Standing at the gas pump, watching the dollar total spin up into the stratosphere, haven't many of us dreamed of owning an electric-powered car that can thumb its hood ornament at the oil companies as it whizzes us ever so quietly by their filling stations? Aside from the promise of oil independence, there's the very real need to reduce environmentally polluting vehicle emissions. For now, electric vehicles are the only cars available to consumers that qualify as zero-emissions vehicles (ZEV) under emerging regulatory standards.
Yet despite some progress in the development and production of electric vehicles, their advancement seems stuck in the stop-and-go traffic jam of battery development. Building pure electric vehicles with a just-adequate driving range still requires a heavy array of expensive batteries, which pushes up the vehicle's sticker price to levels that discourage all but the most gung ho electric-car enthusiasts. In time, that situation may change. But for now, it appears that hybrid-electric vehicles stand a much better chance of electrifying the automotive world.
The current crop of hybrid-electric vehicles (HEVs), both those in production and those in the preproduction prototype stage, supplement the driving power supplied by the conventional internal-combustion engine. They do this by incorporating an electric motor, a more powerful alternator, and a higher-capacity battery into the vehicle's power train. HEVs can be configured as either series, parallel, or series-parallel combinations of gas and electric power (see "Construction And Classification Of Electric Hybrid Vehicles," p. 92).
While these cars don't eliminate vehicle emissions, they do significantly reduce them. Even organizations such as the California Air Resources Board (CARB), which is pushing car makers to ramp up production and sales of electric vehicles, have recognized that HEVs will play a key role in satisfying stricter environmental regulations.
CARB says that carmakers may use HEVs to partially satisfy the CARB mandate for sales of ZEVs. CARB is requiring that zero-emissions vehicles make up 10% of the cars sold in California by 2003. Using a complex formula that takes a vehicle's driving range and fuel efficiency into account, CARB rates HEV models as partial ZEVs. For example, a Toyota Prius rates as 0.3 ZEVs. Note that at the moment, the production of electric vehicles has greatly declined as vendors reassess the viability of their products.
Ford Motor Co. is one automaker that believes the future for electric vehicles may lie in niche markets such as neighborhood vehicles. By developing the Think city vehicles, this maker is developing cars that aren't powerful enough for highway driving but are sufficient for use in urban environments or gated communities.
There are a few reasons why HEVs are now considered better candidates for high-volume production than pure electric vehicles. Above all else, hybrids offer a better ratio of price to performance. This reflects a number of differences between the two vehicle types. Because the hybrid doesn't rely strictly on electric drive, HEVs get by with few batteries and a smaller electric motor, cutting down on both the cost and weight of these elements.
Moreover, a hybrid power train doesn't require lengthy and perhaps inconvenient recharging of batteries. And, despite its limitations in the amount of electric drive power and energy storage, hybrid-concept vehicles are achieving fuel economy ratings of 80 miles per gallon. How do they do it?
HEVs exploit a number of gas-saving techniques. One is stop-start operation, a method that shuts down the gas engine when the vehicle stops, saving fuel consumed during idle periods. When the driver accelerates after stopping, the electric motor kicks in, propelling the car forward and restarting the combustion engine. The electric drive may also provide a boost to the engine as needed. That electric-motor assistance allows the use of a smaller, lighter engine, as it can be sized to accommodate average rather than peak loads. In turn, this allows an improvement in the operating efficiency of the engine.
Another tool available to HEVs is regenerative braking. It recovers the energy used to slow down or stop a vehicle, converting mechanical braking energy from the combustion engine back to electrical energy, which is then stored in the battery. Optimizing all of the electrical and electronic elements of the power train achieves additional gains in fuel efficiency. These components include the electric motor, alternator, batteries, and power-conversion and management circuitry.
Beyond the electrical system, there also is the need to reduce the total weight of the entire vehicle. To this end, HEV designs are turning to special light-weight materials that replace steel autobody components with aluminum and molded plastic parts. With all elements of HEV design, the availability of low-cost manufacturing processes is critical to make the technology feasible. One major incentive for HEV development is that it makes maximum use of the existing automotive manufacturing infrastructure and supply chain.
Another factor favoring hybrids is the increasing demand for electrical power in the car. According to data presented by Delphi Automotive Systems, the steadily rising demand for electrical power will push the level of electrical power consumed in the car past that consumed in the home (Fig. 1). HEVs, which boost the car's power-generating capacity for propulsion, also make more electrical power available to power-train components and the growing list of vehicle accessories. Without a migration to hybrid designs, vehicles will either suffer a degradation in fuel efficiency or else face more-restrictive electrical power budgets.
Hybrid designs will not only make more electrical power available for running the car and its accessories, they also will foster the development of better energy management systems. These will implement higher-voltage systems, improvements in batteries and other energy storage devices, advanced electrical generator design/motor design, more-efficient power-conversion components, and sophisticated control schemes to optimize power management.
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