Mahle GmbH (Ed.) Cylinder Components: Properties, applications, materials

Подождите немного. Документ загружается.

1 Piston rings

1.1 Purpose and function of piston rings

Piston rings fulﬁll the following important tasks for engine operation:

N 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 com-

bustion chamber.

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

N 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 pos-

sible.

These tasks are performed by the piston rings as follows:

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

sure in the combustion cycle, transfer of generated heat to the cylinder surface (see also

Section 1.3.1), and, to a slight degree, 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: Scraping of the oil.

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

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

sumption, and increased blow-by levels.

N 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.

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

that the piston rings get stuck.

N High oil consumption: Determining factors are the conformability (see Section 1.5.1.4) of

the piston rings, and deformation and honing of the cylinder bore.

N 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.

2 1 Piston rings

Axial contact with the ring groove side is substantially generated by the gas pressure applied

to the piston ring side face (Figure 1.1).

When the piston is installed in the cylinder, the piston rings are compressed at their ends to

their gap clearance. In the piston, they are guided in piston ring grooves corresponding to

their dimensions and therefore follow the piston movement. This type, invented in 1854 by

John Ramsbottom, is known as self-tightening and has proved itself from the beginning in

pistons for steam locomotives. It became a basic invention in engine technology, because

reliable sealing of high gas pressures in the combustion chamber was ﬁrst made possible by

this type of ring—up to more than 260 bar today.

The force with which a piston ring presses against the cylinder wall depends mainly on the

difference in diameters of the pre-stressed piston ring and the cylinder. This tangential force

is designed in such a way that the piston ring meets the particular requirements arising from

the combustion process and operating conditions. When the piston ring is installed in the

cylinder, a tangential force is created that in turn generates the contact pressure.

N The radial distribution of the contact pressure is achieved by the shape of the piston ring;

today, piston rings are consistently CNC-turned.

N The radial distribution of the contact pressure depends on the shape of the running

face—straight-faced or taper-faced—and the proﬁle geometry of the piston ring (barrel

shape).

N This is determined by the combustion process.

Figure 1.1:

Forces acting on a piston ring in

the piston ring groove

Light blue: piston ring groove

Medium blue: piston ring

Dark blue: cylinder

Arrows encompassing the piston

ring: forces acting on the piston

ring

: gas pressure above the piston

ring

: gas pressure below the piston

ring

Srad

: radial force and counter-

force

Sax

: axial force and counterforce

caused by friction

T Twist

: countermoment of the

piston ring

1.2 Principles of operation 3

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 1.1). 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 pres-

sure 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 ring materials require:

N good running and boundary lubrication capability,

N elastic behavior,

N mechanical strength,

N high strength at elevated temperatures,

N high heat conductivity, and

N good machinability.

Materials used include untempered and tempered gray cast iron, cast iron with nodular

graphite (tempered), and tempered steel or stainless steel.

To improve running-in characteristics, reduce wear, and prevent scufﬁng, special measures

are taken by coating and reinforcing (protecting) the running surfaces.

Operating behavior depends on many factors, which often makes the optimization of piston

rings difﬁcult and time-consuming:

N type and design of the engine,

N type of combustion, combustion process, pressures, and pressure gradients,

N cylinder design, material, and machining,

N fuel and lubricant,

N piston ring type, material, and running face, and

N operating conditions.

1.2 Principles of operation

As part of the moving boundary of the engine combustion chamber, 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

4 1 Piston rings

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

pressure barrier,” is present. Tests by Felix Wankel 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 crank-

case with each cycle.

1.3 Forces and stresses

1.3.1 Forces and temperatures on piston rings

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 ring-

based solution is the keystone ring (Figure 1.2), developed in the early 1930s by the English

engine manufacturer Napier.

1.3 Forces and stresses 5

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 1.1). 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 introduction of aluminum pistons

for diesel engines used in commercial vehicles at the beginning of the 1930s nearly failed

due to this type of damage, until Dr. Ernst Mahle created an effective solution with the ring

carrier as a groove protector (Figure 1.3).

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 sur-

face. This puts the ﬁrst piston ring into a tribologically critical operating condition.

Figure 1.2: Rectangular ring (left) and keystone ring (right), side clearances

6 1 Piston rings

The piston rings, piston, cylinder surface, and lubricant form a tribological system, where all

sliding parts are responsible for proper operation. For the piston ring, it is the type, detailed

design solution, tangential force (amount as well as axial and radial distribution) and mate-

rial; for the piston, the type and materials, or material pairings, as well as design details; and

for the cylinder surface, it is the material, machining (honing), and contour accuracy (see

Chapter 5). The lubrication depends on the lubricant itself (base oil, additives, viscosity class),

sufﬁcient wetting of the running face, and piston temperature.

Combustion gases contain corrosive components, the worst of which is sulfur dioxide (SO

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:

N surface pressure (tangential load and gas pressure),

N ring width,

N coefﬁcients of friction of the contact surface (coating),

N running face shape (barrel shape),

N 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.

Figure 1.3:

Ring carrier piston

1.4 Types of piston rings 7

1.4 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. This classiﬁcation was made

in DIN ISO 6621, Part 1, corresponding to Figure 1.4.

Figure 1.4: Classification of piston rings per DIN ISO 6621 Part 1, Section 4

8 1 Piston rings

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. Precise machining of the ring groove,

however, is made more difﬁcult. For extreme ratios of radial piston ring width to axial piston

ring width, the piston rings become unstable.

Individual types of engines—gasoline engines, passenger car diesel engines, commercial

vehicle diesel engines, as well as medium-speed four-stroke engines and slow-speed two-

stroke diesel engines—are ﬁtted with piston ring packs where the overall efﬁciency is matched

to the speciﬁc operating conditions by combining and matching different piston ring types.

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. Piston rings are

allowed to cause only minimal wear on the cylinder bore.

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

keystone shape (see Section 1.4.5). The symmetrically barrel-shaped piston ring (see Section

1.4.7) 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 conse-

quence of the more narrow contact surface with the cylinder bore. Oil control is improved by

the wedge effect on account of its shape.

Even if the squareness of the ring groove has slight deviations, the piston ring remains in

its line contact with the cylinder surface. When the piston ring changes direction at the end

of the stroke, contact is maintained between the running face of the piston ring and the

cylinder. Barrel-shaped piston rings cause less wear in the region of the cylinder surface,

where the ﬁrst piston ring changes its running direction. The barrel-shaped piston ring can

be designed with a bevel on its top inner edge, in order to achieve a positive distortion. Strict

requirements regarding lubricating oil consumption, however, have led to the ﬁrst piston ring

taking on part of the oil control task as well. In this regard, the running face is given an asym-

metrical barrel shape. Due to the asymmetry, the center of convexity is shifted in the direction

of the lower half of the ring width. This improves engine run-in and oil control.

1.4 Types of piston rings 9

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. Friction and wear need

to be minimized, of course.

1.4.1 Rectangular ring

Figure 1.5: R-ring

The basic shape of the ﬁrst piston ring is a

rectangular ring with a straight-faced run-

ning face, also known as an R-ring (Figure

1.5). Its task is to seal against the gas pres-

sure in the combustion chamber. Rectan-

gular rings are used for normal operating

conditions, primarily as ﬁrst piston rings in

gasoline engines.

Figure 1.6: M-ring

A slight taper (conicity) to the running face

of the piston ring increases its effectiveness.

Contact between the piston ring and the cyl-

inder 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).

10 1 Piston rings

1.4.3 Piston ring with top internal bevel or internal step

Due to a chamfer on the top inner side of

the piston ring (internal bevel IB), the forces

of the piston ring are modiﬁed such that its

cross section tips about its axis, due to com-

pression during installation of the piston in

the cylinder.

This twist (i.e., a tilted position of the piston

ring under tension) provides a line contact of

Figure 1.8: M-ring with bottom internal bevel

the oil scraping edge of the ring against the cylinder surface, as well as between the lower

inner edge of the piston ring side face and the piston groove side face. The latter reduces

the passage of combustion gases as well as engine oil. When the internal bevel is at the top,

it is referred to as a positive twist. Taper-face rings are also designed with a positive twist.

They have been tried and tested for years and allow control of oil passage and reduction of

blow-by.

Piston rings of this type, also known as R-rings with top IB, are used both as ﬁrst and as

second piston rings (Figure 1.7).

1.4.4 Piston ring with bottom internal bevel or internal step

Figure 1.7: R-ring with top internal bevel

In contrast to Section 1.4.3, moving the

internal bevel to the bottom provides a neg-

ative twist. These piston rings with bottom

internal bevel (IB), also called M-rings with

bottom IB, make contact at the bottom with

the cylinder and at the top inside with the

groove side face (Figure 1.8). Such piston

rings are preferably installed in the second

ring groove and are part of the group of oil

control rings. With regard to oil control, contact of the lower part of the ring face against the

cylinder surface is desired. Oil control rings with greater taper are therefore used to compen-

sate for the twist. The negatively twisted piston ring creates a good seal at the bottom against

the cylinder surface, due to its linear contact, and prevents oil from entering the ring groove.

This is especially important for low pressures in the combustion chamber, as they can occur

when the mixture is throttled in gasoline engines or at charge exchange. Under such condi-

tions, it is superior to piston rings with positive twist. On the other hand, piston rings with

positive twist have a tendency to control blow-by more efﬁciently. Since the passage of oil