I bought a pre-owned 2015 Volt Base with 18,400 miles on it three weeks ago, and I like it very much. At first, I was fascinated by the Power Flow screen, and I kept it on all the time I was driving. However, my enthusiasm for the Power Flow display lasted only about two days. It provides a lovely qualitative cartoon of the energy flow of the Volt, but it lacks any quantitative information about that process. After a few days, watching the little arrows move back and forth was no longer interesting. The Power Flow display could be improved in future Volt generations (or even with Gen 1 and 2 software upgrades) by giving the viewer more information about where the energy is going and how it is being used. Consider the following:
Data values that the Volt is continuously measuring are: (1) the vehicle speed (in miles/hour, convertible to feet/second), and (2) the power flowing between the battery and the propulsion system (in kilowatts, convertible to foot-pounds/second). Dividing the power in foot-pounds/second by the speed in feet/second gives the instantaneous thrust in pounds. This thrust is the force at any instant that is applied through the tires to drive the Volt forward or to decelerate it.
By observing changes with time in the speed of the Volt, the system could monitor the acceleration of the vehicle. A good estimate of the mass of the vehicle can then be derived by observing the thrust needed to provide several independent standing-start accelerations. In particular, if F1 and F2 are the thrusts needed to produce accelerations a1 and a2, then the effective mass M of the Volt would be M = (F1-F2)/(a1-a2). In hilly locations like Seattle, this mass estimate will have to be averaged because uphill or downhill slopes will affect the value. It might take some processing to derive an accurate estimate of the vehicle mass, but it should be a constant and should be accurately estimated in a few minutes of driving.
The thrust has three components, (1) the drag due to the friction in the tires and mechanical system, (2) the force due to air resistance, and (3) the force that produces the instantaneous acceleration. Knowing the mass and acceleration, the acceleration-dependent force could be removed, leaving the combined friction plus air resistance force in pounds. Since the Volt's speed is known, the dependence of that combined force on speed could be used to extract the force due to air resistance and the coefficients of its velocity dependence. (I won’t go into how to do that, but it’s simple algebra.)
These three force components could then be multiplied by the instantaneous speed to give the three power flow components, providing separate power flows (converted back to kilowatts) (1) due to mechanical friction, (2) due to air resistance, and (3) due to acceleration. The Power Flow display could display these three power component values, as well as the net energy going into these three processes (in kW-hrs) since the last charge. Having this information available would be very useful to many Volt owners, because they could directly observe the separate energy consumption effects of tire changes, driving style and mode, and high-speed driving.
