82 Aerodynamic Considerations

8.2.1 Aerodynamics and energy

It is well known that the more aerodynamic is a vehicle, the lower is its energy consumption. Bearing in mind the high cost of onboard electric energy, the aerodynamics of electric vehicles is particularly important, especially at high speeds.

Let us first consider the effect of aerodynamic drag. As seen in Chapter 7, the drag force Fad on a vehicle is:

and the power Padw (Watts) at the vehicle wheels required to overcome this air resistance is:

where p is density of air (kg.m-3), A is the frontal area (m2), v is the velocity (m.s-1) and Cd is the drag coefficient, which is dimensionless.

Electric Vehicle Technology Explained James Larminie and John Lowry Ā© 2003 John Wiley & Sons, Ltd ISBN: 0-470-85163-5

The ideal aerodynamic shape is a teardrop, as achieved by a droplet of water freefalling in the atmosphere as illustrated in Figure 8.1. The coefficient of drag varies with the ratio of length to diameter, and it has a lowest value of Cd = 0.04 when the ratio of the length to diameter is 2.4. Using equation (8.2) and taking air density to be 1.23 kg.m-3, the power required to drive a teardrop shaped body with Cd = 0.04 of cross-section 1 m2 travelling at 100kph (27.8 m.s-1) in clear air will be 664 W. If engineers and scientists could achieve such aerodynamic vehicle shapes they would revolutionise energy in transport. Unfortunately they cannot get near such a low value. However, the ideal teardrop shape is normally an 'aiming point' for vehicle aerodynamicists.

In reality the drag coefficients of vehicles are considerably higher due to various factors, including the presence of the ground, the effect of wheels, body shapes which vary from the ideal, and irregularities such as air inlets and protrusions.

The aerodynamic drag coefficient for a saloon or hatchback car normally varies from 0.3 to 0.5, while that of reasonably aerodynamic van is around 0.5. For example, a Honda Civic hatchback has a frontal area of 1.9 m2 and a drag coefficient of 0.36. This can be reduced further by careful attention to aerodynamic detail. Good examples are the Honda Insight hybrid electric car, with a Cd of 0.25, and the General Motors EV1 electric vehicle with an even lower Cd of 0.19. The Bluebird record-breaking electric car had a Cd of 0.16 (A sphere has a Cd of 0.19.)

As the drag, and hence the power consumed, is directly proportional to the drag coefficient, a reduction from a Cd of 0.3 to a Cd of 0.19 will result in a reduction in drag of 0.19/0.3, i.e. 63.3%. In other words the more streamlined vehicle will use 63.3% of the energy to overcome aerodynamic drag compared to the less aerodynamic car. For a given range, the battery capacity needed to overcome aerodynamic resistance will be 36.7% less. Alternatively the range of the vehicle will be considerable enhanced.

The battery power Padb needed to overcome aerodynamic drag is obtained by dividing the overall power delivered at the wheels Padw by the overall efficiency n0 (power at wheels/battery power).

Padw \pACdv3

n0 n0

The battery mass mb (kg) of a battery with specific energy SE (Wh.kg-1) required to overcome the aerodynamic drag at a velocity v (ms-1) over a distance d (metres) is given by:

The variation of battery power Padb for overcoming aerodynamic drag with speed is shown in Figure 8.2 for vehicles of different drag coefficients and different frontal areas. The battery mass required to provide energy to overcome aerodynamic drag for a vehicle with a range of 100 km travelling at different constant speeds is shown in Figure 8.3. An efficiency n0 of 0.7 is used. Figure 8.3 dramatically illustrates the importance of streamlining, as the battery weight shown in this graph is purely that needed to overcome wind resistance, and for the not very impressive range of 100 km. Figure 8.3 also clearly shows how ill-suited battery electric vehicles are to high-speed driving. Even a well designed car, with a Cd of 0.19, still needs about 400 kg of lead acid batteries to travel just to overcome wind resistance for 100 km when going at 160 kph. If the MATLABĀ® file used in Section 7.4 for the range modelling of the GM EV1 (whose results are shown in Figure 7.15) are adapted for a constant speed of 120 kph, it will be found that the predicted range is less than 80 km However, when driving the SFUDS cycle, which has

20 40 60 80 100

Speed/kph

Figure 8.2 Power requirement to overcome aerodynamic drag for vehicle of different frontal areas and drag coefficients for a range of speeds up to 160 kph

20 40 60 80 100

Speed/kph

Figure 8.2 Power requirement to overcome aerodynamic drag for vehicle of different frontal areas and drag coefficients for a range of speeds up to 160 kph

Speed/kph

Figure 8.3 The effect of drag coefficient and speed on battery mass. The vehicles all have a frontal area of 1.5 m2, and the range is 100 km. The mass is only for the energy required to store the energy to overcome aerodynamic drag; the actual battery mass would need to be higher

Speed/kph

Figure 8.3 The effect of drag coefficient and speed on battery mass. The vehicles all have a frontal area of 1.5 m2, and the range is 100 km. The mass is only for the energy required to store the energy to overcome aerodynamic drag; the actual battery mass would need to be higher plenty of stopping and starting, but no speeds over 60 kph, the range could be over 140 km in good conditions.

It is clear from both Figures 8.2 and 8.3 that there are huge advantages in keeping both the aerodynamic resistance and the vehicle frontal area as low as possible. Bearing in mind the considerable cost saving on both battery weight and battery cost it is well worth paying great attention to the aerodynamic details of the chassis/body. There is great scope for producing streamlined shapes with battery electric vehicles as there is much more flexibility in placing major components and there is less need for cooling air ducts and under vehicle exhaust pipes. Similarly as well as keeping the coefficient of drag low, it is equally important to keep down the frontal area of the vehicle if power requirements are to be minimised. Although the car needs to be of sufficient size to house the passengers in comfort, the greater flexibility in which components can be placed in an electric car can be used to minimise frontal area.

Some consideration also needs to be given to items such as wing mirrors, aerials and windscreen wipers. These need to be designed to minimise the drag. Aerials do not need to be external to the car body and wing mirrors can be replaced by electronic video systems that can be contained within the aerodynamic envelope of the car. Whilst the latter may appear an expensive option at first, the reduced drag will result in a lighter battery with associated cost savings.

8.2.2 Body/chassis aerodynamic shape

The aerodynamic shape of the vehicle will depend largely on the type of use to which the electric vehicle is to be put. If it is a city commuter car or van that will be driven at relatively low speeds, the aerodynamics are much less important than on a conventional vehicle which will be used for motorway driving.

For the latter type of vehicle, to be used at relatively high speeds, a low frontal area and streamlining is vitally important. It is worth having a look at how this was achieved with vehicles such as the Honda Insight hybrid (Cd = 0.25) and the GM EV1 battery car (Cd = 0.19). Although the aerodynamics of high speed battery electric vehicles are vitally important, they are also important for hybrid vehicles, but optimisation can result in a slightly less aerodynamic vehicle with more reliance being placed on the internal combustion engine to achieve range.

Most aerodynamic vehicles at least attempt to copy the teardrop shape and this is true of both these two vehicles. The body shape is also designed to keep the airflow around the vehicle laminar.

On the Honda Insight, shown in Figure 8.15, the body is tapered so that it narrows towards the back, giving it a shape approaching the teardrop. The rear wheels are placed approximately 110 mm closer together than the front wheels, allowing the body to narrow. The cargo area above the wheel wells is narrower still, and the floor under the rear portion of the car slopes upwards, while the downward slope of the rear hatch window also contributes to the overall narrowing of the car at the rear.

At the back of the Insight the teardrop shape is abruptly cut off in what is called the Kamm effect. The Kamm back takes advantage of the fact that beyond a certain point there is little aerodynamic advantage of rounding off or tapering, so it might as well be truncated at this point avoiding long, extended, fairly useless tail sections.

Another important aerodynamic feature is the careful management of under-body airflow. The Insight body features a flat under-body design that smoothes airflow under the car, including three under-body covers. Areas of the under-body that must remain open to the air such as the exhaust system and the area under the fuel tank (it is a hybrid) have separate fairings to smooth the flow around them.

In order to minimise the air leakage to the underside the lower edges of the sides and the rear of the body form a strake that functions as an air dam. At the rear the floor pan rises at a five degree angle toward the rear bumper, creating a gradual increase in the body area that smoothly feeds under-body air into the low pressure area at the rear of the vehicle.

The GM EV1 with its exceptionally low drag adopts a similar approach. It has the advantage that as a pure electric vehicle there is no need for fuel tanks or exhaust pipes. Again the vehicle shape emulates as far as possible the teardrop shape and is as perfectly smooth as possible. The rear wheels on the EV1 are 228 mm closer together, nearly twice that on the Insight. This can clearly be seen in Figure 11.5, which shows several views of this vehicle. With both vehicles abrupt changes in body curvature are avoided, items such as the windscreen are joined smoothly into the shape, and gaps such as that between the wheels and body are minimised. The surface of the wheels blends in with the body shape. This all helps to keep the airflow laminar and thus reduce drag.

On very low speed vehicles (<30kph), such as golf buggies, where tranquility and pure air are more important than rushing around at speed, the aerodynamic shape of the vehicle is almost irrelevant. As discussed earlier, with vehicles such as commuter cars and town delivery vehicles the aerodynamic shape is less important than on faster cars. However, it does have some significance and should not entirely be ignored. There have been attempts to produce both vans and buses with tear drop shapes, but there is a conflict between an aerodynamic shape and low frontal area and the need for maximising interior space, particularly for load-carrying. There is nothing stopping commuter vehicles from being aerodynamic but in the case of vans there needs to be a compromise between the need to slide large items into a maximum space and the desire for a teardrop tail. There is no reason why some of the features used on the Insight and the EV1, such as under-body covers, cannot be used on vans. Careful consideration of the van shape, where possible avoiding rapid changes in curvature, keeping the wheel surfaces flat, minimising gaps and rounding the corners will indeed reduce the drag coefficient, but not down to that of the EV1.

When carrying out initial calculations, as in a feasibility study, the coefficient of drag is best estimated by comparing the proposed vehicle with one of a similar shape and design. Modern computational fluid dynamics (CFD) packages will accurately predict aerodynamic characteristics of vehicles. Most motor manufacturers use wind tunnels either on scale models or more recently on full size vehicles. Some of these now incorporate rolling roads so that an almost exact understanding of drag, lift, etc., can be measured.

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