Transportation Research Part D 15 (2010) 160–167

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Transportation Research Part D

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Notes and comments

Cutting vehicle emissions with regenerative braking

Peter Clarke, Tariq Muneer, Kevin Cullinane *

Transport Research Institute, Edinburgh Napier University, Merchiston Campus, Edinburgh, EH10 5DT, UK

a r t i c l e i n f o

Keywords:Regenerative brakingHybrid driveUltracapacitorDriving cycleCarbon emissions

1361-9209/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.trd.2009.11.002

* Corresponding author.E-mail address: k.cullinane@napier.ac.uk (K. Cul

a b s t r a c t

This paper presents an analysis of vehicle regenerative braking systems as a quick and rel-atively easy means of achieving higher overall fuel efficiency and lowering carbon emis-sions. The system involves the installation of an additional electric motor/generator inparallel to the vehicle’s internal combustion engine and is used in conjunction with a DCDCconverter and ultracapacitor. The system is used to recapture the energy lost in vehiclebraking, significantly reducing a vehicle’s overall energy consumption and lowering vehicleemissions. Experimentally-based evidence is collected and compared for two sample vehi-cles to deduce the potential fuel and emissions saving.

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1. Introduction

To achieve meaningful reductions in future CO2 emissions, what is ultimately required is a change from the use of fossilfuels to alternative zero or low emission energy sources. As one of a raft of interim measures, however, regenerative brakingcan significantly reduce vehicle emissions and increase energy efficiency. Such a system permits a vehicle’s kinetic energy,which is conventionally dissipated by the brakes during braking mode, to be recaptured and stored for later use in acceler-ation mode and can be incorporated into both hybrid drive and electric vehicles.

Some manufacturers have already started to incorporate mild regenerative braking systems into their vehicles. Due tosize limitations on batteries, however, the available charge/discharge capabilities are limited, with large charge/dischargecycles undermining a battery’s operational lifespan and efficiency (BMW, 2009; Toyota, 2008).

Nickel-based batteries yield high energy density, but power density is relatively low. Lithium ion batteries give improvedperformance, but both types of battery lack the high power density required during acceleration and regenerative brakingmodes. Both ultracapacitors and flywheels have the capability of achieving this high power density. However, the ultraca-pacitor has been identified as the most suitable energy storage medium for use in regenerative braking for two reasons.Firstly, ultracapacitors have a much faster charger/discharge period than flywheels and secondly, ultracapacitors are cheaperfor energy storage requirements (McCluer and Christin, 2008).

2. Regenerative braking operation with ultracapacitor cycle

Regenerative braking with the use of ultracapacitors for energy storage could significantly reduce the fuel consumption ofconventional IC-engine vehicles and, most importantly, can be deployed in the immediate future. For electric vehicles oper-ating on batteries or for hydrogen fuel cell vehicles, the system operates in a similar manner but the vehicle’s IC engine isreplaced with a battery bank or fuel cell, and the ultracapacitor operates in parallel to these systems.

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linane).

Fig. 1. Regenerative braking systems operating in parallel with an IC engine.

Fig. 2. Regenerative braking systems for use on an electric vehicle.

P. Clarke et al. / Transportation Research Part D 15 (2010) 160–167 161

With all fuel systems, a DCDC converter and control system is required to control the state of charge of the ultracapacitor,since this is dependent on vehicle speed. For example, at low speeds the state of charge should remain high, enabling energyboosts for acceleration. At higher vehicle speeds the ultracapacitor should remain in a state of discharge, thus allowingregenerative energy to be stored when the vehicle slows. The general layout of a regenerative braking system for an IC engineis shown in Fig. 1, while that for an electric vehicle is shown in Fig. 2.

In addition, to optimise power conversion between the motor/generator and the ultracapacitor, a DCDC converter is re-quired to accommodate the variability of voltage of both the ultracapacitor and the motor/generator. A numerically con-trolled motor controller is used to control the motor power during acceleration, as well as controlling regenerativebraking power to ensure smooth braking.

3. Dynamic analyses of vehicle driving cycle

For the present study, energy consumption was simulated for two vehicles, both when driving the Edinburgh driving cy-cle (EDC) developed by Booth et al. (2001), and for the extra-urban driving cycle for the car. A Skoda Fabia estate diesel 1.9SDi was chosen to represent a typical small family car and an EVT 4000E electric scooter was chosen as a low emission alter-native. The Skoda Fabia when tested was found to emit 140 g/km of CO2 and consume 5.24 l/100 km during an extra-urbandriving cycle. A previous study undertaken on the EVT 4000E electric scooter driving the Edinburgh driving cycle found thatthe scooter consumed 0.056kWh/km with a monetary and environmental cost of £0.0056 and 33.3 g/km. The scooter wasfound to have advantages over the car within the inner city in terms of running costs, environmental impact and reducedcongestion (Muneer et al., 2009).

162 P. Clarke et al. / Transportation Research Part D 15 (2010) 160–167

The simulation software, developed to calculate power and energy requirements for a vehicle during driving, has alsobeen designed to calculate the energy savings obtained from the regenerative braking system when compared directly withthe efficiency of the same vehicle without the system. Simulations take detailed account of energy consumed during levelcruise, acceleration and gradient-climbing modes. For the purpose of auditing the latter driving mode, topography datawas obtained with the use of an onboard altimeter, which has been evaluated to have an accuracy of 97%. Regenerative brak-ing is incorporated into the model as detailed in Figs. 1 and 2, with the energy analysis herein largely based on the work ofRubin (2001).

3.1. Energy and power required for cruise mode

The energy needed by a vehicle travelling along a horizontal surface at constant speed is equal to the force of propulsion(F) multiplied by the distance (d) the vehicle travels:

E ¼ Fd ð1aÞ

Eq. (1b) gives the required power where v is the velocity:

P ¼ Fv ð1bÞ

The two opposing forces on a vehicle are drag and friction. Drag is the aerodynamic force, which resists the vehicle as it trav-els through air. It can be approximated as

Fdrag ¼12

CdAqv2 ð2Þ

where Cd is the drag coefficient, A is the frontal cross-sectional area of the vehicle and q the air density. The second opposingforce is due to rolling friction between the road surface and the vehicles tyres:

Ffriction ¼ lN ¼ lW ð3Þ

Note that W = mg. The total force required to propel a vehicle at constant velocity over a smooth horizontal surface istherefore:

F ¼ 12

CdAqv2 þ lW ð4Þ

Substituting the expression for F into Eqs. (1a) and (1b) respectively gives the relationships for energy and power:

E ¼ 12

CdAqv2 þ lW� �

d ð5aÞ

P ¼ 12

CdAqv2 þ lW� �

v ð5bÞ

3.2. Energy and power for travel up a gradient

When a vehicle is travelling up a gradient, a total of three forces are acting against the direction of motion – Fdrag, Ffriction

and W sin h, where h is the angle of inclination. Thus, the equations for E and P may be written as

E ¼ 12

CdAqv2 þ lW cos hþW sin h

� �d ð6aÞ

P ¼ 12

CdAqv2 þ lW cos hþW sin h

� �v ð6bÞ

3.3. Energy and power required for acceleration

The kinetic energy required to accelerate from an initial velocity of vi to a final velocity of vf is given by,

Eacceleration ¼12

mv2f �

12

mv2i ð7Þ

However, additional energy will be required to overcome road friction and aerodynamic drag. Bearing in mind that therequired energy needed for the latter component will change with changing velocity, the equations for E and P for a vehiclethat is accelerating from rest will be:

Table 1Comparative data for electric scooter and chosen automobile.

Model Scooter AutomobileMoped, EVT Skoda Fabia

Length, m 1.64 3.97Width, m 0.4 1.64Footprint area, m2 0.66 6.5Frontal area, m2 0.56 2.37Purchase cost, £ £1850 £12,000

Fig. 3. Increase in fuel efficiency of Skoda Fabia with varying size of motor/generator. Note: motor used with 50 Wh useful energy storage.

P. Clarke et al. / Transportation Research Part D 15 (2010) 160–167 163

Eacceleration ¼W2gf

v2f þ lWdþ 1

4CdAqv2

dd ð8aÞ

P ¼ E=t ð8bÞ

3.4. Instrumentation, data collection, computer modelling and validation

Table 1 provides details of the scooter and the car used in the trials. Instrumentation fitted on each vehicle records time,speed, acceleration, distance covered and altitude second by second via an onboard data logger. At the end of each test drive,the data collected was uploaded into the simulation model and processed to obtain key parameters such as chronologicalvariation of speed, acceleration, altitude and distance covered. Muneer et al. (2009) in a previous study found that the sim-ulation program returns the cumulative energy values with an accuracy of at least 96%.

The model has been further modified to also incorporate the effects of regenerative braking on the vehicle’s power andenergy consumption. In order to do so, it should be noted that the computed acceleration and hill climbing modes are impor-tant components of the energy estimation, as the main source of energy consumption occurs during these modes. For regen-erative energy estimation, energy recaptured during deceleration mode and hill descent is stored in the vehicle’sultracapacitor. Parameters such as energy storage capacity, maximum regenerative power adsorption and conversion effi-ciency are also taken into account. Stored energy is used to displace the energy requirements of the vehicle’s IC engine,or battery for electric vehicles, reducing overall energy consumption. The overall round trip efficiency for the regenerativebraking system is g 60%.

4. Results

For the purpose of this analysis it is assumed that the car is fitted with auto-stop/start technology, reducing fuel con-sumption to zero when the vehicle is stationary and considerably increasing fuel efficiency in high stop/start situations, suchas city driving. The electric scooter is seen as an alternative form of transport for city travel, suitable for short distances with-in city centres or for commuting to and from park and ride schemes. Thus, scooter trials were only conducted for city drivingmode.

Fig. 4. Increase in fuel efficiency of Skoda Fabia with varying size of ultracapacitor energy storage. Note: this is useful energy storage, not ultracapacitormaximum energy storage and a motor/generator of 12 kW peak has been selected as optimum.

Fig. 5. Averaged power outputs of Skoda Fabia diesel engine, driving at varying speeds with and without regenerative braking.

164 P. Clarke et al. / Transportation Research Part D 15 (2010) 160–167

4.1. Skoda Fabia

The motor/generator and ultracapacitor are required to be sized to provide optimum energy savings, yet only adding min-imum mass and cost to the vehicle’s construction. Fig. 3 shows the gains in fuel efficiency which are achievable with an in-crease in peak motor/generator output for the car. Note that a motor/generator with a peak output of 12 kW has been chosenas the optimum choice for the car for combined city and rural driving. It is found that significant energy savings can be madefor both city and rural driving modes, with 54% and 29% savings respectively.

Fig. 4 shows the increases in fuel efficiency which can be achieved with regenerative energy storage capability. The resultssuggest that, for the city mode of driving, energy consumption is greatly reduced as the regenerative energy storage capacityis increased to 15 Wh, but that there is no significant increase in efficiency if the storage capacity is increased above 15 Wh ofcapacity. However, for rural driving, there is a small increase in efficiency if the storage capacity is increased above 15 Wh.Thus, the optimum regenerative storage capacity is determined to be 15 Wh; increasing the storage capacity above this sig-nificantly increases both the mass and cost of the system.

It should be noted that in practice, when selecting a suitable ultracapacitor, other considerations such as rated power out-put and useable voltage range should also be considered. For example, the Maxwell BMOD165-E048 ultracapacitor selected

Fig. 6. Measured speed, altitude and consumed energy for the car driving within the city of Edinburgh.

Fig. 7. Measured speed, altitude and consumed energy for the car driving on rural roads, including A and B roads.

P. Clarke et al. / Transportation Research Part D 15 (2010) 160–167 165

for the car is rated at 165F and weighs 14.2 kg. Fig. 5 shows the averaged power output of the vehicle’s diesel engine duringcity and rural driving modes. At low speeds, note how both city and rural driving follow a similar trend. It can also be seenthat there are power peaks between the speeds of 50–60 kph and 80–90 kph. This is due to road speed restrictions where theengine power increases to accelerate between speed limits.

Fig. 6 shows a plot of measured speed, altitude and consumed energy for a car driving within the city of Edinburgh, whileFig. 7 shows the same plot for rural roads. From the two figures, the energy savings made with the use of regenerative brak-ing can clearly be seen.

4.2. Scooter

The EVT 4000E scooter does not have regenerative braking capabilities, but retro-fitting regenerative braking couldsignificantly increase its operational range, as well as prolonging its battery lifespan. This study shows simulated resultsfor the EVT 4000E electric scooter with a retro-fitted regenerative braking system, compared to those of the original scooterwith a standard braking system. From Fig. 8, it can be seen that the scooter’s range is increased with the increase inregenerative energy storage capacity. While adding only 1 kg of additional mass, the scooter’s driving range could beextended by 18.1% if a regenerative braking system with 1 Wh of energy storage is incorporated into the scooter’s existingelectric drive.

Fig. 8. Increase in fuel efficiency of EVT scooter with varying size of ultracapacitor energy storage. Note: this is useful energy storage, not ultracapacitormaximum energy storage. Also, the motor/generator is rated at 1.5 kW as supplied by manufacturer.

Table 2Comparative vehicle driving cycle statistics.

Vehicle Modeof driving

Skoda Fabia EVT 4000E scooter

Rural driving City driving City driving

Frequency,% Nonregenerativebrakingaverage ICengine power,kW

Regenerativebrakingaverage ICenginepower, kW

Frequency,% Nonregenerativebrakingaverage ICengine power,kW

Regenerativebrakingaverage ICenginepower, kW

Frequency,% Nonregenerativebrakingaverageelectric motorpower, W

Regenerativebrakingaverageelectric motorpower, W

Acceleration 28.3 24.7 15.8 21.8 19.5 6.3 11.6 2.4 1.6Deceleration 28.2 1.4 0.0 25.2 0.0 0.0 15.6 0.4 0.4Stationary 3.9 0.0 0.0 8.0 0.0 0.0 60.5 0.0 0.0Constant

speed39.5 11.5 10.0 45.0 2.9 2.9 12.4 1.2 1.2

Increase inefficiency,%

29.4 51.7 18.1

166 P. Clarke et al. / Transportation Research Part D 15 (2010) 160–167

5. Conclusions

Table 2 shows the regenerative braking statistics for both vehicles. In terms of environmental impact, the Skoda Fabiaemissions are reduced from emitting 140 g/km of CO2 to 108.4 g/km of CO2 and fuel consumption is also reduced to4.05 l/100 km when operating in extra-urban driving mode. However, the improvement in energy efficiency is achievedin urban driving mode where CO2 emissions are reduced from 144.8 g/km to 67 g/km.

The results also show that energy consumption was significantly reduced by adding regenerative braking to the test vehi-cles. The largest reduction is achieved within city driving, where most of the energy is consumed while accelerating the vehi-cles up to speed, but can then be recaptured during deceleration. In this context, the automobile’s savings are much greaterthan that of scooters because the latter has greater aerodynamic drag relative to its inertia, resulting in smaller energy sav-ings. In rural driving mode, energy consumption is reduced, but to a lesser extent. This is due to the vehicle consuming mostenergy at constant speed which cannot be recaptured by the braking system.

In terms of cost, a prototype system for the automobile costs £3000 per car with no additional running costs ormaintenance required within the vehicle’s life. It is estimated that the cost of a mass-produced system would be reducedby 50%. The additional cost of adding in the system would be recouped within the operational life of the vehicle, with re-duced fuel consumption and a lower road fund tax. During the vehicle’s lifespan, ceteris paribus, a vehicle fitted with a regen-erative braking system would save six tonnes of CO2 in comparison with a standard vehicle. For a similar system installed inan electric or fuel cell vehicle, the additional cost of the system would be counterbalanced by the reduced size of battery orfuel cell required to power the vehicle. For the EVT 4000E electric scooter driving the Edinburgh driving cycle, the energyconsumed would reduce from 0.056 kWh/km to 0.039 kWh/km, with a monetary and environmental cost of £0.0039 and23.6 g/km.

P. Clarke et al. / Transportation Research Part D 15 (2010) 160–167 167

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