Thermal efficiency improvement of internal combustion engines (ICEs) is one of the most important challenges. Engine oil improvement can reduce friction loss and churning loss in ICEs significantly, and it is also effective to improve fuel economy in electrified vehicles such as hybrid vehicles (HV). Viscosity reduction of engine oils can reduce the churning loss, but thinner oil film might raise the concern in reliability due to wear. Advancement of engine design such as direct-injection with turbocharger (DI-TC) requires further development of engine oil technology. It is also important to develop engine oil industry standard such as ILSAC GF-6 and JASO in order to deliver appropriate products for customers.
Technology development was conducted to accomplish both suppression of irregular combustion (Low Speed Pre-Ignition (LSPI)) in DI-TC engines and friction reduction at the same time. The 0W-16 oil and the 0W-20 oil achieved 1.0% of fuel economy improvement (FEI) and 0.5% FEI respectively in comparison to previous 0W-20 product (Fig.1., Fig.2.).
For further viscosity reduction by 0W-8 grade, new oil technology to reduce boundary-lubrication regime, oil film forming polymer, was developed. This resulted in additional 0.7% FEI compared with 0W-16 viscosity grade. Prize recipients also significantly contributed to JASO GLV-1 development to define 0W-8 viscosity grade first in the world in two years.
Fig.1．New Products Fig.2. FEI% of New Products
2. Technical Details
2.1 GF-6 0W-16/0W-20 Engine Oil Technology
Ca detergent has tendency to increase LSPI. Partial replacement of Ca detergent with Mg detergent can reduce the LSPI occurrence. Mg detergent interferes adsorption of Molybdenum di-thio-carbamate (MoDTC) on surface. It was found that boron-containing dispersant was effective to suppress the negative influence from Mg detergent. Following mechanism was confirmed by surface analysis of test pieces after friction rig tests.
(1) MoTDC shows friction reduction by building MoS2 film on phosphate film (POx) derived from di-alkyl-di-thio-phosphate (ZnDTP). (2) Hard particles of MgCO3 contained in Mg detergent remove the phosphate film by abrasion and prevent MoS2 film formation. (3) Addition of boron-containing dispersant forms even harder phosphate-borate film (POx-BOx). MoS2 film can be built on this film and maintain friction reduction effect. (Fig.3)
As shown in Fig.4, the Mg detergent causes the increase of friction coefficient. The boron-containing dispersant can prevent the friction increase even with the presence of Mg detergent. These formulation technologies accomplished LSPI suppression, engine detergency, and fuel economy at the same time.
Fig.3. Mechanism of friction reduction Fig.4. Friction Test Result
2.2 GLV-1 0W-8 Engine Oil Technology
Based on the finding of the interaction between boron-containing component and MoDTC, new boron-containing detergent was developed to maximize low friction film formation under mixed-lubrication regime. Oil film forming polymer was also developed to reduce the boundary lubrication regime by forming polymer film between sliding surfaces (Fig.5., Fig.6.). These two new developments delivered 0.7% FEI at 0W-8 compared with 0W-16. This oil met the new JASO GLV-1 standard (introduced in October 2019) first in the world.
Fig.5. Oil Film Formation Fig.6. Oil Film Thickness
Newly developed 0W-16 and 0W-20 engine oil products were introduced in 2017, and these were qualified as ILSAC GF-6 later. JASO GLV-1 0W-8 engine oil product was introduced in 2020, and contributed further improvement of thermal efficiency of ICEs. Engine oils can be widely applicable not only for new vehicle models but also for existing vehicle models in-use in the market. This also contributes to the CO2 reduction significantly.
*1 Member, Toyota Motor Corporation (1, Toyota-cho, Toyota, Aichi, 471-8572)
*2 Toyota Motor Corporation (1, Toyota-cho, Toyota, Aichi, 471-8572)