Table of Contents
What are piston rings?
A piston is a cylindrical component of an engine that slides back and forth in the cylinder out of the forces aroused by the combustion process.
The piston consists of the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings.
A piston ring is an expandable split metallic ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine usually used to provide a seal between the piston and the cylinder wall.
Piston rings of marine engines are made with the help of pot casting method. In this method, a short cylinder of an oval cross-section is made and the piston rings are then cut and machined from the cylinder.
With this method, a homogeneous and balanced casting is formed around the entire circumference of the ring.
Why are Piston rings needed?
Pistons are usually equipped with piston rings that fit into ring grooves in the piston wall and they assure a snug fit of the piston into the cylinder.
Piston rings are the essential components in the modern internal combustion engine in their optimum tribological performance which have a controlling influence in minimizing frictional power loss, fuel consumption, oil consumption, blow-by, and harmful exhaust emissions.
Main uses of a piston ring
Maintenance of Compressed Gas Between the Piston and the Cylinder Wall– The basic function of the piston ring pack- the collective name for the three or more rings mounted on the piston- is to seal the cylinder in such a way that the combustion gas generated at the time of ignition does not leak from between the piston and the cylinder.
The efficiency of the marine diesel engine depends upon the effective sealing between the piston and liners. Leakage would result in an insufficient power supply. Moreover, it would result in more fuel consumption hampering the efficiency thereafter.
Make Necessary Optimum Lubricating Oil film to Prevent Scuffing– The piston goes up and down innumerable times. A small amount of lubricating oil is poured over the pistons for smooth functioning and avoiding friction arising from metal to metal contact. The piston rings adjust and maintain the proper amount of lubricating oil and provide the necessary lubricating film to prevent scuffing.
Improve Heat Transfer from the Piston to the Cylinder Wall– The temperature inside the piston goes as high as 300 degrees Celsius during ignition. The heat build-up can damage the piston. The piston rings furnish heat transfer from the piston to the cylinder wall and help the excessive heat to escape.
Piston rings also prevent the piston from knocking on the cylinder wall by supporting the piston in the cylinder. Rings act as a barrier that prevents frictional force and eventually engine failure.
The piston rings should have tension properties with which sealing effect is produced.
In olden days, the tension in the engine room was produced by hammering around the circumference of the circular ring.
This tension is achieved by two methods:-
1) Thermally Tensioned Ring
2) Oval Pot Cam Turning Method
Thermally Tensioned Ring
The thermally tensioned ring is one of the cheapest methods to induce tension in the piston rings but is restricted to smaller engines. In this method, the piston ring is machined from the circular pot to the required cylinder diameter.
After making the ring, a gap is cut and a metal piece is inserted in the gap which expands the ring and induces tension in the ring.
After expanding, the ring and the distance piece is placed inside the oven to relieve any stresses induced during the process. The major disadvantage of this process is that the ring loses its tension because of the heat of the engine.
Oval Pot Cam Turning Method
The oval pot cam turning method is expensive but the rings produced from this method retain their tension while working in heat of the engine.
The rings are machined in a cam turning lathe. By changing the cam shape and the oval form, the pressure distribution around the ring is changed and tension is induced.
Video On How Piston Rings Are Made?
Material and Piston Designing Consideration
Piston rings should be made up of material that possesses these properties:
1. Low Friction Coefficient– This property enables the marine piston rings to slide along the cylinder’s surface and make them capable to withstand high pressures at various temperatures under limited lubrication conditions.
2. High Modulus of Elasticity- This property of material enables marine piston rings to provide the required specific pressure upon a cylinder’s surface. Further, it avoids the seizure of piston rings when they are moving in contact with a cylinder.
3. High Yield Strength and Hardness Grey modified cast iron possesses the above properties. Therefore, piston rings are generally made up of cast iron or steel. Moreover, the properties depend on the structure formed during the casting process.
A small amount of chrome, copper, Molybdenum, RIAS, and tin additives are used for plating which enhances the properties.
Piston Rings Design and Arrangement Consideration
The design of the piston rings and the arrangement of rings vary as per the engine type, size, and typical piston assemblies.
Ring Gap
A gap needs to be cut at the ends so that it can be expanded, slip over the piston head, and can be released while releasing it into the piston groove.
This also ensures circumference- wise expansion of the ring at higher temperatures.
The gap is cut with precision because excessive gap results in blow-by and scuffing of rings while lesser gap causes piston rings to butt at higher temperatures which leads to excessive and non-uniform pressure on the cylinder walls and causes excessive wear.
Ideal Gap – 0.30 mm to 0.35 mm
Types of Piston Ring Gap
A. Square Cut
B. Angle Cut
C. Step Cut
D. Title Joint
E. Hook Step
F. Mitre Step
G. Seal Cut
Design Consideration as Per Piston Ring Types
1. Compression Rings – These usually have a rectangular or keystone shaped cross-section. Upper compression rings have a barrel profile for the periphery, while the lower compression rings usually have a taper napier facing.
2. Oil control rings- Made from a single piece of cast iron, multiple pieces of steel, or steel/iron with a helical spring backing that have two scraping lands of various detailed form.
Ring Pack Design as Per Engines
1. 2 Stroke Petrol Engines – 2 Plain Cast Iron Rings
2. 4 Stroke Diesel Engines – Chrome-plated top ring 2 taper faced iron rings 2 scraper rings
3. 4 Stroke Petrol Engines – Plain iron top ring Taper faced iron ring Scraper ring
4. Large 2 Stroke Diesel Engines – 5 plain iron rings
Detailed design of the piston rings depends upon the required elastic pressure, the ring to piston fitting stress, and the ring to cylinder fitting stress.
Plating of piston rings
The piston rings used in the marine engines should be harder than the liner material in which they are used. To provide additional strength to the piston rings, some materials like chromium, molybdenum, vanadium, titanium, nickel and copper are added.
Chrome plating is one of the most common methods of surface treatment. It is generally used on the running surface of the piston rings and on the landing surfaces i.e. in ring grooves.
Chromium has the advantage of high wear resistance, low friction and corrosion resistance properties. The coating done should be of high quality and be able to operate under all conditions inside the engine without damaging or peeling off or breaking.
Plasma coating
It is also one of the methods used for plating the rings. In this method, a gas mixture is passed through an arc generated between the tungsten electrode and a water-cooled copper tube.
A very high temperature is thus generated and the gas molecules start to disintegrate. The plasma state-level carbides and ceramic are sprayed as a fine powder which melts and coats the ring surface.
This plasma coating provides better properties than those provided by the chrome plating.
The disadvantage of chrome and plasma coating is that the thickness of the coating and the bonding to parent metal is limited.
A new method of laser hardening treatment is also done. This produces a wear-resistant layer which is several times thicker than that of the conventional coatings.
Sometimes, copper is plated directly over the chrome layer of the ring. The plating thickness is very thin and the life of this is long enough to cater for running in period. Plasma coated rings are graphite coated to cater to the running-in periods.
Other platings that can be added- Molybdenum, Copper, Nickel
Copper – Copper plating is done either directly on the base metal or on the chromium plating. Tufftriding and phosphating of cast iron also help to ease scuffing problems during running-in. A thin final surface layer of the copper plate is applied thereafter.
Flamesprayed Molybednum– Flamespray is used to deposit molybdenum on the peripheral surface of a piston ring, the resultant coating contains a mixture of molybdenum and oxides of molybdenum.
This coating offers more resistance and is harder than wrought molybdenum which does not contain oxides or porosity. This is considered as the best coating for scuff resistance but tends to oxidize and break up in long-life applications.
Nickel Plating– Nickel ceramic plating is done by conventional electroplating. It was found that Ni–P–BN has better self-lubricating properties than Ni–P–SiC or Ni–P–Si3N4 coatings and exhibits low wear when slid against cast iron and aluminium liner. Ni-P-BN coating is done by screen printing or spray process.
If you liked this article, you may also like to read:
- A Guide to stroke marine engine components
- Types of Piston ring and maintenance.
- Piston Skirt, Piston Rod and Trunk Piston
- How is Marine Engine Repair Done Onboard a Ship?
- Reason for cylinder liner wear and how to measure it?
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