Tuesday, January 11, 2011

The Equivalent MPG conundrum

Here's some Auto stuff..

This is the age of electric and hybrid electric vehicles. Every company is coming up with its HE (hybrid electric) versions of the cars claiming that they improve the fuel economy. Some numbers tell that the Prius has a fuel economy of 48 miles per gallon (mpg) on city cycle, Honda Civic hybrid was rated 40mpg whereas the latest hype is about Chevy Volt, claiming 60 mpg (combined mode) and 93 mpg (electric only mode).

The basic assumption lies in the fact that this equivalent fuel economy is higher because unlike normal vehicles, in HEVs or EVs less amount of gasoline is burnt. They had to find some way for comparing the fuel "burnt" as "electricity" (which produces no emissions) to the fuel that is burnt in conventional vehicles to come up with a number: miles per "gallons" (where there are really no "gallons" of fuel "burnt" in electric vehicles).

"To reach this number EPA created a conversion factor between quantity of electric energy (KWHr) and volume of gasoline (gallons). They did this by dividing the heating value of a gallon of gasoline by the energy in a KWHr of electricity to get a conversion factor of 33.7 gallons/KWhr." ..states the article below

But is this the right way to do it? Should they account for the energy losses in "preparing" that electricity? after all we should be concerned more towards reducing the fossil fuel usage rather than "increasing" the power train efficiency per se

..and if they do what are the new numbers? are they more realistic?

This is a very nice article which sheds light on this issue. One of our profs shared it with us. You guys will find it interesting


  1. It also depends on the major source of electrical energy. In india, all electric cars will produce a bigger carbon footprint because we use coal to produce most of the electricity. If you use solar thermal or geothermal etc., then you can use the mpg numbers the way they are used now.

  2. yes.. agreed. So basically we need to look at the ultimate (well-to-wheels) efficiency of conversion

  3. "They did this be dividing the potential energy or heating value of a gallon of gasoline (115,000 BTUs) by the energy in a KwH of electricity (3412 BTUs) to get a conversion factor of 33.7 gallons per KwH"....its not fraud...is stupidity...not even a 10th standard kid would buy it

  4. "The EPA is therefore giving the electric vehicle a huge break. When we measure mpg on a traditional car, the efficiency takes a big hit due to the conversion efficiencies and heat losses in combustion. The same thing happens when we generate electricity, but the electric car in this measurement is not being saddled with these losses, even though we know they still occur in the system."

    Another way to look at this is that they are doing this to promote more research into the EV area. I would say that more the number of buyers of EVs, the more its popularity, the more money put into researching better mileages. It is like all research / new technologies - early adopters might not be the best, but they get about the community to the threshold required for it to snowball into a movement.

    The baseline is, can EVs be more efficient than conventional vehicles. If the answer is yes, then the misinformation being put out might be justified - else the EVs may never gain enough traction to be a popular thing. An analogy would probably be the iPhone - Android is probably better as a device now, but iPhone definitely changed the 'game'.

  5. yes i agree. EV research is still in its nascent stage. heavy research is going in the material science area to increase the power density of the batteries which may reduce the overall losses. research is going on in power management algorithms which may lead to utilizing the available power in best possible manner.

    as you said, the EV/HEVs in todays market may not be the best options with respect to overall fuel economy, but yes the motivation given by the obama government is definitely working for companies to invest millions to check whether EVs could be the future or not

    (and hence its a nice debate.. IC Engines vs Hybrids!! will IC Engines die? )