Hybrids in Our Future
The hybrid-electric car, with its advertised fuel economy and environmental greenness, has made great inroads into the U.S. passenger-car fleet, as well as the U.S. automotive psychology, even as fluctuating fuel costs and economic uncertainty have caused historic economic strains for traditional automotive manufacturers. But is the hybrid really the Car of the Future? Exactly what is the future for hybrid cars, how will they play a part in our transportation energy future and how will independent research and development play a hand in shaping it?
Technology Today recently talked to Joe Redfield, manager of the Advanced Vehicle Technology Section within Southwest Research Institute’s (SwRI) Engine, Emissions and Vehicle Research Division, about the answers to those and other questions.
TT: Is hybrid vehicle technology beginning to zero in on a particular design paradigm?
It’s not zeroing in at all. It’s getting broader. First you had the very different gas-electric Prius and Insight-type hybrids. Since their initial release, the U.S. OEMs (original equipment manufacturers) have a growing family of hybrid models available. There are new designs under way in the U.S., in Europe, in Japan, and the Chinese auto industry has aggressively pursued hybrid development. But we should also remember that in overall terms, hybrids today only account for about 1 percent of the U.S. Fleet.
TT: What hybrid topologies are SwRI engineers working on?
There are three major areas of hybrid activity under way here at SwRI. First, there’s energy storage, where internal research and development (IR&D) programs are looking at advanced lithium-ion batteries. In addition, we have an active program area in the testing and characterization of all types of battery chemistries used in hybrid energy storage systems. Second, SwRI engineers are working on novel ways to integrate and control electric motors and hybrid drives into transmissions or vehicle drive systems. The third area is modeling and simulation of these advanced hybrid drives for both military and commercial manufacturers. This area can be subdivided into traditional gasoline-electric hybrids, plug-in electric hybrids and hybrid-hydraulic powertrains for large military and commercial trucks.
TT: What makes hybrids, which need two separate powerplants, more efficient than a single, high-efficiency gasoline or diesel?
The exceptional hybrid fuel economy is driven by three fundamental elements. The first is regenerative braking, where kinetic energy from the moving vehicle is captured during braking. Second, tailored controls for both the internal combustion engine and the electric motor create the most efficient power-split available, so that both the engine and the motor are operating at their “sweet spot” of maximum efficiency. Finally, there’s energy storage, where you supplement engine power with stored energy. This means you can downsize the engine, or specially design it to operate in a hybrid mode. Now you’ve fundamentally changed the vehicle powertrain design itself, because you’ve de-coupled engine power from vehicle performance. Every hybrid design takes different factors from those three elements. These hybrid gains are on top of the gains from high-efficiency gasoline and diesel engines, many of which are also being developed at SwRI.
TT: What does that mean for the consumer?
It means that the vehicle drivetrains are more complex. With a hybrid vehicle, there are more things to consider in operating the vehicle than how much gas is in the tank. But there also can be a significant increase in performance compared to gas engines or earlier hybrid designs. The Engine, Emissions and Vehicle Research Division supported one OEM’s electric vehicle design program by performing durability testing. We built a test stand for the drivetrain, and this electric-drive would produce a maximum of 1,200 foot-pounds of torque at each wheel, at any speed. Now, in hot-rod terms, that’s comparable to what the old big-block engines were able to generate at low speeds. In this electric vehicle, you could light the tires up at almost any speed by stepping on the accelerator. Depending on the drivetrain topology, the performance increase over a conventional vehicle can be significant.
TT: That’s great, but what about fuel economy?
The hybrid torque-split between the engine and electric motor is all part of the control system. It is the magic that allows the hybrid to get great fuel economy. In most hybrids, the engine won’t run when it isn’t needed. That is, when you come to a stop the engine turns off and only after the vehicle has started moving again, and the engine power is needed, does the engine start again. The engine-off operation, coupled with kinetic energy recovery and efficient operation of the drivetrain, all combine to give the great fuel economy. Of course, if you’re showing off the performance of the electric drive by smoking your tires, your fuel economy won’t be that great.
TT: Won’t plug-ins get even greater fuel economy?
People talk about plug-in hybrids getting 100, or 200 or even 300 miles per gallon, but that’s something of a misnomer. In a plug-in hybrid, you can’t measure fuel economy the same way as in a conventional drivetrain. A plug-in hybrid has a battery that you charge at home, and the vehicle uses that battery initially for propulsion energy until it’s sufficiently depleted, and then the engine starts. PHEVs (plug-in hybrid-electric vehicles) are rated at 10, 20, 30 or 40, which corresponds to the number of miles they can go on electricity only. If you have a PHEV-40 and your daily round trip is 40 miles or less, you’ll drive your entire commute without ever having to start the engine. In that case, you’ll get a lot of mpg given the current measurement techniques. To be accurate, you still have to account for the electrical energy you started with in the battery. So, a 300-mpg plug-in is a misnomer. Staff at SwRI are participating with industry on the development of new fuel economy measurement standards to accommodate PHEVs.
TT: So the tradeoff is kilowatt-hours versus gallons?
There are a number of factors in that comparison. One of them is energy content. The others are emissions and greenhouse gas emissions. Gasoline, depending on the ethanol content, is equivalent to about 30 kilowatt-hours (kWh) of energy. At $3 per gallon, that’s about 10 cents per kilowatt hour when you convert the gasoline directly into electrical power. That same amount of kWh would cost about $1.80 from the electrical grid. When you consider the overall increased efficiencies of the hybrid drivetrain, the numbers get even more interesting. One of the new ratings being considered specifically for plug-ins and electric vehicles is kWh per mile, instead of miles per gallon. The reduction in the per-mile transportation costs and other advantages of emissions reductions are driving us to the electrification of the drivetrain.
TT: Why have hybrid cars just caught on when electrically driven locomotives have been around for 60 years or more?
It’s true that large machines have been driven electrically for a long time; locomotives and large mining trucks for hauling ore, for example. They’re not new. What is new is capturing the vehicle’s kinetic energy, storing it in batteries and then using it to supplement propulsion.
TT: Speaking of batteries…
Battery technology is moving forward at a very rapid rate. Most of the development has been in battery chemistries. There are advanced lead-acid batteries, nickel metal hydride batteries and a number of different chemistries around the lithium ion battery. Then there are form factors involving cylindrical or planar cells, and the manufacturing techniques for producing them. With each of these batteries there are issues to address like manufacturing ease, durability, reliability, recyclability, safety, and of course, cost. And a whole new world which is just opening up is End-Of-Life Continued Use. Say you have a battery in your vehicle rated at 15 kWh. It holds that much when new, but as the battery ages its capacity may be reduced to, say, five kWh or less. Five kWh may not be enough for the vehicle to operate efficiently, but that battery can still be useful. You might give it a second life as energy storage for a peak-shaving device in your home as a way to defray the battery’s initial cost. There are a number of researchers looking at how to defray the current high initial cost of the battery.
TT: Is there a way to replace batteries altogether?
While battery energy storage is primarily electrochemical based, there is another mechanism for storing electricity. Very large capacitors, called supercapacitors or “ultracaps,” can store large amounts of energy with very high charge/discharge efficiencies and virtually infinite life. You don’t have the losses associated with a chemical battery, where your electricity is a result of a conversion of chemical energy to electrical energy.
TT: Why aren’t we using ultracaps? What’s holding them back?
Right now the energy density of ultracaps is not sufficient for the transportation sector. The energy stored is a function of voltage. The equation for energy stored in a capacitor is • CV2 where C equals capacitance and V equals voltage. Today’s ultracaps store energy at 2-5 volts. If you could go from 3 volts to 30 or 300 volts, you’d increase the energy storage by a factor of 10,000 because of the voltage squared function. If energy were stored at the 300-volt range, the energy density of ultracaps would be approximately four times that of gasoline. As a comparison, the energy density of gasoline is about 50 times that of today’s lead-acid batteries. Even the best lithium-ion battery can store less than one-eighth the energy of gasoline. But we haven’t gotten there yet with ultracaps. When we do get that far, the potential is that transportation energy would primarily come from electricity, and it will be all ultracaps. Basically ultracaps have two conducting plates and an insulator in the middle. The surface area of the conductors, and the space between them, determine capacitance. The more surface area, and the closer together they are, the more the capacitance. The higher the voltage, the more energy can be stored, because of the higher electrical field. But the insulator has to be able to separate the conductors and sustain the electrical field. The limiting factors to high-voltage ultracaps right now center on insulator materials, and on how to manufacture insulators in such a way that they are consistent across a very large surface area. One flaw in the insulator material, and all the energy stored in the capacitor is dumped at that point in a big flash. That’s what the challenge is in the ultracap world: It’s to develop an insulator material that will tolerate the higher voltage.
TT: So, energy storage is the main issue in continued hybrid research?
Part of the challenge is energy storage, but there’s also controllability; or how you control these very different systems to give desired performance and an acceptable life for the system. SwRI is involved in materials research related to battery technology, and also in battery life prediction, hybrid drive topologies and control system development. We’re involved in all the different pieces to support the implementation of hybrid drivetrains.
TT: When and if we develop high-voltage ultracaps, will the internal combustion engine then finally be dead?
The internal combustion engine will be around for a long time, primarily because of the energy density of gasoline and diesel and the ease of refueling. Consider filling up a commercial truck with 40 gallons of gasoline, not uncommon multiple times a day for an active business. The electrical equivalent of 40 gallons would be approximately 1,200 kWh. If you refilled it in, say, five minutes you would need a 125-amp service, and that would take a very thick extension cord. We won’t have that kind of infrastructure at every corner gas station, not to mention what the impact to the grid would be.
TT: Aren’t the plug-in hybrid and the smart electrical grid supposed to take care of that?
I’m going to extrapolate into the future five, 10 or 20 years. Here’s what it looks like: In basic residential transportation there will be significant penetration of plug-in hybrids. Your vehicle plugs in at home, and you leave for work or school and the only noise you hear is the whoosh of air going past the windows and the sound of the tires on the roadway. The engine won’t be running because you will be running on the stored energy in your battery. You’ll come home and you’ll plug back in to your outlet in your garage or parking spot. You may go a week or a month or more without ever putting a drop of gas in your tank. But that’s only half the story. What’s going on behind the scenes is an advanced energy management system where your home and your vehicle play a very critical role in the management of your personal and our nation’s energy system — the smart grid. When you plug in your hybrid, the smart grid will communicate with your car. Your car will know how much energy is stored and how much will be needed for the next trip, and when. The utility, meanwhile, will program and schedule when and how much energy goes into your battery. It’ll efficiently be able to use its cleanest and most efficient energy generation systems, many of which are, by definition, intermittent. In conjunction with renewables, the utility will utilize excess low-cost capacity at night to charge your batteries. The combination of the vehicle and the grid will have a significant efficiency impact on how the grid is utilized and how energy is distributed. And, if you as a car owner decide to participate, you can make your vehicle a distributed generation source to the grid itself, wherever you park it, by plugging in.
TT: Why would you do that if you had to buy the electricity from the grid in the first place?
Money. The load on the utility varies greatly with the time of day and the season of the year. For example, in San Antonio, if you compare the demand for electricity at 2 a.m. in late March versus 4 p.m. in mid-August, the August afternoon demand may be five, 10, or 20 times the 2 a.m. demand during March. The utility has to plan for it. To supply the energy needed in August, the utility might have to use its least efficient, most polluting generation plant. They must keep it maintained and ready to use, even if they only need it for 20 hours in a year. Power costs for utilities typically vary from about $20 per megawatt-hour (MWh) up to $300, $400 and sometimes into the thousands for short intervals of time. If you, as a utility, can get an aggregate of vehicles plugged in, and you have the controls in place and the information on hand to know where they are, you could call on those vehicles to support a localized short-term demand without having to generate electricity or transport it over a long distance. Meanwhile, you as the consumer can buy energy at a low cost in the middle of the night and sell it back in the middle of the day at a profit, and it still saves generation costs for the utility. That brings down the overall cost of energy for all of us.
TT: So the utility company replaces the oil company? And we become sellers as well as buyers of electricity?
I see a future where vehicles are tied to the grid whenever they’re stationary, and your vehicle becomes an integral part of energy generation and energy management infrastructure.
TT: That is quite a vision. When can we expect to see all this happen and what are the SwRI engineers and scientists doing in this area?
The when question is always hardest to answer. With the current U.S. administration’s focus on increased vehicle fuel economy and the goal for 1 million plug-in hybrids on the road, both government and industry are pushing ahead in a number of related areas. Significant funding from the economic stimulus package is going toward advanced battery manufacturing and research here in the U.S. The utility industry is pushing ahead with not only the smart grid interface to vehicles, but also to many of the power-using appliances in homes today. There are many divisions at SwRI involved in this developing area, from battery materials development to advanced utility communications protocols, to power controllers for in-vehicle power management, to the design and operation of the vehicle itself. As this new technology moves forward, you can count on SwRI having a part in its development and implementation.
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Joe Fohn, Technology Today®
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Published in the Summer 2009 issue of Technology Today, published by Southwest Research Institute. For more information, contact Joe Fohn.