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CMU county-level study shows plug-ins have larger or smaller lifecycle GHG than gasoline ICE depending on regional factors

A US-wide county-level study comparing lifecycle greenhouse gas (GHG) emissions from several light-duty passenger gasoline and plug-in electric vehicles (PEVs) has found that PEVs can have larger or smaller carbon footprints than gasoline vehicles depending on regional factors and the specific vehicle models being compared.

The team from Carnegie Mellon University led by Dr. Jeremy Michalek accounted for regional differences in emissions due to marginal grid mix; ambient temperature; patterns of vehicle miles traveled (VMT); and driving conditions (city versus highway). Their open-access paper is published in the journal Environmental Research Letters.

Past studies have shown that life cycle plug-in electric vehicle (PEV) emissions depend heavily on the assumed electricity grid mix, driving patterns (including drive cycle and distance) and climate (including ambient temperature). These factors vary regionally, so PEV emissions implications also vary regionally. Several studies have assessed regional differences in PEV emissions incorporating subsets of these factors—with most focused on regional grid mix, but no study has accounted for the combined influence of consequential grid emissions, driving patterns, and temperature heterogeneity in assessing regionally-specific life cycle implications of PEVs in the US.

—Yuksel et al.

Estimated difference in life cycle GHG emissions (gCO2eq mi−1) of selected plug-in electric vehicles (2013 Nissan Leaf BEV, 2013 Chevrolet Volt PHEV, and 2013 Prius PHEV) relative to selected gasoline vehicles (2010 Prius HEV and 2014 Mazda 3). In each case blue indicates that the PEV has lower GHG emissions than the gasoline vehicle and red indicates that the PEV has higher GHG emissions than the gasoline vehicle. Yuksel et al. Click to enlarge.

The CMU team compared life cycle CO2 emissions of five existing vehicle models:

  • Nissan LEAF BEV
  • Chevy Volt PHEV (EREV)
  • Toyota Prius PHV PHEV (blended)
  • Toyota Prius HEV
  • Mazda 3 (with i-ELOOP)

They selected the vehicles based on availability of Argonne National Laboratory vehicle test efficiency data at high, low, and moderate test chamber temperatures.

They assigned driving conditions to each US county based on urbanization level; assigned vehicle miles traveled (VMT) patterns to counties based on data from the National Household Travel Survey (NHTS) for the corresponding state; and assigned marginal grid emission factors for each North American Electric Reliability Corporation (NERC) region to the counties that lie in that region.

They then estimated the energy consumption rate for each vehicle based on Argonne’s Downloadable Dynamometer Database (D3) temperature-controlled chamber vehicle test data together with information on temperature, drive cycle, and VMT patterns for each county. They used energy consumption and VMT patterns to compute timing and duration of vehicle charging.

They then derived estimated life cycle CO2 emissions for each vehicle type and location by adding vehicle and battery manufacturing emissions, gasoline combustion and upstream emissions (based on computed gasoline consumption), and electricity production and upstream emissions (based on computed electricity consumption, timing, and location).

Framework for the analysis. Yuksel et al. Click to enlarge.

Among their findings:

  • The Nissan LEAF produces lower life cycle GHG emissions than the Prius HEV in urban counties of Texas, Florida, and much of the southwestern US. In most of the rest of the country, the LEAF increases GHG emissions relative to the Prius HEV, with those increases being most notable in the Midwest and in the South.

    The researchers said this is due to the combined effect of grid carbon intensity, highway driving, and regional temperature. The Northern Midwest has a combination of a coal-heavy electricity grid, rural counties (with an assumed highway driving cycle), and cold weather that all contribute to higher relative emissions for the BEV.

  • The Chevrolet Volt PHEV has higher life cycle emissions than the Prius HEV in all counties. This is because the Volt consumes more gasoline per mile in charge-sustaining mode (after the battery is depleted) than the Prius HEV, and it consumes more electricity per mile than the LEAF in charge-depleting (CD) mode (when the battery is charged) at high temperatures.

    In cold weather the Volt consumes both gasoline and electricity in CD mode.

  • The PHEV Prius produces lower life cycle GHG emissions than the HEV Prius in Texas, Florida, and the southwestern US as well as in most urban areas, but it produces higher emissions in many rural areas across the country—especially in the Northern Midwest. This is because the PHEV Prius consumes less gasoline than the HEV Prius in city driving conditions and more gasoline than the HEV Prius in highway driving conditions.

    Differences between the HEV Prius and the PHEV Prius are generally less pronounced than those comparing the HEV Prius to the Volt or the LEAF, the team found.

  • Relative to the Mazda 3 conventional vehicle, (1) the LEAF reduces GHG emissions in urban counties across the US as well as suburban and rural counties in Texas, Florida, the Western US, and New England while increasing GHG emissions in the rural Midwest; (2) the Volt reduces GHG emissions in urban counties across the US while increasing GHG emissions in rural counties of the Midwest and the South; and (3) the Prius PHEV reduces emissions in all counties. In all three cases the GHG emission reductions in urban counties can be substantial.

Assessing their findings for life cycle CO2 emissions for each vehicle in various selected counties, the researchers concluded broadly that:

  • The effects of regional climate and grid mix on emissions become more important for vehicles with higher degrees of electrification. All vehicles have higher emissions in Minnesota, a colder state, compared to California. However, the increase in emissions is largest for the Leaf BEV, whereas only a slight increase is observed with Mazda 3 CV.

    (Both batteries and engines are less efficient when cold, but gasoline vehicles are able to use waste heat from the engine to heat the cabin, while BEVs and EREV PHEVs need to draw energy from the battery to heat the cabin. PEVs thus tend to have larger energy penalties in cold weather regions than conventional gasoline vehicles.)

  • The effect of driving cycle on emissions becomes more prominent for vehicles with lower degrees of electrification. In counties with similar climate conditions and grid mix, they observed that the biggest change in emissions with highway driving compared to city driving occurs with Mazda 3.

Radar chart showing Nissan Leaf life cycle emissions in gCO2eq mi−1 from different cases in selected counties. Yuksel et al. Click to enlarge.

Our results suggest that the GHG-reduction benefits of PEVs have significant regional variability due to grid mix, temperature, and driving conditions as well as differences among vehicle alternatives within each technology class. This suggests that a regionally-targeted vehicle-specific strategy to encourage adoption primarily in areas where specific PEVs provide the largest benefits could increase the GHG reductions achievable under a given budget.

… Broadly, regional policies that are more aligned with the GHG benefits we estimate could be more efficient at achieving GHG reductions, though other factors such as regional consumer preferences, political climate, and other externalities also affect regional policy choices. In general, policies that target GHG reductions directly, such as carbon tax or cap-and-trade policies, rather than favoring specific technologies, are likely to be more efficient at achieving GHG reductions, though support for the development and deployment of new technologies can also have dynamic benefits and potentially lead to large long-term benefits if they enable a fleet transition that would not have happened otherwise.

—Yuksel et al.


  • Tugce Yuksel, Mili-Ann M Tamayao, Chris Hendrickson, Inês M L Azevedo and Jeremy J Michalek (2016) “Effect of regional grid mix, driving patterns and climate on the comparative carbon footprint of gasoline and plug-in electric vehicles in the United States” Environmental Research Letters 11 doi: 10.1088/1748-9326/11/4/044007



The 4500 GM Volt owners in our region are taking a 'Class Action' against GM because their Volts do not operate from the main battery when outside temperature falls below -5C.

Those Volts become ICEVs (only) below -5C with rather poor/high fuel consumption at less than 50% the claimed empg.

Wonder how many Volt owners knew that?


So they compared everything with a Mazda that gets 33mpg (combined) when the US Fleet average is 21.6mpg. So CMU is giving the ICE vehicles a 55% advantage and pretending to do an unbiased study.

Typical. Who paid for this study? Or am I supposed to think people at CMU are that freaking incompetent?


Comparing similar vehicles, not the average vehicle fleet, is proper technique.  A Volt does not fill the same role as a 1-ton pickup truck.


This is the same backwards thinking that encourages people to do nothing!

The problem is the local coal plant Not the EV. If you switch from a coal plant to Wind and solar the Leaf "becomes" the cleanest car!. This immediately points to the coal plant being the problem.

How about requiring all states to get their grids to the point where the entire country is better off with EV's. How about mandating a percentage of clean energy in the next decade!


So an EV is not as good where coal power plants are dominant, we knew that.


EVs still run on domestic energy rather than imported oil, and night-time charging falsifies the claim that base load generators are obsolete.


The Mazda 3 is a compact and the Leaf is a midsize. They're not that big of a difference in size, but a first Gen Leaf compared to a high mileage ICE vehicle is still a bad comparison.

But let's ignore that. The US grid is declining in carbon intensity by about 1.5% per year. This means that during the first 10 years of each vehicles life, the leaf will use a grid that is on average, 8% cleaner than the one we have today. Can the ICE car do that? LOL

If another Democratic President comes in, which seems very likely with Trump and Cruz driving the GOP clown car, then that rate will accelerate.

As you later pointed out, they assume that EV charging is always done in a stupid manner. Many people are smart enough to charge at times when the grid is cleaner and many utilities are giving them incentives to do so.

And also as you point out, EVs run on domestic energy. Ignoring that point is simply stunning to me when you consider the claimed goals of "USA, USA, USA!!!" by the GOP. They're too stupid to know that the oil companies and specifically the Koch brothers have brainwashed them.

It's rather pathetic to watch.

The simple, effective solution for PEV drivers is to make sure your electricity is sourced from clean renewable fuels. This can be direct, with solar, or indirect, though your utility green energy program. Community solar can also be purchased.

As with the car, purchasers have to exercise some diligence to be part of the solution. But it's not really that hard.


It doesn't matter what you buy; if you are charging your EV overnight, PV is not generating the energy for it.  These frauds need to be recognized for what they are.


PV is only one form of renewable energy. Wind, hydro, biomass, tidal, and geothermal all work at night. Even solar thermal can work at night if you store the heat.


Recalculate the Mazda 3 and Prius hybrid when filling up on E30 fuel. Or better yet the Ford focus flex filling up on E85. Corn ethanol without the indirect land us penalty is rated at close to -60% carbon emissions as compared to plain gasoline. The indirect land use penalty is indiscriminately applied only to corn ethanol and is proven to be largely inaccurate. Also, indirect penalties are not applied to competing fuels. Changing to E85 cellulosic with optimized engine per Cummins engineering decreased carbon intensity by 85%. Poet is ramping up their cellulosic to full capacity by year end. Wouldn't this alternative fuel change the math drastically?


To continue to 'grow' liquid fuels for our gas guzzlers if far from being a bright idea? Could be interesting for airline jets, essential chemicals etc.

The ecological mess being created in Alberta to extract oil for tar sands is not much better?

Using Hydro, Wind and Solar energies (with storage) instead of CPPs and NGPPs will be the winning energy mix in the near future.

NPPs could also be interesting and added to the energy mix if the initial cost could be reduced by 50% to 75% with the same safety level.

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