As President Eisenhower said, our “transportation systems are dynamic elements in the very name we bear—United States. Without them, we would be a mere alliance of many separate parts.” (Pres. Dwight D. Eisenhower, Feb 22, 1955). The automobile became a household staple and a powerful symbol of our industrialized economy, such that cars are now intimately intertwined with our lives. As we move into the 21st century and American society begins the transition to new and sustainable forms of energy and growth, we must also reinvent how we fuel the U.S. fleet of approximately 250 million cars .
The aim of any sustainability measure is to reduce the use of and dependence on some critical resource. In the case of automobiles, the goal is to reduce our use of oil refined into either gasoline or diesel. Using less oil means a reduction in our dependence on foreign oil and the ability to counter the costs of higher oil prices . More importantly, it will decrease the emission of carbon dioxide, the primary contributor to climate change, as 30% of emissions come from transportation . This reduction may be accomplished by either increasing the fuel economy of the standard technology, the internal combustion engine, or by a more radical change, replacing gasoline with another energy source, such as a battery or hydrogen fuel cell.
Through a marriage between electric and gasoline vehicle technology, hybrid cars are able to improve efficiency without sacrificing key features like refueling speed, cost, and range (the distance the vehicle can travel without refueling). Unlike electric cars, hybrids do not have to be plugged in – all the energy required to move the car comes from gasoline in the fuel tank. The key innovation of hybrid cars is their battery’s ability to control energy flow in two directions. In a conventional combustion vehicle, the energy released when gasoline is burned is converted into mechanical energy to move the car. This energy conversion is inherently one-way – mechanical energy cannot be turned back into gasoline. In a hybrid car, the battery can be both discharged and recharged while driving. This means power that would normally be lost or used inefficiently can be redirected back to the battery, saving it for a time when the motor needs an extra boost. The electric motor and gas engine are typically connected via a mechanical coupler, or gear box (see Figure 1) .
Figure 1. An example schematic of a hybrid electric drive train. As indicated, mechanical energy can flow either directly from the gas tank/engine or from the battery/electric motor. The battery can be charged either from the wheels (regenerative braking) or via the engine directly. Such a configuration is called a parallel drive train. Adapted from .
When driving around a city, a major source of vehicle energy loss is the frequent starting and stopping, such that 50% of the total energy used to move the car is lost in braking . Technology known as regenerative braking can solve this problem by converting the energy lost in braking back into electricity, which is stored in the battery. Regenerative braking takes advantage of the fact that electric motors can also be run as generators, producing electricity instead of consuming it. Instead of running electricity through the motor to spin the wheels, when you step on the brake the wheels themselves are used to turn the shaft of the motor. Magnetic forces within the motor resist this motion and slow the car, while also generating electricity as a by-product. This electricity is stored in the battery and can be used to power the electric motor, accelerating the car back to full speed once the light turns green.
Hybrid cars also show superior fuel efficiency on the highway, but in this case the improvement is provided by a different mechanism than regenerative braking. Here, the battery and electric motor function more as regulators, allowing the combustion engine to run under optimal conditions. For a gasoline engine, fuel efficiency depends on the engine’s rotational speed. Engines are usually designed to have a “sweet spot” of maximum efficiency at a speed of around 2000-3000 rotations per minute (rpm) . When the car has only a gas engine, the speed of the engine must be matched to the speed of the car, meaning the engine is not always running at its most efficient speed. In a hybrid car, the gas engine can be connected to the electric motor/generator via a mechanical coupling, such that extra speed in the engine can be used to generate electricity and charge the battery. The gas engine always stays in its operational “sweet spot,” generating a little electricity as the car coasts downhill, and reusing that power in an uphill climb when the electric motor is needed to give the engine a boost.
While hybrid cars provide a bridge to reduce our emission of greenhouse gasses, they must ultimately be replaced if we want to make transportation carbon neutral. One way to achieve this is by making cars run solely off batteries and electric motors. The electricity drawn from our grid is produced from several different sources today, many of which are heavy greenhouse gas emitters themselves. This means that in the short term, electric vehicles only show modest reductions (by about 50%) in greenhouse gas equivalent emissions , but they will help us achieve carbon neutrality in the long term, once we obtain most of our electricity from solar, wind, and nuclear power sources.
The three key technological components in an all-electric drive train are the battery, which serves as the “fuel tank” for an electric car; the power electronics, which control the flow of electrical energy between the battery and engine; and the electric motor itself. Current motors are already up to 90% efficient, and compare favorably to gas engines in terms of weight and reliability. Power electronics have made huge advances in the past several years in improving the efficiency of charging and discharging the battery, and the technology is ready for mass deployment. The major hurdle to the full-scale adoption of all-electric vehicles remains the battery pack . An automobile fuel source must be able to hold a lot of energy but be lightweight, a property known as specific energy and measured in units of energy per unit mass, or Watt-hours/Kilograms (Wh/kg). Gasoline is the gold standard with a specific energy of around 12,000 Wh/kg. For reference, a single 18 gallon tank of gas contains enough energy to power the average American home for 19 days. By comparison, a disposable AA alkaline battery has a specific energy of 140 Wh/kg, meaning a “tank” of AA batteries would weigh about 85 times more than an equivalent tank of gas (see Figure 2).
Figure 2. Illustration of the weight imbalance between a gallon of gasoline and the number of disposable alkaline AA batteries needed to carry an equivalent amount of energy. The fact that AA batteries weigh 85 times more than an equivalent amount of gasoline makes it impossible to drive a car on these simple batteries.
The most state-of-the-art battery technology suitable for automobiles today is the lithium-ion (Li-ion) battery, which stores energy by creating positively charged lithium atoms in solution. The specific energy of a Li-ion battery is about 150 Wh/kg, and these batteries have other favorable properties as well, including long lifetimes and the ability to provide large amounts of instantaneous power . However, the unfavorable comparison to gasoline still stands, which explains why electric vehicles currently on the market may have ranges of only 100 miles, a quarter the range of most gas cars, but still have battery packs that take up five times more space than a tank of gas. This range is suitable for most daily commuter driving, but in order for battery power to truly replace gasoline in all automotive applications, entirely new battery chemistries must be invented. These might include lithium-air batteries, which replace the heavy positive terminal with air, thus providing theoretical specific energies comparable to that of gasoline . Battery research is a highly active field, and there is little doubt that a system can be designed to be truly competitive with gasoline cars in the not-too-distant future.
The Short Term Future of Automobiles
In the near term, expect to see the costs for currently available sustainable transportation – hybrids and short-range electric vehicles – to come down as demand increases and technology improves. Advanced automotive engineering is allowing marked efficiency improvements in the combustion engine itself, meaning hybrids and gas-only cars will continue to gain in fuel economy over the next few years. New strong and super-lightweight building materials, such as carbon fiber auto body frames, are becoming cheaper and easier to manufacture, which means they will soon be lightening cars and increasing mileage of all vehicles by as much as 30% . As short-range electric vehicles become more common, we can hope that the model of using these highly efficient cars for daily driving and a gasoline car only for long distance travel becomes the norm. The speed at which this technology develops will depend greatly on the cost of oil and taxes on carbon emissions, but there are clearly many exciting and active fields of research bringing us closer to 100% sustainable, carbon-free personal transportation.
Paul Hess is a Ph.D. candidate in Physics at Harvard University.
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