Hybrid cars are gaining momentum

As car buyers turn to fuel-efficient hybrids that combine gasoline and electricity, even greener hybrids are already on the horizon

By Joseph J. Rohm and Andrew Frank, Scientific American

In the summer of 2005, when fuel prices rose in the United States, hybrid cars, which combine a normal combustion engine with a battery-operated electric motor, began to be stolen from showrooms, a combination that results in fuel savings and improved performance. An average American car travels about 10 kilometers per liter, and in contrast, a full-fledged hybrid car, such as a Toyota Prius for example, reaches double the distance with the same consumption, depending on the nature of driving. Annual sales of hybrid cars in the US doubled from 2004 to 2005 and reached 200,000 units. It is expected that they will increase and pass half a million by 2010. It is likely that by 2020 all car models will come with the option of hybrid drive.

Until then, the next-generation technology, called plug-in hybrids, will improve fuel consumption even more, and will also offer some additional benefits: a cheap battery that will be charged overnight through a simple connection to the power outlet at home or at work, no more than a few stops at the gas station every year and even a chance to sell excess electricity back to the electricity company. Apart from the benefits for the consumer, the new rechargeable cars will help reduce greenhouse gas emissions, which will be emitted instead of millions of exhausts - from power plants. Today the power plants in the US burn local coal and natural gas, and in the future they will be able to generate cleaner electricity from energy sources such as wind, solar and even advanced systems for burning mineral fuel, which bury the carbon dioxide in the soil.

To understand where the hybrid car industry is headed, you have to look back and see what happened to the normal vehicles. For a century, almost all vehicles used an internal combustion engine that burned gasoline or diesel. Already at the beginning of the 20th century, transportation engineers tried to promote the idea of ​​combining a combustion engine with an electric motor running on a battery to add horsepower and improve fuel consumption, but they stopped doing so when internal combustion engines became more powerful and no longer needed improvement. Fuel was cheap and readily available, and gasoline-hungry power plants weren't a real problem. Until the days of the oil crisis of the 70s, it was widely believed among drivers that to achieve good fuel consumption, one had to sacrifice size, weight and performance. Although the small cars received a temporary revival, the vehicles in the US continued to grow and their fuel consumption increased at a tremendous rate. With the onset of the current rise in oil prices, improved fuel economy has returned to being a desirable feature.

Today's hybrid cars combine advanced computerized electronic power control with two engines - normal and electric. This allows for an improvement in fuel consumption and a reduction in gas emissions, along with excellent acceleration and a longer range. Hybrid cars can get by with a smaller internal combustion engine than a regular car, because the battery and electric motor provide more energy when needed, when accelerating or going uphill.

Any technology capable of improving fuel consumption can also be used to increase the number of horsepower. Until now, hybrid car drive systems have improved efficiency so much, and their electric motors develop such high acceleration, that car manufacturers are using the technology to increase horsepower and improve fuel consumption at the same time. The Ford Escape hybrid car, for example, achieves much better fuel consumption but provides almost the same power. Other hybrid vehicles, such as the Toyota Highlander, a 4x4 sport utility vehicle, utilize the hybrid system for modest improvements in both categories.

Due to the additional expense of the large battery, the electric motor and the additional electronic components, the price of a hybrid car is inevitably higher than the price of a normal car. The surcharge for the hybrid cars currently on the US market is from 3,000 to 7,000 dollars, and the average is 4,000 dollars [in Israel, a hybrid car is cheaper than its normal counterpart due to an encouraging taxation policy - the editors]. The battery pack and the electric motor are also paid for by weight: about 5% of the weight of a Honda Accord hybrid, for example. The added load slightly reduces the improvement in fuel consumption.

For a vehicle that travels 24,000 km per year and consumes gasoline whose price is 65 cents per liter in the US, hybrid technology that improves fuel consumption performance from 8 to 12.5 kilometers per liter, you can save the consumer approximately $600 per year [at gas prices in Israel, this is a saving of almost 7,000 shekels per year - the editors]. So at today's hybrid car prices, along with a new federal tax credit for the purchase of hybrid cars in the US, it is possible to recoup the investment in a few years, if you don't take into account the other benefits, such as the increased range.

But with rising gas prices, the improved efficiency of hybrid cars will significantly shorten the payback period. For now, it should be expected that the increasing production volumes and the improvement of battery technology will reduce production costs. From 1997 to 2004, for example, the price of a nickel-metal hydride battery, used in hybrid cars, dropped by half, and so did its weight. Even so, the battery accounts for about 50% of the price increase in today's hybrid vehicles. The Toyota company alone plans to produce almost all of its models in a hybrid version as well and to sell a million hybrid cars every year throughout the world during the next decade. It should be expected that its efforts will increase the production volumes and encourage a decrease in the price.

Types of hybrid cars

The hybrid cars are designed at several levels, depending on the level of fuel economy that the engineer aims for. A "full" hybrid car takes advantage of various fuel-saving methods. In a "mild" hybrid car there are fewer strategies for saving fuel, to save on the costs of the additional systems. Whereas a "micro hybrid" car only turns off the engine at stops for a slight improvement in fuel consumption.

A full hybrid car, such as a Toyota Prius, can provide a 60% or more improvement in fuel consumption. The biggest savings achieved by a full hybrid car comes from regenerative braking, a technology that captures most of the energy that is usually wasted heating the brakes, and turns it into electrical energy. Just as the engine can turn the electrical energy stored in the battery into power torque (the power that turns the wheels and pushes the car), it is possible to reverse the process, and generate electricity from the power torque generated by the deceleration of the moving car and store it in the battery.

City driving, which forces the driver to brake frequently, is the best opportunity to recover braking energy, but the process is also effective when driving up and down hilly terrain. Nowadays, hybrid cars store about half of the braking energy. In the future, improved batteries and more sophisticated systems will allow cars to store even more available energy.

Like any engine, an internal combustion engine also functions most efficiently in a limited range of torques and speeds. Because in a hybrid car the battery and the electric motor can provide the extra power needed for acceleration or going uphill, engineers can downsize the car's combustion engine and bring it to optimal operation by having it operate only at its efficient operating points, where it burns much less fuel. When necessary, the car can electronically integrate the electric motor, so that it provides the necessary extra power.

Some of the manufacturers of full hybrid cars, such as Toyota and Ford, replaced the standard engine based on the process called "Otto cycle", which is used by most cars that run on gasoline, and began to use a more fuel-efficient configuration, based on the "Atkinson cycle". A modern Atkinson cycle engine uses electronic control and intake valve timing to obtain a wider spread of the burning fuel-air mixture in the piston cylinder, allowing the engine to use fuel more efficiently. Until now, engineers have used the Atkinson cycle only rarely, because the improvement in its fuel consumption comes at the expense of the power it produces. But in a hybrid car, the electric motor can compensate for the lost power. When driving on a highway, the combination of the Atkinson engine and the energy savings resulting from braking energy recovery can result in an overall efficiency in the hybrid system that exceeds the efficiency of a modern diesel engine - the leading internal combustion engine in this respect.

In hybrid cars it is possible to avoid additional inefficiencies that plague normal cars where the gasoline engine operates the air conditioner, power steering, water system, oil pump, fan and other energy consuming systems. In a hybrid car, the large battery and cheap electronics power these efficient electric components. On a hot summer day, an electric air conditioner can consume 20% less energy than an air conditioner that uses the engine.

Another huge advantage in terms of fuel consumption in a full hybrid car comes from the ability to use the battery and the electric motor to drive the car without the combustion engine. Then, when the combustion engine burns fuel unnecessarily, for example, at a stop or at low speed, you can run the car as an electric vehicle for everything and save fuel.

Mild hybrid power systems, such as the integrated electric assist engine system of Honda's new Insight, Civic and Accord models, provide savings of up to 35% in fuel consumption. In addition to its role when driving in stop-and-go conditions, the electric motor in mild hybrid cars helps them accelerate, and also stores some of the braking energy.

In a micro-hybrid, or drive-stop hybrid, such as those sold by General Motors, the combustion engine shuts off when the car stops, and a starter-generator system turns the engine on as soon as the driver presses the gas pedal. Such a vehicle, which uses an electric motor to drive the various aids, but not the wheels, achieves a 10% improvement in fuel consumption in urban driving, but only a slight improvement on the highway.

The rule of thumb

It is likely that over time the importance of one of the advantages of the hybrid car will increase: their improved fuel consumption reduces the emission of carbon dioxide, the main greenhouse gas, into the atmosphere. It is very possible that in the future the US federal transportation policy will be affected by the concern about climate change. Many industrialized policies have already tightened their fuel consumption standards to bring about a reduction in carbon dioxide emissions. In the US, in 2002 California passed a law to reduce greenhouse gas emissions from vehicles by 30% until the 2016 model year. Another group of states in the US also aligned with this policy (although car manufacturers filed legal petitions against the move).

Hybrid cars that are charged from the electric grid combine both the advantages of electric drive technology and the advantages of hybrid drive technology. They are fully functional in both electric and hybrid mode, allowing even greater savings in emissions and fuel use. These cars run partially on electricity produced at local power plants, which may reduce the American nation's dependence on oil, and they also provide the power plants with a steady market for electricity during off-peak hours, and drivers with a much cleaner and cheaper fueling option. And like a regular hybrid car, the plug-in hybrid can burn gasoline for a long driving range and quick refueling. Moreover, there is no reason for plug-in hybrid cars to be more complex, heavier or more expensive than the hybrid models available today. First, their internal combustion engines will shrink as batteries and electric motors grow. Second, the prices of batteries and electronic components are steadily decreasing.

At today's gas prices, the cost of operating a normal car in the US is 7.5 cents per kilometer. On the other hand, a plug-in hybrid car will be able to be operated for 2 cents per kilometer using household electricity, the average price of which in the US is about 8 cents per kilowatt-hour. Considering that half of the cars in the US drive no more than 40 kilometers a day, the use of plug-in hybrid cars and a battery capable of providing energy for a 32-kilometer drive will allow up to a 60% reduction in the consumption of oil-produced fuel. Even a person who drives every day in a rechargeable hybrid car to a remote workplace will be able to drive most days on inexpensive electricity that will be stored in an advanced battery that will be charged all night by connecting to the usual electrical outlet at home, and will be recharged at the workplace during the day.

The larger battery of plug-in hybrid cars, combined with the strengthened electric engine, allows for a considerable reduction of the gasoline engine and other mechanical systems related to it. Researchers at the University of California, Davis have built prototype plug-in hybrid cars that can travel nearly 100 kilometers on electricity alone, with a combustion engine less than half the size of a conventional engine. Eight passenger cars and their SUVs are currently undergoing testing. Daimler-Chrysler unveiled in 2005 the first prototype of a plug-in hybrid car produced by a major car manufacturer. It was a hybrid version of a Mercedes van. The upgraded van has an internal combustion engine of 143 horsepower, and an electric motor of 120 horsepower. It can travel a distance of 30 kilometers in full electric mode. Its fuel consumption is 40% lower than the normal van and its acceleration is better. So far, only a limited number of plug-in hybrid vehicles of this model have been put into operation.

As battery technology advances, engineers will be able to develop plug-in hybrid cars that will consume much less fuel than conventional vehicles. The driving range in the city, with a full fuel tank and a charged battery, may reach 1,000 to 1,600 kilometers. The size of the various energy systems in each car will depend on the driving needs of the buyer. People who travel long distances every day may prefer to purchase a plug-in hybrid car with a larger battery, although we think a 30-kilometer range in all-electric drive will satisfy most consumers.

The low fuel bills will offset the high price of the plug-in hybrid car, assuming battery technology continues to develop and costs come down. The price of one electrochemical battery with an increased range can currently reach up to 10,000 dollars. We believe that a nickel-metal hydride battery, or a lithium-ion battery at a reasonable price of less than $3,000 will eventually provide enough energy for ranges of 30 kilometers and more. When the time comes, one can expect that the engineers will extend the life of these batteries to more than 15 years and 240,000 kilometers.

Also, with the help of an improved internal combustion engine, plug-in hybrid cars will be able to run on a mixture of 15% gasoline and 85% biofuel, such as, for example, ethanol produced from cellulose (this ethanol is produced from sources other than corn kernels, such as agricultural waste and dedicated energy crops) . Such vehicles will be able to travel 800 kilometers on a mixture of 4 liters of gasoline and 20 liters of ethanol, thus taking part in a long-term strategy designed to deal with the inevitable depletion of global oil sources, which will come after their exploitation reaches its peak.

Hybrid cars are claimed to have additional unique advantages. Since it is possible to connect them to the electricity grid, you can benefit from the electricity rates during off-peak hours, which are much lower than the normal rates for home consumption. The power plants have excess electricity mainly at night, because the peak loads are due to air conditioning in the summer. It will be possible to charge the hybrid cars that are charged during the night, and feed electricity or provide voltage control services to the grid during the day, when electricity consumption is at its peak. It is even possible that the car owners will be able to receive a credit or payment from the power stations for such a service. According to researchers at the University of Delaware, the potential financial value of such a service is significant, up to $3,000 a year, an amount that could subsidize the cost of purchasing a plug-in hybrid car or its battery. It can be assumed that a power station will lease plug-in hybrid cars to customers or businesses that will agree to connect the vehicle to the grid whenever it is not on the road, and allow the power station to control the charging and discharging times of the battery, depending on the production at the station or the voltage control needs. Such an arrangement will help the power plants to balance loads, for example.

For policymakers concerned about global warming, hybrid cars are claimed to have an advantage over another sought-after green vehicle technology - cars powered by hydrogen fuel cells. Plug-in hybrid cars would be better at utilizing carbon-free electricity, because after all, the process of refueling with hydrogen is expensive and fundamentally inefficient. An efficient hydrogen economy will need infrastructure that can use carbon-free electricity to electrolyze water into hydrogen and oxygen, flow this rarefied gas over long distances and fill cars' high-pressure tanks with it. And all this - to turn the hydrogen into electricity in the fuel cell that will drive the electric motor in the car. The entire process of electrolysis, transportation, pumping and conversion in the fuel cell will leave only 20% to 25% of the original carbon-free electricity to drive the engine itself. On the other hand, in hybrid cars the electricity transmission process is charged, charging the battery in the vehicle and discharging the battery will leave 75% to 80% of the electricity to drive the engine. A plug-in hybrid car will therefore be able to travel three to four times greater distance per kilowatt-hour of electricity from renewable sources than a car powered by a left-hand drive fuel cell.

If the current trends in fuel prices and the concern about climate change remain unchanged, we expect that around 2020 there will be an extensive change in the market, after which most car models will have a version with a hybrid drive. In our opinion, shortly thereafter, the plug-in hybrid cars will gain the status of the ruling alternative vehicles. The speed of the process depends in particular on the increase in oil prices and the government's policy regarding climate change and energy security. When the global transportation system finally works to replace oil as its main energy source, the most likely type of vehicle will be a fuel-flexible plug-in hybrid car that will run on a combination of carbon-free electricity and a mixture of biofuels. If at some point there will be a significant improvement in the functioning of the batteries, it is possible that the drivers will be able to switch to fully electric cars. It would be wise for us to adopt this useful personal transportation technology as quickly as possible.
Overview hybrid cars

A small and growing number of hybrid cars and trucks are powered by a combination of an electric motor and a conventional gasoline-burning engine. This technology, which leads to a reduction in fuel consumption, began to develop rapidly.

The hybrid power system makes the cars more expensive today by several thousand dollars. The car owner has to drive it for a few years until the fuel savings pay back the additional initial investment. But as the battery technology improves and the production volume increases, a sharp drop in the price increase is expected.

The development of better batteries will probably also encourage the commercial production of hybrid cars that will be able to be charged overnight from the electricity grid, and enjoy low peak rates. Since most US power plants generate electricity from domestic energy sources such as coal, nuclear power and hydroelectric power, the transition will reduce the US's dependence on importing transportation oil from foreign countries.

Engineers use several energy-saving strategies in hybrid electric cars, which combine a gasoline engine and an electric motor. A "full" hybrid car is detailed here, such as a Toyota Prius, in which all the described technologies are used. Full hybrid cars can save more than 60% in fuel, while "mild" hybrid cars save up to 35% and "micro-hybrid" cars about 10%.

Using energy-saving electrical components in normal cars, the air conditioner, power steering, water and oil pumps and fans draw mechanical power directly from the rotating engine parts, using belts. But since these are electrical components, they work best with constant battery voltage rather than mechanical systems that must adjust to varying engine speeds. The large battery of the hybrid car makes it possible to supply electricity directly to these components, thus saving energy.

Designing the combustion engine so that it shuts down at stops Normal combustion engines do not operate efficiently at stops at traffic lights and when driving at low load. Hybrid cars reduce these losses by turning off the combustion engine and relying on the electric motor and the battery as the vehicle's energy sources.

Capture of wasted energy

A significant part of the fuel savings comes from a regenerative braking system, which returns part of the energy that would otherwise be lost as heat energy as a result of friction when the vehicle slows down. In principle, when the system is on, the electric motor moves in the opposite direction, like a generator, and converts the energy of the wheels turning into electricity stored in the battery.

Changing the engine type

Some car manufacturers have replaced the normal gasoline engine that operates on the "Oto cycle" method with an engine that operates on the "Atkinson cycle" method. Such an engine burns the fuel much more efficiently, but so far it has not been used much because its power is lower. The hybrid car's electric motor makes up for the lost power.

smaller engine

Combustion engines operate optimally at certain speeds and at some specified levels of torque, but normal cars are forced to operate over a wide power range in day-to-day driving. The electric motor's ability to boost the combustion engine during acceleration or going uphill allows hybrid car designers to downsize the combustion engine.

In the future - electricity from the grid In the next generation of hybrid car models, the car will be able to charge overnight, that is, during off-peak hours, when electricity rates are cheap, using a larger battery and an electric cable with a plug. This way it will be possible to satisfy more than half of the driving needs of the average American using cheaper (and increasingly cleaner) electricity from the central power grid.

About the authors:

Joseph J. Romm and Andrew A. Frank have been supporting the adoption of hybrid car technology for many years. Rom, who received his doctorate in physics from the Massachusetts Institute of Technology (MIT), serves as a principal at the clean energy consulting firm Capital E. His most recent book is The Hydrogen Madness: Facts and Stories in the Race to Save the Climate (Island Press, 2004). In his role as Acting Deputy Secretary of Energy in the late 90s, Rom helped the ministry's efforts to develop and use advanced energy technologies. Frank received his PhD at the University of Southern California. He served as a professor of electrical engineering at the University of Wisconsin in Madison for 18 years, and is now a professor of mechanical and aeronautical engineering at the University of California, Davis. His research interests include advanced hybrid car engineering, including plug-in cars.

Hybrid cars

In the real world, some car buyers have been disappointed because their hybrids did not achieve the excellent fuel economy promised by the US Environmental Protection Agency (EPA) ratings. They are no different than most cars in the field, often achieving significantly worse fuel economy than the EPA rating predicted. Several factors (The EPA announced revisions to its testing procedures in early 2006 that may address some of the problems.) Poor EPA testing procedures. The driving cycle used in government fuel economy tests does not reflect actual driving experience and road conditions because it is based on Unrealistic assumptions about typical driving habits.

For example, the highest driving speed on the highway is 90 km/h in the test, and in contrast, the average driver passes this speed often. Large seasonal increases in consumption. In hybrid cars like the Toyota Prius, a computer program decides when to turn on the combustion engine, when the electric motor and when to charge the battery. The consumption of heating in the winter requires an intervention in the operation of the software to optimize the functioning of the electrical system, because it is necessary to start the engine more often. The result: in hybrid cars there is a greater increase in fuel consumption in the winter than in normal vehicles. There is no escaping the fact that hybrid car owners living in the cold north will see a sharp drop in fuel economy compared to the EPA's rating set in a temperature range of 20°C - 30°C.

Built-in hybrid design.

Because hybrid cars rely on braking energy recovery systems, their fuel consumption is more sensitive to driving style. Drivers who adapt their driving to the hybrid car, for example by coasting to a stop, report fuel consumption figures close to the EPA rating.

Aggressive driving can cause a drop of more than 30% in fuel economy efficiency, but its effect in normal vehicles is much smaller. In general, full hybrids achieve good fuel economy in city stop-and-go driving and a smaller advantage in highway driving.

More on the subject

The Car and Fuel of the Future. Joseph Romm Report for the National Commission on Energy Policy, 2004

Driving the Solution: The Plug-In Hybrid Vehicle. Lucy Sanna in EPRI Journal; Fall 2005

More on plug-in hybrid cars: www.calcars.org
More on hybrid cars: www.hybridcars.com
Andrew A. Frank's technical articles on plug-in hybrid cars: www.team-fate.net