A simulation-based study on regenerative braking for a hydro-pneumatic electric hybrid vehicle

Ulises Diego-Ayala, Keith Pullen, Sejul Shah Mechanical Engineering Department

Imperial College London, UK

u.diego@imperial.ac.uk, k.pullen@imperial.ac.uk, s.shah@imperial.ac.uk

Ricardo Chicurel, German Carmona, Alejandro Gonzalez Instituto de Ingenieria

Universidad Nacional Autonoma de Mexico Mexico, D.F.

rcu@pumas.iingen.unam.mx, pcg@pumas.iingen.unam.mx alex_ozomatl@yahoo.com

Abstract— This paper describes the derivation of a model for the hydro-pneumatic electric hybrid bus VEUNAM, a hybrid passenger vehicle built by the Universidad Nacional Autonoma de Mexico (UNAM). ADVISOR was used to investigate the potential benefits of storing regenerative energy during braking by writing components for the hydro-pneumatic system and adapting an existing ADVISOR electrical vehicle model. Simulations of the hybrid bus following congested routes from Mexico City were conducted.

Keywords: regenerative storage braking energy, hydraulic; hydro-pneumatic; hybrid vehicle; braking; hybrid bus; simulation

I.

II.

INTRODUCTION The growth of vehicle use worldwide has encouraged interest into the development of more efficient powertrains, with significant focus into hybrid vehicles and novel powertrains. Automakers such as Toyota, Honda and Nissan have developed medium-sized commercial hybrid electric vehicles that dramatically reduce fuel consumption and emissions which have no significant drawbacks apart from a higher capital cost. These vehicles achieve this by optimising the component power usage and utilising regenerative braking[1-4], which decreases the energy required to operate the vehicle and is particularly suitable for congested cities. Regenerative braking requires a energy store for capture of the energy until it is required again. Batteries have been the preferred option to date due to their production availability and established capabilities; however, energy storage devices like ultracapacitors, flywheels and hydro-pneumatic systems are being investigated[5-9]. For applications such as passenger buses which involve frequent stops, hydro-pneumatic storage is an option due to space availability [10,11]. Hydro-pneumatic storage converts kinetic energy from the vehicle during braking into potential energy by compressing a reservoir of gas. The vehicle is accelerated by expanding the gas. A hybrid bus (VEUNAM) built by the Universidad Nacional Autonoma de Mexico (UNAM) is described. Unlike most hybrid vehicles, which use conventional combustion engines for the primary energy source and electrical for the secondary, this bus uses an electrical system as primary power source and a hydro-pneumatic system for secondary storage.

Simulations were carried out using ADVISOR, a program written by the National Renewable Energy Laboratory (NREL) [12-15], to investigate the performance of the VEUNAM. In the following sections the regenerative energy hydro-pneumatic system developed for a medium sized electric passenger bus and the ADVISOR model are explained.

THE HYDRO-PNEUMATIC ELECTRIC HYBRID BUS VEUNAM

UNAM developed and built a hydro-pneumatic regenerative energy system for implementation in an electric passenger bus, called VEUNAM (Figure 1). The use of the hydro-pneumatic storage was particularly suitable due to the large space available in the bus and the highly congested driving routes found in Mexico City.

Figure 1. VEUNAM hybrid electric vehicle

The VEUNAM bus is a medium size passenger bus using 52 1180 Wh lead-acid batteries, a 22 kW AC motor and a hydro-pneumatic system. This system utilises compressed nitrogen to assist the motor during acceleration or store energy during braking, and combines regenerative with dissipative braking [10]. It operates based on the performance of a fixed displacement hydraulic pump/motor mechanically connected to the final drive shaft. Depending on the power demanded and the amount of energy contained in the tank, a three-way valve is activated to direct oil towards the tank to store energy or backwards to assist in the acceleration of the bus, as can be seen in the control system diagram shown in Figure 2.

Sponsors: Consejo Nacional de Ciencia y Tecnologia (CONACyT) through the project 27520-A and the loan-scholarship reg. 72126

Figure 2. Control system diagram

The command to activate the three-way valve is determined by the force applied by the driver to the brake and acceleration pedals and the state of charge of the reservoir. A combined brake system senses the force applied to the brake pedal, which is related to the amount of energy required from the driver to stop the bus. If this force is relatively higher than an opposite force from the hydraulic system, then the pump is activated and energy is stored in the reservoir. If the hydro-pneumatic system is not able to store all the energy available then the difference is dissipated as heat by a conventional brake. Preliminary tests conducted on the VEUNAM bus show that it is possible to store regenerative energy from a passenger bus using a hydro-pneumatic system and subsequently use it to accelerate the vehicle[12, 13]. Figure 3 presents the results of a preliminary braking test conducted with the VEUNAM bus. It can be seen that when the brakes are applied and vehicle speed decreases, the pressure in the reservoir increases, thus storing energy in the hydro-pneumatic system[13].

Figure 3. Braking test.

III. THE VEUNAM MODEL IN ADVISOR SIMULINK blocks were written and implemented in ADVISOR, shown grey shadowed in Figure 4, to simulate the hydro-pneumatic electric hybrid VEUNAM bus based on a existing ADVISOR electric vehicle model.

Figure 4. The VEUNAM model in SIMULINK.

Following ADVISOR’s structure, the simulation begins with the basic equation of solid-body motion at the vehicle block. Here ADVISOR calculates the linear force required on the vehicle’s chassis, which will be later converted into torque by means of the wheels, in order to overcome the specific forces that typically act on vehicles [14].

( 1 )

)sin(

21 φmg+ma+AvρC+mgC=F 2

Du

The operation of the wheels is simulated at the wheel and axle block to calculate the power required at the final drive axle to speed up or slow down the VEUNAM bus. The torque and speed of this power demand is analysed at the power distribution block and at the hydro-pneumatic control block.

The hydro-pneumatic control block was designed to determine whether the system is used to store energy or accelerate the vehicle. The torque from the wheels’ axle, the torque required from the pump/motor to operate (hydraulic torque Thyd), reservoir conditions, and whether the vehicle is braking or accelerating determines its operation. During braking, if the torque in the wheels is higher than the torque required from the pump/motor, and the pressure in the tank is below the operational limit, the fixed displacement pump/motor operates as a pump, compressing the nitrogen in the tank and storing energy. When the vehicle is accelerating, if the torque in the wheels is higher than the hydraulic torque, and the volume in the tank is below the limit, the fixed displacement pump/motor operates as a motor, expanding the nitrogen in the tank. When these conditions are not fulfilled, the system does not operate. If the hydro-pneumatic control is active, the torque used to operate the pump/motor of the VEUNAM is provided by the the final drive. The pump/motor produces the volumetric displacement that occurs in the reservoir (see Equations ( 8 ) and ( 9 )).

The hydro-pneumatic control block can be seen in Figure 5.

Analogously, the equation for the torque to operate as a motor during acceleration is

Figure 5. hydro-pneumatic control block.

The energy from the volumetric displacement in the reservoir equals

( 2 )

and, in a discrete time

. ( 3 )

The average pressure for the discrete time was calculated from

( 4 )

with the pressure P2, resulting from the volumetric change during the time step, following a polytropic behaviour

. ( 5 )

The volumetric displacement produced by the pump is calculated from

( 6 )

the volumetric displacement was positive when nitrogen was expanded and negative when it was compressed. Should the system not operate, the volume in the tank remains the same as in the previous time step.

It is considered that the power from the pump affects the change in pressure in the reservoir, which can be expressed as

( 7 )

⋅

The addition of a pump/wheels gear relation yields

( 8 )

( 9 ) ∆tw

η∆VP=T

pump

mpumphyd ⋅

⋅⋅⋅18.56

where

. ( 10 )

vpump

pump

πη∆twX

=∆V21019 6−

Figure 6 depicts the block where the estimation of the hydraulic torque takes place.

dVP=E ∫

∆VP=∆E

)P+(P=P 2121

Figure 6. hydraulic torque estimation block. n)∆V+V

V(=PP

1

1

2

1

Figure 7 shows the VEUNAM bus operation following the control strategy explained above.

π∆tηwX

=∆V vpumppump

21019 6−

mpumppumppump η∆tw

∆VP=T⋅

⋅

Figure 7. Hydro-pneumatic system operation. The pressure, volume and temperature in the tank is calculated in the reservoir block (see Figure 8) from the volume of oil withdrawn or pumped into the reservoir and the volume, pressure and temperature in the previous time step.

mpumppumphyd η∆tw

∆VP=T⋅⋅⋅⋅18.56

Figure 8. reservoir block.

Subsequent to a volumetric change in the tank, the reservoir interchanges heat until the temperature in the gas equalises to that of the surroundings, resulting in a change in the pressure after the volumetric change. shows an example of this condition after a compression of nitrogen in the tank.

Figure 9. Pressure during braking test simulation in the reservoir

A. Polytropic behaviour, B. Heat transfer with surroundings Once the pressure, volume and temperature in the tank are calculated, the hydraulic torque is determined. This value is used in the power distribution block (see Figure 10), to calculate the amount of torque to be requested to the electric batteries of the bus, which is the difference between the torque required by the final drive axle and the torque available from the hydro-pneumatic system.

Figure 10. power distribution block.

Thereafter, during the VEUNAM bus simulation,

ADVISOR calculates the power requested to the batteries in the same way as any electric vehicle modelling.

SIMULATION PARAMETERS IV. ADVISOR 2002 was used to simulate the VEUNAM bus being driven in a typical Mexican drive cycle. This drive cycle was obtained by driving an electric Volkswagen sedan, converted by the Instituto de Ingenieria UNAM, in a round trip from Av. Universidad to Unidad Aragón at peak hours to obtain a drive cycle in Mexico City [15], whose main characteristics are presented in Figure 11 and TABLE 1.

Figure 11. Typical Mexican drive cycle [15].

TABLE 1. MEXICO CITY DRIVE CYCLE.

Parameter Value Distance (km) 18.29

Duration (seconds) [hr] 3924 [1.09] Maximum speed (km/h) 61.27 Average speed (km/h) 16.77

Maximum acceleration (m/sec2) 1.98 Maximum deceleration (m/sec2) -4.15

The vehicle modelled in ADVISOR was set according to the main physical characteristics of the VEUNAM bus, as detailed in TABLE 2

TABLE 2. MAIN PARAMETERS FOR VEUNAM BUS.

Description Value Vehicle Weight (kg) 5236 Frontal area (m2) 7.24 Friction coefficient (--) 0.0149 Aerodynamic coefficient (--) 0.79

TABLE 3 present the main parameters for the reservoir model and its control strategy.

TABLE 3. MAIN PARAMETERS FOR RESERVOIR.

Description Value Maximum volume (m3) .069 Minimum volume (m3) .052 Maximum pressure (MPa) 19 Mechanical efficiency pump (--) .95 Volumetric efficiency pump (--) .98

V. RESULTS AND DISCUSSION An ADVISOR model for the hydro-pneumatic electric hybrid vehicle VEUNAM was developed to simulate typical highly congested Mexico City routes. During the simulation, the hybrid power train followed a pre-determined control strategy, storing regenerative energy in the hydro-pneumatic system during braking and releasing it during acceleration to provide pneumatic assistance for the electric motor. It was found that the hybrid system operated consistently during the drive cycle and that its operation produced a valuable improvement in the energy efficiency of the system. The amount of power required by the VEUNAM during the drive cycle (see Figure 12) was link to several factors, including speed demand, physical characteristics of vehicle and performance of components and power train.

Figure 12. Instantaneous speed and power achieved from vehicle. The hydro-pneumatic system was activated during various sections of the cycle, following the control strategy explained previously. Figure 13 shows an example of the dynamic changes in pressure in the reservoir as the VEUNAM bus was driven and the system operated to compress gas in the reservoir and store regenerative energy or expand the nitrogen and accelerate the vehicle.

Figure 13. Pressure in reservoir.

When the hydro-pneumatic system assisted the electric motor, energy from the reservoir was released to the final drive shaft; and during regenerative operation, part of the available energy from the shaft was stored in the reservoir, as can be seen in Figure 14.

Figure 14. Energy consumed by vehicle and provided or stored by hydro-

pneumatic system. The hydro-pneumatic system operated intermittently along the drive cycle when the vehicle was in motion. Of the total duration of the drive cycle, 3924 seconds, the VEUNAM bus was in motion for 2572 s: 648 s braking and 1924 s accelerating. As illustrated in Figure 15, of the period the vehicle was moving, braking regenerative storage operation occurred for 313 s, compared with 335 s of conventional braking. These results indicate that the conditions for the system to store energy were fulfilled nearly half the time the VEUNAM was braking, which shows good performance in terms of operational frequency utilizing a constant displacement pump. However, during acceleration, the system operated during 432 s of the acceleration period, only 22.4% of the acceleration time. This could be attributed to the considerably higher frequency of acceleration demand and hydro-pneumatic storage capabilities, which did not permit the recharging of the hydro-pneumatic system before being used again.

Figure 15. Frequency of operation for hydro-pneumatic system.

It was found that frequency of operation was related to both the energy consumed from the vehicle and energy requested at any particular moment. The former refers to the energy that was actually provided or stored by the powertrain elements in the system, and the latter to the energy that the vehicle demanded to achieve the requested drive cycle. Energy

requested to slow down the vehicle was normally satisfied by the activation of the brakes, but for a positive demand to accelerate the vehicle, results suggest that the energy requested was not be attainable due to electric and hydro-pneumatic power limitations. An example can be seen around the 1330 seconds mark in Figure 16, where the power requested by the wheels was not fulfilled because the electric motor was not able to provide it. The hydro-pneumatic system did not operate here because it had already released its energy at the beginning of this acceleration segment.An increase in the electric motor power rating could allow the VEUNAM bus to fulfil the demand during the entire drive cycle.

Figure 16. Power requested from wheels, and power provided from electric

and hydro-pneumatic system. Figure 16 also shows various segments with instantaneous positive power demand from the wheels supplied whether by the hydro-pneumatic and electric system working in parallel or solely by electric components. It also displays negative power demand segments when energy was either stored in the reservoir or dissipated as heat by the brakes. During the simulation, braking energy demand was always fulfilled, either by the brakes or in parallel with the hydro-pneumatic system.

Figure 17 displays the distribution of energy consumed during braking and acceleration. Inspection of these figures indicates that the hydro-pneumatic system was able to store up to 3.60 MJ (41%) of the total braking energy, with 3.67 MJ dissipated as heat even though the regenerative system was operating. This substantial amount of energy had to be dissipated because the system was not able to compress more nitrogen in the reservoir during that particular period. Although higher values of instantaneous energy storage could be obtained with higher pressure in the reservoir or higher flow rate from the pump, this might also reduce the operative frequency because of the higher pump torque required to operate the system. With regard to energy consumed by the final drive shaft during acceleration, it was found that when both systems operated in parallel, 2.95 MJ were provided by electric means and 3.07 MJ by hydraulic. This represents practically half the energy required for each system during this period. The contribution of the hydro-pneumatic system thereby enhances the energy efficiency of the system, reducing the electrical

energy consumption at the wheels from 33.55 MJ to 30.6 MJ(8.79%).

Energy consumed during braking

Energy consumed during acceleration

Figure 17. Energy consumed during braking and acceleration. Energy optimisation achievable from a regenerative system depends not only on storage capability, but also on its energy availability: the more energy the system is able to store and the more energy the system has available to be regenerated, the more efficient the system is. In the case of a typical drive cycle, as evident from Figure 18, energy is most frequently required to accelerate the vehicle than to stop it.

Figure 18. Energy consumption.

A1. - Consumed by wheels; A2. - Provided by hydro-pneumatic B1. - Regenerative available at wheels; B2.- Stored by hydro-pneumatic

A simulation of the vehicle being driven only-electric during the same drive cycle was also conducted. Inspection of Figure 19 indicates that power demanded from the batteries was

frequently higher with the VEUNAM bus running only-electric. As expected, when the hybrid mode is in operation, lower energy is demanded of the batteries, which is related to the lower current peaks obtained in our simulation that is thought to prolong the life of the batteries.

Figure 19. Electric power from batteries.

The energy provided by the batteries for these two types of operation is shown in Figure 20. The total electric energy provided with the VEUNAM bus driven only-electric is 36.82 MJ, 8.96% higher than when the vehicle operates with the hydro-pneumatic system. This saving derives from the re-utilization of regenerative energy and a consequent reduction on electric demand during the VEUNAM bus operation.

Figure 20. Electric energy provided for drive cycle.

It was found that the hydro-pneumatic system used in the VEUNAM bus could save up to 41% of the regenerative energy available from the vehicle in a typical Mexican drive cycle, and assist with 10% of the energy required during acceleration, increasing the overall efficiency of the vehicle by 8.96% when compared with an only-electric vehicle operation.

A simulation with the VEUNAM bus weighing 7236 kg was also conducted to study regenerative capabilities with additional weight. The results indicated that the hydro-pneumatic system stored 10% more regenerative energy (3.99 MJ in total) despite having up to 11.26 MJ available to store (28.24% higher than with 5236 kg), as shown in Figure 21. The higher torque available in the shaft, due to the additional weight, caused the hydraulic system to store energy more frequently (371 seconds in operation, 18.53% more time), thus

increasing the total energy stored; however, the regenerative storage was nevertheless limited.

[2] M. Duoba, H. Ng, and R. Larsen, "In-situ mapping and analysis of the Toyota Prius HEV engine, SAE technical paper number 2000-01-3096," presented at SAE Future Transportation Technology Conference and Exposition, Costa Mesa, California, 2000.

[3] K. Fukuo, A. Fujimura, M. Saito, K. Tsunoda, and S. Takiguchi, "Development of the ultra-low-fuel-consumption hybrid car - INSIGHT," JSAE Review, vol. 22, pp. 95-103, 2001.

[4] I. Matsuo, T. Miyamoto, and H. Maeda, "The Nissan hybrid vehicle, SAE technical paper number 2000-01-1568," presented at 2000 Future Car Congress, Arlington, Virginia, 2000.

[5] S. Shah, U. Diego-Ayala, and K. R. Pullen, "Mechanical Energy Storage Hybrid Vehicle – Simulation and Life Cycle Costs," presented at European ELE- Drive Transportation Conference and Exhibition, Estoril, Portugal, 2004.

[6] J. Van Mierlo, P. Van den Bossche, and G. Maggetto, "Models of energy sources for EV and HEV: fuel cells, batteries, ultracapacitors, flywheels and engine-generators," Journal of Power Sources, vol. 128, pp. 76-89, 2004.

Figure 21. Energy consumption with 7236 kg vehicle weigth. A1.- Consumed by wheels; A2.- Provided by hydro-pneumatic

[7] P. F. Ribeiro, B. K. Johnson, M. L. Crow, A. Arsoy, and Y. L. Liu, "Energy storage systems for advanced power applications," Proceedings of the IEEE, vol. 89, pp. 1744-1756, 2001.

B1.- Regenerative available at wheels; B2.- Stored by hydro-pneumatic

VI. CONCLUSIONS AND FUTURE WORK [8] A. F. A. Serrarens, S. Shen, and F. E. Veldpaus, "Control of a Flywheel

Assisted Driveline With Continuously Variable Transmission," Journal of Dynamic Systems Measurement and Control-Transactions of the Asme, vol. 125, pp. 455-461, 2003.

An ADVISOR model of the VEUNAM bus, a hybrid vehicle built by the UNAM, was written in SIMULINK. A simulation-based study was conducted with the VEUNAM bus being driven in a typically highly congested Mexico City drive cycle using hybrid and electric-only operation. Energy consumption and overall performance were compared.

[9] A. Pourmovahed, "Vehicle Propulsion Systems with Hydraulic Energy-Storage - a Literature Survey," International Journal of Vehicle Design, vol. 12, pp. 378-403, 1991.

[10] R. Chicurel, "A compromise solution for energy recovery in vehicle braking," Energy, vol. 24, pp. 1029-1034, 1999.

Results from the simulation showed that the hydro-pneumatic electric hybrid powertrain of the VEUNAM bus operated consistently whilst being driven in a Mexico City route.

[11] B. Wu, C.-C. Lin, Z. Filipi, H. Peng, and D. Assanis, "Optimization of Power Management Strategies for a Hydraulic Hybrid Medium Truck," presented at 2002 Advanced Vehicle Control Conference,, Hiroshima, Japan,, 2002.

The use of a constant displacement pump was examined as an alternative to the more expensive variable displacement pump. It was found that energy was repetitively stored in the hydraulic system and used during accelerations to assist the electric motor, thereby enhancing the overall efficiency of the vehicle.

[12] A. A. Pesaran, "Battery thermal models for hybrid vehicle simulations," Journal of Power Sources, vol. 110, pp. 377-382, 2002.

[13] T. Markel, A. Brooker, I. Hendricks, V. Johnson, K. Kelly, B. Kramer, M. O'Keefe, S. Sprik, and K. Wipke, "ADVISOR: a systems analysis tool for advanced vehicle modeling," Journal of Power Sources, vol. 110, pp. 255-266, 2002.

It was found that the operation of the hydro-pneumatic system was governed by the torque availability from the wheels, the size of pump/motor and the reservoir conditions. This means that particular attention should be paid to the control strategy and the operational settings for the reservoir.

[14] V. H. Johnson, "Battery performance models in ADVISOR," Journal of Power Sources, vol. In press, pp. 1-9, 2002.

[15] [15] S. R. Bhatikar, R. L. Mahajan, K. Wipke, and V. Johnson, "An artificial neural network module for simulation of the energy storage system of a hybrid electric vehicle," Proceedings of the Institution of Mechanical Engineers Part C- Journal of Mechanical Engineering Science, vol. 215, pp. 919-932, 2001.

Ongoing studies are being conducted on the performance of the VEUNAM under different driving scenarios to determine an optimised control strategy suitable for the operational conditions of the hydro-pneumatic system, with further experimental and theoretical analysis planned.

[16] R. Chicurel, G. Carmona, and A. Gonzalez, "Hydro-pneumatic system for regeneration of energy during braking (in Spanish)," presented at VII Congress SOMIM, Celaya, Mexico, 2001.

[17] A. Gonzalez, R. Chicurel, and G. Carmona, "Performance of the hydro-pneumatic braking system of the electric vehicle UNAM (in Spanish)," presented at VIII Congress SOMIM, Monterrey, N.L., Mexico, 2002. REFERENCES

[18] NREL, "ADVISOR main webpage." [1] K. Kenneth and R. Arun, "Benchmarking of OEM Hybrid Electric

Vehicles at NREL," Report of National Renewable Energy Laboratory 2001.

[19] A. Flores, "Study on dynamic performance of lead acid bateries for electric vehicles (in Spanish)," in Instituto de Ingenieria. Mexico: Universidad Autonoma de Mexico (UNAM), 2000.