With high energy prices and the growing urgency to reduce fossil fuel consumption, it makes sense to get the most out of every gallon of gasoline or kilowatt hour of electricity.
A previous article showed that charging an electric vehicle costs about $1.41 per gallon in the United States, providing consumers with significant savings compared to gasoline. Part of the reason electric vehicles are inexpensive to operate is that they use energy with impressive efficiency.
Going deeper, there is a distinct difference between how internal combustion engines and electric motors 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 vehicles will require significantly less energy overall.
Traditional cars and trucks are surprisingly inefficient
Modern gasoline vehicles waste 80% of the energy contained in their fuel. For every gallon pumped into the tank, only slightly more than three cups are used to move the vehicle forward. In economic terms, for a $5.00 gallon of gasoline, only $1.00 gets you closer to your destination.
Most of this waste is an inevitable consequence of thermodynamics. Internal combustion engines ignite liquid fuel to create pressurized gas that drives pistons to spin a crankshaft which eventually spins the wheels of the car. This multi-step process drains energy throughout the process. Most of the energy in the fuel ends up as heat, and only a small fraction reaches the wheels. The concept of waste heat becomes intuitive when you think of the hot air that escapes from the running engine of a car. The engine itself gets hot; a cooling system is needed to manage excess heat; and the heat is dispersed through the radiator and blows out the exhaust. All that heat comes from the gasoline, and none of it helps propel the vehicle.
Other energy uses come from pumps and fans, some of which, ironically, are needed to remove waste heat. This is called parasitic losses. Mechanical friction in the transmission and transmission reduces overall efficiency by another 3-5%. The last loss of energy comes from auxiliary electrical components such as heated seats, headlights, audio system and windshield wipers. Together, these accessories can consume up to 2% of the vehicle’s total energy input.
The net result is that only about 20% of the energy pumped into the fuel tank ends up in the wheels.
Even the most fuel-efficient gasoline-powered vehicles cannot avoid these energy losses. High fuel economy cars are lighter, smaller and more aerodynamic, making the best possible use of the energy that ends up in the drivetrain. Diesel engines have somewhat better thermodynamic efficiency, averaging in the 30s to around 40%. But major thermodynamic losses are a tenacious reality for all combustion engines.
For a more detailed explanation and sources for the figure above, see FuelEconomy.gov.
The simple efficiency of electric motors
Electric vehicles are powered by entirely different mechanisms. Energy enters the vehicle in the form of electricity, which powers the transmission directly: electric vehicles do not need to convert one form of energy to another, which is an important factor in their efficiency.
Electric motors are simple machines with few moving parts, especially compared to the complexity of an internal combustion engine. In an electric vehicle, electricity from the car battery flows through a cylinder which generates a rotating magnetic field. Inside this cylinder is a rotor that spins when driven by magnetic attraction. The spinning rotor turns an axle which drives the wheels.
The whole process also works in reverse: the car’s spinning wheels can spin the rotor and feed electricity back into the battery. This regenerative braking process can recover energy that would otherwise be lost in the form of friction and heat.
Electric vehicles are not 100% efficient, however, and waste energy in several ways. Some energy is lost in the battery charging process and electricity is consumed for vehicle cooling and power steering. Auxiliary electric use is higher in electric vehicles than in combustion engines, mainly due to the electricity needed to heat the car interior in cold weather. In an internal combustion vehicle, waste heat is used to warm the car interior.
In total, the various energy losses of an EV add up to 31 to 35%. Regenerative braking reintroduces 22% back into the system, bringing overall efficiency to around 87% to 91%. Specific numbers vary depending on the type of car and its use, but the overall simplicity and efficiency contrasts with the traditional vehicles that have been the mainstay of the roads for 130 years.
The numbers come from FuelEconomy.gov, and DigitalTrends has a helpful explainer on how the various components of electric vehicles work.
The transition to electric vehicles will reduce the overall amount of energy needed for transport
The fuel efficiency of electric vehicles is an obvious boon to consumers, but it offers an even bigger advantage in the transition to petroleum-based transportation. In the United States, about 8.9 million barrels of motor gasoline are used every day, and about 80% of this energy is wasted in the form of heat and friction. Of the total amount of gasoline burned, only 1.8 million of these barrels (20%) propel vehicles along the road. This means that if the fleet of gasoline vehicles were replaced by electric vehicles, these electric vehicles would only need the energy equivalent of about 1.8 million barrels of gasoline per day, plus the loss of 11% energy in the electric vehicle itself. The rough calculation gives the energy equivalent of about 2 million barrels of gasoline per day, which represents a substantial saving compared to the 8.9 million barrels currently used.
Of course, this raises the question of the efficiency of power stations that charge electric vehicles. Thermal power plants – such as coal, gas or nuclear – face the same thermodynamic challenges as internal combustion engines, but power plants are more efficient than cars. Coal and nuclear are about 33% efficient, and natural gas-fired combined cycle power plants are about 44% efficient. At the top of the scale, hydroelectricity is about 90% efficient. Even if the grid were entirely coal-fired, it would take 31% less energy to charge electric vehicles than to power gasoline-powered cars. If electric vehicles were charged with natural gas, the total energy demand for road transport would almost halve. Add hydroelectricity or other renewables, and the result is even better, saving up to three-quarters of the energy currently used by gasoline-powered vehicles.
But what about batteries? Manufacturing an EV battery consumes the energy equivalent of approximately 74 gallons of gasoline. Over the 10-year (or longer) battery life, the energy investment in the battery is far too low to change the outcome – which is good news.
Decarbonizing the global energy supply is a huge and daunting task. But at least in this case, the job gets easier as road transport moves away from oil. The major improvement in driving efficiency offered by electric vehicles means that vehicles can emit less carbon and less pollution, while reducing overall energy demand. In a world of tough compromises, this one’s an easy win.
Editor’s Note: A future article on this site will explore the efficiency of different types of power generation, including wind and solar power.
Energy loss from gasoline and electric vehicles is from FuelEconomy.gov, with other sources listed here.
Total US gasoline consumption is from EIA.gov.
The efficiency of thermal power plants is determined by their heat rate, which is the btu of energy needed 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 manufacture EV batteries is 41.48 kWh per kWh of battery cell capacity produced.
The average EV battery size is 63.1 kWh.
The multiplication of these two numbers gives the total number of kWh to manufacture the battery. Multiply that number by 3412 to get the number of btu’s, then convert it to the btu content of gasoline.