Overview of automotive engine friction and reduction trends–Effects of surface, material, and lubricant-additive technologies

Abstract

Abstract The increasing global environmental awareness, evidenced by recent worldwide calls for control of climate change and greenhouse emissions, has placed significant new technical mandates for automotives to improve engine efficiency, which is directly related to the production of carbon dioxide, a major greenhouse gas. Reduction of parasitic losses of the vehicle, powertrain and the engine systems is a key component of energy conservation. For engine efficiency improvement, various approaches include improvements in advanced combustion systems, component system design and handling—such as down-sizing, boosting, and electrification—as well as waste heat recovery systems etc. Among these approaches, engine friction reduction is a key and relatively cost-effective approach, which has been receiving significant attention from tribologists and lubricant-lubrication engineers alike. In this paper, the fundamentals of friction specific to the environments of engine components tribology are reviewed, together with discussions on the impact of developing vehicle powertrain technologies, surface and material technologies, as well as lubricant and additive technologies on promises of continuing friction and wear reduction trends. The international accords on climate change require further gains in fuel efficiency and energy sustainability from all industry sectors including those in the automotive and the broader internal combustion engine industries, and the latter encompass off-highway, power generation, marine, and rail industries as well. This paper focsuses on friction reduction in mainly automotive engines, however. The paper starts with a clarification of the common descriptors of mechanical losses and friction in the engine, followed by the topic of lubrication fundamentals such as lubrication regimes. Then the lubrication of the contacting surfaces in each of the major engine subsystems is discussed in turn. These subsystems include the piston assembly: ring-pack/liner, piston-skirt/liner, and piston-pin/connecting-rod contacts; connecting rod and crankshaft bearings; and the valvetrain subsystem. The relative contributions to total friction from the various subsystems are discussed, with the piston-assembly contributing to about half of the total friction. The remainder of the friction comes from the crankshaft, connecting rod, camshaft bearings, and the valvetrain oscillating parts. The bearings are in predominantly hydrodynamic lubrication, in contrast to the valvetrain oscillating components, which are characterized to be mostly in the mixed/boundary lubrication regimes. Despite the title of the paper, a section on emerging powertrain technologies—including that of combustion in gasoline and diesel engines—is also given in the context of the trend towards clean and efficient propulsion systems. The impact of these developing technologies on the reduction of friction and parasitic losses via component, material, and lubricant deisgn will be discussed. These technologies include gasoline direct injection (GDI), turbocharged, and hybrid vehicles and will generate unique green environmental opportunities for future propulsion systems. These technologies are critical to meet fuel economy and reduced emission targets. Specifically, this paper will address the impact of these emerging technologies on future lubricant requirements and advanced tribology research. The connection between these lubricant and tribological requirements will be illustrated by briefly describing the basic lubrication and friction processes at the major engine components incorporating the emerging technologies. Lastly, besides new hardware and material science changes, several advanced additives such as advanced friction modifiers, antiwear additive chemistries, low viscosity lubricants, and the introduction of new VI Improvers all represent possible tribological solutions to the challenge of meeting more stringent energy efficiency requirements and environmental legislation. As original equipment manufacturers (OEMs) seek to accomplish these goals, hardware and emission system changes will place new demands and even greater stress on engine oils. At the same time, engine durability, performance and reliability are of primary importance to vehicle owners and operators. The final section of this paper will discuss the future trends of engine friction reduction and wear control by surface modification such as friction-reducing coatings or surface textures in engine components. The impact of surface coatings or surface textures on engine friction will be reviewed. In addition, the OEMs and lubricant formulation manufacturers will need to respond with novel engine oil technologies formulated to protect the engine, keeping the emissions system working at the optimal fuel economy, while retaining engine durability. In brief, the paper (i) reviews the characteristics of component friction in the environment of the internal combustion engine and the relevant design considerations, (ii) addresses the impact of emerging technologies on engine friction and the tribological changes and requirements, especially on lubricant and additives, and lastly (iii) discusses the interactions between lubricant-additive formulations and material surface engineering, and their effects on friction, wear and engine durability. The increasing importance and interplay between synergistic advancements in component design, material and surface engineering, and advanced lubricant-additive formulation will be fully illustrated.