Comparing ICE and EV Use-Phase Emissions

Introduction

Fig. 1: Side-by-side fuel economy comparison of the 2023 Tesla Model 3 RWD, Tesla Model Y AWD, Toyota Camry LE/SE, and Toyota RAV4. [3] (Courtesy of the DOE)

In 2020, transportation accounted for 27.2% of U.S. greenhouse gas emissions (GHGs) as the leading emitter across all U.S. economic sectors. [1] As efforts to curb carbon emissions intensify, the shift from traditional internal combustion engine (ICE) to electric vehicles (EV) has emerged as a critical component of decarbonizing the transportation sector. EVs are frequently praised for their potential to significantly reduce carbon emissions, especially when powered by renewable energy sources. However, the diversity of energy generation across the U.S. raises the question: where are electric vehicles better for the environment than their ICE counterparts?

We seek to answer this question by comparing the emissions associated with the energy consumption of electric and ICE vehicles in their use-phase operation. It specifically compares the carbon dioxide equivalent (CO₂e) emissions associated with gasoline combustion and vehicle charging across various grid mixes. CO₂e is a unified measure that standardizes the impact of various GHGs on global warming and is calculated by the formula

In this equation, Mass represents the amount of a non-CO₂ GHG under consideration and Global Warming Potential is a dimensionless factor specific to that GHG, indicating its comparative potency to carbon dioxide in warming the atmosphere over 100 years. Using CO₂e allows us to aggregate the environmental impact of diverse gases into a single metric, offering a more holistic and accurate representation of the environmental impact associated with both electric and ICE vehicles.

Method and Scope

We shall focus on four passenger vehicles: the Tesla Model 3, Tesla Model Y, Toyota Camry LE/SE, and Toyota RAV4. The side-by-side comparison illustrated in Fig. 1 shows the fuel economy for each vehicle measured in kWh/mile for the electric variants and gallon/mile for the ICE vehicles. These four vehicles were chosen based on their status as top-selling vehicles during 2022. [2] This selection encompasses the car and sport utility vehicle (SUV) passenger vehicle types, with the Tesla Model 3 and Toyota Camry LE/SE classified as cars and the Tesla Model Y and Toyota RAV4 as SUVs.

Using these specific vehicles fuel efficiencies, we can estimate and compare the use-phase emissions associated with their operation.

Use-Phase Emissions Assessment

To assess the use-phase emissions generated during each vehicles operational lifespan, we take into consideration its fuel economy and energy source using the U.S. Department of Energy's published 2023 Fuel Economy Guide and carbon intensity data from the Environmental Protection Agency (EPA). [3,4]

Because the carbon intensity of electricity generation varies regionally, we use the EPA's 2021 Emissions & Generation Resource Integrated Database (eGRID) for the output emissions associated with electricity generation in lbs CO₂e/MWh across different state grid mixes. Table 2 features the state electricity grid resource mix for West Virginia, Utah, Colorado, California, Vermont, and the U.S. average grid mix in addition to its output emissions.

As illustrated in Fig. 1, the fuel economies for the Tesla Model 3, Tesla Model Y, Toyota Camry LE/SE, and Toyota RAV4, are 25 kWh/100 miles, 38 kWh/100 miles, 3.1 gallon/100 miles, and 3.3 gallon/100 miles respectively. [3] Employing dimensional analysis and the mass of one gallon of gasoline, which is 2.84 kilograms, we determine that the carbon intensity of gasoline upon combustion is 8.93 kilograms CO₂e/gallon. By multiplying the carbon intensities of both gasoline and electricity by the fuel economies specific to each vehicle, we can derive estimates for their respective tailpipe emissions. Table 3 features these derived use-phase emissions estimates in grams CO₂e per mile. For a more reasonable level of precision, values have been rounded to two significant digits.

Results and Conclusion

The results in Table 3 illuminate a substantial difference in use-phase emissions between electric and ICE variants, emphasizing a clear trend of lower use-phase emissions for EVs across grid mixes. Furthermore, Table 3 highlights the considerable variation in EV use-phase emissions across the U.S. due to the diverse energy sources that comprise the electricity mix powering these vehicles.

It is important to recognize that the carbon footprint of an ICE vehicle remains fairly constant during its use phase, primarily due to the stable carbon intensity of gasoline combustion. On the other hand, EVs are powered by electricity grids that are progressively transitioning toward cleaner and more sustainable energy sources, allowing them to become less polluting over time. This underscores that as the U.S. electrifies its transportation sector, a concurrent transition in electricity generation towards less carbon-intensive sources is essential to realize the full decarbonization potential of EVs.

© Haley Stafford. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2020," U.S. Environmental Protection Agency, EPA 430-R-22-003, April 2022.

[2] M. Wayland, "Tesla Breaks into America's Bestselling Cars List For 2022, But Trucks Still Dominate," CNBC, 7 Jan 23.

[3] "2023 Fuel Economy Guide, Model Year 2023," U.S. Office of Energy Efficiency and Renewable Energy, October 2023.

[4] "eGRID Summary Tables 2021," U.S. Environmental Protection Agency, January 2023.