Ash in Stationary Natural Gas Engines
How much ash is too much and how much is not enough?
Among the many operational variables that affect lubrication for natural gas engines in industrial equipment, the level of ash content in the oil is an issue that generates a lot of questions from our customers. Does the application and operating conditions call for an ashless, low- or medium-ash oil? The challenge is that ash can have some benefits but can also cause some problems with excessive deposits in critical components. Let’s look specifically at the role ash plays in the performance and durability of cylinder heads and valves in a natural gas combustion chamber.
First, it’s useful to understand where ash comes from. Ash is generated from the use of calcium and zinc in lubricant additives. Calcium helps neutralize acids, while zinc is effective for anti-wear protection. However, these metals do not burn. As oil burns off, these non-combustible additive components are left behind in the form of ash, which sticks to parts of the combustion chamber. The biggest area of concern is in the valves, specifically the space between the valve face and the valve seat.
Some ash is desirable between the face and the seat. Ash acts as a kind of cushion, reducing metal-to-metal contact. Too little ash will result in measurable surface wear on the face and seat, causing “valve recession” where the valve doesn’t seal properly, and ultimately cutting short the life of the cylinder head. However, if you have too much ash accumulating in this space, a portion of the cushion can break off, interfering with proper sealing between the face and the head and creating a leak path for exhaust gases. Those gases are extremely hot as they rush through the gap and can literally torch or gutter the valve face. The cylinder then cannot produce any compression, and therefore no power.
The challenge is striking that balance between too much and not enough ash – and that is not just a matter of the choice of the oil. It is also a function of how the engine is operating.
Engine Load and Ash Accumulation
A butterfly valve controls the amount of air flowing into the engine. When the engine is running at full load, the butterfly valve is open, producing positive air pressure in the intake manifold. That air pressure will push back up any oil flowing from the top of the cylinder over the valve stem and guide. However, if the engine is throttled down to 50% load, the butterfly valve closes, creating a vacuum in the intake manifold that effectively pulls the oil down past the valve guides. That means more oil is flowing into the combustion chamber and more ash is building up around the valve face and seat.
The engine load, therefore, has a big impact on ash accumulation. Some operators may not realize that running the engine at partial or less than full throttle actually increases the ash buildup in the combustion chamber and on the valves, which can cut short the life of the cylinder head. For this reason, an ashless oil may be a more practical solution than a low ash oil, and it may help extend the head life.
Excessive ash affects other parts of the combustion chamber and beyond – for example, the exhaust and aftertreatment systems. However, issues with the valves are usually the first to come to light. As a rule of thumb, two-stroke engines typically call for an ashless oil, as the smallest amounts of ash can plug up the exhaust and intake ports. For a four-stroke engine, a low-ash oil will do a better job of mitigating valve recession. As always, we recommend discussing your specific engine, application, and operating conditions with a lubrication expert to arrive at the optimal oil decision.