Xin Q. Diesel Engine System Design

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760 Diesel engine system design

by the cylinder pressure gas force and the inertia force acting on the cranktrain

components, and transmitted through the engine mount to the drivertain and

the entire vehicle body. Excessive vibration causes discomfort and even

durability failures of the component.

Harshness usually refers to an unpleasant and subjective characteristic of

noise or vibration, such as a short duration of noise spikes or tactile shock

spikes, and the after-shake oscillatory vibrations.

Noise is perceived by the human ear for both loudness and sharpness

after the air pressure uctuations are converted to the mechanical motion

of the cochlea which responds at different locations depending upon the

excitation frequency of the sound. Sound characteristics can be described

by sound pressure level and frequency with equal loudness contours. The

sound pressure level has a unit of decibel (i.e., one tenth of a Bel), which

in fact is a unit of power. It should be noted that the sound pressure level

is not a measure of the loudness of a sound because the human ear is not

equally sensitive to all frequencies. If the sound pressure of one noise is

concentrated in a higher frequency range where the ear is more sensitive,

it may be perceived louder than another noise with equal sound pressure

level but concentrated in a lower frequency region that is less sensitive to

the ear. In order to obtain the levels which bear a closer relationship to

loudness judgment than the sound pressure levels, three different networks of

frequency weighting (A, B, and C) were incorporated in sound level meters.

The A-weighting most closely mimics the human ear, and it is most widely

used in noise control work. The A-weighted sound level is expressed in

the unit of dB(A) or dBA. A human whispering may be around 1 kHz and

30 dB(A). A normal conversation is at 0.5 kHz and 65 dB(A). A machine

operator can be exposed to sounds around 0.5–5 kHz and 90–100 dB(A).

The noise level in residential areas is around 50 dB(A). A vehicle noise can

be 85 dB(A). Generally, a sound pressure level above 75 dB(A) is regarded

as noisy, and a noise level above 90 dB(A) is considered very high. The

threshold of pain is around 120 dB(A). In order to reduce or prevent hearing

damage, standards organizations (OSHA) and national labor laws stipulate

the limits of noise and impose hearing protection measures.

The audible frequency range (20 Hz–20 kHz) can be divided into bands

with one octave wide. An octave is a frequency interval between two sounds

whose frequency ratio is two (Table 11.1(a–b)). In the frequency analysis

of noise signals, the narrow band spectrum data (e.g., obtained by Fast

Fourier Transfer) provide the highest resolution but are more qualitative in

nature. Usually, 1/3 (one-third) octave, sometimes octave and 1/12 octave,

band spectrum with center frequencies is used to present more quantitative

information in noise analysis. Another type of frequency band, the ‘critical

bands’ of noises, was dened to divide the frequency range into 24 bands

in the unit of Bark (Zwicker denition, Table 11.1(c)) with each band

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Table 11.1 Noise frequency band deﬁnitions

(a) Octave band center frequency (unit: Hz)

31.5 63 125 250 500 1000 2000 4000 8000 16000

(b) One-third octave band center frequency (unit: Hz)

31.5 40 50 63 80 100 125 160 200 250

315 400 500 630 800 1000 1250 1600 2000 2500

3150 4000 5000 6300 8000 10000 12500 16000 20000 25000

Critical band (Bark) 1 2 3 4 5 6 7 8

Center frequency (Hz) 50 150 250 350 450 570 700 840

Bandwidth (Hz) 100 100 100 100 110 120 140 150

Critical band (Bark) 9 10 11 12 13 14 15 16

Center frequency (Hz) 1000 1170 1370 1600 1850 2150 2500 2900

Bandwidth (Hz) 160 190 210 240 280 320 380 450

Critical band (Bark) 17 18 19 20 21 22 23 24

Center frequency (Hz) 3400 4000 4800 5800 7000 8500 10500 13500

Bandwidth (Hz) 550 700 900 1100 1300 1800 2500 3500

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762 Diesel engine system design

having a center frequency from 50 Hz in the lowest band to 13,500 Hz in

the highest band; and it has a bandwidth from 100 Hz in each of the several

low-frequency bands to 3500 Hz in the highest band.

Ideally engine noises are measured at different positions around the engine

in a free  eld environment (i.e., anechoic without sound re ections or echoes)

with electrostatic microphones to measure the sound pressure (a scalar ‘point’

property) or using intensity probes to measure the sound intensity (a vector

‘ eld’ property). Sound power is a measure of the acoustic power radiated

from the source. Sound intensity is the sound power per unit area, given

= p

air

11.2

where the acoustic velocity v

air

is the velocity of the local oscillating motion

of the air particles. The sound power



, sound intensity I

, and sound

pressure p

are related for an engine by the following:



ound

WI =WI

WI WI

· d

AI· dAI

AI =AI

AI AI

A A

WI WI

11.3

where r is the air density and A is the area.

Sound magnitude is characterized by sound pressure or sound power.

Because the perception of the human ear to loudness is proportional to the

logarithmic scale of the sound pressure rather than a linear scale, a logarithmic

scale is usually used to de ne the sound pressure level in the unit of decibel

(dB) as follows:

SPLSP

= 10 · log

= 20 · log

sr,sr

ssrs

sr,sr

11.4

where p

s,ref

is a reference sound pressure, usually equal to 20 micro-pascals

(mPa), which is the minimum sound level the human ear can hear. A sound

pressure of 20 mPa corresponds to a sound pressure level of 0 dB. A

doubling of any value of the sound pressure corresponds to an increase in

sound pressure level of 6.02 dB. A multiplication of the sound pressure by

a factor of ten corresponds to an increase in sound pressure level of 20 dB.

Sound pressure is inversely proportional to the distance from the source. The

sound pressure level decreases by 6 dB for each doubling of the distance

from the source.

The sound power level is given by the following in the unit of dB:



SWLSW

= 10 · log

sr,sr

11.5

where



sr,sr

is a reference sound power, usually equal to 1 ¥ 10

–12

watt.

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763Noise, vibration, and harshness (NVH) in engine system design

The sound power level and the sound pressure level are related by the

following:

pl =pl

– 20 · log

pl – 20 · logpl

– 10.9 +



plWpl

11.6

where l is the distance from the source of the noise in meters, and C

is a

correction term given by Harris (1979).

The sound intensity level is given by the following in the unit of dB:

SILSI

= 10 · log

sr,sr

11.7

where I

s,ref

is a reference sound intensity, usually equal to 1 ¥ 10

–12

W/m

Sound intensity measurement can estimate sound power. Intensity

measurement techniques provide information about the radiation characteristics

of sound sources and their spatial distribution.

Engine vibration can be measured by using the low-mass accelerometers

rigidly mounted to the test structure. The vibration signals, such as

displacement, velocity and acceleration can also be converted to vibration

displacement level, velocity level and acceleration level, respectively, in the

unit of dB as follows:

VDLVD

refre

= 20 · log

11.8

VVL

refre

= 20 · log

11.9

VAC

VACLVAC

refre

= 20 · log

11.10

where l

ref

is a reference displacement, for example 1 ¥ 10

–12

m; v

ref

is a

reference velocity, e.g., 1 ¥ 10

–9

m/s; and a

ref

is a reference acceleration, e.g.,

1 ¥ 10

–6

m/s

. Moreover, mobility refers to the ratio of the velocity response

at the excitation point on the structure to the applied force at the point.

Noise and vibration data are characterized by an overall level and the

levels of detailed signals. The overall level usually refers to either an

average (root-mean-square or RMS) or peak signal, often averaged from four

microphones installed at different locations one meter away from the engine

surface. The detailed signals include a time history or frequency breakdown

of the sound pressure level or vibration data. Often A-weighted overall sound

pressure levels and one-third octave spectra are used in noise testing. For

more information about noise fundamentals, the reader is referred to Harris

(1979) and Hickling (2005).

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764 Diesel engine system design

11.1.2 Objective and subjective assessment of noise

Noise and vibration signals can be measured and evaluated objectively.

However, because the human body has its own frequency response (e.g., the

ear’s response to different frequencies), NVH problems are also largely tied

to subjective impressions or sound quality issues, which are the subject of

the eld of psychoacoustics. The human ear has a logarithmic response to

the magnitude of sound pressure waves and is most sensitive to frequencies

of about 1 kHz. The human ear is sensitive to broad-band impulsive noises.

Poor diesel sound quality may cause subjective annoyance even if the sound

pressure level is low. For more in-depth information about human hearing

and sound quality evaluation methods, the reader is referred to Sasaki and

Nakashima (2007).

Human hearing reacts nonlinearly to sound pressure. The unit of decibel was

conceived partly for that reason. Although the simple frequency weightings

(e.g., A-weighting) try to mimic human hearing response, they do not

provide information about sound quality. Many psychoacoustic parameters

are based on a modied frequency scale, the tonality scale, from 0 to 24

Bark. Psychoacoustic or sound quality parameters include the following:

∑ loudness (in the unit of sone)

∑ sound volume (phon)

∑ severity or sharpness (a weighting that emphasizes the high frequencies

in the unit of acum)

∑ uctuation strength or amplitude (characterizing the extremely low

frequency such as the less than 20 Hz unpleasant sound signals)

∑ roughness (assessing the rough modulations in the 20–300 Hz frequency

range)

∑ tonality (classifying the pure tonal content in the noise)

∑ repetition frequency of periodic sound as perceived periodicity in

Hertz

∑ impulsiveness or knocking measured by kurtosis

∑ harmony or sound pressure distribution

∑ spatial selectivity.

The models and evaluation methods of these psychoacoustic parameters have

been developed to facilitate the design decisions related to sound quality.

The direct injection diesel engine is the most important source of interior

noise for a diesel vehicle. The subjective noise character of the engine can

be evaluated using AVL’s Annoyance Index. The index links the perceived

sound quality to measurable psychoacoustic parameters so that the engine

noises may be described by the index as a function of loudness, perceived

periodicity, sharpness, and impulsiveness.

In engine development the major objectives of noise control are to reduce

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765Noise, vibration, and harshness (NVH) in engine system design

the noise level in dB(A) to meet legislative requirements and to reduce the

annoyance of the noise to satisfy customer requirements on the subjectively

perceived sound quality. The sound pressure level and annoyance are

different attributes. Two different sound levels apart by several dB(A) may

be observed subjectively to have equal annoyance. Measures to reduce

the engine noise level in dB(A) do not necessarily lead to an improved

sound quality. Often it may even result in deterioration in the subjective

noise characteristics (Schiffbanker et al., 1991). More information about

the subjective characteristics of diesel engine noises, noise measurement

methods, and diesel engine noise data examples can be found in Rust et al.

(1989) and Corcione et al. (1989).

11.2 Vehicle and powertrain noise, vibration, and

harshness (NVH)

11.2.1 Noise regulations

As the regulations on pollutant emissions of diesel engines become more

stringent, the limits in vehicle noise regulations have also been drastically

reduced over the last 30 years. NVH development has been subject to

vehicle-level NVH regulations since the rst pass-by noise regulation in

1970 in the EU and in 1971 in Japan and later in 1978 in the US Vehicle

noises are usually tested as the following: (1) the pass-by noise in front

of a by-stander; (2) the noise received by the operator under various load

conditions; or (3) the noise received by the passenger in the cabin. The US

federal noise regulations for trucks require 80 dB(A) at 15 m (EPA CFR Part

205, SAE J366) for pass-by noise (during mid-gear full-pedal acceleration),

and 85 dB(A) at 15 m (EPA CFR Part 325, SAE J1096) for stationary noise

(during no-load acceleration and deceleration, i.e., from idle to maximum

governed engine speed and then to idle).

Pass-by noise measurements are required for new vehicles to fulll the legal

regulations on vehicle exterior noise. The measurement is conducted to obtain

sound pressure levels in dB(A) radiated by an accelerating vehicle. In order

to effectively reduce the vehicle’s exterior noise through design, engineers

need to know the dominant noise sources and their transfer paths.

Correlating the vehicle pass-by noise to a design target of engine noise

is an important engineering practice. The diesel engine is a major NVH

excitation source in the vehicle. When the engine is running, a small portion

of the energy involved is lost through engine vibration to the surrounding air

as sound. To meet the vehicle noise target of 80 dB(A) during acceleration

pass-by, an engineering target lower than 80 dB(A) needs to be established,

for example 78 dB(A) to ensure production audit by reserving a certain

margin to cover tolerances and variations. The contribution of engine noise

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766 Diesel engine system design

to total truck pass-by noise level also needs to be determined, for example

75 dB(A). Note that every 3 dB reduction corresponds to having the sonic

energy decreased by half. Other contributions to the vehicle pass-by noise

come from the tire and the vehicle body. It is important to recognize the

relative importance of the contributions from the engine and the tire to the

total vehicle noise level. If the total noise is low and the tire noise is high,

the engine’s contribution becomes less important.

Engine sound level measurement is normally conducted according to the

SAE J1074 procedure in an engine test cell. A noise target (for example 93

dB(A)) in engine noise control can be set at one meter away from the engine

surface with the engine operating at the steady-state rated power condition.

Such a target may approximately represent the noise level required by the

engine in the vehicle in order to meet the vehicle pass-by noise regulation

level at 80 dB(A). The target cascading from the vehicle level noise on the

road during acceleration to the engine level noise in the test cell running at

steady-state power is largely empirical, and can be very complex. However,

this is an important area where a system engineer can play a major role to

contribute to make this translation more analytically elegant and accurate.

Moreover, it should be noted that the global noise regulations are not directly

comparable because they are tied to different measurement procedures. For

example, the test is conducted at 7.5 m away from the microphone position

under the European noise regulation ISO R362. On the other hand, the same

test is conducted at 15 m away from the microphone position under the US

SAE measurement standards. Previous practice has demonstrated that the

European noise regulation seems more stringent (Wodtke and Bathelt, 2004).

More information on vehicle noise regulations can be found in Reinhart (1991)

and Cherne (1993). The SAE procedures listed in the section of references at

the end of this chapter also provide good fundamental knowledge on vehicle

and engine noises and their measurement methods.

11.2.2 Classiﬁcation of powertrain and drivetrain NVH

problems

NVH is one of the most important vehicle attributes, along with drivability,

durability, ride, and handling. Vehicle NVH can be classied into three

types of issues from a receiver’s perspective, namely acoustic, visual, and

tactile. Examples of those issues include the noise at the driver’s and the

passenger’s ears, the noise perceived by a by-stander, the vibration of the

rearview mirror, the vibration of the steering wheel, and seat shake.

Vehicle NVH can also be classied into two categories, interior and

exterior. The interior NVH issues deal with the noise sound level, sound

quality, and vibration experienced by the occupants inside the vehicle cabin.

The design criterion of the interior cabin noise can be as simple as a sound

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767Noise, vibration, and harshness (NVH) in engine system design

pressure level received by the driver or all occupants at certain vehicle speed

and load, or as complicated as full sound quality metrics (e.g., the Zwicker

sound quality metrics). The exterior noise radiated by the vehicle has a

direct impact on the quality of life for the people who hear the noise. The

exterior noise is subject to the pass-by noise regulation. The diesel engine

contributes to a large extent to both the interior and exterior noises in sound

pressure level and sound quality.

Moreover, vehicle NVH problems can be classied according to the

dynamic excitation sources. The external excitation sources include road

surface, wind and other environmental effects. The internal excitation sources

include powertrain (engine and transmission) and drivetrain forces, such as

engine combustion, engine reciprocating imbalance, engine vibration and

torque cyclicity, engine-driven accessory disturbances, engine mount vibration,

intake and exhaust noises, gear meshing variation, torque converter imbalance,

driveshaft and half-shaft imbalances, tire and wheel imbalance, and brake-

induced forces. Diesel trucks usually have three major noise sources: the

engine (including the exhaust system), the cooling fan, and the tires. Note

that the intake and exhaust aerodynamic noises are generally considered as

a part of the engine noise because its origin is the gas wave dynamics inside

the engine, while the intake and exhaust structure-borne noises (e.g., intake

system mount, exhaust hanger, and heat shield) can be considered as a part

of the vehicle noise.

Engine noise is no doubt the most important noise in powertrain

development. However, transmission noise (mainly gear rattle) sometimes

may become prominent, for example the gear rattle at idle in a manual

transmission. The rattle noise is related mainly to transmission errors and

variations in gear rotation. Gear rattle noise increases if the powertrain

oscillates at its resonance frequency. Note that unique NVH issues exist in

hybrid powertrains, for example the torque blending mismatch between the

engine and the electric motor, the driveline vibrations caused by low-speed

electric motor torque ripple, and the motor gear rattle noise. The powertrain

and drivetrain NVH issues are discussed in more detail by Steyer et al. (2005),

Juang et al. (2006), Wellmann et al. (2007), Tousignant et al. (2009), and

Govindswamy et al. (2009).

Depending on the vehicle speed the major noise sources can be simply

traced to two sources, the engine noise and the road–tire noise. The engine

noise dominates at low vehicle speeds and the road–tire noise dominates at

high vehicle speeds. Wind noise is usually relatively low.

Vehicle noises are classied into two categories, structure-borne and

airborne. Their denitions were given by Anderton and Zheng (1993):

The sound entering the air via an acoustic path is the sound radiated by

an internal sound source, e.g., valve impact which propagates through the

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768 Diesel engine system design

acoustic medium, and is then transmitted through the wall of the (valve)

cover. This type of radiated noise is referred to as airborne sound. The

sound entering the air via a structural path is the sound which has its

source in the vibrational energy that has been transmitted through the

structure (either exural or compressional waves) from an engine’s and

its component’s (including valve mechanism itself) vibrational force. This

type of radiated sound is referred to as structure-borne sound.

The structure-borne noise transmits via the engine mountings, drive shafts

and other connecting elements into the vehicle body. The airborne noise

transmits directly from the engine compartment to pass through the body

shell wall to enter into the vehicle to contribute to the total interior noise.

Automotive structure-borne noise is usually below 500–1000 Hz in a low

frequency range. The airborne noise is usually in a wide range of 300 Hz

to 7 kHz, generally in a high frequency range. Detailed classications of

structure-borne and airborne noises and the source of excitations are detailed

by Riding and Weeks (1991).

According to frequency range the noise can be classied as low frequency

noise and high frequency noise. For instance, the structure-borne noise

which is transferred to the vehicle body via the engine mounts occurs in

the frequency range below 800 Hz and can be regarded as a low frequency

noise. On the other hand, the structure-borne noise radiated from the engine

surface to the surrounding air occurs in the frequency range above 800 Hz

and can be called high frequency noise.

11.2.3 Vehicle powertrain and drivetrain NVH process

Vehicle powertrain and drivetrain NVH development processes are described

in Castillo (2001), Alt et al. (2003), Laux et al. (2005), Mori et al. (2005),

March et al. (2005a, 2005b), and Afaneh et al. (2007). Understanding how

the engine NVH target is cascaded down from the vehicle level is very

important for diesel engine system design.

11.3 Diesel engine noise, vibration, and harshness

(NVH)

11.3.1 Classiﬁcation of engine NVH

The area of engine vibration is very broad, including engine balance, engine

internal excitation forces, and engine mount vibration. Noise is a more direct

performance attribute than vibration for customer satisfaction although they

are related. The engine noise sources originate from combustion processes,

mechanical movements, and air charging/discharging processes. The excited

vibrations are absorbed and transmitted to the structure, and the noise is

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769Noise, vibration, and harshness (NVH) in engine system design

generated from the vibrating surfaces. The engine noise includes that excited

by high-frequency impulsive forces (e.g., combustion pressures, piston

slap impacts) and that excited by low-frequency inertia loads acting on the

components. Engine noise includes the noises from combustion, piston slap,

valvetrain, fuel injector, cranktrain, engine block (including covers and heat

shields), geartrain, accessory, turbocharger, and intake and exhaust ows.

They can be classied into three types: combustion noise, mechanical noise,

and aerodynamic noise. The combustion and mechanical noises are mostly

structure-borne noises.

The mechanical noise is caused mainly by the mechanical impacts between

the moving components due to the cylinder pressure loading and inertia

forces. The impacts happen at the engine bearings, the piston–liner interface,

the valve seat, and the gear drives. The mechanical noise also includes the

noise due to the hydraulic pressure uctuations in the auxiliaries such as

the fuel injection system and the oil pump, and the noise due to the electro-

magnetic mechanisms in the alternator.

The combustion noise is affected mainly by the rate of rise of the cylinder

pressure and the lasting duration of the high rate of rise. It is the noise above

the engine ring frequency and its harmonics in the frequency domain. It

is directly due to the combustion excitation in the cylinder. It is induced

by the in-cylinder gas pressure wave oscillations, and is the radiated noise

from the vibratory surfaces of the cylinder head, the piston, the connecting

rod, the crankshaft, and the engine block.

The aerodynamic noise is mainly an airborne noise induced by the gas

ow pressure waves in the intake and exhaust systems, the turbocharger, and

the cooling fan. Often the fan noise is considered as a part of the vehicle

noise.

The experimental and analytical investigations on engine noise and its

controls are normally conducted in the following areas:

∑ overall sound pressure levels from motoring, no load to full load across

the entire engine speed range

∑ detailed sound pressure levels in the frequency/order spectrum and/or

the time/crank angle domain at different engine speeds and/or loads

(e.g., the contour map of sound pressure levels in the domain of ‘noise

frequency vs. engine speed’ – the Cambell diagram; the contour map of

sound power levels in the domain of ‘noise frequency vs. crank angle’

– the Wigner–Ville analysis)

∑ sound intensity measurement

∑ sound quality measurement and analysis (e.g., binaural head recording,

objective and subjective assessment)

∑ noise source identication for the contributions from the structure-borne

and airborne noises in the frequency spectrum, noise radiation ranking

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