The continuing pursuit for better fuel efficiency stands behind many recent advancements in engine technology. “Downsize and charge” has become the major development paradigm alongside broad acceptance of fuel stratified injection, variable valvetrain systems, cylinder deactivation, 48V electric auxiliaries, powertrain hybridization, use of low-friction coatings and advanced surface finishing methods in component manufacture, etc. The introduction of higher power densities (in excess of 100 kW/L and 200 Nm/L in modern engines) raises performance requirements for engine oil, paralleled by the move towards lower viscosity grades and lower SAPS. This leads to an increasing use of synthetics due to their superior performance. New additives types are also coming to the market to address new performance challenges. In particular, superlubricity additives should be mentioned.
Since the 1980s, fuel economy requirements are included in many engine oil performance specifications, such as ILSAC, and stand behind the introduction of new service categories, such as API SN-RC, API FA-4, ACEA A5/B5, ACEA C5, and future ACEA A7/B7, C6, F8, F11. categories. Lower viscosity oils are broadly adopted due to their superior energy efficiency [2,3]. However, the lower the viscosity, the thinner the lubricant film – and therefore, the greater the risk that some critical engine components will experience boundary friction. The major problem with boundary friction is actually not friction itself but wear. Deployment of anti-wear additives and friction modifiers help alleviate the problem but doesn’t eliminate it completely. High levels of particle contamination, eg due to soot, dust or wear debris, may seriously aggravate the wear problems associated with the deployment of lower viscosity oils [4].
Advanced surface finishing methods, such as self-lubricated hard coatings, mirror-like thermally sprayed bores, mechanochemical surface finishing, helical slide honing, - as well as availability of high-sensitivity testing rigs and “digital twin” simulation tools - create new opportunities for engine tribology optimisation.
The number of official test sequences has also been increasing, sometimes to such an extreme that the testing begins to transform from a development aid to a development bottleneck. The creation and adoption of recent oil performance categories, such as PC-11
and ILSAC GF-6 turned into excruciating marathons costing the industry hundreds of million of dollars. According to industry insiders, HDMO programs at large lubricant companies may easily cost a few million dollars. PCMO programs are less expensive but still can climb to 1 million, depending on base oil interchange and viscosity grade read across guidelines applicability. And despite all the efforts and thousands hours of testing, new oils have brought some new problems to life.
Earlier this year, R.I. Taylor et al have carried out motored strip down friction tests on a 2.0 L i4 M111 gasoline engine and have found high levels of mixed/boundary friction for engine speeds below 1500 rpm [5]. The authors have demonstrated that mixed/boundary friction takes place in the valvetrain system and timing chain, but there is also a significant contribution from the piston assembly for engine speeds less than 1000 rpm. It has also been demonstrated that a SAE 15W-40 lubricant produces more friction at high engine speeds, where a lower viscosity SAE 0W-12 lubricant gives significantly lower friction. However, at low engine speeds, the pattern is inverted. These observations conform to the results presented earlier for 1.6L i4 GDI engine [6], see Fig. 1.
Figure 1: The effect of oil viscosity on engine friction for a 1.6 L i4 GDI engine
Very interesting results proving the validity of these concerns have been recently presented by P. Kleijwegt et al [7], who studied the effect of HTHS viscosity on the friction of Volvo D12D truck engine (see Fig. 2). Heavy Duty Engine Oil (HDEO) lubricants with differing HTHS viscosities were benchmarked against a legacy 15W-40 oil (HTHS 3.7 cP).
Continued on page 22 LUBE MAGAZINE NO.154 DECEMBER 2019 21
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