There were more than 26 million natural gas vehicles operating worldwide in 2018, according to NGV Global, a natural gas industry association. North America had the lowest regional count, with just over 200,000 or less than 1 percent of the global total. These vehicles range from two-wheelers to off-road equipment; include units built as such by original equipment manufacturers, factory-approved conversions and post-sale conversions; and burn all types of natural gas.

Increased use of natural gas is being facilitated by beneficial economics, expanding infrastructure and reduced emissions.

Natural gas prices have generally stabilized over the past few years and are expected to remain relatively low for the foreseeable future. Both supply and demand are growing around the world: In Australia, not only is the consumption outlook stable, but we expect to see an increasing rate of natural gas availability. So much so, in fact, that surplus is exported to the Asia-Pacific region where natural gas demand is increasing. Japan is cutting back on its reliance on nuclear power generation, and both India and China are reducing their dependence on coal.

In the United States, there has been significant growth in natural gas exports due to the commercial use of hydraulic fracturing, or fracking, which makes accessing natural gas reserves more economical than traditional drilling techniques. Overarching these various developments is a trend toward globalization of the natural gas market, which has also helped reduce regional price differentials and led to more price stability in the market.

Another important enabler is improved infrastructure, a barrier that any alternative fuel must overcome. In North America, there is a substantial natural gas pipeline network in place. The number of compressed natural gas and liquefied natural gas fueling stations in the U.S. continues to grow, having climbed from 772 stations in 2009 to more than 1,700 stations in March, according to NGV America. However, their quantity is still quite small compared to conventional filling stations in the U.S., and there is significant room for growth. In contrast, China has over 3,000 LNG stations and is now the leader in the number of LNG-fueled trucks.

Finally, increasing and more stringent emissions regulations help encourage the use of natural gas, which produces less greenhouse gases than diesel. China and India continue to work toward reducing pollution, one example being the impending arrival of China 6 and Bharat VI specifications-in 2019 and 2020, respectively-which are designed to reduce emissions for both light and heavy-duty vehicles. Parts of the U.S. are still out of compliance with government air quality standards. Californias standards for particulate matter and ozone are expected to be exceeded by 2023, prompting the consideration of a new standard that will further reduce nitrous oxide emissions.

Significant improvement in air quality can be achieved by using CNG, LNG and renewable fuels likes biogas. Natural gas combustion is inherently cleaner than diesel, hence tailpipe emissions can be minimized through three-way catalyst technology.

The good news is that these natural gas based alternatives are already in use. CNG is widely used in city-service applications like package delivery, garbage trucks and city buses because it is logistically beneficial. Even though it requires high-pressure tanks on trucks, it is still safer than using diesel fuel because natural gas will quickly vent to the atmosphere whereas diesel spills could potentially ignite. LNG is already preferred by heavy-duty fleets in many longer-haul applications where its higher density allows for a greater distance per gallon of equivalent product.

Sustainable (or renewable) natural gas uses the biomethane emitted from matter like sewage and food waste. It is deliverable by the existing natural gas grid and its use has been documented to reduce greenhouse gas emissions by as much as 85 percent.

Technology Continues to Develop

The increased supply of natural gas combined with emissions benefits from its use have resulted in the development of new engine technology. Cummins Westport, a leading supplier of natural gas engines in North America, has certified three engine platforms as emitting nearly zero NOx. Chinese OEMs such as Weichai, Yuchai and FAW are also large producers of natural gas engines, with plans to expand.

As these original equipment manufacturers produce more advanced natural gas engines, the development of mobile natural gas engine oils is evolving as well. In the 1990s, MNGEOs were governed by the Cummins specification CES 20074, which was augmented in the early 2000s with CES 20085. In 2018, a third generation of standards evolved under CES 20092. Cummins introduced CES 20092 for stoichiometric combustion mid-range and heavy-duty engines and intends it to replace CES 20085 by the end of this year.

Oxidation performance and thermal stability are areas where the difference between a Generation 2 and Generation 3 certified MNGEO is noticeable, and CES 20092 has new engine tests specifically designed for oxidation performance of oils.This should translate into extended drain interval capabilities, a key contributor in helping lower the total cost of ownership.

Mobile Natural Gas vs. HDEO

There are significant differences between the environments in which heavy-duty diesel engine oils and MNGEOs operate. The case for oils that are designed specifically for natural gas vehicles starts with the fact that the fuel is gaseous and practically sulfur-free, which means no issues of fuel wall wetting, atomization of fuel or partially-burned fuel components due to flame diffusion. Acid levels are usually low in natural gas, soot levels are almost non-existent, and there is no fuel dilution. However, since natural gas is comprised primarily of methane and produces a lot of water during combustion, this can lead to emulsion issues in the engine oil.

Diesel engines also operate under compression ignition conditions, and the air-to-fuel ratio can vary over a wide range depending on power needs. As a result, diesel combustion results in soot entering the engine oil, and that soot must be controlled by high levels of dispersants.In contrast, the natural gas engines running in North America are spark-ignited and operate under stoichiometric conditions (the ideal air-to-fuel ratio), so natural gas combustion doesnt result in soot in the oil. This means MNGEOs dont require such high levels of dispersant.In fact, high levels of dispersants in oils used in natural gas engines can result in emulsions forming in the oil, which can dampen the performance of the lubricant.

As shown in the table above, there are many other differences in performance requirements between proper formulations of engine oils used in diesel and natural gas engines, and theseshould be carefully considered when designing the additive package for the MNGEO. A properly formulated MNGEO can help provide extended drain intervals and improved engine durability, which can lead to lower total cost of ownership.

Natural gas will certainly play a significant role as demand continues to grow for cleaner, sustainable fuels for a growing vehicle population. Emissions regulations, combined with favorable economics, are driving new investments in natural gas for mobile applications, particularly in the U.S., China and India. Since the availability, economic viability and price stability of natural gas have improved significantly, MNGEOs will likely receive increasing attention from engine manufacturers, engine oil producers and additive developers seeking to optimize the performance of engines designed specifically for natural gas.

Raj Padmanabhan is Chevron Oronites Asia Pacific product line manager, automotive engine oils & viscosity modifiers. Contact him at praj@chevron.com. Teri Crosby is global product line manager, automotive engine oils & viscosity modifiers.