Let’s get Science-y
The second law of thermodynamics states that heat cannot be converted entirely to mechanical energy, and that some amount of waste heat will always be generated and lost to the environment. The measurement of how much energy is lost as heat can be expressed as thermal efficiency, or the fraction of heat that becomes useful work (i.e. moves the vehicle).
An engine with 100% thermal efficiency, referred to as a Carnot engine, would require a frictionless system operating under constant internal temperature, with an identical external temperature. Such an engine is only possible in theory. In the real world, modern automobile engines can have hundreds or even thousands of moving parts, providing many instances of friction and heat waste. The highest possible thermal efficiency for a combustion engine is 35-40%.
It's not just engine losses, either. Vehicle efficiency is further compromised by drivetrain losses. The drivetrain includes components such as a clutch or torque converter, a gearbox, propeller shaft, differential, and drive shafts. It is typical to see drivetrain losses of roughly 15% for two-wheel drive vehicles, with higher losses in all-wheel drive vehicles. Taken together, only 15-30% of the energy derived from gasoline is translated to the wheels, depending on the drive cycle.
On the other hand, vehicles using electric motors are able to convert roughly 80% of electrical energy into mechanical energy. This is primarily due to the fact that electric motors have far fewer moving parts - sometimes as few as 20. Fewer moving parts mean fewer places for friction to occur. Additionally, electric motors generate significantly less heat than gasoline counterparts, meaning more energy is going directly to moving the vehicle.
In addition to a significantly improved baseline efficiency, EVs also have the ability to recapture energy through regenerative braking. In conventional vehicles, braking converts mechanical energy into heat, which is then lost to the atmosphere. With an electric vehicle, braking can generate a negative torque, allowing the electric motor to behave like a generator, thereby charging the battery. As a result, nearly every EV on the market is more than twice as efficient as the most efficient gasoline vehicles.
Can we compare EV efficiency to gas car efficiency?
In order to directly compare the efficiency of an EV to a gasoline vehicle (i.e. fuel consumed per mile driven), we must first understand the basics of electricity as a fuel source. Electricity consumption in electric vehicles is identical to the consumption with other electronics, such as laptops, but varies by order of magnitude. For a quick refresher on this topic, see our article on kWh for electric vehicles.
The simplest way to compare the efficiency of a gasoline vehicle to an EV is through MPGe. According to the EPA, the top performing gasoline vehicle is the Hyundai Elantra Hybrid Blue, rated at 54 MPG combined, over twice the efficiency of the average vehicle (note: the non-hybrid version drops down to 37 MPG combined). Depending on vehicle choice and drive style, modern EVs can exceed 130 MPGe, as is the case with the Tesla Model 3.
Of course, not all EVs are created equal, and efficiency can be significantly impacted by factors such as driving style and environment. However, using an official standard, such as MPGe, allows us to compare several examples. As mentioned, the most energy-efficient EV in the market is currently the Tesla Model 3, which boasts an official rating of 132 MPGe combined. On the other end of the spectrum, the GMC Hummer EV pick-up is rated for 47 MPGe combined. This particular EV is still more efficient than the best non-hybrid gasoline vehicle, despite it being a poor performer in the EV space.
Does efficiency even matter?
At this point, you may find yourself asking: if EVs are inherently more efficient and non-polluting, why should we care about efficiency? Although not a simple answer, there are a few major points to consider:
- EVs require critical minerals that must be mined from the earth. The acquisition of such minerals jeopardizes the health of the environment and local communities that often face direct contamination and climate change impacts.
- The majority of EVs on the road today use the existing electric grid for charging. Although an increase in EVs is not the biggest threat to the grid, it is facing significant challenges from natural disasters and climate change. The more efficient EVs are, the less of a strain on the grid.
- More efficient EVs can use the same size battery to get more range, which is a major selling point. Not only does this make EVs more cost-effective for consumers, it also reduces the amount of resources needed to produce each vehicle, keeping costs low.
Electric motor efficiency is already considered to be almost as good as possible, given the limitations of physical materials.
If automakers want to increase EV efficiency, the two major ways to accomplish it are improving aerodynamics and tire selection.
Improving Efficiency: Aerodynamics
Aerodynamics is the study of how moving objects interact with the air. The two main components that can affect efficiency are downforce and drag. Downforce helps keep the car grounded and improves handling and brake response, particularly at high speeds. Drag, on the other hand, counteracts forward movement and makes the vehicle require more power to maintain a particular speed. Both of these forces are dependent on speed, the shape of the front area of the vehicle, and an experimentally-determined mathematical coefficient based on the specific shape and design of the vehicle. In short, vehicles that looks slicker and more streamlined often are designed to have lower drag.
Drag is measured using a drag coefficient that is determined by testing. With its excellent efficiency, it should be no surprise that the Tesla Model 3 is one of the most aerodynamic cars in the world, with a drag coefficient of 0.23. A standard automobile has an average drag coefficient of 0.30 to 0.35. The Hyundai Elantra mentioned above has a drag coefficient of 0.28, while the Hummer EV comes in above 0.50.
Studies indicate that the aerodynamic drag of a vehicle is responsible for a large part of a vehicle’s fuel consumption and may contribute up to 50% of the total vehicle fuel consumption at highway speeds. In more applicable terms, a 10% reduction in aerodynamic drag will improve highway fuel economy by roughly 5%. More aerodynamic vehicle design helps manufacturers meet fuel efficiency regulations.
Are aerodynamics more important for EVs than for gas vehicles?
Arguably, yes, for two main reasons.
- As discussed above, gasoline engines are quite inefficient, but can be improved using newer technological advances (e.g. cylinder deactivation) and the inclusion of hybrid systems. Advances in electric motors are unlikely, as they’re already incredibly efficient. This means that other areas need to be improved to increase EV efficiency.
- The efficiency of gasoline vehicles at highway speeds is better than at low speeds, despite increases in drag force. This is because energy usage is highest when idling, accelerating, and decelerating. Simply put, gasoline vehicles are most efficient when they remain at a constant moderate speed.
For EVs, the motor efficiency is less speed-dependent. Since the drag force increases in proportion to the square of the speed, an electric car vehicle becomes exponentially less efficient at high speeds.
Improving Efficiency: Tires
While manufacturers don’t need to reinvent the wheel, so to speak, they do face particular challenges with tires for EVs. EVs are significantly heavier than similarly sized gasoline vehicles, given the large batteries strapped to the frame. Additionally, electric motors generate large amounts of engine torque, which places additional strain on the tires. Therefore, tires need to be able to carry a heavier load and withstand high torque, while maintaining comfort, minimal rolling resistance for optimal range, and traction when needed most.
Increased fuel economy through energy efficient tires matters for all vehicles, but is particularly important for EVs because of their high efficiency motors. Pirelli estimates that the tire's influence on an EV's range is anywhere from 20 to 40%, whereas the same influence on an ICE vehicle is between 15 and 20%. Similarly, Michelin claims ~5% of the energy an ICE vehicle generates goes to the tires, whereas tires on an EV consume ~16%. Many major brands offer EV-specific tires, which have become increasingly popular as consumers shift towards environmental friendliness and look for ways to reduce fueling expenses as gas prices fluctuate. These tires offer unique construction materials and design patterns that decrease rolling resistance while maintaining traction and safety under various road conditions. The materials used for EV tires are also significantly more important for EVs, where it is common to see a 20% decrease in tire lifespan. While this is a common argument for why EVs aren’t more environmentally-friendly than ICE vehicles, improved material science will help EV tires last longer.
Why Efficiency Matters: Energy Sources
It is estimated that roughly 60% of electricity generation in the US is derived from fossil fuels, including coal, natural gas, petroleum, and other gasses. Generally speaking, this means that simply driving an EV does not decouple you from dirty energy sources. It does, however, reduce your overall carbon footprint, primarily through the increased fuel efficiency. Additionally, some areas of the country are advancing the clean energy movement at a rapid pace, further reducing reliance on fossil fuels and improving the carbon footprint of EVs.
Let’s compare this to gasoline, the primary fuel for vehicles on the road today. The production of gasoline is complex and energy-intensive, to say the least. The process begins with extraction of its main ingredient: crude oil. Extracted oil is then transported to refineries, where fractional distillation is used to break crude oil down into various distillates, one of which is gasoline. Following refinement, gasoline must then be distributed through pipelines and tankers, where it can finally arrive at the station. Although difficult to measure precisely, it is clear that this process requires large amounts of energy, with estimates of 1-6 kWh of electricity per gallon of gasoline produced.
Setting aside pollution, the use of dangerous chemicals, and the complexity of this process, might it be more efficient to simply produce electricity that can be directly used to power a vehicle?
The answer is yes.
Why Efficiency Matters: Conservation
Electricity demand is expected to increase dramatically in the future with more extreme temperature fluctuations related to a changing climate, increased adoption of electric vehicles, and increased reliance on electronics (e.g. computational work, crypto mining). Areas of the country with heavy dependence on dirty energy sources ought to pay particular attention to the efficiency of all vehicles, including EVs. In Louisiana, for example, only 4% of net electricity generation comes from renewable sources. Relatedly, Louisiana ranks 50th in the US for air pollution.
Compare this to the state with the least air pollution, Vermont, which makes nearly 100% of its electricity from renewable resources.
Although driving an EV in Louisiana will significantly reduce your carbon footprint and likely reduce fueling expenses, it does not eliminate your dependence on fossil fuels. Therefore, to minimize your contribution to both energy usage and pollution, efficiency is of the utmost importance, even when driving an EV.
Written by Brandon August, a lifelong explorer of all things academic. After obtaining an undergraduate physics degree and a doctoral degree in biomedical, he began to explore various professional fields in health and wellness, rideshare work, freelance writing, and day trading. On the recreational side, he has always been involved in the automotive field, owning various vehicles across the years. After a recent move to California, he entered the EV space, purchasing both a Chevrolet Bolt EV and a Bolt EUV for his household.