- Email: [email protected]
Powertrain preheating system of tracked hybrid electric vehicle in cold weather
Applied Thermal Engineering 91 (2015) 252e258
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
Powertrain preheating system of tracked hybrid electric vehicle in cold weather Rui Wang a, *, Yichun Wang a, Chaoqing Feng a, b, Xilong Zhang a a b
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China College of Energy and Power Engineering, Inner Mongolia University of Technology, Hohhot 010051, China
h i g h l i g h t s A novel preheating method was proposed for heavy duty tracked HEV. Thermal energy in preheating system is produced by the PMSM in driving system. This method can achieve preheating target by its own components without any adding. Analyzing low temperature performance of power battery and select its capacity.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2015 Accepted 12 August 2015 Available online 22 August 2015
In order to make sure that the heavy duty tracked vehicle can work in various conditions, especially severe cold weather, preheating system of powertrain should be adopted, and a novel preheating system is presented for the tracked hybrid electric vehicle (HEV) in which heat is generated by the low-speed drive motor. The new preheating system can meet the need of cold start without adding any additional device. The characteristic of heat generation by motor is tested when the rotor of motor is rotated in very low speed. The heat loss from power cabin to external environment has been simulated, and the relevant test has been done to verify the simulation results. Combining the characteristic of heat generation and heat loss situation about preheating system, the heat transfer model of preheating system was implemented by MATLAB. The total energy required for preheating in different ambient temperature was calculated by this model. The results showed that: the minimum heating power was 70 kW and energy required was about 180 MJ when the HEV worked in 46 C. If lithium ferrous phosphate (LFP) battery was used in power system, the minimum battery capacity is about 290 A h. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Heavy duty tracked HEV Cold weather Preheating system Heat generated by PMSM Low temperature performance of battery
1. Introduction In order to make sure the heavy duty tracked vehicle can work in various conditions, especially severe cold weather, cold start problem in winter must be resolved in design stage of vehicle. Generally, the auxiliary heating method is used to solve the cold start problem in cold weather [1,2]. Under the condition of very low ambient temperature, if the vehicle is not started and parked out of door for a long time, the lubricating oil of vehicle would have higher viscosity and lower lubrication, and starting the engine in this condition would bring fatal damage to the engine and the power train would suffer serious attrition, which will have tremendously
* Corresponding author. E-mail addresses: [email protected], [email protected] (R. Wang). http://dx.doi.org/10.1016/j.applthermaleng.2015.08.027 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
impact on the performance of vehicle [3e6]. Based on the different structures and working environments of vehicle, the auxiliary heating method is widely used [7,8]. For the traditional heavy duty tracked vehicle, fuel fired heater is a universal method of auxiliary heating. The fuel is sprayed into boiler and fired to generate heat, and then with turning on the cycling pump, the heat is transferred to the engine and other transmission components by cooling liquid. When the engine and transmission component are heated to a proper temperature for engine to start and work normally, the fuel fired heater stops working. Compared to traditional propulsion systems, hybrid electric vehicle (HEV) has many advantages such as improved fuel economy, better acceleration performance, low acoustic signature and exportable electric power [9], which are very suitable for the tracked vehicle. For the tracked HEV, installing a traditional fuel fire heater can meet the need of cold start in winter, but this kind of
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
preheating system will occupy some space and make the crowded power cabin more crowded. Authors in this article propose a novel preheating method for heavy duty tracked HEV in which heat is produced by the permanent magnet synchronous motor (PMSM) in driving system. When the rotor of motor is rotated in very low speed, most of input electric energy is transformed into thermal energy [10]. Base on the cycle of lubricating and cooling system, the heat generated by motor was transferred to the whole power train and preheat every components. With this method, we can achieve the same purpose of preheating by using its own components without adding any additional device. Motor electric preheating technology can eliminate all parts of traditional fuel heating system, such as fuel fired heater, fuel supply tube, air inflow and exhaust gas outflow pipes. There are lots of merits about motor electric preheating method, such as non-complicated system, high heating power supply, easy control, resource reusing, space and money saving, and so on. In the preheating process of heavy duty tracked vehicle power train, lots of heat will be lost through the surface of power cabin. That heat loss process exists in several different heat transfer forms. The heat will be transferred from engine and gear box to power cabin surface by means of radiation, conduction on joints and small space nature convection, and then the heat transfers to environment by cabin surface. The way of heat transfer mentioned above can be neglected in vehicle cooling system design, but it is the main way for preheating system to lose energy in cold weather, so it is necessary to consider the lost energy accuracy when designing the preheating system. In this article, structural analysis of dual-mode electromechanical compound transmit system which is used in power train of heavy duty tracked HEV has been done, the cooling system of tracked HEV which matches well with transmit system is designed to realize the electric preheating by motor, only adding some valves and pipes. The characteristic of heat generation by motor is tested when the rotor of motor rotates in very low speed. In order to obtain the heat loss rate from power cabin to external environment, CFD has been used to simulate the heating process and the simulation result has been verified by experiment. Combining the heat generation and heat loss situation of preheating system with parameters of engine cabin components, the heat transfer model of preheating system implemented by MATLAB is used to calculate the total energy required in preheating process under different ambient temperature. At the same time, energy losses and heating efficiency can be obtained for different heating powers. Finally, the minimal battery capacity of preheating system can be determined. 2. Working principle of powertrain in heavy duty tracked HEV The dual-mode electromechanical compound transmit system is used in power train of heavy duty tracked HEV, Fig. 1 displays the structure of the transmit system. From Fig. 1, it can be seen that the dual-mode electromechanical compound transmit system is mainly composed by three planetary gears and two motors. Through different connection statuses of clutch CL1 and CL2, transmission can be switched between two modes respectively. Transmit mode one is a low speed mode in which CL2 is engaged and CL1 is released. In this case, the transmission ratio is determined by the speeds of motor A and motor B. In the condition of same engine speed, the speed of wheels can change with the speed of motors. The other transmit mode is a high speed mode in which CL1 is engaged and CL2 is released, and the working principle of this mode is same as that of mode one.
253
R1
Engine
CL0
R2 Motor A
C1
C2
S1
S2 P1
P2
CL1
R3
CL2
C3 S3 P3
Motor B
Wheel
P-planetary gear, S-sun gear, R- ring, C-carrier, CL-clutch,
Fig. 1. Structure of dual-mode transmit system.
3. Design of cooling system and preheating system Aiming at the heavy duty tracked HEV, the cooling system is designed with two coolant cycles and two independent cooling fans. One cycle is the high temperature cycle which is cooling the engine and charge air cooler. Another cycle is the low temperature which is cooling the motors, lubricating oil and motor controller. The cooling system schemes are designed and shown in Fig. 2. For the purpose of preheating by reusing cooling system components, some pipes and valves are added. The working states can be switched between cooling and preheating by controlling the status of valve 1 and valve 3. In the driving condition, cooling system is working; motor radiator is connected with cooling cycle. When the power system need preheating in cold weather, the preheating cycle is connected with low temperature cycle by changing the state of valve 1 and valve 3. The heat which is generated by motors and controller is transferred to the engine and lubricants heat exchanger, so the power train preheating can be realized. 4. Heat generation characteristic of motor Using permanent magnet synchronous motor (PMSM) in the driving system of heavy duty HEV is the trend in recent years, which has many merits such as high efficiency, high power density, better performance on speed control and so on [11]. On the running process of motor, the power losses include core loss, copper loss, mechanical loss and miscellaneous loss. Core loss is generated in permanent magnet rotor and copper loss is generated in stator coil. Those power losses are changed into thermal energy mostly. When the rotor of motor rotating in a low speed, core loss and copper loss are the main power loss, other losses are ignored in the motor heating generation process.
Fig. 2. Cooling cycle of motor heating preheating system.
254
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258 Table 1 Main parameters of motor. Name
Value
Maximum power [kW] Maximum speed [r/min] Rated power [kW] Rated speed [r/min] Maximum torque [N m]
550 6000 350 3200 1600
In order to study the core loss and copper loss in the rotor of motor which rotating in a low speed (about 5 r/min), an experiment system is designed and heat generation characteristic of motor has been tested. The two motors in transmit system have the same parameters, so motor A was selected in the experiment. The permanent magnet rotor of motor is made by samarium cobalt, and the main parameters are shown in Table 1. For PMSM, when the rotor of motor is rotating in a low speed, the input electric energy almost changed into thermal energy, and heat generation power in motor is changed with input voltage. The experiment has been done to obtain the heat generation condition of motor when the rotor is rotating in a low speed, and experimental result is shown in Fig. 3. The heat generation of controller was tested by the cooling water cycle. The copper loss can be calculated by electric current and resistance of coil. The total heating power of PMSM was equal to the heat obtained by cooling water, it can be tested by the mass flow rate and temperature different between inlet and out let of cooling water. The core loss is equal to total heating power minus copper loss if other loss of PMSM (mechanical loss etc) is neglected. Form the experimental results, it can be seen that quantity of heat generated by controller accounts for about 50% of all, and the remaining half of heat is generated by motor. About the heat generation in motor, core loss and copper loss account for about 55% and 45% respectively. Maximum total heating power can achieve about 8% of the motor rated power. 5. Heat loss in the preheating process
1. The air flow is uncompressible; 2. Surface temperature in each heat source was consumed with its average temperature; 3. The properties of material are isotropic. Power cabin and a part of atmosphere around cabin are selected as simulation domain and the gridding division has been done for simulation. After importing the mesh grid into FLUENT software, the boundary conditions are set as follows: inlet of air flow is in the front of vehicle and set as velocity inlet, and it depend on air flow rate in environment; outlet of air flow is behind the vehicle and set as outflow; bottom of simulation area is ground and set as wall; top of simulation area is atmosphere. Power cabin surface connected with atmosphere is set as couple, and surface between power cabin and human cabin install thermal insulation material, so it is set as thermal isolation wall. The surface of heat sources such as engine, gear box and radiator are set as constant temperature, and each heat source has the same temperature in its surface, and those temperatures are determined by the outflow temperature of cooling water. K-epsilon two equation model was selected as viscous model, and convergence criteria is that residuals of energy equation is 106, others are 104. 5.2. Verification of simulation by experiment
5.1. Numerical simulation There are mainly two ways for heat transfer between components and environment, convection and radiation. For the preheating process of tracked vehicle, heat sources in the power cabin transfer the heat to cabin surface and then the heat is released into
35
In order to make the temperature test more convenient, idling was selected as the working condition in experimental verification. Non-contact infrared thermometer was used in experiment, its measuring range was 22 to þ110 C, resolution ratio was 0.1 C and precision is ±1%. The temperature test point was located on power cabin surface shown in Fig. 5. The environment temperature was 7 C, and air flow rate in environment was 0.5 m/s. Before the experiment, the vehicle should keep working in idling status more
total heat generation controllor heat generation core heat generation copper heat generation
30
Heat generation power / kW
environment. The quantity of that heat will determine the heat loss ratio in preheating process. In order to describe the heat transfer between components and environment, CFD numerical simulation has been done for the heating process. Because the object of study is power cabin, the other parts of vehicle are unnecessary in numerical simulation. So the simplified geometric model of power cabin is built and it is shown in Fig. 4. The main heat sources in the power cabin include engine, gear box, radiator and kinds of pipes. The pipes are ignored in the geometric model because they are much smaller in size than the other heat sources. In order to simplify the simulation model, the following are the assumptions made in the physical model.
25 20 15 10 5 0 0
100
200
300
400
500
Voltage / V Fig. 3. Experiment results about heat generation power in different input voltage.
Fig. 4. Geometric model of power cabin.
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
255
Fig. 5. The temperature test point location on power cabin surface.
Fig. 7. The relationship between heat transfer power and temperature difference of heat source and environment.
Fig. 6. Temperature distribution in the surface of power cabin.
than one hour to make sure that the temperature distribution is stable. The value of each test point was recorded three times and the final result came from their average values. Air flow rate, environment temperature and heat source temperature in the experiment are set as boundary conditions in numerical model. Using the CFD model, the experimental working conditions were simulated and the temperature distribution result on the surface of power cabin is shown in Fig. 6. The test results and simulation results of each test point are displayed and compared in Table 2, so do the errors of each point. From the Table 2, it can be seen that the test results are higher than simulation results in every test point. It is because that the coolant pipes and exhaust pipe are neglected in simulation model, the total heat source of power cabin is less than real situation, it caused the simulation results are lower than test results. But the relative errors of every test point are smaller and less than 1%, it indicates that the simulation model can describe the heat loss situation accurately. Exceptional test points (point 2, point 4 and point 7) have bigger absolute errors. Combining with the location of each test point, the reasons are analyzed and shown as follows: test point 2 is located on the up surface of cabin which is far away from engine, gear box and radiator. For the real vehicle, the exhaust pipe is located behind test point 2, but in simulation model the exhaust pipe is neglected, so the test temperature is higher than simulation result at this point and absolute error reach to 2.5 K. Test point 4
located at flank of power cabin and outside of this point are track and wheel which will reduce the air speed in test point 4. In simulation model, track and wheel are deleted, so the air flow speed in simulation model is higher than real situation, the low air flow speed decreases the heat transfer rate and increases the temperature in real test. Test point 7 is located at the bottom of power cabin, heat sources, such as engine and gear box, are connected with bottom of power cabin, so the heat will transfer to the power cabin by heat conduction and it will increase the temperature, in simulation model the fixed connection are neglected, so the temperature at point 7 in simulation result is lower than test result. When the simulation model was verified by experiment, the heat transfer rate in the preheating process can be calculated by the simulation model. When the power train is preheating, the temperature of engine and gear box is rising gradually. Different temperatures of heat sources will lead to the power cabin have different heat transfer rates to environment. So we should calculate the heat transfer power that changing with the different temperatures of heat sources and environment. The surface temperature of engine and gear box, and other temperatures are set to 46 C (the environment temperature in most atrocious weather), the relationship between heat loss power and temperature difference of heat source and environment is retrieved and shown in Fig. 7. Because the heat transfer between power cabin and environment is mainly composed by convection and radiation, so the simulation data in Fig. 7 is fit for quartic polynomial. The fit result is shown as formula (1), fitting determination coefficient is R2 ¼ 0.9997.
Qe ¼ 0:00005Td4 0:0096Td3 þ 0:6988Td2 1:6471Td
(1)
where Td is temperature difference between heat source and environment, K; Qe is heat exchange power, W.
Table 2 Test and simulation results comparison of power cabin surface temperature. Test point
1
2
3
4
5
6
7
8
Test result/[K] Simulation result/[K] Absolute error/[K] Relative error %
286.1 285.5 0.6 0.2
283.8 281.3 2.5 0.9
282.3 281.6 0.7 0.2
287.8 285.9 1.9 0.7
288.6 288 0.6 0.2
286.8 286.3 0.5 0.2
288.2 286.5 1.7 0.6
282 281.4 0.6 0.2
256
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
Fig. 8. Simulation flowchart of preheating system.
6. Power component temperature rise in preheating process The heat generation and heat loss results of preheating system have been obtained by experiment and simulation. Combined with parameters of engine cabin components, the heat transfer model of preheating system is implemented by MATLAB. Simulation flowchart of preheating system is shown in Fig. 8. The total energy required for preheating in different ambient temperatures can be calculated by this model, the energy losses and heating efficiency can be obtained at different heating powers. In the MATLAB model, the weight, heat capacity and other parameters of components are input into model; weight and physical parameters of coolants and each kind of lubricant oils are set according to the cooling system design. In the calculation of heat exchange of preheating process, the relationship between heat loss power Qe and temperature difference Td of heat sources and environment which is shown in formula (1) should be imported into the model. Changing the environment temperature and heating power in simulation model, the preheating time and total energy consumption will be obtained in different working condition. Based on the power system design and requirement of engine start, the target temperatures of each component to preheat are shown in Table 3.
Table 3 Preheating target temperature of power system. Components
Target Components temperature/[ C]
Engine body 30 Lubricant of engine 35 Lubricant of gear box 35
Target temperature/[ C]
Coolant of engine 40 Gear box body 30 Lubricant of motor 60
Thermal energy transfer of low temperature preheating system mainly depends on coolant and lubricating cycle, so the temperature of coolant and lubricant oil should be higher than the components temperature. For preheating system, ambient temperature (46 C) is recognized as the worst working condition, so it is used as the initial temperature. With different heating powers, temperature rise characteristics of engine are simulated and shown in Fig. 9. From the Fig. 9, it can be seen that it takes shorter and shorter time for components in power cabin to reach to the target temperature with the heating power increasing. The temperature rising curves are nearly linear. Making 40 kW heating power as an example, the slope of temperature rising curve decreases with the heating time increase, but the change is very trivial. It indicates that heat loss power is very smaller than the heating power. When the component temperature increases and the difference of temperature with that of environments becomes larger, heat loss power will increase and it affects the heating process obviously, so in latter stage of heating process, the slope of temperature rising curve decreases quickly. Because the heat loss power is very small, the difference between slopes of temperature rising curves for heating power in 80 kW and 60 kW is not obvious. In order to analyze the working characteristic of preheating in different ambient temperatures, many simulations have been done. The results are shown from Figs. 10e13. It can be known from the Fig. 10, the heating time varies with the ambient temperature changes and its tendency is approximately linear. In the worst working condition (ambient temperature is 46 C), and in different heating powers, 80 kW, 60 kW and 40 kW, preheating times are 37 min, 51 min and 78 min respectively. We made lots of simulations about preheating time in different heating power. Based on those results, it can be found that: to make sure the heating time is not more than 45 min, the minimum heating power is 70 kW. The Fig. 11 shows the total energy consumption in different heating powers. The energy consumption increases with heating power decreases, but the difference is small. Higher heating power caused shorter heating time, so the energy consumption is less than lower heating powers. In the worst working condition, the total energy consumptions are 180 MJ, 182 MJ and 185.5 MJ respectively when the heating powers are 80 kW, 60 kW and 40 kW. Total energy loss condition is shown in Fig. 12 and the heating efficiencies in different heating powers are shown in Fig. 13. It can be seen that low ambient temperature will increase the heating time and the energy loss. The heating efficiencies are more than 93% in three different heating powers. The higher heating power is, and the lower heat loss will be. If the heat power is 80 kW, the heating efficiency of preheating system could reach to 96.5%. 7. Capacity matching about power battery Total energy consumption has been calculated by simulation in different ambient temperatures. And then, in the process of preheating system design, two important factors should be considered: the one is that power battery could provide sufficient energy in cold weather; the other is that power battery can discharge that energy in given power. Energy discharge characteristic of battery is influenced by temperature, so further analysis should be done about the battery in preheating system. The results of battery discharge performance can be used to calculate the capacity which is needed in preheating system. Rated voltage of power battery is 800 V in the heavy duty HEV, and based on the minimum heating power which can meet the need of preheating, thus the discharge current can be calculated. The low temperature performances of Lieion batteries were studied by
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
257
Fig. 9. Powertrain temperature rise characteristics with different preheating powers.
many researchers [12e14], from those study we can see that: when the environment temperature is below 40 C, and under the 1/3 C discharge current, the capacity of LiFePO4 battery only can reach 30% of the nominal capacity. Combined with the characteristic of lithium battery, parameters of battery are shown in Table 4. Lithium ferrous phosphate (LFP) battery and lithium manganate (LM) battery are wildly used as
power battery in HEV. So the parameters comparison between two kinds of battery is shown in the Table 4. From the Table 4, it can be seen that low temperature performance of LFP battery is slightly better than LM battery, so LFP battery is selected as the power battery of heavy duty HEV. In the worst working condition (ambient temperature is 46 C), the capacity of battery should reach 292 A h at least. In this case, the
Fig. 10. The heating time varies with the ambient temperature in different heating power.
Fig. 12. Total energy loss varies with the ambient temperature.
Fig. 11. Total energy consumption varies with the ambient temperature.
Fig. 13. Heating efficiency varies with the ambient temperature.
258
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
Table 4 Battery parameters that meet the needs of preheating. Battery parameters
Values
Minimum heating power/[kW] Discharge current/[A] Capacity of LFP battery/[A h] Capacity of LM battery/[A h] Total energy for preheating [MJ] Needed discharge rate
70 87.5 292 (841 MJ) 308 (887 MJ) 181 21.5% 20.4%
LFP battery LM battery
preheating system cannot meet the need of heating time until the battery discharge rate reaches to 21.5%. The evidence indicates that, in 46 C, LFP battery discharge rate can reach 21.5% [15]. Besides, internal resistance of the battery consumes power and generates thermal energy in the process of discharge, so the temperature of battery will be increased in preheating process, which is conducive to improve the performance of the battery.
heat is generated by the drive motor running in very low speed. The new preheating system can meet the need of cold start and without adding any additional devices. The total energy required for preheating in different ambient temperature was calculated by simulation model. Besides, energy loss and heating efficiency can be obtained at different heating power. The results show that: the minimum heating power which can meet the need of heating time is 70 kW and total energy consumption is about 180 MJ. Combine with the low temperature discharge characteristic of power battery, if LFP battery is used in power system, the required heating power and battery capacity were calculated in preheating process. The minimum battery capacity is about 290 A h when the ambient temperature is 46 C. The bad performance of battery in low temperature causes the excessive battery capacity supply. So some discussions have been taken about heat preservation method of battery. Increasing the temperature has positive influence on the battery performance and decreasing the battery capacity.
8. Discussions When the battery was selected to meet the need of vehicle, if ignore the energy consumption of preheating system, the 200 A h power battery can meet the need of power system designed for the heavy duty HEV mentioned in this article. In order to provide sufficient energy for preheating system, the capacity of battery should be enlarged to 292 A h at least. The battery capacity increases about 50% and the space occupied increases about 50% too. The bad performance of battery at low temperature causes the excessive battery capacity supply. So some heat preservation methods should be adopted to increase the temperature of battery in cold winter. The popular method in recent years is that: firstly, thermal insulation layer should be laid on the surface of battery cabin; secondly, the electric heating wire is installed around the battery, and it heats the battery by the energy of itself. Through the battery temperature control system, the self-heating device can keep the battery temperature not very low. If it can raise the battery temperature to 25 C when the ambient temperature is 46 C, the discharge rate could be increased from 25% to 55%. In that case, the capacity of power battery can be reduced about 50%. For the battery cabin which has good heat insulation method, comparing with the discharge rate reduction in low temperature, the energy consumption of self-heat conserving system is smaller. In order to solve the problem about bad discharge performance of battery in low temperature, some companies had developed a novel LFP battery which can keep discharge rate in 75% at 41 C. If this battery can be used in power system of HEV in future, it will decrease the capacity of battery which is selected to meet the need of preheating in cold weather. The preheating system which is designed in this article can achieve the preheating purpose by using its own components without adding any additional device. The cost of this novel preheating system only includes some pipes and valves, is about $200. The cost of traditional preheating system which used fuel fire heater is about $1000. 9. Conclusions For the tracked HEV, to solve the cold start problem in winter, a new preheating system of power train was designed that the
Acknowledgements This work is supported by the National Preparatory Research Project of China (No. 104010201).
References [1] J.E. Clark, C.A. Wu, D.B. Goldstein, The role of technology development in the global positioning system, in: IEEE International Frequency Control Symposium, 2002, pp. 488e492. [2] P. Efthymiou, M.H. Davy, C.P. Garner, et al., Insights into cold-start DISI combustion in an optical engine operating at 7 C, SAE Int. J. Engines 6 (2) (2013) 1059e1074. [3] M. Park, D. Jung, M. Kim, et al., Study on the improvement in continuously variable transmission efficiency with a thermal management system, Appl. Therm. Eng. 61 (2) (2013) 11e19. [4] J. Kim, K. Choi, C.L. Myung, et al., Experimental evaluation of engine control strategy on the time resolved THC and nano-particle emission characteristics of liquid phase LPG direct injection (LPG-DI) engine during the cold start, Fuel Process. Technol. 106 (2013) 166e173. , Cold start characteristics of an ethanol-fuelled [5] L.C. Monteiro Sales, J.R. Sodre engine with heated intake air and fuel, Appl. Therm. Eng. 40 (2012) 198e201. [6] R.H. Chen, L.B. Chiang, C.N. Chen, et al., Cold-start emissions of an SI engine using ethanol gasoline blended fuel, Appl. Therm. Eng. 31 (8e9) (2011) 1463e1467. [7] C. Ji, C. Liang, B. Gao, et al., The cold start performance of a spark-ignited dimethyl ether engine, Energy 50 (2013) 187e193. [8] Y. Cui, H. Peng, K. Deng, et al., The effects of unburned hydrocarbon recirculation on ignition and combustion during diesel engine cold starts, Energy 64 (2014) 323e329. [9] G. Burban, V. Ayel, A. Alexandre, et al., Experimental investigation of a pulsating heat pipe for hybrid vehicle applications, Appl. Therm. Eng. 50 (1) (2013) 94e103. [10] M.S. Choi, S. Um, Y.D. Chun, Electrothermal effects of advanced electromagnetic materials on electrical loss and thermal characteristics of synchronous rotational machines, J. Mech. Sci. Technol. 28 (8) (2014) 3335e3343. [11] S. Morimoto, S. Ooi, Y. Inoue, et al., Experimental evaluation of a rare-earthfree PMASynRM with ferrite magnets for automotive applications, IEEE Trans. Ind. Electron. 61 (10) (Oct.2014) 5749e5756. [12] S.S. Zhang, K. Xu, T.R. Jow, The low temperature performance of Lieion batteries, J. Power Sources 115 (1) (2003) 137e140. [13] L. Wang, X. Zhou, Y. Guo, Synthesis and performance of carbon-coated Li3V2(PO4)3 cathode materials by a low temperature solid-state reaction, J. Power Sources 195 (9) (2010) 2844e2850. [14] S. Wu, M.S. Chen, C.J. Chien, et al., Preparation and characterization of Ti4þ doped LiFePO4 cathode materials for lithiumeion batteries, J. Power Sources 189 (1) (2009) 440e444. [15] Li Zhe, Han Xuebing, Lu Languang, et al., Temperature characteristics of power LiFePO4 batteries, J. Mech. Eng. 47 (18) (2011) 115e120.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
Powertrain preheating system of tracked hybrid electric vehicle in cold weather Rui Wang a, *, Yichun Wang a, Chaoqing Feng a, b, Xilong Zhang a a b
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China College of Energy and Power Engineering, Inner Mongolia University of Technology, Hohhot 010051, China
h i g h l i g h t s A novel preheating method was proposed for heavy duty tracked HEV. Thermal energy in preheating system is produced by the PMSM in driving system. This method can achieve preheating target by its own components without any adding. Analyzing low temperature performance of power battery and select its capacity.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2015 Accepted 12 August 2015 Available online 22 August 2015
In order to make sure that the heavy duty tracked vehicle can work in various conditions, especially severe cold weather, preheating system of powertrain should be adopted, and a novel preheating system is presented for the tracked hybrid electric vehicle (HEV) in which heat is generated by the low-speed drive motor. The new preheating system can meet the need of cold start without adding any additional device. The characteristic of heat generation by motor is tested when the rotor of motor is rotated in very low speed. The heat loss from power cabin to external environment has been simulated, and the relevant test has been done to verify the simulation results. Combining the characteristic of heat generation and heat loss situation about preheating system, the heat transfer model of preheating system was implemented by MATLAB. The total energy required for preheating in different ambient temperature was calculated by this model. The results showed that: the minimum heating power was 70 kW and energy required was about 180 MJ when the HEV worked in 46 C. If lithium ferrous phosphate (LFP) battery was used in power system, the minimum battery capacity is about 290 A h. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Heavy duty tracked HEV Cold weather Preheating system Heat generated by PMSM Low temperature performance of battery
1. Introduction In order to make sure the heavy duty tracked vehicle can work in various conditions, especially severe cold weather, cold start problem in winter must be resolved in design stage of vehicle. Generally, the auxiliary heating method is used to solve the cold start problem in cold weather [1,2]. Under the condition of very low ambient temperature, if the vehicle is not started and parked out of door for a long time, the lubricating oil of vehicle would have higher viscosity and lower lubrication, and starting the engine in this condition would bring fatal damage to the engine and the power train would suffer serious attrition, which will have tremendously
* Corresponding author. E-mail addresses: [email protected], [email protected] (R. Wang). http://dx.doi.org/10.1016/j.applthermaleng.2015.08.027 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
impact on the performance of vehicle [3e6]. Based on the different structures and working environments of vehicle, the auxiliary heating method is widely used [7,8]. For the traditional heavy duty tracked vehicle, fuel fired heater is a universal method of auxiliary heating. The fuel is sprayed into boiler and fired to generate heat, and then with turning on the cycling pump, the heat is transferred to the engine and other transmission components by cooling liquid. When the engine and transmission component are heated to a proper temperature for engine to start and work normally, the fuel fired heater stops working. Compared to traditional propulsion systems, hybrid electric vehicle (HEV) has many advantages such as improved fuel economy, better acceleration performance, low acoustic signature and exportable electric power [9], which are very suitable for the tracked vehicle. For the tracked HEV, installing a traditional fuel fire heater can meet the need of cold start in winter, but this kind of
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
preheating system will occupy some space and make the crowded power cabin more crowded. Authors in this article propose a novel preheating method for heavy duty tracked HEV in which heat is produced by the permanent magnet synchronous motor (PMSM) in driving system. When the rotor of motor is rotated in very low speed, most of input electric energy is transformed into thermal energy [10]. Base on the cycle of lubricating and cooling system, the heat generated by motor was transferred to the whole power train and preheat every components. With this method, we can achieve the same purpose of preheating by using its own components without adding any additional device. Motor electric preheating technology can eliminate all parts of traditional fuel heating system, such as fuel fired heater, fuel supply tube, air inflow and exhaust gas outflow pipes. There are lots of merits about motor electric preheating method, such as non-complicated system, high heating power supply, easy control, resource reusing, space and money saving, and so on. In the preheating process of heavy duty tracked vehicle power train, lots of heat will be lost through the surface of power cabin. That heat loss process exists in several different heat transfer forms. The heat will be transferred from engine and gear box to power cabin surface by means of radiation, conduction on joints and small space nature convection, and then the heat transfers to environment by cabin surface. The way of heat transfer mentioned above can be neglected in vehicle cooling system design, but it is the main way for preheating system to lose energy in cold weather, so it is necessary to consider the lost energy accuracy when designing the preheating system. In this article, structural analysis of dual-mode electromechanical compound transmit system which is used in power train of heavy duty tracked HEV has been done, the cooling system of tracked HEV which matches well with transmit system is designed to realize the electric preheating by motor, only adding some valves and pipes. The characteristic of heat generation by motor is tested when the rotor of motor rotates in very low speed. In order to obtain the heat loss rate from power cabin to external environment, CFD has been used to simulate the heating process and the simulation result has been verified by experiment. Combining the heat generation and heat loss situation of preheating system with parameters of engine cabin components, the heat transfer model of preheating system implemented by MATLAB is used to calculate the total energy required in preheating process under different ambient temperature. At the same time, energy losses and heating efficiency can be obtained for different heating powers. Finally, the minimal battery capacity of preheating system can be determined. 2. Working principle of powertrain in heavy duty tracked HEV The dual-mode electromechanical compound transmit system is used in power train of heavy duty tracked HEV, Fig. 1 displays the structure of the transmit system. From Fig. 1, it can be seen that the dual-mode electromechanical compound transmit system is mainly composed by three planetary gears and two motors. Through different connection statuses of clutch CL1 and CL2, transmission can be switched between two modes respectively. Transmit mode one is a low speed mode in which CL2 is engaged and CL1 is released. In this case, the transmission ratio is determined by the speeds of motor A and motor B. In the condition of same engine speed, the speed of wheels can change with the speed of motors. The other transmit mode is a high speed mode in which CL1 is engaged and CL2 is released, and the working principle of this mode is same as that of mode one.
253
R1
Engine
CL0
R2 Motor A
C1
C2
S1
S2 P1
P2
CL1
R3
CL2
C3 S3 P3
Motor B
Wheel
P-planetary gear, S-sun gear, R- ring, C-carrier, CL-clutch,
Fig. 1. Structure of dual-mode transmit system.
3. Design of cooling system and preheating system Aiming at the heavy duty tracked HEV, the cooling system is designed with two coolant cycles and two independent cooling fans. One cycle is the high temperature cycle which is cooling the engine and charge air cooler. Another cycle is the low temperature which is cooling the motors, lubricating oil and motor controller. The cooling system schemes are designed and shown in Fig. 2. For the purpose of preheating by reusing cooling system components, some pipes and valves are added. The working states can be switched between cooling and preheating by controlling the status of valve 1 and valve 3. In the driving condition, cooling system is working; motor radiator is connected with cooling cycle. When the power system need preheating in cold weather, the preheating cycle is connected with low temperature cycle by changing the state of valve 1 and valve 3. The heat which is generated by motors and controller is transferred to the engine and lubricants heat exchanger, so the power train preheating can be realized. 4. Heat generation characteristic of motor Using permanent magnet synchronous motor (PMSM) in the driving system of heavy duty HEV is the trend in recent years, which has many merits such as high efficiency, high power density, better performance on speed control and so on [11]. On the running process of motor, the power losses include core loss, copper loss, mechanical loss and miscellaneous loss. Core loss is generated in permanent magnet rotor and copper loss is generated in stator coil. Those power losses are changed into thermal energy mostly. When the rotor of motor rotating in a low speed, core loss and copper loss are the main power loss, other losses are ignored in the motor heating generation process.
Fig. 2. Cooling cycle of motor heating preheating system.
254
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258 Table 1 Main parameters of motor. Name
Value
Maximum power [kW] Maximum speed [r/min] Rated power [kW] Rated speed [r/min] Maximum torque [N m]
550 6000 350 3200 1600
In order to study the core loss and copper loss in the rotor of motor which rotating in a low speed (about 5 r/min), an experiment system is designed and heat generation characteristic of motor has been tested. The two motors in transmit system have the same parameters, so motor A was selected in the experiment. The permanent magnet rotor of motor is made by samarium cobalt, and the main parameters are shown in Table 1. For PMSM, when the rotor of motor is rotating in a low speed, the input electric energy almost changed into thermal energy, and heat generation power in motor is changed with input voltage. The experiment has been done to obtain the heat generation condition of motor when the rotor is rotating in a low speed, and experimental result is shown in Fig. 3. The heat generation of controller was tested by the cooling water cycle. The copper loss can be calculated by electric current and resistance of coil. The total heating power of PMSM was equal to the heat obtained by cooling water, it can be tested by the mass flow rate and temperature different between inlet and out let of cooling water. The core loss is equal to total heating power minus copper loss if other loss of PMSM (mechanical loss etc) is neglected. Form the experimental results, it can be seen that quantity of heat generated by controller accounts for about 50% of all, and the remaining half of heat is generated by motor. About the heat generation in motor, core loss and copper loss account for about 55% and 45% respectively. Maximum total heating power can achieve about 8% of the motor rated power. 5. Heat loss in the preheating process
1. The air flow is uncompressible; 2. Surface temperature in each heat source was consumed with its average temperature; 3. The properties of material are isotropic. Power cabin and a part of atmosphere around cabin are selected as simulation domain and the gridding division has been done for simulation. After importing the mesh grid into FLUENT software, the boundary conditions are set as follows: inlet of air flow is in the front of vehicle and set as velocity inlet, and it depend on air flow rate in environment; outlet of air flow is behind the vehicle and set as outflow; bottom of simulation area is ground and set as wall; top of simulation area is atmosphere. Power cabin surface connected with atmosphere is set as couple, and surface between power cabin and human cabin install thermal insulation material, so it is set as thermal isolation wall. The surface of heat sources such as engine, gear box and radiator are set as constant temperature, and each heat source has the same temperature in its surface, and those temperatures are determined by the outflow temperature of cooling water. K-epsilon two equation model was selected as viscous model, and convergence criteria is that residuals of energy equation is 106, others are 104. 5.2. Verification of simulation by experiment
5.1. Numerical simulation There are mainly two ways for heat transfer between components and environment, convection and radiation. For the preheating process of tracked vehicle, heat sources in the power cabin transfer the heat to cabin surface and then the heat is released into
35
In order to make the temperature test more convenient, idling was selected as the working condition in experimental verification. Non-contact infrared thermometer was used in experiment, its measuring range was 22 to þ110 C, resolution ratio was 0.1 C and precision is ±1%. The temperature test point was located on power cabin surface shown in Fig. 5. The environment temperature was 7 C, and air flow rate in environment was 0.5 m/s. Before the experiment, the vehicle should keep working in idling status more
total heat generation controllor heat generation core heat generation copper heat generation
30
Heat generation power / kW
environment. The quantity of that heat will determine the heat loss ratio in preheating process. In order to describe the heat transfer between components and environment, CFD numerical simulation has been done for the heating process. Because the object of study is power cabin, the other parts of vehicle are unnecessary in numerical simulation. So the simplified geometric model of power cabin is built and it is shown in Fig. 4. The main heat sources in the power cabin include engine, gear box, radiator and kinds of pipes. The pipes are ignored in the geometric model because they are much smaller in size than the other heat sources. In order to simplify the simulation model, the following are the assumptions made in the physical model.
25 20 15 10 5 0 0
100
200
300
400
500
Voltage / V Fig. 3. Experiment results about heat generation power in different input voltage.
Fig. 4. Geometric model of power cabin.
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
255
Fig. 5. The temperature test point location on power cabin surface.
Fig. 7. The relationship between heat transfer power and temperature difference of heat source and environment.
Fig. 6. Temperature distribution in the surface of power cabin.
than one hour to make sure that the temperature distribution is stable. The value of each test point was recorded three times and the final result came from their average values. Air flow rate, environment temperature and heat source temperature in the experiment are set as boundary conditions in numerical model. Using the CFD model, the experimental working conditions were simulated and the temperature distribution result on the surface of power cabin is shown in Fig. 6. The test results and simulation results of each test point are displayed and compared in Table 2, so do the errors of each point. From the Table 2, it can be seen that the test results are higher than simulation results in every test point. It is because that the coolant pipes and exhaust pipe are neglected in simulation model, the total heat source of power cabin is less than real situation, it caused the simulation results are lower than test results. But the relative errors of every test point are smaller and less than 1%, it indicates that the simulation model can describe the heat loss situation accurately. Exceptional test points (point 2, point 4 and point 7) have bigger absolute errors. Combining with the location of each test point, the reasons are analyzed and shown as follows: test point 2 is located on the up surface of cabin which is far away from engine, gear box and radiator. For the real vehicle, the exhaust pipe is located behind test point 2, but in simulation model the exhaust pipe is neglected, so the test temperature is higher than simulation result at this point and absolute error reach to 2.5 K. Test point 4
located at flank of power cabin and outside of this point are track and wheel which will reduce the air speed in test point 4. In simulation model, track and wheel are deleted, so the air flow speed in simulation model is higher than real situation, the low air flow speed decreases the heat transfer rate and increases the temperature in real test. Test point 7 is located at the bottom of power cabin, heat sources, such as engine and gear box, are connected with bottom of power cabin, so the heat will transfer to the power cabin by heat conduction and it will increase the temperature, in simulation model the fixed connection are neglected, so the temperature at point 7 in simulation result is lower than test result. When the simulation model was verified by experiment, the heat transfer rate in the preheating process can be calculated by the simulation model. When the power train is preheating, the temperature of engine and gear box is rising gradually. Different temperatures of heat sources will lead to the power cabin have different heat transfer rates to environment. So we should calculate the heat transfer power that changing with the different temperatures of heat sources and environment. The surface temperature of engine and gear box, and other temperatures are set to 46 C (the environment temperature in most atrocious weather), the relationship between heat loss power and temperature difference of heat source and environment is retrieved and shown in Fig. 7. Because the heat transfer between power cabin and environment is mainly composed by convection and radiation, so the simulation data in Fig. 7 is fit for quartic polynomial. The fit result is shown as formula (1), fitting determination coefficient is R2 ¼ 0.9997.
Qe ¼ 0:00005Td4 0:0096Td3 þ 0:6988Td2 1:6471Td
(1)
where Td is temperature difference between heat source and environment, K; Qe is heat exchange power, W.
Table 2 Test and simulation results comparison of power cabin surface temperature. Test point
1
2
3
4
5
6
7
8
Test result/[K] Simulation result/[K] Absolute error/[K] Relative error %
286.1 285.5 0.6 0.2
283.8 281.3 2.5 0.9
282.3 281.6 0.7 0.2
287.8 285.9 1.9 0.7
288.6 288 0.6 0.2
286.8 286.3 0.5 0.2
288.2 286.5 1.7 0.6
282 281.4 0.6 0.2
256
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
Fig. 8. Simulation flowchart of preheating system.
6. Power component temperature rise in preheating process The heat generation and heat loss results of preheating system have been obtained by experiment and simulation. Combined with parameters of engine cabin components, the heat transfer model of preheating system is implemented by MATLAB. Simulation flowchart of preheating system is shown in Fig. 8. The total energy required for preheating in different ambient temperatures can be calculated by this model, the energy losses and heating efficiency can be obtained at different heating powers. In the MATLAB model, the weight, heat capacity and other parameters of components are input into model; weight and physical parameters of coolants and each kind of lubricant oils are set according to the cooling system design. In the calculation of heat exchange of preheating process, the relationship between heat loss power Qe and temperature difference Td of heat sources and environment which is shown in formula (1) should be imported into the model. Changing the environment temperature and heating power in simulation model, the preheating time and total energy consumption will be obtained in different working condition. Based on the power system design and requirement of engine start, the target temperatures of each component to preheat are shown in Table 3.
Table 3 Preheating target temperature of power system. Components
Target Components temperature/[ C]
Engine body 30 Lubricant of engine 35 Lubricant of gear box 35
Target temperature/[ C]
Coolant of engine 40 Gear box body 30 Lubricant of motor 60
Thermal energy transfer of low temperature preheating system mainly depends on coolant and lubricating cycle, so the temperature of coolant and lubricant oil should be higher than the components temperature. For preheating system, ambient temperature (46 C) is recognized as the worst working condition, so it is used as the initial temperature. With different heating powers, temperature rise characteristics of engine are simulated and shown in Fig. 9. From the Fig. 9, it can be seen that it takes shorter and shorter time for components in power cabin to reach to the target temperature with the heating power increasing. The temperature rising curves are nearly linear. Making 40 kW heating power as an example, the slope of temperature rising curve decreases with the heating time increase, but the change is very trivial. It indicates that heat loss power is very smaller than the heating power. When the component temperature increases and the difference of temperature with that of environments becomes larger, heat loss power will increase and it affects the heating process obviously, so in latter stage of heating process, the slope of temperature rising curve decreases quickly. Because the heat loss power is very small, the difference between slopes of temperature rising curves for heating power in 80 kW and 60 kW is not obvious. In order to analyze the working characteristic of preheating in different ambient temperatures, many simulations have been done. The results are shown from Figs. 10e13. It can be known from the Fig. 10, the heating time varies with the ambient temperature changes and its tendency is approximately linear. In the worst working condition (ambient temperature is 46 C), and in different heating powers, 80 kW, 60 kW and 40 kW, preheating times are 37 min, 51 min and 78 min respectively. We made lots of simulations about preheating time in different heating power. Based on those results, it can be found that: to make sure the heating time is not more than 45 min, the minimum heating power is 70 kW. The Fig. 11 shows the total energy consumption in different heating powers. The energy consumption increases with heating power decreases, but the difference is small. Higher heating power caused shorter heating time, so the energy consumption is less than lower heating powers. In the worst working condition, the total energy consumptions are 180 MJ, 182 MJ and 185.5 MJ respectively when the heating powers are 80 kW, 60 kW and 40 kW. Total energy loss condition is shown in Fig. 12 and the heating efficiencies in different heating powers are shown in Fig. 13. It can be seen that low ambient temperature will increase the heating time and the energy loss. The heating efficiencies are more than 93% in three different heating powers. The higher heating power is, and the lower heat loss will be. If the heat power is 80 kW, the heating efficiency of preheating system could reach to 96.5%. 7. Capacity matching about power battery Total energy consumption has been calculated by simulation in different ambient temperatures. And then, in the process of preheating system design, two important factors should be considered: the one is that power battery could provide sufficient energy in cold weather; the other is that power battery can discharge that energy in given power. Energy discharge characteristic of battery is influenced by temperature, so further analysis should be done about the battery in preheating system. The results of battery discharge performance can be used to calculate the capacity which is needed in preheating system. Rated voltage of power battery is 800 V in the heavy duty HEV, and based on the minimum heating power which can meet the need of preheating, thus the discharge current can be calculated. The low temperature performances of Lieion batteries were studied by
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
257
Fig. 9. Powertrain temperature rise characteristics with different preheating powers.
many researchers [12e14], from those study we can see that: when the environment temperature is below 40 C, and under the 1/3 C discharge current, the capacity of LiFePO4 battery only can reach 30% of the nominal capacity. Combined with the characteristic of lithium battery, parameters of battery are shown in Table 4. Lithium ferrous phosphate (LFP) battery and lithium manganate (LM) battery are wildly used as
power battery in HEV. So the parameters comparison between two kinds of battery is shown in the Table 4. From the Table 4, it can be seen that low temperature performance of LFP battery is slightly better than LM battery, so LFP battery is selected as the power battery of heavy duty HEV. In the worst working condition (ambient temperature is 46 C), the capacity of battery should reach 292 A h at least. In this case, the
Fig. 10. The heating time varies with the ambient temperature in different heating power.
Fig. 12. Total energy loss varies with the ambient temperature.
Fig. 11. Total energy consumption varies with the ambient temperature.
Fig. 13. Heating efficiency varies with the ambient temperature.
258
R. Wang et al. / Applied Thermal Engineering 91 (2015) 252e258
Table 4 Battery parameters that meet the needs of preheating. Battery parameters
Values
Minimum heating power/[kW] Discharge current/[A] Capacity of LFP battery/[A h] Capacity of LM battery/[A h] Total energy for preheating [MJ] Needed discharge rate
70 87.5 292 (841 MJ) 308 (887 MJ) 181 21.5% 20.4%
LFP battery LM battery
preheating system cannot meet the need of heating time until the battery discharge rate reaches to 21.5%. The evidence indicates that, in 46 C, LFP battery discharge rate can reach 21.5% [15]. Besides, internal resistance of the battery consumes power and generates thermal energy in the process of discharge, so the temperature of battery will be increased in preheating process, which is conducive to improve the performance of the battery.
heat is generated by the drive motor running in very low speed. The new preheating system can meet the need of cold start and without adding any additional devices. The total energy required for preheating in different ambient temperature was calculated by simulation model. Besides, energy loss and heating efficiency can be obtained at different heating power. The results show that: the minimum heating power which can meet the need of heating time is 70 kW and total energy consumption is about 180 MJ. Combine with the low temperature discharge characteristic of power battery, if LFP battery is used in power system, the required heating power and battery capacity were calculated in preheating process. The minimum battery capacity is about 290 A h when the ambient temperature is 46 C. The bad performance of battery in low temperature causes the excessive battery capacity supply. So some discussions have been taken about heat preservation method of battery. Increasing the temperature has positive influence on the battery performance and decreasing the battery capacity.
8. Discussions When the battery was selected to meet the need of vehicle, if ignore the energy consumption of preheating system, the 200 A h power battery can meet the need of power system designed for the heavy duty HEV mentioned in this article. In order to provide sufficient energy for preheating system, the capacity of battery should be enlarged to 292 A h at least. The battery capacity increases about 50% and the space occupied increases about 50% too. The bad performance of battery at low temperature causes the excessive battery capacity supply. So some heat preservation methods should be adopted to increase the temperature of battery in cold winter. The popular method in recent years is that: firstly, thermal insulation layer should be laid on the surface of battery cabin; secondly, the electric heating wire is installed around the battery, and it heats the battery by the energy of itself. Through the battery temperature control system, the self-heating device can keep the battery temperature not very low. If it can raise the battery temperature to 25 C when the ambient temperature is 46 C, the discharge rate could be increased from 25% to 55%. In that case, the capacity of power battery can be reduced about 50%. For the battery cabin which has good heat insulation method, comparing with the discharge rate reduction in low temperature, the energy consumption of self-heat conserving system is smaller. In order to solve the problem about bad discharge performance of battery in low temperature, some companies had developed a novel LFP battery which can keep discharge rate in 75% at 41 C. If this battery can be used in power system of HEV in future, it will decrease the capacity of battery which is selected to meet the need of preheating in cold weather. The preheating system which is designed in this article can achieve the preheating purpose by using its own components without adding any additional device. The cost of this novel preheating system only includes some pipes and valves, is about $200. The cost of traditional preheating system which used fuel fire heater is about $1000. 9. Conclusions For the tracked HEV, to solve the cold start problem in winter, a new preheating system of power train was designed that the
Acknowledgements This work is supported by the National Preparatory Research Project of China (No. 104010201).
References [1] J.E. Clark, C.A. Wu, D.B. Goldstein, The role of technology development in the global positioning system, in: IEEE International Frequency Control Symposium, 2002, pp. 488e492. [2] P. Efthymiou, M.H. Davy, C.P. Garner, et al., Insights into cold-start DISI combustion in an optical engine operating at 7 C, SAE Int. J. Engines 6 (2) (2013) 1059e1074. [3] M. Park, D. Jung, M. Kim, et al., Study on the improvement in continuously variable transmission efficiency with a thermal management system, Appl. Therm. Eng. 61 (2) (2013) 11e19. [4] J. Kim, K. Choi, C.L. Myung, et al., Experimental evaluation of engine control strategy on the time resolved THC and nano-particle emission characteristics of liquid phase LPG direct injection (LPG-DI) engine during the cold start, Fuel Process. Technol. 106 (2013) 166e173. , Cold start characteristics of an ethanol-fuelled [5] L.C. Monteiro Sales, J.R. Sodre engine with heated intake air and fuel, Appl. Therm. Eng. 40 (2012) 198e201. [6] R.H. Chen, L.B. Chiang, C.N. Chen, et al., Cold-start emissions of an SI engine using ethanol gasoline blended fuel, Appl. Therm. Eng. 31 (8e9) (2011) 1463e1467. [7] C. Ji, C. Liang, B. Gao, et al., The cold start performance of a spark-ignited dimethyl ether engine, Energy 50 (2013) 187e193. [8] Y. Cui, H. Peng, K. Deng, et al., The effects of unburned hydrocarbon recirculation on ignition and combustion during diesel engine cold starts, Energy 64 (2014) 323e329. [9] G. Burban, V. Ayel, A. Alexandre, et al., Experimental investigation of a pulsating heat pipe for hybrid vehicle applications, Appl. Therm. Eng. 50 (1) (2013) 94e103. [10] M.S. Choi, S. Um, Y.D. Chun, Electrothermal effects of advanced electromagnetic materials on electrical loss and thermal characteristics of synchronous rotational machines, J. Mech. Sci. Technol. 28 (8) (2014) 3335e3343. [11] S. Morimoto, S. Ooi, Y. Inoue, et al., Experimental evaluation of a rare-earthfree PMASynRM with ferrite magnets for automotive applications, IEEE Trans. Ind. Electron. 61 (10) (Oct.2014) 5749e5756. [12] S.S. Zhang, K. Xu, T.R. Jow, The low temperature performance of Lieion batteries, J. Power Sources 115 (1) (2003) 137e140. [13] L. Wang, X. Zhou, Y. Guo, Synthesis and performance of carbon-coated Li3V2(PO4)3 cathode materials by a low temperature solid-state reaction, J. Power Sources 195 (9) (2010) 2844e2850. [14] S. Wu, M.S. Chen, C.J. Chien, et al., Preparation and characterization of Ti4þ doped LiFePO4 cathode materials for lithiumeion batteries, J. Power Sources 189 (1) (2009) 440e444. [15] Li Zhe, Han Xuebing, Lu Languang, et al., Temperature characteristics of power LiFePO4 batteries, J. Mech. Eng. 47 (18) (2011) 115e120.