Gas Engine Reliability: Understanding Engine Oil Consumption

This paper explains the mechanisms that affect lubricating oil consumption in stationary gas engines. It explains the positive and negative consequences of oil consumption. It discusses the importance of considering oil ash content when selecting a gas engine lubricating oil and emphasizes the importance of oil consumption monitoring in daily operations.

The lubricating oil in a stationary gas engine performs multiple critical functions. It lubricates surfaces in relative motion to each other by providing separation of these surfaces through fluid viscometrics, seals the piston ring/cylinder liner interface, and cools engine parts, such as pistons and bearings. With the help of additive technology, the lubricating oil also provides protection against wear during startup and against corrosion by acidic species, originating from fuel or from oil degradation processes. Finally, yet importantly, it keeps the engine clean.

A typical gas engine lubrication circuit consists of an oil sump, oil pump, oil cooler, pressure and temperature control valves, full-flow oil filters, and secondary filtration systems. The oil pump draws in fluid from the sump through a strainer. Next, the oil is cooled down in the oil cooler; a thermostatic valve determines how much oil goes through the cooler and how much bypasses it in order to achieve the desired oil inlet temperature. Oil flows through the oil filters with a fineness that may range between 20 microns (nominal) to 40 microns (absolute). From the oil filters, oil is fed to the engine, supplying the main and rod journal bearings, camshaft and follower, gear train, turbochargers, rocker arms and shafts, pistons, and liners. The valve stems and valve guides are normally not included in the pressurized oil system as they receive oil from the rocker arms flowing freely over the valve deck. All oil eventually flows back to the oil sump at about 10C–15C (50F–59F) hotter than when it entered the engine.

The oil film provides complete separation of the bearings and crankshaft journals. A hydrodynamic wedge is formed upon startup, where the faster the shaft runs in the bearing, or the heavier the fluid viscosity is, the thicker the oil film will be. Reduced speed, reduced viscosity, and increased load decrease the oil film thickness. A hydrodynamic wedge also builds between the piston ring and liner wall interface to provide separation of these metal surfaces; this oil film also acts as a seal for combustion gases.

A minimum amount of oil flow is required for proper lubrication of the intake and exhaust valve stems and guides. Via the clearance between guide and valve stem, a small amount of oil will enter the gas flow. This oil will reach the intake valve seats and protect these. However, on the exhaust side, the exhaust gas flow will prevent it from reaching the exhaust valve seats. Instead, these are protected with ash from oil burnt in the combustion chamber.

Oil consumption in stationary gas engines is a normal and necessary function for healthy operation. Gas engine manufacturers consider this in the design of their engines, as a specific amount of oil consumption is required. Each engine manufacturer has their own range of what is or is not acceptable relating to the rate of consumption, often provided to owners in grams per kilowatt-hour (g/kWh). In modern engines, oil consumption rates of 0.05 g/kWh to 0.15 g/kWh are typical, and tend to increase as an engine gets closer to the planned overhaul interval. Oil consumption rates in lightly loaded engines are typically somewhat higher.

The rate of oil consumption can also have an impact on the useful service life of an oil fill. Low oil consumption can mean little fresh oil makeup, which can reduce the potential oil life. Higher engine oil consumption increases the rate of fresh oil makeup, which can help provide longer oil service life, as long as the higher oil consumption does not coincide with increased blow-by.

The oil consumed by the engine will pass into the combustion chamber, where it is burned. However, some metal-containing additives are incombustible and remain as ash. The ash from stationary gas engine oils is often made up of calcium, magnesium, zinc, phosphorus, boron, and molybdenum used in detergent and anti-wear additive chemistries.

Oil consumption in combination with ash content will determine the overall ash throughput through the combustion chamber, turbocharger, exhaust gas catalysts, and heat recovery boiler. Therefore, stationary gas engine lubricants are often classified according to their ash content. There are gas engine oils with so-called "low ash" content of about 0.5% to 0.6% ash (by weight). For engines that need more protection, gas engine oils with "medium ash" content are available, with ash levels up to 1.0% ash (by weight).

Beneficial Effects of Lube Oil Ash. The primary reason for a lubricating oil containing ash-producing additives is the role these additives play in overall protection of the engine. The ash-producing additives are:

The second beneficial role of lubricating oil ash is the protection of the exhaust valves. A small amount of oil flow is needed for proper lubrication of the intake and exhaust valve stems and guides.

On the intake side, oil seeping out between the valve and guide will be carried by airflow to the valve face and will lubricate the seating surface. However, on the exhaust side, oil will be burned before it reaches the valve seat. Therefore, the exhaust valve seating surface is lubricated with dry oil ash, originating from lube oil burned in the combustion chamber. The ash that remains can deposit or sinter onto the valve seat and face surfaces to protect them from wear and recession. Figure 1 shows an unprotected recessed exhaust valve. Figure 2 shows a valve with small ash pearls or speckles on the seating surface, providing evidence of adequate dry lubrication of the seating surface, preventing valve recession.

1. Recessed exhaust valve. Courtesy: Petro-Canada Lubricants

2. Well-protected exhaust valve. Courtesy: Petro-Canada Lubricants

Detrimental Effects of Lube Oil Ash. Ash from burned lubricating oil will largely escape with the exhaust gases. However, a fraction will deposit on combustion chamber components (Figures 3 and 4). Deposits on the piston crown and firing deck can increase the compression ratio of an engine, which can influence the chances of knocking (detonation). Knocking can severely damage the engine. Therefore, sensors are installed to detect it. When knocking occurs, the engine control system will first retard ignition timing (negatively impacting the engine efficiency) and eventually reduce load or switch off the engine completely (negatively impacting production).

3. Ash deposit on piston top. Courtesy: Petro-Canada Lubricants

4. Ash deposit on valve deck. Courtesy: Petro-Canada Lubricants

Deposits formed on piston crowns and top lands can act as an insulator, thereby not allowing sufficient heat to transfer. The combustion chamber temperature goes up, which can also contribute to knocking.

Excess deposits formed on spark plug electrodes can bridge the plug gap, causing poor or no spark (spark plug fouling). Whereas some ash is required to provide dry lubrication of valve seats, too much ash deposit on exhaust valve seating surfaces can prevent complete closure of the valve, leading to valve torching (Figure 5).

5. Torched exhaust valve. Courtesy: Petro-Canada Lubricants

Oil ash and carbon deposits can form on the piston top land. When the piston top land deposits grow significant enough to contact the liner wall, the deposits disturb the lube oil film on the liner and can rub and wear away liner crosshatch, resulting in bore polish. The oil film thickness in the hydrodynamic regime is a function of speed x viscosity / load x surface roughness, so when the surface roughness decreases (crosshatch being worn away), this smoother surface allows a thicker oil film to form. Because of this, more oil is transported into the combustion chamber by the ring pack, while at the same time the thicker oil film cannot withstand the combustion pressure, resulting in increased blow-by. As this wear mechanism progresses, this creates increased deposit formation, which leads to increased wear, more blow-by and more oil consumption.

To reduce harmful bore polish, modern gas engine manufacturers can utilize an anti-polish ring. An anti-polish ring is a ring that sits at the top of the liner that has a slightly smaller inside dimension than the liner. This ring continually cleans the piston top land of any harmful deposit by not allowing the top land deposit to bridge across to the liner wall and cause bore polish. The effect of this is that oil consumption will not increase.

Loosened ash deposit and carbon particulate from the oxidized base oil can be returned to the crankcase via blow-by. The larger particles are generally filtered off in the main oil filters and smaller particles are removed through the oil centrifugal filter if the engine is so equipped.

In cogeneration applications, boiler fouling can occur, as the boiler is cooler than exhaust gases, promoting condensation of oil vapors that capture oil ash. Increased oil consumption will lead to increased fouling of the boiler, resulting in less heat being recovered, forcing more frequent boiler cleaning.

Many gas engines are installed with exhaust emission abatement systems in the form of catalysts to reduce CO and NOx emissions. Oil ash deposits can mask the catalyst reactive surface. Chemical deactivation can also occur due to a reaction between the catalyst and different elements found in the lubricating oil ash such as phosphorus and sulfur. This results in less efficient conversion of harmful emissions in exhaust gas catalysts. Increased oil consumption often results in reduced catalyst lifetime. Masking by ash can also lead to failure of emission sensors.

The above explains that the ash throughput must be controlled by finding the correct balance between oil consumption and oil ash content. The target is to provide sufficient ash throughput and ensure the lifetime of the cylinder heads while at the same time keeping ash throughput low enough to prevent excessive fouling of the combustion chamber, heat recovery boiler, and exhaust gas catalyst, all of which could cause engine downtime and cost significant money to rectify.

Several mechanisms can affect oil consumption in a stationary gas engine.

Piston Ring Pack. In modern engines with low oil consumption, this mechanism is the predominant cause of oil consumption. A natural consequence of an oil film on the liner wall is that some oil will enter the combustion chamber through oil throw-off from the ring pack near top dead center. The thicker the oil film, the more oil is transported upward by the ring pack. All oil transported above the top dead center point is lost by means of inertial throw-off into the combustion chamber and will be burned. This means that for good combustion sealing and low oil consumption, a thin oil film is desired. For wear protection, however, a certain oil film thickness is required. Therefore, a solution needs to be identified where the oil film is as thin as possible, but thick enough to prevent wear.

Oil film thickness varies over the piston stroke length. Mid-stroke the piston speed is high and the rings travel on the liner surface in a fully hydrodynamic regime. As the piston slows down and enters either turnaround zone, the hydrodynamic effect lessens and the piston rings start pressing themselves through the oil film. The buffering effect will prevent the rings from reaching the liner before the piston starts moving again and the rings re-enter the hydrodynamic regime. The minimum required oil film thickness is determined by the amount of buffering that is needed. In practice, a partly flooded lubrication regime provides sufficient buffering effect to avoid top dead center ring/liner wear, and will help to improve combustion chamber sealing and to reduce inertial throw-off (reduce oil consumption).

At a given oil viscosity, and a given oil and liner temperature, it is the oil control ring, together with the liner crosshatch surface finish, that determines the oil film thickness on the liner, and, therefore, the engine oil consumption via inertial throw-off. As such, the oil control ring prepares a well-defined oil film for the other rings to run on. At a given oil film thickness, the compression ring profiles (asymmetric barrel, trapezoid), piston ring stiffness and bendability, ring radial tension, and combustion gas pressure in the ring groove determine how much oil is being scraped upward to the combustion chamber and lost by inertial throw-off. The honing pattern of the cylinder walls helps to keep the oil film thin and reduces inertial throw-off. For a given mechanical design, a higher-viscosity fluid will contribute to a thicker oil film and lead to greater oil consumption. Low-viscosity oils, and oils with good viscosity control in operation, can help keep oil consumption through inertial throw-off low (Figure 6).

6. Fully flooded and partly flooded lubrication regime. Courtesy: Petro-Canada Lubricants

Valve Stems and Turbocharger Seals. Another contributor to oil consumption is oil loss, for example, between valve stem and guides or via the turbocharger seals. This is also influenced by viscosity, however, in the opposite way: A low-viscosity oil leaks away easier and can result in increased oil consumption.

Modern engines are equipped with valve stem seals that help meter the required amount of oil flow into the crevice between stem and guide. This helps to prevent excessive oil consumption via the valve stems. If valve stem seals and guides become worn, the loss of oil via leakage over valve stems can be more significant. Also, in older designs without metering seals, oil consumption via valve stems can be significant. In cases of lower engine load, oil carryover via the valve stems can be more pronounced due to lower charge air or intake manifold pressure; this reduced backpressure more readily allows oil to flow down the guide/stem. In order for the turbocharger oil seals to work well, it is necessary that axial play remains small. This should be monitored during regular maintenance.

Reverse Blow-By. Reverse blow-by can happen when the pressure in the pockets between the piston rings is higher than the pressure in the combustion chamber, such as at mid-stroke or toward the end of the power/expansion stroke. The pressure in the combustion chamber has fallen due to expansion, and the pressure in the inter-ring pockets lags behind. This can occur if such pockets are relatively large, or if the engine is operated at partial load. In such a case, the inter-ring pressure can propel oil past the ring pack and toward the combustion chamber, contributing to oil consumption.

A number of design measures can help to avoid reverse blow-by. For example, reducing the number of compression rings to two, reducing the height of the lands between the piston rings, and proper sizing of the ring slot are beneficial in this regard.

Crankcase Ventilation. Crankcase ventilation can be a major cause of oil consumption. The crankcase exhaust gases contain oil mist and vapors; therefore, crankcase gas ventilation systems are equipped with coalescer-type filters to remove oil mist from the crankcase gases. If such filter is saturated or overloaded, crankcase gases can escape unfiltered, significantly increasing the perceived oil consumption of the engine. The same can happen if the drain line of the filter is blocked, stopping the return flow of separated oil back to the sump.

Lube Oil Volatility. As far as the oil is concerned, oils of different qualities, base oil types, and viscosities will have different levels of volatility when exposed to high temperatures. Oils with higher volatility will lose a higher fraction due to evaporation, contributing to oil consumption. A gas engine oil formulated with a straight run base stock will have an advantage over oils using a blend of heavier and lighter base stocks, as the lighter fractions will more easily evaporate.

The natural oil consumption of an engine may be an important criterion when selecting the right lubricating oil for the engine. This is because oil consumption determines ash throughput. For example, consider an engine that consumes 5 liters of oil per day (4.4 kilograms/day). Assuming this is a low-ash oil containing 0.5% ash (by weight), then approximately 22 grams of ash is passing through the combustion chambers each day.

If that same engine would consume 9 liters per day, this would mean approximately 39 grams of ash being put through the engine each day. This increased ash throughput could also be achieved in another way, such as if the engine consuming 5 liters of oil per day would be lubricated with a medium-ash oil containing 0.9% ash (by weight). This also translates into an ash throughput of 39 grams per day.

The increased ash throughput is undesired for the reasons mentioned above, unless it is required to protect the exhaust valves. The increased protection of exhaust valves (and therefore longer lifetime of cylinder heads) is traded off against the detrimental effects of higher ash throughput. As mentioned above, one has to account for a higher rate of combustion chamber fouling, and therefore, more frequent cleaning, the risk of higher liner wear rates, an increased fouling rate of the exhaust heat recovery boiler, and a reduced lifetime of the catalyst. The lubricating oil ash content is therefore an important property to consider when selecting the lubricating oil for a gas engine.

Oil consumption is a great indicator of the health of an engine, especially that of the piston ring pack and liner. Firstly, it will signal filling of the top ring grooves with carbon, which will disturb the functioning of the top ring and will inevitably result in blow-by and increased oil consumption. Secondly, it will signal bore polish of the liner: When the honing pattern is worn away by carbon deposits on the piston top land, the oil film thickness will increase and the oil consumption will go up (Figure 7). Thirdly, it can help to indicate anomalies such as a broken piston ring.

7. Heavy top land carbon deposits (left) and corresponding bore polish (right). Courtesy: Petro-Canada Lubricants

An increased oil consumption rate not only means increased expense on fresh oil, it also means an increased ash throughput, deteriorating engine condition and causing other detrimental effects as discussed above. It is therefore critical to monitor the rate of oil consumption. It is ideal to utilize a low-flow oil meter designed to monitor lubricant top-up flowrate, usually mounted after the fresh oil makeup tanks and before the engine's oil level controller. This meter should be read and recorded at regular running hour intervals to document oil consumption.

It is good practice to report top-up volumes when submitting an oil sample for analysis. When oil sampling is conducted at regular running hour intervals, the top-up volumes should remain the same. An increase in oil consumption can easily be noticed in this way and an investigation initiated by means of a borescopic inspection, for example.

This paper has explored the mechanisms of oil consumption in gas engines, including pathways through the piston ring pack via inertial throw-off and reverse blow-by, valve stem and turbocharger seals, crankcase ventilation, and lube oil volatility. We have demonstrated the influence that the lubricating oil film thickness plays: A thicker oil film contributes to more oil being carried past the ring pack and a thinner oil film allows more oil to pass through the valve stem/guide interface and turbocharger seals.

For a stationary gas engine, choosing a lubricant with the correct ash level is extremely important in order to provide sufficient engine protection while not producing excessive ash deposits. The total amount of ash passing through an engine is not only a function of oil ash content, but also very much a function of oil consumption. Understanding and monitoring daily oil consumption rates provides critical insight into engine reliability and helps in identifying conditions that may contribute to premature wear. It is a necessary tool that complements used oil analysis, filter analysis, and visual inspections.

—Thijs Schasfoort and Clinton Buhler are senior technical services advisors for Petro-Canada Lubricants.

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