Units of measure don't always make sense when left in the hands of bureaucrats.
Industry standards don't always make sense. A standard may be a requirement by a specific industry. Specs for some connectors, obscure specifications (such as GM's long-lived wheel arch clearance for extreme snow), or federal requirements created to provide a measure of something technical come to mind. (I think we can all agree non-technical people should not be making decisions on technical issues.)
Enter MPGe, or Miles Per Gallon "Equivalent."
The problem originated a few years ago when the EPA had to provide a measure of efficiency for the Nissan LEAF when it was introduced. You may not know what a "Monroney" label is, but if you have ever looked at a new car you've seen one; it's the EPA label displaying a vehicle's fuel efficiency.
"Monroney" Efficiency Label for an EV.
As engineers, we tend to overcomplicate things. We like to analyze things quantitatively, using formulas and empirical methods to measure the quantifiable, then extrapolate trends and break things down to the lowest common denominator. We most likely agree that the best "technical" method to determine efficiency of any system: take the ratio of the unit output or end product to the amount of energy or power input. Right?
While the familiar "miles per gallon" may not appear on the surface to fulfill this, consider it taken in a more literal form:
Where η represents System Efficiency, Wout represents output power (the amount of energy to move a car a specified distance, in miles), and Win represents input power (the amount of energy contained in a specified quantity of fuel, in gallons).
While we may not think of this commonly used term in this way, that's essentially what it represents. So why not use the same formula for an EV, in commonly accepted units? You would find that Wout represents output power (the amount of energy to move a car a specified distance, in miles) and Win represents input power (the amount of energy utilized by the EV, in KWh).
So, you simply end up with miles per KWh. Simple and descriptive, right?
Well, a few early vehicles did get away with this on their efficiency labels. But, as with any good regulatory and political organization, the EPA shortly thereafter decided to conduct a variety of focus group studies to determine what the public would accept as a standard definition. While a variety of definitions were provided, MPGe was identified as the clear winner in this battle. The EPA settled on that, and that figure now is provided on every EV sold in America.
A Chevrolet Volt at a charging station.
The MPGe figure was originally created as a measure of the efficiency of a non-gasoline, alternatively fueled vehicle for Progressive's X-Prize in 2007. The EPA currently defines this as the efficiency of a vehicle, assuming an equivalency of 33.7 KWh per gallon of gasoline, measuring the energy consumed when driven on one of their standard test cycles (as with a normally fueled vehicle).
I'm not sure what's more disappointing: that we now have a technically accurate but very non-descriptive definition of efficiency for EVs, or that I expected a different result from a bunch of bureaucrats. One thing is for certain, I will continue to use miles/KWh as a measure of efficiency for my own EV. It makes more logical sense, and it doesn't make my engineer brain swarm with questions, such as, "Just really how much energy is in a gallon of fuel? What sort of fuel is it on the test? What if a vehicle calls for 91 octane premium unleaded but most Americans will just put in the cheapest available? What if you have a Flex Fuel vehicle and you run E85?" You get the idea.
What silly specifications have you had to deal with lately? What odd specifications do you have to convert back to "your" specification in your head?
This article was originally published on The Connecting Edge.