Lately we’ve been hearing more questions about which oils to use with CNG engines, as more and more companies and municipalities start using natural gas-powered vehicles in their fleets due to their low emissions (not to mention, when fuel prices are higher, there’s also a well-defined cost benefit to using CNG). While some people think that all truck engines are relatively the same, CNG engines operate at higher temperature levels than diesel engines, and require specially formulated oils to meet their more demanding operating requirements—including the right viscosity grade, balanced ash levels, and antioxidant concentrations.
To help you select the best oil for your CNG engine, here are 4 considerations to keep in mind when choosing a CNG engine oil:
1) OEM Requirements
Whenever it’s time to select a fluid for your engine or machinery, the first place you should always go for answers is the OEM requirements. As the manufacturer who built the engine, they know what will make it run well. While there aren’t API spec levels for CNG oils, engine manufacturers have developed a few specifications themselves, such as Cummins CES 22074, 20081, and 20085, as well as Detroit Diesel DDC 93K216. If your engine calls for oil that meets these specifications, you’ll want to make sure you’re using one that does.
Because CNG engines typically have higher RPM ranges, increased horsepower, and higher combustion temperatures, finding an oil with the ideal viscosity is critical. Higher combustion temperatures can lead to damaging viscosity increases, as well as chemical oxidation and nitration in the oil. That being said, the application of the CNG engine is what determines which viscosity level is required for the oil. For example, 15W-40 oil is typically recommended in larger engines with heavier payloads, due to the higher workload and temperatures. On the other hand, 10W-30 may be better for smaller engines (e.g., fleet vans) that don’t pull heavy loads.
3) Sulfated Ash Level
CNG engines don’t produce soot when the gas burns; instead, due to the detergent additives in the oil, they produce ash—similar to wood ash in a fireplace. Because gaseous fuels are “dry” and provide no lubricant value your engine oil must provide a soft ash deposit on the exhaust valves to prevent recession. That being said, too much ash can cause piston deposits—especially on the piston crown area—which can harm engine efficiency and cause irregular combustion.
In order to create just the right amount of ash in your engine, it’s critical that lubricant manufacturers and blenders take great care when determining the proper quantities of anti-wear and detergent additives. Typical fluids have between .9 and 1% sulfated ash, but levels must be much lower in CNG oils in order for the fuel to completely combust. The recommended level of sulfated ash varies by model and manufacturer. Typically you'll want a low or medium ash level fluid, but it is always best to refer to the OEM recommendation. If you're experiencing a more specific issue, contact your fluid supplier.
4) Phosphorus Limit
One of the main differences between CNG and diesel engines is that CNG engines have a specific limit on the amount of phosphorus used in their additive packages in order to protect the catalyst. To ensure that your oil doesn’t cause any damage, make sure the fluid you’re using is below the given phosphorous limit for your engine.
Looking for a high-quality CNG engine oil that meets most manufacturer specifications, and helps extend engine and component life? Our THRIVE CNG Mobile 15W-40 is a high performance CNG engine oil recommended for CNG, LNG, and LPG applications where CES 20074 and DDC 93K216 is required. It provides superior bearing corrosion protection, increasing the uptime and operational efficiency of your engine.
To learn more about how U.S. Lubricants can help you formulate the ideal CNG engine oil, contact Tony Springer at TSpringer@uslube.com or by phone at (800) 490-4900 ext. 8823.