Engine Mount System Design.docx

Engine Mount System Design.docx

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EngineMountSystemDesign

PowerplantMountSystemDesign&Development

PurposeofMountSystem

TypicalSystem–3PointTorqueRollAxis（TRA）

MountSystemTerminologyandConventions

EngineOrderForces,CouplesandTorques

SuddenTorqueChange

ForceReduction（Isolation）ByPowerplantMounts

OneDimensionalSystemIsolationSystem

ForceTransmissibility

TorqueTransmissibility

Simplified3-PointPendulumTRASystem

Typical3-PointTRAPendulumSystem

S-18Example

Typical4-PointTRAPendulumSystem

Damping

LowFrequencyModalStrategy

HighFrequencyIssues

DesignIssues

DesignStrategy

PurposeofMountingSystem

1.SupportEngine

2.Restrainmotionduetostarterandconstant（DC）enginetorque

3.Reduceenginevibrationforces（orderforces）actingonvehiclebody

4.Avoidvibrationproblemsfromsuddentorquechanges.

TypicalSystem–3PointTorqueRollAxis（TRA）

2

1

Z;

Yaw

Y;Roll

X;Pitch

3

Mount3resiststheenginetorqueinconjunctionwithmounts1and2.Mounts1and2alsosupporttheweightoftheengine.

MountSystemTerminologyandConventions

CoordinatesandRotations

Hereweusevehiclecoordinates:

X-axisisfore-aft;PitchisRotationabouttheX-axis;PitchcouplesarecouplesabouttheX-axis

Y-axisissidetoside;RollisRotationabouttheY-axis;TorquesarecouplesabouttheY-axis（crank）

Z-axisisvertical;YawisRotationabouttheZ-axis;YawcouplesarecouplesabouttheZ-axis

TorqueRollAxis（TRA）

Thetorquerollaxisistheaxisonwhichafree-freepowertrainwouldrotateifsubjectedtoanoscillatingtorqueaboutthecrankshaft.Thisaxisdependsonlyontheinertiapropertiesofthepowertrain.Thecenterofgravityislocatedonthisaxis.

Principal（Static）RollAxis

Ifthemountsystemisdesignedwithsufficientcarethen:

-Thereisaprincipalrollaxissuchthatastatictorqueproducesonlyroll

-Aprincipalyawaxissuchthatyawcouplesproduceonlyyaw

-Aprincipalpitchaxissuchthatpitchcouplesproduceonlypitch

Theseaxesdependonmountstiffnessandgeometry.

Forcesinthedirectionoftheseaxesproduceonlymotioninthesamedirection.

TorqueRollAxisSystem

Ifthetorquerollaxis（controlledbyinertia）andtheprincipalrollaxis（controlledbystiffness）coincidethenanoscillatingenginetorquewillcausethepowerplanttoroll;therewillbenoothermotion.

EngineOrderForces,CouplesandTorques

BasicDefinitionsandConcepts

Engineorderforces（Fn）couplesandtorquesoscillateatfrequencieswhicharemultiplesofthecrankshaftrotationfrequencies:

-Theoscillationfrequencyisfn;

-GasTorquesdueenginefiringdependonthrottlesetting

-Inertiaforces,couplesandtorquesareduepistonandcon-rodoscillations

-Inertiaforces,couplesandtorquesareproportionaltoRPM2

-Qn/√2istheRMSvalueusedinNVH

Thefrequenciescorrespondingtoimportantordersmustbetakenintoaccount.Herewegiveabriefsummary:

Order

600

660

720

780

840

900

960

1020

1080

1120

1200

RPM

1

10

11

12

13

14

15

16

17

18

19

20

Hz

1.5

15

16.5

18

19.5

21

22.5

24

25.5

27

28.5

30

Hz

2

20

22

24

26

28

30

32

34

36

38

40

Hz

3

30

33

36

39

42

45

48

51

54

57

60

Hz

EngineOrderTypes

Therearefourtypes:

A.NormalEngineFiringOrderTorques（allenginesofgiventypehavethem）

Description

Engine（Crankshaft）Order

I3–FiringTorques

3/2,3,9/2……

I4–FiringTorques

2,4,6,…

V6–FiringTorques

3,6,9,…

V8–FiringTorques

4,8,12,…..

Usuallythelowestorderisthemostimportant,3/2forI3,2ndforI4,etc.

AtwoliterI4shouldhavea2ndordergastorqueofabout80Nmat1000rpm,idleingearwithautomatictransmission.TheprimaryfiringtorqueofV6’sandV8’saresmallerrelativetotheirsize.

B.NormalInertiaCouplesandForces

Theseforcesandcouplesresultfromtheaccelerationsofthepistonsandconnectingrods.Usuallythecrankshafthasextrabalanceweightstoreducepistonforcesby½（50%balance）butthisproducesnewforcesinthedirectionperpendiculartothecylindersandcrankshaft.

Description

Engine（Crankshaft）Order

I3InertiaCouple（pitchandyawaxes）

1,2

I3InertiaTorque（crankaxis）

NoInertiaTorque

I4InertiaTorque（aboutcrankaxis）

2

I4VerticalInertiaforce

2

90V6inertiacouple

1

60V6&90V8inertiacouples

2

AtwoliterI4hasa2ndorderinertiatorqueofapproximately20Nmat1000rpm.Its2ndorderverticalinertiaforceisapproximately400Nat1000rpm.Notethatinertiaforcesandcouplesincreaselikethesquareoftherpm.

C.UnexpectedForcesDueFiringVariationetc

Theseforcesresultfromunexpected（unwanted）sourcessuchascylindertocylindergaspressurevariation.

Description

Engine（Crankshaft）Order

Cylindertocylinderfiringvariation

½,1,3/2,..

Ordersdueengineblockdeflection

Primary±1/2

Ordersduecrankdeflection

Primary±1

Torqueconverter/Flywheelimbalance

1

Alternatorimbalance

Pulleyratiodependent

V8Banktobankdifference

1

Unequalexhaustorintakerunners

½,1,3/2,..（mainlyairborne）

SuddenTorqueChange

Suddentorquechanges,asforexamplewithgearchangeandclutchengagement,produceaimpulsivetorqueswhichcanexciteenginerigidbodymodesandtherebycauseshakeandjudder（fore-aftshake）.

ForceReduction（Isolation）ByPowerplantMounts

OneDimensionalSystemIsolationSystem

Aonedimensionalsystemshowsmostoftheimportantfactors

EquationofMotion

F

Thefigureshowsasingledegreeoffreedomsystemwith

viscousdamping.

Theequationofmotioniswrittenintwoways

or

isviscousdampingratio,cistheviscousdampingcoefficient;

Figure:

SDOFSystem.

Theresonantfrequencyofthesystemisfr;

F

ForceTransmissibility

WhenaForceFisappliedtothemassthespringanddampertransmitaforceF’

to“ground”.TheforceF’isgivenby

M

C

K

Theforcetransmissibilityis:

;

F’

Fromthiswecanseethefollowingresults:

Figure:

ForceTransmission

{j=sqrt（-1）}

forverylargef

Figure:

EffectofζonTransmissibility

Themountingsystemresonancefrequencyshouldbemuchlessthanthatofthelowestfrequencyproblemresonance（fL）thatcanbeexcitedbytheengineforces.BecauseT（√2fr）=1itissometimessuggestedthatthemountresonantfrequencyshouldbelessthanfL/√2（

）.

Caution:

（a）Thestiffnessofelastomericmountstendstoincreasewithfrequencysohighfrequencyisolationmaynotbeasgoodasexpected

（b）Highfrequencynoiseisusuallyabiggerproblemthanlowfrequencynoisesohighfrequencyisolationisimportant–particularlyforvehicledrivenathighspeedonsmoothroads.

TorqueTransmissibility

Enginetorqueisthemostimportantsourceofvibration.Atwo-dimensionalmodelillustratesthekeyideas.Thelargerectangularblockrepresentstheengine（viewedalongcrankshaft）.Twomountssupporttheweightandresistthetorsionalforces（typicalrearwheeldrivearrangement）.Wehave:

TorsionalSpringRate:

BodyAttachment

WhenaTorqueTisappliedthemountstransmitatorqueT’

to“ground”.Thecalculationsarethesameasformassandforce,andsowecanwritethetorquetransmissibilityas:

TheforceateachmountisjustF’=T/2w.Soforthemountforces:

Sowecanreducethetransmittedforcebyincreasingw.

Thistypeofsystemalsohasavertical“bounce”modeat:

Simplified3-PointPendulumTRASystem

VehicleFore-Aft

EndViewY-DirectionalongCrank

CG

PlanView–ViewfromAbove

Thepicturesaboveshowasimplifiedversionofa“pendulum”typetorquerollsystem.

-Theuppermountssupporttheengineweightandprovideisolationfromverticalforces

-Theuppermountsalsoresistfiringtorque

-Theroll-restrictormountonlyresistsenginetorque.

Theimportantmotionsare:

-Roll

-Fore-aft（X）

-Pitch

-Verticalmotion（Z）

ModeDe-coupling

Thefollowingrelationshipsprovideapproximatemodede-coupling:

Todecouplethefore-aft（X）motionfromroll:

Todecouplevertical（Z）motionfrompitch:

Todecouplefore-aft（X）motionfromyaw:

（rollrestrictorignored）

Thedecoupledresonantfrequenciesarethen:

Typical3-PointTRAPendulumSystem

Thenextfiguresshowsasideviewofatypical3-pointTRAsystem.

Front-BlockMount

SideViewofTypical3-pointTRASystem

Wenotethefollowing:

-Thetorquerollaxisisatanangletothecrankaxisbecauseofthemassdistribution

-Theprinciplerollaxisisalsoatanangletothecrankaxisbecauseofthemountarrangement

-Theprincipalrollaxisissimilartothetorquerollaxisbutcannotbemadeexactlythesame

-Thefrontblockandtransmissionmountsareclosetothetorquerollaxis.

Thefollowingpictureshowsaplanview（viewfromabove）.

OutputShaftAxis

Top（Plan）ViewofTypical3-PointTRAPendulumSystem

S-18Example:

AsanexampleofhowthisallworkswegivetheS-18parametersandshowhowCAEcalculatedfrequenciescomparetothesimplifiedcalculations.Wealsogiveexperimentalresults.

PowerplantInertiaData:

Mass=131KgJxx=3.65Kg.m.mJyy=5.77Kg.m.mJzz=4.59Kg.m.m

MountStiffnesses（N/mm）

Xyz

RightUpper9540125

LeftUpper24055110

RollRestrictor24000

MountPositions

Wr=.209mWb=.153（averagetwomounts）L1=.407mL2=.417m

Decoupling

Todeco