Electrifying transportation reduces emissions AND saves massive amounts of energy » Yale Climate Connections


With high energy prices and increasing urgency to reduce fossil fuel burning, it makes sense to get the most out of every gallon of gasoline or kilowatt-hour of electricity.

A previous post showed that charging an EV costs around $1.41 per gallon in the US, offering consumers a major savings over gasoline. Part of why EVs are cheap to operate is that they use energy with impressive efficiency.

Delving deeper, there’s a stark difference between the way internal combustion and electric engines use energy. The bad news is that combustion engines are fundamentally inefficient. But the good news is that electric motors offer vast improvements and save money and energy. Even better: Replacing traditional vehicles with electric ones will require far less energy overall.

Traditional cars and trucks are surprisingly inefficient

Modern gasoline-powered vehicles waste a whopping 80% of the energy in their fuel. For each gallon pumped into the tank, only a bit more than three cups go to moving the vehicle forward. In economic terms, for a $5.00 gallon of gasoline, only $1.00 of it gets you closer to your destination.

Most of this waste is an inescapable consequence of thermodynamics. Internal combustion engines ignite liquid fuel to create a pressurized gas that pushes pistons to turn a crankshaft that ultimately spins the car’s wheels. This multistep process bleeds off energy all along the way. Most of the energy in the fuel ends up as heat, and only a small fraction reaches the wheels. The concept of wasted heat becomes intuitive when one thinks about the hot air wafting off a car’s running engine. The engine itself gets hot; a cooling system is needed to manage excess heat; and heat is dispersed through the radiator and blows out the exhaust. All of that heat comes from gasoline, and none of it helps propel the vehicle.

Further energy uses come from pumps and fans, some of which, ironically, are needed to carry away waste heat. These are called parasitic losses. Mechanical friction within the transmission and drivetrain lops another 3 to 5% off the overall efficiency. The final loss of energy is from auxiliary electrical components like heated seats, lights, the audio system, and windshield wipers. Taken together, these accessories can consume up to 2% of the vehicle’s total energy intake.

The net result is that only around 20% of the energy that’s pumped into the fuel tank ends up at the wheels.

Even the most fuel-efficient gasoline-powered vehicles can’t sidestep these energy losses. Cars with high fuel economy are lighter, smaller, and more aerodynamic, thereby making the best possible use of the energy that ends up in the drivetrain. Diesel engines have somewhat better thermodynamic efficiency, averaging in the high 30s to around 40%. But major thermodynamic losses are a stubborn fact of life for all combustion-based engines.

For a more detailed explanation and sources for the figure above, see FuelEconomy.gov.

The simple efficiency of electric motors

Electric vehicles are propelled by entirely different mechanisms. Energy enters the vehicle as electricity, which directly powers the drivetrain: EVs need not convert one form of energy to another, which is a big factor in their efficiency

Electric motors are simple machines with few moving parts, especially compared with the complexities of an internal combustion engine. In an EV, electricity from the car’s battery flows into a cylinder that generates a rotating magnetic field. Inside that cylinder is a rotor that spins as it gets pulled along by the magnetic attraction. The spinning rotor turns an axle that drives the wheels.

The whole process works in reverse, too: The car’s spinning wheels can turn the rotor and feed electricity back into the battery. This process of regenerative braking can recapture energy that would otherwise be lost as friction and heat.

EVs are not 100% efficient though, and they lose energy in a few ways. Some energy is lost in the process of recharging the battery, and electricity is consumed for the vehicle’s cooling and power steering. Auxiliary electric use is higher in EVs compared with combustion engines, mostly due to the electricity needed to heat the car’s interior in cold weather. In an internal combustion vehicle, waste heat is used to warm the car’s cabin.

In all, the various energy losses in an EV add up to 31% to 35%. Regenerative braking adds 22% back into the system, making the overall efficiency around 87% to 91%. The specific numbers vary based on the type of car and how it’s used, but the overall simplicity and efficiency is a contrast to traditional vehicles that have been the mainstay of the roadways for 130 years.

The numbers are from FuelEconomy.gov, and DigitalTrends has a helpful explainer for how various components of EVs work.

Transition to EVs will reduce overall amount of energy needed for transportation

The energy efficiency of EVs is a clear boon for consumers, but it offers an even more significant benefit in the transition away from petroleum-burning transportation. In the US, about 8.9 million barrels of motor gasoline are used every day, and around 80% of that energy is wasted as heat and friction. Of the total amount of gasoline burned, only 1.8 million of those barrels (20%) propel vehicles along the road. This means that if the gasoline vehicle fleet was replaced with EVs, those EVs would need the energy equivalent of only around 1.8 million barrels of gasoline per day, plus the 11% energy loss within the EV itself. The rough math pencils out to the energy equivalent of around 2 million barrels of substantial gasoline per day, which is a savings over the 8.9 million barrels currently used.

Of course, this begs the question of the efficiency of electric power plants that charge EVs. Thermal power plants – such as coal, gas, or nuclear – face similar thermodynamic challenges as internal combustion engines, but power plants are more efficient than cars. Coal and nuclear are around 33% efficient, and combined cycle natural gas power plants are about 44%. At the top end of the scale, hydropower is approximately 90% efficient. Even if the grid were entirely fueled by coal, 31% less energy would be needed to charge EVs than to fuel gasoline cars. If EVs were charged by natural gas, the total energy demand for highway transportation would fall by nearly half. Add in hydropower or other renewables, and the result gets even better, saving up to three-fourths of the energy currently used by gasoline-powered vehicles.

But what about batteries? Manufacturing an EV battery consumes the energy equivalent of about 74 gallons of gasoline. Over the 10-year lifespan (or more) of the battery, the energy investment in the battery is far too small to change the outcome – which is good news.

Decarbonizing the world’s energy supply is an enormous and daunting task. But at least in this case, the job gets easier as highway transportation shifts away from oil. The major improvement in driving efficiency offered by EVs means that vehicles can emit less carbon and less pollution, while also lowering overall energy demand. In a world of tough tradeoffs, this one is an easy win.

Editor’s note: An upcoming article at this site will explore the efficiency of different types of power generation, including wind and solar.

To dig in further …

Energy loss from gasoline powered vehicles and electric vehicles is from FuelEconomy.gov, with further sources listed there.

Total gasoline use in the US is from EIA.gov.

Efficiency of thermal power plants is determined by their heat rate, which is the btus of energy required to generate one kWh. EIA.gov lists the heat rate for different types of power plants.

The efficiency of hydroelectric generation is listed by several sources as 90% (US Bureau of Reclamation, Killingtveit, 2020).

The energy required to make EV batteries is 41.48 kWh per kWh of battery cell capacity produced.

Average EV battery size is 63.1 kWh.

Multiplying those two numbers together gives the total number of kWh to manufacture the battery. Multiply that number by 3,412 to get the number of btus, then convert to the btu content of gasoline.

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