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Design principles for an automotive muffler
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International Journal of Applied Engineering Research
ISSN 0973-4562 Volume 9, Number 4 (2014) pp. 483-489
© Research India Publications
http://www.ripublication.com
Design Principles for an Automotive Muffler
Vinod Sherekar1 and P. R. Dhamangaonkar2,
1
PG Scholar, Mech. Engg., College of Engineering Pune, Maharashtra, India
2
Mech. Engg., College of Engineering Pune, Maharashtra, India
Abstract
Mufflers are important part of engine system and commonly used in exhaust
system to minimize sound transmissions caused by exhaust gases. Design of
mufflers is a complex function that affects noise characteristics, emission and
fuel efficiency of engine. Therefore muffler design becomes more and more
important for noise reduction. Traditionally, muffler design has been an
iterative process by trial and error. However, the theories and science that has
undergone development in recent years has given a way for an engineer to cut
short number of iteration. This paper discusses the various functional
requirements of automotive muffler like Insertion loss, Size of muffler,
Backpressure, Cost and Weight. The objective of this study is to give general
design guidelines for designing various parameters of muffler. This paper
suggests suitable guidelines for selection of muffler grade, muffler length and
diameter, Tail-pipe length according to engine cubic capacity and calculation
of pressure drop in muffler on the basis of exhaust gas flow rate. It also
explains the checking criteria for suitability of muffler for particular
application.
Keywords: Back pressure, Insertion loss, Muffler, Tail-pipe,
Introduction
Internal combustion engines are generating the acoustic pulse by the Combustion
process. This noise is controlled through the use of silencers and mufflers. A silencer
has been the traditional name for noise attenuation devices, while a muffler is smaller,
mass-produced device designed to reduce engine exhaust noise. Continuous
development has been made in improving performance of the silencers used for
automotive exhaust systems. Exhaust mufflers are widely employed to muffle the
noise of an engine body or the noise of other predominant sources in vehicles. In
order to maintain a desired noise and comfortable ride, the modes of a muffler need to
be analyzed. The intensity and magnitude of the noise will vary greatly depending
484
Vinod Sherekar and P. R. Dhamangaonkar
upon engine type i.e. naturally aspirated or turbocharged, horse power developed,
means of scavenging, type of fuel used, number of cycles whether 2 cycle or 4 cycle
engine etc.
Basic requirement of muffler design
There are various requirements that should be considered when designing a muffler
for a automotive application. Such functional requirements may include adequate
insertion loss, backpressure, size, durability, desired sound, cost, weight,compact
shape and style.Some of them requirements are given in detailed below.
Adequate Insertion Loss(IL): An effective muffler will reduce the sound pressure of
the noise source to the required level. In the case of an automotive muffler the noise
in the exhaust system, generated by the engine, is to be reduced.A mufflers
performance or attenuating capability is generally defined in terms of insertion loss or
transmission loss. Insertion loss is defined as the difference between the acoustic
powers radiated without and with a muffler fitted. The transmission loss is defined as
the difference (in decibels) between the sound power incident at the entry to the
muffler that of the transmitted by muffler.The muffler designer must determine the
required insertion loss so that a suitable type of muffler can be designed for the
automotive application. [1,2]
Selection of size: The proper selection and sizing of the silencer is of utmost
importance to ensure that pressure drop, acoustical performance and other specific
design criteria are met. The selection of the correct type of engine exhaust silencer is
determined by the type of engine, the end use of the engine and the degree of
silencing required. Also, the silencer size selected must accommodate the specified
volume of exhaust gas flow keeping the back pressure within the limits specified. The
available space has a great influence on the size and therefore type of muffler that
may be used. A muffler may have its geometry designed for optimum attenuation
however if it does not meet the space constraints, it is useless. [2]
Backpressure: Backpressure represents the extra static pressure exerted by the
muffler on the engine through the restriction in flow of exhaust gasses. Generally the
better a muffler is at attenuating sound the more backpressure is generated. In a
reactive muffler where good attenuation is achieved the exhaust gasses are forced to
pass through numerous geometry changes and a fair amount of backpressure may be
generated, which reduces the power output of the engine.Backpressure should be kept
to a minimum to avoid power losses especially for better performance of vehicle. [3]
Cost and Weight: Generally the larger a muffler is, the more it weighs and the more
it costs to manufacture. For a performance vehicle every gram saved is crucial to its
performance.Effectively supporting a muffler is always a design issue and the larger a
muffler is the more difficult it is to support. A Muffler’s mounting system not only
needs to support the mufflers weight but it also needs to provide vibration isolation so
Design Principles for an Automotive Muffler
485
that the vibration of the exhaust system is not transferred to the chassis and then to the
passenger cabin. This vibration isolation is usually achieved with the use of hard
rubber inserts and brackets that isolate or dampen vibration from the muffler to the
chassis. Therefore small light weight muffler is desirable [4].
Design parameters of muffler
Selection of muffler grade: There are following exhaust muffler gradesavailable and
one can choose according to required insertion loss and specific requirement. The
procedure for selection of muffler grade is also explained below:[5, 6]
Industrial/Commercial:
IL
Body/Pipe
Length/Pipe
= 15 to 25 dBA
= 2 to 2.5
= 5 to 6.5
Residential Grade:
IL
Body/Pipe
Length/Pipe
= 20 to 30 dBA
= 2 to 2.5
= 6 to 10
Critical Grade:
IL
Body/Pipe
Length/Pipe
= 25 to 35 dBA
=3
= 8 to 10
Super Critical Grade:
IL
Body/Pipe
Length/Pipe
= 35 to 45 dBA
=3
= 10 to 16
Step-1: Unsilenced noise level (e.g. UNL=104 dB@1m)
The overall noise level from most unsilenced engine exhaust systems varies from
about 100 dBA to 120 dBA, when measured 1 meter from the pipe outlet.
Step-2: Calculate exhaust noise criteria [6]
ENC= RNC – 5 (dBA)
(1)
(E.g. To meet a noise level of 65 dBA,design a muffler for 60 dBA)
Step-3: To calculate the unsilenced exhaust noise at the receiver location. The
following equation provides a correction for distance assuming free-field
spreading.[6, 7]
Lp (Xr) UNL= Lp (Xo) – 20 log (Xr/ Xo)
(2)
486
Vinod Sherekar and P. R. Dhamangaonkar
Where
Xr
Xo
= the reflection distance consider for calculation
= the distance at which unsilenced noise level (UNL) is measured
Step-4: The required insertion loss of the muffler is determined by subtracting
the receiver noise criteria from the unsilenced receiver noise level. Note that a 5
dB safety factor is recommended to account for the fact that actual muffler
performance often falls short. By calculating the Insertion loss selects the
suitable type of exhaust grade from available grades as stated above.
IL = UNL-ENC + 5
(3)
Exhaust engine tones: One needs to calculate the target frequencies to give more
concentration of higher transmission loss. For calculating the target frequencies
engine max power rpm is required and calculation follows: [6,8]
Cylinder firing rate (CFR)
CFR= RPM/ 60 -For 2 stroke engine
CFR= RPM / 120 - For 4 stroke engine
(4)
Engine firing rate (EFR):
EFR = n* CFR (n= no of cylinders)
(5)
Chamber length selection
a) Selection of chamber length from considering exhaust gas temp:[5]
Maximum exhaust gas temp: (If not available then consider following)
T = 1200oF
– for gasoline engine
o
T = 900 F
– for diesel engine
Figure 1: Chamber length selection
Design Principles for an Automotive Muffler
49.03*√oR
49.03*√oR
2*π*f
2*π*f
0.5 (
)≤ L1 ≤ 2.6 (
) (oR- Rankin temperature)
487
(6)
By performing sound analysis with frequency analyzer find out dominating
frequency (f) and used it for calculation.
b) Chamber length according to ASHARE Technical committee 2.6
Suppose if the requirement of selected muffler grade matches with supercritical grade
then
(Similar procedure follows for other grades) (Exhaust pipe diameter=silencer inlet
pipe diameter)
IL
= 35 to 45 dBA [5,6]
Body/pipe
=3
Length/pipe = 10 to 16
Length L1:
10* exhaust pipe diameter ≤ L1 ≤ 16 *exhaust pipe diameter
(7)
Selection of Body diameter(S2)
a) Body diameter from supercritical grade [5]
Suppose if the requirement of selected muffler grade matches with supercritical grade
then (Similar procedure follows for other grades)
Body diameter (S2) = 3* exhaust pipe diameter from engine
(8)
Figure 2: Selection of body diameter
b) Body diameter from muffler volume calculation
∗ ∗
π*L1*(S2) 2
Muffler volume (Vm) =
=
[8]
(9)
488
Vinod Sherekar and P. R. Dhamangaonkar
Volume of silencer must be at least 12 to 25 times the volume considered. Volume
canbe adjusted depending on the space constraint. According to available of space
choose the factor more than 12.
c) Body diameter from muffler volume calculation[1]
Vm =
∗
∗
∗√ ∗
=
π*L1*(S2) 2
(VP=Engine swept volume, T=no. of stroke, N=RPM)
(10)
Q= constant according to different requirement can be chosen between 5 and 6
Tail-pipe length: Exhaust tail pipe will have resonances that can amplify engine
tones.To avoid amplification of tones use short tail pipe. [6, 7]
S3
c
- )
(11)
Optimum tail-pipe length (L2) = (
4*f 1 2
(f1=fundamental firing frequency, c= speed of sound in fps)
Pressure drop:Theoretically it is verydifficult to calculate exact pressure drop
because of complex inner structure of silencer but following equations gives
approximate pressure drop and it should not exceed by the specify limits. The
pressure drop calculated can be check with the pressure drop calculated from
commercial FEM software [7, 9]
(
Exhaust flow rate(CFM) =
∗
)∗
∗
∗(
.
.℉
)
(12)
Efficiency = 0.85 for naturally aspired engine, 1.4=for turbo charged,
C=1 for two –cycle engine,C=2
for four- cycle engine
(
)
(13)
Exhaust gas velocity (V) =
(
2
)
Pressure drop(∆P) = C*(
) *(
)
℉
C= pressure drop coefficient, ∆P=pressure drop inches of water
(14)
Conclusion:
An automotive muffler should be designed to meet all necessary requirements as
mentioned above, like adequate insertion loss, minimal backpressure, space
constraints, be durable, light weight, be cost effective. There will be many possible
muffler design solutions for a particular situation and many possible ways to predict a
mufflers insertion loss. The muffler can be designed by various methods to achieve
good performance some of them are stated above but the design is proven by its
performance on an automobile itself.
Acknowledgement
It is pleasure to thank our Honorable H.O.D. (Mechanical Engg. Department)
Dr.S.N.Sapali for their suggestion, co-operation and encouragement. I express my
deepest gratitude towards Prof.A.P.Bhattu and Mr. Mahesh Patil who helped me in
the completion of this paper.
Design Principles for an Automotive Muffler
489
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