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The Aerodynamics of Future Electric and Hydrogen Fuel-Cell Cars Essay

This document will investigates the influence of global warming and peak oil including the consequences that affect the vehicle industry. Much research and development have been done in recent years to either “electrified” or to implement fuel cell onto normal road vehicles. The use of fuel cell on a road vehicle might be the solution but it is considered too implausible to mass-produced in today’s technology (Suplee, 2009). Electric vehicle, on the other hand, are more practical and easy to develop.

This paper will focus on the technical review of a purely electric car. The performance, styling, comfort and cooling of an Electric Vehicle (EV) is different from most Internal Combustion Engine (ICE) vehicles as they are more energy efficient and the way electric vehicle gets its energy are dissimilar to ICE vehicle. In addition, the mass use of such vehicle is relatively new and is not as matured as ICE vehicle. The study of aerodynamics and aero-acoustic of an electric car is important as both factors can improve the car’s range and usability.

Various methods of reducing the overall drag of the car and the potential wind noise that would affect the occupants inside the vehicle are analysed in this paper, as well as ways to optimise the usage of the battery will be examined. INTRODUCTION The effect of global warming and peak oil has prompted car manufacturers to look for an alternative for vehicle propulsion. Alternative propulsion such as bio-fuel, petrol-electric hybrids and turbo-charged diesel have been proposed and manufactured in recent years.

All these substitutes can reduce carbon emission to a certain extent but these are not long term solutions for solving global warming. A new concept of vehicle propulsion is therefore required to solve the carbon build up in the atmosphere. Electric vehicle is the only answer to this problem for the majority. Better engine design can improve the overall efficiency of the vehicle, however, other aspect such as aerodynamics cannot be ignore as reduction of drag can dramatically reduce the power required to propel the vehicle.

Much effort has been done in streamlining the front of electric vehicle but majority of the drag induced on cars are pressure drag. A good rear vehicle design is needed to reduce the overall drag of the vehicle as well as to prevent unwanted lift produced by downwash effect. According to Ahmed (1984), the drag coefficient of a vehicle reaches a maximum with a 30o rear slant angle. Steps have to be taken to ensure a gentle real slant angle is achieved or an implementation of a device to recover the pressure on the roof surface.

Besides refining the back of the vehicle, the aerodynamic of an electric car can further improve by optimising the underfloor. In order to convince the majority of the society that electric vehicle should be the correct step towards clean energy, creative measures has to be done in order to optimise the usage of electricity, such as battery swap by Better Place and integrating car body with battery by Volvo. EVs run quieter than ICE counterpart due to the fact that they do not have pistons, crankshaft and combustion happening within the vehicle.

This however, has its drawback as the lack of noise covering from the vibration of the engine has place more emphasis in sealing doors and windows to prevent disturbing wind noise from entering the cabin. Special design has to be placed on EVs to ensure it produces sufficient noise to warn nearby pedestrians and yet the sound will not enter the cabin. EFFECT OF GLOBAL WARMING Much of Earth’s average temperature rose in the last century due to the fact that more carbon dioxide (CO2) has been produced by various human activities. The burning of fossil fuel creates CO2 and various gases.

The presence of CO2 in the atmosphere has an effect of warming up the planet by trapping heat from the sun in the atmosphere, but surplus of this gas would prevent solar radiation escaping into space, forming a ‘warm blanket’ around the planet. The concentrated CO2 increases the rate of heat absorption, warming the planet even further. The increase in Earth temperature has an impact on the polar regions and glaciers. These natural light reflectors shrink in size and cannot reflect excess solar rays. This cycle is a never ending process.

According to the United State’s National Academy of Science and NASA, Earth’s temperature has risen 0. 8oC since the beginning of 20th century and is projected to rise further of 1. 1oC to 6. 1oC, if steps are not taken to reduce carbon emission into the atmosphere. Figure 1: Earth’s temperature in the last 120 years (NASA, 2011) A computer simulation study by Saitoh et. al. (2005) has shown that the average temperature in the center of Tokyo would rise beyond 43oC at 6pm in year 2030. This scenario would be the same for all megacities around the world, if human continue to use inefficient ICE vehicle in everyday’s transportation.

Other negative impact of global warming includes more frequent hurricanes, floods and droughts. The melting of polar caps would trigger a rise in sea level. This would have dire effect on most major cities, as majority of them are situated near coastlines. In order to tackle this problem, one must look into the cause of the carbon emissions. Transportation, in particular, contributes significantly 14% of the total amount of carbon emission. Figure 2: Carbon emission by various sector (ATAG, 2011) Out of the 14% of carbon emission (Figure 2), more than 80% are produced by motor vehicles (Figure 3).

Figure 3: Amount of CO2 contributed by various mode of transportation. (Higgins, 2003) Governments around the world are tightening their reins on carbon emission, including Australia. This had an impact on the automotive industry as they try to manufacture and design cars to have better fuel economy but this does not solve the problem totally, as they still produce CO2 in their pipeline. A new, revolutionary concept of road transportation, therefore, is required and this concept is none other than going electric.

One of the selling point of EV is that it has zero carbon emission, provided the electricity it gets are from clean energy sources, such as solar, wind, hydroelectric and geothermal or to a lesser extent, nuclear power. PEAK OIL Fossil fuel such as petroleum is non-renewable resource and this would mean the precious supply will run out one day. The ever increasing production of ICE vehicle further adds on to the growing consumption of such resources. According to Brandt (2006), oil production around the globe would reach its peak somewhere in near future, after which it will have a sudden drop of production.

This would inflate the price of oil, causing more political uncertainties around the world. Most of the world’s oil fields which can be easily extracted are already running low, forcing oil companies to dig deeper to search for the black gold and this is the reason why the price of oil has been on a constant rise in the last 10 years and this trend is likely to continue. The amount of fuel burnt in everyday transportation can be eliminated if the use of electric vehicle are used by the masses. The media should also dispel the negativities about electric vehicle so that the public are more educated about its benefits.

ELECTRIC VEHICLE AERODYNAMICS In order to improve the range of an electric car, automotive engineers can either: optimise the aerodynamic of the vehicle, thus reducing the overall drag or putting more batteries onto the vehicle itself, making the car go further on a single charge. The latter idea seem impractical as installing more batteries would increase the weight of the vehicle dramatically, reducing its overall efficiency and the time required to fully charged the batteries. The only option would be to improve the vehicle’s aerodynamics.

There are several aerodynamics advantages EVs enjoyed over the ICE counterpart. Firstly, an electric motor is a very efficient system whereby its energy efficiency is usually more than 90%. There are less waste heat produced in the running of the engine, thus a smaller frontal cooling area is required. Also, the underside of the vehicle can be fully faired as they do not require exhaust pipes, muffler, fuel transfer pipes and fuel tank. In addition, an EV require less volume for its engine compartment as it does not require to have pistons and crankshaft in the operation of the system.

According to Ishihara et. al. (2011), a smaller motor compartment would allow aerodynamicist to optimised the geometry of the frontal area and improve the overall aerodynamics performance of the car. Several ways of optimising the aerodynamics of a EVs has shown to the world when Nissan introduced Leaf – a pure electric hatch that is affordable by the majority. Some of the good points of aerodynamics design should be learn so that it can be applied to future EVs. Figure 4: Nissan Leaf – A pure electric car (Ishihara et. al. , 2011) Sharp front corners According to the designer of Nissan Leaf (Ishihara et.

al. , 2011), it is crucial to have a sharp front corners in EV as it can direct air to flow along the side of the front tires thus reducing airflow separation. Figure 5: CFD simulation of air flow at the front of Nissan Leaf (Ishihara et. al. , 2011) This can be seem in the circle region in Figure 5, where the flow velocity varies very little at the region. Rear slant angle Besides optimising the frontal area to minimise the overall drag of the EV, it is important to focus on the flow separation at the back of the vehicle. The rear slant angle of any car is found to have a huge influence on drag and lift.

Much of the drag produced by cars are due to the pressure difference between the front and the rear. As the air flows along all surfaces of the car and separates at the blunt rear ends, it creates a set of longitudinal steam-wise vortices called C-pillar vortices. The separation of these vortices would induce an increase in local velocity. As such, the pressure at the back of the vehicle drop and the high pressure accumulated at the front of the vehicle would have a negative force opposite to the motion of the vehicle, creating unwanted drag to the whole car.

The design of rear slant angle is thus an important issue for reducing the overall drag of the vehicle. Research done by Ahmed (1984) has shown that drag coefficient of a car begin to rise as the slant angle increases. (Figure 6 and 7) Figure 6: Effect of rear slant angle (x-axis) on drag coefficient (y-axis) (Ahmed et. al. , 1984) Figure 7: Drag coefficient between two different rear slant angle (Watkins, 2011) To overcome the unwanted drag, the designers of EVs should introduce a large roof spoiler to compensate the pressure loss at the back of the vehicle.

The roof spoiler also aims to minimised the turbulence and wake occurring at the back of the vehicle as much as possible. (Ishihara et. al. , 2011) Figure 8: Visualisation of air flow at the rear of Nissan Leaf by CFD (Ishihara et. al. , 2011) Sharp rear corners To further reduce the overall drag of an EVs, it is recommended to introduce sharp rear corner. The purpose of this is to deflect airflow away from the rear of the car and to prevent air from flowing round the rear corner of the car, which can cause significant pressure drop. (Ishihara , 2011)

Much can be learnt from one of the lowest drag coefficient production car – Toyota Prius. According to the official review of Toyota Prius, it has a drag coefficient of 0. 25. (Toyota Motor Sales, 2011) Figure 9: Toyota Prius (The Motor Report, 2010) Figure 10: Visualisation of air flow at the rear corner of Nissan Leaf by CFD (Ishihara et. al. , 2011) It can be seen in Figure 9 and 10 that both vehicles (Prius and Leaf respectively) have low drag coefficient in common. Both employ sharp rear corners and an extended spoiler.

Future EVs should make full use of this design to minimise the power required to propel the vehicle. Underfloor fairing Most ICE vehicles have energy efficiency of around 20 – 30%. (Carter, 2011) The remaining percentage of energy conversion are waste heat and friction. Heat is generated when energy is converted from one form to another. In the case of EVs, it has a higher energy efficiency – usually above 90% (CSIRO, 2007) The high efficient electric motor in EVs would mean that they generate less heat during operation.

EVs generally do not require a large front cooling air inlet as oppose to ICE vehicles. (Figure 11, circled in yellow) Figure 11: General Motors EV1 (Cogan, 2008) Figure 12: Side view of cooling airflow through ICE in motion (Watkins, 2011) ICE vehicle usually disperse the heat via the under surface of the car. As such, there will not be any fairing at the underside of ICE vehicles, as the lack of cover would increase the rate of heat dispersion. The complex surface also generate higher turbulence, resulting additional drag of the car.

Modern EVs, on the other hand, do not require to have a large open surface for heat dispersion. Because of that, front fairings can be applied to almost all EVs. This would reduce the presence of turbulence occurring at the underside, reducing the overall build-up of drag. Figure 13: Underside of Nissan Leaf (Ishihara et. al. , 2011) In addition, EVs do not have an exhaust pipe running underneath the vehicle which allow the fairing to be applied throughout the entire underfloor. The lack of muffler and exhaust in EVs enable designers to fully cover the rear bottom of the vehicle with a diffuser.

This application further improve the aerodynamics performance of an EV. (Ishihara et. al. , 2011) Figure 14: Flow visualisation of the underside of Nissan Leaf (Ishihara et. al. , 2011) The aerodynamics performance does not have much influence on the consumers’ decision to buy a car. The strongest factor to persuade buyers to purchase an electric vehicle is its design and its own appeal to the buyer. The next factor that the buyer consider is the price of the car. The aerodynamics of a EV, however, would determine whether if the car can travel longer with minimum power.

ELECTRIC VEHICLE AERO-ACOUSTICS Much of the noise created by EVs are due to the contact between the wheel and road surface, as well as the running of the electric motor. The noise produced by EVs are very much different to ICE vehicles due to the fact that firstly, there are less vibrations occurring in the engine, secondly, the lack of a large radiator fan and thirdly, there is no combustion occurring in the operation of an EV. The vibrations in ICE are usually caused by the movement of pistons and crankshaft.

EVs generally produces lesser noise than ICE vehicle, but this has its drawback as there would not be any noise ‘covering’ from the engine to mask the wind noise when the vehicle is travelling at high speed. The airflow around the side mirrors will produce disturbing noise to the cabin when the vehicle is in motion. To overcome this problem, engineers designing EVs has to reshape the front exterior. The engineers who had designed Nissan Leaf has made exceptional effort to reduce the disturbing wind noise when the car is travelling.

They had installed headlamps that are projected upwards to deflect airflow from striking the side mirror casings. This design should be implemented in future electric car design. (Figure 15) Figure 15: Headlamp of Nissan Leaf deflects air flows around the side mirror housing (Ishihara et. al. , 2011) According to Ishihara (2011), this configuration reduces the wind noise produced by 3dB, credited to the improved headlamp design. Research done by Govindswamy and Eisele (2011) deduced that the noise level of a ICE vehicle is similar to that of EV when it is travelling at constant speed and in coasting down condition.

The reason is because the noise that are produced in this scenario is dominated by the tire friction, wind and transmission noise. At acceleration, however, the electric motor begins to produce high frequency noise. Another challenging acoustic issue for electric car designer to tackle is that EVs are quiet at low speed and is difficult for pedestrians to notice its presence. Research has shown that electric car is hard to hear when it is travelling below 32km/h (The Economist, 2009) A solution to this would be installing a warning sound that will activate when the vehicle is travelling at low speed.

(Figure 16) The drawback of this device is that the sound produced by the speaker can only travel in the one direction and that is only audible to the people who are at the front of the vehicle, unlike ICE vehicle, where the vibration of the engine propagates evenly to all directions at periodic frequency. Figure 16: Illustration of sound warning system (Toyota, 2010) OPTIMISING ELECTRIC VEHICLE The most important component of an EV is its battery. Enhancing this would deliver a better performance for EVs. The improvement in battery technology in recent years have greatly enhance the range offered by EV.

The Nissan Leaf and Renault Fluence Z. E. have around 100 miles of driving range on a full charge. However, the idea of electric car still has not received a warm response from the majority of drivers due to the “range anxiety” effect. Also, the time required to fully charged an EV is much longer to fill up a fuel tank of a ICE vehicle. Companies which affiliate to the development of EVs have devised various methods to improve the concept of battery charging and energy storage. Better Place has introduced the idea of battery swapping.

This could be the solution for the time required to charge a battery. This idea involves building various battery swap stations across the city and the EV that has a depleted battery can opt for a swap at the station. The time taken to swap the battery is comparable to filling up a full tank of petrol. Figure 17: Battery swap station (McDonald, 2011) Volvo has been researching the possibility of integrating battery into the car body, saving overall vehicle weight and increasing the energy storage. They claimed that the weight of the car can reduce up to 15%. (Firth, 2011).

Figure 18: Battery integration into car body (Firth, 2011) COMPARISON BETWEEN RENAULT FLUENCE Z. E. AND NISSAN LEAF In this section, comparison will be drawn between two EVs with similar driving range – Renault Fluence Z. E. and Nissan Leaf. The former without significant aerodynamics improvement while the latter with entire new design to enhance its driving range. Figure 19:Fluence Z. E. (Top) and Leaf (Bottom) (Electric Cars Report, 2011) (Ishihara et. al. , 2011) Parameters| Fluence Z. E. | Leaf| Average Driving Range| 100 miles| 100 miles| Engine Power| 70 kW| 80kW|.

Design| Taken from ICE cousins chassis and shape| A new design to optimise aerodynamics performance| Weight| 1543kg| 1521kg| Top Speed| 135 km/h| 145 km/h| Table 1: Parameters of Fluence Z. E. and Leaf (Renault, 2011) (Nissan, 2011) In order to achieve a longer driving range for Fluence Z. E. , the engine power was cut down to 70 kW and the top speed was tune down to 135 km/h. One of the reason why it was done is because Fluence Z. E. is not concern on the aerodynamic performance than that of Leaf. (Cole, 2010) On the overall packaging design, Fluence Z. E.

might be slightly better than Leaf due to the fact that the entire body was originated from ICE version. Unlike Leaf, which was developed from sketch on the basis of optimising aerodynamic performance. CONCLUSION The aerodynamics of future electric cars must be optimised in order for it to travel further and faster, using examples such as; sharp front and rear corners, underfloor fairings and reducing the pressure loss at the back of the vehicle as much as possible. The sound produced by electric vehicles while travelling is very much different from internal combustion engine vehicle.

Innovative design such as the one used by the designers of Nissan Leaf can be learnt to provide a pleasant ride for its occupants. The aerodynamics of an EV is indeed an important factor on the performance of the vehicle. ACKNOWLEDGMENTS The author would like to thank the lecturer for this module, Professor Simon Watkins for his guidance and dedication throughout the semester. I would also like to extend my appreciation to Mr Mark Thompson for his assistance throughout the course.


Suplee C. , 2009, Hydrogen-powered car still seems improbable, Washington Post, viewed 2nd October 2011, 2. Saitoh T. , Yamada N. , Ando D. & Kurata. , 2005, ‘A grand design of future electric vehicle to reduce urban warming and CO2 emissions in urban area’, viewed 2nd October 2011, Science Direct 3. National Academy of Science, 2008, ‘Understanding and responding to Climate Change’, viewed 2nd October 2011, The National Academies 4. ATAG, 2011, Aviation’s role in climate change, viewed 2nd October 2011,

5. Higgins P. , 2003, Reducing Australia’s Greenhouse Gas Emissions from the Transport Sector, Emergent Futures, viewed 4th October 2011, 6. NASA, 2011, GISS Surface Temperature Analysis, NASA, viewed 4th October 2011, 7. Brandt A. , 2006, Testing Hubbert, Energy and Resources Group, viewed 4th October 2011, 8.

Ishihara Y. , Takagi H. & Asao K. , 2011, ‘Aerodynamic Development of the Newly Developed Electric Vehicle’, SAE paper 20117230 9. The Motor Report, 2010, Toyota Australia Announces Prius Plug-in Hybrid Test Fleet, The Motor Report, viewed 8th October 2011, 10. Toyota Motor Sales, 2011, PRIUS 11 Specs, Toyota, viewed 8th October 2011, 11. Carter S. , 2011, Engine Efficiency: Good or Bad? , Physic 212, viewed 9th October 2011, 12. CSIRO, 2011, Energy efficiency explained, CSIRO, viewed 9th October 2011, 13. Cogan R. , 2008, 20 Truths About GM EV1 Electric Car, GREENCAR. com, viewed 12th October 2011, 14. Govindswamy K. & Eisele G. , 2011, ‘Sound Character of Electric Vehicles’ SAE paper 20111728 15. The Economist, 2009, The sound of silence, The Economist, viewed 13th October 2011.

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