Heavy-duty Diesel Engine Oil Developments and Trends
The main driving force since 1990 for the development of the entire API commercial “C” diesel engine oil classifications (for example, CF-4, CG-4, etc.) is the concern over the environmental impact of diesel engine emissions. The number 4 indicates that these apply to 4-stroke diesel engines. In 1997, the Environmental Protection Agency (EPA) adopted stringent emissions standards for both NOX and particulate emissions with the aim of reducing emissions to 0.2 gram per brake horsepower hour (g/bhp-hr) for NOX and 0.01 g/bhp-hr for particulate emissions by 2010 for on-road diesel engines. To further control emissions, the EPA also set lower limits on diesel fuel sulfur levels that are used for on-highway (15 ppm sulfur in 2006) and off-highway (500 ppm sulfur in 2007, 15 ppm Sulfur in 2010) applications. Consumer demand for longer lasting oils and the concern over increased engine and oil sump temperatures due to current and future engine designs to meet these emissions standards have further driven the development of new engine oil service categories
Engine Oil Developments and Trends
Beginning January 1, 2007 on-highway diesel engines faced tougher emission standards for NOX (1.2 g/bhp-hr) and particulate matter (PM) (0.01 g/bhp-hr). Over the course of the next three years, NOX emissions will trend down toward the 2010 standard, mentioned above. This phase-in provision allows the engine manufacturers to concentrate on reducing NOX. On-highway fuel sulfur levels have dropped to 15 ppm, beginning in 2006 because even relatively small amounts of sulfur add particulate exhaust emissions.
To achieve these emissions limits, OEMs are using a combination of cooled exhaust gas recirculation (EGR) at higher rates and exhaust aftertreatment devices such as catalytic diesel particulate filters and oxidation catalysts. This has resulted in a new generation of engine oils that provide emission control system durability, prevent catalyst poisoning and particulate filter blocking, while still offering optimum protection for control of piston deposits, oxidative thickening, oil consumption, high-temperature stability, soot handling properties, foaming and viscosity loss due to shearing.
To provide these aspects, the American Petroleum Institute (API) has joined efforts with OEMs and the American Society for Testing and Materials (ASTM) to develop a new diesel engine oil classification for 2007, designated as API CJ-4.
Emission Design Strategies for 2007
Cummins, Detroit Diesel, International-Navistar, Mack and Volvo North America have employed the use of heavy EGR (30 to 35 percent) closed crankcase ventilation and diesel particulate traps to remove soot and other particulates.
Caterpillar uses its advanced combustion emission reduction technology (ACERT), an advanced combustion process called clean gas induction (CGI), closed crankcase ventilation and diesel particulate filters. CGI employs the use of remote EGR, a closed crankcase ventilation system and diesel particulate filter system with active regeneration. CGI draws clean inert soot-free exhaust gas from downstream of the particulate filter and then puts the clean gas into the intake system. This clean gas does not induce engine wear and the low intake manifold gas temperature of the CGI contributes to lower NOX emissions. The particulate filter uses a walled-flow filter technology. Regeneration is necessary to activate the process of oxidation that eliminates the soot that collects along the inlet walls of the filter. To aid regeneration, the exhaust gas is heated by auxiliary means. The regeneration process takes place only when needed. Caterpillar engines that are 500 horsepower or less will require one diesel particulate filter. Engines of 550 horsepower or more will require dual particulate filters.1
All the OEMs chose closed crankcase ventilation in order to remove harmful vapors generated in the crankcase. These vapors are discharged into the engine’s intake system, (usually via the intake manifold), where they are burned as part of the combustion process rather than being discharged into the atmosphere.
Diesel Particulate Filters
Diesel particulate filters (DPF), sometimes referred to as traps, which resemble large mufflers are aftertreatment devices used to remove 90 percent of the particulate matter from diesel exhaust. They are porous filters generally made of a high-temperature ceramic structure or densely packed ceramic and metal fibers (Figures 1 and 2).
Diesel particulate filters physically capture particulates in the diesel exhaust and prevent their discharge from the exhaust pipe, while allowing exhaust gases to escape. To keep the particulate filters from clogging, collected particulates such as soot must be removed from the filter by burning them off at elevated temperatures. This process is known as regeneration. Regeneration, which can be done either periodically or continuously, involves either the use of electrical heaters, passive heat from the exhaust or injection of a small amount of diesel fuel into the exhaust stream into the filters to burn off the collected particulate matter completely. Any remaining residue and ash is blown against the exhaust flow and into a container trap for disposal. These traps must be cleaned out periodically to keep the diesel particulate filters from clogging. Onboard diagnostics monitor the particulate trap’s condition and manage regeneration. If regeneration is needed and the vehicle is idle, onboard fuel reformers that convert diesel fuel to a more hydrogen-rich, hotter burning fuel will be activated by onboard diagnostics that signal the operator to initiate regeneration. An alternative to onboard regeneration is the cleaning of the diesel particulate filters at a maintenance facility. The EPA requires that diesel particulate filters operate for at least 150,000 miles before they need cleaning. Engine emissions must comply for 435,000 miles.
Impact of Emissions Strategies
The 2007 engines will cost more. Diesel particulate filters and related hardware are projected to increase the price of a new truck by $6,000 to $10,000. Diesel particulate filter service, which may be required every 150,000 miles, is expected to cost $150 or less each time. Ultra sulfur diesel fuel (15 ppm S) will cost more, have a lower heat (BTU) content resulting in increased fuel consumption, and could cause premature injector failure due to its lack of lubricity.
The 2007 engines will generate more soot and experience higher peak cylinder temperatures due to the higher levels of EGR. This will cause the engine to run hotter and require an engine oil with improved oxidation resistance. To protect the aftertreatment devices, the engine oil will have to contain lower sulfated ash, sulfur and phosphorus contents, while still offering optimum protection for control of piston deposits, oxidative thickening, oil consumption, high-temperature stability, soot handling properties, foaming and viscosity loss due to shearing.
API CJ-4 represents the latest in a series of engine oil upgrades for heavy-duty diesel engine oils. Development of API CJ-4 was completed in 2006 and finalized as API CJ-4 for licensing on October 15, 2006.
To ensure protection of the aftertreatment devices, chemical limits were set for the first time ever for heavy-duty diesel engine oils. The chemical limits for API CJ-4 target the engine oil’s sulfated ash, phosphorus and sulfur content, commonly referred to as SAPS. These chemical limits include the following:
In addition to these chemical limits, a volatility limit of 13 percent maximum as determined by the NOACK Volatility Test Method ASTM D5800 has been set for API CJ-4.
SAPS are found in or derived from components (additives and base oils) in engine oil formulations. These various components are used to help provide extended oil drain intervals, base number (BN) retention and to protect against wear, oxidation, corrosion and piston deposits. Although SAPS contribute significant performance benefits, they can cause problems in the 2007-compliant and future engine designs needed to meet the 2010 emission requirements if they are too high.
The most concern for proper functioning of the 2007-compliant emission engine lies in the impact sulfated ash has on aftertreatment devices such as diesel particulate filters.
The term sulfated ash relates to the amount of metallic elements in engine oils, which are mostly derived from the engine oil’s detergent and antiwear additive chemistry. These additive packages contain multiple components based on metals such as calcium, magnesium, zinc, etc. Because a 100 percent seal between the piston rings can never be achieved, a certain amount of engine oil will enter the combustion burn.
As the engine oil enters the combustion chamber and burns, its residue forms an ash-like material. This ash-like material contributes to deposits in the crown land above the piston ring as well as to deposits in the ring grooves. These deposits can lead to rubbing wear on the cylinder liner and cause the piston rings to not operate freely. Ultimately, as the cylinder liner-to-ring interface is compromised high oil consumption can occur.4 In addition to these deposits, inorganic compounds from the lubricating oil’s additives can become oxidized during combustion and generate metal oxide particles. These particles can be carried downstream with the exhaust and collect on the diesel particulate filter. These ash particles cannot be removed by filter regeneration because they are not combustible. As the ash particles accumulate, they result in filter blockage that increases back pressure to the engine, increasing fuel consumption and decreasing power. Ash particle buildup also necessitates more frequent cleaning of the particulate filters by mechanical means such as compressed air or water-pulse methods.
An engine oil’s sulfated ash content also directly relates to an engine oil’s acid neutralization capabilities (BN), because most of an engine oil’s BN comes from the metal-containing detergent additives. Generally, the higher an engine oil’s BN, the higher its ash content and the greater its ability to prevent acidic corrosion in the engine. Fortunately, with the mandated use of ultra low sulfur diesel fuel in on-highway applications, corrosion from fuel sulfur will require less of a need for BN control and thus a lower ash content.
Figure 3. Impact of SAPS on Particulate Filters
Sulfur’s Source and Its Impact
Heavy duty-diesel engine oils are comprised of approximately 75 to 85 percent base oil with the remainder made up of additive systems. The sulfur concentration in the base oil can range from zero (synthetic base fluids such as PAOs) to as high as 0.5 percent by weight (Group I base stocks). Sulfur content in a base oil can be reduced by the use of refinery hydrotreating and hydrocracking methods to levels ranging from less than 0.1 to less than 0.3 percent by weight. The additive systems used are also major sources of sulfur. The sulfur-containing additives used in the formulation of heavy-duty diesel engine oils include the detergents, antiwear agents (primarily from zinc dithiophosphate, ZDTP or ZDDP), corrosion inhibitors, friction modifiers and antioxidants.
It has been estimated by the EPA that any amount from 1 ppm to 7 ppm of sulfur can be contributed to the diesel engine’s exhaust, when the engine oil enters the combustion chamber and burns. The worst-case estimate of 7 ppm is based upon nominal heavy-duty diesel vehicle fuel and oil consumption rates of 6 miles per gallon and 1 quart per 2,000 miles respectively.
During normal operation, only a small percentage of the engine oil consumed by open crankcase ventilation heavy-duty diesel engines travels past the rings and burns in the combustion chamber. The remainder of the consumed oil is lost through evaporation by being emitted through the crankcase ventilation tube and is not combusted. If an engine oil that contains a
sulfur level of greater than the 0.4 percent maximum limit for API CJ-4 were used in a 2007-compliant engine, the closed crankcase ventilation system would recover the evaporated oil and carry it through the exhaust stream.5,6,7
Once in the exhaust stream, sulfur can inhibit the effectiveness of the particulate filters by poisoning the catalysts. This poisoning of the catalyst can increase the conversion of sulfur oxides to sulfates, which increases particulate emissions and accumulation of particulate material. Accumulation of particulate material can lead to reduced engine performance, due to increased backpressure and ultimately failure of the trap. To clarify sulfur, once it poisons the catalysts, it desensitizes them and blocks active sites on the catalysts. This causes the sulfur oxides to be converted to sulfate particulates which increases particulate emissions and also leads to the buildup of particulate matter in the aftertreatment devices.
Figure 4. North American Market Scenarios through 2009
Phosphorus Source and Impact
The primary source of phosphorus in heavy-duty diesel engine oils comes from the antiwear agent zinc dithiophosphate (ZDDP). Phosphorus also comes from corrosion inhibitors, friction modifiers, corrosion inhibitors and antioxidants. Typically, pre-2007, CI-4 Plus type heavy-duty diesel engine oils contained 0.11 percent to 0.15 percent by weight of phosphorus. New CJ-4 oils have a 0.12 percent phosphorus maximum.
Once in the exhaust stream, phosphorus can reduce the efficiency and deactivate the noble metal catalysts by coating and building up on the active catalyst sites, causing irreversible damage that accumulates over time. As a result, increased levels of harmful emissions such as NOX, carbon monoxide and hydrocarbons pass through the catalytic converter unchanged, resulting in an increased level of NOX, CO and hydrocarbon emissions.
Paradigm Shift in Engine Oil Technology
One goal of API CJ-4 was backward compatibility with the oil formulations of the older API CI-4 and CI-4 Plus classifications. However, the restriction in SAPS has resulted in a paradigm shift in engine oil technology. The reduction in ash levels from the norm of 1.3 percent to 1.5 percent to the mandatory maximum of 1.0 percent and the additional reduction in sulfur levels of the base oil and additives to 0.4 percent maximum will require replacing conventional metal-containing additive chemistries with alternative additive chemistries that are low in metallic content, sulfur and in some cases ashless. The use of these alternative additive chemistries has reduced the engine oil’s BN to a level ranging from 8 to 10. This lowering BN can reduce oil drain intervals in off-highway diesel engines that will still be allowed to use low sulfur diesel fuel (500 ppm maximum) till 2010. For on-highway diesel engines, this reduction in BN is not anticipated to affect current oil drain intervals because the use of ultra low sulfur diesel fuel (15 ppm maximum) will be the balancing factor in the oil drain interval equation. These factors could result in a differentiating of two different engine oils: one for off-highway diesel engines and the other for on-highway diesel engines (Figure 4) for the next few years. Further, it is anticipated that some OEMs will require BN minimums depending upon the application their engines are used in.
CJ-4 can be used with the 500 ppm sulfur fuel to be used in off-road diesel from 2007 to 2010, but there may be a reduction in oil drain intervals from previous oils.
To meet the API CJ-4 limit of 0.12 percent phosphorus, the amount of ZDTP used in the formulation of heavy-duty diesel engine oils has been reduced. This reduction in ZDTP will require the use of alternative ashless antiwear agents to protect the valve train from wear.
The reduction in sulfur levels to 0.4 percent maximum along with the NOACK volatility limits of 13 percent maximum and the need for increased oxidation stability due to the increased thermal stress placed on the engine oil from the use of heavy EGR rates and aftertreatment have resulted in an increased use of Group II, Group III and Group IV basestocks.
Figure 5. Future Engine Technology 2010
ASTM has mandated backward compatibility with existing API CI-4 and CI-4 Plus engine oils for the API CJ-4 classification. To ensure backward compatibility, a combination of existing CI-4 and CI-4 Plus laboratory and engine bench tests are used in conjunction with new engine sequence tests that utilize ultra low sulfur diesel fuel. The new engine sequence tests for API CJ-4 include the following:
Engine Technology for 2010
In 2010, on-highway diesel emissions will be further reduced to 0.2 g/bhp-hr for NOX, and particulate emissions will remain at 0.01 g/bhp-hr. On-highway diesel emission levels, in conjunction with the mandated use of ultra low sulfur diesel fuel for off-highway diesel engines beginning June 2010, will result in further engine design changes and the use of additional aftertreatment technologies as well as the use of the current diesel particulate filter technology. The additional aftertreatment devices that will be used are:
The use of these aftertreatment devices will result in further chemical limits being placed on future heavy-duty diesel engine oils to ensure catalyst compatibility. This will result in the development of a new engine oil service classification for heavy-duty diesel engine oils that will require a careful balancing act in providing engine durability to existing engines while still providing aftertreatment compatibility and life. This new engine oil service classification must be in place and ready for use by the end of 2009. It is anticipated that API, in conjunction with the OEMs and ASTM, will begin work on PC-11 (possibly to be referred to as API CK-4) sometime in late 2007 or early 2008.
Heavy-duty Diesel Engine Oil Developments and Trends