CHOICES FOR DRIVING A CLEAN CAR TODAY

Car manufacturers say that super clean fuel-cell vehicles are unlikely to enter mass production within the next 10 years.

But as the pressure to reduce vehicle emissions and fuel consumption continues to rise hybrid electric vehicles offer a short term solution that is both technically proven and economically viable.

There are other technologies which also offer environmental benefits such as the ultra-efficient IC engines, clean turbo-diesel engines, ethanol-fueled IC engines, and advanced hydrocarbon fuel technologies, are also evolving. In most cases these alternative technologies are less expensive and appear less risky to the automakers.

In the competitive race to improve drivability, comfort, and safety, while reducing fuel consumption and emissions, automotive engineers are discovering that many of the prospective solutions will require increased electrical power, which reinforces the desirability of at least some level of vehicular hybridization.

The hybrid cars of today cover a range of technologies, each characterized broadly by the extent to which electrical power is used for propulsion in the vehicle. At one end of the spectrum is the 'micro-hybrid', a car that features a "beefed-up" starter, in which fuel is saved during vehicle idle stop, and mechanical energy is captured during braking.

At the other end of the range is the 'plug-in hybrid', in which an electric motor is capable of propelling the car on its own for 5 to 50 miles, and supplements the power of the internal combustion engine in most acceleration events.

To date the most successful hybrids on the market are the strong "full" hybrids. These vehicles employ a 30 to 70-kW electric motor that is engaged frequently during the drive cycle and is powered by an advanced high-power battery, which is charged on board by the IC Engine and by the kinetic energy captured during deceleration and braking of the vehicle.

Central to the discussion regarding the relative merits of the various hybrids is the big box that stores the energy to propel the electric motor-the battery. It is evident that the battery is a key to achieving (or failing to achieve) technical and commercial success with any of the various hybrid architectures.

In fact, the battery is responsible for 25 - 75% of the increased weight, volume, and cost associated with the various hybrid configurations. Even more critical are battery life, reliability, and behavior under abuse as they present the largest threat to the commercial success of hybrid technology. Batteries store electrical energy, which is measured in kWh.

Today's mild, moderate, and strong hybrids on the market utilize batteries with rated capacities of 0.6 to 2 kWh. In general mild hybrids require smaller batteries than do strong hybrids.

The rated energy capacity of the battery is dictated by the battery's level of usage (the duty profile), and includes a significant margin for life, to meet the 10-year minimum life requirement of the automotive market.

In today's hybrid batteries, only about 10% of the rated battery capacity is used frequently, and up to an additional 30% is accessed under extreme driving conditions. The remaining capacity is in place to ensure adequate service life.

All hybrids with moderate to significant power-train hybridization employ a NiMH battery as the main electrical-energy storage device. NiMH batteries are a reliable power source for hybrid cars and field results suggest long life.

However, NiMH batteries are not an ideal energy-storage device for cars. Their limitations include moderate energy conversion efficiency, which translates to some energy loss and significant heat production in normal usage, reduced life with high depth-of-discharge (DOD) cycling, and unsatisfactory performance at high and low temperatures. NiMH battery packs for HEVs are priced at $900 to $1500 per kWh, which brings the price of today's pack to between $600 and $3,000 per vehicle.

The 2006 NiMH battery market for HEVs is estimated at $600 million.

Although NiMH is currently the most economical (and only proven) power source for the application, it has limited potential for cost reduction as production volume further increases.

Lithium-ion batteries offer higher power and energy per unit weight and volume, and better charge efficiency than NiMH batteries. However, the reliability of lithium-ion technology for automotive applications is still not proven and its current cost is higher than that of NiMH.