ic-engine source characterisation and exhaust system simulations1583508/... · 2021. 8. 7. ·...

IN DEGREE PROJECT VEHICLE ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2021

IC-Engine Source Characterisation and exhaust system simulations

An investigation into the sensitivity of the source characteristics and the inclusion of source characteristics in exhaust simulations

PONTUS GRÄSBERG

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ENGINEERING SCIENCES

IC-Engine Source Characterisation and exhaustsystem simulations

An investigation into the sensitivity of the source characteristics and the inclusion ofsource characteristics in exhaust simulations

PONTUS GRASBERG

Degree Project in Technical AcousticsSupervisor and Examiner: Hans Boden, KTH Royal Institute of Technology

Supervisor: Naveen Indolia, SCANIA CV ABMay 19, 2021

Abstract

To be able to predict the sound pressure level emit-ted by a exhaust system one must be able to describe thesource. The source in the form of an engine can linearly bedescribed as a source strength and a source impedance. AnIC-engine can acoustically have a non-linear part mean-ing that the source characteristics have a dependency onthe load. The first part of this work investigates throughsimulation’s in GT-Power how these characteristics are af-fected by the load connected to the source. The Secondpart of the work combines the source characteristics withsimulations of a muffler and compares to different methodsof getting the pressure at the outlet of the exhaust. Thefirst method is direct simulation of the muffler in COMSOLMultiphysics and the second is a transfer matrix based cal-culation. How sensitive the results at the outlet are tochanges in the source impedance is also tested.

It is concluded that using five loads for the multiloadmethod in the form of five different lengths on the pipeconnecting the engine and muffler works when the pipe havethe same length as would be seen in reality. Furthermore,the pipe lengths should have a small range, 100 mm betweenlargest and smallest pipe length giving good results. Thesource characteristics were at least above 1000 RPM stableenough as to not significantly change the sound pressurelevel at the outlet.

Keywords: IC-engine, source characteristics, Multiloadmethod, one-port source, exhaust system

ReferatBränslemotor källkarakterisering och

avgassystemssimuleringar

For att kunna modellera ljudtrycket som avges fran ettavgassystem behover man kunna beskriva kallan. Kallani form av en branslemotor kan linjart beskrivas som enkallstyrka och en kallimpedans. En branslemotor kan dockha en akustisk ickelinjar del vilket medfor att kallan kanvara beroende utav vilken last i form av ljuddampare denar kopplad till. Forsta delen av detta arbete undersokergenom motorsimuleringar i GT-Power hur lasten paverkarkallkarakteristiken. Den andra delen av arbetet kombine-rar kallkarakteristiken med simuleringar av ljuddamparenoch jamfor olika metoder for att fa ljudtrycket vid utloppetav avgassystemet. Den forsta metoden for detta ar direktsimulering av ljuddamparen i COMSOL Multiphysics darkallkarakteristiken inkluderas och den andra metoden artransfermatris baserad. Det testas ocksa hur kansligt ljud-trycket vid utloppet av ljuddamparen ar for variationer iimpedansen.

For kallkarakteristiken anvands fem laster per utrakningoch slutsatsen dras att lasten i form av roret mellan mo-tor och ljuddampare samt ljuddamparen bor vara sa liksom mojligt det riktiga systemet. Utover det dras slutsat-sen att en liten variation i det kopplande rorets langd gerbattre resultat och att en variation mellan storsta och mins-ta roret pa 100 mm ger bra resultat. Till sist dras slutsat-sen att for varvtal over 1000 RPM ar kallkarakteristikentillrackligt stabil for att ge stabila resultat vid utloppet avljuddamparen, medans under 1000 RPM kan det vara sta-bilt nog men har ar validering viktigare.

Acknowledgements

First, I would like to thank my supervisor at SCANIA CV AB Indolia Naveen forgiving me the opportunity to do this thesis, for being a guiding hand throughout thethesis work and for discussions on the subject. Furthermore, I would like to thankKim Pettersson at SCANIA CV AB for ensuring a good working environment andalways being helpful.

I would also like to thank my supervisor at KTH Hans Boden for suggestionsand advice on the work in the thesis.

Finally, I would like to thank Johan Dahlqvist, Sciuto Fabrizio and JorgenMardberg at SCANIA CV AB for both supplying engine models in GT-Power aswell as advice on the use of GT-Power.

Contents

1 Introduction 31.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Theory 72.1 Two-port model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 IC-engine source characteristics . . . . . . . . . . . . . . . . . 72.2 Multiload Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3 Non-linear effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.4 GT-Power simulations . . . . . . . . . . . . . . . . . . . . . . . . . . 92.5 Sound pressure level after exhaust outlet . . . . . . . . . . . . . . . . 10

2.5.1 COMSOL Multiphysics simulations . . . . . . . . . . . . . . . 102.5.2 ESA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.6 Frequency missmatch . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.7 Pass-by requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.8 Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3 Method 133.1 Engine Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.1.1 Simulation outline . . . . . . . . . . . . . . . . . . . . . . . . 143.2 FEM Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2.1 COMSOL model Setup . . . . . . . . . . . . . . . . . . . . . 16

4 Results & Discussion 194.1 Source sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.1.1 Average pipe length . . . . . . . . . . . . . . . . . . . . . . . 194.1.2 Pipe length range . . . . . . . . . . . . . . . . . . . . . . . . . 234.1.3 Engine Muffler tests . . . . . . . . . . . . . . . . . . . . . . . 26

4.2 Exhaust simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.2.1 Feasibility of PML boundary condition . . . . . . . . . . . . . 304.2.2 Influence of source impedance . . . . . . . . . . . . . . . . . . 304.2.3 COMSOL and ESA . . . . . . . . . . . . . . . . . . . . . . . 31

4.2.4 CBE1 and CBE1u engine comparison . . . . . . . . . . . . . 334.2.5 Engine load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5 Conclusions 37

6 Future work 39

Bibliography 41

Appendices 42

A Set up of Multiload simulations in GT-Power 43A.1 Simulation setup in GT-Power . . . . . . . . . . . . . . . . . . . . . 43A.2 Set-up of separate multiload calculation . . . . . . . . . . . . . . . . 45

B Import source characteristics 49

C Waterfall Plots 52

List of Figures

1.1 Schematic representation of engine and exhaust system. . . . . . . . . . 4

2.1 Electro-acoustic analogies representation for a source and a load. Figuretaken from [5]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Example of a truck muffler with an integrated outlet. . . . . . . . . . . . 112.3 Schematic representation of a pass-by test[4]. . . . . . . . . . . . . . . . 12

3.1 Example of engine simulation model in GT-Power. This is a CBE1/CBE1umodel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2 Boundarys used for boundary conditions in COMSOL Multiphysics sim-ulations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.1 Total source strength Lp for same engine and muffler with different casesfor connecting pipe. Made using MATLAB. . . . . . . . . . . . . . . . . 20

4.2 Real source impedance [Pa ∗ s/m3] over 3rd, 6th and 9th engine orders.Made using MATLAB . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.3 Imaginary source impedance [Pa ∗ s/m3] over 3rd, 6th and 9th engineorders. Made using MATLAB . . . . . . . . . . . . . . . . . . . . . . . . 22

4.4 Total source strength Lp for same engine and muffler with different casesfor connecting pipes. Made using MATLAB. . . . . . . . . . . . . . . . 23



4.7 Total source strength Lp for same engine and muffler with different muf-flers connected. Made using MATLAB. . . . . . . . . . . . . . . . . . . 26



4.10 IL loss for two sizes of the sphere for the PML boundary condition. . . . 304.11 SPL at the opening of the outlet for different percentage differences in

the source impedance. From COMSOL . . . . . . . . . . . . . . . . . . 31

4.12 Comparison of the spectrum for CBE1/CBE1u using COMSOL Multi-physics and ESA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.13 SPL spectrum for CBE1 and CBE1u for different RPMs . . . . . . . . . 344.14 Total SPL using COMSOL Multiphysics for CBE1 and CBE1u . . . . . 344.15 Engine load from previous pass-by test. . . . . . . . . . . . . . . . . . . 354.16 Total SPL for CBE1 and CBE1u at different Engine loads. . . . . . . . 36

A.1 Example of connecting pipe element in a GT-Power model. . . . . . . . 44A.2 Example of settings for the connecting pipe . . . . . . . . . . . . . . . . 44A.3 necessary settings in the plots section of the connecting pipe. . . . . . . 45A.4 Example of the Excel file for the case setup. . . . . . . . . . . . . . . . . 46A.5 Example of the data in GT-Power solution file. . . . . . . . . . . . . . . 46A.6 Case setup for Multiload calculations . . . . . . . . . . . . . . . . . . . . 47A.7 Example of produced result files. . . . . . . . . . . . . . . . . . . . . . . 47

C.1 Real part of the source strength for different lengths of the connectingpipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

C.2 Imaginary part of the source strength for different lengths of the con-necting pipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

C.3 Real part of the source impedance for different lengths of the connectingpipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

C.4 Imaginary part of the source impedance for different lengths of the con-necting pipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

C.5 The source strength for different ranges of the connecting pipe lengths. . 56C.6 The source impedance for different ranges of the connecting pipe lengths. 57C.7 The source strength for different connected mufflers. . . . . . . . . . . . 58C.8 The source impedance for different connected mufflers. . . . . . . . . . . 59

LIST OF FIGURES

Nomenclature

Name Description UnitPS Source strength PaPL Pressure at muffler inlet PaLs Source strength sound power level dBZS Source impedance Pa ∗ s/m3

ZL Load impedance Pa ∗ s/m3

Q Volume flow rate m3/sTii Transfer matrix coefficient 1T12 Transfer matrix coefficient Pa ∗ s/m3

T21 Transfer matrix coefficient m3/(Pa ∗ s)Lp Sound pressure level dBf Frequency HzT Time s

1

LIST OF FIGURES

List of Abbreviations

Abbreviations DescriptionFFT Fast Fourier TransformRPM Revolutions Per MinuteIC Internal CombustionSPL Sound Pressure LevelPML Perfectly Matched LayersBEM Boundary Element MethodIL Insertion LossDOE Design Of ExperimentCFD Computational fluid dynamics

2

Chapter 1

Introduction

1.1 Background

When simulating an exhaust system, one could get an insertion loss or transmissionloss for the muffler. While this tells what change in the sound pressure level canbe expected when adding the muffler to the engine it does not give what the soundpressure level will be and when comparing to a new engine the necessary insertionloss in unknown. To be able to ease testing of new muffler designs and not have toinclude the engine in all new simulations the engine and muffler parts need to beseparated. In an ideal case with regards to the design process these two-parts wouldbe independent. It has however been indicated that the engines acoustic propertiesis affected by the muffler, this means that it is of importance to see how dependentthe engine characteristics are to know when new simulations of the engine is neededfor changes in the muffler design.

1.2 Purpose

The work has 2 main parts, the wanted end result is to predict the sound pressurelevel after an exhaust. This will be done through simulations of the engine, andthen connecting these to simulations of the muffler.

An exhaust system is described in figure 1.1. Such a system could be describedas a one-port source connected to a 2-port system i.e. the engine, connected to aload being the connecting pipe and muffler. For such a system one could linearlydescribe it as the source consisting of a source strength and a source impedance.For a completely linear system these source characteristics would be independentof what load they are connected to, but an IC-engine has non-linear effects. Whenrunning simulations in GT-Power it has been indicated that the source character-istics change when the load is changed. Since the source characteristics affect thecalculated insertion loss and inlet acoustic pressure of a muffler, they should not beunreasonably different from the true characteristics of this source load combination.As to not be forced to get new engine characteristics for every change of a muffler it

3

CHAPTER 1. INTRODUCTION

Figure 1.1: Schematic representation of engine and exhaust system.

is of interest to know how sensitive the characteristics are to the load. Investigatingthis sensitivity is the first part of the work.

The second part of the work is to connect the engine characteristics to the 3Dsimulations done on the muffler in order to get the sound pressure level after anexhaust. There are different ways to achieve this. Since the source is described asa one-port it would be reasonable to use a transfer matrix for the muffler togetherwith an end radiation impedance. One problem with this approach is the use ofan integrated outlet for the muffler, this means there is no tailpipe for the mufflerwhich is needed for the calculation of a transfer matrix.

The limitations in this work means that the studies will be carried out throughsimulations. Measurements will not be done in this work, but further validationwould be necessary.

1.3 MethodologyTo test the sensitivity simulations will be carried out using the program GT-Power.In GT-Power the whole engine and exhaust system can be modelled with 1D ele-ments. Simulations will then be made for one engine and muffler combination, butwith changing length of the connecting pipe. The connecting pipe changes the load

4

1.3. METHODOLOGY

impedance, this can then be used to get the engine characteristics for different loads.These engine characteristics can then be compared to see when it is necessary tocalculate new engine characteristics for a muffler. After this, simulations will alsobe carried out with different mufflers and a different engine to see how these largechanges affect the system.

Lastly, different methods for obtaining the sound pressure level at the outlet ofthe exhaust will be tested and compared to see the effect of different approaches.

5

Chapter 2

Theory

In this chapter methods and considerations used for the work will be described.When measuring source characteristics there are two characteristics of interest in alinear time-invariant case, that is the source strength and the source impedance.

The Source characteristics need to be combined with or imported as boundaryconditions to exhaust system simulations in order to get the sound pressure level(SPL) at some point in or after the muffler.

2.1 Two-port model

A multi-port is a system where there is a relationship between a set of input variablesand a set of output variables. A Two-port is a variant of this where there are twoinput and two output variables, it is also referred to as a four-pole because ofthere being four variables. The two-port is a linear relationship between a set ofinput variables and a corresponding set of output variables and can be a sufficientdescription of a system that is linear and time-invariant. Most commonly the volumevelocity q and the pressure p is used for a two-port model as described in equation2.1 [1]. [

P2Q2

]=

[T11 T12T21 T22

] [P1Q1

](2.1)

Where qi is the volume flow and pi is the pressure.

2.1.1 IC-engine source characteristics

For sources that are disconnected between the outlet and inlet the source can bedescribed as a one-port. The one-port can then be modelled as a source strength andsource impedance for a linear time-invariant system [2]. These source characteristicscan then be combined with a transfer matrix to get the SPL at the outlet of anexhaust system.

7

CHAPTER 2. THEORY

This can be handled in a direct way by using something such as a two-microphonemethod and have two different excitation’s of the same system. In the case of anIC-engine the sound power level from the engine is high and there is therefore hardto achieve a high enough external excitation as to be measured over the engine.Turning of the engine would solve this but then the source characteristics changesignificantly. Lastly even if a high enough external excitation is achieved there arethen likely to be strong non-linear effects making results uncertain [3].

Another way to get the source characteristics is to change the load of the sourcei.e. changing the muffler or connection to the muffler. This is a multiload methodand is described in further detail in section 2.2.

2.2 Multiload Method

When using the multiload method an electro-acoustic analogy can be used to setup the equations. Two ways of formulating this analogy is shown in figure 2.1. Inthe figure marked as a in 2.1 the analogy is formulated as a pressure source with asource impedance ZS and a load impedance ZL. The case marked as b is equal tothe analogy in a, but here the source is formulated as a volumetric flow rate source[4].

Figure 2.1: Electro-acoustic analogies representation for a source and a load. Figuretaken from [5].

The cases as set up in figure 2.1 is assuming a linear case of the source and load.Using this and formulating the relationship between pressure and volumetric flowrate we get equation 2.2.

PL = PS − ZSQ, Q = PLZL

, (2.2)

Combining the two equations in equation 2.2 one can get equation 2.3.

PLZL = PSZL − ZSPL (2.3)

8

2.3. NON-LINEAR EFFECTS

If the pressure PL is known and that either the impedance of the load ZL orindirectly the volumetric flow rate Q is also known there are now two unknowns.These two unknowns are the source characteristics PS and ZS . Marking differentload cases with index i we get equation 2.4.

PL,iZL,i = PSZL,i − ZSPL,i (2.4)

To determine these two, at least two different load cases i are needed, using morecases creates an overdetermined system which could be solved as the least squaresolution.

2.3 Non-linear effects

Using the electro-acoustic analogy, it would be possible to describe the system welland the source would have one solution. The analogy and two-port model doeshowever rely on the assumption that the system is linear and time-invariant. Inthe case of an IC-engine the source has a non-linear part. This means that thecharacteristics of the source can be affected by the load it is connected to.

It is then of importance to have a proper selection of loads. Since the loadsaffect the source, in an ideal case we would want one load for the investigation, butsince the only real alternative is using the multiload method these need to be chosencorrectly. The loads need to be sufficiently different as to provide a non-singularsystem. Secondly the loads should be similar enough, especially with regards to theback pressure as to not effect the source significantly [6].

The back pressure could be included in several ways. One could simulate or mea-sure the back pressure and set this as the ambient pressure in an engine simulationwithout a muffler, but this would not include non-linear interaction between themuffler and the engine. A second alternative is to include a 1D model of the mufflerdirectly in the simulations of the engine. This has been done in the simulationshere.

2.4 GT-Power simulations

Simulations are carried out in a software called GT-Power. GT-Power simulate thewhole engine in the time domain by building an engine model from one dimensionalelements [7] and solving the Navier-Stokes equation in one dimension. The fact thatthe engine and muffler model is made up of one dimensional elements means thatit can only describe the plane waves [1] meaning that results are poor when planewaves does not dominate the field [8]. GT-Power does non-linear calculations andtherefore it is possible to capture non-linear interactions between the engine andmuffler.

9

CHAPTER 2. THEORY

2.5 Sound pressure level after exhaust outletFor many purposes the source characteristics are not themselves interesting, whatis of interest is the pressure levels produced at and after the outlet of the exhaust.Calculating this can be done in several ways, one way is to use the electric-acousticanalogy as presented in figure 2.1 a together with a two-port model of the muffler.In this case an analytical model of the exhaust radiation must be applied in orderto get the pressure outside of the muffler. The internal software ESA at SCANIACV AB does these kinds of simulations and will be compared.

Another possibility is to do a FEM calculation of the muffler where the sourcecharacteristics are included as boundary conditions. This approach has the addedbenefit of being able to capture effects of the outlet if the situation is different fromthe analytical radiation model, which could be the case in some designs of integratedoutlets.

Lp = 20 ∗ log10(p/pref ) (2.5)

Lp,tot = 10 ∗ log10(∑

10Lp,i/10) (2.6)The total SPL Lp,tot can be calculated according to equation 2.6.

2.5.1 COMSOL Multiphysics simulationsSimulations of the muffler was carried out using COMSOL Multiphysics. COMSOLMultiphysics is a commercial FEM program in which the simulations of the mufflerwere carried out.

2.5.2 ESAExhaust System’s Acoustics (ESA) is an internal transfer matrix based acousticcalculation software at SCANIA CV AB. By using COMSOL Multiphysics to getthe Transfer matrix of the muffler this can then be used as an input for ESA togetherwith the source characteristics. For the radiation from the outlet of the muffler ESAuses an assumption of a free field monopole source. For an exhaust of an IC-enginethis is the type of source to be expected. A monopole source is omnidirectional andfor low frequencies this is theoretically the case. However, the presence of the flowfield can cause directivity in the actual radiation from the source. This cannot beincluded in ESA and may affect the results [9].

For the calculation of a transfer matrix a tailpipe is necessary. Some muffler usesan integrated outlet meaning that there is no tailpipe which can result in effectsfrom the outside part of the integrated outlet. Some of these integrated outletsare indented into the muffler meaning that the effects of the outside is stronger,an example of a muffler with an integrated outlet that is not indented is shownin figure 2.2. This can affect both the end impedance and the radiation from theexhaust. These effects are therefore not included in the transfer matrix or theradiation model.

10

2.6. FREQUENCY MISSMATCH

Figure 2.2: Example of a truck muffler with an integrated outlet.

2.6 Frequency missmatch

The data of from the GT-Power simulations will be the source data of the enginein the frequency domain. The frequencies of these characteristics match multiplesof the engine orders meaning that the frequencies will change with different RPMsas described in equation 2.7 where fo is the frequency of the order.

fo = order ∗ RPM

60 (2.7)

Since it is of interest to connect these characteristics to 3D simulations of amuffler and doing 3D simulations for all frequencies for all RPM is time consumingit is important to interpolate to the right frequencies or to limit the number ofcases. For a Transfer matrix approach to the problem the source data needs to beinterpolated to the frequencies used for the transfer matrix. For the impedance, aspline interpolation works well. For the source strength interpolating the frequenciesis not correct since this is the same as inserting energy into the system. This partcan instead be handled by constructing a time signal with the sampling frequencyand length according to what is needed to connect to the transfer matrix. Thendoing an FFT to get the frequency spectrum One way to construct such a timesignal is equation 2.8 and this is the way it is handled in the Internal SCANIA CVAB software ESA.

P (t) =f∑

P (f) ∗ e2πif∗T (2.8)

Where T is a time vector according to the time length and sampling frequencynecessary.

11

CHAPTER 2. THEORY

2.7 Pass-by requirementThe current legal requirement for the noise from trucks to be sold within the EUis reg. (EU) 540/2014, this is a pass-by noise measurement. For these tests, thedriving conditions for the truck is specified meaning that we know at what RPMrange the truck will be running. The RPMs as the truck passes the microphoneas seen in figure 2.3 is in the legislation specified as being 85% -89% of the enginespeed at the rated maximum power. The RPMs are therefore dependent on wherethe maximum power of the engine is, for example if the rated maximum is at 1800RPM the RPM range for the test becomes approximately 1530-1600. Furthermore,a speed and a load in the form of some weight on the truck is specified meaningthat the running conditions can be specified [10].

Figure 2.3: Schematic representation of a pass-by test[4].

In figure 2.3 the set-up for the measurements is described.

2.8 EnginesFor comparisons two engines have been simulated. The two engines are physicallyidentical the difference lies in the engine tuning. The first engine is the CBE1(Common Base Engine 1) which is a four-stroke six-cylinder diesel engine and thesecond one is a CBE1u which is its replacement because of updated legalization.The specific ones used for simulations here are 475 Hp version made for usage inNavistar trucks.

12

Chapter 3

Method

When trying to investigate the sensitivity of the engine source characteristics withregards to a load it was decided that this should be handled by varying the con-necting pipe length as seen in figure 1.1. The benefit of this is that the change inthe load is well controlled, steady and understood.

Apart from the connecting pipe changes there is also simulations for differentmufflers and an open end for the same engine to see the effect when there are largerdifferences in the acoustic load. The engine load and set-up for the engine was inall cases identical, the engine load was 100%.

Lastly there are simulations of the muffler including the source characteristics ofthe engine. These simulations tested the impact of the source impedance, comparedthe two engines and the impact of the engine load.

3.1 Engine Simulations

To investigate the sensitivity of the engine characteristics a simulation programcalled GT-Power was used. The benefit of doing simulations instead of measure-ments are that it allows for flexibility when deciding what cases to test as well asallowing for more tests since measurements takes time and requires both space andequipment. The engine simulations were each done for 5 load cases and the multi-load method presented in section 2.2 is used to acquire the source characteristics.It has been found that a calibrated 1D engine model such as the ones that has beenused here provide reasonably accurate results for low frequencies [11].

13

CHAPTER 3. METHOD

Figure 3.1: Example of engine simulation model in GT-Power. This is aCBE1/CBE1u model.

In figure 3.1 an overview of a model used for simulations is shown. In this figurethe variable pipe is shown which is used by changing its length to create the neededload cases. The left side of the pipe contains the engine model while the right is a 1Dmodel of a muffler used for the engine. Since there can be non-linear characteristicsfrom an engine it is important to take the characteristics at a case which is similarto its actual use case. For this reason, a model of the muffler creating the backpressure and possibly other effects was used in the simulations.

The set up procedure used for calculating the source data is outlined in appendixA.

3.1.1 Simulation outline

In the simulations it was of interest to see how sensitive the system was to a changein the load i.e. the muffler. For this reason several simulations were made for thesystems with changes in the load, The first scenario was to change the length of theconnecting pipe, since a change in the pipe length is also used for the different loadchanges this in practice meant that the average length of the pipe was changed. Intable 3.1 the lengths used for the different simulations are listed.

The second scenario that was tested was to keep the average length of theconnecting pipe constant while changing the range of pipe lengths used. In table3.2 the lengths used for this part is listed. In all cases the average length of theconnecting pipe is 1050 mm.

In the last scenario of the engine simulations large changes in the acoustic loadwas tested, for these simulations there were three cases. Using an engine with aCAS1 silencer, a medium muffler and lastly with an open end.

14

3.2. FEM SIMULATIONS

0-2 m 2-4 m 4-6 m 6-8 m 8-10 m 10-12 m Unit100 2100 4100 6100 8100 10100 [mm]575 2575 4575 6575 8575 10575 [mm]1050 3050 5050 7050 9050 11050 [mm]1525 3525 5525 7525 9525 11525 [mm]2000 4000 6000 8000 10000 12000 [mm]

Table 3.1: Lengths used for testing the connecting pipe lengths impact

Range: 1000 mm 200 mm 100 mm550 950 1000800 1000 10251050 1050 10501300 1100 10751550 1150 1100

Table 3.2: Lengths used to test the connecting pipe length range impact

3.2 FEM Simulations

The simulations of the exhaust were carried out on a muffler using COMSOL Mul-tiphysics. Since using all the frequencies and all the RPMs result in a number ofcases too large to be practical, solving the muffler for all the source data was nota practical alternative. If RPMs between 800 and 2300 was used and there wereapproximately 30 frequencies per RPM this would be 4300 cases. This results intwo possibilities, either simulating the muffler to get a transfer matrix and thencombining this with the source data to get the pressure and volume flow at theoutlet for an engine case. Another possibility was to set up the FEM simulationswith the source data but to limit the number of cases. The result from these twoapproaches will be compared.

Limiting the number of cases could be done by 2 ways, first the frequencies rangeand resolution could be limited and secondly the RPM range could be limited. Thefrequencies from the engine simulations are multiples of half the first engine ordermeaning that they are on all the engine orders which contain the most acousticenergy. This is the frequency resolution that was used. Secondly in the RPM rangeof interest there was no interest in going more than 400 Hz since there are no strongengine orders in that range. For reference the 9th engine order at 2300 RPM canbe calculated from equation 2.7 as 345 Hz. With these limitations one can still notinclude all the RPMs as there would be to many cases therefore the RPM range alsoneeds to be limited for the simulations to be practical. The legal requirement asdescribed in section 2.7 is at a specified RPM range. Using this range and a RPMresolution of 10 there ends up being around 10 RPM cases. This together with thateach RPM has around 30 frequencies to be calculated results in approximately 300

15

CHAPTER 3. METHOD

calculations on the muffler.When doing the FEM simulations including the source strength and the source

impedance of the engine It was tested how the impedance affected the system.Firstly, by running the simulation with the source impedance from engine simula-tions times some constant.

After this, simulations were also done comparing the acoustic performance of thetwo engines CBE1 and CBE1u by calculating the SPL at the outlet both directlyin COMSOL Multiphysics as described above as well as by using transfer matrix.

3.2.1 COMSOL model Setup

The internal part of the muffler is in the COMSOL Multiphysics simulation set upaccording to the established method at SCANIA CV AB. The conditions at theinlet and outlet are changed according to what is needed for the simulations. Sinceit is of interest to include the source characteristics of the engine both an incidentwave and an interior impedance needs to be defined at the inlet for the muffler.

At the outlet a free field condition was necessary, for this boundary elementmethod (BEM) at the outlet surface shown in figure 3.2 a, could be used. TheBEM resolves the field around the muffler and does therefore include the effect ofreflections at the outlet. Another alternative was to use perfectly matched layers(PML) which is a reflection free condition, since this condition is reflection freeit cannot be applied directly at the outlet since this would remove the effects ofreflections here. PML is instead applied on a sphere around the outlet meaningthat possible effects of the outlet are included. This is shown in figure 3.2 b.

(a) (b)

Figure 3.2: Boundarys used for boundary conditions in COMSOL Multiphysicssimulations.

Since BEM resolves the fields outside of the muffler it becomes computationally

16

3.2. FEM SIMULATIONS

heavy. For this reason, PML was used to investigate the SPL at the outlet. Thiscondition could be replaced by the BEM condition to get the pressure after theexhaust outlet.

To get the Transfer matrix simulations on the muffler was done by creating twocases. These two cases were made by exciting the muffler with an incident pressurewave once from the inlet and once from the outlet. Then the waves were measuredby means of the three-microphone method at both the inlet and outlet. Since thiswas only a secondary calculation done in order to get a comparable result and themethod is well established and validated it will not be discussed in detail here.

17

Chapter 4

Results & Discussion

In this section the results from the simulations described in section 3 is presentedand discussed. This is done in two parts, the first parts concern the sensitivity ofthe source characteristics that is acquired from GT-Power simulations. The secondpart is the usage of the source characteristics to calculate the SPL at the outlet ofthe muffler.

4.1 Source sensitivity

It is established that the acoustic source characteristics of an engine is dependenton the connected load. This means that the loads must be selected with regardsto what the engine will be connected to. For this reason, the simulations in thissection were done in order to investigate how sensitive the engine is to the changesin load.

4.1.1 Average pipe length

In this part several simulations have been performed where the engine model and themuffler model has been the same. The only difference in these simulations is in theconnecting pipes length. The connecting pipe lengths used are according to table3.1 in section 3. In figure 4.1 the total source strength at each RPM is presented.Here the total length of the connecting pipe has been changed. For all cases 5 pipelengths has been used creating an overdetermined system. In figures C.1 to C.4 inappendix C the waterfall plots for the source characteristics corresponding to thosein figures 4.1 to 4.3 are presented.

19

CHAPTER 4. RESULTS & DISCUSSION

Figure 4.1: Total source strength Lp for same engine and muffler with different casesfor connecting pipe. Made using MATLAB.

Figure 4.1 shows that the total connecting pipe lengths does not have a greatimpact on the total source strength Ls. This indicates that small changes to themuffler does not greatly affect the engine characteristics. The maximum differenceobserved is around 1 dB which can be considered small. This difference is at lowRPM.

For a six-cylinder engine such as has been simulated here most of the acousticpower is on the 3rd and 6th engine order, meaning that these RPM/frequency pointsare acoustically the most important. For that reason, the source impedance overthese engine orders has been plotted. In figures 4.2 and 4.3 the simulated impedancefor the cases corresponding to the source strength in figure 4.1 is presented. Thisis the source impedance divided by the area [kg/sm2]=[Pa ∗ s/m3]. In figure 4.2the real part for the engine order is plotted while the imaginary part is plotted infigure 4.3.

Since the frequency of an engine order is dependent on the RPM as in equation2.7 each RPM has a corresponding frequency for each engine order. For this reasonthe plots in figures 4.2 and 4.3 use both corresponding x-axis.

20

4.1. SOURCE SENSITIVITY

Figure 4.2: Real source impedance [Pa ∗ s/m3] over 3rd, 6th and 9th engine orders.Made using MATLAB

21


Figure 4.3: Imaginary source impedance [Pa ∗ s/m3] over 3rd, 6th and 9th engineorders. Made using MATLAB

No steady change in the impedance over the changes in connecting pipe lengthcan be observed. The impedance is more stable for higher RPMs while the impedancecan have significant changes for RPMs below 1000. This is well illustrated in the6th order in figure 4.2 as well as in figure 4.3.

Outside of the main engine orders the acoustic power is significantly lower mean-

22


ing that the signal to noise ratio is in most likelihood larger. This could at leastpartly explain why the stability is worse outside of the main engine orders.

Since the instability is observed at the same RPM range at all orders and notthe same frequencies range it is likely that it is an effect of the engine behavior atlower RPM and not just an effect of geometric parameters such as resonances inthe connecting pipe. This could be some non-linear or time-variant effect whichwould not be captured well by the linear multiload method. In this case it becomesimportant to use the parts according to how it would be used. It may also be that itis sensitive to small deviation and that even this might not give results that coincidewith the actual system. To confirm if the source characteristics can be used and ifso, what considerations should be had verification measurements are necessary.

4.1.2 Pipe length range

The plot in figure 4.4 is the total source strength Ls that uses the same engine andexhaust model as in figure 4.4 to 4.6. The different plots here correspond to changesin the length of the connecting pipe while keeping the average pipe length constantat 1050 mm. For this 3 different ranges were tested with the lengths according totable 3.2 in section 3. In figures C.5 and C.6 in appendix C waterfall plots of thesimulations in figures 4.4 to 4.6 can be seen.

600 800 1000 1200 1400 1600 1800 2000 2200 2400

RPM

155

160

165

170

175

180

Ls [dB

]

CAS1 Center length 1050 mm

Range: 1000

Range: 200

Range: 100

Figure 4.4: Total source strength Lp for same engine and muffler with different casesfor connecting pipes. Made using MATLAB.

23



24



Here as well there is only minor changes in the total source strength in figure4.4. In the impedance in figures 4.5 and 4.6 there is changes to the impedance

25


outside of the low RPM range. These changes may be because of improved resultsas we move closer to the actual system. Considering that they get closer with adecrease in the range this is likely. It can also be seen that when the average rangeis constant the low end is stable over the scenarios. Since the low RPM range doesnot here change it indicates that this range though sensitive to changes gives correctvalues for the specific connecting pipe length. To be able to draw conclusions withcertainty measurements would be needed.

4.1.3 Engine Muffler tests

In section 4.1.1 and 4.1.2 the changes in the load has been relatively small. It isknown that the load has an impact on the source characteristics and in this section,it will be investigated if the effects are large when the change in the load is large.For these simulations, a 475 hp CBE1u engine was simulated while connected toa CAS1 silencer, a medium silencer and lastly with no muffler and an open end.These cases provide large differences in the back pressure and other acoustic effects.

900 1000 1100 1200 1300 1400 1500 1600 1700 1800

RPM

169

170

171

172

173

174

175

176

177

178

179

Lps [dB

]

Total source strength Lps

CAS1

No muffler

medium muffler

Figure 4.7: Total source strength Lp for same engine and muffler with differentmufflers connected. Made using MATLAB.

In figure 4.7 the total source strength Ls is shown. Here the Ls are significantlydifferent and the right muffler should be attached to give results corresponding to

26


how the engine in practice is run.Figure 4.8 and 4.9 shows the real and imaginary parts of the source impedance

over the 3rd, 6th and 9th engine order. Here also a large difference can be ob-served between the different mufflers. The difference in both source strength andimpedance are significant with regards to what SPL they would cause in the muffler.Waterfall plots for the data presented in figures 4.7 to 4.9 can be seen in figures C.7and C.8 in appendix C.

27



28



29


4.2 Exhaust simulationsThe source characteristics are often in themselves not interesting. What is of interestis the SPL at the outlet and outside of the muffler. For this reason, simulation inthese sections has been carried out including the source characteristics.

4.2.1 Feasibility of PML boundary conditionPreviously BEM has been used to create the free field conditions at the outlet. Thismethod becomes computationally heavy and for that reason PML was used to setup the free field condition. This condition is an alternative for calculating IL, TM,and the SPL in the muffler or at the outlet, but for calculations far away from theoutlet the BEM is better suited. BEM is better suited for instance if one want thecontribution from the exhaust at the distance corresponding to a pass-by test, asseen in figure 2.3 this requires a domain to be at least 7.5 m long which is large. Inthis part it is tested if the PML condition is feasible.

Figure 4.10: IL loss for two sizes of the sphere for the PML boundary condition.

The PML was tested by changing the size of the sphere that the condition isapplied to. The sphere can be seen in figure 3.2 b and the radius were 0.3 m and0.5 m. Since the condition is independent of the sphere size if it creates a reflectionfree condition both solutions should give the same results. For this comparison, theInsertion loss (IL) was calculated by taking the power at the outlet of the mufflerand the power at the inlet. The resulting IL can be seen in figure 4.10. Sinceboth simulations give the same IL and a faulty boundary condition would causereflections and in turn cause a higher power at the outlet the condition is feasible.

4.2.2 Influence of source impedanceFor all the simulations in this section the same source data was used. This sourcedata was imported into COMSOL Multiphysics using the application builder inCOMSOL and the code in appendix B. After this, simulations on a muffler were

30

4.2. EXHAUST SIMULATIONS

carried out with this as input. The data presented in figure 4.11 is the average SPLat the opening of the outlet pipe for a varying source impedance.

Figure 4.11: SPL at the opening of the outlet for different percentage differences inthe source impedance. From COMSOL

In figure 4.11 it can be seen that a uniform percentage change in the sourceimpedance affects the spectrum as expected with a slight increase in the SPL for adecrease in the impedance.

Z/Zs : 0.85 0.9 0.95 1 1.05 1.1 1.15Lp[dB] : 121 120.7 120.4 120 119.7 119.4 119.1

Table 4.1: Table of the total SPL for different impedances Z

Table 4.1 shows the difference in the total SPL for different impedance’s Z aspercentages of a simulated source impedance Zs. It can be seen that a differencein the source impedance at the level the source characteristics of this engine is ateffects the level at the output as approximately 1 dB for 15% difference in theimpedance. Considering the stability of the source characteristics and how muchthe impedance affects the SPL at the outlet it seems that the source characteristicsare stable and useful above 1000 RPM, while below there is some uncertainty andfurther investigation and validation are needed.

4.2.3 COMSOL and ESAThis section contains results from simulations done with the same set-up for twoengines used in Navistar trucks. These are the same engines used in section 4.2.4and described in section 2.8. Figure 4.12 shows the SPL at the outlet for the twomufflers as calculated from both COMSOL and ESA with source data for 1500RPM. The results from COMSOL has a coarse frequency resolution since it uses

31


the frequencies that is outputted from GT-Power directly. The result of them nothaving the same frequency resolution is the same total energy does not result in thesame level of a specific frequency.

Figure 4.12: Comparison of the spectrum for CBE1/CBE1u using COMSOL Mul-tiphysics and ESA.

ESA CBE1 COMSOL CBE1 ESA CBE1u COMSOL CBE1u117.6 dB 120.2 dB 115.5 dB 120 dB

Table 4.2: Total SPL from COMSOL Multiphysics and ESA at 1500 RPM

In table 4.2 the total SPL for the different engines and calculation methodsat the outlet is presented. While the simulations using ESA are quick once onehas the transfer matrix, simulating the transfer matrix is still a large simulation.Theoretically the main benefit of using ESA compared to COMSOL in the currentstate of simulations is that ESA which is transfer matrix based simulations can

32


handle the temperature outside the muffler being different from the temperaturein the exhaust. This could be handled in COMSOL as well if one imports a flowfield simulation with a temperature gradient, but that has not been done here.The COMSOL method on the other hand can include directivity and effects of theoutside part of the opening which can occur especially when using an integratedoutlet where there is no tailpipe.

Furthermore, a possible difference is the inclusion of the source characteristics inthe COMSOL model. The source characteristics are included as an incident pressurewave and as an interior impedance at the inlet. These two conditions cannot be atthe same place in the inlet pipe and this means that it could be possible to havesome added effect because of the distance between them.

The difference between engines in the total SPL as seen in table 4.2 is not thesame between methods. Since they do not show the same trends for a RPM be-tween engines it is necessary to have measurements to investigate which alternativeproduces the better results.

4.2.4 CBE1 and CBE1u engine comparison

In figure 4.13 the SPL for several RPMs which is in a possible range for a pass-bytest is presented. As expected, the SPL for different RPMs is similar over the RPMswith the major difference being the frequency shift of the peaks caused be the ordersbeing dependent on the RPM. There is a difference in the levels over the RPM rangewhich is further illustrated in figure 4.14. In figure 4.14 we see the total RPM forthe two engines CBE1 and CBE1u. Over 100 RPM the total SPL at the outlet canchange significant amount, the CBE1 changes 1 dB and the CBE1u 2 dB. Thismeans that to get good comparisons of the pressure at the outlet at a pass-by testthe RPMs need to be correct for the engine. The lower RPM range which it hasbeen indicated is more sensitive, is not relevant for the legal requirement, but areof great interest when designing a premium product.

33


Figure 4.13: SPL spectrum for CBE1 and CBE1u for different RPMs

Figure 4.14: Total SPL using COMSOL Multiphysics for CBE1 and CBE1u

Considering the steady changes over RPMs observed in the SPL in figure 4.14

34


it may be unnecessary to have a fine RPM resolution. Instead using a coarserresolution can give acceptable results.

4.2.5 Engine load

Previous engine simulations have been run with 100% load. This is not the case ina pass-by test and in this section simulation results with the engine load accordingto the engine load from a pass-by test will be presented. If 100% engine load isused the results are not directly comparable to a pass-by test, but if the differencebetween two engines is the same using the wrong engine load may still show thetrend of the a new engine when comparing to an old engine.

1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600

RPM

0.93

0.935

0.94

0.945

0.95

0.955

Lo

ad

[%

]

Engine Load

Figure 4.15: Engine load from previous pass-by test.

The engine load from a previously conducted pass-by test of a Representativeengine can be seen in figure 4.15. The engine load is high around 95%. In figure4.16 the resulting SPL at the outlet can be seen for both CBE1 and CBE1u for 3different loads. The pass-by test load is high and results in a SPL that is effectivelythe same as the SPL from running the engine at 100% engine load. It was alsotested to run the engine at 50% engine load , this resulted in a significant changefrom the 100% engine load case. The trend between the engines can also be seen isdifferent from the 100% engine load case. This means that when comparing engines,it is not enough to run them at one load to get a trend for all load scenarios.

35


1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600

RPM

116.5

117

117.5

118

118.5

119

119.5

120

120.5

121

121.5

SP

L [

dB

]Total SPL

CBE1 load 100%

CBE1 Pass-by load

CBE1 load 50%

CBE1u load 100%

CBE1u Pass-by load

CBE1u load 50%

Figure 4.16: Total SPL for CBE1 and CBE1u at different Engine loads.

36

Chapter 5

Conclusions

The results for the engine characteristics indicate that the lower RPM range ismore sensitive to changes in the connecting pipe. This part therefore needs extracare if it should be used. The range of the pipe lengths can be small withoutproducing large differences. The differences observed can be better for low ranges,but validation would be needed. A range of 100 mm in the connecting pipe lengthwas sufficient for the getting Representative results. Difference occurs between theengine characteristic dependent on the load and therefore the engine should bemodelled connected to a muffler with the connecting pipe and muffler similar to theones actually used. Modeling with different muffler or with an open end produceslarge deviations and should be avoided.

The source impedance affects the SPL at the outlet of the exhaust system. A15% deviation in the source impedance for a truck muffler at these levels resultedin approximately 1 dB difference. This is in most cases an acceptable error andtherefore deviations of 15% for the source impedance for all frequencies is acceptable.

The results from ESA and direct COMSOL simulations gives similar results. Thetotal level and trend between engines are not the same and it is therefore necessaryto have validation to conclude which gives the better results. Using ESA is howeverwell suited if one wants a waterfall plot or a high RPM and frequency resolution.The direct COMSOL simulations does however have the possibility of improvementby importing a flow field and temperature gradient from CFD simulations.

Since the engine load at a pass-by test is high the SPL at the outlet is similarto the results given when simulations are run at 100% engine load. For this reason,it works well to run simulations at 100% engine load when the pass-by test is ofinterest. A load of 50% does show significant difference in the SPL at the outletboth in absolute values and in trends. Because of this if a scenario with lower engineload is of interest simulations should be run at a lower engine load.

Lastly the total SPL from the CBE1u have in the simulations been indicatedas being lower or equal to the total SPL from the CBE1. Therefore, if the legalrequirement remains the same the CBE1u can be expected to pass the requirementwhere the CBE1 does.

37

Chapter 6

Future work

To be able to use the methods reliably one would need validation measurements.These measurements are of extra importance if one wants to look at the low RPMrange since the characterization did not here produce fully stable results. Sincethe use of 1D simulations together with the multiload method has previously beenshown to produce reasonably accurate results the measurements could be doneindirectly by comparing the results from different predictions and measurements inthe exhaust system.

One alternative to the variation of the loads is to use a quarter-wave resonatorwhich would not affect the back pressure immensely but would allow for a differencein the load. This quarter-wave resonator could be attached to the pipe connectingthe muffler and the engine and changing the load would then be done by changingthe quarter-wave resonators length [6]. Testing whether this or some other approachto the varying of the load gives good result could also be investigated.

When doing simulations for the SPL at a pass-by test, for the engine the RPMrange as well as the load on the engine is important. The load of the engine can beacquired from experiments, but since the simulations are mostly of interest beforetest are done it would be good to be able to calculate this. Since the regulationsstates how the truck should be driven in the test it should be possible to calculatethis engine load and this would be good to be able to predict how a new engine orjust different engine tuning would affect the exhaust contribution to in the pass-bytest.

Furthermore, it has been seen that some elements of the 1D engine simulationssuch as the turbo can in some regards be insufficient for the acoustic modeling.Further work could here be done to improve the model from an acoustic view.

The tuning of the engine has an impact on the acoustic performance of theengine. It could therefore be of interest to investigate how different parameterseffect the acoustic performance of the engine to see if there here is room for helpingthe acoustics by directly affecting the source.

Lastly the inclusion of a flow field and temperature gradient in the FEM sim-ulations of the exhaust system could be included to improve the prediction of the

39

CHAPTER 6. FUTURE WORK

SPL at and after the outlet of the engine.

40

Bibliography

[1] Mats Abom. An Introduction to Flow Acoustics. KTH The Marcus WallenbergLaboratory, 2006. Chap. APPENDIX.

[2] Hans Boden. “On multi-load methods for determination of the source data ofacoustic one-port sources”. In: Journal of Sound and Vibration 180.5 (1995),pp. 725–743. doi: https://doi.org/10.1006/jsvi.1995.0111.

[3] Antti Hynninen. Mats Abom. “Acoustic Source Characterization for Predic-tion of Medium Speed Diesel Engine Exhaust Noise”. In: Journal of Vibrationand Acoustics 136.2 (2014). doi: https://doi.org/10.1115/1.4026138.

[4] H P Wallin. U Carlsson. M Abom. H Boden. R Glav. Ljud och Vibrationer.KTH Farkost och Flyg/ Marcus Wallenberg Laboratoriet for ljud- och vibra-tionsforskning, 2017. Chap. 3.3.6.

[5] Hans Rammel. Hans Boden. “Modified multi-load method for nonlinear sourcecharacterisation”. In: Journal of Sound and Vibration 299 (2007), pp. 1094–1113. doi: https://doi.org/10.1016/j.jsv.2006.08.013.

[6] V Mavian. A J Torregrosa. A Broatch. P C Niven. S A Amphlett. “A view onthe internal consistency of linear source identification for I.C. engine exhaustnoise prediction”. In: Mathematical and Computer Modelling 57.7-8 (2013),pp. 1867–1875. doi: https://doi.org/10.1016/j.mcm.2011.12.018.

[7] Gamma Technologies. GT-SUITE Flow Theory Manual. Version Version 2020.Gamma Technologies. 2020.

[8] Gamma Technologies. GT-SUITE Acoustics Application Manual. Version Ver-sion 2020. Gamma Technologies. 2020.

[9] Maarten Hornikx. Wim De Roeck. Thomas Toulorge. Wim Desmet. “Flowand geometrical effects on radiated noise from exhaust pipes computed bythe Fourier pseudospectral time-domain method”. In: Computers Fluids 116(2015), pp. 176–191. doi: https://doi.org/10.1016/j.compfluid.2015.04.017.

41

https://doi.org/https://doi.org/10.1006/jsvi.1995.0111

https://doi.org/https://doi.org/10.1115/1.4026138

https://doi.org/https://doi.org/10.1016/j.jsv.2006.08.013

https://doi.org/https://doi.org/10.1016/j.mcm.2011.12.018

https://doi.org/https://doi.org/10.1016/j.compfluid.2015.04.017

https://doi.org/https://doi.org/10.1016/j.compfluid.2015.04.017

BIBLIOGRAPHY

[10] The European Parliament and the Council of the European Union. Regulation(EU) No. 540/2014 of the European Parliament and of the Council of 16 April2014 on the sound level of motor vehicles and of replacement silencing systems,and amending Directive 2007/46/EC and repealing Directive 70/157/EEC.2014.

[11] Antti Hynninen. Raimo Turunen. Mats Abom. Hans Boden. “Acoustic SourceData for Medium Speed IC-Engines”. In: Journal of Vibration and Acoustics134.5 (2012). doi: https://doi.org/10.1115/1.4006415.

42

https://doi.org/https://doi.org/10.1115/1.4006415

Appendix A

Set up of Multiload simulations inGT-Power

This part handles the setup and running of the multiload method when given acalibrated engine and exhaust model in GT-Power. The multiload is in this casebased on changing pipe length between the exhaust break and the inlet of themuffler. The simulations are handled in two steps, first there is the simulationof the engine and exhaust system and secondly the multiload calculation is doneexternally. The reason for doing this is to allow the engine simulation to run inparallel which is not allowed when the multiload is included in the first GT-Powermodel.

A.1 Simulation setup in GT-Power

Once a calibrated engine model is available the first step is to decide on the RPMsthat is of interest. To add RPMs some engine data needs to be interpolated, sincesome of the data is step like functions a spine interpolation in the form of an Akimainterpolation may be suitable. Some engine data that is required to be positive mayif interpolated become negative and should in this case be 0.

The next step is to add the muffler and connecting pipe. the connecting pipeshould have the same diameter as what it is attached to, in this case the exhaustbreak. The connecting pipe should then have the same characteristics as a regularpipe in the model. An example of the connecting pipe is shown in figure A.1, toaccess its settings right click and choose edit properties.

An example of the settings of the pipe is shown in figure A.2, To create thevariable for the DOE simply write its name within brackets in the length box andthen add to main.

43

APPENDIX A. SET UP OF MULTILOAD SIMULATIONS IN GT-POWER

Figure A.1: Example of connecting pipe element in a GT-Power model.

Figure A.2: Example of settings for the connecting pipe

44

A.2. SET-UP OF SEPARATE MULTILOAD CALCULATION

Figure A.3: necessary settings in the plots section of the connecting pipe.

In The plots section of the pipe setting it is important that the pressure (static)and Volumetric Flow Rate (at the Boundary) is selected as the necessary data forthe multiload will not be saved otherwise. The location of these plots should alsobe set to 0.0 as this means that it will take the data from the inlet of the pipe whichis the same place relative to the engine in all cases. These settings are shown infigure A.3.

Lastly the DOE should be setup, this can be found under case setup. Drag thelength variable in main to the DOE, after this specify the minimum and maximumvalue of the variable as well as the number of cases to be used. Lastly apply theDOE changes, bofor running the model it is good to go into plot setup and removeunnecessary plots as to reduce the solution file size. If the model is run on thecluster, set the maximum number of cases per packet to 1 and maximum numberof licenses per simulation to below 20 as to avoid clogging the cluster.

A.2 Set-up of separate multiload calculation

For the setup of the multiload there are already 2 templates setup for doing this.The first template is an excel file for the case setup in GT-Power. This data thatis needed for the case setup could of course be copied manually, but the excel fileeases this process. In figure A.4 an example of the excel file can be seen. the onlydata that needs to be modified is the lines in blue. For the density and soundspeedone needs to copy these lines from the solution file of the engine model. This datacan be found by going into the solution file under the connecting pipe element, CaseRLT as is shown in figure A.5

45

APPENDIX A. SET UP OF MULTILOAD SIMULATIONS IN GT-POWER

Figure A.4: Example of the Excel file for the case setup.

Figure A.5: Example of the data in GT-Power solution file.

The data in the excel file shown in figure A.4 should then be copied into thecase setup of the multiload calculation file. the case setup is shown in figure A.6.Once this is done the template can be run, this can be done locally as it is notcomputationally heavy. Note that it will create 1 text file per RPM for the resultsin the folder that the multiload template is located.

The results from the multiload is in these text files and are on the form as theexample in figure A.7.

46

A.2. SET-UP OF SEPARATE MULTILOAD CALCULATION

Figure A.6: Case setup for Multiload calculations

Figure A.7: Example of produced result files.

47

Appendix B

Import source characteristics

St r ing path = ” I n s e r t Path” ;S t r ing Source s t r eng th = ”a” ;S t r ing Source impedance = ”a” ;S t r ing f f = ”a” ;

i n t rpm start = 1500 ;i n t rpm step = 10 ;i n t rpm stop = 1600 ;i n t fmax = 400 ;

f o r ( i n t i = rpm start ; i < rpm stop +1; i = i+rpm step ) {

model . component ( ”comp1” ) . func ( ) . c r e a t e ( ” i n t ”+(( i−rpm start ) / rpm step+1) , ” I n t e r p o l a t i o n ” ) ;

with ( model . component ( ”comp1” ) . func ( ” i n t ”+(( i−rpm start ) / rpm step+1) ) );

s e t ( ” source ” , ” f i l e ” ) ;

s e t ( ” f i l ename ” , path+i+” . txt ” ) ;s e t Index ( ” funcs ” , ” S o u r c e s t r e n g t h r e a l ”+i , 0 , 0) ;s e t Index ( ” funcs ” , 1 , 0 , 1) ;

s e t Index ( ” funcs ” , ” Source s t r ength imag ”+i , 1 , 0) ;s e t Index ( ” funcs ” , 2 , 1 , 1) ;

s e t Index ( ” funcs ” , ” Source impedance rea l ”+i , 2 , 0) ;

s e t Index ( ” funcs ” , 3 , 2 , 1) ;s e t Index ( ” funcs ” , ” Source impedance imag ”+i , 3 , 0) ;s e t Index ( ” funcs ” , 4 , 3 , 1) ;

s e t ( ” argun i t ” , ”Hz” ) ;s e t ( ” nargs ” , 1) ;

49

APPENDIX B. IMPORT SOURCE CHARACTERISTICS

endwith ( ) ;model . component ( ”comp1” ) . func ( ” i n t ”+(( i−rpm start ) / rpm step+1) ) . l a b e l

( i+” RPM” ) ;model . component ( ”comp1” ) . func ( ” i n t ”+(( i−rpm start ) / rpm step+1) ) .

importData ( ) ;

}

f o r ( i n t i = rpm start ; i < rpm stop +1; i = i+rpm step ) {

i f ( i == rpm start ) {Source s t r eng th = ” ( S o u r c e s t r e n g t h r e a l ”+i+” ( acpr . f r e q ) ”+”+”+” i ∗Source s t r ength imag ”+i+” ( acpr . f r e q ) ) ”+” ∗(rpm==”+i+” ) ” ;

} e l s e {Source s t r eng th = ” ( S o u r c e s t r e n g t h r e a l ”+i+” ( acpr . f r e q ) ”+”+”+” i ∗Source s t r ength imag ”+i+” ( acpr . f r e q ) ) ”+” ∗(rpm==”+i+” )+”+Source s t r eng th ;

}i f ( i == rpm start ) {

Source impedance = ” ( Source impedance rea l ”+i+” ( acpr . f r e q ) ”+”+”+” i∗ Source impedance imag ”+i+” ( acpr . f r e q ) ) ”+” ∗(rpm==”+i+” ) ” ;

} e l s e {Source impedance = ” ( Source impedance rea l ”+i+” ( acpr . f r e q ) ”+”+”+” i

∗ Source impedance imag ”+i+” ( acpr . f r e q ) ) ”+” ∗(rpm==”+i+” )+”+Source impedance ;

}long omax = Math . round ( fmax∗60/ rpm start ) ;i f ( i == rpm start ) {

f f = ” range ( 0 . 5 ∗ rpm/60 ,rpm∗0 .5/60 , ”+omax+”∗rpm/60) ”+” ∗(rpm==”+i+” ) ”;

} e l s e {f f = ” range ( 0 . 5 ∗ rpm/60 ,rpm∗0 .5/60 , ”+omax+”∗rpm/60) ”+” ∗(rpm==”+i+” )+

”+f f ;}

}

model . component ( ”comp1” ) . v a r i a b l e ( ) . c r e a t e ( ” var1 ” ) ;with ( model . component ( ”comp1” ) . v a r i a b l e ( ” var1 ” ) ) ;

s e t ( ” Source s t r eng th ” , Source s t r eng th ) ;s e t ( ” Source impedance ” , Source impedance ) ;

endwith ( ) ;

model . study ( ” std1 ” ) . c r e a t e ( ”param1” , ” Parametric ” ) ;model . study ( ” std1 ” ) . f e a t u r e ( ”param1” ) . s e t Index ( ”pname” , ”rpm” , 0) ;model . study ( ” std1 ” ) . f e a t u r e ( ”param1” ) . s e t Index ( ” p l i s t a r r ” , ” range

(1500 ,10 ,1600) ” , 0) ;model . study ( ” std1 ” ) . f e a t u r e ( ” f r e q ” ) . s e t ( ” p l i s t ” , f f ) ;

}

50

APPENDIX C. WATERFALL PLOTS

Appendix C

Waterfall Plots

Figure C.1: Real part of the source strength for different lengths of the connectingpipe.

52

Figure C.2: Imaginary part of the source strength for different lengths of the con-necting pipe.

53


Figure C.3: Real part of the source impedance for different lengths of the connectingpipe.

54

Figure C.4: Imaginary part of the source impedance for different lengths of theconnecting pipe.

55


Figure C.5: The source strength for different ranges of the connecting pipe lengths.

56

Figure C.6: The source impedance for different ranges of the connecting pipelengths.

57


Figure C.7: The source strength for different connected mufflers.

58

Figure C.8: The source impedance for different connected mufflers.

59

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