What Does The Power To The Wheels Do?

The three uses of Power to the Wheels are overcoming Wind Resistance and Rolling Resistance, and recovering from Braking. The data below show that Electric vehicles are fairly similar to Gasoline vehicles, except Braking in City driving, where the regenerative braking makes a decisive difference, 2% vs. 7-10%. As expected, braking plays the largest role in City efficiency, and the smallest role in Highway efficiency. In City driving, the energy regained from regenerative braking compensates for the other factors for Electric vehicles. Rolling Resistance is higher for Highway efficiency because the power required grows linearly with speed. The biggest difference between City and Highway efficiency is Wind Resistance, which grows as the cube of speed.

Next we look at some calculated values of Wind Resistance and Rolling Resistance for a Tesla Model S. Recall that the Tesla Model S has the lowest Coefficient of Wind Resistance of any production car, so the Wind Resistance would be even higher for most other cars.

The Power to overcome Rolling Resistance grows linearly with speed, and linearly with weight. Techniques for reducing the weight of Autonomous Vehicles are a major focus in the next sections. We have already mentioned going to direct power to eliminate the main battery – the battery pack in a Tesla Model S weighs 1,323 pounds, or 29% of the total weight, and is the most expensive component. Rolling Resistance varies greatly with tires and road surface type, we could potentially gain a factor of 10 improvement over car tires on existing roads[http://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html accessed 11/13/2015] – this emphasizes the importance of optimized surfaces in A-Ways, and protection of the surfaces from wear, weather and dirt.

The Power to overcome Wind Resistance is the product of the cube of the speed, the frontal area of the vehicle, the Coefficient of Drag of the vehicle, and the density of the medium, air in our case. As you see from the graph, at higher speeds Wind Resistance dominates, and gets dramatically higher as speed continues to increase: at 240 mph it will be 8 times as great as at 120 mph.

For higher speeds, Wind Resistance will drive the design of Autonomous Vehicles. For example, to minimize frontal area, long vehicles with a small front are highly desirable (the coefficient of drag may increase somewhat as the length increases, but less than linearly) – this implies that we want to use convoys of vehicles, sharing many loads.

To minimize Coefficient of Drag, streamlining is crucial, which is evident in the sleek shape of the Tesla Model S. For convoys we want to control airflow at the junction between vehicles – streamlining can lead to some counter-intuitive looking vehicles. The values of Drag Coefficients[http://www.engineeringtoolbox.com/drag-coefficient-d_627.html accessed 11/13/2015] below show range values – note that the Dolphin goes through water which is much more dense than air, so Drag Coefficient is even more critical; the requirement to ride on the ground exacts a considerable penalty, partially explaining why the aircraft can do so much better.

For very high speeds, it makes sense to reduce the density of air by evacuating it – A-Ways provide the opportunity to accomplish this; reducing air density by ½, that is at the altitude the top of Denali, will reduce the Wind Resistance to one-half. Another technique is to move the air in the direction of the vehicles, since it is the relative velocity that matters – note in A-Ways, the vehicles will tend to push the air ahead of them, so this is a natural effect to be explored.