9 Piston rings

9.1 Purpose and function of piston rings

Piston rings fulfill the following important tasks for engine operation:

Sealing of the combustion chamber, in order to maintain the pressure of the combustion

gas. The combustion gas must not enter the crankcase, and oil must not reach the

combustion Chamber.

Transfer of heat built up in the piston to the cylinder surface.

Controlling the oil balance, where a minimum of oil is needed on the cylinder surface to create

a hydrodynamic situation, while oil consumption needs to be kept as low as possible.

These tasks are performed by the piston rings as follows:

1st piston ring: Compression of combustion air or gas mixture, and the resulting gas pressure in the

combustion cycle, transfer of generated heat to the cylinder surface and scraping of the residual oil

from the cylinder surface.

2nd piston ring: Support of the remaining gas pressure due to blow-by past the 1st piston ring,

scraping of oil from and transfer of generated heat to the cylinder surface.

3rd piston ring: oil control ring.

The following points must be considered in the design of piston rings:

Scuffing: Partial seizure process leading to severe wear, poor sealing, increased oil

consumption, and increased blow-by levels.

Ring flutter: Occurrence of radial and axial vibrations. The gas pressure acting radially on the

piston ring in the groove root drops off, and the piston ring is no longer tightly guided.

Ring sticking: At excessive piston temperatures, the oil in the ring grooves cokes up, so that

the piston rings get stuck.

High oil consumption: Determining factors are the conformability of the piston rings, and

deformation and honing of the cylinder bore.

Friction: The piston rings have a large part in the friction of the piston group.

Piston rings are mostly single-piece, slotted, and self-tightening. Their basic shape is a thin walled,

axially short circular cylinder. To generate the necessary contact pressure against the cylinder wall,

the piston rings are in the shape of an open circular spring. The spring force acting radially in the

installed state is greatly amplified by the gas pressure behind the piston ring.

9.2 Principles of operation

The piston ring fulfills various tasks. To maintain the cycle of the thermodynamic process, it must be

ensured that the gas pressure in the cylinder is maintained and does not drop off. This is the task, in

particular, of the first piston ring. One premise is that lubrication, acting as a “gas-sealing oil

pressure barrier,” is present. Tests by Felix Winkle had demonstrated that without such a fluid layer,

higher gas pressures cannot be sealed against moving parts.

The motion of the piston ring develops a hydrodynamic pressure that is greater than the gas

pressure. This is why it is so important for the function of the piston ring that the cylinder surface is

sufficiently coated with lubricating oil.

Coarse metering of this oil quantity is performed by the oil control ring, while fine control is achieved

by the first piston ring.

The arrangement of several piston rings in series forms a system of throttle chambers, in which the

pressure of leaking gases is further decreased by throttling and swirling. It is unavoidable, however,

that a small portion of combustion gases, compressed mixture, or air will pass by the piston rings

and enter the crankcase (blow-by gas).

The width and tolerance of the ring gap has a significant effect on the blow-by rate. The piston ring

seals against the side faces like a valve. Leakage points are most noticeable at the running face,

because the blow-by gas breaks through the oil film. This amount of blow-by gas should, of course,

be minimized. Nevertheless, gases comprising up to 5% of the displacement enter the crankcase

with each cycle.

9.3 Forces and stresses

9.3.9 Forces and temperatures on piston rings

Forces acting on a piston ring in the piston ring groove: po: gas pressure above the piston ring pu: gas pressure below the piston ring FSrad: radial force and counterforce FSax: axial force and counterforce caused by friction MT Twist: countermoment of the piston ring

Figure 9.1

The radial pressure applied by the piston ring to the cylinder bore is small in comparison to the gas

pressure applied via the ring groove in the piston to the inner side of the piston ring (Figure 9.1). In

diesel engines, with their high gas pressures, the piston ring running face is, in many cases, shaped in

such a way, that the gas pressure building up on the running surface acts against the pressure from

the back of the ring, which reduces the contact pressure of the ring onto the cylinder surface.

Despite every effort, the piston ring cannot form a perfect seal. Leakage occurs at the ring gap, at

the side faces, and at the contact faces to the cylinder.

Piston rings are highly stressed mechanically, thermally, tribologically, and corrosively.

Piston rings must fulfill their task at combustion gas temperatures of up to 2,600 °C and combustion

pressures of up to 260 bar.

About 25 to 60% of the heat absorbed by the piston is transferred to the cylinder wall by the piston

rings.

The limit of the temperature load on the first piston ring is reached when the oil in the first piston

ring groove starts to carbonize due to excessive temperature. The movement of the first piston ring,

which is a requirement for its reliable function, is thereby limited. It can no longer maintain its

proper contact to the cylinder surface, and ring sticking occurs. One ringbased solution is the

keystone ring (Figure 9.2).

Figure 9.2: Rectangular

ring (left) and keystone

ring (right), side

clearances

Effective piston cooling is essential, as it significantly helps to reduce the thermal loads of the piston

rings. Depending on the type of piston cooling, the heat flowing into the piston rings can be reduced

to less than one-third.

During one revolution of the crankshaft, the piston moves from the top to the bottom (BDC) and

back to the top dead center (TDC). It travels twice the stroke distance. During this motion, it is

accelerated and decelerated. Due to its inertia, the piston ring moves in the ring groove relative to

the piston. Due to frictional forces at the cylinder surface, it tends to tilt as it moves (Figure 9.1).

Upon impact, it can exert high forces on the side faces of the ring groove.

In diesel engines, this effect is increased further by the high gas pressure. Wear of the groove side

faces degrades the function of the piston rings, until it causes ring scoring, ring fracture, and, as a

result, piston seizure.

The high gas temperatures to which the first piston ring, in particular, is subjected, even if only for a

short time, make its function more difficult, in that together with the gas pressure, they burn off or

blow away the lubrication between the first piston ring and the cylinder surface. This puts the first

piston ring into a tribologically critical operating condition.

Combustion gases contain corrosive components, the worst of which is sulfur dioxide (SO2).

Sulfur dioxide promotes corrosive wear of the cylinder surface, mainly in the region of the

TDC. The ring running face is also affected. Poorer fuels (heavy fuel oils) used to run large bore

engines (medium-speed four-stroke and slow-speed two-stroke engines) intensify this problem and

require special measures on the ring, piston, and cylinder. The motion of the ring pack generates

friction and thus mechanical losses. Between 10 and 20% of the total engine friction loss is caused

by the ring pack.

Friction is determined mainly by the following factors:

surface pressure (tangential load and gas pressure),

ring width,

coefficients of friction of the contact surface (coating),

running face shape (barrel shape),

Surface condition of the counterpart (cylinder surface).

Reduction of friction losses can be achieved primarily by minimizing surface pressure, i.e., by

reducing the tangential load and ring width.

9.4 Types of piston rings

The various tasks of the piston rings can no longer be met by a single ring type. Thus, it became

necessary to classify the piston ring types in use today.

Figure9.3: Classification of piston rings

9.4.1 Piston ring over view

In recent years, the width of the piston rings has been drastically reduced. It is now only 1.2 to 1.0

mm for gasoline passenger car engines. For comparison: In the 1930s, the ring width was two to

three times greater. Axially lower piston rings have lower mass, require less installation space, and

allow a lower compression height of the piston. They also show better operating behavior in terms

of friction, ring flutter, and blow-by.

The first piston ring is closest to the combustion chamber. This means that it is exposed to the

highest mechanical and thermal loads. In order to ensure good temperature resistance, nodular cast

iron or steel materials are used as the base material in these piston rings. They are also coated or

specially treated, in order to reduce friction and wear.

The first piston ring for highly loaded commercial vehicle diesel engines generally has a keystone

shape The symmetrically barrel-shaped piston ring is preferred for use in highly stressed engines,

due to its better run-in characteristics and good lubricating oil and blow-by control. Due to the

barrel shape the contact surface area on the cylinder bore is reduced, which leads to greater contact

pressure as a consequence of the more narrow contact surface with the cylinder bore. Oil control is

improved by the wedge effect on account of its shape.

The second piston ring has a double function, depending on its type: It must seal against gas

pressure while scraping oil off the cylinder wall; at the same time, sufficient lubrication of the first

piston ring must be ensured. The second piston ring features a reinforced design with regard to its

scraping effect, based on its additional function as an oil control ring. Its effectiveness is based on

the contact pressure, the shape of the scraping surface (land), and the method of removal of scraped

oil. This requires good conformability, i.e., the ability to adapt as smoothly as possible to the

continuously changing cylinder deformation while maintaining the required contact pressure against

the cylinder wall.

9.4.2 Compression rings

9.4.2.1 Rectangular ring

The basic shape of the first piston ring is a rectangular ring with a straight-faced running face, also

known as an R-ring (Figure9.4). Its task is to seal against the gas pressure in the combustion

chamber. Rectangular rings are used for normal operating conditions, primarily as first piston rings

in gasoline engines.

Figure 9.4: R-ring

9.4.2.2 Rectangular ring with taper-faced running face

A slight taper to the running face of the piston ring increases its effectiveness. Contact between the

piston ring and the cylinder wall is reduced to a narrow line. This line contact increases the contact

pressure of the piston ring against the cylinder bore and ensures that contact is maintained with the

bore, even if the cylinder is deformed. 1.4.2 Rectangular ring with taper-faced running face

The run-in phase is thereby shortened. It also provides a downward scraping effect, which supports

the oil control function of the oil control rings. This type of ring, also called a taper face ring or M-

ring, is typically employed as a second piston ring (Figure 1.6).

Figure 9.5: M-ring

9.4.2.3 Keystone ring

Keystone rings are divided into single- and double-

sided types. On a single-sided keystone ring, also

known as an ET-ring, only one side has a taper-

faced design; on a double-sided keystone ring, also

known as a T-ring, both sides do (Figure 9.9).

These piston ring geometries reduce oil carbon

build-up in the ring groove. The radial motion of

the piston ring in the ring groove keeps it clear of

oil carbon. Keystone rings of both types are used as first piston ring.

Figure 9.9: ET-ring (top) and T-ring (bottom)

9.4.2.4 First piston ring with barrel-shaped surface

The barrel shape, better hydrodynamic lubricating conditions are achieved, and the axially shorter

contact surface at the cylinder surface improves sealing. In

addition, the negative effects of cylinder deformations

during engine operation can be better compensated.

Piston rings of this type, also known as R-ring B, are used as

first piston rings (Figure 9.7).

Figure 9.9: R-ring B

9.4.3 Oil control rings

In addition to the task of the compression rings to seal off the combustion chamber from the

crankcase, there needs to be some mechanism to distribute the oil evenly onto the liner. The

number of oil control rings in a ring pack is one or two. Normally a single oil control ring is sufficient

but on occasions a second ring may be required. The appearance of the oil control ring differs from

that of the compression ring. (See figure 9.8)

The oil control ring is perforated by slots in the peripheral direction which provides a way for the

excess oil to leave the ring pack area. The scraped oil is collected in the oil control ring groove and

transported through the piston back to the crankcase. The

scraped oil may run through the possible gap between the

liner wall and the piston skirt. With the latter alternative,

the oil is forced in front of the oil control ring. The oil control

rings may have a coil spring inserted, as the pre-tension of

the ring is not sufficient in all instances. The additional force

on the oil control rings causes them to have the most

extreme lubrication conditions, even though these are the

rings that control the oil film.

Figure 9.8

9.4.3.1 Slotted oil control ring

The slotted oil control ring contacts the cylinder surface with two lands. Slots are machined in the

center web between the two lands, through which the scraped-off oil can enter the ring groove

behind the slotted oil control ring, and from there can enter the interior of the piston through drilled

bores (S-ring). The smaller total contact surface increases the contact pressure against the cylinder

surface. This is necessary, because no gas pressure can build up behind the slotted oil control ring.

The contact pressure of the oil control rings thus arises from their tangential force. Further reduction

in the size of the land surfaces resulted in the beveled-edge oil control ring (D-ring), with chamfers

on the lands, and the double-beveled oil control ring.

9.4.3.2 Spring-loaded oil control ring

1.4.11.1 Coil spring loaded ring

To improve conformability and increase contact pressure, oil control rings are preloaded with a

cylindrical spring (coil spring) on the inside of the ring (SSF-ring). The ends of the spring are butted

against each other (Figure 9.9). Oil control rings with widths of 2.0, 2.5, 3.0, and 3.5 mm are typical in

new diesel engines. As with springless piston rings, beveled-edge oil control rings with coil spring (DSF-

ring) and double-beveled oil controlling with spring (GSF-ring) are used (Figure 9.10).

Figure 9.9: SSF-ring

Figure 9.10: DSF-ring (top), GSF-ring (bottom)

One of the most important characteristics of oil control rings is the specific surface pressure.

Generally speaking, the greater the specific surface pressure, the lower the lubricating oil

consumption. In order to reduce lubricating oil consumption during engine run-in, the two-part

lands of the piston rings are angled at the running face. The angled running face provides greater

contact pressure during run-in, which reduces the normally higher lubricating oil consumption in this

stage. After a certain period of operation, the angled profiles wear down and take on a cylindrical

shape.

I-shaped ring

The I-shaped oil control ring, made of steel (Figure 9.99), is a new development. In contrast to oil

control rings made from cast iron, these are made from I-shaped steel wire. It is rolled, cut to length

in this shape, and then finished. In order to

increase wear resistance, the I-shaped oil control

rings are usually nitrided. I-shaped oil control

rings are recommended particularly for high-

speed diesel engines, as well as for highly loaded

diesel engines.

Figure 9.99: I-shaped oil control ring made of steel

9.9.9 selection of piston rings

We have 4 compression rings and 1 oil control ring as follow:

First piston ring: single-sided keystone ring.

Second piston ring: piston ring with barrel-shaped surface.

Third and fourth piston ring: Rectangular ring with taper-faced running face.

Fifth piston ring: The I-shaped oil control ring.

9.5 Materials, coatings, and surface treatment

Note: all materials, coating and surface treatment processes will mention below its recommended

for our diesel engine rings.

Piston ring materials require:

Good running and boundary lubrication capability,

Elastic behavior,

Mechanical strength,

High strength at elevated temperatures,

High heat conductivity, and

Good machinability,

Wear resistance.

Cast iron

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

Piston ring coatings and surface treatments provide improved wear resistance and seizure

resistance, along with low cylinder wear and favorable lubrication properties. Nanotechnology

processes are also employed in this connection. Nitrided cast iron, chrome-based coatings such as

hard chrome and chrome-ceramic, plasma-sprayed molybdenum, plasma-sprayed cermet and high-

speed flame-sprayed coatings meet the most demanding service life and run-in requirements.

9.5.1 Selected materials for piston rings and coatings:

For first piston ring

Martensitic nodular cast iron as a base material:

Martensitic nodular cast iron

Alloying elements: Ni, Mo

ISO 6621-3: Subclass 53

First piston ring with high ultimate tensile strength

Bending strength: min. 1,300 MPa

Hardness: 28 to 42 HRC

Chrome-ceramic

Galvanically applied

For second ,third and fourth piston rings

Martensitic alloyed gray cast iron

High wear resistance

Alloying elements: Mo, Nb, V, W

ISO 6621-3: Subclass 25

High ultimate tensile strength with good wear resistance for second compression rings in

diesel engines

Bending strength: min. 650 MPa

Hardness: 37 to 45 HRC

Oil control ring

Steel alloyed with chromium and silicon

ISO 6621-3: Subclass 62

Heat-resistant springs in two-piece oil control rings in diesel

Tensile strength: 1,800 to 2,000 MPa

9.6 design calculation and details

9.6.1 Dimensions calculations of piston ring

Radial thickness (t1)

𝑡1 = 𝐷√3 𝑃𝑤

𝜎 = 130√

3∗0.042

110 =4.4 mm

Pw: gas pressure is limited between 0.025 N/mm2 to 0.042 N/mm2,

D: cylinder bore in mm,

𝜎: Allowable bending (tensile) stress taken from 85MPA TO 110 MPA.

Axial thickness (t2)

𝑡2 = 0.7 ∗ 𝑡1 = 0.7*4.4 = 3 mm.

Gap of the ring at Free State

G1= 3.5*t1 =15.4 mm

Gab at installation state

G2= 0.002*D =0.26mm

9.6.2 Stress analysis calculation

9.6.2.1Stresses on piston ring

Piston rings are subjected to the greatest stress during installation, when they mounted on the

piston. The installation stress (SA) and the stress arising during engine operation (Sw) can be

calculated as follows:

Sw : Stress during engine operation

Sa: Installation stress

E: Young’s modulus of the piston ring material

ty : Radial distance from the neutral axis to the ring running face

m : Free gap in relaxed state

s1 : Gap clearance in installed state

d1 : Nominal diameter of cylinder liner

a1 : Radial dimension of piston ring

m1: Installation opening (normally, m1 = 8 · a1)

𝑆𝑎 =8

3.𝜋 .

100∗109∗(0.0044−0.0022)∗(8∗0.0044−0.00154)

(0.130−0.0044)2 = 398 Mpa

𝑆𝑤 =8

3∗𝜋 .

100∗109∗0.0022∗(0.00154−0.00026)

(0.130−0.0044)2 = 179 Mpa

For complex piston ring cross sections, such as two-piece oil control rings, the stresses are typically

determined by finite element analysis.

.9.6.2 2 Conformability

In the course of a combustion cycle, the heat flow changes, which results in high temperature

gradients in the piston and the cylinder liner. These, in turn, cause varying distortions in the cylinder

surface. The piston ring needs to adapt to these deformations, in order to keep blow-by and oil

consumption low. The conformability of a piston ring is expressed by the coefficient k.

In the case of Hill and Newman (1985) conformability relation

𝐾 =𝐹𝑡 ∗ (𝑑1 − 2 ∗ 𝑇𝑦)2

4 ∗ 𝐸 ∗ 𝐼 𝐹𝑡 = 𝑝 ∗ 𝑅 ∗ 𝑤

K: Coefficient of conformability

Ft: Tangential load of the piston ring

I: Axial moment of inertia of the piston ring cross section

P: radial pressure cause by installation

W: axial thickness of the ring

R: radius of the cylinder

∴ 𝐹𝑡 = 0.078 ∗ 106 ∗ 0.065 ∗ 0.003 =92.59 N,

∴ 𝐼 = 0.00443∗0.003

12= 2.1 ∗ 10−11 𝑚𝑚4 ,

∴ 𝐾 =15.21∗(0.130−2∗0.0022)2

4∗100∗109∗2.1∗10−11= 2.85 ∗ 10−4

The greater the value of the coefficient k, the better the conformability of the piston ring. The ability

of a piston ring to make contact with the cylinder surface can be estimated as follows, according to

Tomanik:

𝑈𝑚𝑎𝑥 =𝑘 ∗ 𝑑1

10 ∗ (𝑖2 − 1)

Umax: Maximum cylinder deformation that the piston ring can adapt to

I: Order of deformation (i = 9, 5, 3…)

𝑈𝑚𝑎𝑥 =2.85∗10−4∗0.130

10∗(22−1)= 1.235 ∗ 10−6 M

9.6.2.3 Radial pressure

There is a radial pressure cause of the spring force due to the installation of the ring (pretension

force) and the pressure of the gas acts on the rear of the piston ring, the sum of these forces cause

the radial pressure acts on the cylinder liner surface.

the radial pressure which cause by the installation process :

𝑃1 =𝐺 ∗ 𝐸

7.06𝐷(𝐷/𝑡1 − 1)3

P: Wall pressure, lbf/sq.in.

G: difference between the free and closed gaps, INS.

E: Young's Modulus, ibf/sq.in.

D: Ring Dia. ins.

T1: radial thickness, ins.

∴ 𝑃1 =0.596∗14.5∗106

7.06∗5.118∗(5.118

0.1732−1)3

= 10.277 Ibf/sq.in = 0.078 Mpa

Radial pressure caused by gas behind the ring P1

Assume the pressure under the ring equal brake mean effective pressure

Bmep = 𝑏𝑟𝑎𝑘𝑒 𝑝𝑜𝑤𝑒𝑟

𝑣0=

372.845∗103

𝜋

4∗0.156∗0.1302∗

2000

60∗2∗6

= 18 bar = 1.8 Mpa

P (total radial pressure) = P1+P2= 1.878 Mpa

3.6.9 Determination of friction loss by the piston rings

9.6.3.1Calculation involving only the residual stress of the installed piston ring

There are 4 compression piston rings for each piston (+1 oil control ring which is a different design).

Assume all rings have the same contact area with cylinder surface.

The nominal contact area per cylinder is 4 x ring width x outer periphery (80mm bore):

4 x 0.003x 3.14159 x 0.130 =4.9 ∗ 10−3 𝑚2

If the normal pressure is (p1) 78000N/m2 the total radial (or normal) force per cylinder is

= 4.9 ∗ 10−3 ∗ 78000 = 380 𝑁

Assume Friction coefficient μ = 0.10. An according to test for Glidewell and Korcek

Which the test was the almost the same condition for our piston ring material and coatings

∴friction force = μ *radial force =0.1*380 = 38 N

Assuming the engine crank is rotating at 5000 rpm

Mean piston speed = 2∗𝑠𝑡𝑟𝑜𝑘𝑒∗𝑁

60= 2 ∗ 0.156 ∗

2000

60= 10.4 M/sec,

So in 1 sec the piston will travel 10.4 m

∴ The power loss in one piston = 10.4*38=395.2W

FOR TOTAL ENGINE FRICTION POWER LOSS DUE TO RING INSTALLATON = 395.2* NO OF CYLINDER

= 2.3 KW.

9.6.3.2 Calculation involving only the residual stress of Effect of Gas Pressure behind Piston

Rings.

If it is assumed that the brake mean effective pressure in the cylinders is about 1.8MPa, then this

pressure can be assumed to be the mean pressure pushing the rings against the cylinder walls.

This gives rise to a force between each piston and cylinder of = 4.9 ∗ 10−3 ∗ 1.8 ∗ 106 =8.8 KN

∴ Friction force = 8800 *0.1 =882 N

The power loss in one piston = 882* 4.09 =9.9 kW,

For the total power loss due to the Effect of Gas Pressure behind Piston Rings = 9.9 *6 =22 kW.

So the total friction power loss by piston ring = 64 kW

The percentage of loss power in piston ring from the total power = 14 %