Gas compression engines operating at a lower load can lead to greater oil consumption. Here’s why – lower loads result in lower intake manifold pressures in the cylinder, which means more oil is drawn down the intake valve guides and past the piston rings. This increases ash and carbon deposit formation. 

Lower loads also mean lower combustion temperatures. This lower combustion temperature compounds the ash and carbon deposit formation in the combustion chamber. Excessive deposit formation leads to valve sticking, valve torching, ash pounding, engine wear and reduced engine performance. All of this can add up to extra cost for maintenance and lost productivity.

Drain intervals of lower-output engines can be impacted in a few ways. On the positive side, reduced load means reduced pressure and heat in the combustion chamber, so the severity of service for the lube oil is reduced. On a low-load unit, there may be greater ability to reduce levels of nitrogen oxide (NOX) by altering exhaust emissions through air fuel ratio adjustments, which has a major impact on reducing nitration of the engine oil and extending oil life. Lower combustion temperatures can also result in insufficient heat for NOX to form when compared to a fully loaded unit.

While it is true that lower loads do reduce the amount of thermal loading on the engine oil, it does not have to become a problem. The minimum oil temperature is typically maintained by a thermostat, and, in most cases, the temperature can be kept high enough to flash off water vapors and minimize the absorbance of NOX into the lubricant oil film. Hence, it is important to install the correct thermostat in order to maintain the required minimum oil temperature during service. This sustained minimum temperature, along with cooler maximum oil temperatures, can prolong drain intervals and increase oil life.

Problems related to lower loads in natural gas compression engines include: valve sticking, valve torching, backfiring, and, most significantly, excessive deposit formation in the combustion chamber, which causes wear, such as ash pounding. Although more prevalent in higher loads, exhaust valve recession can still be a challenge in lightly loaded engines.

Ash pounding is an event that occurs due to deposit build-up to a point where it becomes trapped between the edge of the piston crown and cylinder head deck. This causes a mechanical deformation of the crown, which interrupts the lube oil film locally and can eventually lead to a piston failure and, in extreme cases, a piston seizure. (This phenomenon has been observed in some lean-burn Waukesha engines.)

Exhaust valve recession occurs when too little ash is produced in an engine. (This phenomenon occurs in all gas engines to some degree and is commonly seen in G3400 Caterpillar and turbocharged White Superior Engines.) It is necessary for an ideal amount of ash to be generated as the ash will form a protective layer on the valve seating surfaces, which will provide protection from valve recession as the engine operates. However, too much ash will contribute to valve torching. As this is a fine balance, it is essential to select a lubricant formulated with the correct amount of ash.

Running at a lower load requires special consideration for lubricants. Operators need a lubricant with strong oxidation, nitration and acid control, paired with the right amount of ash and detergency to protect the engines. For operators who are consistently running loads below 60%, they may need to choose a lubricant with a lower ash content. In a lower-load situation, a low-ash lubricant will protect from deposits and contribute to cleanliness.

Conducting regular oil analysis is also essential with low-load engines. Periodic inspection of combustion chambers can also assist in determining whether or not the chosen lubricant is creating an acceptable level of ash.

The right lubricant has the ability to combat issues stemming from lower loads. Higher-quality products with a higher-quality base stock could extend the life of the oil. The ash content, detergent system and volatility of the lubricant are also essential factors to consider when evaluating a lubricant to be used in engines that consistently operate at lower loads.

The SENTRON line-up from Petro-Canada Lubricants has a variety of products to meet the various needs of gas compression engines. Understanding what your engine requires based on the application, ash requirement and load will help determine the best fit.

SENTRON VLA 40 is a "very" low ash stationary gas engine oil designed for engines running at low loads (<60%). Formulated with a very low ash content of 0.3% to prevent ash buildup that causes engine damage and failure, SENTRON VLA 40 is the choice for low-load engine performance. In engines challenged with harmful deposit formation, products such as SENTRON LD Synthetic Blend and SENTRON Ashless 40 should be used as they are specifically designed to improve combustion chamber cleanliness.

When dealing specifically with ash pounding, SENTRON LD Synthetic Blend helps with control of deposit formation. It is a partially synthetic multi-grade product with a very high viscosity index (VI). Because of its low volatility, oil consumption can potentially be reduced, correspondingly resulting in less deposit formation in the combustion chamber. SENTRON’s robust detergent system also contributes to optimal overall engine cleanliness and protection.

SENTRON has a line-up of products with a variety of strengths including very strong nitration resistance, oxidation stability and acid control. So whatever the problem that's shortening the drain, there is a product in the SENTRON line-up that can address it and help.

Contact Senior Technical Services Advisor Clinton Buhler. Our team is more than happy to help you find a solution. We've got the technical expertise and experience that comes with having a complete line-up of specialty fluids, oils and greases that can help maximize your equipment performance, savings and productivity.

Clinton Buhler is a Senior Technical Services Advisor for Petro-Canada Lubricants Inc. with more than 15 years experience helping customers with rotating equipment challenges. He currently specializes in gas plant, pipeline and power generation lubricant applications. Clinton is based in Alberta, Canada.