Final Term Report
Engine Lubricant Trends Since 1990
Submitted to
Dr.Simon C.Tung
Mechanical Engineering Department
Wayne State University
Prepared by
Shi Cheng
Xihu Dong
ME5730 Tribology and Lubricant Technology Course
July 16, 2007
Objective
This report is stated of the important trends in engine lubricant performance tests that have taken place over the last 10–15 years. Lubricant formulations are impelled by industry standard specifications, original equipment manufacturer requirements, and consumer needs. The report includes the important specifications and associated performance tests, and how these have impacted on lubricant development. The following are four trends of engine lubricant, firstly, needing to improve fuel economy, secondly, needing to improve oxidation stability, thirdly, needing to improve handling of contaminants (e.g. soot), and, fourthly, moving to lubricants containing low levels of sulphur, phosphorus, and sulphated ash, for after-treatment device compatibility reasons in recent years.
Introduction
Engine lubricant formulations change with changing industry standard specifications, original equipment manufacturer (OEM) requirements, and consumer needs. Industry standard specifications are often driven by legislation (in particular, legislation on vehicle emissions), which usually requires new engine technologies to be introduced. When new technologies place particular challenges on lubricant formulation, these new technologies often appear as engine performance tests (or bench tests) in industry standard specifications. Industry standard specifications are set by the Association des Constructeurs Europe´ens d’Automobiles (ACEA) in Europe [1, 2] [although individual tests within a specification sequence are developed by Coordinating European Council for the Development of Performance Tests for Lubricants and Engine Fuels (CEC)]. In the USA, the American Petroleum Institute (API) sets specification requirements [3] [and individual tests with the specification sequence are developed by the American Society for Testing and Materials (ASTM)]. In Japan the lubricant specification body is set by the Japan Automobile Standards Organization (JASO). Lubricant marketers are free to choose the mix of specifications that their products meet, and whether their products just meet the specifications or exceed them greatly, and they are also free to develop products which meet consumer needs that are not explicitly covered by industry standard specifications. The regional specification standards bodies (ACEA, API, and JASO) are also starting to develop global lubricant specifications now. The International Lubricant Specification and Approval Committee (ILSAC) set specification standards that are mainly used in the USA and Japan (e.g. ILSAC GF- 4). Global specifications are in place for heavy-duty diesel engine lubricants (DHD-1, DHD-2, and the proposed DHD-3). The whole process of developing new specifications is complex, and a good overview has also been given by Caines and Haycock [4].
Since 1990, the main trends in requirements for lubricant formulation have been as follows.
1. The need for improved oxidative stability is necessary to control deposit formation, to minimize sludge formation, and to ensure that viscosity is controlled, as oil drain intervals and engine power outputs (and hence oil temperatures) have trended upwards.
2. The need for improved fuel economy performance has been driven by legislative controls, such as Corporate Average Fuel Economy (CAFE) in the USA, and the CO2 emission limits that the European Union (EU) has mandated for introduction in 2008, and related fuel economy initiatives in Japan; there are also fuel economy engine tests in ILSAC and ACEA lubricant specification sequences which will be considered in more detail later.
3. The need for improved soot-handling capabilities for heavy-duty diesel engine oils is required. Increasingly stringent legislative limits on nitrogen oxide (NOx) emissions have led to increased lubricant soot levels, and since 1990 there has been a need to improve the soot-handling capabilities of these oils. In Europe and the USA this has led to increased dispersancy levels in the oil.
4. It is essential to reduce vehicle emissions. The need for reduced tailpipe emissions from vehicles has led to the widespread introduction of after-treatment devices (e.g. catalytic converters) for passenger car vehicles, and this trend is expected to spread to heavy-duty diesel engines over the next few years. The OEMs clearly want lubricants to be compatible with these increasingly sophisticated devices. Currently there is much debate on whether chemical limits should be imposed on lubricant formulations to limit the amount of sulphur, phosphorus, and sulphated ash. Note that, since one of the most important antiwear additives contains both sulphur and phosphorus and contributes to the sulphated ash level, there are some interesting formulation challenges to meet these chemical limits while still ensuring satisfactory lubrication performance.
5. New improved additives and base oil technologies are required. New additive and base oil technology clearly impacts lubricant formulation. For example the appearance of group II base oils in the USA in the 1990s enabled fuel economy lubricants to be formulated at a cost-effective level. Towards the latter part of this decade, the expected appearance of gas-to-liquid base oils will be significant for the widespread production of low-sulphur lubricants. These trends will be considered in more detail below and, where appropriate, the evolution of engine tests since 1990 will be described, together with the improved lubricant performance that these tests have necessitated. Some of the future challenges are also discussed.
Discussions and experimental approaches
1. OXIDATION STABILITY
Engine lubricants include mainly base oil (depending on performance level the base oil content will be 80–95 percent of the lubricant) with the remainder being lubricant additives (5–20 percent) [5, 6]. Since the lubricant can stay in the vehicle for a long time period (a minimum of 3 months in the USA, and from 6 months to 2 years for a passenger car in Europe) and operates at elevated temperatures (sump temperatures can be as high as 130 ºC in a passenger car under high-load operation, e.g. towing a caravan up a steep hill, and piston temperatures can be 250 ºC or higher) in the presence of aggressive gases (combustion gases, NOx, high pressures, etc) the lubricant formulator needs to ensure that oxidation is controlled. Figure 1 shows schematically the process of lubricant oxidation.
Fig. 1 Schematic diagram of the lubricant oxidation mechanism
The base oil, which is predominantly a branched hydrocarbon, produces ketones, alcohols, esters, and (mostly) acids during the oxidation process. It is also possible that higher-molecular weight products are formed by polymerization. These oxidized products are responsible for the change in viscosity during an oil drain interval; viscosity increases large when the antioxidants in a lubricant are used out. Exception of viscosity increases, the oxidation products can become precursors for sludge and deposits. Since acids are formed during the oxidation process, the total acid number (TAN) of oil will also increase as oxidation progresses. The increased acid content of the lubricant will cause bearing corrosion. Since acids form during lubricant oxidation, the fresh lubricant contains a certain amount of ‘base’, which is included for the neutralization of any acid products that are formed during oxidation. This is quantified using the total base number (TBN).
Figure 2 shows the typical effects of lubricant oxidation [7] (in terms of viscosity increase, TBN decrease, and TAN increase).
Since 1990, specific power outputs of engines have gone up, so have oil drain intervals. Therefore better antioxidant performance is required in engine lubricants for these reasons, which is why industry standard specifications are requiring improved oxidation performance. An ‘oil stress factor’ (OSF) may be defined [8]. The usual definition of such an oil stress factor is:
OSF =(P / VD)(ODI / VS) (1)
Where P is the engine power (kW), VD is the engine displacement (l), ODI is the oil drain interval (km), and VS is the volume of lubricant in the sump (l). The OSF thus has units of kilowatt kilometers per square litre. (Note that alternative OSF equations are also used, some of which take account of oil consumption rates.)
Using the above definition of OSF, Fig. 3 shows schematically the way in which the OSF has increased in recent years. (Note that this figure is for European gasoline passenger cars, with high-performance sports cars omitted.)
Figure-4 shows that one of the side effects of increased antioxidant performance in passenger car engine lubricants (in response to oil specification tests and increased OSFs) has been, in general, cleaner engines.
Figure-2 The effect of lubricant oxidation on viscosity, TBN and TAN for three different oils.The data were generated in a laboratory screener test designed to mimic the sequence IIIE engine test
Fig. 3 Schematic indication of the way in which the OSF is increasing (# BP/Castrol 2004)
Fig. 4 Effect of antioxidant treat rate on engine cleanliness: the antioxidant treat rate ranges from high to good to average (left to right)
There are industry standard tests by which we assess: viscosity increase of oxidized oil [9, 10], sludge formation in engines [11, 12], deposit formation and piston cleanliness [13], and acid corrosion. There are also limits on the volatility of the oil (as measured using the NOACK volatility test), which impact on the base oils that may be used (clearly, if the oil is too volatile, a large fraction of the lubricant could evaporate, which is undesirable).
The following are the relevant industry standard engine tests.
1. The Sequence IIIF engine test [9] for viscosity increases because of oxidation, and also deposits, piston cleanliness, cam wear, and oil consumption. The test uses a 1996 Buick V6 gasoline engine, with a displacement of 3.8 l. The test duration is 80 h. The test uses unleaded fuel, and the sump oil temperature is 155 ºC. The lubricant viscosity increase (this is the increase in kinematic viscosity as measured at 40 ºC) has to be less than 275 percent to pass performance. In this test, piston skirt varnish and weighted piston deposits are rated; cam and lifter wear and oil consumption are also measured (there are pass–fail limits on each of these factors. This test is used in ILSAC GF-3 and API SL specifications. Note that, for ILSAC GF-2 and API SJ, the Sequence IIIE engine test was used; this test duration is 64 h and pass–fail limits for the viscosity increase was 375 percent. Clearly the increased duration in the current test and the tightened limits require improved oxidation performance of the lubricant. In comparison with the MHT-4 thermo-oxidation engine oil simulation test (TEOST) (see below) this more stringent performance requirement has led to engine lubricants requiring higher-quality base oils and/or increases largely in antioxidant levels, by as much as 50–75 per cent. In ILSAC GF-4, the Sequence IIIG engine test is used, and in this test the pass–fail limit on the lubricant viscosity increase (increase in Vk 40) has been tightened even further to just 150 percent.
2. In Europe, in order to assess lubricant oxidation, the CEC uses a pressure differential scanning calorimeter (PDSC) test [14] (the test method is CEC L-85-T-99). The oxidation induction time of the lubricant is measured in this laboratory test. The test conditions are as follows: air at 100 lbf/in2; no flow, 40 8C for 2 min, 40–50 8C at 58C/minute, 50 8C for 5 min, 50–210 8C at 408C/min, and 210 8C isothermal for 120 min; aluminum sample pans; sample mass 3.0+0.5 mg. Samples generally give an oxidation plot with a primary and secondary peak. The onset time for the primary peak is recorded as the oxidation induction time. For ACEA E5-02 performance this oxidation induction time has to be longer than 35 min.
3. The Sequence VG engine test [11] evaluates the lubricant performance in combating sludge and varnish formation under low-temperature conditions (e.g. such as those found in ‘stop–go’ driving cycles). A Ford 4.6 l V8 gasoline engine is used in the test (this engine is also used for US fuel economy tests) and the test is run for 216 h for a range of engine speeds and loads, with the sump oil temperature varying between 45 and 100 ˚C.
4. In Europe the CEC use the Mercedes Benz M111 sludge test [12] (this engine is also used for the European fuel economy engine test). This is a test for black sludge. The engine is a 2.0 l gasoline engine, the test duration is 224 h, the engine speed varies from 750 to 6000 r/min, the load varies from 0 to maximum, and the oil temperature varies from 37 to 140 ºC. The test method is CEC L-53-T-95.
5. For high-temperature deposit formation, the TEOST MHT-4 bench test is used [15]. In this test, the amount of deposits (in milligrams) formed in a laboratory glassware test are assessed. For the current specification (ILSAC GF-4) the pass–fail limit is 35 mg whereas for ILSAC GF-3 and API SL the pass–fail limit was 45 mg maximum. There has been some argument about the correlation of this test to field performance; whatever the merits, the test does impact on lubricant formulation.
6. In Europe, the Peugeot TU3 and TU5 tests have been used to evaluate piston deposits, ring sticking tendency and viscosity increase of lubricants (for gasoline engine oils) [10]. The TU3 engine test uses a 1.36 l four-cylinder single-point injection engine, the test duration is 100 h, and the oil temperature varies between 40 and 100 ºC. The TU5 test uses a 1.6 l four-cylinder multipoint injection engine, with a test duration of 96 h, and an oil temperature of 80–150 ºC.
7. A large number of European tests are put in place for assessing diesel engine oil performance (both heavy-duty and passenger car diesel engines). The OM364LA test is used to test bore polishing, piston deposits, sludge, liner wear, and oil consumption (the test uses a Daimler Chrysler OM364LA engine; the test method is CEC L-42- T-99). The OM602A test is used to look at cam wear, viscosity increase, bore polishing, cylinder wear, sludge, and oil consumption (it uses a Daimler Chrysler OM602A engine; the test method is CEC L-51-T-98). The OM441 LA test is used to evaluate piston deposits, bore polishing, wear, oil consumption, valve train condition, and turbocharger deposits (the test uses a Daimler Chrysler OM441LA engine; the test method is CEC L-52-T-97). The Peugeot XUD-11 test is used for assessing lubricant-related piston deposits and viscosity increases (the test uses a four cylinder IDI T/C I/C 2.0 l diesel engine; the test method is CEC L-56-T-99). The Volkswagen 1431 test is used for testing piston deposits, varnish, and ring sticking (the test uses a four-cylinder T/C I/C 1.6 l engine; the test method is CEC L-46-T-93). The Volkswagen 1453 test is used for evaluating piston deposits, ring sticking, and viscosity increase (the test uses a four-cylinder T/C I/C 1.9 l engine; the test
method is CEC L-78-T-97 (or CEC L-78-T-99)).
8. ILSAC GF-4 uses the Sequence VIII test for bearing corrosion protection. The pass–fail limits for maximum bearing mass loss are 26 mg (this is essentially the same as ILSAC GF-3 where the pass–fail limit was 26.4 mg). In ILSAC GF-2 and API SJ specifications, however, the pass–fail limit for maximum bearing mass loss was 40 mg. (However, the test severity is in fact the same, since unleaded fuel is used in the GF-3 and GF-4 tests, whereas leaded fuel was used in the GF-2 test, and the limits were altered to maintain test severity.)
9. In ILSAC GF-3 and API SL (and also ILSAC GF-4) there is a constraint on lubricant volatility. The volatility (as measured by ASTM D 5800 using a NOACK volatility test) must be less than 15 percent, whereas in ILSAC GF-2 and API SJ the limit was less than 22 percent. Clearly this change has led to a need to use superior base stocks in lubricant formulations (particularly for the lower-viscosity lubricants required for improved fuel economy performance, as discussed later). NOACK volatility limits are also used in ACEA specifications.
In addition to industry standard specifications, OEMs often use the same tests (for their own in-house lubricant specifications that are often required for factory fill and service fill lubricants) but may persist in tighter limits. For example, Ford require a double-length Sequence IIIF test (160 h) or a double-length Sequence IIIE test (128 h) with a maximum viscosity increase of 200 percent (the corresponding limits for the single-length tests are 275 percent maximum (IIIF) and 375 percent maximum (IIIE)). In addition, Ford’s limit for high-temperature deposits in the TEOST MHT-4 test is 30 mg (while the ILSAC GF-4 limit is 35 mg). Clearly, oils meeting the Ford requirement exceed the antioxidant requirement for minimum performance in ILSAC GF-3 and API SL (and will require the use of high-quality base stocks and increased antioxidant treat rates).
Finally fuel sulphur levels are considered. If fuel sulphur levels are very high (e.g. greater than 500 ppm), then there can be a detrimental effect on lubrication (in fact the API have specifications for lubricants which are used with high-sulphur fuels; API CF). However, fuels with very low levels of sulphur can also cause problems. In Europe, fuel sulphur levels are currently set at 50 ppm, but there will be a move to 10 ppm levels later this decade (some EU countries are already at this level). The USA is also moving to low-sulphur fuel; by 1997, 95 percent of fuel in the USA will be at the 15 ppm sulphur level, and all fuel in the USA will be at this level by 2010. Sulphur in fuel can help a lubricant’s antioxidant performance, and reduced fuel sulphur levels can also cause lubricity problems in fuel pumps. Clearly, the fuel and lubricant do interact with each other, although the precise mechanisms are not well understood.
2. FUEL ECONOMY
More and more countries and automotive manufacturers hope to reduce emissions that include CO2 emissions that arise from burning fuel in internal combustion engines. There is a direct correlation between a vehicle’s fuel consumption and the CO2 emitted from that vehicle. The EU have set a ‘voluntary’ target that the fleet average CO2 emissions for manufacturers selling vehicles in Europe must be lower than 140 g/km by 2008 (this is about 25 percent lower than current levels). Many countries have also wanted to reduce their CO2 emissions by signing the Kyoto Agreement. Since one of the main sources of CO2 emissions arises from the transport sector, improved fuel economy is a major target of many OEMs [16–19]. In the USA, there are CAFE targets that an OEM must meet, averaged over their entire fleet. The fuel consumption of a vehicle depends among other factors on the friction that must be reduced in the engine [20–26]. Clearly the friction depends on the engine design. However, friction in the engine can also be affected by lubricant viscosity. If the lubricant (dynamic) viscosity is denoted by η (mPa s), then for those parts of the engine that are lubricated hydrodynamically (this is where an oil film completely separates the moving metal surfaces) the friction varies as η . For parts of the engine where there is boundary lubrication (e.g. the valve train) the metal surfaces can touch, and the use of surface-active ‘friction modifier’ additives (such as molybdenum dithiocarbamate, or organic-based friction modifiers such as glycerol mono-oleate) can largely reduce friction in these areas. Hence, the need for improved fuel economy has led to lower viscosity lubricants containing friction modifier additives.
Table 1 shows typical viscosities for different lubricant SAE viscosity grades (note that the viscosity grade assigned to a lubricant is determined by SAE J300 [27]). In 1990 the most commonly used viscosity grades, in the USA were SAE-10W/30 (34 percent), SAE-10W/40 (25 percent) and SAE 15W-40 (22 percent). These viscosity grades are still widely available today, on a global basis. However, an increasing proportion of SAE 5W-30 and SAE 5W-20 grades are now available in the USA (SAE 5W-20 is the recommended viscosity grade for new Ford vehicles in the USA, whereas the recommended viscosity grade in Europe is SAE 5W-30). Clearly these lower-viscosity grades have more lower viscosity than SAE 15W-40 lubricant. In Japan, some OEMs are now using SAE 0W-20 lubricants as factory fill oils for new vehicles. In the USA and Japan the main driving force leading to lower-viscosity lubricants has been the ILSAC GF-2, GF-3, and GF-4 fuel economy engine test. In Europe, since 1990 there has been a trend towards 0 W (and 5 W) grade lubricants (to give fuel economy benefits at low temperatures) while at the same time having a 30 or 40 grade specification at high-temperatures (in order to give an enough thick oil film under high-temperature conditions). For example, Volkswagen specify SAE 0W-30 lubricants as their preferred viscosity grade.
Table 1 Typical viscosities of commonly available lubricants [as classified by the Society of Automotive Engineers (SAE) J300 classification system].
| SAE viscosity grade | VK 40 (cSt) | VK100 (cSt) | Estimated dynamic viscosity at 215 8C (mPa s) |
| SAE 30 | 91.3 | 10.8 | 3950 |
| SAE 20W-50 | 144.8 | 17.8 | 5870 |
| SAE 15W-40 | 114.3 | 14.9 | 2940 |
| SAE 10W-30 | 72.3 | 10.8 | 1900 |
| SAE 5W-30 | 57.4 | 9.9 | 1090 |
| SAE 0W-20 | 44.4 | 8.3 | 690 |
According to SAE J300, the minimum high temperature high-shear viscosity (HTHSV) (evaluated at a temperature of 150 8C and a shear rate of 106 s21) is 2.6 mPa s for xW-20 grades, is 2.9 mPa s for xW-30 grades (and also for 0W-40, 5W-40, and 10W-40 grades), and is 3.7 mPa s for 15W-40, 20W- 40, 25W-40, and 40 grades, and also for xW-50 and xW-60 grades. (Note that for xW-10 grades there is no minimum HTHSV value defined.) For ILSAC GF-3 (and also for the proposed ILSAC GF-4 sequence) the only allowed viscosity grades are SAE 0W-20, SAE 5W-20, SAE 5W-30, SAE 0W-30, and SAE 10W-30 (note there are no such viscosity grade restrictions for API SL). ACEA also use the SAE J300 viscosity classification but allow HTHSV to be as low as 3.5 mPa s (rather than the 3.7 mPa s specified for the relevant viscosity grades in SAE J300).
The first US fuel economy engine test was the Sequence VI test in terms of engine tests. This used an engine with a sliding valve train system (i.e. the cam slid against the follower). The test ‘appetite’ was found to be that lower-viscosity lubricants gave improved fuel economy, and friction modifiers were also important. The fuel economy improvement of candidate oils was evaluated against a 20W-30 ASTM HR reference oil. The Sequence VI-A engine test was introduced as below. (used in ILSAC GF-2). A Ford 4.6 l V8 gasoline engine was used (the same engine as used for the Sequence VG sludge test). The engine had a roller follower valve train system (i.e. the cam contacted a roller, and for the majority of operation the contact was rolling rather than sliding), and was found to have an appetite for lower viscosities, but the use of a roller follower valve train meant that there was very little boundary friction in the engine, and so friction modifiers only had a small effect on fuel economy. The Sequence VI-B engine test [28]was used the same engine as for the Sequence VI-A test, but operating conditions were changed to increase the proportion of boundary friction. However, the leading way to improve fuel economy in this test is simply to reduce lubricant viscosity. For the Sequence VI-B test, the fuel economy of the test lubricant was evaluated under both ‘fresh’ and ‘used’ conditions (the Sequence VI-A test just measured the fuel economy benefit of the fresh lubricant). The lubricant was aged in the engine for the equivalent of 5000 miles (which is typical of a US oil drain interval), when the fuel economy of the used oil was once again measured. The Sequence VI-B fuel economy engine test is used in both ILSAC GF-3 and ILSAC GF-4.
Tables 2 and 3 show the pass–fail limits for the Sequence VI-A (for GF-2 specification) and Sequence VI-B engine tests (for both GF-3 and GF-4 specifications) respectively. Clearly, there has been a need for improved fuel economy performance each time that the specification has been upgraded.
Table 2 Pass-fail limits for the VI-A(ILSACGF-2) tests. Fuel economy improvements (FEI) (%) in the tests are evaluated relative to the ASTM BC reference oil
| Viscosity grade | FEI (% min) |
| SAE 0W-20 | 1.4 |
| SAE 5W-20 | |
| All other SAE 0W-x SAE 5W-x | 1-1 |
| SAE 10W-x | 0.5 |
The Mercedes Benz M111 engine is used for the fuel economy test in Europe [29]. The test duration is 24 h, and a combined ECE-15 (4 cycles) + European Urban Driving Cycle (EUDC) is used (Fig. 5). The oil temperatures in the test are 20 ºC, 33 ºC, and 75 ºC. No aged oil fuel economy measurement is implemented. The fuel consumption of the test oil is compared with standard reference oil, RL-191 (an SAE 15W-40 lubricant with an HTHSV of 3.9 mPa s). The pass–fail limit is 2.5 percent. It is found that this engine has both an outstanding viscosity and a friction modifier effect. For oils whose HTHSV is greater than 2.6 mPa s, it is not possible to pass this test without a friction modifier.
An attempt was made recently by the CEC to develop a fuel economy engine test for aged oil fuel economy (the CEC TDG-L-089 Test Development Group) [30]. The proposed test used a Ford Duratorq 2.0 l passenger car diesel engine using an ageing cycle that would be representative of 30,000 km of European driving. The Test Development Group found that the fuel economy evaluated in the test (compared with various reference lubricants) was linearly dependent on the HTHSV, and the role of friction modifiers was small (this contrasts with the M111 fuel economy test where friction modifiers are found to have a significant effect). According as these results, no new test was introduced and the Group recommended that improved fuel economy may be achieved in this engine by simply using lubricants with lower HTHSV.
Table 3 Pass–fail limits for the VI-B (ILSAC GF-3 and GF-4) tests. Fuel economy improvements (FEI1 and FE2) (%) in the tests are evaluated relative to the ASTM BC reference oil
| Viscosity grade | FEI1 (16 h) (% min) | FEI2 (96 h) (% min) |
| Sequence VI-B pass–fail limits, as used in ILSAC GF-3 |
| SAE 0W-20 | 2.0 | 1.7 |
| SAE 5W-20 | | |
| SAE 0W-30 | 1.6 | 1.3 |
| SAE 5W-30 | FEI1 + FEI2 ≥ 3.0 |
| SAE 10W-30 | 0.9 | 0.6 |
| and higher | FEI1 + FEI2 ≥ 1.6 |
| Sequence VI-B pass–fail limits, as used in ILSAC GF-4 |
| SAE 0W-20 | 2.3 | 2.0 |
| SAE 5W-20 | | |
| SAE 0W-30 | 1.8 | 1.5 |
| SAE 5W-30 | | |
| SAE 10W-30 and other grades not above | 1.1 | 0.8 |
Fig. 5 Driving cycle used for the CEC M111 fuel economy engine test
Besides bench tests, fuel economy lubricants are also evaluated on vehicles running on chassis dynamometers, running on standard driving cycles. In Europe, Volkswagen use the driving cycle shown in Fig. 5, but the oil temperature at start-up is 27 ºC (the Volkswagen PV1451 fuel economy test). Other OEMs, e.g. Daimler Chrysler, also use the same cycle but with a starting oil temperature of around 20 ºC. In Japan, the driving cycles are similar to the European cycles. The Japanese 11 mode cycle uses a cold start (the oil temperature at start-up is around 25 ºC) and is used to simulate urban driving conditions, whereas the 10–15 mode cycle is used for a fully warmed-up engine (the sump oil temperature is approximately 100 ºC) and attempts to simulated urban and higher speed driving cycles. Both these driving cycles are illustrated in Fig. 6.
Fig. 6 Japanese driving cycles
In the USA, the Federal Standard Urban Driving Cycle and the Federal Standard Highway Driving Cycle are available for use. These are more highly transient than the European and Japanese driving cycles and so will have proportionately high fuel consumption (although they may be more realistic with respect to real driving conditions). These driving cycles are shown in Fig. 7.
Fig. 7 US driving cycles
An attempt has also been made by the CEC to develop a fuel economy test for heavy- duty trucks (the CEC IL-87 Heavy Duty Diesel Fuel Economy Group). , by changing the lubricant the Group concluded that the scope for fuel economy improvement in heavy-duty diesel engines is small but measurable. The Group concluded that the main constraint on using lubricants to improve the fuel economy performance is ACEA’s insistence that the HTHSVs of heavy-duty diesel engine lubricants should be greater than 3.5 mPa s. The Group concluded that it would be useful if recommended codes of practice were introduced for operators who wished to conduct valid fuel economy field trials (along the lines of US guidelines for running fuel economy field trials using trucks issued by the US Truck Maintenance Council).
In terms of what impact these fuel economy tests have had on lubricants, Taylor [22] has concluded that, for the USA, the appetite of the Sequence VI-B engine test has led to the following formulation trends: lower-viscosity grades (SAE 5W-20, etc); the use of shear unstable viscosity modifiers; the use of higher-viscosity-index (VI) base stocks; the use of effective friction modifiers; lower HTHSV (2.6 mPa s); lower cold cranking simulator viscosities.
Since there is a need for many European lubricants to achieve longer oil drain intervals than in the USA, the additive treat rate tends to be higher than for many US lubricants, and as such it is difficult to formulate a ‘European-style’ long oil drain interval lubricant that also performs well in US fuel economy tests.
There is also a limit on lubricant volatility (as measured by NOACK) in API and ACEA specifications, and so the move to lower-viscosity lubricants meant a shift towards higher-quality base stocks. In the USA, SAE 5W-x fuel economy grades are mainly formulated using group II base stocks. In Europe, 0W-x grade lubricants are formulated using polyalphaolefins. Clearly, the final price of the product depends on the choice of base stock (polyalphaolefins are considerably more expensive than group II base stocks).
3. HEAVY-DUTY DIESEL ENGINE OIL SOOT CONTROL
More stringent emissions legislation was introduced in the USA and Europe during the 1990s. This has resulted in radical changes in heavy-duty diesel engine design, which in turn have had a significant impact on the lubricant environment, affecting the relationship between oil formulation, piston deposits, soot dispersion, and wear control [31, 32]. 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 US diesel engines). The category aims to approve lubricants which balance after-treatment life with engine durability. In this specification, it is likely that some ‘chemical limits’ will be imposed in addition to engine test requirements (this will be discussed in more detail in the next section).
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; Delphi non-thermal plasma. For NOx reduction, there are the following devices: passive NOx catalysts; storage catalysts*; selective catalytic reduction. There are also some devices which combine more than one of the above options: the Degussa GD-KAT*; Johnson Matthey SRT*.
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 Scandinavia, on trucks using standard lubricants.
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 (%) | Sulphur (%) |
| 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 VIs in excess of 130, pour points lower than 240 ºC and volatilities (as measured by NOACK) less than 10 percent).
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.
