Electric vehicle
Consideration of Vehicle Mass
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| 9.5 Consideration of Vehicle Mass The mass of an electric vehicle has a critical effect on the performance, range and cost of an electric vehicle. The first effect of the mass on rolling resistance and the power and energy to overcome this has already been discussed in Section There are two other effects of mass. The first concerns a vehicle climbing a hill and the second is the kinetic energy lost when the vehicle is accelerating and decelerating in an urban cycle. In Equation (8.3) of Chapter 8 it was seen that the force F hc in newtons along the slope fora car of mass m(kg) climbing a hill of angle ψ is given by F hc = mg sin ψ (9.7) It follows that the power P hc in watts fora vehicle climbing a slope at a velocity v(m s −1 ) is given by P hc = F hc × v = mgv sin ψ (9.8) Figure 9.10 shows the total power needed to travel at a constant 80 kph up slopes of varying angles up to for vehicles of two different weights, but otherwise similar. They are based loosely on the GM EV1 electric car studied in Chapter 8. They both have a drag coefficient of 0.19 and tyres with a coefficient of rolling resistance of 0.005, and the frontal area is 1 .8 m. We can see that the 1500 kg car, which is approximately the weight of the real GM EV1, has to provide approximately 12 times as much power at than is needed on the at. With the 800 kg vehicle the power needed increases greatly, but only by about eight times. Looking at Figure 9.10 we see why the GM EV1 electric car needs a motor of power about 100 kW. In the SFUDS simulation we noted that the maximum power needed was only 12 kW, as in Figure 8.16. Taking heavy vehicles uphills requires high power. The results shown in the graph send a clear message. Considerable power is required for hill climbing, and such terrain will restrict the range of electric vehicles relying solely on rechargeable batteries. When designing electric vehicles the effect of hills must betaken into account, though there are no agreed standard hills for doing this. It is not too difficult, after a little experience, to add gradients to the simulation driving cycles considered in the previous chapter. This is usually done with a specific journey in mind. The effect of the vehicle mass when accelerating and stopping in town and city conditions is another area where the mass of the electric vehicle will have considerable influence on vehicle performance. There area variety of simulated urban driving cycles 228 Electric Vehicle Technology Explained, Second Edition 20 30 Motive power/kW 50 60 70 0 1 2 3 Slope angle/degrees 5 800 kg car kg car 7 8 9 10 0 Download 3.49 Mb. Share with your friends:
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