04-04-2013, 04:23 PM
Piston ring tribology
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Abstract
The tribological considerations in the contacts formed by the piston skirt, piston rings and cylinder liner have attracted much attention over several decades, not least indicated by the large number of articles published on this topic in recent years. Recent studies include modelling, miniaturised experimental work and full-scale engine testing.
This literature survey, covering over 150 references, aims to shed new light on the tribological issues related to the piston assembly. The work is intended as a compact reference volume for internal combustion engines in general, with particular emphasis on diesel engines.
Central topics discussed in this work are the basic functions of the piston and the piston rings, the design and the materials of the components, mechanical and thermal loads on the rings, the contact pressure between ring and liner, the sealing action, blow-by leakage, hot gas erosion damage, exhaust emissions, the lubrication conditions and the influence of combustion products, the coefficient of friction and the friction force, the wear of the sliding surfaces and surface technology for wear reduction.
Introduction
There are two entirely diverse points of view that make dynamic seals particularly demanding in a tribological sense. Firstly, dynamic seals support the cleanliness of a lubricant and a tribological element to be protected from external contamination, and thus contributes to suppressing wear by three-body abrasion caused by contaminant particles. Simultaneously the seal suppresses the leakage of lubricant from the tribosystem, which is an issue of increasing relevance for environmental protection and cleanliness. Secondly, the counter surfaces of a dynamic seal operate under the same tribological laws as any sliding couples, though with the requirements of low friction and low wear and a long service life. Seals for linear motion are particularly challenging as their counter surface smoothness, direction of sliding, speed and lubrication tend to vary more than in the closed contact forming a seal for a rotating motion.
Piston rings
In the early steam engines no piston rings were used. The temperatures and the steam pressures were not as high as the corresponding parameters in today’s internal combustion engines, and the need for considering thermal expansions and clearances was smaller. Increasing power demands required higher temperatures, which caused stronger heat expansion of the piston material. This made it necessary to use a sealant between the piston and the cylinder liner to allow a decrease in the clearance in cold conditions, i.e. when the clearances were at their maximum. Keeping the clearance between the piston and liner wall at a minimum considerably reduces the combustion gas flow from the combustion chamber past the piston.
The first piston rings used in an engine had the sole task of sealing off the combustion chamber, thus preventing the combustion gases from trailing down into the crankcase.
Main functions of piston rings
The functions of a piston ring are to seal off the combustion pressure, to distribute and control the oil, to transfer heat, and to stabilise the piston. The piston is designed for thermal expansion, with a desired gap between the piston surface and liner wall. The 10 rings and the ring grooves form a labyrinth seal, which relatively well isolates the combustion chamber from the crankcase. The position and design of the ring pack is shown in Fig. 2.1. The ring face conforms to the liner wall and moves in the groove, sealing off the route down to the crankcase. The sealing ability of the ring depends on a number of factors, like ring and liner conformability, pre-tension of the ring, and gas force distribution on the ring faces. Piston rings forces are discussed in greater detail in Section 5.2. Some of the combustion chamber heat energy is transferred through the piston to the piston oundaries, i.e. the piston skirt and rings, from which heat transfers to the liner wall. Furthermore, the piston rings prevent excess lubrication oil from moving into the combustion chamber by scraping the oil from the liner wall during the downstroke. The piston rings support the piston and thus reduce the slapping motion of the piston, especially during cold starts where the clearance is greater than in running conditions. The rings are generally open at one location, at the ring gap, hence easily
assembled onto the piston.
Ring categories
Piston rings form a ring pack, which usually consists of 2–5 rings, including at least one compression ring. The number of rings in the ring pack depends on the engine type, but usually comprises 2–4 compression rings and 0–3 oil control rings. For example, fast speed four-stroke diesel engines have 2 or 3 compression rings and a single oil control ring. The oil control rings used in diesel engines are two-piece assemblies and sparkignited engine oil control rings may be three-piece assemblies as well. In addition to the general compression rings and oil control rings there are scraper rings, which have the tasks of both sealing and scraping off the oil from the liner wall.
Piston ring materials and coatings
A piston ring material is chosen to meet the demands set by the running conditions. Furthermore, the material should be resistant against damage even in emergency conditions. Elasticity and corrosion resistance of the ring material is required. The ring coating, if applied, needs to work well together with both the ring and the liner materials, as well as with the lubricant. As one task of the rings is to conduct heat to the liner wall, good thermal conductivity is required. Grey cast iron is used as the main material for piston rings (Federal Mogul, 1998). From a tribological point of view, the grey cast iron is beneficial, as a dry lubrication effect of the graphite phase of the material can occur under conditions of oil starvation. Furthermore, the graphite phase can act as an oil reservoir that supplies oil at dry starts or similar conditions of oil starvation (Glaeser, 1992). Coatings for rings are widely used. One example of such a coating is chromium, which is used in abrasive and corrosive conditions where running conditions are severe. Hard chrome plating is particularly relevant for the compression ring. Piston ring surfaces are, in addition to chromium plating, thermally (plasma) sprayed with molybdenum, metal composites, metal-ceramic composites or ceramic composites, as a uniform coating or an inlay coating material (Mollenhauer, 1997).