“LUBE OIL CONSUMPTION ANALYSIS OF DI ENGINE USING RICARDO RINGPAK

SOFTWARE”

Ashwinkumar S. Dhoble, R. P. Sharma

R& D Centre, Mahindra & Mahindra Ltd.,

Nashik

ABSTRACT

As the emission standards have become stricter, inrecent years, the diesel engine technology hascontinued to advance rapidly in the field ofemission control. The focus of engine industryresearch and development is on reducing the engineout emissions CO, HC, NOx and particulate matter(PM).

The particulate matter in the air has become thesubject of increased attention due to concern ofpotential health effects. To meet the ever-demanding needs to reduce PM, lube oilconsumption reduction is one of the effective ways.Therefore, it is necessary to better understand theeffects of ringpack parameters on engine lube oilconsumption.

In order to evaluate and or optimise the ringpackperformance, a parametric study of ringpack iscarried out using Ricardo RINGPAK ver. 3.1.1simulation package.

This simulation package uses a completelyintegrated approach through the intricate couplingof various sub models which include ring axial andtwist dynamics, inter –ring gas dynamics,lubrication module, oil consumption, ringconformance etc.,

The ringpack model of DI engine is prepared in thepre –processor of the RINGPAK, which consists ofpiston land profile, groove profile, ring profile withall geometrical data along with the materialproperties and surface finish.

The effect of the top ring end gap and engine speedon oil consumption, wear rate of ring and liner faceand power loss in friction were studied for reductionof oil consumption.

The oil consumption was predicated using theexisting ring configuration with the bore distortionand model was tuned to match the measured oilconsumption values. The sensitivity study was doneto reveal the trend of oil consumption with top ringend gap increase and engine speed.

The results were obtained for each iteration ofringpack parameter; in the form of inter-ring gaspressures, ring-liner minimum oil film thickness,maximum instantaneous ring pressure and ringdisplacement. From the results of oil consumption,it was seen that the top ring end gap increase has thepositive effect on the reduction of oil consumption.The ring displacement prediction gave no indicationof either the ring flutter or ring sticking during thecomplete engine cycle. Similarly ring –linerminimum oil film thickness plot was useful to makesure that ring is not operating in the oil starved orfully flooded condition.

Hence from the sensitivity analysis of ringpack, itwas possible to optimise the ringpack configuration.

INTRODUCTION

Piston rings in a piston engine play an importantrole in the performance and endurance of theengine. Piston rings have to provide optimumsealing function, low wear and friction all at thesame time. The lube oil consumption and blow-byin an engine is directly related to the sealingfunction of the piston rings.

The need to meet stricter emission norms demandsreduction in the lube oil consumption because lubeoil consumption has very high impact on theemission of the particulate matter from dieselengine. Achievement of low oil consumption is

sometimes a matter of trial and error method. Thebetter way to understand the piston ring dynamicsand complex phenomenon of gas dynamics; is touse a simulation tool having a fair degree ofaccuracy to prepare a model of the piston, pistonrings and liner for analysis. Ricardo’s RINGPAKsoftware was used as a simulation tool forinvestigating the lube oil consumption and ringdynamics in the existing DI engine ring packconfiguration.

RINGPAK SOFTWARE

RINGPAK consists of basically two modules one isRINGPRE (Pre-processing plus solver) and theother is RINGVIS for post processing. The pre-processor module RINGPRE generates the inputdata file for simulation purpose.

The solver is a complex integration of the followingsub models,1) Ring axial and twist dynamics2) Inter-ring gas dynamics3) Ring radial dynamics4) Lubrication modules (Hydrodynamic, Mixed

and Boundary) at the ring-liner interface5) Liner oil transport6) Oil consumption7) Ring face and liner wear8) Ring conformance

The solver calculates the results as a function of,

1) Piston ring pack configuration/geometry2) Material properties of the components3) Surface finish parameters of the components4) Lubricant properties5) Engine operating condition (example speed,

cylinder and crankcase pressures andtemperatures)

6) Piston/Ring/Liner temperatures7) Distorted bore shape

MODELING

The modeling part of the pre-processor mainlyconsists of defining land profile points for pistongeometry, grooves, ring and ring face. The landprofile co-ordinates were used to model the pistonand grooves. The modeling of the piston rings wasdone by specifying the cross sectional dimensionsand then the face profile. The face profile can bespecified in two ways, one by specifying the profilepoints and second one is by parametric method i.e.

by using curves and bevels. The second method wasused for modeling face profile of the rings as therings face profile can be easily generated in themodel and can be converted into ring profile points.

The following were the input requirements,1) Engine features (e.g. Bore, stroke, connecting

rod length, speed)2) Land and groove geometry3) Ring geometry and ring face profile4) Surface roughness features of rings, grooves and

liner5) Lubrication properties of oil (e.g. viscosity,

density, distillate fractions)6) Temperatures of surfaces7) Cylinder pressure and temperature8) Oil supply information9) Distorted bore shape

The geometric data and material data for the pistonland, groove and the rings were taken from thecomponent engineering drawings. The ring faceprofile was modeled using the parametric option,i.e. by using curves to model the face barrel. The oilof grade SAE 30 was used for the analysis. Thesoftware has a data bank for most of the commercialoils. In addition the RINGPRE also has a facility foruser defined oil data input. The P-Theta diagram isspecified at the simulation speeds.

The surface roughness and the hardness data for theland, grooves and rings were obtained bymeasurements in the Metrology and the Metallurgylaboratories.

SIMULATION RESULTS

The effect of engine speed on the oil consumption isshown in Fig.1. It is observed that with increase in

the engine speed the oil consumption increasesalmost linearly.

Fig. 1.0., Effect of Engine Speed on Oil Consumption

20

22

24

26

28

30

32

1500 2000 2500 3000 3500

Engine Speed (rpm)

Oil

Con

sum

ptio

n (g

/h)

This is primarily due to increase in the number ofcycles per hour with the increase in speed. Also theinertia effects are more dominant at higher enginespeeds. The trend of the blowby is as shown inFig.2

OIL CONSUMPTION

The total oil consumption phenomenon consists ofthe oil consumption due to oil throwoff from the topring into the combustion chamber, blowback and

evaporation. Throw off from the top ring has amajor share on the total oil consumption for a givenset of conditions. Fig.3 shows the comparativemechanism of oil consumption contributing to thetotal oil consumption. The oil consumption dataplotted above corresponds to full load condition atthat engine speed.

Effect of Top Ring End Gap

Traditionally it is experienced that the increase inthe top ring end gap helps in reducing the oilconsumption. The cause of reduction is the decreasein the oil throw off due to reduction in thecircumferencial oil accumulation area. This reducesthe oil reaching the combustion chamber, due toless oil accumulation. The effect of top ring end gapis shown in Fig.4. Changing the end gap from 0.25mm to 0.45 mm gave an improvement of 20% at3200 rpm.

The improvement in the oil consumption due toincrease in the top ring end gap has two sides, whenthe top ring end gap is increased, for a given speedand load setting, cylinder pressure values decrease.This is due to the fact that during the compressionstroke, as the piston is coming up, the compressedgases start flowing through the enlarged end-gap,thus leading to a lower volume of gas available forcombustion. This results in lower cylinderpressures.

However, for the same given speed and load setting,since more gases are flowing into second landregion, the second land pressures are higher withincreasing top ring end gap sizes.

Consider the situation during expansion stroke,when the cylinder pressure values start droppingbelow the second land pressures, there is a reverseflow of gases leading to entrainment consumptionmode. With increasing end gap sizes; there arehigher second land pressures and lower cylinderpressures, and in general one would expect to seeslight increase in entrainment oil consumption valuesince there are higher reverse gas flow rates duringexpansion.

The other is effect is that, since the cylinderpressures are lower with increasing end gap sizes,the gas loads acting behind the top ring are lowerand hence, the top rings has lower scraping actionand accumulates less oil. Thus the throw off oilconsumption value could be expected to be lower

Fig.2., Effect of Engine Speed on Blow by

0.01

0.0125

0.015

0.0175

1500 2000 2500 3000 3500

Engine Speed (rpm)

Blo

wby

(g/

s)

Fig.3. , Modes of Oil Consumption

28.5

6

0.55 1.13

30.2

4

0.48

8

1.10

4

28.3

52

23.5

2

0.33 0.87

24.7

126.7

6

0

5

10

15

20

25

30

35

Throw-Off Blowback Evaporation Total

Oil

Con

sum

ptio

n (g

/h) End_Gap_.25mm @ 3200rpm

End_Gap_.25mm @ 2600rpm

End_Gap_.25 mm@ 1920rpm

Fig.4., Effect of Top Ring End Gap on Oil Consumption

28.5

6

0.55 1.13

30.2

4

22.3

6

0.76

0.91

24.0

4

0.48

8

1.10

4

28.3

52

20.0

4

0.58

0.90

21.5

2

26.7

6

0

5

10

15

20

25

30

35

Throw-Off Blowback Evaporation Total

Oil

Con

sum

ptio

n (g

/h)

End_Gap_.25mm @ 3200rpm

End_Gap_.45mm @ 3200rpm

End_Gap_.25mm @ 2600rpm

End_Gap_.45mm @ 2600rpm

with increasing top ring end gap size. Since theaccumulated oil is also a source of entrainmentmode (coupling between the 2 modes), this canhave the decreasing effect on the entrainment mode.

For a given speed and load condition and increasingend gap size, cylinder pressure drops, second landpressure increase, reverse gas flow-rates through theend gap increase, oil accumulation at the top ringleading edge decrease (throw off drops). Althoughreverse gas flow rates increases, source of oil maybe lower (entrainment has a balancing act; couldincrease but typically decreases)

Thus in general oil consumption should be lowerwith increase in top ring end gap size but in somecases of high oil availability could show a slightincrease due to above reasons. The case of DIengine illustrated here has an effect of decrease inoil consumption with the increase in top ring endgap which is experimentally validated.

Ring –Liner Lubrication

The lubrication module in the software predicts; theminimum oil film thickness prevailing between thering and liner throughout the engine cycle.

The Fig.5 gives the minimum oil film thicknessvariation for complete engine cycle. Generally thefirst ring is more critical for the oil film thickness

because it encounters the maximum pressure riseduring the combustion. The ring operation shouldbe such that there should not be metal to metalcontact between the liner and ring face as it willlead to very rapid wear of the ring. The ring linerlubrication regime can be divided in threecategories a) Fully flooded ring, b) Partially floodedring and c)Completely starved ring. All thementioned conditions of the rings are shown inFig.6.

a) Fully Flooded ring b)Partially flooded ring

c) Fully starved ring

Fig.6, Regimes of ring – liner lubrication

In fully flooded ring the entire ring face islubricated by oil and the loads are borne by oil filmand asperity forces only. Whereas in the partiallyflooded ring only a fraction of the ring face islubricated by oil, with remaining portion being gaslubricated. As a result loads are borne byoil/asperity forces and gas forces. This is thepreferred regime of lubrication, which ensures noaccumulation of oil as well as no metal to metal

contact. Consider third situation, this conditionhappens due to low lubricant availability on theliner or a high pressure gradient across the ring.This leads to detachment of oil film from the ringface. The loads on the ring are supported by onlythe gases flowing below the ring face. In thiscondition metal to metal contact occurs and willlead to excessive gas blow-by or high wear rate ofring face. From Fig. 5 it is seen that for the existingcondition there is no metal to metal contact takingplace; hence wear can be expected within the

Fig. 7 Wear along Liner

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

Distance along liner [m]

Wea

r L

oad

[MW

/m^

2]

Fig. 5.,Minimum Oil Film Thickness

0123456789

10

-180 0 180 360 540

Crank Angle

Min

. oil

fil

m th

ickn

ess

in

mic

ron

Ring 1Ring 2Ring 3Ring 4

ring2ring 1

ring 3&4

durability limit. The Fig. 7 shows the graph of wearacross the liner span.

The wear load as seen from Fig. 7, is maximum atthe TDC position; due to combustion . Thus higherwear rates are expected at this position due toreversal of direction of motion of top ring and inaddition to this high –pressure values alsocontribute to this effect.

Ring Flutter

The phenomenon of ring flutter is more prone to thefully starved condition of lubrication due to highpressure gradient across the ring. Since from theminimum oil film lubrication graph, it was statedthat the rings are operating in partially floodedlubrication regime, and again this can be made surefrom the ring axial movement plots as given in Fig8. There are no discontinuities or shattering in theaxial movement graphs, it can be firmly said that

there are no signs of ring flutter throughout theengine operating cycle.

EXPERIMENTAL VALIDATION

The oil consumption test was done on the enginewith the increased end gap of 0.45mm the resultsobtained shows the close matching with thepredicted oil consumption with the software in theaccuracy of around 8 to 10 % at 3200 rpm. Theaccuracy of the results is also attributed to the inputdata of the software, which is a critical parameter togovern the prediction.

CONCLUSION

The simulation model developed using softwareRINGPAK gave an insight of the performance

parameter of the ringpack for the existing directinjection diesel engine configuration.

• The oil consumption increases with the increasein engine speed as the same way blow –by alsoshows an increasing trend. The oil consumptionincreases by 16.5% from engine speed of 1800to 2600 rpm and also shows an increase by 7%from engine speed of 2600 to 3200 rpm. It isclear that the rate of increase of oil consumptionfalls with the increase in the engine speed.

• Oil- throw off from the top ring has the majorshare in total mechanism of oil consumption

• There is no oil starvation during the completeoperating cycle of the engine this avoids themetal to metal contact throughout the engineoperating.

• With an increase in the top ring end gap, it isseen that the oil consumption has come down.The change of top ring end gap from 0.25 to0.45 mm gave an improvement of 20% at 3200rpm and at around 24% at 2600 rpm.

ACKNOWLEDGEMENT:

The authors would like to express sincere thanks toDr. P. K. Goenka, Vice President (R&D), Mahindra& Mahindra Ltd., Ricardo Consulting EngineersUk, and Management of Mahindra & MahindraLimited for their support and guidance to publishthis paper.

REFERENCE

1. Y. Chung, H. J. Schock and L. J. Brombolich, “Fire Ring Wear Analysis for a Piston Engine”,SAE Paper 930797 (1993)

2. Hubert M. Herbst and Hans H. Priebsch,“Simulation of Piston Ring Dynamics and TheirEffect on Oil Consumption”, SAE Paper2000-01-0919 (2000)

3. Hans H. Priebsch and Hubert M. Herbst,“Simulation of Effects of Piston RingParameters on Ring Movement, Friction, Blow-by and LOC” , MTZ 60 (1999)

4. Jinglei Chen, and D. E. Richardson,“Predicted and Measured Ring PackPerformance of a Diesel Engine”, Paperpresented at 4th Ricardo Software InternationalUser Conference, Detroit, March 5,1999

Fig. 8, Ring Outer Dia. Axial Movement

0

0.2

0.4

0.6

0.8

1

-180 0 180 360 540

Crank Angle in deg

Axi

al M

ovem

ent i

n m

m

Ring 1

Ring 2

Ring 3

Ring 4

ring2

ring1

ring3 &4