With the advent of alternative fuels (ethanol, biofuel, electricity, etc.) and hybrid vehicles, the traditional MPG (miles-per-gallon of gasoline) will soon be obsolete as a one-size-fits-all measure of fuel economy.
Replacing MPG is not straightforward – in addition to the complexities introduced by the new fuels and vehicle technologies, the fuel economy that a consumer can expect will become a stronger function of how they drive and how far they drive (“drive cycle”). For example, measuring fuel economy over the traditional EPA city and highway drive cycles does not suffice for plug-in-hybrid electric vehicles (PHEVs) because the fuel economy of PHEV's depends very strongly on driving distance – driving 30 miles in the city typically will result in vastly different fuel economy than driving 70 miles in the city (which is not the case for pure gasoline-powered vehicles).
One approach to dealing with these issues is to define a new “MPG” in a manner that incorporates aspects of the drive cycle (“MPG” in quotes to distinguish it from the traditional MPG that consumers understand). An example is the J1711 standard under development by SAE International, which includes “utility factors” that distinguish between periods when vehicles are in charge-depleting (vehicle running on battery power only) vs. charge-sustaining (gasoline fuel consumed) modes.
For the Progressive Insurance Automotive X PRIZE, we are not taking this approach. As discussed elsewhere on this blog (here and here), we are using MPGe (miles per gallon energy equivalent) as the basic figure of merit for fuel economy:
MPGe = (miles driven) / [(total energy of all fuels consumed)/(energy of one gallon of gasoline)])
It’s important that the basic figure of merit for fuel economy has a simple definition, so that consumers can understand it. MPGe is easy to explain, it accounts in a neutral manner for any combination of fuels, and it reduces to the familiar MPG in the case of gasoline fuel only.
Another advantage of MPGe is that it’s defined independent of any particular drive cycle. This also makes MPGe relatively simple to explain. And it allows the drive cycle to be reported as additional information, which is appropriate given the increasing complexity and importance of drive cycles mentioned above.
Here’s an analogy based on the traditional MPG for pure gasoline cars: Consumers understand MPG easily, as it only depends on how far they drive and how much gas they use - just divide the miles driven by the gallons of gas consumed. When EPA publishes city and highway MPG numbers, the MPG is computed in exactly this way, but based on driving carefully-chosen city/highway drive cycles. Similarly, MPGe is a simple efficiency measure that’s computed independent of drive cycle, and can be reported together with information about the drive cycle over which it’s measured.
In PIAXP we will measure fuel economy using MPGe, but will be choosing competition stage lengths and other driving constraints that in effect achieve drive cycles that are good representations of consumer driving.