Engine Mount System Design.docx
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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
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