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Mechanical Men - Noise and vibration refi nement of powertrain

Create The Future
نویسنده :مهدی کاظمی
تاریخ:چهارشنبه 16 اسفند 1391-06:10 ب.ظ

Noise and vibration refi nement of powertrain

Abstract:The noise level and sound quality of the powertrain system is
arguably one of the most signifi cant infl uences on the customer’s
perception of a new car. This chapter describes the process for
integrating the powertrain into passenger vehicles.The fi rst step in
the
process is to set targets for the vibration source levels and isolation
of paths to meet the noise level requirements. The second step is to
incorporate enablers to tailor the induction and exhaust sound quality
in order to meet the customer’s expectation of the vehicle.The chapter
concludes with a review of future powertrain trends and the challenges
they pose for noise and vibration refi nement.
Key words:nois/vibration,powertrain,engine,transmission,driveline
.exhaust,induction

 Introduction 
The primary motivation to improve noise and vibration in motor vehicles
has traditionally been to eliminate any unusual or unexpected noises. The
customer perceives a quiet car as a well-built quality vehicle, and any unexpected
noises can be a concerning sign of impending failure. The noise level
and sound quality of the powertrain system is arguably one of the most
signifi cant infl uences on the customer’s perception of his or her new car.
The owner of an expensive luxury saloon expects to be propelled along by
a whisper-quiet and silky-smooth engine. At the other extreme, the owner
of a shiny new sports car expects a loud and exciting note from the exhaust
on every stab of the throttle. Thus powertrain noise and vibration refi nement
does not merely involve the elimination of unusual noises, but increasingly
involves sound quality engineering to mould the aural excitement of
a new car.
This chapter begins with a review of the key powertrain noise targets and
how the targets are rolled down from vehicle-level to powertrain systemlevel
to achieve the noise level requirements. A refresher on order tracking
analysis is also covered, since this is the language of describing powertrain
noise and vibration performance.
The powertrain hardware is introduced in a series of tables documenting
 the sources and paths of powertrain noise. These sources and paths sum  together to create the net engine sound level and character received by the
driver and the occupants in the vehicle. A review of the many hardware
enablers to ensure refi nement of the engine, transmission, driveline, induction
and exhaust systems is presented along with numerous and interesting
examples.
Powertrain systems are continually evolving, and this chapter considers
the future trends in hardware enablers for enhanced engine sound quality.
But the biggest infl uence on noise and vibration refi nement in the future
will no doubt be the ever-increasing push for better fuel economy. The
challenges this presents to the noise and vibration engineer are outlined at
the end of the chapter.
12.2 Principles and methods
12.2.1 Requirements
The requirements or targets for powertrain noise and vibration performance
in a new vehicle program can be considered as the minimisation of
any unusual or unexpected noises under all driving conditions. At the
highest level, the requirements can be summarised as:
• Smooth idle
• Quiet cruising (under light engine loads)
• Smooth and linear acceleration (under high engine loads)
• A sound character appropriate for the class of vehicle.
Unexpected noises are often due to a system resonance being excited by
the powertrain under a specifi c operating condition. For example, an
exhaust boom might be noticeable to the driver on each occasion when he
or she drives away from a standstill. On further investigation, it could be
apparent that the boom noise is always evident when the engine speed is
1200 rpm, and seems to be more severe under higher engine loads. A V6
engine has an engine fi ring frequency of 60 Hz at 1200 rpm, and in this case
the forces from the engine were found to excite a 60 Hz bending mode of
the exhaust system. This example illustrates that a powertrain refi nement
problem will typically involve a natural resonance of a system (whether it
is structural or acoustic) that is excited by forces from the powertrain when
the frequencies of both coincide. Targets and requirements of powertrain
refi nement are set during the development of a new vehicle program to
defi ne acceptable levels as well as to provide strategies to avoid these issues.
12.2.2 Development process
As mentioned in Chapters 1 and 2, development of a refi ned powertrain
for a new vehicle has the same key processes as the general vehicle
Copyrighted Material   
development process. The key steps in the integration and development of
a refi ned powertrain are:
1. Target setting
• Set vehicle-level targets:
– Defi ne noise and vibration levels at driver interfaces over the
operating range of the powertrain.
• Set powertrain system and component-level targets:
– Set targets for powertrain components based on the vehicle
requirements.
– Vehicle body transfer functions are utilised to calculate system
source and isolation requirements.
– Component targets allow for systems to be developed without
the need for a vehicle, which is very important for external suppliers
and keeping development costs low.
2. Development and design of components
• Mathematical and CAE tools are used to develop components.
• Experimental tests are run on a rig or a prototype vehicle.
3. Verifi cation
• Confi rmation tests are run to show that the system meets the targets.
They are usually experimentally verifi ed on the vehicle.
Component noise and vibration targets need to be defi ned as objective measurements
so that mathematical analysis and experimental analysis can be
carried out interchangeably. Objective targets also allow for clear verifi cation
results so that pass/fail criteria can be based on an objective measurement.
12.2.3 Measurement methods
To assess the noise and vibration refi nement of the vehicle’s powertrain,
measurements need to be made across the range of operating conditions.
Run-up sweeps are performed by ramping up the engine speed over a short
period of time, thus covering a range of operating conditions in one measurement
set. The run-up sweeps can be performed at low load and repeated
at full load to further extend the range of operating conditions. If the engine
speed is recorded at the same time as the noise and vibration signals, then
a spectral order map can be generated for the run-up sweep as shown in
Fig. 12.1. An order is defi ned as a harmonic corresponding to the rotational
speed of the engine. For run-up sweeps, orders are a useful alternative to
frequency measured in hertz, since engine forces are synchronised to the
rotating speed of the engine. To calculate the frequency of the Nth order
at a particular engine speed, Equation 12.1 is used:
Frequency of the th order Hz
rotational speed rpm
N
N
( )
= × ( )/60(seconds)
For example, a V6 engine running at 3000 rpm has a fi rst-order frequency
of 50 Hz and a third-order frequency (engine fi ring rate) of 150 Hz (see
Fig. 12.1).
The spectral order map consists of the computed spectrum at each engine
speed increment during the run-up sweep. Overlaid on the diagram are
order lines that indicate harmonics of the engine rotational speed at each
engine speed increment. A high level of noise at a constant frequency in
the spectral order map is indicative of a resonance in the system (evident
as visible horizontal bands in Fig. 12.1). When the engine speed sweeps
through a resonance and the engine forces excite it, we get a resulting
increase response in noise level. This response can create that unexpected
noise that can alert the driver that his or her new car is not a quality engineered
car after all.
Whether noise and vibration development is to be carried out experimentally
or by computer-aided mathematical analysis, the use of run-up
sweeps and order tracking for powertrain measurements is a fundamental
tool. However, if a particular problem has been identifi ed at a steady operating
speed and load, steady-state measurements can also be taken and the
narrowband FFT calculated for analysis.
12.2.4 Acquiring engine speed
For experimental measurements, an engine speed signal is necessary to
perform order tracking of the noise and vibration data. Engine speed is
acquired using an engine tachometer. Alternative tachometers available for
order tracking include:
• Optical tachometers. These typically require a refl ective mark placed
on the crankshaft. They can be the most precise, and can also be used
to provide a phase signal. However, they can take the technician some
time to install and set up on a vehicle.
• The engine management tachometer signal used by the engine computer
can sometimes be tapped into (if you know how) but it might not
be very accurate.
• Vehicle data bus tachometers. These generate an engine speed pulse
from the vehicle’s data bus (e.g. CAN) and are convenient to use since
they are simple to plug into the connector under the steering wheel. The
downside is that they generate an artifi cial signal to replicate the engine
speed and thus cannot be used for phase measurements.
Order tracking measurements are commonly referenced to the engine
speed, since engine forces generate many vehicle noise and vibration problems.
However, when working on transmission or driveshaft problems, it
makes sense to refer orders back to the transmission or driveshaft speed.
In this case, you will need a tachometer that measures driveshaft speed
rather than engine speed. As an alternative, some analysis software allows
the application of a multiplier to the engine tachometer to derive another
tachometer signal.
12.3 Powertrain noise sources and paths
12.3.1 Source, path and receiver model
The focus of this section is primarily on how to integrate the powertrain
system into the vehicle. This is presented by identifying the sources ofvibration from each powertrain subsystem and the airborne and structureborne
paths of each into the vehicle.
The source/path/receiver model of noise and vibration is a powerful
concept in engineering powertrain refi nement. This fundamental concept is
used in the target setting process, and extends into the development of vehicles
and as a tool in the analysis of noise and vibration problems.
and Applications, New Yor

References
Baret C., Nierop G. and Vicari C. (2003), New ways to improve vibro-acoustic
interactions between powertrain and car body, Graz, Austria, 2nd Styrian Noise
Vibration and Harshness Congress
Beranek L. (1960), Noise Reduction, New York, McGraw-Hill
Beranek L. and Ver I. (1992), Noise and Vibration Control Engineering, Principles
k, Wiley-Interscience



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