DEVELOPMENT
You will find theSealings
figures mentioned in this article in the German issue of MTZ 7-8/2005 beginning on page 566.
Kompressionskolbenringe
in Otto- und Dieselmotoren
– Stand der Technik
und Ausblick
Compression Piston
Rings in Gasoline and
Diesel Engines
Current Engineering Practice and Outlook
The demand of the customer for higher engine performance and
lower fuel consumption and the need to reduce the environmen-
tal impact of motor vehicle traffic requires a huge development
By Johannes Esser, commitment at the gasoline and diesel engine manufacturers.
Steffen Hoppe, The supplier Federal Mogul Burscheid GmbH makes a major
Rudolf Linde and contribution to accomplishing these goals at compression piston
Frank Münchow rings by optimizing and improving the key functional features like
base material, wear protection and design.
20 MTZ worldwide 7-8/2005 Volume 66
DEVELOPMENT Sealings
1 Introduction havior, are characterized by further opti- fully used, Table 3. Improved side wear re-
mizations of the spherulitic graphite struc- sistance especially on the bottom side is
The piston rings for modern internal com- ture which are particularly manifested in achieved by selecting the harder spheroidal
bustion engines, especially the top piston an increased bending strength under static graphite cast iron (GOE 56) or 18 % Cr steel.
rings, will in future have to meet the in- (installation characteristics) and dynamic However, steel rings – especially axially
creasingly tough demands placed on them loading. Additionally, bainitic matrix struc- narrow rings – carry the risk of increased
by continued increases in engine perfor- tures will enable lower wear rates on the groove wear, therefore in terms of the cu-
mance, lower legal emission limits and sides and running face to be achieved. mulative wear of the groove and ring side
longer life expectancy combined with low- Reductions in piston overall height there is not always a difference.
er maintenance costs. The challenge will es- along with a narrower compression ring A distinct switch to steel materials for
sentially be to fulfil the core functions of axial width in gasoline engines, and in- the axially wider compression rings in
piston rings, namely creased mechanical demands especially be- diesel engines has not occurred. This is due
■ sealing yond 200 bar peak pressure in diesel en- partly to the very good material match be-
■ transferring heat while assuring scuff gines, call for greater strength reserves and tween the cast iron ring and ring carrier
resistance improved wear resistance of the ring sides. material and also to the very good machin-
■ controlling oil consumption Steel materials are particularly well suited ability of cast irons – particularly for creat-
■ prolonging engine life by limiting wear to meeting these demands. A typical fea- ing sharp bottom running face edges.
rates through the use of suitable materials, ture of the improved dynamic loadability of Experiences with spheroidal graphite
coatings, designs and production processes steels compared to cast irons is the in- cast iron compression rings in the top
at reasonable costs under series production creased fatigue strength under pulsating groove in truck diesel engines have always
conditions. bending stresses. Figure 1 plots the en- been very favorable. This is reflected in the
durable stress amplitudes of the base mate- high proportion of cast iron rings used in
2 Critical Design Features rials used for piston rings. The difference in European diesel engines, Table 3. However,
dynamic strength between cast iron and since the 1960's good results in series pro-
The following observations provide an forged steel diminishes in some cases sig- duction have also been achieved with coat-
overview and describe the outlook for the nificantly as a result of the effect of coat- ed compression rings made from 18 % Cr
design of top and second compression pis- ings and surface treatments. Investigations steel in truck diesel engines, where there is
ton rings in terms of base materials, types show that the dynamic strength of nitrided very low axial wear. Additionally, with the
and axial widths. Design features like run- steel rings increases by about 30 % after an introduction of cylinder pressures signifi-
ning face wear protection (coatings), run- additional chemical treatment (the CPS cantly exceeding 200 bar, it is expected that
ning face design, conformability and side process). steel piston rings will be specified increas-
armoring to meet special requirements are The preferred steels are high-chromium ingly often for the top groove.
also addressed. martensitic steels with a chromium con-
tent of 13 or 18 %, the wear strength being 2.2 Piston Ring Types
2.1 Base Materials governed by the formation of finely distrib- Vehicle gasoline engines operate with rec-
Fundamentally, the materials used for pis- uted chromium carbides and by a signifi- tangular section rings in the top groove at
ton rings today are cast irons and steels of cantly increased case hardness resulting 100 %. The running faces are symmetrically
varying qualities. Turning first to the cast from additional nitriding treatment. If heat barrelled, asymmetrically barrelled or ta-
irons, the entire spectrum of available cast treated, low-alloy Cr-Si steels are to be used, pered depending on oil consumption and
iron qualities, Table 1, is used depending then the ring running face will need to be blow-by requirements. About 30 % of the
on the stress profile, which is characterized coated. European passenger car gasoline engines
by the material strength, elastic strain In the past 15 years there has been a shift are designed with either an asymmetrical
properties, fatigue strength and wear be- worldwide away from cast iron toward barrel or a taper on the running face in or-
havior. For top compression rings a steel for the top compression rings in gaso- der to improve oil consumption.
spherulitic graphite structure with very line engines. The nitrided steel ring is pre- The majority of passenger car diesel en-
high bending strengths and elastic moduli ferred, particularly in Europe and Japan, gines are also equipped with rectangular
is particularly preferred, the martensitic Table 2. In the high speed operating envi- rings. The proportion of keystone rings in
matrix for increased hardness determining ronment of the gasoline engine, the axially passenger car diesel engines has remained
the resistance against side wear. narrow steel ring has now become the stable over the past 25 years at about 30 %.
For uncoated piston rings in the second standard for the top groove. Over 90 % of With increasing bore diameter, the propor-
groove, fine-flaked cast irons in a heat- engines currently under development are tion of keystone rings also increases due to
treated condition were developed. Excel- fitted with nitrided steel rings in the top combustion-related effects, Figure 2.
lent wear resisting properties are achieved groove. For reasons of cost the second
on account of the special carbides formed groove is typically equipped with cast iron 2.3 Axial Width of Rings
from the elements chromium, vanadium, rings, their design and running face coating Over the past 20 years a distinct trend to-
manganese and tungsten and the marten- being governed by the prevailing function- ward axially narrow top compression rings
sitic matrix structure. The malleable cast al requirements. in gasoline engines has been observed
iron GOE 44 is a material with a specific In European passenger car diesel en- world wide, Figure 3. Such smaller widths
residual carbide content in a fine-pearlitic gines – including highly rated engines with are necessary for technical reasons due to
matrix and combines the potential for high specific outputs greater than 50 kW/l – top increasing engine speed levels and the re-
tangential loads with good wear resistance. compression rings made from a class 52/56 sulting need to reduce piston mass and pis-
Development trends, driven by a de- material (spheroidal graphite cast iron) and ton dimensions. Development of the nitrid-
mand for greater material strength and fa- second rings made from a class 32 heat ed steel ring is an essential prerequisite for
tigue strength as well as improved wear be- treated wear-resistant material are success- the successful use of axially narrow rings in
MTZ worldwide 7-8/2005 Volume 66 21
DEVELOPMENT Sealings
the top groove. Ring widths of 1.0 and 1.2 3.2 Thermal Spray Coatings
mm are today preferred sizes in gasoline
Figure 7: Micro-
engine development for the top groove and
structure of the
of 1.2 to 1.75 mm for the second groove.
HVOF coating
The shift toward small axial ring widths
has not occurred in diesel engines on ac-
count of the greatly increasing peak cylin-
der pressures, Figure 4 and Figure 5. Re-
duced axial widths are most likely to result
from the trend toward diesel engines with
bores smaller than 75 mm. In truck diesel
engines there is even a tendency toward
larger ring widths because of the increas-
ing peak cylinder pressures. If axially nar-
rower rings are being considered within
the context of reducing friction losses, then
it is important to bear in mind a possible ef-
fect on the axial wear behavior.
3 Running Face Wear Protection
Under the demands of modern fuel injec-
tion and combustion strategies, top piston ing the thermal loadability of chromium- zone and the related precipitation of special
rings undergo significantly increasing ther- based coatings still further without suffer- nitrides. To increase the application of ni-
mal and mechanical stressing. Improving ing any major drawbacks related to system triding it was vitally important, through
running face armoring is therefore the focal wear and economy. technological process development, to en-
point of development efforts, as following able the build-up of the nitride case to be
methods show. 3.2 Thermal Spray Coatings carefully controlled. Figure 8 describes the
Thermal spray coatings have been used for microstructure of the formed nitride case
3.1 Electrochemical Coatings compression rings in internal combustion for typical gasoline engine applications.
Standard hard chromium coatings are to- engines for a long time. Plasma spray coat- A further advantage is the additional
day mainly used as wear protection for sec- ings in particular have a very high ceramic side wear protection provided by the for-
ond rings and oil control rings. With a high- content, making them highly suitable for mation of an all-over nitride case and by
er thermal loadability and improved wear reducing scuffing caused by heavily in- the improved compatibility of the piston
performance the chromium-ceramic coat- creased adhesive wear. Plasma spraying, ring side to the groove side achieved by
ing CKS from Goetze/Federal-Mogul, which however, is not suitable for depositing hard means of superfine grinding of the side
has been used with great success for many metal-like structures, which further im- contact surfaces. The wear resistance of ni-
years, is closely linked to the successful de- prove the wear characteristics. The process trided steel rings and their comparatively
velopment of modern, high performance developed to enable this, known as the low thermal loadability are generally not
diesel engines. high velocity oxygen fuel spraying (HVOF), sufficient for applications in the top groove
For even tougher requirements a new allows powdery materials like CrC, WC and of modern diesel engines.
coating system was developed. This con- metallic Ni-Cr-Mo alloys to be shot onto the
sists of a hard chrome matrix reinforced piston ring surface and sintered in an ultra- 3.4 PVD Coatings
with minute diamond particles that are sonic flame. This is done at relatively mod- The most recent generation of piston ring
firmly embedded in the extremely fine erate temperatures of about 3000 °C. The coatings is represented by the coating sys-
crack network, which has a special sub- resultant coating structure, Figure 7, which tems produced in the physical vapor depo-
structure, Figure 6. The chromium-dia- is under residual compressive stresses, has sition (PVD) process. These coating systems,
mond coating, designated GDC from Feder- submicroscopic carbides embedded in a Ni- Figure 9, predominantly of a chromium ni-
al-Mogul, exhibits the lowest coating wear Cr-Mo matrix. Low porosity, extremely high tride (CrN) basis, are chiefly defined by
of any known coating on the market. The adhesive strength and hardnesses of 750 to characteristics such as an extremely high
GDC coating can be produced with a sharp 1000 HV define the coating properties. Be- hardness between 1800 and 2200 HV, ex-
bottom running face edge for low oil con- sides the improved scuff behavior stem- ceptionally low coefficients of friction and
sumption and blow-by and has the benefits ming from the ceramic coating structure, a ceramic crystalline structure. The CrN
of high thermal loadability and wear resis- the HVOF coatings known by the Federal- coatings are additionally characterized by
tance. Taking all of its positive attributes Mogul proprietary name MK-Jet reduce low wear rates, a very high thermal load-
into account, the coating is an innovation wear compared to plasma spray coatings ability and a high chemical stability.
for future generations of engines [1]. by a factor of 3 to 4. Piston ring applications for PVD coat-
In terms of process engineering the use ings are restricted by the coating process,
of electrochemical coatings has a large ap- 3.3 Nitrided Coatings which is derived from “thin film” technol-
peal because of the relatively high deposi- When high chromium alloy martensitic ogy. In coating thicknesses greater than 50
tion rates. Long-term research projects con- steels are nitrided, the amount of wear in μm, for example, problems with coating
cerned with electrochemical coatings are the system 'piston ring running face / cylin- adhesion and cracking are observed due to
focusing on new material combinations der bore' reduces markedly due to the sig- the extremely high residual stresses in the
and surface textures with a view to increas- nificantly greater hardness in the surface coating. These considerations call for steel
22 MTZ worldwide 7-8/2005 Volume 66
DEVELOPMENT Sealings
to be used as the base material for PVD- increases. Figure 12 plots the distribution 6 Side Armoring
coated piston rings, and it should addition- of piston ring coatings in European passen-
ally be nitrided to reduce the residual ger car diesel engines from 1980 to 2005. The demands of new engine generations
stress gradient between the coating and may necessitate side armoring for reasons
piston ring material. Common coating 4 Running Face Design of wear or material compatibility. The use
thicknesses for gasoline engine applica- of nitrided steel rings is not always a suit-
tions are 10–15 μm, while in diesel engines The running face design especially of top able remedy. Chromium plating of the
the thicknesses are 30–50 μm due to the compression rings is becoming an increas- sides has proved very successful in heavy
higher wear stresses. ingly important consideration in modern fuel engines. The development of a new
diesel engines. The asymmetrical barrel coating process enables the sides of rectan-
3.5 Coating Characteristics and face has been produced for more than 20 gular and keystone rings to be chromed up
Market Overview years and is a standard design for diesel en- to a plating thickness of about 10 μm. The
Figure 10 shows an overview of the rela- gines in Europe. A small barrel at the bot- time-consuming remachining necessary
tive wear resistance of the coatings under tom running face edge is critical to the with the traditional process can therefore
this discussion. The lowest wear rates are functional behavior. Though in itself a com- be largely omitted. Rings produced using
currently achieved with the GDC running mon feature, barrelling in large-scale pro- the new “Blitz Chrome” technology will be-
face coating. Thermal loadability / scuff re- duction places the highest demands on gin service in truck engines in 2005.
sistance is required especially in diesel en- modern production engineering.
gines. Chromium-nitrided and nitrided A high-quality running face design is 7 Summary
steel rings fail to meet the requirements of combined with the sharpest possible bot-
modern diesel engines and for this reason tom running face edge to achieve optimum The further increases in mechanical and
are generally not used. Testing of more oil scraping. Compared to non-machined thermal engine loading anticipated in the
heavily loadable coatings reveals no clear- running face edge radii, an inlaid (semi-in- future call for innovative products and
cut distinction in current typical engine ap- laid) design with a sharp base material manufacturing processes for piston rings.
plications. Even so, by means of controlled edge improves oil consumption by up to Base materials will be further improved
generation of critical loads an indication of 60 %. with respect to strength and wear resis-
the thermal loadability of coatings can be tance. In the future, steel materials will be
given. The CKS coating meets the require- 5 Conformability used alongside cast irons for the top com-
ments of current production engines, its pression rings in diesel engines.
range of application being extended by If the conformability of spring loaded oil The currently available electrochemical,
GDC. MK-Jet and PVD are at the better top control rings is dependent on the cross-sec- thermal spray and PVD coating technolo-
end of the coating scale with respect to tion and tangential load setting, then the gies have not yet reached their fundamen-
thermal loadability. nominal value for the conformability of tra- tal application limit. Solutions tailored to
Since becoming established in European ditional one-piece compression rings with individual applications to match the pe-
gasoline engines in the early 1990's, nitrid- a constant wall thickness is governed pri- ripheral conditions will be employed.
ed steel rings have proved to be a highly marily by the free gap. For given geometri- To address the demand for minimal oil
trusted and reliable component. Figure 8 cal dimensions the free gap defines the clo- consumption and blow-by, piston rings
showed the cross-section of a nitrided steel sure stress and the opening stress. Conse- with an optimized geometry, such as inlaid
ring with the corresponding hardness quently, relatively tight limits to the con- running faces or the form-optimized FO
curve at the surface. Any increase in scuff formability are set by appropriate stress ring, will be introduced as and when re-
resistance that may be needed is achieved conditions and assembly capability. quired.
by additionally coating with CKS or PVD. The conformability of one-piece rings is
Figure 11 shows the coatings used for top not uniform around the circumference but
piston rings in European series production is at its greatest opposite the gap. Owing to
Reference
gasoline engines in the period from 1985 to the lack of bending moment at the ring
2005. ends, the local conformability at the gap [1] Esser, J.; Linde, R.; Münchow, F.: Diamantbe-
For the use in diesel engines, action had falls to zero. Figure 13 shows a schematic wehrte Laufschicht für Kompressionsringe. In:
to be taken very early on to attain the re- comparison of the conformability of a clas- MTZ 65 (2004), No. 7-8/04, pp 582–585
quired wear and scuff resistance. At the be- sic piston ring and the FO ring around the
ginning of the 1990's the widely used circumference. By introducing variations in
chromium coating reached the limit of the radial wall thickness near the gap, a
thermal loadability. Plasma coatings, form-optimized piston ring – the FO ring –
though scuff resistant, cause considerable is obtained, Figure 14. This has the effect of
cylinder wear at the top ring reversal significantly improving the local flexibility
point. A quantum leap in scuff resistance and adaptability of the piston ring to irreg-
was made with the development of the ular bore deformations.
CKS coating. This coating, despite an ever Test results obtained so far in gasoline
increasing power density, has essentially and diesel engines demonstrate significant
provided the required scuff resistance to- potential for oil consumption reduction. To
gether with low cylinder wear through to implement the FO ring concept a complete-
the present day. The chromium-diamond ly new manufacturing process was devel-
coating GDC extends the application range oped. The first FO ring was introduced in se-
of electrochemical running face coatings to ries production during the first quarter of
handle the expected future performance 2005.
MTZ worldwide 7-8/2005 Volume 66 23
