Electric vehicle
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| Figure 9.1 Aerodynamic ideal shape, a teardrop of aspect ratio 2.4 where ρ is the density of air (kg m −3 ), A is the frontal area (m 2 ), v is the velocity (m s −1 ) and C d is the drag coefficient, which is dimensionless. The ideal aerodynamic shape is a teardrop, as achieved by a droplet of water freefalling in the atmosphere and illustrated in Figure 9.1. The coefficient of drag varies with the ratio of length to diameter, and having the lowest value of C d = 0.04 when the ratio of the length to diameter is 2.4. Using Equation (9.2) and taking air density to be 1 .23 kg m −3 , the power required to drive a teardrop-shaped body with C d = 0.04 of cross-section 1 m 2 travelling at 100 kph (27.8 ms ii 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 fora saloon or hatchback car normally varies from to 0.5, while that of a reasonably aerodynamic van is around 0.5. For example, a Honda Civic hatchback has a frontal area of 1 .9 m 2 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 C d of 0.25, and the General Motors EV1 electric vehicle with an even lower C d of 0.19. The Bluebird record-breaking electric car had a C d of As the drag, and hence the power consumed, is directly proportional to the drag coef- ficient, a reduction of C d from 0.3 to 0.19 will result in a reduction in drag of that is 63.3%. In other words, the more streamlined vehicle will use 63.3% of the energy to overcome aerodynamic drag compared with the less aerodynamic car. Fora given range, the battery capacity needed to overcome aerodynamic resistance will be less. Alternatively the range of the vehicle will be considerable enhanced. The battery power P adb needed to overcome aerodynamic drag is obtained by dividing the overall power delivered at the wheels P adw by the overall efficiency η 0 (power at wheels/battery power): P adb = P adw η 0 = 1 2 ρAC d v 3 η 0 (9.3) Design Considerations 219 35 30 25 20 15 10 5 0 0 20 40 60 80 100 120 140 Power to overcome drag/kW A = 2 m 2 Cd = A = 1.5 m 2 Cd = A = 2 m 2 Cd = 0.19 Speed/kph A = 1.5 m 2 Cd = 0.19 Download 3.49 Mb. Share with your friends:
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