Contributed by Bob Chabot
Diesel Fuel: Down, But Not Out
Lubricity additives bolster diesel fuel performance, engine hardware and drivability
Although lubricity additives have been used successfully in diesel fuel for over 20 years, recent changes at the refinery level and to vehicle hardware mean that understanding additive performance is even more essential than ever before. To help provide more understanding about the impact of lubricity improver chemistry on fuels, MOTOR visited with Sally Hopkins, Infineum Global Lubricity team leader, and Ian Kershaw, the managing director of Ricardo Strategic Consulting.
Diesel fuel additives are chemicals that do not naturally occur in distilled crude oil-based diesel fuels, but are blended with the fuel to improve its properties. The table above shows some common additives and their purpose. For a complete list of registered diesel fuel additives, visit the EPA website. (Image — U.S. Environmental Protection Agency)
First Up: An Additive Primer
Until the latter part of the twentieth century there was little or no use of diesel fuel additives. Due to the versatility and robustness of the diesel engine, suitable diesel fuel could be produced from a blend of straight-run atmospheric distillation components.
Diesel fuel demand continued to increase over this timeframe, but the advent of low sulfur blends and tightening specifications required additional processing. Depending on the crude oil source, different refining processes were employed and the use of diesel fuel additives began to ramp up.
Although there is no rigorous definition of what constitutes an additive, as opposed to a blending component, it is generally accepted by the industry that an additive is something added at less than 1% w/w (i.e. 10,000 mg/kg or 10,000 ppm). Because of this low treat rate of additives, the physical properties of the fuel, such as density, viscosity and volatility, did not change significantly.
The basic types of diesel fuel additives available are used to:
- Improve handling and distribution — For example, low temperature operability additives include flow improvers, wax anti-settling additives, dehazers, de-icers, antifoam additives, corrosion inhibitors, deodorants and others.
- Stabilize — Examples include antioxidants, biocides, metal deactivators, dispersants and other stabilizers.
- Protect engines — This category includes corrosion inhibitors for vehicle fuel systems, injector cleaners and lubricity additives.
- Enhance combustion — These additives include ignition improvers, smoke suppressants and combustion catalysts.
Kershaw cited several examples of why an additive might be used. To increase the yield of diesel fuel, refiners must cut deeper into the crude feedstock, which necessitates the use of flow improvers to restore the low temperature performance of the fuel. Increasing consumer demand for improved ignition quality and increasing cetane number specifications led to the use of ignition improver additives. In addition, as legislation specifying ultra-lower fuel sulfur levels spread, the ability of the diesel fuel to lubricate the fuel injection equipment diminished; to offset that issue, fuel suppliers began using lubricity additives.
Additives may be added to diesel fuel at three different stages: (1) at the refinery, (2) in the fuel distribution system and (3) after the fuel has left the control of the producer. Additives of the latter group, when added by the end user or a reseller, are called aftermarket additives. A word of caution is warranted for users of aftermarket additives. Some aftermarket additives are aggressively marketed, with performance claims that are often undeliverable; in most cases, they are not needed and should be avoided, especially in the case of modern high technology diesel engines.
Reputable fuel marketers of good quality diesel fuels ensure regional diesel blends contain all the additives needed and that they have been extensively tested to minimize the possibility of adverse interactions between different additives and/or fuel components. At the very least, check with the engine manufacturer, since inappropriate use of additives may have adverse effects on the engine and void engine warranties. For example, several engine manufacturers require that alcohol-based de-icers not be used.
Hopkins shared some of the trends shaping diesel fuel lubricity additive chemistry. (Image — Infineum International Ltd.)
Additives That Improve Lubricity Are Essential for Modern Diesel Engines
“Low sulfur diesel fuels need to be treated with lubricity improver (LI) additives to protect the high-pressure components of the diesel fuel system,” Hopkins emphasized. “Their extensive use in the field over the past 20 years might suggest that new technology development work in this area would be limited. However, over that period, significant changes have taken place in the refining and automotive markets. This means that continuing research to understand the additive appetite in the latest fuel types and combinations is still needed today.”
With so much change in the market, she noted Infineum has conducted a series of studies to help identify the best LI chemistries to ensure safe application from the refinery through the transportation infrastructure and into the vehicle. The fuels studied were representative of typical diesel grades found in the marketplace and the lubricity and cetane improver additives represented commercially available components.
"One of the key assessments for lubricity improver additives is the High Frequency Reciprocating Rig (HFRR) performance," she noted. “For example, Infineum has been examining the quality of diesel fuels in its Winter Diesel Fuel Quality Survey for over 30 years. Using the results obtained from 70 fuels collected from 2004 to 2015, our HFRR performance tests of ester and acid LI additives was compared over a range of treat rates. The ester LI provided a much deeper response, with wear scar diameters (WSD) below 350 µm achieved at realistic market treat rates. For mono-acid LI, the HFRR response was good but tended to plateau as the treat rate was increased. It was possible to relate the difference in performance at higher treat rate to the average film coverage, with the acid LI having much lower average film coverage, which resulted in higher wear scar diameters. Of note, the deeper HFRR response provided by ester LIs can be beneficial in real world applications.”
Hopkins then provided several examples of how this deeper HFRR response could be applied. At the refinery, it can provide additional security where there are diesel fuel pools that contain intermittently severe fuels. In addition, it may also help to ensure internal specifications are consistently met. In the vehicle, it can be useful in providing increased protection for high-pressure common rail systems, where injection pressures already exceed 2,000 bar and are expected to increase to 3,000 bar by 2025.
Automakers are expected to continue the long-term trend toward using downsized internal combustion engines and more advanced technologies. (Image — Ricardo Strategic Consulting)
Maintaining Consistent Ignition Properties in the Marketplace
In the real world, the production of automotive diesel fuel uses a range of refinery streams. So-called “cracked components”, when used in diesel blending, can result in fuels with low cetane values. The addition of cetane number improvers (CNI) is the most common way to restore the fuel’s ignition properties, according to Hopkins.
“To test the impact of CNI on lubricity, test fuels are treated with either ester or acid LIs and CNI was added. HFRR results indicated that, in the presence of CNI, acid LIs have a large negative effect on HFRR performance compared with ester LIs.
Acid lubricity improvers, in the presence of CNI, have a negative effect on HFRR performance. The findings were further substantiated by using film coverage analysis. This showed that the acid LI films were relatively unstable compared to the film coverage for the ester LI, which appeared to be more robust with less variability over the duration of the test.”
A growing real world challenge is the export and global movement of diesel fuels increasing. For example, different fuels — such as diesel and jet fuels — can be transported from refineries to users through multiproduct pipelines (MPPs), which have the potential for residual diesel additives to be transferred from one type of fuel to the other. To prevent contamination, the updated Energy Institute EI1535 standard applies, which sets out the minimum criteria to determine the acceptability of additives for use, as well as contamination and water mapping tests to verify fuel purity.
“In addition, additive selection is now playing an increasingly complex and active role in these multiproduct pipeline applications,” Hopkins added. “The use of certain LIs approved by Energy Institute, up to a specified maximum treatment rate, addresses some concerns. For example, the addition of ester LIs, which demonstrate the most robust HFRR performance, can ensure specification limits are met regardless of the source refinery and end market use. To avoid the risk of other undesirable additives cross-contaminating or degrading another fuel type, other LIs are added at end distribution terminals rather than passing through the MPPs.”
Bad Diesel Fuel Chemistry Puts Advanced Fuel Injectors at Risk
To date, there is no published data on whether there is the potential for metal contamination to occur prior to the vehicle fueling. The problem lies afterwards. Infineum, Ricardo, and others have extensively studied the potential for diesel fuel lubricity additives to solubilize metals. In some field tests using fuels treated with monoacid and ester LIs, there have been mixed reports of the build up of zinc and calcium. In addition, the use of fatty acid methyl ester (FAME) in the fuel may also impact metal pick up.
For example, Infineum recently completed a study where it monitored diesel fuels for several years in a market where the fuels contained a maximum of 100 ppm sulfur and no FAME for the entire period. Samples were collected from 26 service stations at the same time each year. For each fuel, it was possible to ascertain whether it was treated with an ester or an acid lubricity additive. On analysis, zinc pick up was predominantly found in fuels that had been treated with acid LIs, while for fuels containing ester LIs average zinc values were close to zero. Hopkins shared the study’s conclusion: “When fuels are contaminated with zinc at levels as low as 1 ppm — predominantly found in fuels that had been treated with acid lubricity improvers — large levels of spray-hole and internal injector deposits can be generated. In addition, other metals such as calcium and sodium can be associated with the formation of deposits.”
“This underscores the value of LI chemistry analysis. With ester LIs, there is essentially no zinc pick up and less potential for injector deposit formation than for acid LIs. In Infineum’s view, carefully selected ester LIs are the most robust choice to consistently meet diesel fuel lubricity specifications in an increasingly complex market.”
Good News and Bad News
“There are three well-established drivers that will continue to shape automotive technology,” shared Kershaw, whose firm specializes in addressing high-impact issues and solving operational problems for the automotive and transportation sectors. “These are air quality, climate change and urbanization. With OEMs focused on reducing tailpipe emissions and improving fuel economy, there has been a very strong trend in recent years to both downsize internal combustion engines and to use more advanced boosting technologies to improve their performance.”
For those who service diesel engines, there is some good news worth noting. There are also some changes that will have to be adapted to. “Things are about to change beginning this year, in response to recent amendments to European emissions legislation — specifically the introduction of a new drive cycle and the testing of real-world driving emissions (RDE). Look for similar regulations in other jurisdictions to have a similar effect. Internal combustion engines of every flavor have to become cleaner from a noxious emissions point of view, while also enabling the vehicle to become even more economical or to have lower CO2 emissions.
With Europe historically being the lead market for diesel engines in light–duty applications, Kershaw was asked what these trends mean in terms of the balance of powertrain technologies. He suggested the historically strong and attractive performance of diesel engines will continue to make them a popular choice for the region’s consumers, but expect to see a shift away from diesel in smaller sized light duty segment A and B vehicles I the short term, and over the much longer term, a shift from diesel fuel to other alternatives, primarily gasoline.
“What we are seeing with the latest emissions regulations is that the cost of diesel engines is becoming increasingly prohibitive in the smaller car segments. This means that diesel engines are being phased out and replaced by downsized gasoline engines, or even micro gasoline hybrids. However, in the larger vehicle and sport utility vehicle segments, expect diesel to still have a central role for many years to come.”
In the light duty vehicle segment, expect a slow, steady shift away from diesel over the long term. (Image — Ricardo Strategic Consulting)
Implications for Fuels and Lubricants
“On the fuels side, for light-duty vehicles, we see a slow, but steady shift away from diesel to gasoline in the coming years," Kershaw projected. "In addition, the gasoline fuels in use may well have a richer biofuel blend. That said, because diesel engines remain the core powertrain for heavy-duty commercial vehicles, there will also be continued strong demand for diesel fuel. Expect markets outside of Europe to follow a similar course.”
In terms of the lubricants market, Kershaw forecasts a positive outlook. “In Europe, we see the number of internal combustion engines being maintained, or even growing, as the population of vehicles on the road continues to increase out as far as 2040. In addition, as OEMs demand better fuel economy and engine protection in more severe operating conditions, they are tending to maintain lubricant volumes, despite the smaller engine displacements. Together, these factors mean that lubricant volumes may be stable for some time to come.”
At the same time, he believes OEMs will be looking for more from the lubricant. “Expect the technology content of engine oil to rise as the automotive industry turns to the lubricants industry to help them meet the performance challenges they face. Look for the requirements for lubricants to deliver wear protection, friction reduction, contamination and high temperature tolerance to all continue to rise over time.”
“Finally, emissions compliance is moving from being primarily an engineering challenge to being a whole vehicle challenge, Kershaw concluded. “This will increasingly require stakeholders to work together to develop cleaner, sustainable and affordable transport that meets future market needs. It’s no longer a debate. Diesel engines will have to be cleaner, more fuel efficient and application-specific based on purpose rather than just price alone.”
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