Automotive lubricants: recent advances and future developments

Citation

Wilson, B. (1999), "Automotive lubricants: recent advances and future developments", Industrial Lubrication and Tribology, Vol. 51 No. 2. https://doi.org/10.1108/ilt.1999.01851bab.012

Automotive lubricants: recent advances and future developments

Automotive lubricants: recent advances and future developments

The first Tribology Group seminar of the season was staged at the Institution of Mechanical Engineers headquarters, London, on 29 October 1998, co-sponsored by the Automobile Division and the Combustion Engines Group of the IMechE.

Presentations were made covering passenger car and truck engine lubricants, lubricant additives, crankshaft bearings, and the measurement techniques thin layer activation for wear and laser-induced fluorescence for fluid film thickness. The final contribution of the day described the impact of lubricants on the environment.

Passenger cars

Mr J. H. May, Rover Group, outlined the recent changes affecting the lubrication of European passenger car engines. To qualify as an engine lubricant in Europe, lubricating oils must pass a number of "sequence" tests as specified by the ACEA (Association des Constructeurs Européen d'Automobiles) in 1996 and 1998. These are known as levels A l, 2 and 3 for petrol engines and B l, 2 and 3 for passenger car diesel engines. For truck or lorry diesel engines the sequences are E l, 2, 3 and 4.

Full details of these tests were summarised in the September/October 1998 issue of this journal.

Oils for the future will have to pass tests for:

• •

  fuel efficiency;

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  emissions reduction and catalyst compatibility;

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  high temperature capability;

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  light duty cycles;

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  extended drain intervals;

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  environmental impact;

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  effect of alternative fuels;

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  effect of new engine technology fuel efficiency.

By 2008 the average carbon dioxide level in engine exhausts must be down to 140g/km. This indicates an average fuel consumption for all cars of 17km/litre or 48 miles per gallon. Currently only the Fiat Seicento approaches this level of economy from a petrol engine. Moreover, many design changes being made aimed at reducing exhaust emissions also tend to increase overall fuel consumption. Engine friction losses typically account for 7.5 per cent of a vehicles energy use. The lubricant can reduce this by reducing the HTHS (high temperature high shear) viscosity and through the incorporation of friction modifiers.

Oils with a dynamic HTHS viscosity of 2.6 mPas are already in use although in 1996 it was considered that the minimum should be 2.9 as the safe limit for crankshaft bearings when taking the high shear rates into account. It is likely that values of 2.2 or 2.4mPas are feasible without reversing the improvement in fuel economy. The long term maintenance of fuel efficiency of lubricants will be a future requirement of the ACEA sequences.

Emissions reduction

Motor vehicle legislation covers the emissions of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter (FM). Action to reduce emissions affect the lubricant, and the oil itself can affect the emissions directly or indirectly. Every year until 2007 there is some change in the emissions requirements that manufacturers have to meet. The changes needed to meet the 2000 and 2005 limits will undoubtedly have some impact on the lubricants.

For example, the use of "common rail" or "unit injector" direct injection of diesel fuel to meet these limits places additional burdens on the oil. Worries about the poisoning of exhaust catalysts by phosphorus additives have decreased now that it has been shown that metallorganic detergents can counteract any adverse effect. However, there is still concern over the effect of phosphorus, silicon and halogen-containing additives on the oxygen sensors of vehicle on-board diagnostics (CED) due to be introduced into Europe next year. Another concern is the possible evaporation or desorption of hydrocarbons from lubricating oil adding to evaporative emissions as the standards are reduced to near zero.

High and low temperature capability

Current trends in technology and design are continuing to increase the operating temperature of lubricants, not helped by exhaust catalyst temperatures of up to 85°C. At the other extreme there is light duty operation or "shopping trolley" cycle. Such duty cycles involve short journeys with frequent stops which can result in fuel dilution of the engine oil building up to high levels because the lubricant rarely reaches sufficiently high temperatures to counteract the effect. This can cause inadequate oil film thicknesses leading to wear, sticking valves from the deposits formed and oil gelation. It could be that light cycles are now becoming the limiting factor on increasing oil drain intervals.

Drain intervals and the environment

In response to customer pressure, oil drain intervals are being increased with 30,000 to 40,000km (19,000 to 20,000 miles) being proposed. Peugeot are reported to recommend 20,000 mile drain intervals and Cadillac 100,000 miles for their ranges of petrol-engined cars.

In the UK most of the used oil drained from vehicles goes for burning. The draft EU proposals on End-of-Life Vehicle Disposal puts pressure on manufacturers to use some re-refined base stock, but this can conflict with the need for longer drain intervals, higher temperature stability and improved fuel efficiency.

The other main environmental concern is the impact of the chemistry of the components of the oil and the production processes throughout the supply chain.

Alternative fuels

The fuels directive for the year 2000 and beyond will change the composition of both petrol and diesel in Europe. Evidence to date from the USA and from the use of "city" diesel and petrol suggest no significant affect on lubricants. There are no major lubricant-related problems from the use of LPG (liquid petroleum gas) but limited experience suggests that lubricant formulations do need to be changed to cope with CNG (compressed natural gas).

The need for bi-fuelling to allow the use of both liquid and gaseous fuels may require additional compromises. CNG is the better option for reducing both carbon dioxide and emissions. LNG (liquid natural gas) and hydrogen are other alternatives in the longer term. Hydrogen would remove the problem of fuel dilution but its inherent lack of lubricity might pose new challenges for engine lubrication.

Fuel cells which can generate electricity by using hydrogen as a fuel are being developed. Mercedes Benz is on record claiming that it aims to have l00,000 fuel cell cars on the road by 2004, emitting only water from their exhausts. The snag for the oil industry is that a fuel cell does not use any lubricant.

New engine technology

Efforts focus on charge optimisation, electronic controls and aftertreatment. The most significant future development is the introduction of the direct injection of petrol where petrol is injected into the cylinder under high pressure. Flow patterns and swirl are very important for correct mixing of the charge so combustion chamber deposits could become even more of an issue. Variations in combustion mixtures and the need for de-NOx catalysts may have significant interactions with the lubricant.

Additives

Mr P.G. Carress, Lubrizol Adibis, added to Mr May's presentation and concentrated on the role of additives. The additive content of engine oils has increased from 10 per cent in the 1980s to 12 to 15 per cent . Of this 50 to 60 per cent are dispersants. The high temperature performance of detergents has become more important as temperatures have increased. Recently OEMs (original equipiment manufacrurers) in Europe have been introducing test requirements in addition to those agreed by industry bodies such as the ACEA, adding to the high cost of around $500 in Europe for meeting these specifications. The challenge for the lubricant industry is to provide drain intervals, fuel economy, reduced emissions and less oil consumption for passenger cars. There is little pressure to use re-refined oils. There is no need to go away from the traditional additive elements although chlorine, potassium and zinc are not welcome. Ash content is not such a problem for passenger car engines now.

Truck engines

The high performance engine lubricants currently offered mean that 100,000km drain intervals for trucks are achievable.

Mr P.E. Brett, Castrol International, provided an international view of truck engine lubrication. He predicted that the different emissions standards of the USA, Japan and Europe would have converged by about 2004 (see Table I).

Table I Exhaust emissions legislation

		NOx	Particulates
	Year	(g/kwh)	(g/kwh)
Euro 1	1983	8	0.35
Euro 2	1996	7	0.15
Euro 3	2000	5	0.10
Euro 4	(2004/5)	3	Still under
				discussion

The principal target for the next century would be the further reduction in NOx, the main changes in the API engine lubricant specification tests for 1998 (Figure 1) have been the introduction of:

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  the Mercedes Benz 441LA full size truck engine test for piston and cylinder durability along with turbocharger performance;

  

  Figure 1 Development of API diesel specifications

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  Cummins M11 for the effect of soot on valve train wear, viscosity and oil filter pressure drop;

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  Mack T9 for engine durability and ring/liner wear.

The Cummins and Mack tests from the API specifications have also been included in the ACEA 1998 E4 and E5 categories (Figure 2).

Figure 2 Performance tests for ACEA

To reduce emissions the design of pistons has been changed to reduce the top land (height above the top piston ring) to 9mm. Top ring groove temperatures are now over 320ºC in the latest American designs for 2002. These two piece pistons have steel crowns and different lubricant requirements to reduce piston groove deposits. Deposits in turbocharger compressor volutes are another problem with ACEA E3 quality oils.

The major problem of soot contamination of lubricants in low emission, heavy duty diesel engines can be mitigated by optimising the dispersant properties of the oil to control soot particle agglomeration. The effect of soot in the oil, addressed by the Mack T8 test in the API and ACEA sequences, is increased viscosity and wear. Particle size is more significant than soot content. Wear has been shown to decrease when the particle size of the suspended soot particles, for example, was reduced from greater that 250nm to less than 125nm.

Fuel efficiency

Engine design changes to reduce exhaust emissions have tended to increase overall fuel consumption. Fuel efficient lubricants are being adopted to counteract this, although engine friction accounts for only about 7 per cent of engine energy consumption. Optimum performance of the lubricant depends on the base oil type, its viscosity and the presence of friction modifiers (Figure 3). Viscosity has the greatest effect, although the HTHS viscosity is limited to 3.5mPas because of durability concerns. (Compare this view with the 2.2 to 2.3 minimum suggested for car engines by the previous speaker ­Ed.)

Figure 3 Factors affecting fuel efficiency

EGR (exhaust gas recirculation)

To meet future NOx emissions and fuel consumption limits, EGR in truck engines is being adopted for US2002 model year trucks, brought forward from 2004. This timetable is demanding substantial effort from the oil industry and engine manufacturers to combat the adverse effects of increased wear, corrosion and deposits. Test engines for lubricant development should be available next year.

(The aim of exhaust gas recirculation is to displace some of an engine's induction air with largely inert exhaust gas as a way of achieving leaner, less NOx inducing, combustion ­ Ed.)

Diesel oil formulation

Mr R. Mainwaring, Shell Additives International, chose the role of soot in viscosity control and wear as his main theme.

Primary soot particles have diameters of 30 to 60nm depending on the engine combustion process. A 5 per cent loading has a concentration of 10¹⁵ particles per cubic centimetre. Such a large number will influence the flow characteristics of an oil, but by adding dispersant molecules to the oil the extent of soot particle aggregation and hence the amount of viscosity increase can be controlled. Soot laden oils have viscosities which vary depending on their history and the measuring device. Wear tests are run on the GM6.5 roller follower rig and the Cummins M11 crosshead tester. Dispersants were found to reduce wear by enhancing oil film thickness rather than by any effect on soot aggregates. Wear can be reduced if the minimum oil film thickness is greater than the primary particle size of the soot.

Crankshaft bearings

Mr D.D. Parker, Glacier Vandervel Bearings, described the plain bearing as acting like a fuse in a bearing system. A heavy duty plain bearing comprised a l.5mm steel backing, 0.25mm of copper lead with a 0.02mm thick lead based overlay on the surface. Bearings are continually being subjected to increasing loads and have to operate with thinner oil films. As the wear mechanism of engine plain bearing materials is mainly abrasive, the presence of anti-wear additives in the lubricant is less effective. A lead/tin/alumina composite overlay was developed to increase the wear resistance of bearings.

Power loss owing to bearing viscous friction could be reduced by reducing their diameter while increasing their length to preserve the same oil film thickness. The use of EGR will increase the soot content of diesel engine lubricants, accelerating bearing wear unless more efficient filtration systems are provided. It has been suggested that dispersancy additives can cause bearing wear by forming "oil balls" several micrometres in diameter which are sufficiently hard to embed into the surface of the bearing. There is also evidence that mixing different oils can lead to increased bearing wear and corrosion where the oil additives were incompatible.

Cavitation erosion damage of bearings can be minimised if engine manufacturers design and make lubrication systems, oil drillings, galleries and components so that air entrapment and vapour cavities are minimised.

More collaboration is needed between the industries concerned to optimise the durability and energy efficiency of plain bearings.

TLA (Thin Layer Activation)

Dr J. Asher, AEA Technology, described the TLA method of on-line measurement of wear from engineering components. Components are treated to make their surfaces mildly radioactive to a depth of between 10 and 100 micrometres. Measurements are made using computers and gamma radiation detectors. Wear measurement of internal components can be made from outside the engine or machine. Sensitivity is better than 1 per cent of the irradiated layer depth.

One application was owing to a difficult problem of piston ring rotation and sticking.

LIF (Laser Induced Fluorescence)

Professor Arcoumanis, Imperial College, advocated the use of LIF as a non-intrusive technique for directly measuring the lubricant film thickness between piston rings and liners in reciprocating engines while running. Some molecules, particularly polycyclic aromatic hydrocarbons, will fluoresce for up to l00ns when irradiated by light. In a Castrol-sponsored PhD student project, the measurements were made using optical fibres in the cylinder liner of a Lister Petter PHW1 direct injection, single cylinder, 2,000rpm, diesel engine. The blue coloured light source was an argon-ion 300mW laser. Laser light travelled down the optical fibre to the oil film and the fluorescence light with a different wave length travelled back the same way along with some reflected laser light. No clear relationship between lubricant viscosity and film thickness under the top piston ring was found. Additive chemistry appeared to be a more important factor. Residual films were seen on start-up. The oil film under the top compression ring was always less than 5 micrometers thick but never fell to zero.

Environmental considerations

Mr C.I. Betton, Burmah Castrol Trading, reviewed the impact of lubricants on the environment. He considered in turn the use of base mineral oils, polyol esters, poly alpha olefins, hydrocracked mineral oils and additives, concluding that, in practice, the formulated oil did not present a toxic hazard. A table of aquatic toxicity of additives was given. Estimates in 1985 were that, of the total EU sales of 4.5mt of lubricating oil sold:

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  15 per cent were recycled;

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  17 per cent were burned;

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  13 per cent were unaccounted for; and

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  2 per cent went down the drain.

Estimates in 1992 of oil input into the North Sea were:

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  shipping (illegal discharges) 132,000mt;

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  accidental spills 48,000mt;

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  atmospheric deposition 36,000mt;

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  road run-off 146,000mt;

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  natural seepage 28,000mt;

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  exploration 8,000mt.

Studies of roadway run-off, offshore pollution and biodegradability were reported. The best environmnental options for oil disposal were listed, including re-refining, processing for use as a fuel, burning in cement kilns and road stone coating plants, and gasification.

Unacceptable disposal methods were burning in space heaters, re-refining using acid clay technology and road oiling.

The present surprisingly small environmental impact of lubricant losses could be further reduced when required by the inevitable legislative pressure.

More than 50 delegates attended the seminar, a good level of support at a time when some technical meetings are being cancelled owing to lack of support. There were useful discussions of the papers which added to the benefit derived by the delegates. Volumes of the papers will be available for purchase from the IMechE.

Bill Wilson