TECHNOLOGY TODAY
Present and future hybrid engines will depend on fuel use and availability
The first regular production hybrid car, the Toyota Prius, went on sale in Japan in December 1997.When the Honda Insight entered the U.S. market in 1999, these were the only two hybrids available anywhere.
Today, Toyota s Prius, Highlander hybrid and Lexus 400h still account for more than 75 percent of the world's hybrid sales, but that's changing fast. Eight additional hybrids from various manufacturers are scheduled for release by the end of this year, and by the end of the decade there could be more than 20 different hybrid models on American roads. According to a recent J.D. Power study, U.S. hybrid sales will top half a million in five years, but many in the industry think that estimate is conservative. Toyota alone is planning to sell 100,000 per year, and they'll be competing with Ford, General Motors (GM), DaimlerChrysler,Volkswagen, Nissan, Honda and quite possibly Mitsubishi, Subaru and even Porsche.
The hybrid market's quick but unexpected success has caught Toyota and Honda with very little production capacity, and most other automakers with no hybrid technology at all-or at least nothing that's ready for production. In order to get into the market quickly, some have gone to outside engineering firms for help, and even directly to Toyota.
Nissan is purchasing components from Toyota to develop a hybrid prototype of the Altima.When launched in late 2006, it will have a Nissan 2.5L fourcyl. engine and powerful lithium-ion batteries instead of the nickel-metal-hydride (NiMH) batteries used in other hybrids. Nissan plans to build 50,000 Altima hybrids a year in Smyrna, Tenn., and it will compete directly with the new Toyota Camry hybrid scheduled for production next year in Georgetown, Ky.
It's difficult to tell whose technology Ford is using in the new Escape Hybrid. Ford has announced the development of its own patented system, but Toyota has announced a technology licensing agreement with Ford. Either way, the Ford Escape Hybrid and its upscale sibling, the Mercury Mariner, are the first SUV hybrids to reach the market and the first with four-wheel-drive.
The front-wheel-drive Honda Accord hybrid with a 3.0LV6 engine will soon be joined by the rear-drive Lexus GS 450h with a 3.5LV6 engine. With combined engine/motor horsepower ratings of 255 in the Honda and "more than 300" in the Lexus, these cars are aimed at the near-luxury car market. Like all other hybrids, their real world fuel mileage is expected to be about 20 to 25 percent better than their conventional counterparts, but the cash saved as a result of higher fuel mileage is unlikely to cover the higher purchase price. Most hybrids cost $2,500 to $3,500 more than standard models.
Technology
At this point there are two basic drivetram designs for hybrid vehicles: series and parallel. In the series design, which is used by Honda, the electric motor is between the engine and transmission. The motor is actually a motor/generator, and it's bolted directly to the crankshaft. Almost any time it's turning, it's either drawing current from the batteries and adding power to the drivetrain, or it's taking power from the engine to recharge the batteries. It also switches over to generator mode during braking, using the power of the vehicles motion to recharge the batteries.
Compared with the parallel design, the series hybrid drivetrain is lighter, more compact and far simpler. It can be adapted to existing vehicle platforms more easily, and if the electric portion of the hybrid drivetrain fails, the vehicle can still be driven on just the engine.
The one major disadvantage is that the crankshaft must rotate to move the vehicle, so it can't be driven on just the electric motor. Honda has addressed this in the next generation Civic hybrid. The motor is still bolted to the crankshaft, but the engine has a new valvetrain and software that deactivates all the cylinders when creeping in a traffic jam or during low-speed city driving, so the crank can rotate without the engine actually running.
The parallel drivetrain can move the vehicle with the crankshaft stopped because the motor, generator and engine are all separate units connected by a planetary gear set. There is no transmission: The planetary gear's ring gear engages the motor and the final drive differential, and the control unit manages torque flow through this one gear set.
The motor is the prime mover, supplying torque to the final drive while the engine turns the generator to charge the batteries. The control unit also can send the engine's torque to the final drive when needed.
While this system provides potentially greater fuel mileage, the vehicle will not move if the electric portion of the gasoline/electric hybrid powertrain fails. Except for Honda, most of the hybrids in production or being developed now use a parallel hybrid drivetrain.
Future technology
Today's production hybrid vehicles use a NiMH battery. Like lead acid batteries, they release hydrogen if overcharged, and although the control unit can easily manage this, the battery compartment has its own ventilation system. NiMH batteries are relatively environmentally friendly because the materials can be easily recycled.
The lithium-ion (Li-Ion) batteries being used for the first time in the next Honda Civic hybrid have about twice the energy density, but they're more expensive. They also carry a risk of fire and explosion if ruptured in a crash; however, that risk is probably no greater than a tank of gasoline.
While both of these batteries have higher energy density than lead acid batteries, the lead acid battery can still discharge its energy to the motor faster when connected to identical motors, meaning it has a higher power density. Despite decades of development, the Holy Grail of a practical battery with both high energy density and high power density has still not been achieved. That's why even the most advanced battery-powered electric cars have a top speed and maximum range of about half the typical gasoline-powered car.
Given these limitations, the best way to power an electric car is to generate the electricity onboard. The ultimate goal is to do this with fuel cells, but they won't be ready for about a decade.
Meanwhile, engineers are currently experimenting with a hybrid with a small piston engine that runs all the time. Its only function is to turn a generator, so none of its power is used to move the vehicle or power any accessories.
In addition to batteries, the vehicle would also have a bank of ultra-capacitors. Similar to start-up capacitors used on electric motors, they are an electronic storage device-not a chemical battery. Ultra-capacitors hold a limited amount of energy, but they can be charged very quickly and, most importantly, can discharge that energy extremely quickly. A small bank of ultra-capacitors can deliver enough current to accelerate a vehicle with all the "oomph" of a very powerful gasoline engine.
Energy and power density
Hybrids generate their own electricity for charging their batteries. Some environmental groups are pressuring car companies to equip hybrids for recharging batteries from a wall socket. While this would be a welcome convenience feature for the car's owner, it's expensive and ultimately does nothing for the environment. It takes energy to generate electricity, and transporting that electricity over miles of wire is far less efficient than generating it as needed on the vehicle, especially since the vehicle can generate some of its electricity without using fuel. Understanding the real benefits and limitations of hybrid vehicles will be easier if we understand the concepts of energy, energy density, power and power density.
Energy is defined as the ability to make something move. It can't be created from nothing. But it can be converted from one form to another, and it can be stored. For instance, lets look at the energy conversions in our simplest one-stroke engine, otherwise known as a gun.
When the gun is cocked, human muscle provides the energy to compress the spring. Now the spring contains stored mechanical energy. When the spring is released, its energy moves the firing pin. When the pin strikes the primer, the chemical energy stored in the fuel (gunpowder) is released when the fuel is ignited. That chemical energy pushes against the bullet to make it move, and is therefore converted to mechanical energy. Other engines and motors do basically the same thing: convert one form of energy to another in order to make something move.
Power is the rate at which energy is released. If the firing pin's spring is released slowly, the pin won't strike the primer hard enough to cause ignition. If the gunpowder is spread out in a line like a fuse and burned a little bit at a time instead of all at once in the firing chamber, its chemical energy is released too slowly to make anything move. The faster the energy is converted from one form to another, the more power is generated.
In a piston engine, crankshaft torque is a measure of the fuel's chemical energy that's converted to mechanical energy. Horsepower is a measure of how fast that energy is converted. Even though the engine's torque peaks as engine speed increases, horsepower continues to increase with speed because the rate at which fuel is converted to mechanical energy is also increasing.
Energy density is the amount of stored energy in a given mass. Gasoline has a higher energy density than alcohol, because burning a pound of gasoline will produce more heat energy than burning a pound of alcohol.
Power density is the amount of power produced for the mass of whatever is producing the power. A 300-pound engine that makes 300 horsepower has a power density of one horsepower per pound. A 15-pound engine that makes 30 horsepower has a power density of 2 horsepower per pound. A 10-pound battery that can flow 30 amps has more power density than a 10-pound battery that can flow 20 amps.
In any powered vehicle, fuel plays a part in power density calculations because the weight of the fuel must be considered part of the weight of the whole powertrain. For instance, the average gasoline-powered car is built to accelerate to 60 niph in about 10 seconds and travel about 300 miles on one tank of fuel.
If we increase the vehicles maximum weight, increase the range requirement, require quicker acceleration or use a fuel with less energy density, the total fuel weight must increase to accomplish the same performance goals. The weight of the engine plus fuel that will accomplish the intended speed/ load/range-or the weight of the motor plus batteries that will accomplish the intended speed/ load/range-equates to the powertrain's power density.
Hybrid economics
A hybrid vehicle has two different power plants and two different energy storage systems. This is a high percentage of powertrain weight, or low power density, compared with conventional vehicles. Even by adding the acceleration of both powertrains together, it's hard to get a power density that's equal to a vehicle with only one powertrain.
However, with regenerative braking, the second powertrain can also recover the car's energy of motion (mechanical energy) and store it in the batteries. This adds to the car's overall energy density, but how much it adds depends on how the vehicle is used. Still, even under the least favorable conditions, today's hybrid vehicles get better fuel economy than similarly sized gasoline powered vehicles. So even though hybrids have a lower power density, they still have a higher energy density than conventional vehicles.
Official fuel mileage figures for the Prius are 60 mpg in the city, where the engine is used less, and 55 on the highway. However, it's obvious a different mileage test is needed because most owners are reporting combined mileage figures in the low-to-mid 40s. That's still impressive, but no better than today's diesel-powered Volkswagen Jetta that costs only $1,000 more than its gasoline counterpart. That's one reason that half the new vehicles sold in Europe last year were diesel-powered.
While this has not prevented Toyota from selling gasoline-electric hybrids in Europe, sales there are far lower because small Euro-market cars' fuel mileage is already close to that of today's small hybrids. However, Mercedes Benz has already shown a concept S-Class with a diesel-electric hybrid powertrain, showing that they are confident there will be a large-car hybrid market.
DaimlerChrysler and General Motors have announced a partnership to develop a hybrid powertrain for even larger vehicles, and both plan to introduce hybrid versions of existing SUV platforms some time in 2007. They believe that large hybrid vehicles have a strong future for two reasons. First of all, even though there are already signs of a slowing truck market, they're betting it will remain strong for at least the next few years.
Second, both manufacturers believe it makes more sense to improve fuel mileage on large vehicles than small ones, and in a way they're right. Boosting a truck's mileage by 25 percent, from 20 mpg to 25 mpg, saves one gallon of fuel per 100 miles. Improving a small car's mileage by 25 percent, from 30 to 37.5, saves only two-thirds of a gallon per 100 miles. Multiply that one-gallon improvement by 100,000 vehicles, and the nation's fuel saving over one year is quite significant.
End game
So far we've only seen the first generation of hybrids. The technology will improve and so will the fuel savings, but ultimately it's a doomed market. According to "The Hydrogen Economy" written and published by the National Academy Press, at the end of 2003, sales of new vehicles with conventional powertrains will begin declining sharply and hybrid powertrain sales will increase sharply about seven years from now. They expect hybrid sales to peak with roughly 60 percent of the new car market by 2023.
As hydrogen-powered vehicles reach the market, they expect hybrid sales to decline to almost nothing by 2035. Even though the market is expected to last about 40 years, some companies will choose not to invest in this technology and move straight to hydrogen power instead.
Hybrid vehicles represent, quite literally, the beginning of the end of a transportation system powered by hydrocarbon fuels. While aircraft and large cargo haulers will continue to use traditional fuels for at least the foreseeable future, the personal people movers that consume the vast majority of today's fuel products will be replaced with vehicles powered by hydrogen. It's hard to say exactly when because there are still a lot of problems to be worked out, but the automotive industry as a whole is narrowly focused on that one goal. They're pouring vast amounts of money and resources into developing the technology, and they are actively supported by most of the industrialized nations' governments.
The ultimate goal is sustainable energy. For the past 150 years, we've run our engines on a variety of economically viable hydrocarbon fuels. But the number of engines in the world is now so great and increasing so rapidly, the only fuel economy that really matters is the Earth's ability to absorb what comes out of the exhaust pipe.
Engineers, environmentalists and most of the world's leaders recognize that pumping carbon into the air is not a sustainable energy policy, and they're working hard to change it. The hydrogen economy is inevitable.
Even though we're only seven years into the hybrid market, the end of that market is already certain. But it's still early. Even if this is your first day in the automotive industry, hybrids will probably be around for your entire career.
[Author Affiliation]
By JACQUES GORDON, Contributing Editor
