Electric vehicles (EVs) are often disparaged for being short on range, but some critics also question whether they’re truly “greener” than their internal combustion engine (ICE) counterparts. (For the purposes of this article, “greener” refers to lower greenhouse gas emissions.) Such complaints about electric vehicles typically address three problems: 1) the environmental impact of battery manufacturing, 2) the source of electricity used to charge the vehicles, and 3) the power-line losses associated with distributing electricity. It’s true that mining for battery materials is a dirty business, and it’s also a fact that making batteries is an energy-intensive process. Furthermore, a fair portion of our electricity comes from burning fossil fuels—effectively moving the tailpipe from the car to the power plant. And finally, the transmission and distribution of electricity isn’t 100 percent efficient, nor is the charging process itself, so not all of the power that’s generated at the plant makes it to the EV’s powertrain.

Figure 1. Which vehicle is greener: gasoline or electric?

EV enthusiasts will counter these criticisms by pointing out that it takes energy to extract, transport and refine crude oil—not to mention the energy used by the military to protect U.S. oil interests around the globe. It’s also worth noting that large power plants—even those fired by fossil fuels—are more efficient than automobile internal combustion engines. Lastly, a hefty amount of fuel is burned by the tanker trucks that deliver fuel from the refineries to your local filling station.

Both sides make valid arguments. Ultimately, it comes down to the math, so engineering.com explored the research and found a few scientific studies that shed light on the issue of whether EVs are in fact greener than ICEs. Having examined the numbers, we can say definitively, without hesitation … (drumroll, please) … that it depends. Let’s take a look at the factors.

Manufacturing-Related Emissions

The Union of Concerned Scientists (UCS) conducted a “cradle-to-grave” study comparing the environmental impact of electric vehicles with that of ICE vehicles. UCS considered nearly every variable, including mining for raw materials, processing the materials, manufacturing the vehicles, fueling them for a typical life span, and reusing/recycling/disposing of the materials at the end of the vehicle’s useful life. Figure 2 shows the results of the UCS study.

Figure 2. Life cycle greenhouse gas emissions of various vehicle types. (Image courtesy of the Union of Concerned Scientists.)

For the most part, the researchers determined that the environmental impact of manufacturing a battery electric vehicle (BEV) is similar to that of building an ICE vehicle, with the exception of the batteries, which contain rare materials and require energy-intensive processing. Manufacturing an ICE vehicle, on average, generates about seven metric tons of greenhouse gases, while the typical BEV manufacturing releases 15 percent more than that. UCS determined that the “carbon payback period”—the amount of time it takes for the BEV to make up for the higher manufacturing emissions with lower driving emissions—is about one year, depending on the source of electricity used to charge the BEV batteries. The best-case scenario was 3,700 miles and the worst-case scenario was 13,000 miles. Long-range batteries increase the carbon payback period to 15,000 miles (clean grid) or 39,000 miles (dirty grid).

The study was somewhat limited in that it didn’t examine specific models of each vehicle type, but instead used industry averages. In other words, rather than comparing a Nissan Leaf to a Ford Focus, the researchers created generic BEV and ICE models based on average numbers for each vehicle classification. As we’ll see later, the researchers also didn’t take into account different driving conditions.

Refueling and Driving-Related Emissions

EV critics will argue that an EV’s tailpipe is the smokestack from the power plant. While that might be true, it’s also true that a fair amount of electricity is generated without a smokestack—a trend that will certainly continue, as even fossil fuel companies recognize the need to switch to renewable energy. Figure 3 shows that over the past decade, dirty coal has lost a significant share of the market to cleaner-burning natural gas and emission-free renewable energy.

Figure 3. Electricity sources in 2009 compared to 2018. (Image courtesy of EIA.gov.)

UCS found that with even a small amount of renewable energy on the grid, a BEV produces less greenhouse gas than an ICE vehicle, and in places with high levels of renewable energy, the difference is quite dramatic. The results are shown in Figure 4.

Figure 4. Life cycle BEV emissions based on source of electricity. (Image courtesy of the Union of Concerned Scientists.)

To determine the carbon footprint of fossil fuels, UCS researchers included the effects of drilling for oil, transporting crude oil to refineries, refining petroleum, delivering gasoline to filling stations, and burning fuel in the car’s engine. For electric vehicles, the researchers examined the emissions caused by extracting the fuel (mining for coal, drilling for natural gas), delivering fuel to power plants, burning fuel in power plants, power losses due to transmission and distribution, and EV efficiency.

Driving-Related Emissions by Region

The UCS study examined the sources of electricity by region and used that data, plus the aforementioned numbers, to calculate the miles per gallon equivalent (MPGe) of a BEV. When we say that a BEV’s MPGe rating is 45, it means that the BEV produces the same amount of greenhouse gas emissions as an ICE car that gets 45 MPG. (This is for driving only, not manufacturing.)

As you can see in Figure 5, driving an EV in New York, where most electricity comes from hydro, nuclear, and natural gas, is equivalent to driving an ICE vehicle that gets 135 MPG. In the coal-heavy Midwest, EVs don’t fare as well. Note that these numbers are from 2012, and as mentioned earlier, the proportion of electricity generated by coal is steadily decreasing. It’s safe to say that the Midwest figures are better today, especially with the proliferation of wind farms in that region.

Figure 5. MPG equivalence by region in 2012. (Image courtesy of the Union of Concerned Scientists.)

The US Department of Energy’s Alternative Fuels Data Center has an online calculator that lets users compare the total emissions for different types of vehicles based on each state and its mix of electricity sources. The national averages are given in Figure 6. Note that this data includes two kinds of hybrid vehicles, whereas the UCS study examined only BEVs and ICEs.

Figure 6. Annual emissions based on vehicle types. (Image courtesy of the U.S. Department of Energy.)

Another Factor: Temperature

While the aforementioned numbers address nearly every factor, they don’t account for the climate in which the cars are being driven. Why is that important? On a cold day when you turn on the car’s heater, where does the heat come from? In an ICE vehicle, it’s the waste heat generated by the engine. Normally that’s dissipated into the atmosphere by the car’s radiator, but there’s another radiator—the heater core, which is located closer to the passenger compartment. Turning on the heater in an ICE doesn’t technically turn anything on; it simply redirects some of the waste heat into the passenger compartment.

Since a BEV’s motor is more than 90 percent efficient, there’s not much waste heat to redirect to the passenger compartment, so most EVs use a heat pump—effectively an air conditioner running in reverse—which uses electricity and, consequently, decreases the car’s range. Many EVs include seat and steering wheel heaters, which employ resistive heating. Although heat pumps are more efficient than resistive heating, seat heaters use less power overall, since they warm the person instead of the air inside the car. Either way, any heater will decrease the range of a BEV, forcing the driver to charge up more often.

To address the effects of regional climate on EV performance, researchers from Carnegie Mellon University (CMU) conducted a study that took into account all of the aforementioned factors, plus the local climate. CMU researchers also compared the performances of hybrid and plug-in hybrid vehicles, in addition to ICEs and BEVs, and rather than evaluating industry averages for each type of vehicle, they chose specific models. The framework the researchers used for their evaluation is shown in Figure 7.

Figure 7. CMU framework. (Image courtesy of Yuksel, Tamayao, Hendrickson, Azevedo, and Michalek via Creative Commons.)

Figure 8 compares the emissions from four types of vehicles in two regions of the U.S.: the Western Electricity Coordinating Council (WECC) and the Midwest Reliability Organization (MRO). WECC represents the cleanest sources of electricity, while MRO represents the dirtiest.

Following are a few definitions and points to assist with your review of Figure 8:

● CV is an abbreviation for “conventional vehicle”—what we’ve been calling an ICE vehicle in this article.

● The Mazda 3 is a pretty efficient CV (28 MPG city/38 MPG highway).

● Gasoline upstream refers to emissions associated with extracting, transporting, refining, and delivering fuel to filling stations.

● Gasoline use refers to emissions associated with burning fuel in the car’s engine.

● Electricity upstream refers to emissions associated with extracting, transporting, refining, and delivering fuel to power plants.

● Electricity use refers to emissions associated with the burning of fuel to produce electricity and the transmission and distribution of electricity to charging stations.

Figure 8. Vehicle emissions based on vehicle type and region. (Image Courtesy of Yuksel et al. via Creative Commons.)

Not surprisingly, the conventional vehicle is the heaviest polluter in almost every instance except for highway driving in very cold climates. What is surprising is that hybrids and plug-in hybrids often produce fewer emissions than BEVs. Bear in mind that this study was conducted in 2015. As the upper Midwest moves to more renewable sources of electricity, BEVs will fare much better in this region.

So, What’s the Verdict?

Obviously, it’s not a cut-and-dried decision, but it’s safe to say that if you’re in the market for a new vehicle, an ICE is probably the worst kind of vehicle you can buy in terms of greenhouse gas emissions. Hybrids are a safe bet in most parts of the U.S., and BEVs are the cleanest in regions that make more significant use of renewable energy. As always, your mileage may vary.

If you’re concerned with the additional cost of a hybrid or electric vehicle compared to an ICE, see my analysis of the total cost of ownership (TCO) of an EV vs an ICE. The article includes a downloadable spreadsheet that allows you to compare the TCO of any two vehicles.

Click here for an infographic that summarizes this article.

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