KNOW YOUR RINGS…
We’re not talking about the wedding variety; we mean those precious little gems that make your engine run sweet. We quizzed our resident egg-spert Neil on the topic…
FB: What are piston rings?
NC: Piston rings are a type of seal. They are fitted to the outer circumference of an engine’s pistons. They are positively located by machined grooves – invariably near to the piston’s top surface (the piston crown). Piston rings are usually made from either steel or iron, but occasionally more exotic metals such as chromium or titanium alloys are employed.
Modern piston rings are coated or plated with a myriad of special materials to increase surface hardness, wear and corrosion resistance and to improve their surface fififinish. Seal rings, almost identical in design to piston rings, are also utilised to seal rotating and sliding shafts in other applications too.
FB: So what do rings do?
NC: A piston ring’s primary function is to prevent pressure in the combustion chamber from leaking past the piston. However, not all piston rings are the same; four-stroke engines usually have three separate piston rings per piston. The upper ring for sealing combustion pressure and the bottom ring to control the amount of engine lubrication oil left clinging to the surface of the cylinder bore, while the centre ring does a little of both these tasks. Piston rings also perform the important task of conducting heat away from the piston to the cylinder bore surface. It’s worth noting that two-stroke engines only have two (or only one on racing engines) piston rings per piston.
FB: How do rings form a seal while moving?
NC: To provide an effective seal, the piston rings must be in firm contact with one side of the piston groove and the cylinder bore surface. Contact with the piston groove side comes from gas pressure forces, inertial forces and from friction forces originating from ring to bore contact. Contact between the rings and bore surface is a function of the ring’s inherent spring force and gas pressure forces pushing the ring radially
outwards. The ring’s inherent spring force exerts around 0.12 to 0.25N/mm2, but the additional force from combustion gas pressure can increase this figure exponentially.
FB: How do rings last so long?
NC: Contact between the ring and cylinder bore is not entirely metal-to-metal. A thin film of oil should be present on the bore surface to facilitate what is known as ‘mixed friction’. The thickness of this oil film is critical; too thin and the rings will scuff, rapidly wear and also damage the bore; too thick and the engine will smoke and suffer from excessive oil consumption. The oil film also aids sealing and helps to keep the rings cool. Curiously, excessively thick oil films actually increase ring (and piston) friction, reducing engine power output and fuel efficiency. The ideal oil film thickness depends on engine design, piston, ring, and bore material, ring and bore surface finish, piston speed and the oil itself. FB: How do you get the right oil film thickness? NC: As the piston and rings move down the bore they encounter engine oil on the bore surface. This oil will be present as a result of either splash and ‘throw-off’ from the crank and con rod, or from an oil jet. The amount of oil clinging to the bore can be substantial and it is the job of the bottom ring to scrape the bulk of the oil off the bore surface (scraper or oil control ring). The centre ring also removes an amount of oil from the bore, and it is the combined effect of the bottom and centre ring that leaves a precise thickness of oil that enables the heavily loaded top ring to perform its job correctly.
FB: Why are there three rings?
NC: The top compression ring frequently has a rectangular cross-section with a slightly barrelled outer face. Occasionally, wedge-shaped keystone rings or Dykes L-shaped rings are used as compression rings. Keystone rings are less prone to jamming in the piston groove and Dykes compression rings are only really used in two-stroke engines. The centre ring either has a chamfered outer face or more frequently a stepped recess at the bottom of the outer face. Napier rings provide even more effective oil scraping thanks to an undercut recess. The bottom oil control ring tends to be of a two or three-piece design incorporating expander springs that increase ring to bore force and keep the upper and lower sections of the ring apart.
FB: Why are rings coated?
NC: Piston rings are treated and coated for several reasons; to achieve a specific surface finish; to increase surface hardness and reduce wear and friction; to reduce surface hardness and aid lubrication (during run-in); to reduce corrosion and to prevent the ring from sticking in its groove. Some coatings are applied to stop the ring reacting with its surroundings or ‘micro-welding’ itself to the piston.
FB: What coatings are used?
NC: Tin or copper can be used to reduce surface hardness, as can zinc or manganese phosphate. These coatings are relatively soft and are removed by wear after the run-in period. An extremely thin layer or iron oxide (black oxiding) affords the ring some corrosion resistance. The ring sides can be chrome plated or nitrided to ‘armour’ them against the piston grooves. This helps to reduce ring sticking, and ‘micro-welding’ of the ring to the piston. Ring outer faces can be coated with molybdenum or metal-ceramic composites. Molybdenum forms a hard crystalline structure during its application process. Specialist coatings even include chrome plating containing microscopic diamond crystals. Nitriding and nitrocarburising are the most popular methods of increasing ring hardness in motorcycle four-stroke engines. Heating the ring to 450-5500ºC in a nitrogen atmosphere produces a hard ‘case’ on the surface of the ring.