Cummins details natural gas fuel quality standards for CNG truck engines, from filters to methane number

Cummins builds a long lineup of engines that burn natural gas, and it monitors fuel quality the same way diesel and gasoline are watched. Alongside American Society for Testing and Materials (ASTM) standards, Cummins has developed its own fuel standards for different engines and applications. Customers who are unfamiliar with a natural gas supplier or worried about fuel quality can ask that supplier to test the fuel using Cummins' Natural Gas Analysis Tool, and Cummins service technicians can also test gas in the field if they suspect it is not meeting engineering standards. With U.S. natural gas supplies at historic levels, more fleets and owner-operators are turning to engines such as the 15-liter X15N, reporting lower fuel costs while keeping the power and performance their operations require.

Natural gas suppliers use ASTM testing methods to determine overall fuel composition, removing impurities such as water, oil, hydrogen sulfides, carbon dioxide and other content that can affect performance before the gas enters pipelines. Underground pipelines deliver the gas to fueling stations, where compressors pressurize it and filters remove contaminants, water and compressor oil before it is pumped into vehicles as compressed natural gas (CNG). If the fuel comes from sustainable sources such as dairies, sewage plants and landfills, suppliers may label it renewable natural gas (RNG). On the truck, serviceable filters act as another line of defense against oil, water and other impurities, and drivers should drain fuel filters each time the truck is filled, reporting any unusual water or oil. Filter life depends on application: severe-duty cycles require replacement every 15,000 miles, while normal use can extend life up to 65,000 miles.

Engine knock can be a sign of fuel trouble, but it may not warrant serious concern. Scott Baize, regional technical platform leader at Cummins, said a relatively high amount of ethane speeds up combustion and can cause knock or detonation; the engine's knock sensors detect that and adjust combustion to prevent knocking. Nitrogen has the opposite effect, slowing combustion, and the engine can adapt within published limits that are wider than with gasoline. Propane or higher hydrocarbons can also cause knock, and exceeding the published limits may trigger a fault code or derate. At some processing sites, hydrogen may be added to natural gas to decarbonize it, but the high price of hydrogen increases cost and Cummins does not recommend it for its engines; David King said the company has hydrogen limits and is not a fan of high concentrations. Customers near natural gas processing plants may also encounter high levels of siloxanes, which can cause fuel system corrosion and spark plug failures, and fuel testing can check for that and other impurities.

Certarus, a virtual pipeline company, offers an example of field evaluation. Mel Lara, senior RNG commercial manager, said the company focuses on compression, transport and delivery of CNG, and delivered 230 million diesel gallon equivalents in 2025. Certarus compresses gas at hubs or third-party sites, loads it into trailers up to 4,000 psi, and transports it to end users. After testing Cummins' X15N on existing routes, Lara said drivers, who compared the CNG truck to diesel trucks, told the company that 'this performance matches the diesel performance.'

Cummins recommends a proactive and preventive approach to natural gas fuel management, especially for first-time users unfamiliar with natural gas in their area. David King said fuel suppliers, fleet managers and drivers should all do their part to be on the lookout for fuel-related issues, and Cummins prepares customers before they take delivery of natural gas trucks by doing a fuel analysis to ensure they have the right fuel, just as it helps them gear the truck properly and spec a fuel delivery system with the capacity needed for their routes. The company also has a tool to help customers make sure the fuel meets the fuel specification. With decades of experience in natural gas engines and fuel testing, Cummins remains eager to work with customers and suppliers to keep fuel quality up to par.