modelling of heavy duty truck systems for nvh analysis

OTEKON’14
7. Otomotiv Teknolojileri Kongresi
26 – 27 Mayıs 2014, BURSA
MODELLING OF HEAVY DUTY TRUCK SYSTEMS FOR NVH ANALYSIS
Birkan Tunç
Ford Otosan A.Ş., Product Development, Gebze, Kocaeli
ABSTRACT
Heavy commercial truck area is an important part of vehicle industry. Increasing competition in the market
requires newly developed vehicles of comanies to succesfully handle different kind of attribute requirements beyond
durability performance of trucks. While major design criteria of trucks is mainly based durability considerations,
passenger cars are commonly designed with increased NVH and dynamic performance. Recent developments show that
comfort of trucks becomes more important to achive best product in the market.
NVH investigation of vehicles can be divided into two main categories such as airborn noise contribution and
structure born vibration and acoustic path analysis. There are powerful CAE tools in the market used to model structural
behavior of the vehicles and adress issues based on the models developed for NVH simulations. This paper introduce
modelling approach of a heavy commercial truck with a finite element based CAE tool to simulate response of the
vehicle under different powertrain loading conditions such as idle and wide open throttle situations.
Keywords: Truck NVH, mathematical models, vehicle NVH, virtual prototyping, simulation, NVH CAE
present a methodology to develop NVH model of a truck
by using Ford internal tool based on Nastran modal
solution to prepare NVH models. This tool uses internal
solver to simulate transient and freuency response of the
assembly model. Another objective of this paper is to
explain loads acting on the vehicle due to engine
excitation. In this paper, simulation results such as
vibration response of vehicle at some specific points such
as steering wheel, seat track and acoustic response at ear
locations have been presented for wide open throttle load
condition. Test results of the vehicles are presented as
well to show the degree of correlation of the model
developed. Since the correlation of the model is an
ongoing process, detailed investigation of the study has
not been given in this paper.
The paper is organized as follows: first, the details of
the truck full vehicle NVH model are presented in
Section 2. Next, generation of engine loads for a 6
clyinder internal conbustion engine is presented. Finally,
simulation results have been introduced and compared
with the test data.
1. INTRODUCTION
Modelling of a system’s structural behavior for
different aspects are highly important especially to reduce
test expenses and to increase the design process timings.
Structural modelling of a vehicle for NVH allows design
engineers to assign structural vibration targets, seperation
of critical eigenfrequencies from each other and main
excitation sources and helps better adressing of issues
which occur during test phase of vehicle development
with a better understanding of modal behavior of the
system.
There are different kinds of tools using finite
element technic to model structures such as Nastran,
Abaqus, Ansys etc. Those tools yield quite accurate and
correlated results of structural behavior with a systematic
approach of modelling and allow fast ivestigation of
design changes on vehicle performance. In recent years,
these software environments are increasingly used to
model and design of the heavy duty truck systems.
Beyond those general tools engineers develop specific
programs to make specific assessments of systems for
NVH to be carried out easily such as noise path analysis,
system and compnant modal contributions and design
optimisation studies. The objective of this paper is to
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Figure 1: Vsign Graphical User Interphase
Each FE model is a modal model with its modal analysis
data and those information is used to form global
stiffness and mass matrix of the complete system. Modal
data is given by Nastran analysis and converted to a
format that can be used in the tool. System of equations
are solved wit internal solver to find the response of the
structure to a specific loading history. Each three
dimentional model is presented as 2D shapes in graphical
user interface.
Full vehicle NVH model of the heavy commercial
tractor consists of the following models:
2. DEVELPMENT OF NVH MODEL
Multibody modelling of full vehicle for NVH
simulation consists of different types of modelling
approaches. This models are mainly used to simulate
vehicle vibration and acoustic response under engine,
road and wheel imbalance exciations. Development of a
model as simple as possible allows engineers to
investigate the effect of design modifications in a short
time period which is crucial at design stage. A typical
model of a full vehicle consist of rigid bodies such as
powertrain, finite element models of main components as
vehicle body, chassis, tires etc., beams (anti roll bars,
struts, rods, etc.) and three or six degree of freedom
spring and damping elements to model bushings which
connects subsystems to form full vehicle model. Mass,
stiffness and damping matrices are then easily generated
for the system of equations to be solved.
Geometry data from a CAD model is used to
generate FE models of structural parts. This process
requires great deal of effort since a vehicle body structure
consists of at leatt 300 different sheet metal parts and
many plastic parts of instrument panel and seating
systems.
-
FE Model of Cabin and Chassis
FE Model of Suspensions
Rigid Model of Powertrain
Beam Model of Drivetrain
Beam Model of Suspension Componentes
Beam Models of Steering System
Tire Models as Spring Elements
Details of these models will be presented in the following
sections.
2.2 Trimmed Body and Chassis Modelling
2.1 Full Vehicle Modelling Tool
A trimmed body model consists of sheet metal of the
body structure so called body in white structure, doors,
instrument panel, seats, hood, mirrors, exterior trim parts
such as bumpers etc. Most of the structural components
are represented by finite element models forming more
than one million degree of freedom system. While
structural parts are FE models since the main objective of
modelling of the system is to calculate vibration response
of steering wheel, seat track and some other critical
points, many of the trim parts are modeled as mass
elements with representative mass and inertia properties
to reduce degree of freedom of the system. Higher
number of degree of freedom increases solution time and
output data size and reduces post processing performance
of the computers.
In this paper Ford internal NVH modelling tool is
used to develop a full vehicle model of a heavy
commercial tractor. The tool allows users to use general
finite element models of the structures and generate euler
beams, rigid body components, spring and bushing
elements and mass elements etc. Each individual
component are then connected to each other as simple
constraints to form the full vehicle model. The
powerfullness is based on the fact that the engineer does
not need to run a simulation of the whole assembly of
finite element models once eigenvectors and eigen
frequencies are available for each FE model when design
modifications to be investigated.
Figure 2: Trimmed Body of a Heavy Truck Cabin
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Figure 2 gives a general overview of a trimmed body of a
heacy commercial truck cabin. Similarly figure 3 shows
examples of instrument panel and seat system inside the
body structures.
Figure 5: Chassis Finite Element Model
Figure 5 shows chassis finite element model of a heavy
commercial tractor. Chassis consists of rails and cross
members which are the structural parts supplying the
stiffness and where all the accessories such as muffler,
fuel tank, fenders and bumpers are attached. Dynamic
behavior of the chassis mainly depends on the mass and
inertia of the subsomponents and modelling approach.
Many of the strutural components are modeled as finite
element models since heavy components have low
frequency resonances which might effect the vehicle
vibration and acoustic characterictic. As in the same case
in trimmed body modelling the nummber of degree of
freedom so the mesh size need to be correctly determined
by means of structural tests to improve the solution
performance, post processing time and effectiveness of
the CAE models.
Another approach to improve post processing times
besides reducing the number of degrees of freedom is to
use display models for which only for a number of
degrees of freedom the solution results are recorded.
Display models are generated in a way that the engineers
understand the structural behavior without missing
important resonance frequencies and mode shapes of sub
components. Thus the finite element model is divided
into large group of panels with representative elements
and exact output nodes of the original model.
Figure 3: Instrument Panel and Seat Models
Besides structural modelling of trimmed body
structure, acoustic cavity model is generated as well to
investigate the acoustic response of the vehicle such as
sound pressure levels at driver ear position. Since the seat
cushions are the sound absorber matrials, they are
modeled as FE models and connected to cavity model
with coupling elements. Cavity model is connected to
trimmed body structure model with couling elements at
the surface area of the cabin or cavity and called wetted
surface. Structural vibrations are transfered to cavity
nodes as boundary conditions to calculate soun pressure
levels.
Figure 4: Cavity Model of a Heavy Truck Cabin
Cavity model of a heavy commercial vehicle is shown in
figure 4. CAE models in this phase are powerful tools to
determine contribution of each panel to acoustic reponse
and gives directions to engineers to improve structural
performance by mean of design modifications.
In order to investigate the vibration characteristic of a
heavy commercial truck chassis needs to be correctly
represented by finite element models with high degree of
confindence since the engine and suspensions which are
the main excitation sources are directly connected to
chassis and the forces are transmitted to body structure
via vibrations paths along chassis system.
Figure 6: Display Model of Truck Cabin
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Display model of a heavy commercial truck cabin is
shown in figure 6. Original model is presented as a coarse
mesh model having randomly defined output nodes and
elements. The points where measurement results need to
be considered such as steering wheel output points or seat
track nodes are added to display model manually.
2.3 Suspension FE Modelling
Heavy commercial truck might have different number
of suspension systems depending on the design objective
of the vehicle. The vehicle considered in this paper has a
heavy commercial tractor with front and rear suspension
and a cabin suspension between chassis and cabin
structures. The critical point in modelling of structures is
the resonance frequencies of the systems. If a subsystem
has a resonance frequency between the frequency range
of interest such as engine excitation frequency flexible
modelling of the system is a must to correctly calculate
the response of the whole structure. Rigid body
modelling is just an approach if there is no resonance
frequency of the component in the frequency range of
interest of close the this frequency range to reduce the
effort of finite element modelling and improving solution
and post processing time.
Flexible modelling consists of finite element
modelling for components with complex geometries and
beam and spring element representation of components
having simple geometrical properties such as tierods,
strutrs, antiroll bars etc.
Figure 8: Front Suspension FE Model
All of the components of the truck suspensions with
complex geometries have been modelled as finite element
models and the remainings are modeled as beam and
bushing elements and to be presented in the next section.
Front suspension model consists of leaf springs, antiroll
bar, knuckle and spindle system. Leaf spring has been
model with shell element with different thickness to
better represent variable cross section of the leafs.
Figure 9: Rear Suspension FE Model
Rear suspension of this tractor consists of air springs
which have been represented as spring and damping
elements. Rear suspensin model consists of rear axle
housing and rear spindles FE models.
Figure 7: FE Model of Cabin Anti Roll Bar
Figure 7 shows finite element model of cabin anti roll
bar. Beam modelling of the roll bar is an example for this
part but not considered here.
2.4 Modelling of Powertrain
Vehicle development is a part of team work where
system and susbsystem level targets are clearly
determined and cascaded to each related engineering
team. Critical part of NVH development is to seperate
resonance frequencies from the frequencies of excitation
sources and resonance frequencies of other main
components to prevent harshness. A part of this
powertrain should be desinged in a way that the structural
resonance frequencies need to to be higher then engine
excitation. Regarding this information a vehicle design
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engineer just needs the mass and inertia properties of the
engine to investigate the effect of rigid body resonances
of the powertrain and the effect of mass and inertia of the
engine to vehicle response under engine excitation. Thus,
powertrain can easily be modeled as a rigid body with
correct mass, inertia and engine mount stiffness
information. Figure 10 shows an example of heavy tractor
engine inertia properties.
data to correlate the full vehicle model against test data
by changing the stiffness with those tolerence region.
Since the bushings are on the transfer path of vibration
and they are isolation members, reducing the stiffness of
those members to problem solution is a general approach
considering the trade off between vehicle dynamic
requirements.
2.6 Full Vehicle Model
Availability of all the information and models
presented in the preceeding sections allows to generate
full vehicle NVH model of a heavy commercial truck.
Individual components are connected to each other to
form the complete assembly of the system.
Figure 10: PT Mass & Inertia Properties
2.5 Modelling of Bushings
Most of the subcomponents in a vehicle system such
as suspensions, antiroll bars, engines are connected to
body or chassis structure with rubber bushings for better
isolation of the body structure from excitation sources.
This heavy commercial tractor model have more than 60
bushings at different attachment points. Each of these
bushings exhibits dynamic stiffness characteristics
depending on the frequency.
Figure 12: Full Vehicle NVH Model of Heavy Tractor
Figure 12 shows the assembly of a full vehicle heavy
commercial tractor. All of the finite element models, rigid
body model and euler beams are presented as display
models. The only missing model is the modal models of
the tires. Generic approach to tire modelling is to create
finite element models of the tires in deformed
configuration due to the weight of the vehicle and run
modal analysis at this configuration to obtain modal
behavior of the tire. Another simple approach used in this
case is to model the tires as spring and bushing elements
with measured stiffness values.
Figure 11: Dynamic Characteristic of a Suspension
Bushing
Dynamic stiffness properties of a suspension bushing is
given in figure 11. Those bushings are paths of vibration
in a vehicle structure. It is well known that the stiffness of
the bushings may change up to %20 because of the
inhomogenity of the rubber materials used and production
tolerances and differences. Once the stiffness of all the
bushings are measured and experienced engineer use the
Figure 13: Beam & Rigid Body Models of Tractor
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Figure 13 shows the suspension, engine, driveline and
steering system models ogf the tractor. Driveline and
steering system is modeled as mostly euler beam
elements. This approach allows engineers to run design
iterations faster and generate response since it is easy to
change section properties of the beams in the tool quickly
and run simulations without solution of a million degree
of freedom system again since the modal behavior of FE
components are available. Engine and transmissin models
are rigid body models and shown as visual elements to
investigate the behavior under engine excitation.
number and orientation of cylinders someof the forces
corresponding to engine orders cancel each other. Those
forces are meaningless since they do not generate
vibration.
3. GENERATION OF ENGINE LOADS
Internal combustion engines are one of the main
excitation sources of the vehicles due to the oscilating
and rotating masses of the cranktrain and combustion
pressures. Because of the geometry of the crank train and
the oscilating piston and connecting rod masses there
exist a mass torque around crankshaft axis and shaking
forces perpendicular to crankshaft axis of an internal
combustion engine.
Figure 16: Cranktrain of a 6 Cylinder Inline Engine
Heavy commercial vehicle presented here has a six
cylinder inline engine and crank train is shown in figure
16. Calculation of the forces and moments with the
formulas similar to given in figure 15 shows that the main
inertia loads are the 3rd engine order inertia torque for a
six cylinder engine. Although 6th engine order inertia
torque has a considerable effect, remaning torques and
shaking forces can be neglected. It is shown that the
engine mass torque is depend on the engine speed and
can be normalized according to RPM with the following
formula:
Another important excitation of the engine comes
from the combustion pressure inside the cylinders.
Figure 14: Forces Acting on a Single Crank Mechanism
Figure
17:
Cumbustion
Pressure
of
a
Cylinder
Combustion pressure in the cylinders depend on the
engine speed and need to be considered seperately while
generating a load case to simulate wide open throttle
behavior. Combustion torque of the engine is calculated
by the multiplication of the piston area and crank train
geometry.
Figure 15: Calculation of Mass Force of a Single Crank
Mechanism
Those inertia forces can be expended to fourier series
giving components of orders of engine speed. Each of the
components are called engine orders. Based on the
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Figure 18: Fourier Representation of Combustion Force
Figure 20: Sound Pressure Levels (CAE Simulation)
Combustion pressure is similarly expended to fourier
analysis to see the dominant engine orders. For a six
clinder engine there are two combustion process per
revolution of the engine. Regarding this information and
fouriser analysis 3rd engine order combustion torque is
found to be dominant load acting on cranktain and
engine.
Calculation of those forces have been performed by
Excite simulation tool. Engine load data are then obtained
for each RPM of the engine for wide open throttle case
and applied in full vehicle model to cranktrain and engine
block to obtain the vehicle response.
Figure 20 shows the calculated sound pressure levels of
two vehicles under wide open throttle acceleration
situation. Blue curve representes the first vehicle and
purple color represents the second vehicle. The degree of
correlation for acoustic response calculated with CAE is
somewhat limited because of the modelling approach of
the cavity such as interaction of the cavity with body
structure and lack of absorbtion properties in CAE
models. But simulation and test results show good
agreement at low and high frequencies. For the low
frequency first vehicle exhibits higher respnose and for
the higher frequencies second vehicle exhibits higher
response in both simulations and test data. All of the
response is the 3rd engine order content since the
dominant order has significant effect on the results.
4. SIMULATION RESULTS
In this section test and simulation results of two vehicles
having different powertrains will be presented. Full
vehicle models and engine load cases have been
generated for each model.
Figure 21: Steering Wheel Velocity (Overall)
Figure 19: Sound Pressure Levels (Test Data)
Figure 19 shows the sound pressure levels of two
vehicles under wide open throttle acceleration situation.
Blue curve representes the first vehicle and green color
represents the second vehicle.
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simulation technics have been presented. Simulation
results have been compared to test results to see the
degree of correlation. Detailed investigation of
correlation process have not been given here. It is shown
that the modelling approach gives quite relaible results
and allows engineers to understand the bacics of truck
NVH behavior.
CONTACT
Figure 22: Steering Wheel Velocity (3rd EO)
[email protected]
Figure 21 presents test data of the steering wheel
vibration response and figure 22 shows the 3rd engine
order content of the test data. Green curve in figure 22 is
the overall velocity of the steering wheel while red curve
is the dominant engine order.
Vehicle NVH
Ford Otosan
Gebze Engineering Center
D-Block Tubitak Serbest Bölge
Gebze / Kocaeli
Tel: 02620 677 92 46
Figure 23: Steering Wheel Velocity (3rd EO CAE)
It is shown in figure 23 that the velocity of the steering
wheel response of the first vehicle is higher compared to
the velocity of the second vehicle around low engine
speed. This trend is shown in the test results as well
showing the degree of correlation of the simulations
between test data. Test results shows higher accelerations
at medium speed for the first vehicle but this is not the
contribution of the 3rd engine order thus the confidence
of the simulation results are maintained.
4. CONCLUSIONS
Development of full vehicle models for NVH analysis
and simulations by means of CAE tools are powerful
sources which helps engineers understand dynamic
behavior of structures. To be able to reduce test expanses
and increse product development phase speed is an
important aspect in engineering design. In this paper
modelling approach of a heavy commercial truck and
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