To familiarize the Customers with terms and marks related to piston rings as well as parameters essential for their correct functioning, we have provided information about particular rings types, structural details, coil springs types, coatings as well as materials used for production of MAR-MOT piston rings.
- Piston rings and their application
- Basic parameters
- The piston ring types
- Construction details
- Piston ring joint types
- Coil spring types
- Materials
- Coatings
Piston rings and their application
Piston rings are metal seals which are to prevent leakages and gas blow-by between the combustion chamber and the crankcase as well ensure heat transfer from the piston to the cylinder. Moreover, the piston rings prevent the oil excess necessary for correct functioning of the system piston ring – piston – cylinder from leaking to the combustion chamber and create a uniform oil film on the cylinder working surface (see Fig. 1.3).
In order to do that, the piston rings must be in contact with the piston working surface and with side surface of the piston groove. Radial action is provided mainly by natural spring force of the ring. Gas pressure in the combustion chamber increases radial (to the piston groove) and axial thrust, which improves the ring compression properties. Axial pressure alternates between the upper and the lower piston groove surface due to the action of gases, inertia and the friction force (see Fig. 2.3).
The piston ring operational properties, that is their compression efficiency, result from the structure of the engine, piston and cylinder, their finish, applied oil and fuel and above all their technical quality and properly selected piston ring types.
Basic parameters
For appropriate and unambiguous understanding of the definitions given below, please have a look at the following figures (see Fig. 17.1))
| PARAMETER | SYMBOL | UNIT |
|---|---|---|
ring width a) parallel sided rings – distance between the ring side surfaces measured in any point perpendicular to the reference surface (see Fig. 1.1, Fig. 2.1 and Fig. 18.1) b) keystone rings – distance between the ring side surfaces measured in a6 distance from the piston ring side (see Fig. 3.1) |
h1
h3
|
mm
mm
|
radial wall thickness – distance between the inner and outer ring surface (see Fig. 4.1 and Fig. 19.1) | a1 | mm |
total free gap a) distance between the ends of the ring in free state, measured in the middle of the radial width (see Fig. 5.1) b) on the piston rings with internal lock the gap in free state is measured at the point marked as p (see Fig. 6.1) | m p | mm mm |
closed gap – distance between the ends of the ring closed in the cylinder with nominal diameter appropriate for the ring, measured in the narrowest point of the gap (see Fig. 7.1) | s1 | mm |
tangential force – the force required to maintain the one-piece (see Fig. 8.1) as well as multi-piece (see Fig. 9.1) rings in closed state | Ft | N |
ovality – the difference between two perpendicular diameters d3 and d4 when the ring is in closed state. It can be positive (d3 > d4) as well as negative (d3 < d4) (see Fig. 10.1) | U | mm |
light tightness – light tightness of the working surface edge of the ring installed in the cylinder with its nominal diameter (see Fig. 11.1) | percents | |
taper on periphery – the taper is an intentional deviation from the perpendicularity of the ring working surface to the base surface (see Fig. 12.1) | degrees | |
barrel on periphery – the barrel is the intentional working surface convexity deviation from the perpendicular line in reference to the base surface (see Fig. 13.1) | t2, t3 | mm |
coating thickness – distance between the outer coat surface and the surface of the material, of which the ring is made (see Fig. 14.1) | mm | |
keystone angle – the angle between both side surfaces of the keystone ring (see Fig. 15.1) | degrees | |
land width – height of the working surface contacting the piston surface in the oil rings (see Fig. 16.1) | h4, h5 | mm |
The definitions given above are conforming to ISO 6621/2 – Internal combustion engines – Piston rings – Part 2: Inspection measuring principles.
The piston ring types
Piston rings can be divided into three groups depending on their function. The groups are described below with specification of the most common piston rings from these groups.
Compression rings
The rings designed mainly to prevent gas blow-by between the piston side surface and cylinder working surface. The rings act as the combustion chamber seal. It is assumed that the rings are responsible for 80% of compression.
| DESCRIPTION | ISO DESIGNATION* | PL DESIGNATION** | FIGURE |
|---|---|---|---|
| straight-faced (rectangular ring) | R | R |  |
| taper-faced rectangular ring | M | RS |  |
| straight-faced keystone ring 6° | T | T6 |  |
| straight-faced keystone ring 15° | K | T15 |  |
| straight-faced half keystone ring 7° | HK | T7J |  |
Scraper rings
Rings without holes, designed to scrap oil excess as well as the carbon deposit remains from the cylinder working surface. The rings act also as the second compression ring.
| DESCRIPTION | DESIGNATION ISO* | PL DESIGNATION** | FIGURE |
|---|---|---|---|
| scraper ring (stepped) | E | ZW |  |
| scraper ring (stepped), taper-faced | EM | ZS |  |
| napier ring (undercut step) | N | N |  |
| napier ring (undercut step), taper-faced | NM | NS |  |
Oil control rings
Rings with holes or equivalent openings to scrap the oil excess from the cylinder working surface and to drain it to the crankcase through openings in the piston. The rings are designed as one-piece (without coil spring), two-piece (with coil spring) and multi-piece rings, the so called oil control rings, used in petrol engines and divided into three groups, depending on the construction (the coil spring shape): U / S / H – flex.
| DESCRIPTION | ISO DESIGNATION* | PL DESIGNATION** | FIGURE |
|---|---|---|---|
| slotted oil control ring | S | OW |  |
| bevelled-edge oil control ring | D | OD |  |
| double-bevelled oil control ring | G | OJ |  |
| coil-spring-loaded slotted oil control ring | SSF | OWE |  |
| coil-spring-loaded bevelled-edge oil control ring | DSF | ODE |  |
| coil-spring-loaded double-bevelled oil control ring | GSF | OJE |  |
| coil-spring-loaded bevelled-edge oil control ring, chromium-plated and profile ground | DSF-C | OE |  |
| coil-spring-loaded bevelled-edge oil control ring, chromium-plated, not profile ground | DSF-CNP | OE |  |
| expander/segment oil control ring | ES | U / S / H – flex |  |
* ISO 6621/4 – Internal combustion engines – Piston rings – Part 4: General specifications
** PN-90/S-36507 – Silniki spalinowe – Pierścienie tłokowe żeliwne o średnicach do 200 mm
Construction details
By implementing the piston ring cross section modification consisting in additional shaping of its edge, it is possible to influence contact ability of the ring working surface to the cylinder working surface. The phenomenon is defined as the piston ring torsion. Depending on the construction detail location on the piston ring edge, the torsion may be positive (IF and IW), increasing pressure of the lower piston ring edge on the cylinder working surface, or negative (IFU and IWU), providing increased pressure of the upper piston ring edge on the cylinder working surface. Piston rings of this type are used in diesel and petrol engines mainly as compression rings (possibly scraper rings).
| DESCRIPTION | ISO DESIGNATION** | FIGURE |
|---|---|---|
| peripheral edges chamfered | KA |  |
| inside edges chamfered | KI |  |
| internal bevel (top side) | IF |  |
| internal bevel (bottom side) | IFU |  |
| internal step (top side) | IW |  |
| internal step (bottom side) | IWU |  |
* ISO 6621/4 – Internal combustion engines – Piston rings – Part 4: General specifications
Piston ring joint types
The piston ring joint is a cut along the ring radius. The cut can be of different shape depending on the working conditions and the tasks for which the ring is designed. In two-stroke engines protections (notches) are used in the piston ring joints to avoid axial rotation of the ring on the piston (i.e. locks B4 and B9). The rings with angle (B2), hook (B3) and overlapping (B8) joints are used in hydraulic devices.
| DESCRIPTION | DESIGNATION | FIGURE |
|---|---|---|
| butt joint | B1 |  |
| angle joint | B2 |  |
| hook joint | B3 |  |
| joint with internal notch | B4 |  |
| overlapped joint | B8 |  |
| joint with side notch | B9 |  |
Coil spring types
The coil springs are integral part of two-piece oil-control rings. The springs are used to increase pressure of the oil-control ring working surfaces (lands – see Piston rings basic parameters) on the cylinder working surface.
In MAR-MOT piston rings mostly the CSE type coil springs are used. The spring is characterised by variable lead and ground surface which provides uniform pressure distribution on the whole piston ring circumference and, thanks to increased pressure between the ring and the spring, it facilitates cooperation of the elements preventing seizing of the spring in the oil control ring groove.
The coil spring types used in piston rings, their marking and technical drawing are given below.
Additionally, the photographs present the correct way of the CSE type spring installation in the ring groove (see photo. 34.2).
| DESCRIPTION | ISO DESIGNATION* | FIGURE |
|---|---|---|
| coil spring with constant pitch | CSN | rys. 31.2 |
| grounded coil spring with constant pitch | CSG | rys. 32.2 |
| grounded coil spring with variable pitch | CSE | rys. 33.2 |
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* ISO 6626 – Internal combustion engines – Piston rings – Coil-spring-loaded oil control rings
Materials
Piston rings are subject to variable mechanical conditions, high temperature and high pressure. The factors mentioned above may cause deformations, loss of elasticity and premature mechanical wear, which as a result leads to deterioration of the ring-piston-cylinder system operational properties.
To guarantee optimal and long-lasting operation the MAR-MOT piston rings are made from the following materials.
Grey cast iron
|  | ||||||||||||||||||||||||||||||||
| 100:1 | |||||||||||||||||||||||||||||||||
 | |||||||||||||||||||||||||||||||||
| 500:1 (trawiony HNO3) |
Ductile cast iron
|  | ||||||||||||||||||||||||||||||||||
| 100:1 | |||||||||||||||||||||||||||||||||||
 | |||||||||||||||||||||||||||||||||||
| 500:1 (trawiony HNO3) |
Steel
| MAR-MOT designation: | St-A | ||||||||||||
| ISO 6621-3 designation: | MC 63 | ||||||||||||
| application: | compression rings | ||||||||||||
| chemical composition [%]: |
| ||||||||||||
| mechanical properties | |||||||||||||
| hardness: | 50~60 HRC | ||||||||||||
| bending strength: | 1700~2200 MPa | ||||||||||||
| modulus of elasticity: | ≥205 GPa |
| MAR-MOT designation: | St-B | ||||||||||
| ISO 6621-3 designation: | MC 68 | ||||||||||
| application: | pierścienie olejowe składane (blaszki) | ||||||||||
| chemical composition [%]: |
| ||||||||||
| mechanical properties | |||||||||||
| hardness: | 45~55 HRC | ||||||||||
| bending strength: | 1400~2000 MPa | ||||||||||
| modulus of elasticity: | ≥205 GPa |
| MAR-MOT designation: | St-C | ||||||||||
| ISO 6621-3 designation: | MC 67 | ||||||||||
| application: | expander/segment oil control ring (expander) | ||||||||||
| chemical composition [%]: |
| ||||||||||
| mechanical properties: | |||||||||||
| bending strength: | 1000~2000 MPa | ||||||||||
| modulus of elasticity: | ≥190 GPa |
Coatings
The MAR-MOT piston rings are supplied with hard coatings as well as protective coatings, depending on the Customer’s requirements. The coatings are specified and described below.
Hard coatings:
To increase the piston ring and the cylinder working surface life, the hard coatings are used. They include: chromium and molybdenum coating.
Chromium coating (Cr)
The chromium coating is the most popular coating characterised by high abrasion resistance. It is used mainly on working surfaces of the compression rings with the highest temperature and pressure and the oil control rings due to the high unit pressure.
Molybdenum coating (Mo)
Molybdenum coating is characterized by extraordinary abrasion resistance and efficient heat conduction, which facilitates heat transmission from the piston to the cylinder and, due to the slightly porous surface which absorbs the oil molecules it prevents the piston ring from seizing in the cylinder.
Protective coatings:
To improve the operational properties and to protect the piston rings against corrosion the following protective coatings are used: phosphate coating, tin coating and oxidation.
Phosphate coating (P)
Phosphate coating is a soft coating enabling quicker running in of the piston rings in the early stage of operation and protects the surface against corrosion.
Oxidation (Ox)
Oxidation is applied mostly on the surfaces of the multi-piece oil control ring side plates. The coating is thin and protects the surface against corrosion.
Tin coating (Sn)
The Tin coating is very soft and facilitates the ring operation on the piston by lowering the friction force.