FE Simulation of Engine head block for structural durability

By PRADEEP PAWAR.

GuideProf. N. S. Hanamapure

INTRODUCTIONWhat is Durability?

Simply it describes overall life requirementExperimental Durability Tests and their

limitationsDevelopment of CAE toolsEver increasing power density requires …

highly optimized coolant flow (minimized volume)detail knowledge about material behavior and

modelingunderstanding failure mechanismadvanced and robust simulation processes

Durability analysis of Engine Head block

Mechanical BC’sMechanical BC’s Stress / strain analysis(durability testing, duty cycle)Stress / strain analysis(durability testing, duty cycle)

Stresses /StrainsStresses /Strains

Gasket pressureGasket pressure

Durability(HCF, LCF, TMF)Durability(HCF, LCF, TMF) OK?OK?

no

Heat transfer analysis(nucleate boiling)Heat transfer analysis(nucleate boiling)

Temperature fieldHeat fluxTemperature fieldHeat flux OK?OK?

Simulation variant

noSimulation variant

Material- filling & solidification- behavior, modeling- testing- process

Material- filling & solidification- behavior, modeling- testing- process

yes

yes Design Release(Durability)Design Release(Durability)

Deformations(liner, …)Deformations(liner, …)

Heat transfer BC’sHeat transfer BC’s

CFD simulationInner cylinder flow, coolant flowCFD simulationInner cylinder flow, coolant flow

Flow field(HTC, combustion)Flow field(HTC, combustion) OK?OK?

yes

noSimulation variant

CAE Models(FE-Model, complete engine), (CFD models coolant jacket, combustion chamber & ports)CAE Models(FE-Model, complete engine), (CFD models coolant jacket, combustion chamber & ports)

Flow BC’sFlow BC’s

Multidisciplinary Simulation Approach

Thermodynamic Analysis Thermodynamic 1D calculationsCFD simulationIn-cylinder combustion simulation Coolant flow simulationThermal or Heat Transfer Analysis

Conjugate heat transfer analysis Inputs – Surface heat flux, under-hood HTC and T Outputs – Solid temperatures

Thermo-structural Analysiscarried out to account for the different operating

conditions of the engine as per the real world usage

pattern. This includes the following,

Cold assembly

Cold firing

Warm-up

Warm-up with peak firing load

FE Model - first activity

Material Data - elasto-plastic curves

Fatigue AnalysisFatigue analysis always begin at a crackStages of Fatigue Failure

Crack initiation(life till crack of 2mm is detected)

Crack propagationSudden fracture due to unstable crack growth

S-N or ε-N curves are base Modes of failure

HCF – Endurance factor of safety

LCF – Damage

High Cycle Fatigue

Low Cycle Fatigue

105 2*105

Endurance strength

No. of cycles

Alternating stress

Various Approaches for Fatigue Analysis

Stress Life approachOld MethodStress or S-N diagram is baseUsed for HCF applications (i.e. assembly expected

to last for 100000 cycles)

Strain Life approachStrain or ε-N diagram is baseUsed for LCF applications

Fracture Mechanics approachUsed for calculation of remaining life i.e. crack

propagation life

Case StudyEvaluation of strength of engine cylinder

head and crankcase assemblytwo cylinder four stroke engineThem0-dynamic AnalysisCFD simulations

Coolant flow simulationIn-cylinder – HTC & Fluid temperature

Under-hood- HTC & Air temperature

Conjugate Heat transfer approach is used

Thermo-structural AnalysisLoad cases

Assembly Load case Cold Firing Load case Engine Warm-up Hot firing load case

FE ModelConsist two cylinders

Fatigue AnalysisHCF analysis is carried out to calculate

fatigue safety factorStress life approach is used

Design ModificationLow safety factors were observed in some of

the regions. Modifications were suggested to improve the safety factors in those regions.

Case StudyEngine discussed hear is a diesel enginedue to increased power output, cylinder head

is susceptible to High cycle Fatigue (HCF) cracks as well as Low Cycle Fatigue (LCF) cracks at weaker areas like water bracket area, valve bridge on fire deck, fuel line boss, intake port bridge, etc.

FE ModelTwo complete cylinders (#5 and #6)

Boundary Conditions Symmetric BC are applied at cutting planes AA and BB.

The full speed full load durability test on diesel engine developed showed that cylinder head shows more cracks in head bolt boss area and intake port bridge due to HCF.

The stress analysis under assembly loads, thermal loads and high cycle firing loads is carried out in ABAQUS.

FEMFAT Version 4.6b is used for calculations of fatigue factor of safety. Safety Factor is less than 1 in some areas is observed therefore some design modifications are required.

The supplementary rib is provided at intake port bridge and augmented thickness is added at head bolt boss area

This design modification increases bending stiffness at crack location. The modified design at intake port bridge and head bolt boss is effective to improve the durability (increase of about 13% in safety factor).

ConclusionSuccessful mechanical development of

today's increasingly high loaded engine designs demands new sophisticated simulation techniques.

Cracks can be investigated by FE analysis and optimization design can be obtained.

It is possibly increasing to replace the classic durability test in engine development .

CAE is essential method in rapid development process.