In Part 2 of her series entitled Diesel emissions – A breath of fresh air, Wearcheck’s technical manager, Steven Lumley, talks about lubricant viscosity and the different roles of additives. MechChem Africa presents the key takeaways.
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Driving the market for cleaner diesel engines are three interacting developments: better emission system compatibility; improved fuel economy; and greater engine durability. Fuel and lubricant manufacturers play a key role in achieving these drivers.
Improved fuel economy
Lubricant manufacturers make use of multiple complex physical and chemical strategies to improve fuel economy, the most common being to reduce the viscosity of the oil, which often necessitates the selection of higher quality base oil, combined with the use of additives such as viscosity index improvers and friction modifiers.
The fuel consumption of an engine is affected by, among other factors, the friction that must be overcome in the engine, for which the engine lubricant plays an important role. In principle, every engine runs more smoothly and economically with a less viscous (thinner) oil. Yet the thinner the oil, the weaker the oil film needed to prevent mechanical contact between the moving metal surfaces. Film stability can be off-set, however, by using friction modifiers and viscosity index improver additives.
Lower oil viscosity grades such as 5W30 and 10W30 are now commonplace in heavy-duty diesel engines. The SAE grading system defines both low and high temperature viscosity requirements – typically kinematic viscosity – which is a measure of an oil’s resistance to flow under the force of gravity at specific temperatures.
There is another important type of viscosity, however: high temperature high shear (HTHS) viscosity, which is a fluid’s resistance to flow under conditions resembling highly-loaded journal bearings in firing internal combustion engines.
In an operating engine the lubricant is required to protect engine components under hotter and more severe operating conditions, and HTHS dynamic viscosity best predicts the oil’s behaviour in these operating conditions. Engine lubricants can have the same SAE viscosity grade but different HTHS viscosity, however, so understanding the relationship between these readings is becoming more important as more engine manufacturers move towards lower HTHS engine oils.
The American Petroleum Institute (API) base oil classification system groups base oils according to their purity and viscosity Index. The system uses physical and chemical parameters to divide all base stocks into five groups – Groups I, II, III, IV and V. Group I, II, and III are mineral oil derived from crude oil, Group IV is a fully synthetic oil, and Group V is for all base oils that are not included in one of the other groups.
Group I base stocks are high in aromatics, sulphur and nitrogen, all of which have a negative impact on lubricant performance, which makes them unacceptable for most modern diesel engine oil formulations. These issues have led many formulators to focus on Group II/II+ and Group III base oils due their lower volatility, aromatic and sulphur contents, better oxidation stability and higher viscosity index.
Friction modifiers (FMs)
Friction modifiers are typically used in engine oils to lower metal-to-metal friction between interacting component surfaces, to reduce wear and improve fuel economy. However, their effectiveness is dependent on the lubrication regime within the engine, which is also affected by engine design.
The typical regimes encountered are hydrodynamic or full-film lubrication, where two metal surfaces are completely separated by an unbroken lubricant film; boundary lubrication, where occasional metal-to-metal contact takes place between surfaces and mixed lubrication, which is a combination of the two. For engine components lubricated hydrodynamically, the friction is governed by the viscosity of the oil: thinner oil results in less the friction. For engine components experiencing boundary or mixed lubrication, FMs are used to more effectively reduce friction. For example, engines with roller follower valve train systems have relatively little boundary lubrication and friction modifiers may not deliver significant fuel economy benefits, while engines without roller followers may show significantly more benefit.
Viscosity Index Improvers (VIIs)
Viscosity Index Improvers (VIIs) are large polymer additives that partially prevent oil from thinning (losing viscosity) as operating temperatures increase, thus improved fuel economy. They are also responsible for better oil flow at low temperatures, resulting in reduced wear.
Properly-designed VIIs support higher lubricant viscosities in the hotter engine operating environments for robust wear protection, while maintaining lower viscosities in moderate engine temperature environments, which provides fuel economy benefits. They allow lubricant formulators to meet the minimum HTHS viscosity while lowering kinematic viscosity at the same time.
Emission system compatibility
The most important aspect of modern-day diesel engine oil formulation is its compatibility with exhaust aftertreatment technologies. The chemical composition of a diesel engine oil contains, among other things, sulphated ash, phosphorous and sulphur. These substances provide attributes such as detergency; neutralisation of acids, anti-wear properties and anti-oxidants.
Unfortunately, these chemicals are problematic for current emission technology and as a result, lubricant manufacturers are facing lubricant formulation restrictions aimed at protecting emission control systems.
This has given rise to a new class of low sulphated ash, phosphorus and sulphur (low-SAPS) engine oils. These oils are also designated ‘low-ash’ due to their reduces tendency to form ash.
While some additives have organic alternatives containing little or no sulphur and phosphorous and which do not contribute to sulphated ash, some important anti-wear and detergent additives do not have organic alternatives.
Until effective replacements are found for these, a careful balancing and reduction in the concentrations
of SAPS-contributing additives is required to ensure that the engine oil meets all the performance requirements that engines and emission systems demand.
There are three major mechanisms for possible interference between the lubricants’ components and aftertreatment devices: poisoning, deactivation and accumulation of ash deposits.
Sulphated ash: The term sulphated ash relates to the amount of incombustible metallic ash that remains as a result of engine oil combustion. This ash is mostly derived from the engine oil’s calcium and magnesium-based detergent and zinc-based anti-wear additives.
Ash from the small amount of oil burnt as part of normal engine operation is trapped in the diesel particulate filter (DPF). During regeneration to remove particulate matter (PM) from the filter, the already burnt ash portion cannot be oxidised and remains in the filter, causing the DPF to become irreversibly blocked. Low SAPS oils are formulated to limit the maximum sulphated ash allowed in the oil, primarily to protect against
DPF blockage.
Phosphorus: Anti-wear, anti-oxidation additives known as ZDDPs have been a mainstay of diesel engine oil formulation for more than 60 years. Unfortunately, ZDDPs contain two of the limited substances in low SAPS oils – ash and phosphorus.
Diesel oxidation catalysts (DOCs) are degraded by phosphorous, which deactivates the noble metal catalysts by building up a coating on the active catalyst sites, causing irreversible damage over time. This enables harmful emissions such as NOx, CO and HCs to pass through the catalysts unchanged.
Catalyst poisoning by phosphorous can also significantly decrease filtration efficiency of both catalysed (C-DPF) and uncatalysed DPF substrates, which also results in reduced soot regeneration efficiency.
Sulphur: Sulphur emissions in an diesel engine originate from two sources: from the fuel and from the lubricant. Lubricant-derived sulphur emissions are under increased scrutiny because of the lubricant’s contribution to total SO2 emissions, which has a tendency to significantly hinder NOx adsorber catalyst (NAC) performance.
Heavy-duty diesel engine oils are composed of approximately 75 to 85% base oil. The sulphur concentration in these base oil can range from zero, for synthetic base fluids such as polyalphaolefins, to as high as 0.5% by weight in Group I base stocks.
Additive systems such as anti-wear agents (ZDDPs), corrosion inhibitors, detergents and friction modifiers are also a major source of sulphur in lubricating oils. Once in the exhaust stream, sulphur can inhibit the effectiveness of the DOC, C-DPF and SCR systems, while also increasing particulate emissions, which leading to blockages of the NAC and reduced engine performance.
Diesel fuel also contains sulphur derived from the original crude oil source, which can still be present after the refining process. About 98% of this sulphur in diesel fuel oxidises in the combustion process to sulphur dioxide (SO2) that contributes to the formation of smog and acid rain.
Euro V to VI-rated diesel engines have advanced aftertreatment systems for particulates and NOx, but these systems are sensitive to the sulphur content in diesel fuel. For this reason, most engine manufacturers have progressively limited fuel sulphur content to 10 ppm, which is known as ultralow-sulphur diesel.
DPF regeneration is also affected by higher sulphur because it decreases NOx formation in DOCs. This leads to performance loss in passive DPF systems that depend on upstream NOx from the DOC to oxidise the soot. Higher back pressure and more frequent active regeneration result in higher fuel consumption.
South Africa’s fuel improvement initiative, in support of global greenhouse gas reduction agreements, were planned to meet Euro V standards by 2017 through the Clean Fuels 2 (CF2) programme, but the programme stalled due to uncertainty around the cost recovery mechanism for refinery upgrades, which in 2009 was estimated at US$3.9-billion.
Sasol introduced 10 ppm diesel to the market in late 2013 as part of the initial roll-out strategy, but to date 10 ppm is still not widely available. This limits the availability of new engine technology as low sulphur fuels are key to enabling advanced control technologies and fuel-efficient designs.
Durability and extended drain intervals
There are also additional trends affecting future diesel engine oil formulations such as increased oil drain intervals, smaller sump levels, higher running temperatures and shear forces, all of which put increased stress on the lubricants. Additives tasked with achieving all of this include:
Detergents: Detergents are cleaning agents that contain metals. They work at high temperatures in pistons, rings, liners and valves to reduce or remove deposits on surfaces and in the bulk of the oil. They also neutralise acidic compounds formed during the combustion of diesel or due to base oil oxidation. The Total Base Number (TBN) of the oil is an expression of this neutralisation ability.
The majority of metallic detergents are based on either calcium or magnesium attached to an oil-soluble organic soap, typically sulphonates, phenates or salicylates. However, because magnesium-based detergents provide a higher TBN per unit of sulphated ash produced, they are now favoured in formulations.
A reduction in TBN is expected with many low-SAPS oils. While the TBN of new oil is important, the ability of oil to retain TBN over extended drain intervals is arguably more critical than the absolute value in the new oil.
Dispersants: Dispersants are non-metallic, ashless cleaning agents that inhibit sludge-formation by keeping insoluble contaminants such as soot dispersed in the lubricant and preventing them from coating metal surfaces. The soot particles themselves are sub-micron in size when formed, but with progressive fuel usage these particles will eventually agglomerate.
EGR system recirculate a small amount of cooled exhaust gas, which in turn reduces the NOx gases. However, recirculating exhaust gas also creates a multi-pass opportunity for soot to accumulate in the engine oil, causing sludge to form on rocker and front engine covers, bearings to fail, valve bridges and fuel injection links to wear, and filters to plug – and this is further exacerbated by extending oil drain intervals.
In response to this issue, lubricant blenders have had to increase the treat rate of this additive and dispersants are typically one of the major components % of the additive package. However, the thickness of the polymeric-based dispersant becomes problematic, resulting in the use of lighter base stocks, resulting in higher volatility lubricants.
Anti-oxidants: Oxidation is a form of irreversible chemical deterioration of the lubricant. It is caused by the base oil combining with oxygen, sulphur and nitrogen to form harmful compounds. Oxidation creates oil-insoluble, high-molecular-weight molecules that increase the viscosity of the lubricant, accelerate wear and eventually lead to varnish-formation, typically on pistons and valves in engines. The use of EGR systems can increase the rate at which the oil oxidises as many of these systems rely on the engine’s coolant system to reduce exhaust gas temperatures, which increases the engine running temperature.
Anti-oxidants are a group of additives that minimise oxidation and deposit-formation by decomposing reactive hydroperoxides and free radicals before they can lead to oxidation of the base oil. There are two types of antioxidants: primary and secondary antioxidants. Primary antioxidants are free radical scavengers typically comprised of aromatic amines and hindered phenolics. Secondary antioxidants are peroxide decomposers typically composed of phosphites and certain sulphur-containing compounds.
Ashless-type oxidation inhibitors have helped to replace the oxidation performance of ZDDPs, with recent additive systems making use of aminic and phenolic chemistries. The use of molybdenum-based chemistry for improved antioxidancy performance has also gained popularity in recent years.
The API has introduced two new standards to take into account the latest technology in diesel engines. API CK-4 and FA-4 first appeared in the API service symbol donut in 2017. These new service categories improve upon existing standards by providing enhanced protection against oil oxidation, engine wear, piston deposits, shear stability as well as providing better compatibility with emission-controlling devices.
API CK-4 was introduced to reflect the upgraded performance benefits beyond API CJ-4 for engine lubricants with a minimum HTHS viscosity of 3.5cP. New API CK-4 lubricants must pass more stringent oxidation and aeration limits with increased shear stability, providing greater protection for heavy-duty diesel engines. CK-4 is backward-compatible with older API categories such CJ-4.
Conclusion
It is widely acknowledged that there is more to be done in the drive to further reduce harmful gases, improve air quality and mitigate the effects of global warming. As former UN secretary General Ban Ki-moon famously said: “There is no plan B because there is no planet B”.
How far we have come, though. 30 years ago, one heavy on-highway truck produced the same level of particulate matter as 100 heavy goods vehicles produced in 2019. Now isn’t that a breath of fresh air!
