Tightening of the NOx legislation led to a widespread retardation of injection timing, which has led to increased levels of soot loading of the engine lubricant, which has been associated with increased wear levels. Besides, the drive for reduced a little emissions has led to a large reduction in oil consumption levels. Therefore the aged oil is likely to be replenished (by ‘top-up’) less often and by smaller amounts. Therefore contaminant levels have tended to increase. Increased soot levels in lubricants can lead to soot-induced oil thickening, engine sludge (in the rocker cover and sump), oil filter blocking, and reduced cold-start pumpability.
During the 1990s many engine tests designed to test the performance of soot-loaded lubricants were implemented. The basic tests introduced were as follows.
1. The Mack T8 test [33] was introduced into the API CG-4 oil performance category as a test designed to assess oil’s ability to ‘handle’ soot. During this engine test (250 h for API CG-4 and 300 h for API CH-4), which uses a 12 l Mack E7-350 horsepower diesel engine, soot is deliberately accumulated in the lubricant. The engine is overfuelled, with retarded timing of 9.5 8, and an engine speed of 1800 r/min. For CG-4, when the soot level reaches 3.8 percent (by mass), the viscosity of the lubricant is measured, and for a passing result the oil viscosity is required to increase by less than 11.5 cSt (as measured at 100 ºC). For API CH-4 the same viscosity increase limit is used, together with a relative viscosity increase limit measured at 4.8 per cent soot by mass. The Mack T-8 test led to oils with increased dispersancy and it was also found that a careful choice of base oil was needed in order to ensure passing performance. The Mack T8 test became the Mack T-8E test in CH-4 (the same test but over a longer time (300 h) and with the viscosity increase evaluated at 4.8 per cent soot level). In API CI-4, the Mack T-8E test is also used but there is a lower allowed relative viscosity increase at 4.8 percent soot (compared with API CH-4) for passing performance. A Mack T11 test [which uses a low
swirl head, and low rates of exhaust gas recirculation (EGR) for 300 h] is used in MACK EO-N and the new API CI-4 Plus specification that is currently under discussion. The passing criterion of the Mack T-11 test is a maximum viscosity increase of 12 cSt at 6 per cent soot level.
The minimum viscosity is defined after 90 cycles in the Kurt Orhahn test. (Most oils that were qualified in the Mack T-8E non-EGR with limits of 4.8 percent soot would fail the Mack T11 test). API CI-4 oils require even high levels of dispersancy performance than CH-4 or CG-4 oils.
2. The GM 6.5 l roller follower wear test forms part of the US API CG-4, CH-4, and CI-4 diesel oil performance specifications [32, 34, 35]. A steel tire rests directly on individual inlet and exhaust cams and rolls over the cam surface throughout the engine’s operating cycle. The engine is run at 1000 r/min and high load for 50 h and generates 4 percent of soot. The depth of the wear scar on the axle where it contacts the needle bearings is assessed. Wear has to be controlled to less than 11.4 mm for the oil to satisfy the API CG-4 performance level. The oil film thickness between the needle bearings and the axle, under GM6.5 test conditions, was just 13 nm. It was much smaller than the surface roughness of the two surfaces, and about five times smaller than a typical size soot particle. Therefore it is likely that soot particles in the lubricant which enter the needle bearing–axle contact will make heavy simultaneous contact with the moving surfaces and maybe promote high levels of wear.
3. The Cummins M11 cross-head wear test forms part of the API CG-4 and CH-4 oil specification tests [32, 34]. The cross-head is a component of the Cummins M11 valve train system (each of the inlet and exhaust rocker arms actuate two valves with the load transferred by a simple bridge piece, known as a cross-head). In the M11 engine test the cross-head is weighed before and after each engine test and the weight loss used to assess a sooted oil’s wear performance.
Figure 8 gives a visual representation of how a lubricant’s performance level (with respect to soot loading) can impact on cleanliness of the rocker cover. Soot loading levels have increased even more significantly in recent years as legislated NOx emission have decreased even further, and OEMs have introduced technology such as EGR. The latest API CI-4 specification [35] was specifically designed to assess lubricant performance in engines equipped with EGR.
Work is currently ongoing for proposed category PC-10 [36]. This new category is planned to be in place by 2007 (which is when diesel particulate traps will be required on all
Fig. 8 The impact of oil performance level on rocker cover sludge in a Cummins heavy-duty diesel engine
4. AFTER-TREATMENT DEVICE COMPATIBILITY
Over the last decade, legislation on the allowed emissions from vehicle tailpipes has tightened significantly. For passenger car vehicles, the emissions of concern are carbon monoxide (CO), hydrocarbons (HCs) and NOx. Legislation is now so tight that all passenger car gasoline and diesel engines required to meet US, European, and Japanese targets will use after-treatment systems of some sort [37, 38]. In addition to emissions targets do not take any account of vehicle size in the passenger car sector. Since emission levels increase with increasing vehicle weight, these fixed targets are more demanding for larger vehicles. Therefore ‘cutting edge’ after-treatment systems will be demanded by these vehicles first.
For heavy-duty diesel engines, the main emissions targeted by legislation are particulates and NOx HC and CO emissions are also limited but at present these are not challenging targets). After-treatment systems are not widely used in the heavy-duty sector at the present time, but emissions legislation scheduled for introduction in 2005 will force their use.
There are many different after-treatment device options.
For passenger car vehicles equipped with conventional gasoline engines (i.e. stoichiometrically fired engines), three way catalysts can control HC, CO and NOx emissions very well at present. However, these catalysts take time to warm up, and this warming-up period is usually indicated by a light. Once the light is off, emissions are controlled to meet legislated limits. It is the emissions that occur during the pre-light-off period that dominate the total emissions level. Therefore the current emphasis is on promoting quick light-off and then ensuring that the catalysts perform well over the lifetime of the vehicle. The target light-off time is currently less than 15 s. in order to get this, options being explored include close coupling of catalyst to engine, starter catalysts, and heaters. For lubricants, there is pressure on the phosphorus content since phosphorus is thought to lead to coating of the catalyst, affecting catalyst performance. Increasingly, there is also pressure to reduce sulphur levels in the lubricant (note that the widespread use of after-treatment devices in passenger car vehicles has been partly responsible for the introduction of low-sulphur fuels in the USA and Europe with fuel sulphur levels less than 50 ppm, and in some parts of Europe less than 10 ppm) and questions are being asked about the contribution of sulphur from the lubricant. For stratified charge gasoline direct-injection engines, which operate under ultra-lean conditions (i.e. high air–fuel ratios), conventional three-way Catalysts will not work because the NOx cannot be reduced due to the oxygen-rich atmosphere. Hence NOx emerges at engine-out levels considerably in excess of legislated levels. CO and HC control depends largely on the promptness of the light off time. The use of stratified charge engines will only remain viable if after-treatment systems can be developed which will reduce NOx to N2. Options for such after-treatment devices include the following.
1. Passive NOx control. HCs are accrued on a zeolite substrate during low-temperature operation, creating a rich microclimate with appropriate stoichiometry; these HCs are liberated during higher-temperature operation to enable reduction of NOx species.
2. Active NOx control. NO is converted to NO2 during lean operation over a platinum catalyst. The NO2 is stored on barium storage sites embedded on the catalyst substrate and, when the barium is saturated, the engine switches to rich operation, liberating NO2, which is reduced in the presence of HC and CO. This is feasible since gasoline engines will run stoichiometrically.
Option 1 has no known issues for lubricants; however, efficiencies are low (less than 30 per cent) and there are other technical problems which makes it unlikely that these systems will see widespread use.
Option 2 is complex but has efficiency levels greater than 90 percent; however, these systems require low-sulphur fuels and, since fuel sulphur levels are low, attention is now being focused on the lubricant sulphur level.
For light-duty and heavy-duty diesel vehicles, as mentioned before, the important emissions to control are particulates and NOx. Unlike passenger car vehicles, emission targets are expressed in units of grams per brake horsepower hour, which deletes any bias towards larger vehicles. There are many more options for after-treatment devices for heavy-duty diesel vehicles. In the lists of devices given below, those indicated asterisks are sensitive to sulphur. To reduce particulate levels there are the following: oxidation catalysts; simple traps; Johnson Matthey continuously regenerating trap*; Englehard regenerating trap; AEA Electrocat;
Particulate traps are made of cordierite material with porous walls, silicon carbide, or sintered metal sheets. The device can be thought of as a large number of drinking straws, which have porous walls. Half of the ‘straws’ are open at the inlet end and closed at the outlet end, whereas the other half are closed at the inlet end and open at the outlet end. Exhaust gases enter the device, and the gas has to diffuse through the walls before it can escape. Any particulates do not diffuse through the wall and are kept within the trap.
Clearly, the particulate trap will eventually become blocked (the engine back pressure can be monitored to give warning of when the trap is becoming blocked). These devices are effective at trapping particulates. However, it is clear that these devices may be sensitive to the amount of sulphated ash present in the lubricant. In practice, these devices will also need to be regenerated quickly and relatively cheaply when they become blocked.
The Johnson Matthey continuously regenerating trap is a commercially available system that removes CO, HCs and particulates. This device requires ultralow-sulphur fuel to operate effectively. This device has been in widespread used in Scandinavian countries where low-sulphur fuel is readily available. The device can be fitted to both new and old engines. The device captures lubricant ash, but the accumulation of ash in the device is not sufficient to cause a rise in back pressure. After every 100 000 km, the trap is simply turned around, and this clears ash out of the system. These devices have been used successfully in
For NOx reduction, the Siemens selective catalytic reduction SiNOx system uses urea to remove NOx from the exhaust gas. The main issue for this system is that a separate tank of urea is needed on the truck (the specific urea consumption amounts to about 4 per cent of fuel consumption). Also there clearly needs to be an infrastructure to ensure the supply of urea. A urea tank of 45 l would have a range of roughly 3000 km. This route is preferred by European OEMs. This system does not require the use of low-sulphur lubricants, but there may be issues arising from the need to provide a urea infrastructure to customers, and how urea is handled.
The Degussa GD-KAT is a combined particulate and NOx control device. It consists of a pre-catalytic converter, (which reduces CO, HC and particulates), a hydrolysis catalytic converter (which removes some NOx), a reduction catalytic converter (which removes more NOx) and an oxidation catalytic converter (which removes NH3, produced from the
use of urea in the hydrolysis catalytic converter).
In conclusion, there are a large number of after-treatment options for passenger car and commercial vehicles. Some of these systems are thought to be sensitive to lubricant sulphur level and lubricant sulphated ash level [39, 40]. In addition, for threeway catalysts used on conventional gasoline engines, there is evidence that phosphorus from the lubricant forms a coating on the catalyst. However, direct links between these inferred sensitivities and a reduction in after-treatment systems performance have not yet been made.
Despite this, in forthcoming industry standard lubricant specifications, chemical limits for sulphur, phosphorus and sulphated ash are being discussed. In ILSAC GF-4, for example, the limits given in Table 4 are used.
Table 4 Chemical limits for ILSAC GF-4 (introduced in January 2004)
| | Phosphorus limit (mass %) | Sulphur limit (mass %) |
| All grades | 0.06 minimum, 0.08 | - |
| 0W, 5W grades | - | 0.5 maximum |
| 10W grades | - | 0.7 maximum |
Note that, in Table 4, there is both a minimum phosphorus limit and a maximum phosphorus limit. Some OEMs, during the ILSAC GF-4 negotiation process, wanted a minimum limit to ensure maintenance of engine durability.
For ACEA E6-2004 and JASO DH-2, Table 5 gives the limits on sulphated ash, phosphorus, and sulphur that have been imposed for engine oils used with an exhaust after-treatment system.
Table 5 Chemical limits (maximum values) on sulphated ash, phosphorus, and sulphur for ACEA E6-2004 and JASO DH-2 engine lubricants
| | Sulphated ash (%) | Phosphorus (%) | |
| ACEA E6-2004 | 1.0 | 0.08 | 0.3 |
| JASO DH-2 | 1.0 | 0.12 | 0.5 |
Clearly, there is some conflict between the chemical limits that may be imposed for after-treatment compatibility and the lubrication demands of modern engines. For good antiwear performance, the most cost-effective and reliable additive is zinc dialkyl dithiophosphate (ZDDP), which contains ash, sulphur, and phosphorus. For heavy-duty diesel engines equipped with EGR, oils with higher TBN are needed to neutralize the strong acids that are likely to be formed; most additives used to increase TBN will contribute to the lubricant sulphated ash level. For long oil drain intervals, increased levels of dispersant and detergent will be required, the latter contributing to ash levels. Basically, most base oils contain sulphur at present. There is currently great activity ongoing to find low-sulphur, low-phosphorus, and low-sulphated-ash additives that give good antiwear performance and good dispersant–detergent performance.
The other major issue is that most industry standard specifications assume that oil that meets the latest specification is ‘backward compatible’ with previous specifications. If chemical limits are imposed, it is almost inevitable that they will be tightened in the future for meeting emissions legislation limits. The backward compatibility may not be possible; it means different lubricants depend on the engine and after-treatment technology.
Finally, it should be mentioned that the use of after-treatment devices does impact on vehicle fuel consumption. Diesel particulate filters are expected to result in higher fuel consumption. Catalytic reduction systems will be chosen in order to give lower fuel consumption.
5 THE IMPACT OF NEW BASE OIL AND ADDITIVE TECHNOLOGIES
While lubricant performance requirements, as indicated by specification tests, have changed over the last 15 years, so the lubricant formulator can select the range of base oils and additives which are available.
Many US ILSAC GF-3 lubricants made use of group II base oils. These base oils had the right mix of volatility and viscosity grade to formulate fuel economy lubricants for ILSAC GF-3. These base oils are produced by the further hydroprocessing of conventional base oils
There is the prospect of large volumes of zero-sulphur, low-volatility base stocks manufactured from gas [41] in the future. These ‘gas-to-liquid’ base oils may enable some of the sulphur limits to be met for future passenger car and heavy-duty diesel vehicles. (These base oils are likely to have
A large number of new additives have been used besides base oil developments over the last 15 years. For fuel economy applications, molybdenum dithiocarbamate has been widely used as a friction modifier. However, there has been a move to organic friction modifiers at present (since they will not contribute to the sulphated ash level of the lubricant) such as glycerol mono-oleate.
New dispersant polymethacrylate viscosity modifiers have appeared [42], which appear to be surface-active VI improvers, and these additives have found use in some engine oil applications.
by the chemical limits that are being proposed for future lubricant specifications, there is much activity ongoing to develop low ash, low-sulphur and low-phosphorus-containing additives [43, 44] (especially antiwear and friction modifier additives).
6 FUTURE CHALLENGES
For the future, the major challenge will be to continue to develop lubricants, which adequately lubricate engines with higher power outputs, operating at higher temperatures, with longer oil drain intervals, and which are compatible with after-treatment devices.
As previous statement, compatibility with after-treatment devices is currently being done by chemical limits being placed on sulphur, phosphorus, and sulphated ash. Lubricants may not only meet future specifications that have these chemical limits but also need to be backward compatible with older vehicles. This will become a more difficult challenge when chemical limits are tightened.
Conclusions
The above are the main trends in engine lubricants over the last 10–15 years. We have highlighted a need for lubricants that have better antioxidant performance, better dispersant–detergent performance, and improved fuel economy performance and that are compatible with after-treatment devices.
These requirements match appropriate industry standard (and OEM) engine and bench tests. Today’s lubricants perform at a far higher level than those of 10–15 years ago. At the same time, the most modern technology only represents a small fraction of the passenger cars and trucks. It is necessary to lubricate adequately not only new-technology engines, but also the older-technology engines that are still on the road. For this reason, lubricants not only meet new specifications but also enable backward compatible with older specifications. it will be a challenge to continue to develop this when chemical limits on sulphur, phosphorus, and sulphated ash (designed for the protection of after-treatment devices) are tightened.
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