While there is a gradual spread of cars driven by electric motors, there are still a number of studies and developments in progress to further improve fuel efficiency of IC engine cars. Tribology is very important, and the automobile is one of the most visible applications that show the great impact of tribology on energy and environmental issues facing the globe. A number of fundamental studies on friction materials and lubrication for automobiles are conducted and published. However, most of us do not know well about the applications of these fundamental studies and tribological matters in practical systems, because they are not available in the form of scientific or technical papers.
The Japanese Society of Tribologists (JAST) publishes monthly the Journal of Japanese Society of Tribologists. Every issue contains a collection of review papers that introduces research and development trends on a selected theme. The themes are sometimes very basic and sometimes very practical. The journal uses Japanese language, and the majority of the readers are domestic members of JAST. However, many of these review papers are interesting and include information worth introduced to engineers and researchers outside Japan. The February issue in 2016 focused on “Automotive Resource Savings and Energy Savings” in Commemoration of the 60th Anniversary of JAST. In the issue, the technologies for improving fuel efficiency of cars are shown in the light of various aspects of tribology.
The editorial board of JAST has decided to publish English translated versions of these review papers on automotive tribology in this special issue of Tribology Online. The authors of five among seven review papers kindly accepted this plan. We also have eight original papers on automotive tribology, which have been newly submitted and accepted by peer reviews. We hope the readers in the world will find these papers interesting and useful.
Tribology Online is an electronic online journal publishing peer reviewed original papers. For the benefit of authors and readers, Tribology Online is open to everyone for free access through the Internet. In order to ensure effective use of copyright, all papers published in Tribology Online in and after this special issue are open access and to be distributed under the terms of the latest version of the Creative Commons attribution license: Creative Commons Attribution- NonCommercial- NoDerivatives (CC BY-NC-ND).

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The study conducted by a study group on the prediction of car fuel savings realized through tribology development organized by the Japan Society of Tribologists showed that the fuel loss caused by tires was 7.5% for constant velocity running at 60 km/h. Furthermore, the Japan Automobile Tyre Manufacturers Association estimates that the contribution rate of the reduction in tire rolling resistance to fuel savings is between 20 to 25% in the case of constant velocity running, and these figures cannot be explained if we consider the fuel loss of tires alone. The study group adopted a threefold improvement in fuel economy, as it reduces both the exhaust and cooling losses in the same ratio. In addition, the study group proposed a novel retroactive effect, which states that reduction of downstream elements’ losses decreases the upstream elements’ losses. The contribution rates of the individual elements to the fuel consumption can be calculated by taking the threefold improvement and the retroactive effect into consideration.

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According to the International Energy Agency (IEA), it is predicted that internal combustion engines (ICEs) will function as important power sources for plug-in hybrid vehicles or hybrid electric vehicles until 2050. To improve ICE performance, the utilization of technology that is focused on reducing heat loss and friction loss is critical. To achieve friction reduction, the application of low-viscosity oil for pistons and the realization of lower coefficients of friction between piston rings and cylinders serve as effective boundary conditions. The application of these conditions generates high seizure resistance as well. Many research and evaluation studies have been performed using the single cylinder floating liner engine method. However, there are no engines for which a quantitative evaluation is possible, so the causes behind friction losses are difficult to determine, and a mechanical analysis is difficult to perform. In this study, the trend of power sources analyzed by the IEA, prediction of the reduction in the coefficient of friction in automobiles by the European research center, and the concepts of each engine sliding surface condition are introduced. In addition, the difficulty of quantitative evaluation is also described, and the importance of a standard for the floating liner engine is explained.

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Currently, the demand for improved fuel consumption rates in automobiles has increased because of growing concerns related to global warming and energy conservation. Because of these concerns, the number of automobiles equipped with hybrid systems, also known as hybrid electric vehicles (HEVs), is increasing. In hybrid electric vehicles, a reduction in engine bearing friction is required to improve fuel economy, particularly under the operating conditions with low oil temperature. In this study, two cutting-edge technical methods used for reducing engine-bearing friction are described. One method utilizes a resin coating as a low-friction material, which is including graphite in the coating instead of MoS₂ and CaCO₃. The other utilizes a Partial Twin Groove (PTG) bearing as a surface design. PTG bearing has an effect of quick warm-up oils on the bearing surface by reducing oil leakage. Narrow grooves located on the bearing surface enhance the suction of oil from the outside of the bearing to the inner surface. As a result, engine oil recirculates in the bearing and the oil temperature is quickly raised, particularly under cold conditions. This design offers a fuel efficiency of approximately 0.14%.

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The role of rolling bearings is to support rotational motion with low friction. Although the friction is low in rolling bearings, it does not mean that friction does not occur at all. Many rolling bearings are used in automobiles. Therefore, if it is possible to reduce the friction loss in the rolling bearings, it can contribute to reducing the fuel consumption of the automobile. In this paper, we introduce representative techniques for reducing the friction in automotive bearings.

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Reducing automobile tire rolling resistance is an effective measure for achieving fuel cost savings and reducing CO₂ emissions, which contributes to global warming. In this report, mechanisms to represent tire rolling resistance and the methods to control it will be explained.

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This study proposes a novel approach for controlling frictional performances at paper clutch systems by a combination of organic friction modifiers (OFMs). The OFMs are commonly used for modifying the frictional properties at the paper clutch. Although the effect of the OFMs tends to be dependent on temperature due to their working mechanism based on the surface adsorption, the frictional properties are preferable to be stable in all the operating temperature for the consistent and precise clutch control. Aiming to modify the temperature dependence of the OFM effect, an OFM with an Advanced Concept (FMAC) was newly developed, and the impact on the clutch frictional performance was investigated using Low Velocity Friction Apparatus (LVFA). Only with the conventional OFM, the friction values experienced excessive reduction at 80 or 120°C, while an optimal property was achieved at 40°C. In the presence of the FMAC, it was possible to inhibit the conventional OFM selectively at the high temperature conditions preventing the friction reduction, leading to the ideal frictional property in all the temperature conditions. The surface analysis revealed that the FMACs were capable of adsorbing on the substrate more intensively at high temperature, which should be the reason of the temperature dependent competitive effect of the FMACs.

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Wear of DLC coating has been great concern in DLC/Steel sliding surface lubricated by molybdenum dithiocarbamate (MoDTC) containing lubricants. However, in order to design lubrication system with DLC and MoDTC, the mechanism of this issue is needed to be clarified. In this study, the roller-on-disc oscillation friction test by SRV friction tester was conducted to provide the hypothesis about the mechanism of wear of DLC with MoDTC. In friction test, DLC coatings (a-C:H and ta-C) were coated on discs. Counterpart was the SUJ2 bearing steel roller. It was found that MoDTC in lubricants accelerated the wear of the both two DLC coatings and it was dramatic especially for a-C:H coating. In order to clarify the enhancement mechanism of MoDTC for wear of DLC coatings, the surface analysis on DLC coatings and steel roller was conducted with XPS (X-ray Photoelectron Spectroscopy) and surface profile meter. As a result, sp² ratio increased layer was observed on the a-C:H coating with XPS and the surface roughness of wear track was larger than out of wear track after sliding with MoDTC containing lubricant. On the other hand, ta-C did not show the increasing of sp² ratio and the surface roughness of wear track decreased after sliding with and without MoDTC containing lubricant. For both DLC coatings, tribofilm generated from MoDTC was also observed on the counter SUJ2 roller surface with XPS. Depth profile of chemical elements of the tribofilm on the steel roller with XPS contributed to clarify the distribution of Molybdenum compound as MoO₃ and Mo₂C. Top surface was MoO₃ and the interface between the SUJ2 roller and tribfilm was Mo₂C. The authors provide the hypothetical model about a tribo-chemical reaction between both DLC coatings as ta-C or a-C:H and tribofilm from MoDTC.

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The first piston ring application of a hydrogen-free diamond-like carbon (H-free DLC) coating to a mass-production gasoline engine reduces friction. It is found that the friction reduction effect between the cylinder bore and piston is 18%. When H-free DLC is applied to top ring, friction is over 10% reduced. The effect is larger than H-free DLC applied to oil rings. Because DLC as low friction coating is effective in a thin oil film and there are few oil at a large area near piston Top Dead Center (TDC). The friction reduction effect is enhanced when lower viscosity engine oil is used. In this study, the adhesion strength of H-free DLC coating on piston rings is also improved. It is shown that the relation between the adhesion strength and 2 coating parameters; i.e. providing substrate roughness and coating thickness. Adhesion strength is verified by test results using actual parts considered to piston ring environment by gasoline engine. Piston rings with low friction and high reliability are developed.

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In this study, we investigated tribological properties of “tetrahedral” Si-containing hydrogenated DLC coating (TMS coating) during sliding against steel or cast-iron lubricated with engine-oil containing MoDTC and ZnDTP additives. TMS coatings derived from only tetramethylsilane were prepared using PACVD with high-bias process. TMS coating had the highly carbon sp³ bonded structure induced by Si (over 20 at.%) with high hydrogen content (30 at.%). TOF-SIMS analysis showed that TMS coating could form the additive-derived tribofilm on both of the non-ferrous coating surface and the ferrous counter surface to promote Mo-sulfide formation effectively, leading a low friction coefficient. The tribo-chemical reactions were related not only to Mo-sulfides formation, but also reduction of Mo-oxides. Mo-oxides could react with DLC material and induce hydrogen evolution and transformation into further weak carbon structure, causing an increasing wear of hydrogenated DLCs. TMS coating showed no significant wear in spite of plenty hydrogen content. Si induced sp³ bond and could act so as not to induce clustering of the sp² phase. This could inhibit increasing wear of the DLC in the presence of MoDTC. The effectiveness of TMS coating in friction and wear behavior was shown to depend on tribo-chemical reactions and transformation of carbon bond structure.

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DLC films are of significant interest for automobile parts, because they possess the potential to improve the friction property under oil lubricated conditions. In order to obtain further low friction, we focused on the combination of boron doped DLC (B-DLC) and the Mo-trimer as a lubricating oil additive, and the influence of the boron concentration on the friction property of the B-DLC films was evaluated by a block on ring type test. Compared to DLC films without boron, the B-DLC films exhibited an extremely low friction coefficient below 0.03 ranging from 2 to 12 at% in boron concentration under the lubricating condition using the engine oil containing the Mo-trimer. It was found that the surface roughness of the B-DLC films rubbed in the oil containing the Mo-trimer became particularly smooth below 2 nm measured by an atomic force microscope. On the wear surface of the B-DLC films, Mo₃S_7^- derived from the Mo-trimer was selectively confirmed as having a uniform distribution by time-of-flight secondary ion mass spectrometry. Based on the obtained results of surface measurements and analyses, it was suggested that the B-DLC films exhibited a low friction property primarily caused by the nano-scale smooth surface, disturbing the adsorption of the other additives and the transfer reaction products from the counter parts by forming a selective adsorbed Mo-trimer film on the B-DLC surface.

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The frictional force measuring method of a piston-piston ring cylinder system at engine operation condition by using floating liner method has some device structures, such as using plural one dimensional load cells (load washer) and six or eight three-component (triaxial) force sensors method. However, in some cases the qualitative and quantitative values of the measured data differed due to differences in the structure of the device, differences in settings and differences in the method of processing the measured data. This research described a typical device structure of the floating liner engine, and described the case of the operation data processing method of piston frictional force, the reproducibility of data, and the zero point of frictional force. Furthermore, by the combustion pressure seal method of the floating liner, it showed clearly that momentary swing back force occurs in a frictional force curve and considered the cause.

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In the area of transmissions, continuously variable transmissions (CVTs) have been adopted for cars with various engine displacements mainly in Japan and North America because of their better fuel economy and smooth shift feeling. Continuously variable transmission fluids (CVTFs) with lower viscosity and optimal friction property for chain and pulley will be effective for improving fuel saving performance in CVT. In this study, compared with a market chain CVTF, the developed oil had a lower viscosity, a higher metal friction coefficient, and higher clutch transmission capacity, while its anti-seizure property was better than a market chain CVTF. As to higher metal friction coefficient, small islands were observed on friction surface by atomic force microscopy (AFM) .It is supposed that those islands promote a shift to the boundary lubrication regime, contributing to a higher friction coefficient. The developed oil containing some additives demonstrated 10% higher variator torque capacity compared with a commercial CVTF.

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Texturing on sliding surface of resin seal ring was attempted to improve lubrication in this study. Numerical analysis of hydrodynamic flow at the sliding surface was conducted by Reynolds equation using the finite differential method. Frictional torque was measured at various sliding speeds and pressures and the oil film thickness was measured using LIF (laser-induced fluorescent) technique. Theoretical and experimental results are discussed and compared. The results show that the developed textured seal ring can be used at higher pressures and sliding velocities compared to a standard design thanks to the better lubrication that results from the hydrodynamic structures. Moreover, the textured seal rings show remarkably low frictional torque. Compared to conventional rings, friction is reduced by up to 70% with optimized texture dimensions.

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