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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 59, 2017 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Zhuo Yang, Junjie Ba, Jing Pan 
Copyright © 2017, AIDIC Servizi S.r.l. 
ISBN 978-88-95608- 49-5; ISSN 2283-9216 

Design and Research of Automobile Exhaust Heat Energy 
Conversion Device 

Yuelei Wang 
Hainan Institute of Science and Technology, Hainan, China 
wangylei@163.com 

With the progress of the times, the car has become an indispensable part of people's lives. However, the 
development of the automobile industry has brought enormous benefits to human beings, while it also 
consumes a lot of energy and pollutes our living environment. Therefore, this paper intends to use automobile 
exhaust heat combined with thermoelectric generator to recover part of the energy consumption of 
automobile. The energy can be converted into electric energy to supply the muffler with purification function, 
so as to achieve the purpose of energy saving and emission reduction. This paper summarizes the 
advantages and disadvantages of the first-generation exhaust gas temperature difference generator. In order 
to improve the power generation, the second-generation exhaust gas temperature difference generator is 
designed. The utility model has the characteristics of a hexahedron structure and is internally provided with a 
fin vortex structure. The utility model can increase the airflow disturbance, thereby improving the heat transfer 
efficiency and the generation power. In addition, using GT-POWER software, a small and medium sized 
vehicle engine is used as the research object. The simulation results show that the noise value of the engine 
is between 3500 and 6000r/min. Peak noise is in the 110-124dB interval, and the corresponding frequency of 
high noise values is in the 180-200HZ, which increases with the speed changes. The simulation results 
provide the basis for the design of the muffler. Based on the optimized design, the exhaust gas temperature 
difference generator is processed in real time. The experimental results show that the exhaust gas 
temperature difference generator designed in this paper has higher power generation efficiency, and it can 
meet the demand of power supply for the tail gas purification. 

1. Introduction 
With the sustained and in-depth development of China's social economy, modern industry has created 
enormous wealth for mankind. Meanwhile, it has brought serious pollution to the ecological environment, 
especially the air environment. All kinds of poisonous gas, pollution gas and automobile exhaust are the main 
causes of air pollution (Ibrahim, et al., 2010; Zhao et al., 2016; Arapatsakos et al., 2015; Gattuso et al., 2016; 
Cannistraro et al., 2016; Lozano et al., 2016). However, the contribution rate of automobile exhaust pollution 
to the air pollution index of large cities is more than 60%. Data show that 98%-99% NOx generated by 
automobile exhaust is directly discharged into the atmosphere by the exhaust pipe, and the total amount of 
NOx emissions within a year can reach 5.21×107t. This kind of NOx can lead to the formation of 
photochemical smog and acid rain and other hazards under certain conditions, resulting in serious damage on 
human health and ecosystems (Liu and Li, 2015). At present, governments often use strict control measures 
of vehicle exhaust emissions, vehicle management, traffic control, and fuel oil restrictions to curb the 
production of NO (Rehman, et al., 2016). However, due to the rapid and sustained growth of China's car 
ownership (nearly 264 million vehicles by the end of 2014), the management is not standardized, resulting in 
the concentration of NO into the urban atmosphere exceeded two. Therefore, the NO pollution caused by 
motor vehicle emission has become a thorny problem which has to be faced with. 
After entering in the 21st century, with the rapid development of automobile industry in our country, the energy 
consumption of vehicles is increasing day by day. Unfortunately, the vehicle fuel energy has not been fully 
utilized (Jeng, et al., 2016). Therefore, in order to effectively achieve the purpose of energy saving and 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1759007

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Yuelei Wang, 2017, Design and research of automobile exhaust heat energy conversion device, Chemical 
Engineering Transactions, 59, 37-42  DOI:10.3303/CET1759007   

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emission reduction, this paper intends to design a high thermoelectric conversion efficiency of gasoline engine 
exhaust gas temperature difference generator and exhaust gas purification muffler. The exhaust gas 
temperature difference power generation device is used to recover the exhaust gas of the automobile, and the 
automobile exhaust gas is preheated and replaced with a power supply, which is provided with a muffler with 
exhaust gas purifying function (Li, et al., 2016). 
In 1847, Tektronix developed the first thermoelectric generator, but its power generation efficiency was very 
low. In the following time, new explorations were made. However, with the passage of time, people gradually 
discovered the value of the recovery of automobile exhaust gas. The thermoelectric generation technology is 
applied to the automobile, and the exhaust gas is used as the heat source to generate electricity. The 
thermoelectric generator designed by Nissan Motor Company in Japan is installed in the middle of the 
gasoline engine exhaust pipe, and the exhaust gas temperature is up to 8000 °C. The design installs 72 
modules on the outer wall of the rectangular area on the inner channel (Fang, et al., 2016). A module consists 
of 8 pairs of thermocouples (Chen, et al., 2015). When the temperature difference reaches 563 K, the output 
power of a single module is 1.2 W. Under the test condition that the car climbs at 60 km/h speed, the exhaust 
gas temperature difference power generation device can convert 11% of the energy in the exhaust gas, and 
its power generation amount is 35.6 W. At home, Wuhan University has also studied the automobile exhaust 
temperature differential power generation. Their design is to exhaust the exhaust gas from the engine and 
convert it through a thermoelectric generator. The electric energy is used to charge the battery or to the 
electrical equipment through the battery management system. The thermoelectric generation section is fitted 
with the 41.44kw output power when the engine speed is 3000 r/min (Hendricks, et al., 2016; Zhou, 2015). 

2. Car muffler 
Muffler is a part of the automobile exhaust system, its main role is to effectively reduce engine exhaust noise, 
and ensure that the smooth exhaust and engine exhaust resistance is small. 

2.1 Classification of automobile silencer 

(1) Resistance muffler 
The basic principle of the resistance muffler: Noise absorbing material is combined with the muffler structure 
to absorb noise, which is usually good for medium and high frequency noise. Generally speaking, the effect of 
the muffler is related to the nature of the material used. If the density of the material is large, then the sound 
absorption effect is better in the initial stage. With the increase of the density of the material, the effect of the 
noise reduction will be decreased with the increase of the density of the material. Therefore, there is an 
optimum range for each kind of material. At present, the main sound absorption materials are glass fiber 
filament, low carbon steel wire mesh and wool chain (Schmidt, et al., 2016). 
(2) Reactive muffler 
The basic principle of the reactive muffler: The flow of air through a sudden enlargement or reduction of the 
cross section of the muffler will produce wave reflection and interference, which can reduce the energy of 
sound waves. In general, the effect of the muffler on the low and middle frequency noise is good, and the 
effect of the high frequency noise is poor (Wei, et al., 2017).  
(3) Impedance compound muffler 
The basic principle of the muffler is the superposition of the resistance and the resistance, so as to achieve 
the purpose of the low, medium and high frequency noise. On the basis of fully understanding for the noise, 
usually, the resistance muffler is designed at the inlet end of the airflow, and the resistive muffler is designed 
inside the muffler. 
(4) Resonator muffler 
The basic principle of resonance resistance muffler is to use resonance system to eliminate noise. The 
resonance system consists of a pipe with a certain number of small holes and a closed cavity outside the tube. 
In noise elimination, part of the sound wave is absorbed by the small hole in the pipe and is rubbed against 
the inner wall, so that the sound energy is converted into heat energy and consumed. The other part will be 
reflected back by the resonant cavity wall, so that the energy of the acoustic wave is attenuated to achieve the 
purpose of silencing. 

2.2 Evaluation index of muffler 

The evaluation of muffler is mainly from two aspects: acoustic performance and aerodynamic performance. 
The evaluation indexes of acoustic performance mainly include acoustic attenuation, radiation noise, insertion 
loss, tail pipe noise and transmission loss. The evaluation indexes of aerodynamic performance mainly include 
pressure loss and resistance coefficient. Acoustic performance evaluation index is to ensure that the muffler 
can have a sufficient frequency range and the amount of noise elimination (Tao, et al., 2015). However, the 

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evaluation index of aerodynamic performance is to ensure that the air flow resistance in the process of noise 
elimination should be small as soon as possible. The following is the main acoustic performance of two 
mufflers: 
Tailpipe noise: Muffler tail pipe noise is one of the key evaluation indexes to evaluate the performance of 
muffler. Its specific manifestation is the ripple noise, and it is composed of air noise and air friction noise. The 
proportion of air noise and air friction noise depends on the size and speed of airflow in the muffler. When the 
engine speed is low, the flow velocity of the muffler is small and the flow rate is low. At this time, the air noise 
accounts for main proportion. If the engine speed is high, then the flow rate is high and the flow rate is high. At 
this time, the friction noise accounts for main proportion. Therefore, we should always monitor the engine 
speed in the design of muffler as far as possible to reduce the friction noise muffler. 
Radiated noise: The radiated noise of the muffler is produced by the vibration of the muffler shell and other 
mechanical components. According to this definition, we can see that when the sound wave is excited, a 
forced bending vibration wave is generated along the shell and pipe. At a certain frequency, the wavelength of 
the bending wave is equal to that of the incident wave in the air. The two wave produces resonance, and the 
shell produces a coincidence effect. At this time, the radiation noise of the muffler reaches the maximum 
value, then the lowest frequency fc of the anastomosis effect can be expressed by the Eq (1): 

E

pc
f c

12

22

22

πt

c
=

B

M

π
=    (1) 

In the Eq (1), C is the sound velocity (m/s). M is the surface density of the shell or pipe (kg/m2). B is the 
acoustic stiffness. t is the thickness of the shell or pipe (m). p is the density (kg/m2). E is the elastic modulus 
(N/m2). 

3. Engine exhaust noise simulation based on GT-POWER 
3.1 Introduction to GT software 

GT-power is a simulation module in the GT-suite software package, which is mainly used for engine 
performance and noise simulation. At the same time, it can also complete the design of the engine electronic 
control function. GT-suite is also known as the virtual engine, which is widely used in the field of engine 
performance simulation. It includes GT-power, GT-drive, GT-cool, GT-crank, GT-vtrain. Each module is based 
on the same operating platform, and the data between modules are compatible with each other. 
The detailed model of all the critical conditions of the engine is included in the GT-power software. The 
software can simulate the actual operation of the engine in different conditions. In GT-power, there are many 
thermodynamic models that can accurately simulate the operation of an engine, such as chemical models, 
combustion heat transfer and fluid mechanics models. It is convenient to build engine models by using GT-
power. Because there are a lot of off the shelf modules for us in GT-power, such as turbocharger module units 
and intake and exhaust module units, these are very practical. For some complicated, compressible, nonlinear 
and unsteady one-dimensional flow models, GT-power is simulated according to the principle of conservation 
of mass, the principle of finite difference and the principle of conservation of energy. It guarantees the 
accuracy of the calculation. In addition, GT-power software can also carry out detailed and accurate simulation 
analysis of different combustion products. 

3.2 Calibration of engine model based on GT-POWER 

After the completion of the engine model, it is impossible to determine whether the software model can 
accurately reflect the real engine operation process, so it needs to be verified. By running the model, the 
power and torque curves of the model can be obtained. Then the load characteristics and speed 
characteristics of the engine are tested, and the experimental data are compared with the simulation results of 
the GT-POWER model. The torque error and power error are within 5%, which proves that the model is 
accurate. 

3.3 Analysis of engine exhaust noise based on GT-POWER 

As shown in figure 1, the engine exhaust spectrum and 4000r/min noise spectrum can be obtained according 
to the engine model. It can be seen that the noise value of the engine is between 3500 and 6000r/min when 
the engine exhaust spectrum is analyzed. The spectrum is about 124 dB, which is an important basis for the 
design of muffler. At the same time, it can be seen that the noise level of the engine in the 4000r/min band is 
very high in the 180-200HZ band. The exhaust noise of engine is very obvious, so the noise in the frequency 
range should be eliminated. 

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Figure 1: Exhaust spectrum and 4000r/min noise spectrum 

The hearing range of the human ear is between 20-20000Hz. In order to study the distribution of noise at 
different frequencies, the frequency range can be divided into many frequency intervals. It is called the 
frequency band, frequency range or frequency band. Each frequency range has an upper and lower limit, 
which is represented by fu and f1, respectively. 

12 ff
n

u =  (2) 

When n=1/3, the band is called the 1/3 octave. In practical engineering applications, the 1/3 octave is often 
used to study the energy distribution of noise. The center frequency of the 1/3 octave can be calculated by the 
Eq (3): 

1ff nc =f  (3) 

It can be seen that the peak noise of engine exhaust is in the range of 110-124dB. In n=4800r/min, 
n=5200r/min, the noise has 2 peaks, in addition, the other speed is basically unchanged for the frequency of 
the peak noise. The corresponding frequency of high noise value is between 180-200HZ. With the increase of 
the rotation speed, the noise value of the 200HZ frequency also increases. It can be concluded that the main 
muffler focus in the 180-200HZ frequency band. 

3.4 Analysis of stable power supply performance of automobile exhaust gas generator 

In order to ensure that the electric energy generated by the electric generator can supply electricity to the 
plasma of the muffler effectively, the voltage boost control test of the exhaust gas waste heat generator is 
carried out. In the experiment, the Single Ended Primary Inductor Converter (SEPIC) type DC voltage 
converter is designed, and the voltage of the generator is boosted and regulated to DC 12V to achieve the 
same phase boost and voltage. Figure 2 shows the test results of the power generation voltage rise of the 
exhaust gas generator. 
The test results show that the voltage of the 6 thermoelectric power generation components used in the test 
can be stabilized at about 14V. It is proved that the design of the generator can ensure the power 
consumption of the muffler. But there is a lack of stability of the voltage. The main reason is: Firstly, in the 
process of operation, the hydrocarbons and carbon particles in the exhaust gas are deposited on the surface 
of the tail gas tube, which hinders the heat transfer and reduces the performance of the generator. Secondly, 
thermoelectric material performance decreased with time, and the temperature of the thermoelectric material 
increased. It can cause the reduction of the cross-sectional area and the internal resistance of the 
thermocouple. Meanwhile, excessive temperature rise may also lead to failure of hot end materials. Finally, 
the thermal expansion rate of the contact interface may lead to insufficient interface contact, which can 
increase the contact resistance and influence the heat transfer function. The thermal contact resistance of 

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thermoelectric materials increases with the increase of operating time, which results in the degradation of 
thermoelectric properties. 

-5 0 5 10 15 20
0

2

4

6

8

10

0

2

4

6

8

10

12

14

16

18

20

O
ut

pu
t v

ol
ta

ge

In
pu

t v
ol

ta
ge

Time (m)

 Input voltage
 Output voltage

 

Figure 2: Pressure test results 

4. Conclusions 
The design principle of the impedance muffler and the technology of plasma cleaning are studied by adding a 
new vehicle purifier. Through the rational design, the automobile tail gas residual heat generator and muffler 
with purifying function are produced. Therefore, the utility model can not only recover the residual heat of 
automobile exhaust, but also purify the automobile emission. The exhaust noise of a small and medium sized 
vehicle engine was simulated by using the simulation technology and GT-POWER software. The simulation 
results show that the noise value of the engine is high between 3500 and 6000r/min. Peak noise is in the 110-
124dB interval, and the corresponding frequency of high noise values is in the 180-200HZ, which is increasing 
with speed changes. It can be concluded that the main muffler focus in the 180-200HZ frequency band. After 
optimization design, the exhaust gas temperature difference generator and the muffler with purification 
function were processed. The experimental results show that the exhaust gas temperature difference 
generator designed in this paper has higher power generation efficiency, and which can meet the demand of 
power supply for the tail gas purification. At the same time, the purification function of the muffler with 
purification function is mainly affected by the supply voltage and the engine speed. Through this experiment, 
we summarized some areas which need to be improved:  
(1) The temperature of the cold end of the generator needs the control device, and is suitable for the 
automobile exhaust temperature. The generator needs to keep the constant temperature difference as far as 
possible, so that the power supply is stable. 
(2) The heat transfer efficiency of the inner wall of the generator is low, so it is necessary to design the 
exhaust gas passage with higher heat transfer efficiency, so as to obtain a higher temperature difference and 
obtain a larger power generation efficiency. 
(3) Due to the addition of plasma generator in the muffler, the structure of the muffler becomes complicated, 
and the exhaust resistance is increased, thus reducing the power of the vehicle. 
(4) Due to the processing precision and the limit of the equipment, the structure and the design dimension of 
the purified muffler are different. Therefore, we hope to improve the processing technology in the future. 

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