At Edison2 we try very hard to talk facts and only facts. For us, the numbers the Very Light Car made in coastdown testing both confirmed what we had hoped and opened some new vistas.
We’ve discussed the meaning of ABC coastdown numbers in previous blog posts; now let’s talk about the consequences.
By definition, a horsepower is 33000 ft.lb/min; it’s the work that a good horse could reliably deliver when, say, lifting coal out of a mine. A fit man can expend about a horsepower for short bursts: for example, a 200 lb man running up a 10 ft flight of stairs in 3 seconds (200 x 10 x (60/3) = 40000 ft.lb/min = 1.2 hp).
The beauty of ABC coastdown numbers is that we can calculate the overall drag for any given speed. From there we can then calculate the power required to drive the car and the energy needed to cover a given distance.
Consider the spreadsheet below. Based on our coastdown ABC numbers, this tabulates the power required for the VLC to run on a level road at a range of speeds.
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VERY LIGHT CAR |
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A |
B |
C |
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EFFICIENCY (%) |
BATTERY KWhr |
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6.31 |
0.1862 |
0.00433 |
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84 |
16 |
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SPEED |
DRAG |
POWER |
ENERGY |
RANGE |
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mph |
lb |
hp |
KW |
KWhr/100 Miles |
Miles |
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0 |
1.4 |
0.0 |
0.00 |
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10 |
8.6 |
0.2 |
0.17 |
2.04 |
785 |
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20 |
11.8 |
0.6 |
0.47 |
2.79 |
574 |
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30 |
15.8 |
1.3 |
0.94 |
3.74 |
428 |
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40 |
20.7 |
2.2 |
1.65 |
4.90 |
327 |
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50 |
26.4 |
3.5 |
2.63 |
6.26 |
255 |
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60 |
33.1 |
5.3 |
3.95 |
7.83 |
204 |
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70 |
40.6 |
7.6 |
5.65 |
9.61 |
167 |
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80 |
48.9 |
10.4 |
7.78 |
11.58 |
138 |
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90 |
58.1 |
14.0 |
10.41 |
13.77 |
116 |
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100 |
68.2 |
18.2 |
13.57 |
16.16 |
99 |
The power numbers are really quite astonishing – only 3.5 hp to drive the VLC at 50 mph. The great virtue of such a low power requirement is that energy requirements are also spectacularly low. Using our platform, the range of an electric car stops being a problem.
In the table, for a given speed in the left column, the ABC numbers are used to calculate overall vehicle drag in pounds; knowing drag and speed, we can calculate power in horsepower and kilowatts (1 hp = 0.746 KW). Knowing the power requirement in KW and speed allows us to calculate energy use per 100 miles. For example, at 50 mph, it would take 2 hours to drive 100 miles, so the energy required is 2 hours x 2.63 KW = 5.26 KWH.
Where this gets interesting is the Range column. The numbers here are calculated from energy use and battery size and efficiency. Here we’ve used 16 KWH for the battery size (like the Chevy Volt) and, based on our research and some very competent advice, assumed 84% for overall efficiency for the battery, controller and motor.
It is significant how this works out: 204 mile range at 60 mph reminds us why we chost an internal combustion engine for the X Prize. For the competition we needed to (and easily did) demonstrate a 200 mile range with our Mainstream cars, and 204 miles with a battery cuts it far too close. We could have fitted a larger battery, but the extra weight would decrease overall efficiency.
The numbers don’t lie and wishful thinking doesn’t cut much ice with physics. So the consequences are, if you want an electric car with realistic range it’s going to need to look pretty much like the Very Light Car. If it looks like an ordinary car, it’s not going to go very far before it needs recharging.