Hybrids Keep Getting Better

Production hybrid vehicles have been rolling on U.S. highways for a mere six years. While carmakers are well-known for their reuse of hardware over several model years, new hybrid components have frequently appeared with each new model. This is due, at least in part, to the current low volume of hybrid vehicle sales and lack of dedicated high-volume assembly for the unique components in these vehicles. However, production hybrids come in a variety of different vehicle types from high economy vehicles designed specifically as hybrids, to hybrid versions of passenger cars, sport utility vehicles (SUVs), and trucks — each requiring unique hardware. Perhaps the most aggressive changes have occurred in the batteries but other parts have advanced as well. The ultimate success of these and the many new models being promoted by every major carmaker hinges on improvements in performance, as well as weight, size, and, most importantly, cost reduction.

A Battery of Changes

Looking inside a few hybrids reveals the component complexity and diversity. However, the battery is one of the more-difficult-to-find components. In many hybrids, the battery is located under the seat. About the only thing standard about today's hybrid batteries is the use of nickel metal hydride (Ni-MH) cells that provide 1.2V per cell. Each vehicle requires unique performance and packaging. The 2000 U.S. Prius had battery improvements over the initial models sold only in Japan from 1997 to 1999. The revisions have continued in each new Toyota hybrid.

The Lexus battery system in the 2006 RX 400h consists of 30 Ni-MH modules — each with eight cells that develop 9.6V. The cells have higher power density than the previous generation with the total battery pack providing 288V and a peak power of 45 kW. By revising the metal case, Toyota improved the heat rejection, and reduced the size and the module height by 22 percent. The batteries are split into three groups for easier mounting in the vehicle with a dedicated cooling fan for each group.

	The transmission in GM's two-mode full hybrid system integrates two electric motors into the transmission that work with traditional transmission gears and electronic controls to provide two modes or ranges of infinitely variable gear ratios.

According to Naritomo Higuchi, assistant chief engineer, Honda R&D, the 2005 Honda Accord Hybrid's improved battery efficiency resulted from reduced internal resistance between cells (19.6 to 12 mV) by increasing the number of weld points from 56 to 104. This allowed Honda to increase the output by 20 percent and reduce the size by 10 percent compared to the 2003 Civic Hybrid.

Belts and Suspenders

Since the engine is not running at idle in hybrids, eliminating all of the traditional belt-driven components is a key design strategy. This means that the alternator, power steering pump, water pump and ac compressor require an alternate

source of energy. The alternator was easy to replace since it and the starter motor are among the parts that are eliminated in most hybrids. The other units required additional design effort.

Toyota and Denso co-developed a high-voltage electric ac compressor to eliminate the belt-driven compressor. An integrated motor in the electric compressor uses new structures and control methods for the motor and inverter resulting in a 40 percent smaller and 50 percent lighter unit than conventional electric compressors. In vehicles such as the Lexus RX 400h, high voltage from the battery powers the compressor.

Some future mild hybrids, such as GM's 2006 Saturn VUE and 2007 Chevy Malibu, will buck the beltless trend and use a belt-driven 42V starter generator. This allows the continued use of traditional belt-driven components. Other future GM hybrid vehicles appear to have belt-drive options. The two-mode full hybrid being developed by GM, DaimlerChrysler and BMW uses a special transmission that includes two motors. The electrically-variable transmission has two hybrid drive modes, one operating up to 50 mph and the second at higher speeds.

	The 2006 Mercury Mariner components include a 300V battery pack, a constant variable transmission (CVT), electric traction motor and integrated generator/starter.

The first hybrid SUV, the 2005 Ford Escape Hybrid and now the 2006 Mercury Mariner Hybrid use a 300V battery pack, a constant variable transmission (CVT), electric traction motor and integrated generator/starter that supports an Atkinson cycle 4-cylinder engine. A four wheel drive (FWD) SUV hybrid, such as Toyota's 2006 Highlander, uses three motors that Toyota identifies as MG1, MG2, and MGR. MG2 is the front electric drive motor. MG1 is the engine-driven generator/starter and MGR is the rear electric motor. Both MG1 and MGR recharge the battery but the 50-kW MGR is used only for the 4WD version providing the additional power for the rear wheels.

While individual hybrid components may come from different suppliers they are frequently combined in highly integrated subassemblies. Honda Motor Company's Integrated Motor Assist (IMA) has its permanent-magnet electric motor mounted between the engine and transmission. The controls, including a built-in ac inverter, the inverter for the motor, dc to dc converter for reducing high voltage to 12V, the battery and motor and battery electronics, are all packaged in an air-cooled assembly mounted behind the rear seat in the 2006 Civic Hybrid. The modular assembly requirements add to the unique and custom aspects of hybrid components.

Competition Heats Up

	The 2006 Honda Civic's Hybrid Intelligent Power Unit co-packages key components for easier mounting and cooling.

About the only negatives associated with hybrids today revolve around cost — the added cost of the system to buyers around $3,000 per vehicle and replacement cost of the batteries. With Toyota being the most successful hybrid manufacturer, it comes as no surprise that Japanese suppliers, such as Denso, Panasonic, Sanyo and Aisin AW, provide most of the new components. However, U.S.-based Energy Conversion Devices and Johnson Controls have developed battery technology for hybrid applications.

Perhaps the most aggressive competition to Japanese component suppliers is Continental Temic. The creator of the integrated starter alternator damper (ISAD) though acquisition, Continental is focused on supplying complete hybrid solutions. Partnering with ZF Friedrichshafen AG, a leading supplier of vehicle driveline products, Continental plans on providing mild hybrid engines, storage management and the associated hybrid electronics.

The new component suppliers promise to make future hybrids even more attractive. As Larry Nitz, executive director General Motor's Global Hybrid Powertrains observes, "Right now, hybrid vehicles are an emerging technology. As the technology becomes more mature, the supply base will develop, competitive pressures will increase, consumer demand will grow and costs will decline."