Measuring EV Efficiency

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June 11, 2013

Sometimes industry standards don’t make sense. It may be a requirement by a specific industry (specs for some connectors come to mind), obscure specifications (such as GM’s long-lived wheel arch clearance for extreme snow), and sometimes it’s a federal requirement created to provide a measure of something technical (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 is the EPA label displaying a vehicle’s fuel efficiency.

"Monroney" Efficiency Label for an EV

“Monroney” Efficiency Label for an EV

As engineers, we have a tendency to overcomplicate things. We like to analyze things quantatively, using formulas and empirical methods to measure the quantifiable and extrapolate trends and break things down to the lowest common denominator. I think we can all agree, though, 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=Wout/Win

ƞ represents System Efficiency (Greek letter “eta,” I admit I had to look this one up)

Wout represents output power (the amount of energy to move a car a specified distance, in miles)

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, it is 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)

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 out with this on their efficiency label. 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.

Measuring EV Efficiency 2

The MPGe figure was originally created as a measure of the efficiency of a non-gasoline alternatively-fueled vehicle for Progressive’s X-Prize back 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 am not sure what is more disappointing; that we now have a technically accurate but very non-descriptive definition of efficiency for EV’s, 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… 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? What if…

So, 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?

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