Ring pack to HDD engines focused on low fuel consumption

RING PACK TO HDD ENGINES FOCUSED ON LOW FUEL CONSUMPTION

Rafael Antonio Bruno1, Andre Ferrarese

1, Daniel Lopez

2 and Eduardo Nocera

1

1MAHLE Metal Leve S.A.

2MAHLE GmbH

E-mails: [email protected], [email protected],

[email protected], [email protected],

ABSTRACT

Low fuel consumption is a target for all engine developers. This demand is aligned with many

others: high power output, improved mechanical efficiency, low emissions etc. These

demands are linked to economic, politic and environmental aspects and they all come to a

key-demand: low friction components.

Low friction concept is based not only in materials with low friction coefficient. It is also

given through low friction designs. This work will present and discuss the possibilities of

doing it for a piston ring pack applied in a Heavy Duty Diesel (HDD) engine without

jeopardizing the performance on blow-by and lube oil consumption.

Basically the sum of tangential force of the ring pack divided by the bore diameter is a factor

able to express the friction potential of the ring pack. Today engines on Euro 4 and Euro 5

versions present this value between 0.9 and 1.1 N/mm. The use of low friction concepts like

low width, materials with low friction coefficient and optimized oil ring design enables to

reach the level of 0.5 and 0.6 N/mm, i.e. almost 50% of potential friction reduction. Fuel

consumption in a preliminary dyno test showed around 0.4% reduction using optimized ring

pack compared to Euro 5.

This paper presents the main achievements on low friction ring pack on HDD engines using

rig and dynamometer tests. It is used and presented results on engines focused on Euro 5 and

Euro 6 versions.

1. INTRODUCTION

Global environmental concerns are driving internal combustion engine designs to higher

efficiency. New legislations are setting up milestones to reach commercial vehicle emission

goals and limit NOx and particulate material emission levels to achieve more environmentally

friendly engines. Latest developments are heavily focused on the reduction of pollutant

emissions through the improvement of the combustion efficiency or the introduction of after

treatment systems. However, the using of low friction components as a means to reach fuel

economy and, consequently low emission targets, seems to be not so explored so far. But such

application could represent a significant potential. Figure 1 shows an overview of the

legislations concerning NOx and Particulated Material (PM) and how it is spread on several

regions. Meeting these emission targets will require the introduction of new Power Cell Unit

(PCU) technologies and a more detailed system understanding. Low friction Power Cylinder

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Units (PCU) can contribute to lower engine fuel consumption. On the other hand, the

introduction of a low friction PCU in modern engines must not jeopardize the lubricant

control (oil consumption), combustion gas sealing (blow-by), wear and/or scuff resistance,

especially for the piston rings.

Figure 1 - Emission legislation map for HDD engines.

Following this trend, this paper presents the development of an advanced piston ring pack that

applies innovative technology. It enables the engine to present similar lube oil consumption

with reductions on total ring pack load up to 50%.

In Section 2, the new operational environment of the ring pack is explored. Section 3 presents

the potential of friction reduction from rings in a HDD engine. The discussions about the ring

design and technologies to achieve low friction, as well as the innovative concepts are

presented in Section 4. Then engine tests validating the proposed design to reduce fuel

consumption are shown in Section 5, following conclusions in Section 6.

2. TECHNOLOGICAL CHALLENGES FOR RING PACKS IN MODERN HEAVY

DUTY DIESEL ENGINES

To successfully implement a low FMEP (Friction Mean Effective Pressure) PCU in Heavy

Duty Diesel (HDD) engines, it is critical to identify the major existing or future engine

technologies that will significantly influence the piston ring pack environment. Figure 2

shows the major technological evolutions affecting the PCU that must be considered as

additional constraints in the efforts to reduce ring pack friction power losses.

They can be classified in three categories: those related to engine technology, fuel and

lubricant. The increase of thermal and mechanical loads that results in higher power density

demands higher piston ring durability. This loading also leads to higher cylinder bore



deformations which requires high ring conformability properties [4]. This increase on engine

load also affects piston grooves and lands, which demands specific ring design features to

guarantee optimum management of gas and oil in the piston ring belt area. It helps to maintain

an acceptable blow-by level and to avoid excessive carbon build-up.

Figure 2 - Engine technological evolutions and their impact on Power Cell Unit.

The piston ring pack lubrication is necessary to maintain its efficiency over the engine

lifetime, which is also greatly affected by several new engine solutions. Protective lubricant

properties already challenged by higher operating temperatures are further degraded due to

fuel interactions that frequently result in higher fuel dilution levels (post-injection strategy for

particulate filter regeneration...). The latest combustion strategies and the extensive use of

exhaust gas recirculation to reduce NOx emissions also pollute the oil with increased levels of

soot and mixed combustion products. The challenge to develop future power cylinders

consists of reducing friction power losses and weight while maintaining traditional

functionality such as gas sealing, oil control, and durability. Identifying solutions might

require compromises in component design that only a system approach and a full

understanding of the engine boundary conditions will allow.

3. MECHANICAL LOSSES IN A HDD ENGINE

Mechanical losses in a diesel internal combustion engine represents about 4 – 15% from the

consumed fuel, depending on the engine type and the loading operation regime. It is

noticeable the contribution of the piston and the piston ring pack in this loss, once 1/3 of this

value is attributed to these components (see Fig.3).



Figure 3 - Friction loss in HDD engines [1].

According to [2], as presented in Fig. 4, there is certain relation of friction reduction and fuel

consumption. From the Fig. 4, it is possible to get a simple rule in Diesel engines that every

3% reduction in friction can lead to 1% of fuel consumption. Considering that ring pack

friction is proportional to the total ring pack load, reducing 50% on ring pack load can lead to

50% of ring pack friction which means 5 to 12% of friction reduction. In this way, there will

be an expected fuel consumption reduction around 1.5 to 4% depending on the engine and its

operation conditions.

Figure 4 - Fuel saving estimative due to friction reduction in an ICE @ 2000rpm [2].



4. MINIMIZING ENGINE MECHANICAL LOSSES TROUGH A LOW FRICTION

RING PACK

As basic rules to reduce ring pack Friction Mean Effective Pressure (FMEP) [5 and 6], the

design changes needed are the following:

- Ring pack tension reduction.

- Ring axial width reduction.

- Use of coatings or treatments with small coefficients of friction and high wear

resistance.

- Use of optimized ring profiles to enhance hydrodynamic lubrication with cylinder

bores.

Following these directions without considering the engine architecture, its operating

conditions, or the other components of the PCU system, there will be limited potential of

FMEP savings once the traditional ring pack functions are jeopardized.

The multiple interactions between the components of the system require the implementation

of a suitable methodology for the development of low friction ring packs. Combining

investigation techniques and tools, environment understanding, and component expertise is

essential. Figure 5 shows the ring pack boundary conditions and the affected ring properties.

It is important to differentiate characteristics like cylinder pressure, bore distortion and

temperature that are defined by the engine OEM’s during the early stage of the engine

development from those such as bore topography and piston deformation that are part of the

PCU system, allowing for component design synergies and compromises for optimization. A

suitable low friction ring pack design methodology should divide the analysis of the system

into two parts. One is related to the optimization of the entire ring pack in terms of ring

dynamics, combustion gas flows and blow-by. The other is related to oil transport in the PCU.

Figure 5 - System Analysis – Ring Pack Boundary Conditions and its affected properties.



The lubrication function of the ring pack is to supply a controlled amount of oil to areas of

critical surface interfaces, namely the liner and the piston groove. A minimum liner oil film

thickness is required to allow hydrodynamic lubrication and therefore limit friction and wear.

Oil is also needed in the first piston groove to lubricate the ring and groove contact surfaces to

avoid excessive wear. High temperatures limit the residence time of oil in the top ring groove

and increase the risk of carbon deposits and ring sticking. Lubrication refreshment should be

done with a continuous oil supply to the groove. This quantity should be limited and

controlled in order to avoid oil loss and therefore excessive oil consumption. Optimization of

the entire PCU system must be done by controlling oil supply and release in each region of

influence to achieve efficient lubrication while minimizing friction power losses.

4.1 Changes on Ring Pack Design

As mentioned before, there are many ways to reach friction reduction changing ring pack

characteristics. The chart below (fig.6) aims to summarize the possibilities of doing it and the

expected functional impact (risk) of each one.

Figure 6 - Possible changes on ring pack design and its expected functional risk.

Design change possibilities to reduce

friction

1) Reduce Tension

2) Reduce Axial width

3) Barrel shape more asymmetric

4) Face Coating with a lower coefficient of friction

Functional risk

Expected Impact on ring pack friction

reduction

Existing MAHLE Ring Pack Design

Top

Rin

g2n

d R

ing

5) Reduce Tension

6) Reduce Axial width

7) Face Coating with a lower

coefficient of friction

1) LOC (low conformability) -

Assembly (low free gap)

2) Side Wear LOC (Face wear –

oil up scraping) Carbon Deposits,

assembly

3) Side Wear (gas pressure is less

supported by face)

4) LOC (low conformability)

Assembly (low free gap)

5) Side Wear (low torsion resistance)

LOC (Face wear – oil up scraping,

no axial flutter at high rpm-full load)

6) Needs to switch to PVD.

LOC (PVD – no sharp bottom face

corner)

++

+++

++

++

++

+++

++

++

++

+++

+

+

+

Expected Impact on ring pack function

Oil

Contr

olR

ing

+++

+++

++

++

++

+++

--

--

++

8) Reduced tension

10 ) Reduced Axial Width

11 ) Reduced Land width (h5)

12 ) Face coating with a lower coefficient of friction

9) Ring material / Cross section

e.g. I-Shape – Higher conformability

7) LOC (Oil scraping-Unit pressure,

Axial and Circumferential

conformability)

10) Assembly (Resistance –

PCU stuffing into liner)

9) Assembly (Ring stiffness)

8 ) LOC (Oil scraping-Unit pressure,

Axial and Circumferential Conformability). I-Shaped already

implemented



4.1.1 Ring Pack Tangential Load

Ring pack tangential load is the most evident parameter to be minimized when friction

reduction is pursued. However, as friction reduction was not considered an explicit need for

HDD engines so far, historically we can not observe any kind of improvement on this

characteristic, mainly because the strategies chosen to reduce emissions based in after

treatment systems and exhaust gas recirculation methods. Fig. 7 shows an overview of the

ring pack tangential load of the engines from different legislations. The ring pack tangential

load is presented divided by the bore diameter. As the plot suggests, by the opposite it is

noticed a slight trend to increase the ring pack tangential load.

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

1,10

1,20

1,30

1,40

EU 04 EU 05 EU 06

Su

m o

f F

t / d

1 (

N/m

m)

Figure 7 - Ring Pack tangential load for engines from different emission legislations.

MAHLE proposed ring pack presents a tangential load 37% lower than EU05 engines. The

main contribution for this is the using of V-shape Oil Control Ring (to be better explained

ahead).

Figure 8 - MAHLE Ring Pack solution for low friction HDD engines.

0.20 max

Oil Ring Design:“V”-Shape – Nitrided Steel

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1 2 3

To

tal

Rin

g T

en

sio

n (

N)

/ B

ore

Dia

mete

r (m

m)

1 2 3 4 5Benchmark

Series HD Applications

EU06Friction optimized

EU05

- 37 %

Oil Ring Design:“V”-Shape – Nitrided Steel

( ~ 50 % reduction of Ft)



4.1.2 Ring Pack Width

Ring axial width is a quite effective way to reduce friction in ring pack, mainly when applied

to the top ring, but with noticeable effects in other grooves also. The challenge on reducing

the top ring axial width is to support the mechanical load due to high PCP levels. Benchmark

does not show a significant trend of reduction in HDD engines.

4.1.3 Ring Material

For top rings, the well-known chrome-ceramic plating has been a typical solution until now

[7]. Nevertheless with the above-discussed challenges imposed by the diesel engine

technology this coating is now in its application limit. For the coming years MAHLE has

invested in the development and in production capacity to supply PVD (Physical Vapor

Deposition) coating technology to the market.

The PVD coating is composed of a 100% ceramic chromium nitride (CrN) with hardness up

to 1,600 HV0.05 and very low friction coefficient. The 30 to 50 micrometers thick coating is

deposited onto a nitrided 17wt% chrome stainless steel substrate with a pure chrome

interlayer to improve adhesion [8].

It is known that the excessive radial wear brings many drawbacks to ring performance, as Ft

reduction, gap increase and elimination of the original ring barrel shape profile. All these

effects contribute to blow-by increase and Lube Oil Consumption (LOC) increase too.

Durability tests in HDD engines has proven the higher wear resistance of the PVD coating

with values up to 4 times lower than the usual chromium as presented in [8]. For higher

durability it is provided a nitrided case as a secondary running layer. Beside the benefits for

the running face, the nitrided layer also provides protection to avoid damage to the lateral

sides of the ring.

Besides the excellent wear resistance, the PVD coating presents as well a high scuffing

resistance [9] and great compatibility with the main liner materials and surface finishing [8],

which leads to minimized wear to the tribological pair (Fig. 9).

Figure 9 shows the running face aspect of top rings of different materials when run against

different liner materials. The results went from 300h engine tests (12L, PCP of 180bar, Power

density of 29kW/L).



Perl

itic

CI

( la

me

lla

r)

Perl

itic

CI

( ve

rnic

ula

r)

Ba

init

icC

I

(la

me

llar)

Lin

er

Mate

rials

Top Ring Coatings

GDC50 Competitor MCR236 Cr-Ceramic MIP230 PVD MIP230 PVD

Ste

el

Lin

er

Ste

el

Lin

er

(nit

rid

ed

)

Lin

er

Mate

rials

Figure 9 - CrN (PVD) showed the best compatibility and bearing pattern with all tested

materials.

The plot in Fig. 10 was built with results from field test (engine 14.8L HDD, PCP 220bar) and

express the liners wear at the region of top dead center. This region is mainly affected by the

top ring material.

0

5

10

15

20

25

30

35

40

45

0 50000 100000 150000 200000 250000 300000 350000

Mileage km

TD

C w

ea

r m

ax

µ

m

Competitor Diamond Cr- Coating

MAHLE PVD Coating

Figure 10 - Diamond-Cr (competitor) showed higher liner wear (higher system wear) and

some functional borderlines.

4.2 Cylinder Honing

Liners finishing tend to be smoother for the next years [3, 10 and 11]. Smoother finishing led

to lower oil retention, and thus the lube oil consumption is decreased. Based on that, it is

necessary to provide robustness on ring’s characteristics to get a safe performance, especially

for top rings, which are additionally under high thermal and mechanicals loads. In [8], was

showed results from 500h HDD engine tests were performed with PVD on different cylinder

honing combined with higher EGR (around 20%). Besides reaching significant reduction on

LOC, it was observed a very good compatibility in terms of ring and liner wear.



4.3 New OCR Profile

In Heavy Duty Diesel (HDD) engines, the reduction of the oil ring tension shows a big

potential to reduce ring pack FMEP, since this effect keep constant during all operation

conditions of the engine. For a successful application of low friction oil rings, high

conformability oil rings with narrow and highly wear resistance outer diameter contact

surfaces are needed to conform to distorted bores and apply reasonable unit pressure with a

low tension. MAHLE developed a friction optimized oil ring to fulfill these new

requirements: The V-Shape Ring. See Figure 11.

min10°

Cross Section @ 180°

Typical

0,12 – 0,15 mm

Detail of land

Cross Section @ 180°

Typical

0,12 – 0,15 mm

Detail of land

Figure 11 - MAHLE low Friction Oil Ring Design: V-Shape for HDD Engines.

Reduced widths, as well as axially narrower OCRs contribute to increase the conformability

of the oil ring. This property is essential to ensure the ring will fit properly the cylinder even

with the distortions caused by thermo-mechanical loads. Another way to increase ring

conformability is increasing ring tangential load, which goes against the low friction needs.

Fig. 12 illustrates how the ring width reduction (and consequently some radial dimension

reduction) can contribute to increase the ring conformability. Another important parameter of

the OCR is its unitary pressure (P0), which means the nominal pressure applied against the

cylinder wall to scrap the oil film. As long as the ring touch the cylinder properly (good

conformability), the higher unitary pressure, the higher OCR scraper efficiency. The demand

for scrapping is directly linked to the type of finishing (honing) applied to the liner. Unitary

pressure depends on Ft, ring diameter and the land size. The best way to increase P0 is to

reduce ring land size (h5) and this effect is illustrated by Fig. 13.



Figure 12 - Ring width influence on conformability (k-Factor).

0,4

1,2

2

2,8

3,6

4,4

5,2

30 40 50 60 70 80 90

Tangential Force (N)

Un

ita

ry P

res

su

re-

P0

(N

/mm

2)

Cylinder Honing

h5= 0,15

h5= 0,20

h5= 0,32

Baseline

Figure 13 - Influence of land width on tangential force reduction vs. unitary pressure.

0,2

0,6

1,0

1,4

1,8

2,2

2,6

30 40 50 60 70 80 90

Tangential Force (N)

k-F

ac

tor

( )

Bore Distortion

(reduced land)

Baseline



V-Shape Ring provides lower contact width allowing proper unit pressure to guarantee a good

oil film control between the ring and cylinder wall while reducing its tension and hence

friction power losses. The design combines higher tapered angle lands at the contact face

(minimizing the radial wear effect) with special finishing operations process such as

brushing/lapping to provide good oil control during engine “break-in” period and ensure a

homogeneous contact pattern.

The concept allows the combination of a nitrided base material with reduced cross section for

a better conformability, and reducing lands contact surface width to values below 0.2 mm,

without creating an over nitrided case of the lands, that could result in fragile surface with

subsequent risk of land breakage, especially during PCU assembling. With the V-Shape

concept lower land width contact can be ensured, by keeping a robust design with reduced

stress concentration.

Furthermore, the symmetric orientation of lands maintain the ring/bore contact net force

aligned with ring gravitational center improving axial conformability and avoiding negative

effects of operating down-tilt (temperature effect) that might lead to differentiate contact force

between upper and lower lands.

This type of profile allows significant tension reduction (and hence FMEP) while maintaining

good oil control. Higher wear resistant coatings like PVD are available if the durability needs

to be increased.

In a first approach, a reduction of 50% tangential force in the oil ring was successfully applied

by using the V-Shape Design, keeping the same oil consumption levels and piston cleanliness

compared to the base line. The concept allowed a total reduction of the ring pack tangential

force of around 37% compared to the baseline. Figure 14 shows an example in the evolution

of ring packs according to emissions levels, and the improvements achieved in terms of Total

Ring Tension/Bore diameter in the EU6 application compared to its EU5 predecessor by

applying the V-Shape Design in a low friction optimized ring pack.

Figure 14 - Optimized MAHLE ring pack design to Euro 6 applications.



5. ENGINE TEST RESULTS

It was performed an engine test (800h) overload cycle (PCP 215 bar) in a 13L engine

(340kW) in order to assess oil consumption and oil ring performance. As general comment,

all rings showed a very good appearance and very regular contact pattern after test. Oil

consumption results were satisfactory (similar to baseline assembly), which can be confirmed

by piston visual aspect (see fig. 15).

Figure 15 - Piston cleanliness after 800h engine test.

Figure 16 shows the running face aspect (5 positions) of a V-Shape oil control ring used in

this engine test. It is noticeable that the ring presented a very good contact pattern.

Top

max

. 0.2

0

10

°

3.0

or 3

.5m

ax

. 0.2

0m

ax

. 0.2

0

10

°

3.0

or 3

.5

0o 90o 180o 270o 360o Figure 16 - V-Shape OCR running face after 800h engine test.



Figure 17 shows a representative plot with the contact band width after 750h engine test

(same platform from the figures above, but other engine test). There was no contact failure

and the contact band width kept very low when compared to the baseline design. This

characteristic indicates that the ring had a more effective scraper action along the engine life,

minimizing the oil consumption.

Lands contact width, ring #25

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0° 90° 180° 270° 360°

Circumferential direction

wid

th [

mm

]

lower land upper landlower land new upper land new

lands total widthspecification

(lap area)

gap gap

3.0

0.0

5 …

0.2

0

10

°

3.03.0

0.0

5 …

0.2

0

10

°

0.0

5 …

0.2

0

10

°

0.0

5 …

0.2

0

10

°

Figure 17 - Contact band width after 750h engine test.

6. CONCLUSIONS

As it was presented in this paper the innovative MAHLE oil control ring, V-Shape, enables

significant ring pack force reduction. Experiences were made using an Euro5 engine, which

showed similar lubricant oil consumption with 50% less oil ring force. The much reduced

running contact face was not jeopardized even after 750h engine test. It represents that the

contact pressure is maintained for longer time than conventional oil ring profiles.

Several other ring and PCU characteristics are important to reduce friction. In a summary the

main ways are:

- Reduced ring pack force (enabled by V-Shape)

- Reduced ring pack width (compromise with carbon build up)

- Use of coatings with lower friction coefficient (PVD CrN on top rings)

- Reduced liner roughness (observed as a market trend)

First assessment in terms of fuel consumption measurements in a dyno test showed around

0.4% reduction compared to a Euro5 version by the application of V-Shape oil ring. Further

tests are ongoing to explore the benefits on different engine running conditions and ring pack

configurations. The theoretical evaluations point potential reductions on fuel consumption and

CO2 emissions from 1.5 to 4.0% depending on the engine configuration.

7. REFERENCES

[1] RICHARDSON, Dan E. Review of Power Cylinder Friction for Diesel Engines, ASME

Transactions, vol.122, October, 2000.



[2] BASSHUYSEN, Richard van; SCHAFER, Fred. Internal combustion Engine

Handbook, SAE International, 2004.

[3] JOCSAK, Jeffrey. et al. The effects of cylinder liner finish on piston ring-pack friction,

ASME ICEF2004-952, Sacramento, USA, 2004.

[4] TOMANIK, Eduardo. Improved Criterion for Ring Conformability Under Realistic

Bore Deformation, SAE 2009-01-0190, Sao Paulo, Brazil, 2009.

[5] FERRARESE, André; TOMANIK, Eduardo. Low Friction Ring Pack for Gasoline

Engines, ASME ICEF2006-1566, Sacramento, USA, 2004.

[6] EHNIS, Holger; DEUß, Thomas.; FREIER, Rudolf. Friction Mapping –

Reibleistungsmessungen am befeuerten Vollmotor. In: Tagungshandbuch 11. Symposium

Dieselmotorentechnik, Technische Akademie Esslingen, 2008.

[7] SOARES, Edmo; et al. NanoBor - Reinforced Chromium Top Ring Coating for Diesel

Engines Application, SAE2009-36-0179, Sao Paulo, Brazil, 2009.

[8] WARKENTIN, Thomas; NOCERA, Eduardo; BANFIELD, Robert. New Ring Pack for

Heavy Duty Diesel Engines, SAE2007-01-2846, Sao Paulo, Brazil, 2007.

[9] MAIER, Olaf; BINDER, Ingolf. Burning Mark Test for Piston Rings with High Speed

Diesel and Gasoline Engines - An Engine Test Procedure to Rate the Scuff Resistance,

SAE2008-36-0072, Sao Paulo, Brazil, 2008.

[10] UEHARA, Samantha. Overview of the New Surface Finishings for SI Bores,

SAE2007-01-2823, Sao Paulo, Brazil, 2007.

[11] TOMANIK, Eduardo. Friction and wear bench tests of different engine liner surface

finishes, Tribology International Vol.41 No.11, p1032, Elsevier, November, 2008.



Ring pack to HDD engines focused on low fuel consumption

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