MPGe and the Confusing World of Auto Efficiency
Friday, August 26, 2011 at 04:05PMA problem in today’s world of automobiles is how to fairly compare the efficiency of vehicles with differing types of engines and fuels. One solution is Miles per Gallon Energy Equivalent (MPGe), used by the X Prize for their automotive competition and by the EPA to rate electric vehicles and plug-in hybrids. MPGe is a useful standard: for example, it brought the Nissan Leaf’s initial claim of 367 MPG down to a more realistic 99 MPGe.
MPGe compares the energy content of various fuels to that contained in a gallon of gasoline: approximately 115,400 BTUs, as shown in the following table:
BTU FUEL QTY UNIT
115,400 Gasoline 1.000 US Gallon
115,400 E85 1.414 US Gallon
115,400 Diesel 0.899 US Gallon
115,400 Electricity 33.7 kWh
Edison2 chose to run our X Prize cars on E85 for technical reasons, and because of the MPGe metric there was no penalty for the fuel’s inherently low energy content, just as there was no advantage for someone running a higher energy content fuel like diesel. It’s the energy content of the fuel compared with a gallon of gas that’s at stake, not the volume.
The bottom line is that a gallon of gas has the same energy content as about 1.4 gallons of E85 or 0.9 gallons of diesel. How far you go on this quantity of any of these fuels is your MPGe figure. Here’s how this works out:
FUEL MPGe MPG Gallons / 100 miles
Gasoline 100 100 1.000
E85 100 70.7 1.414
Diesel 100 111.2 0.899
With electricity the process is similar. Fully charge the battery pack, run the car, and measure with a calibrated meter the electricity needed to fully recharge. This method – “plug to wheels” – was used by the X Prize and is used by the EPA.
But some people, including those of us at Edison2, see a problem with the “plug to wheels” metric: it fails to take into account generation and transmission losses associated with electricity. For example, in 2004 6.4% of the electricity generated in the US was “lost” in transmission and distribution (this figure is higher in many countries, such as 15.8% in Mexico), and half or more of the energy generated at a coal-powered electric plant is lost as heat. Thermal conversion in an internal combustion engine, of course, occurs under the hood, but a plug-to-wheels accounting for EVs does not consider energy generation.
But there are metrics that look at the bigger picture of efficiency and measure vehicles “well to wheels”. Argonne National Lab’s GREET model (Greenhouse gases, Regulated Emissions and Energy use in Transportation) looks at the upstream efficiency of fuel sources – such as transmission and generation losses – and was used by the X Prize in calculating emissions. Likewise the CAFE standards for emissions will gradually factor in the upstream emissions.
But these issues with MPGe are only the tip of the iceberg when it comes to confusing or even misleading claims for MPG, due in part to the differences world-wide in assessing efficiency. As an example, VW made headlines this year with their 313 MPG diesel plug-in hybrid XL1 concept. However, this admittedly very efficient two-seater is rated using imperial gallons, which are 25% larger than US gallons, on the European NEDC testing cycle, which does not take into account the energy stored in the car’s 5 kWh battery. Considering these factors, and the higher energy content of diesel, the XL1 gets in the neighborhood of 106 MPGe (but because NEDC is a different drive cycle than the stricter EPA cycle an accurate comparison is really not possible).
MPGe may not be perfect, but it is a good and reasonably fair place to start in comparing automobile efficiency. Adding in upstream energy losses – for all fuels – would make it even better.
Reader Comments (9)
Wow, your opening up a big can of worms with this blog post. Perhaps you were looking to get some additional traffic to the site?
I prefer looking at the vehicles efficiency fuel independent. So far that has been the company line from Edison2. We can power our efficient vehicles from any fuel source and our cars will go farther than any other vehicle on the planet. That is the beauty of lightweight efficient vehicle. The efficiency speaks for itself and as different folks see merit in various power sources the vehicle can accommodate that source. Build and EV, build a diesel, gasoline, fuel cell...flux capacitor, whatever.
I do fully agree with the last paragraph of the post. We have established a starting point for comparing various fuel source efficiencies, and we just need to keep having civil conversations to try and improve the way we compare them.
Thanks for laying out a very clear explanation regarding MPGe. A comprehensive look at any future automobile must include the various elements which comprise MPGe. Other factors must be included as well if the vehicle is to be manufactured and widely used -factors that are advantageous to a manufacturer as well as a consumer; factors involving manufacturing cost and profit as well as safety, reliability, sales price, cost and convenience of operation.
Surely I am clueless to a great deal in life, but I am amazed that some of these factors were not better thought out in the case of the Nissan Leaf. I don’t think people have a clue as to how far away the electric car actually is. Imagine for example that the cost of the batteries could be reduced significantly to the point where the Leaf’s cost could be reduced from $28,000 to that of the Versa, say $11,000. One could even make the cost of the Leaf less, say $9,000. Which of you, standing in the Nissan showroom, is going to choose a 50 -100 mile range with an 8 hr refueling time compared to a 350-400 mile range with a 5 minute refueling time?
If you really want to the entire picture of energy consumed by the car, you would certainly need to look at the energy it takes to find, extract, process, and transmit/transport the fuel. In addition, you would also need to look at the amount of energy it takes to manufacture, maintain and replace the vehicle. Once that is accomplished (along with other calculation that I have probably forgotten), then you can have a truly complete look at the energy consumption of a vehicle.
While, this is a massive undertaking, I think it is necessary to understand the true energy costs of owning and operating a vehicle. If a car is built so lightly that parts are constantly breaking and in a couple of years the entire car must be replaced, then it hardly matters if it gets incredible fuel milage. Beyond the aggravating inconvenience of having to replace things all the time, this would be an extremely energy intensive car. As long as the Very Light Car is durable, then I suspect that it would perform extremely well under such a test of energy efficiency.
The diagram for the gas car is misleading because it does not show the many upstream energy costs and externalized social and environmental costs of a gallon of gas. The true cost of gas is estimated to be around $15 dollars per gallon. Energy costs per gallon of gas must include the energy used in exploration, extraction, refinement and delivery, as well as the massive externalized energy costs associated with oil wars, military policing of oil supply channels, compromised health, and environmental degradation.
The diagram for the electric car could have simply shown a solar panel recharging the car. Really it makes good economic sense and would be great environmental and energy security policy to sell every electric car with a matched set of solar panels or some other form of sustainable energy supply. Ford already plans to offer a specially designed set of solar panels with its Focus EV.
The VLCe has the major advantage of needing only half the solar panels and battery capacity to achieve even greater range than the Focus or Leaf.
Wouldn't it be delicious if, for a given drive distance, the electric car consumes less electricity than the amount used in refining gasoline for an internal combustion car? I wonder if that is the case for the VLC versus a gas guzzler SUV.
I asked my chemical engineering friend who used to work for a petroleum company what he thought of the numbers below. He got a little upset, refused to answer and told me that in our area, most electricity is made from burning petroleum. Then suggested that if I wanted to be really environmentally friendly I should eat my dog.
http://gatewayev.org/how-much-electricity-is-used-refine-a-gallon-of-gasoline
interesting comparison, but I think the only relevant thing to consider is vehicle efficiency. The fuel source is pretty much irrelevant. That's the nice thing about a hyper-efficient vehicle like the VLC. Everybody has their reasons for wanting a particular power-plant, but in the end, if the platform itself is efficient, then whatever you choose, you get the most bang for your buck.
Gasoline doesn't appear out of thin air.
As time goes on, it takes more and more energy to find and extract oil from the ground, and it yields less gasoline per barrel -- heavy sour crude or worse yet, tar sands bitumen take far more energy to produce each gallon of gasoline.
The best numbers I have seen are that it takes ~7.5kWh of electricity FOR EACH GALLON OF GASOLINE. This amount of electricity if used directly in a Nissan Leaf will take it about 24 miles, so all the rest of the carbon footprint (for the oil itself and the other energy used (natural gas and the fracking to get *it*) would be saved. Oh, and the 10% ethanol probably has more energy overhead than it contains.
E85 has an even bigger carbon footprint than gasoline.
Besides, you can get electricity from your own rooftop or from a wind turbine or other renewable source. So ALL the inefficiency of a heat conversion plant goes away, completely.
Neil
the error is that electricity on your roof top is 'free'. The collection of materials (mining), the transportation of those, the refining, the delivery, the manufacturing, the product delivery etc. all cost energy. If solar energy was so cost-effective, we'd be seeing a LOT more of it -- the market is pretty good about sorting that out. EV's are NOT cost-effective now, nor will they be in the near future, AND, rarely is a apples to apples comparison done between an EV, with it's very aero shape, low rolling resistance and light-weight materials to an IC of equal basic construction. As the VLC has shown 100 mpg is certainly possible for an ICE-powered vehicle. That will be a pretty tough act to follow for an EV. Still waiting on those miracle batteries, and the 5 minute charge...
Wow, your opening up a big can of worms with this blog post. Perhaps you were looking to get some additional traffic to the site?