“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, January 11, 2014


The Nissan Leaf is rated by the EPA to achieve a so-called "MPGe" (miles per gallon gasoline equivalent) of 129 city, 102 highway, and 115 combined (note that the EPA sticker to the left is for an earlier year of the Leaf, I couldn't find a 2014 version). What is this "MPGe" of which they speak? While the linked Wikipedia article gives a thorough explanation, I want to briefly cover a couple of aspects.

First, the EPA makes the assumption that a gallon of gasoline is equivalent to 33.7 kWh (kilowatt hours) of electrical energy. In a straight comparison of chemical potential energy to electrical potential energy, this is accurate to within the variability of the myriad blends of gasoline available. The energy content of various fuels is listed here (though not in units I prefer).

I pay $0.16/kWh, so the 33.7 kWh would cost me $5.39, whereas a gallon of gasoline (I bought 8.6 of them yesterday) would cost me $3.639. On the other hand, that gallon will take me a trifle more than 50 miles whereas the 33.7 kWh will (by the EPA's reckoning) take a Nissan Leaf 115 miles. And my driving habits enable me to better the EPA rating of 42 m.p.g. for my car by about 21% so I might be able to drive the leaf 140 miles. Thus, I spend about 7.1 cents per mile for energy in my CT200h versus the 3.8 cents/mile I might spend in a Leaf.

What gives? The fact is that the battery to electric motor to driving wheels efficiency of the power train in the Leaf is going to be close to 90%, while the fuel to heat to mechanical motion to wheels in the internal combustion engine will be well under 30% on a good day. Over 70% is exhausted to the environment as waste heat.

It must be kept in mind that, neither in the EPA sticker ratings nor in my calculations for my car, is the energy used in getting the battery or tank filled included. That is, the energy to generate and transmit the electricity and the energy to extract crude, refine it to gasoline, and deliver it to the gas station is not included. It counts only what's in the vehicle. Supposedly, the CAFE ratings DO utilize the so-called "well-to-wheel" efficiency. There's a very nice comparison (albeit written by Tesla employees but still very credible) of the well-to-wheel efficiencies of a few examples of internal combustion engine vehicles, hybrids (non-plug in) and the Tesla battery electric vehicle here. The Tesla is about twice as efficient as a Prius on that basis. And DAMN those Tesla Model S roadsters look good!

No comments: