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Active Muffler for Single Cylinder Engine, Using Electronic Throttle Control for
Formula Student Cars
Conference Paper · July 2017
DOI: 10.4271/2017-28-1935
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Active Muffler for Single Cylinder Engine, Using Electronic
Throttle Control for Formula Student Cars
2017-28-1935
Published 07/10/2017
Praveen V V and P Baskara Sethupathi
SRM University
CITATION: V V, P. and Sethupathi, P., "Active Muffler for Single Cylinder Engine, Using Electronic Throttle Control for Formula
Student Cars," SAE Technical Paper 2017-28-1935, 2017, doi:10.4271/2017-28-1935.
Copyright © 2017 SAE International and Copyright © 2017 SAE INDIA
Abstract
Formula SAE is a prestigious engineering design competition, where
student team design, fabricate and test their formula style race car,
with the guidelines of the FSAE rulebook, according to which the car
is designed, for example the engine must be a four-stroke, Otto-cycle
piston engine with a displacement no greater than 710cc.
According to FSAE 2017 Rule Book [1], ARTICLE 3, IC3.2 and
IC3.3 state that the maximum sound level should not exceed 110 dBC
at an average piston speed of 15:25 m/s (for the KTM 390 engine,
which has 60 mm stroke length, the noise level will be measured at
7500 RPM) and 103 dBC at Idle RPM. So, the active muffler which
works as a normal reflective muffler till the 7500 RPM range, after
which an electronic controlled throttle mechanism is used to reduce
the backpressure (since after 7500 RPM the noise level doesn't matter
in FSAE) by using tach signal from the engine to control the throttle
(two position). The electronic throttle will be controlled using an
ardino board and control the backpressure, with respect to the Engine
RPM. This ensures that Noise level rules are met without
compromising on the performance of the engine.
Further research is required in utilizing the active throttle setup for
continuously variable position (with respect to the RPM), in order to
tune the exhaust system according to individual requirements and for
achieving the required acoustic tuning of the exhaust system.
Introduction
Being a race car the weight and space constrains have to be
minimalistic for the muffler and but the major concern is to reduce
the backpressure of the muffler, for any engine the mass flow rate of
the exhaust gas increases with respect to the increase in engine RPM
and in any fixed geometry muffler, the backpressure increases with
respect to increase in mass flow rate of the exhaust gas.
In Formula SAE, the exhaust system is designed to get the maximum
output from the engine, but in teams using single cylinder Otto-cycle
engines, it has to be a compromise in terms of performance from
exhaust system, due to the low frequency sound waves, it is difficult
to design a muffler with considerably less backpressure, weight and
volume, Engine and exhaust system specifications are shown in Table
1 and Table 2. So, in most cases one or more of the parameters are
compromised in order to get the required noise level.
With the help of the active muffler, the weight and dimensional
constrains are met without compromising on the backpressure, while
meeting the noise level requirements. Since the active setup can
control the backpressure with the help of a lighter and compact
muffler design, with the use to electronic or vacuum actuated valves
in order to reduce the backpressure, under certain conditions.
Table 1. Engine specifications
Table 2. Exhaust system specifications
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Process Involved in Designing and Testing
of the Muffler
The testing methodology is based on designing and fabricating
mufflers of different sizes, geometries and noise attenuating principles,
with an increasing backpressure (from muffler 1 to muffler 6), till the
noise level requirements are met, then the minimum backpressure
required to meet the noise level requirements is found. Based on
which the lightest and least volume consuming muffler is designed
and fabricated. Based on which an active muffler is designed.
The testing setup, consists of the car, with engine running on same
conditions for the test and the noise level measured with a dBC scale
sound level meter. The setup for the noise level test is shown in
Figure 2. All the tests were conducted in an Anechoic chamber under
same air temperature. Which is shown in Figure 1 and Figure 2.
•
A CAD model of the muffler is made.
•
It is simulated in ANSYS® Fluent, to get the backpressure at
idle RPM and 7500 RPM.
•
It is then fabricated, weighed and then fixed to testing setup.
•
The noise level is measured at idle and 7500 RPM.
•
The readings are used to design the next muffler, which has an
improvement in terms of weight or volume or noise level.
Figure 1. Testing setup to measure the exhaust gas temperature in inlet and
outlet of the muffler
The Minimum Required Backpressure to Attenuate the
Noise Level
The design process is started with a simple free flow muffler (Muffler
1) of comparatively small volume of 2.9 liters, the free flow muffler
is of absorption type with a 40 mm OD perforated tube and a shell of
100 mm diameter and 380 mm length, filled with noise absorption
material. This resulted in a very high sound level during the sound
level test, with 109.5 dBC at idle RPM and 120.8 dBC at 7500 RPM.
The sound level was used to make a benchmark in designing the next
muffler. The muffler and its respective sound level measurements are
shown in the Figure 3.
Figure 3. Muffler 1 and its respective sound level measurements
Then in the next free flow muffler volume was increased to 4.0 liters,
but with a shorter perforated tube and the same noise level test is
done with same engine parameters and was found to be too high in
the range of 126 dBC. Muffler 2 and its respective sound level
measurements are shown in Figure 4.
Figure 4. Muffler 2 and its respective sound level measurements
In the next free flow muffler volume was further increased to 4.9
liters, with a longer perforated tube and 36% more porosity. but the
noise level was still in the range of 118 dBC. Muffler 3 and its
respective sound level measurements are shown in Figure 5.
Figure 2. Testing setup to measure the sound level of the muffler
Figure 5. Muffler 3 and its respective sound level measurements
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In the next muffler, the volume was kept constant, but the
backpressure was increased by creating a 180 degree bend in the
tube, there by doubling the perforated pipe length, then the muffler
was simulated using the mass flow rate of the exhaust gas at 750° C
and 7500 RPM and the backpressure was found to be 1.4 kpa at 7500
RPM, but the noise level was still 113.5 dBC at 7500 RPM (all the
free flow mufflers were packed with the same noise absorption
material of 128 kg per m^3 density). Muffler 4 and its respective
sound level measurements are shown in Figure 6.
This resulted in a muffler which was compact (5 liters) and
comparatively light (weighing 2.78 kg) with a simulated backpressure
value of 2.8 bar (at 7500 RPM and gas temperature of 750° C) then
for experimental testing the muffler was tested in a cold air flow
bench, which is shown in Figure 9. Where the air is pumped into the
muffler with different mass flow rate and its respective backpressure
is measured using piezoelectric sensors, which resulted in a
backpressure of 5.5 kpa at cold air flow condition and the noise level
to 97.8 dBC at 1800 RPM and 107.1 dBC at 7500 RPM.
Figure 6. Muffler 4 and its respective sound level measurements
From the experiment, it was concluded that the backpressure had to
be increased significantly to attain the required noise level. In the 5th
muffler along with a free flow absorption muffler, an expansion
chamber of 3.8 liters volume was added and was tested with similar
engine parameters, now the sound level reduced considerably to
108.6 dBC, when analyzed, the backpressure was found to be 14 kpa,
but with the volume of 8.9 liters and weight 4.7 kg of the muffler was
far too big to be used in a formula student car. Muffler 5 and its
respective sound level measurements are shown in Figure 7.
Figure 7. Muffler 5 and its respective sound level measurements
Figure 9. Cold air flow bench Backpressure measurement setup
Design of the Active Muffler
In order to save weight and make the muffler compact without
increasing the backpressure, an active muffler had to be used. The
muffler was designed with 3 chambers with a diameter of 150mm and
length 0f 350mm with a perforated tube (of 38mm OD and 35mm ID
with 104 4mm diameter holes and 120 3mm holes) with two blocks
(first block in the 1st chamber, between 4 and 3mm holes and the
second block in 2nd chamber, between 3 and 4mm holes) as the inlet
tube, which extends from inlet to the 3rd chamber, the free flow outlet
(of 38mm OD and 35mm ID) tube extends from 1st chamber to
outside of the muffler, acting as one of the two tail pipes. The second
outlet tube (of 38mm OD and 35mm ID) starts from the second baffle
plate (where a small chamber of 220 ml volume is welded to the
baffle plate) and extends outside the muffler, acting as the second
outlet which always remains open and the other outlet which is
shown in Figure 11, where the active ETC mechanism is fixed. The
internal structure of the muffler is shown in the Figure 10.
In the 6th muffler a complicated 3 chamber design with perforated
tube with two blocks, absorption material packing in 3rd chamber,
followed by an expansion chamber, which changes the flow direction
of the gases and a long tail pipe of 400 mm length was used. Muffler
6 and its respective sound level measurements are shown in Figure 8.
Figure 10. Internal structure of the muffler (without the outer shell)
Figure 8. Muffler 6 and its respective sound level measurements
Downloaded from SAE International by P Baskara Sethupathi, Wednesday, July 05, 2017
Figure 11. Active throttle assembly
Figure 13. Backpressure Vs Mass flow rate graph for the active muffler (with
the throttle closed) in cold air flow bench
Figure 12. Active muffler assembly, with the ETC
The active throttle body consist of a butterfly valve which is controlled
by a servo motor (via. Gear mechanism, to achieve the required torque
and isolate the heat from the servo motor) which is shown in Figure 11
and Figure 12. The throttle body is connected to one of the outlets of
the muffler, when backpressure is minimal. the system is active with
two positions, which normally remains closed, when engine is off and
when engine RPM is less than 7600, when the engine RPM reaches
above 7650, the valve fully opens in 0.47 second and closes again
when engine RPM drops below 7600. The engine RPM value is taken
from the Pe3 ECU to control the servo motor.
When the muffler was simulated, it produced a backpressure of 2.72
bar (at 7500 RPM with a gas temperature of 750° C and ETC closed),
shown in Figure 14 and when tested in the cold air flow bench, it
shows a comparatively less backpressure of 5 kpa (measure with the
active throttle closed at 7500 RPM), shown in Figure 13, and when
the muffler was simulated with both outlets open (throttle open
condition), at 7500 RPM with a gas temperature of 750° C it had a
backpressure of 0.67 bar, shown in Figure 15. The transition loss of
the active muffler with the throttle closed is shown in Figure 16.
With this kind of setup, the noise level rules are met with reduction in
volume and weight of the muffler. This reduction in backpressure has
also increased the torque figures of the engine after 7600 RPM range
by a considerable margin (when specifically tuned, with respect to
reduction in exhaust backpressure).
Figure 14. Pressure difference with one outlet open (Throttle closed condition)
Figure 15. Pressure difference with both outlets open (Throttle open
condition)
The muffler was tested on the car (with similar engine conditions as
the previous mufflers) and it gives a noise level of 99.1 dBC at idle
RPM and 109.2 dBC at 7500 RPM and when the ETC opens at 7650
RPM the noise level increases to 118 dBC at 8000 RPM. The noise
level readings are shown in the Figure 17. The testing setup is shown
in Figure 18.
Downloaded from SAE International by P Baskara Sethupathi, Wednesday, July 05, 2017
Further research has to be made to utilize the ETC, with continuous
variable throttle, which changes according to the change in engine
RPM and drive modes. Which will help in further tuning of the
muffler to get maximum power output from the engine.
Conclusions
This paper empathizes the importance of exhaust system in today’s
motorsports world and new ways to design and validate the exhaust
system. With a new methodology to design, test and validate a
muffler and using that data to further develop the next muffler’s
design, particularly in student level motorsport events.
References
Figure 16. Transition loss graph for the active muffler, with throttle closed
(with various RPM)
Figure 17. Noise level values of the active muffler (measured with different
intervals, and the ETC opening at 7650 RPM)
Figure 18. Picture taken during the testing of the ETC and noise level
measurement at different RPM
Further improvements that can be done to the active muffler
The muffler along with the ETC setup weighs 3.21 kg (where muffler
and throttle mechanism is made out of stainless steel 304), but if the
same muffler and throttle mechanism is made out of titanium T6,
with thinner shell, tubes and baffle plates, the weight can be reduced
by 1.45 kg.
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Contact Information
Mailing address: No. 24 Ranganathan Gardens, 15th Main Road,
Anna Nagar West, Chennai, 600040, India
vvpraveen4215@gmail.com
Contact number: +91 8939880672
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Definitions/Abbreviations
RPM - Rotations per minute
ETC - Electronic throttle control
FSAE - Formula SAE®
CAD - Computer aided design
OD - Outer diameter
ID - Internal diameter
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