Wang X. Vehicle Noise and Vibration Refinement

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Noise and vibration reﬁ nement of powertrain systems in vehicles 255

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

Engine speed (rpm)

(a)

1500

1000

100

Frequency (Hz)

2000 2500 3000 4000 4500 5000 55003500

1st Order

3rd Order

at 3000 rpm =

150 Hz

10 100 1000

Frequency (Hz)

(b)

Interior sound pressure level (dB(A))

6th Order

at 3000 rpm

= 300 Hz

6th Order

12.1 (a) A spectral order map, and (b) a 3000 rpm spectral cut for

interior sound pressure level during a full-load run-up sweep.

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256 Vehicle noise and vibration reﬁ nement

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 engi-

neered car after all.

Whether noise and vibration development is to be carried out experi-

mentally 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 oper-

ating 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 com-

puter 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 prob-

lems. 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 of

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Noise and vibration reﬁ nement of powertrain systems in vehicles 257

vibration from each powertrain subsystem and the airborne and structure-

borne 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 vehi-

cles and as a tool in the analysis of noise and vibration problems. Figure 12.2

Receiver Paths

Powertrain mounts

Structure-borne noise

and vibration

Interior powertrain

noise

Engine forces

Transmission forces

Final drive forces

Driveshaft forces

Exhaust forces

Exhaust mounting

Driveline mounting

Airborne noise

Engine radiating

surfaces

Transmission

radiating surfaces

Exhaust radiating

surfaces

Induction radiating

surfaces

Induction snorkel

Target setting

Vehicle

performance

target

Component

isolation/insulation

performance

Component

source level

requirements

Induction pressure

waves

Exhaust pressure

waves

Exhaust tailpipe

Sources

12.2 Simpliﬁ ed powertrain noise source/path/receiver model.

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258 Vehicle noise and vibration reﬁ nement

Table 12.1 Deﬁ nition of frequency ranges

Term Frequency range Typical sensation

Low-frequency Sub-ﬁ ring frequency range,

typically <20 Hz

Vibration – shake

Noise – inaudible

Mid-frequency Firing frequency range (idle to

red line), typically 20–400 Hz

Vibration – roughness,

harshness

Noise – boom, moan

High-frequency Super-ﬁ ring frequency range,

typically >400 Hz

Vibration – tingling

Noise – whine, whistle

presents a simpliﬁ ed diagram of some powertrain noise and vibration sources,

and their paths to the receiver inside the vehicle.

12.3.2 Noise sources

Table 12.1 provides the deﬁ nitions of low, mid and high frequency ranges

relating to powertrain noise and vibration used in this chapter. Table 12.2

is a summary of the sources of forces produced by the powertrain systems

(refer to Eshleman (2002) and Wowk (1991) for further examples relating

to general rotating machinery). Included in the table is a guide to the fre-

quency range of forces, and this information will be used later in this section

to assist in developing a vehicle mode map to ensure that vehicle and system

response modes are well separated from excitation forces.

The mechanical structure of the powertrain systems will have vibration

modes (natural resonances) that are likely to be excited by the powertrain

forces described in Table 12.2, or in some cases from other inputs such as

a rough road or wheel imbalance. Table 12.3 lists some signiﬁ cant power-

train system modes that must be managed to ensure adequate reﬁ nement

of a passenger vehicle.

In setting the design parameters in order to minimise the source and

ampliﬁ cation of powertrain forces for a future model vehicle, the source

excitation frequencies should be kept separated from the powertrain modes

where possible. By combining the information of the source forces in Table

12.2 and the system mode responses in Table 12.3, a mode placement map

can be produced. Figure 12.3 shows an example of a powertrain mode

placement map for a typical passenger car vehicle.

12.3.3 Noise paths

Table 12.4 lists the typical path of the powertrain forces into the vehicle.

An airborne path refers to noise radiated from the powertrain components

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Table 12.2 Sources of powertrain noise and vibration

Component Direction of

force

Excitation frequency Low-frequency

range

Mid-frequency

range

High-frequency

range

IC engine

Engine rotating unbalance Translational First-order shake/vibration ×

Engine reciprocating

unbalance/unbalanced

inertial forces

Translational First-order and harmonics

(dependent on cylinder

conﬁ guration)

××

Engine reciprocating

unbalance/unbalanced

moments

Moment Nth order and harmonics

(dependent on cylinder

conﬁ guration)

××

Engine reciprocating

unbalance/inertial torques

Torsional Nth order and harmonics

(dependent on cylinder

conﬁ guration)

××

Combustion gas pressure Torsional Firing order (and

harmonics)

××

Combustion gas pressure

– random misﬁ re

component

Torsional Broadband ×

Combustion gas pressure

– driver commanded

step-input

Torsional Broadband ×

Engine combustion noise Translational Broadband ×

Fuel injector and valve train Translational Broadband ×

Forced induction pumps

(turbo or superchargers)

Translational Blade pass frequency ×

Engine accessories Multiple Pulley ratio and vane/

blade/pole passing

××

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Component Direction of

force

Excitation frequency Low-frequency

range

Mid-frequency

range

High-frequency

range

Transmission

Transmission gear whine Multiple Gear tooth meshing

frequency (= shaft

speed × number of

teeth)

Driveline

Coupling misalignment Multiple Shaft speed (and

harmonics)

××

Driveline imbalance Translational Shaft speed × ×

Final drive module gear

whine

Multiple Gear tooth meshing

frequency (= shaft

speed × number of

teeth)

××

Exhaust and induction

Exhaust noise Wave Firing order (and

harmonics)

Flow induced noise Wave Broadband ×

Induction noise Wave Firing order (and

harmonics)

Table 12.2 Continued

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Noise and vibration reﬁ nement of powertrain systems in vehicles 261

Table 12.3 System-level structural vibration modes

Powertrain

mode

Mode type Low-frequency

range

Mid-frequency

range

High-frequency

range

Powertrain

mounting

Rigid body ×

Driveline/

propshaft

Rigid body ×

Driveline

surge

First elastic

torsional

Exhaust

ﬂ exural

Elastic

bending

Final drive

module

(FDM)

mounting

Rigid body ×

Driveline

bending

First elastic

bending

Gear rattle Second

elastic

torsional

Gear whine Third elastic

torsional

into the surrounding air that can ﬁ nd its way into the cabin unless it is suf-

ﬁ ciently insulated. Structure-borne paths refer to forces transmitted directly

into the vehicle body where they can directly generate noise and vibration

of the vehicle. The structure-borne paths are usually isolated by elastomeric

mountings to minimise force input into the vehicle structure. Development

of a future model vehicle involves setting suitable speciﬁ cations for insulat-

ing and isolating each of these paths.

12.3.4 Noise receiver

The noise receiver of interest is usually the occupants inside the vehicle

cabin. The reader is directed to Chapter 13 for a description of the sound

propagation characteristics inside the cabin.

12.4 Enablers and applications

12.4.1 Internal combustion engine

This section discusses the key powertrain subsystems and the design factors

that inﬂ uence noise and vibration reﬁ nement. The powertrain subsystems

covered include the engine, transmission, driveline, induction and exhaust

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Frequency

Idle

firing

frequency

Cruise

firing

frequency

Primary combustion firing forces

Primary rotating and reciprocating forces

Maximum

firing

frequency

Powertrain input forces

Other vehicle systems force inputs

Powertrain rigid body modes

Powertrain elastic modes

Powertrain

mounting

Driveline

surge

Accessory

drive torsion

Gear rattle

Exhaust

structure

Final drive

mounting

Accessory

mounting

Driveline

bending

Suspension

hop modes

12.3 Target setting mode placement map.

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Table 12.4 Paths for each powertrain source

Noise source Airborne path Structure-borne path

IC engine

Engine rotating unbalance Engine and transmission

mounting

Engine reciprocating unbalance/

unbalanced inertial forces

Engine and transmission

mounting

Engine reciprocating unbalance/

unbalanced moments

Engine and transmission

mounting

Engine reciprocating unbalance/

inertial torques

Engine and transmission

mounting

Combustion gas pressure Engine and transmission

mounting

Exhaust mounting

Driveline and axle

mounting

Combustion gas pressure –

random misﬁ re component

Engine and transmission

mounting

Combustion gas pressure –

driver commanded step-input

Engine and

transmission

Driveline and

axle

Engine and transmission

mounting

Driveline and axle

mounting

Engine combustion noise Engine block and

covers

Engine and transmission

mounting

Exhaust mounting

Driveline and axle

mounting

Fuel injector and valve train Engine block and

covers

Forced induction pumps (turbo

or superchargers)

Induction inlet

Induction ducts

Engine accessories Accessories

housing

Brackets

Engine and transmission

mounting

Transmission

Transmission gear whine Transmission

housing

Engine and transmission

mounting

Driveline and axle

mounting

Driveline

Coupling misalignment Driveline and axle

mounting

Driveline imbalance Driveline and axle

mounting

Final drive module gear whine Driveline and axle

mounting

Exhaust and induction

Exhaust noise Exhaust tailpipe

Mufﬂ er shells

Exhaust mounting

Engine and transmission

mounting

Flow-induced noise Exhaust tailpipe

Induction noise Induction inlet Induction mounting

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264 Vehicle noise and vibration reﬁ nement

systems. Countermeasures or enablers to aid in reﬁ nement are introduced

for each subsystem, and their use is explained with examples.

The reciprocating four-stroke internal combustion engine is the class of

engine installed in the majority of today’s gasoline and diesel passenger

cars. Not surprisingly, the reciprocating engine is a signiﬁ cant generator of

forces that need to be managed systematically to avoid any unexpected

noises that might upset the owner of a new car.

Forces generated by the rotating crankshaft and forces generated by the

reciprocating motion of the pistons generate vibration loads that are pro-

portional in amplitude and frequency to the engine speed. These forces are

generated in all directions (translation, moments and torsion) and vary

depending upon the number of cylinders, the conﬁ guration of the engine,

and the mass and inertia of the reciprocating components. To counteract

some of these forces, additional balance weights can be applied to the

crankshaft. One noticeable exception is that the reciprocating force gener-

ated by a four-cylinder in-line engine require balance shafts (however, to

save costs, balance shafts are often not included in the design of engines

smaller than 2 litres displacement). Table 12.5 provides a summary of the

forces and methods used to counterbalance the forces. Refer to Rao (1986)

for derivation of the unbalance forces and Bosch (1986) for a summary of

the magnitudes.

The valve train also generates inertial imbalance forces as the engine

rotates. For example, the rotating cam lobes on the camshaft can induce an

unbalance moment, but these are generally not a signiﬁ cant concern.

The combustion gas pressure forces that act on the piston during ﬁ ring

generate a signiﬁ cant torsional vibration in the crankshaft. This is due to

increase in rotational speed of the crankshaft during each downward expan-

sion stroke of the pistons. While the frequency of the gas forces is depen-

dent on the engine speed, the amplitude of the torsional moment is

proportional to the torque or load output of the engine. Table 12.6 shows

the periodic torsional moments due to engine combustion for various cyl-

inder conﬁ gurations.

In addition to the periodic combustion gas forces, Table 12.6 also shows

that additional forces can be generated due to irregularities in the combus-

tion process. These irregularities can be periodic or random in nature

depending upon what is causing them.

The low and midfrequency forces generated by the rotating and recipro-

cating motion of the engine are capable of exciting the vibration modes of

engine components, so they must be designed to avoid resonances in the

engine operating range. Signiﬁ cant considerations include:

• Frequency of the ﬁ rst bending mode of the engine block and transmis-

sion. Besides driving targets for engine block and transmission casing

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