Nearly 25 years BEFORE Nicholas Otto invented the spark ignited internal combustion engine (the type of engine most cars use), an English Engineer by the name of John Ramsbottom invented the piston ring in 1852, and he did it for steam engines. So, before we had cars or gasoline, the industrial revolution brought us steam locomotives. Prior to Mr. Ramsbottom’s stroke of genius, steam engine pistons featured grooves packed with hemp or cotton to improve sealing. Of course, this didn’t work that well, which led to John Ramsbottom’s invention of the metallic, split ring design in 1852. Over the next decade, he continued to refine his design, and eventually a steam engine could go 4,000 miles before needing to replace the piston rings. That was a significant improvement in performance and durability. 170 years later, the piston ring is still evolving and enabling greater engine performance and durability.
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Interestingly, Britain is not only the birthplace of the piston ring, but it is also the home of several of advancements in piston ring design. English engine manufacturer D. Napier & Son is often credited for the invention of Keystone style top ring and eponymous Napier style rings. Both developments took place during the 1930’s, and these designs are commonly used today in diesel and gasoline engines respectively.
If that wasn’t enough, the UK is also home of the most advanced piston rings, which are found in Formula 1 engines. With over 50% thermal efficiency, today’s F1 engines are the most efficient combustion engines ever built (rivaling combined cycle gas turbines). That’s nearly double the efficiency of a regular petrol engine and 50% better than a turbo diesel. The folks at Mercedes AMG made some videos about this achievement, which can be found on YouTube.
One of the reasons for the incredible efficiency of these engines lies in the fact that piston rings account for nearly 40% of all engine friction. That makes piston rings the number 1 source of friction in an engine, so the best way to improve engine performance and durability goes right back to Mr. Ramsbottom’s invention - the piston ring. The evolution of piston ring materials and coatings has allowed for innovation in piston ring sizes and designs. Today’s piston rings are dramatically thinner, lighter and stronger than your Grandfather’s piston rings.
For decades, cast iron was the material of choice for piston rings. However, the low hardness of grey cast iron, up to 22 Rockwell C, allowed for higher wear, which shorten engine life. The advent of hard Chrome plating allowed cast rings to achieve a face hardness of 72Rockwell C. These hard chrome rings lasted much longer, but were much tougher to break-in. This led to rougher cylinder bore finishes and many lubrication “tricks” that enabled the hard chrome faced rings to break-in.
By the 1960’s Plasma Moly rings appeared, which replaced the hard chrome face with a softer, porous moly face coating. The much softer Moly coating, 80 on the Rockwell B scale, did not require the rougher cylinder bore finishes or lubrication “tricks” to achieve proper break-in. However, the sprayed on “Moly” coating was susceptible to flaking in higher horsepower applications. Also, the cast iron or ductile iron base ring could only be so thin before they became too brittle.
As such, steel rings that feature PVD(chemically bonded) face coatings were developed in the early 2000’s, which allowed for thinner piston rings with greater strength than the iron forefathers.
With the advent of better piston ring materials and coatings, the potential performance of piston rings skyrocketed!
Because of the instant reduction in friction and increase in durability, professional racing teams quickly adopted the thinner, steel piston rings.
Here’s some real-world proof of that statement. Between my Dad’s 20 years in NASCAR as a driver and my 15 years working for NASCAR teams, I’ve seen the evolution of piston rings firsthand. Back in 2001, the state-of-the-art piston ring package in NASCAR was an .043, .043, 3mm Ductile Moly set, and those rings lasted one 500-mile race. It was typical for the engine to be down 5 to 8 HP after that one race. Today’s NASCAR engines use .5mm , .6mm, 2mm PVD coated steel rings that last over 1,500 race miles without losing a single horsepower!
It’s not just racing engines that take advantage of thin ring technology. OEMs around the world have embraced the efficiency of thinner piston rings. A 1972 Chevy V8 used a 5/64, 5/64, 3/16cast iron ring package. 50 years later, turbocharged, direct injection engines making twice the horsepower per cubic inch of that old Chevy utilize 1.2, 1.0, 2.0mm steel rings.
More efficient piston rings increase horsepower, reduce engine temperature (both water temperature and oil temperature) and extends motor oil life. All of which means engine life and performance increase. I think John Ramsbottom would be proud to see today’s piston rings.
A piston ring is a metallic split ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine.
The main functions of piston rings in engines are:
Most piston rings are made from cast iron or steel.
Design
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Piston ring configurations:Piston rings are designed to seal the gap between the piston and the cylinder wall.[2] If this gap were too small, thermal expansion of the piston could mean the piston seizes in the cylinder, causing serious damage to the engine. On the other hand, a large gap would cause insufficient sealing of the piston rings against the cylinder walls, resulting in excessive blow-by (combustion gases entering the crankcase) and less pressure on the piston, reducing the power output of the engine.
The sliding motion of the piston ring inside the cylinder wall causes friction losses for the engine. The friction caused by piston rings is approximately 24% of the total mechanical friction losses for the engine.[3][4] The design of the piston rings is therefore a compromise between minimising friction while achieving good sealing and an acceptable lifespan.
Lubrication of piston rings is difficult and has been a driving force to improvements in the quality of motor oil. The oil must survive high temperatures and harsh conditions with a high-speed sliding contact. Lubrication is particularly difficult as the rings have an oscillating motion rather than continuous rotation (such as in a bearing journal). At the limits of piston movement, the ring stops and reverses direction. This disrupts the normal oil wedge effect of a hydrodynamic bearing, reducing the effectiveness of the lubrication.
Rings are also sprung to increase the contact force and to maintain a close seal. The spring force is provided by either the stiffness of the ring itself or by a separate spring behind the seal ring.
It is important that rings float freely in their grooves within the piston, so that they can stay in contact with the cylinder.[5] Rings binding in the piston, usually due to a build-up of either combustion products or a breakdown of the lubricating oil, can cause engine failure and is a common cause of failure for diesel engines.[citation needed]
Number of rings
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Sealing is often achieved by multiple rings, each with their own function, using a metal-on-metal sliding contact. Most pistons have at least two piston rings per cylinder.
Automotive piston engines typically have three rings per cylinder.[6] The top two rings—known as compression rings—are primarily for sealing the combustion chamber. The bottom ring—known as the oil control ring—is primarily for controlling the supply of oil to the cylinder wall, in order to lubricate the piston skirt and the oil control rings.[7]
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Ring construction
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The compression rings in an automotive engine typically have a rectangular or keystone shaped cross-section. The upper compression ring typically has a barrel profile for the periphery, while the lower compression ring typically has a taper napier facing. Some engines also use a taper facing for the top ring, and simple plain-faced rings were used in the past.
Oil control rings are typically made from either a single piece of cast iron, multiple pieces of steel, or steel/iron with a helical spring backing to create the tension required for a close seal. Cast iron oil rings and rings with a helical spring backing have two scraping lands of various detailed form. On the other hand, multi-piece steel oil control rings usually consist of two thin steel rings (called rails) with a spacer-expander spring between them to keep the two rails apart and provide radial pressure.
The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore. Ring gap shapes include square cut, angle cut, tite joint, step cut, hook step and mitre step.[8]
History
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Steam engine with 3 piston rings at location D Spring-loaded piston ringsEarly steam engines used a hemp packing to seal the combustion chamber,[9] which caused high frictional resistance and did not provide a very effective seal.
The first use of a piston ring in the cylinders of a steam engine appears in 1825 by Neil Snodgrass, a Glasgow engineer and mill-owner, for use in his own machines. This used springs to keep the seal steam-tight. From use within the mill this was experimented on the steamer "Caledonia" which plied the Gareloch.[10][11]
The modern design of a metallic split-ring was invented by John Ramsbottom in the 1850s. Ramsbottom's initial design in 1852 was a circular shape, however these wore unevenly and were not successful. In 1854, a revised design was claimed to have a lifespan of up to 4,000 mi (6,437 km).[12] This was based on the discovery that a perfectly round (prior to installation) ring with a split in it does not exert an even pressure on the cylinder walls once installed. The revised piston ring was manufactured to an out-of-round shape, so that it would exert even pressure once installed in the cylinder. An 1855 patent documented this change. The switch to metallic piston rings dramatically reduced the frictional resistance, the leakage of steam, and the mass of the piston, leading to significant increases in power and efficiency and longer maintenance intervals.
Engine wear
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Piston rings are subject to wear as they move up and down the cylinder bore, due to their own inherent load and due to the gas load acting on the ring. To minimize this, they are made of wear-resistant materials, such as cast iron and steel, and are coated or treated to enhance the wear resistance. Coatings used in modern motorcycles include chromium,[13] nitride,[14] or ceramic coating made by plasma deposition[15] or physical vapour deposition (PVD).[16][17] Most modern diesel engines have top rings coated with a modified chromium coating (known as CKS or GDC),[13][dead link] which has aluminium oxide or diamond particles respectively included in the chrome surface.
In two-stroke engines, the port design is also an important factor for the lifespan of the piston rings.
See also
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References
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