Design and Analysis of Hybrid Power Systems with Variable Inertia Flywheel

The purpose of this study is to analyze and to design for the hybrid power systems with variable inertia flywheel while the hybrid power system consists of 1 power plant, 1 variable inertia flywheel which is used as the energy storage device, a planetary gear set, and a set of actuators. First, the kinematic and kinetic equations of the hybrid power system are developed in order to establish the relationships of the speed and torque of all elements, and the specific speed and torque of the output needed for the vehicle can be found by using the software “ADVISOR” in various operation modes. Then, with the prescribed mechanical brake energy recovery system model and a control model, a comprehensive analysis can be achieved. Finally, various driving modes, such as ECE, ECE+EUDC and New York Bus driving mode are investigated in order to demonstrate the characteristics of the hybrid power system. The numerical results show and conclude the effectiveness of the variable inertia flywheel, and the improvement on the efficiency of hybrid power systems. Copyright Form of EVS25.


Introduction
The researches on hybrid electric vehicles (HEV) are becoming more important in recent years, because the advantages of HEV are the significant reduction in fuel consumption and low emissions.Today, the cost of state-of-the-art batteries suitable for vehicular application remains high, and the use of a motorgenerator and battery to transmit and retrieve the mechanical energy needs complicate systems for energy conversions [1].In the search for a simple and practical hybrid system, the usage of a flywheel to retrieve energy in a hybrid system could be a possible design concept.
The use of flywheels in hybrid vehicles has been proposed by many researchers [1][2][3][4][5][6].Flybrid Systems Company, in 2009, proposed a hybrid flywheel system, connected with drive shaft on the power train by continuous variable transmission, and then connected with flywheel by a clutch [2].Diego-Ayala used only a planetary gear set to connect the output with the flywheel [1] and R.M. van Drute used a planetary gear set and continuous variable transmission to connect with a flywheel [3][4][5][6].In all these concepts, the inertia of the flywheel is constant.It is possible that if the variable inertia flywheel is used, the energy storage and retrieve might become more effective and efficient.
The purpose of this study is to investigate the effectiveness and improvement of using a variable inertia flywheel in the hybrid power system for either electric vehicles or traditional internal combustion engine vehicles.

Hybrid Power Systems
The motor and transmission are the general vehicle components.In the hybrid power system, a variable moment of inertia flywheel, a planetary gear set and a set of actuator are added on to the system, as shown in Figure 1, [1].The angular velocity and torque of the system can be expressed as the following equations: where N 31 is -N 3 / N 1 and N 3 is gear number of sun gear, and N 1 is gear number of ring gear.In the equations, there are three variables, either for the angular velocities, or for the torques.Which means two variables should be assigned, or so called, controlled, and the system is said to be a two degree-of-freedom system.
Figure 1: The Hybrid Power System Structure In the system, there are actuators used to make the ring gear of planetary gear decelerate, so that the brake kinetic energy will be transferred to flywheel.When flywheel speed is high enough to drive the vehicle, the actuators will make the ring gear of planetary gear decelerate, and the flywheel kinetic energy will be

Equations of Hybrid Power Systems
The performance of the Hybrid Pow transferred to the wheel to drive the vehicle.The operating logic is shown as Figure 2.
er System can be the lo calculated by using equations ( 1) and (2).However, sses at the gearbox should also be included by accounting for its efficiency ηGB.Depending on the direction of the energy flow, the torque loss T fw_loss at the sun and the torque loss T Ring_loss at the ring can be determined as the following.( 1) The losses of torque in the transferring are: ( ) 2. If T Out < 0 and ω Out >(1be decelerated by the flyw A)ω fw , the vehicle will heel.The torque of the flywheel and the torque of the ring gear can be expressed as: ( 1) The losses of the torques in the transferring are: ( ) ) For a given torque T fw acting o change of the angular velocity of t determined by the rotational equa is: n the flywheel, the he flywheel can be tion of motion, which ( ) fw where△ω fw , I fw , and T fw_loss are the change in speed, the inertia, and the torque flyw respectively.The kinetic energy of the flywheel and the a (13) The operating lo combined with "ADVIS simulation software [ be losses of the heel ring gear stored or released re: gic for the hybrid power system OR" which is numerical 7].They can be arranged to come a modular numerical simulation mode.The "ADVISOR" give the required torque and necessary angular velocity to the operating logic for the hybrid power system, and the operating logic for the hybrid power system will then determine the motor to be On or Off.The process is shown as in Figure 3.

Variable Inertia Flywheel Structure
In t an be various with respect to the speed of the flywheel [8].
will ch his study, the inerti of the flywheel c a The changing of the position of the lumped-mass ange the inertia of the flywheel as Figure 4.
ass of variable inertia flywheel is express the following equation: The m ed by After calculation, the stored energy of a variable inertia flywheel is show as Figure 5. n In Figure 5, the X-axis is the speed of the flywheel th energy could be restored.As shown, the relationship of th , e Y-axis is the inertia, and Z-axis shows how much e inertia and the changing of the inertia of the flywheel with the overall efficiency is implicit, and need some detailed numerical simulations to evaluate all design parameters and their interaction.Now if projection of Figure 5 is on X-Y flat and with a different function, which the inertia is varied with the angular velocity, is selected, the way of changing inertia of the flywheel is shown in Figure 6.Appendix.The motor of 137(W) is shown as Figure 7.The X-axis is torque, the Y-axis is revolutions per minute (rpm), and Z-axis is the efficiency in transferring energy.Whereas the flywheel's losses map, including on and wind resistance x Suzuki [9].These values change dynamically as the simulation runs, whose main data is shown as Figure 8.
To assess the hybrid vehicle under city driving conditions, three widely accepted driving cycles were selected: the ECE cycle [10], the ECE+EUDC cycle[10 d New York bus cycle [11], whose main data are shown in Table 1 and the driving cycles are shown as Figure 9~11.
The EI index is showing the improvement of t compared hybrid vehicle.Its physical meaning is that ho to r he red lin low down.When the sp celerate and the ring gear begins slo e is the driving mode whose unit is km/hr, and the angular speed of the planetary gear set with the reduction ratio is the black line.The green line is the angular velocity of the flywheel.The blue line is the angular velocity of the ring gear.
At 49.2 sec, the bus shall start to accelerate, and the flywheel and the ring gear will s eed of flywheel and the speed of bus with the reduction ratio get very close, the actuator will then stop action, and the bus will be driven by original power source (here it is the motor) and so the flywheel is free spinning now.
When 85.2 sec, the bus shall decelerate, and the flywheel begins to ac wly down.When the speed of flywheel and the speed of bus with the reduction ratio get very close, during the deceleration, again the actuator would stop action.Than the bus will be decelerated by its original brake system and flywheel will be free spinning again.For the hybrid power systems with fixed inerti ywh eed, which is range of operation of the flywheel while driving the vehicle, will clearly affect the SOC remained .To compare the results of all variations, we have the simulation of running the ECE cycle for 5850 sec, and results are shown in Figure 13.The X-axis is the operation speed of the flywheel, the Y-axis is the reduction ratio of the planetary gear set, and the Z-axis is SOC remained .We find the reduction ratio of the planetary gear set, 0.89 and operation speed, 11 rad/sec will give the most SOC remained .These parameters will be set up for electric bus of the hybrid power systems with fixed inertia flywheel for further discussion.
Use the same way to search the better parameters for the hybrid power systems with variable inert he w farther, in percentage, the tested hybrid vehicle can go while using the same SOC as the no-recharged EV.
Here we set up A 15,433 kg electric bus of the hybrid power systems with fixed inertia flywheel, and simulate run the ECE cycle for one time.We set the fixed inertia flywheel of I=24.1.For the planetary gear set, A=0.89, N 31 = -N 3 /N 1 = -0.1236=-11/89, gear number of sun gear is 11, gear number of ring gear is 89 and gear number of planeta y gear is 39.
We focus on 45sec to 100 sec for the acceleration and the deceleration, as shown in Figure 12.T We find the set of parameters gives the most SOC remained is the inertia of the flywheel using the function I(ω)= ω 1/6 , as shown in Table 2.The increased EI is mai ause e na the New York bus cycle.In the New York bus cycle, th and retrieve en nly bec of th ture of ere are accelerations and decelerations 11 times in 10 minutes, and during 90% of the cycle time, the vehicle speed is lower than 20 km/h, and about 67% of the cycle time the bus is stalled.Since the electric machine of EV will operate in low efficiency when the vehicle is at low speed and demands high torque, the electric machine used in EV bus for braking energy recovery will not be very suitable.It only has 3% improvement, even less that in the case of ECE cycle.
For the New York bus cycle, when using the variable inertia flywheel (I = a + bω 1/6 ) to store own as Figure 14.The green line is the hybrid power systems with variable inertia flywheel, whose SOC remained is the greater than the other hybrid vehicles.The blue line is the hybrid power systems with fixed inertia flywheel, whose SOC remained is also greater than the EV with regenerative brake, which is the red line.Now we will simulate the hybrid power systems with a variable inertia flywheel to run the ECE cycle fo ergy, the large amount of energy input and output can be coped with the vehicle speed, and provide better arrangement of the energy.While in low speed, the small inertia flywheel can be accelerated quickly to the operation speed and spinning into a large inertia flywheel, which is more suitable for kinetic energy 50 sec, ECE+EUDC for 3675 sec and the New York bus cycle for 100 minutes and compare the results, as shown in Table 3.
There is an EI improvement, upto 33.7% for the EV with the variable i 1/6 World Electric Vehicle Journal Vol. 4 -ISSN 2032-6653 -© 2010 WEVA storage and release.However, for the flywheel which is fixed inertia, the flywheel with large inertia will not speed up quick enough, and with small inertia just cannot store enough energy to drive the bus.Various types of hybrid power systems are investigated and co evaluate the effectiveness of their energy recovery performance in different driving patterns.2.
On the energy recovery systems, the electric bus using variable inertia flywheel in the Ne bus Cycle simulation can improve up to 33.8% efficiency better than the EV bus without energy regeneration, and also better than other hybrid power system evaluated in this study.

3.
The hybrid power system with variable inertia flywheel is satiable for a heavy-vehi stop-and-go with severe acceleration-anddeceleration operation.With the aid of the variable inertia flywheel, the motor, or the power plant, can be operated more efficiently and energy recovered is also increased with the help of the variable inertia flywheel if the range of operation, size, and the adjustment of the inertia of the flywheel have been selected and designed properly.

Figure 2 :
Figure 2: Operating logic for the System Figure 3: modular numerical simulation mode

Figure 4 :
Figure 4: Variable Inertia Flywheel structure To adjust or to change the inertia of the flywheel, c n be in m ect the ability of the flywheel in storing or releasing kinetic en a any different ways.It will certainly aff ergy.In this case, the inertia of variable inertia flywheel is expressed by the following equation:

Figure 5 :
Figure 5: variable inertia flywheel of stored energy map

1 A
Figure 6: Different function of the flywheel on the energy map

Figure 7 :
Figure 7: Motor characteristic Figure 8: Energy losses at the flywheel

Figure 12 :
Figure 12: ECE cycle for each component in 45~100 sec a fl eel, adjusting the reduction ratio and the operation sp

Figure 13 :
Figure 13: Optimization of parameters map for fixed inertia flywheel Figure 14: SOC remained of vehicles using NewYork bus cycle How Co. Ltd. to iated, and the special th U., Martinez-Gonzalez, P., McGlashan, nical hybrid vehicle: an investigation of a [2] osium From our simulati the HEV with varia provement, EI index can be from 16.1% upto 33.8%, especially in case of New York bus cycle.Here are some summarized remarks.1.

Table 1 :
Data of driving cycle

Table 3 :
Simulation results