as chair of the API Category Life Oversight Group, where he inspired many others to assume active roles within various API and ASTM forums. My thoughts were that Thom could always keep the discussions objective and civil.

Angela probably summed it up best, saying, I will miss his warm and welcoming personality.

-Steve Swedberg