AI Online


Hybrid Driven

Toyota aims the 2004 Prius at a mass-market audience and gives us a glimpse of things to come.

Prius’ slipery shape sets it apart from the rest of the Toyota pack.
While the rest of the automotive world is struggling to make a business case for hybrids, Toyota Motor Company is making money. According to Toyota, the last few month’s worth of the current generation Prius brought in a small profit (industry insiders say about $1,100 dollars per vehicle).

Toyota credits the profit to a savings in manufacturing costs, (The first generation Prius shared the Motomachi-Sumi, Japan, assembly line with Rav-4 — the 2004 Prius will be built in Tsusumi on the same line with Camry) and the licensing of the technology to Nissan, who will buy transmissions and power electronics from Toyota, supplying its own Atkinson cycle engine. Nissan plans to sell 100,000 hybrid vehicles over a five-year period starting in 2006.

Dave Hermance, executive engineer environmental engineering says that Toyota has also had conversations with Ford, GM and DaimlerChrysler about the possible joint-use, joint-development and patent sharing of Toyota’s hybrid technology.

“We had a technology collaboration agreement with GM,” says Hermance. “It hasn’t yet generated a product and it might not. Our President (Fujio Cho) has said that Toyota is open to licensing this technology to anyone that wants to use it — obviously not for free.”

It’s the development of this technology that will continue to put money in the bank as the volume of hybrid electric vehicles (HEV) continues to climb. President Cho has said that Toyota has plans for global production of 300,000 HEVs a year by 2005-2006, with about half of those slated for North America.

To reach those numbers, Toyota will need to sell hybrid technology to mainstream consumers. Enter, the second-generation 2004 Prius. While the original car proved that hybrid technology made real-world sense, the 2004 Prius is designed to introduce the benefits of hybrid technology to a much larger audience.

The interior is one of the classiest that Toyota has done.
The 2004 Prius (Prius II) is classified as a midsize car by the EPA. It has a six inch longer wheelbase and is five inches longer overall than the previous car. The biggest growth spurt comes in the interior where space is increased from 101 to 112 cubic feet. The sharply-styled interior has seats that fold flat into a bed and a 60-40 split folding rear seat that allows for storage of longer items while keeping a third passenger seat.

Though the ’04 car is bigger, engineers managed to keep the weight gain to a minimum thanks to the extensive use of aluminum for the hood, rear hatch, front and rear bumper reinforcements and brake and suspension components. Prius II also features a lightweight plastic fuel tank and Toyota says that the greater reliance on the regenerative braking system allowed them to reduce the size of the front discs and go to a composite caliper.

The steel unibody has fascias made of recyclable TSOP plastic. Toyota refers to the design as a triangular monoform, which not only gives Prius its contemporary stylish look but a 0.26 coefficient of drag as well. The fifth-door design should appeal to European customers who have shied away from the previous generation car.

“The European arm of Toyota told us that the reason the ’01 and ’03 Prius didn’t sell well in Europe had nothing to do with hybrid versus diesel,” says Hermance. “The European market strongly demanded a hatchback.” The 1998 Prius concept, designed by Calty, was a hatchback, but the production car was changed to a sedan.

The ride is greatly improved by trading in the low rolling-resistance tires for P185/65R15 all-weather radials mounted on aluminum alloy wheels.

While both Ford and Honda have decided to market hybrid technology as an option in standard production vehicles, Toyota will keep Prius unique.

“Part of the vision,” says Hermance, “is that Prius will be a technology demonstration vehicle.”

Hatchback design makes for much cargo room. All seats fold flat with a 60/40-split rear seat. The hatchback should appeal to European buyers as well as young Americans.
The ’04 Prius is loaded with optional telematics goodies that are sure to whet the appetites of those interested in a vehicle like Prius. The Smart Entry/Smart Start system uses an embedded chip in the key fob to communicate with the vehicle enabling the lock, which opens when you touch the back of the door handle and allows you to start the vehicle without inserting a key.

There is also an optional Bluetooth-enabled hands-free cell phone system and a 7-inch touchsensitive LCD screen that displays standard features like the energy monitor, radio, HVAC controls and optional features such as Toyota’s nextgeneration navigation system and the hands-free telephone functions. The power-flow screen now displays instantaneous fuel economy.

What North Americans won’t get is a selfparking option and the electric-drive-only option that will be available on the Japanese car. The ’04 Prius features true by-wire technology. Both throttle and shifter are full bywire systems. Hermance says that the brake system is cooperative.

“When you step on the brake pedal, you generate hydraulic pressure,” Hermance says, “but it’s not metered to the wheel cylinders until the computer decides that it needs hydraulic help.” The system is designed to utilize the regenerative system as much as possible and cover the shortfall with the hydraulic brakes. In an emergency, the system will pass the hydraulic pressure on immediately.

The ’04 Prius uses a new version of regenerative braking called electrically controlled braking. The system combines the benefits of ABS, electric brake distribution and brake sensors, increasing the regenerative capability of the system.

Another unique feature is the electricinverter air conditioning. The compressor is driven by an integrally-mounted 3-phase 200 volt A/C electric motor allowing it to operate without the engine running at idle, increasing fuel efficiency and reducing emissions. In addition to cooling the car in the summer, the system can also be used to defrost windows during the winter.

The fourth-generation nickle-metal hydride battery bolts neatly under the package shelf. The battery is warranteed for 8-years or 80,000 miles. Toyota says it should cost about $1,000 dollars to replace.  
The heart of the 2004 Prius and the future of Toyota powertrain lie in the second generation Toyota Hybrid System now branded as Hybrid Synergy Drive.

The Toyota Hybrid System is a power split system, similar to the system Ford developed for the Escape and, according to Hermance, the same type of system that will be used in the 2005 Saturn view. Basically, the power split system takes an automatic transmission, (a sequentially-shifted automatic or a CVT) and provides it with two separate inputs, one for the electric system and one for the engine. The two functions are combined in the transmission providing the ability for either ICE, electric drive or optimally, the best of both worlds.

Engineers made several improvements to the Hybrid Synergy Drive system. Peak power to the motor was increased from 33Kw to 50Kw (a 50 percent increase) and torque was increased by about 15 percent to 400 Nm. Improved motor control improved mid-range torque by about 30 percent. Maximum speed has been increased to 6,700 rpm.

The generator was structurally increased allowing it to turn at a higher speed for a more aggressive control of the engine. The result was higher engine-off operation while still being able to start the engine. This also improved the balance between the electrical system and the engine system. A high-voltage converter boosts power to 500-volts allowing for the use of the more powerful motor and generator while maintaining the same current level.

“When you buy electronic components, you pay for the ability to carry current, not the ability to withstand voltage,” says Hermance, “so if you reduce the current in the system, you can reduce the cost.”

Because of the increased efficiency of the system, engineers were able to reduce the size of the power switching devices (the eight insulated bi-polar transistors in the power electronics) by about 20 percent.

“Not only is that a savings in cost and space,” says Hermance, “it also increases efficiency because there’s less heat rejection from the devices.”

Engineers added the new circuits for the electric air conditioning and the high-voltage output while still maintaining the same size package. The system runs off of Toyota’s latest generation 200-volt nickel-metal hydride battery.

The modules inside the battery pack have been increased in specific power by about 35 percent and the number of modules was reduced from 38 to 28. The same power output was maintained by using the higher power density (21 kW) batteries.

The power rating of the battery is 1,250 watts per kilogram, more than a kilowatt in each kilogram module in the system. Hermance says that’s a power density higher than any other nickel-metal hydride technology. The 1.5L Atkinson Cycle engine is a carryover from the previous generation vehicle. Engineers increased rpm by 500 (from 4,500 to 5,000) which in turn increased maximum power output by 5 kW.

The combination of the more powerful engine and electrical system and increased efficiency of the control system reduced the amount of engine operation needed for city driving. During the U.S. certification (FTP) testing, 3.6 miles of the 7.5-mile test were run with the engine off.

Changes were also made to the emissions technology. The previous generation car had a hydrocarbon-absorber ring around the outside of the under-floor catalyst and a diverter valve. At cold start the hydrocarbons were stored in the absorber and once the catalyst came up to temperature, the valve changed position and the stored hydrocarbon was bled off through the catalyst system.

Hermance says that this was a relatively complex mechanical system and Toyota engineers were concerned about the durability. Prius II is equipped with a coolant heat storage system, essentially a three-liter vacuum bottle storing high temperature coolant.

After the ready light comes on and before the engine starts, the heated coolant is released into the head, substantially increasing the core temperatures in the head avoiding fuel condensation during cold starts. This in turn significantly reduces the amount of engine hydrocarbons at the engine out position making them easier to control in the inverter position.

Once the engine is running, the vacuum bottle is bypassed and when the engine comes up to temperature, high-temperature coolant is diverted into the bottle for the next cold start. If, for some reason, the coolant in the vacuum bottle isn’t hot enough, the system waits for the hot soak event to occur and uses an external pump to pump the hotter coolant into the vacuum bottle.

When you slide behind the wheel of the Prius II, press the brake pedal and hit the start button, the IP lights up but nothing else happens. Push down on the accelerator pedal and you begin to move.

Prius is launched by the electric motor the engine starts when needed by spinning the generator. The engine can be started at any positive vehicle speed as long as it doesn’t exceed the 10,000 rpm limit of the generator. There is no reverse gear in the transmission. Reverse is an electrically-operated function that turns the electric motor backwards.

“The beauty of the electric CVG-type system”, says Hermance, “is that at a fixed vehicle speed, you can control engine speed by changing the generator speed.”

The continuously-variable operation runs the engine at its most efficient without the complications of belts or hydraulically-actuated pulleys. The majority of the drive force comes from the electrical side, either as energy moving from the engine, to the generator, to the motor or from the battery to the motor.

Peak power is provided electrically allowing for a more efficient use of the gasoline engine. Prius II has a top speed of 103 mph and will accelerate from 30 to 50 mph about a second quicker than a Camry. The advertised peak power is lower than a conventional drivetrain, yet it achieves the acceleration performance of a much more powerful system.

“Based on conventional wisdom,” says Hermance, “this vehicle shouldn’t accelerate as fast as it does. But this is non-conventional wisdom.” Prius II will not only offer dual-stage front airbags and optional side and side curtain airbags, but will be the first Toyota vehicle to rely on an entirely electric and electronic active safety package.

A centralized computer will link the electric steering and braking systems along with ABS, EBD (electronic brake distribution) and VCS (vehicle stability control). A total electronic system will make it possible to achieve quicker reaction times than a system that relies on a traditional hydraulic circuit for brake and steering assist. Gas mileage is EPA rated at 51 city 59 hwy for an average of 55 mpg, 15 percent better fuel economy than the previous car. The 11.9 gallon fuel tank gives it a 600 mile range.

Why put so much money and effort into hybrid technology? Toyota sees its hybrid strategy as a bridge to fuel cell technology. A remarkable amount of Toyota’s hybrid technology is adaptable to fuel cells. The battery pack, power electronics, regenerative braking and motor technology are identical.

“The total system power requirement in the fuel cell is a little higher,” says Hermance “so you need a little more current, but we’ll have more current flow in the RX(330) version.

“A lot of the control system logic is the same,” Hermance says. “The specific code is different but the general learning of how to bolt two systems together in a seamless manner is very much similar.”

Hermance says that you could basically take out the ICE and replace it with a fuel cell stack. “You can also take the power split device out,” says Hermance, “because you don’t need to run that mess, you’d just have pure electric drive.”

Toyota is already the world’s largest producer of insulated-gate bipolar transistors, and they are working right now on the fourth generation nickel-hydride battery.

Hermance smiles, “Who’s going to be better positioned to have cost effective product when the fuel cell stack is ready?”

Hybrid systems may also be poised to take the place of diesels in Europe once the new emissions take effect in 2007.

“Right now there’s a big hump to get over with the new NOx restrictions,” says Lindsay Brooke, Senior Manager, Market Assessment at CSM WorldWide.

Brooke says that the technology needed to bring European diesels up to 2007 specs would put a 200 percent cost penalty on diesel engines. “The cost of hybrid technology will continue to go down as the cost of diesels goes up,” Brooke insists. He says that we’re going to see worldwide, a coming together of U.S., Japanese and European emissions standards.

“There’s huge value in a vehicle that’s PZEV Tier 2 Bin3.

“It’s going to open a lot of people’s eyes,” says Brooke, “and help Toyota’s efforts in convincing European consumers that it’s an alternative to diesels.”

Previous posts

Next posts

Fri. May 17th, 2024

Share this post