As the population of natural gas (NG) engines grows every year, so does the risk of comingling engine oils in shops with diesel and NG fueled fleets. To better understand the implications, this article will discuss the differences between the two types of engine oils and the ramifications of contamination with the incorrect engine oil, which can be compounded when oil drain intervals are extended.
Engine oil formulations have been impacted as emissions regulations continually tighten. Because small amounts of engine oil can enter the combustion chamber and be consumed, there can be a negative impact on engine emissions and after-treatment devices.
To mitigate this impact, the EPA and engine manufacturers have placed chemical limits on the types and amount of additives in engine oils. Combusted oil additives also leave behind ash, which can damage exhaust catalysts, EGR coolers, and diesel particulate filters. The challenge for engine oil blenders is to maximize the performance of these lubricants while operating within the established chemical constraints.
While there are numerous shared requirements of engine oils, diesel engines have several divergent needs versus those of NG engines. Diesel engines have lower combustion temperatures to reduce Nitrogen Oxides (Nox) and, as a result, tend to generate soot.
Soot is a result of incomplete fuel combustion. Blow-by pushes some of this soot into the crankcase. Soot particles have an affinity for each other and tend to agglomerate. Higher levels of detergent and dispersant additives are required to disperse the soot particles and hold them in suspension to prevent viscosity increase, varnish, sludge, and wear. Diesel engine oils tend to be formulations with higher Sulphated Ash (“Ash”) to accommodate the need for higher levels of detergent and dispersant additives.
Natural gas engines have very similar engine oil requirements as diesel, but with a few important differences. NG engines have higher combustion and operating temperatures. Fuel combustion is more complete and natural gas engines do not generate soot. The need for detergent and dispersant additives is significantly lower than with diesel engines.
Due to higher operating temperatures, oil oxidation and nitration stability are primary needs. Higher combustion temperatures in NG engines make exhaust valves more sensitive and prone to ash deposits. While this ash can help protect against valve recession, high-ash levels can create deposits that can lead to valve torching.
Therefore, natural gas engine oils are blended to be lower-ash formulations. Higher quality base oils can play a significant role in compensating for the lower additive levels in these formulations by providing better inherent oxidation stability… especially under extended drain environments.
Comparison of Performance Needs of Diesel and Natural Gas Engines
The impact of mixing engine oils is directly related to the engine type and amount of comingled oil.
To mitigate the chances of comingling products, the proper training of maintenance staff is vital. Also, bulk tanks and dispensing equipment should be clearly marked to identify the different products and applications.
Regular oil sample intervals will provide tell-tale signs of any mixture thru wear metal and physical property alerts. Most importantly, additive levels in used oil sample results should be reviewed to identify if there are any changes. If a significant comingling of engine oils is identified, be sure to schedule an oil change as soon as possible… regardless of the engine type.