All oils degrade over time. They can be considered consumable items as they must be replaced over time. Diesel engine oils are no different except that they may be susceptible to certain mechanisms that turbine oils are not. The diesel engine is often placed under a lot of pressure to deliver power while keeping cool and managing emissions.
The critical areas for lubricant performance in a diesel engine usually include:
- Viscosity control
- Alkalinity retention, base number (BN)
- Engine cleanliness control
- Insoluble control
- Wear protection
- Oxidation stability
- Nitration
Typically, these factors are monitored in these types of oils to ensure that they remain in a healthy condition.
Several factors affect oil degradation in a diesel engine. According to The International Council on Combustion Engines (The International Council on Combustion Engines, 2004), these include specific lube oil consumption; specific lube oil capacity; system oil circulation speed; NOx content in the crankcase atmosphere; and influence on the lubricant, fuel contamination in trunk piston engines, deposition tendency on the cylinder liner wall, metals in lubricant systems, and oil top-up intervals. These can further be divided into systemic conditions (which cannot be easily altered) and environmental conditions (because of processes occurring within or to the system) as shown in Figure 2.
Systemic Conditions
While lubricant degradation can be caused by environmental strains being placed on the lubricant, there are times when the operating design of the system also encourages degradation. Three such cases for diesel engine oils are specific lube oil consumption, specific lube oil capacity and system oil circulation speed.
Specific lube oil consumption (SLOC, g/kWh) is defined as the oil consumption in grams per hour per unit of output in kilowatts (kW) of the engine (The International Council on Combustion Engines, 2004). Over the years, there has been a reduction in the SLOC for engines with special rings inset into the upper part of the cylinder liner. These reduce the rubbing of the crown land against the cylinder liner surface.
With reduced oil consumption, oil top-ups, which would have introduced fresh oil into the system, are consequently reduced. This fresh oil would have increased the presence of additives and helped in maintaining the required viscosity of the current oil. However, since the SLOC is reduced, the oil does not get a “boost” during its lifespan and will continue to degrade at its current rate. Hence, a lower SLOC may encourage the degradation of diesel engine oil.
Specific lube oil capacity, also known as the sump size, which is the nominal quantity in kilograms (kg) of lubricant circulated in the engine per unit of output in kW. According to The International Council on Combustion Engines, the specific oil capacity does not directly affect the equilibrium level of degradation. However, it can influence the rate at which deterioration occurs as smaller sump sizes can increase the rate at which degradation achieves an equilibrium level. Typically for dry sump designs, the specific oil capacity is around 0.5 kg/kW to 1.5 kg/kW. These values are closer to 0.1 kg/kW to 1.0 kg/kW for wet sumps.
System oil circulation speed refers to the time taken for one circulation of the total bulk oil. In diesel engines, lubricants are usually subjected to blow-by gas (including soot and NOx) during their time in the crankcase. If the lubricant spends a longer time in the crankcase, it can become degraded at a faster rate. Typically, the time required for one circulation of bulk oil averages between 1.5 minutes to 6 minutes. However, we have seen the trend toward smaller sump sizes and, by extension, shorter circulation times, which should reduce the degradation rate.
Environmental Conditions
The environmental conditions that lubricants must endure can also influence their degradation. These conditions can either be enforced through the system, its operating conditions or from conditions outside the system. There are a few environmental conditions which must be addressed (The International Council on Combustion Engines, 2004).
NOx content in the crankcase atmosphere and influence on the lubricant has more applicability to gasoline engines compared to diesel engines but they should not be fully ruled out. Diesel engines are more susceptible to sulfur-derived acids (caused by the burning of diesel fuel). However, NOx can be produced by the oxidation of atmospheric nitrogen during combustion, which can affect degradation.
Field studies show a correlation between nitration levels, an increase in viscosity and an increase in acid in the oil. NOx can also behave as a precursor and catalyst that promotes oxidation through the formation of free radicals in the lubricant. On the other hand, there can be direct nitration of the lubricant and its oxidation products to produce soluble nitrates and nitro compounds. These can eventually polymerize to form similar by-products of oxidation. This can lead to increased acidity (lowering the BN) and increased viscosity of the lubricant.
Fuel contamination in trunk piston engines happens quite often in diesel engines. If the fuel injectors are defective or the seals do not effectively seal to keep fuel out, fuel enters the oil. When fuel is in the oil, oil can become degraded quickly, often causing the viscosity to reduce to a value that compromises the ability of the oil to form a protective layer inside the component. The fuel dilution test can quantify the content of fuel in the oil. Depending on the type of engine, the tolerance levels will differ.
Deposition tendency on the cylinder liner wall is usually caused by unburnt fuel or excess oil in this area or the chamber. Typically, the piston rings scrape these deposits back into the oil, leading to an increase in the volume of insolubles. This also increases the viscosity of the oil, and it appears a darker color.
Reducing the SLOC also decreases the deposits on the liner wall because special rings (near the top of the liner) are installed to have controlled clearance of the piston crown. This reduces the crown land deposit which can also minimize bore polish and hot carbon wiping.
In addition, with a reduction in SLOC, the number of oil top ups is also reduced. As such, the replenishment rate of additives (in particular the BN) is not as frequent. Therefore, the degradation of the oil will advance at a slightly faster rate due to the lower SLOC which affects the rate of top up.
Metals in lubricant systems can also act as a catalyst for the degradation of the oil. During the oxidation process, copper is one of the most common catalysts in addition to other wear metals (such as iron) which can increase the rates of oxidation. As such, the presence of these metals increases the degradation rate as well.
Oil top-up intervals must be managed in such a way that it does not disturb the balance of the system. Typically, when the sump level falls below 90% to 95% (depending on the manufacturer), a top-up is needed. When fresh oil enters the system, it replenishes some additives and breathes new life into the oil. However, with this change in temperature of new oil coming into the system (especially in large quantities of about 15%), the deposits held in suspension tend to precipitate.
Additionally, foaming (caused by the increased concentration of some additives) can occur if too much fresh oil is added at once. As such, oil top-up intervals must be managed to avoid further degradation.
