It's no secret that hybrid electric vehicles get pretty incredible fuel economy. In fact, the top three vehicles in EPA fuel economy ratings these days are hybrids, topped by the Honda Insight with its amazing 68 miles-per-gallon highway figure.

How do these vehicles accomplish such a seemingly impossible feat? Really, the answer is as simple as the current hybrid powerplants from Honda and Toyota are elegant.

Historically, vehicles have been equipped with engines designed to handle all possible driving needs. While it may be that your daily driving might consist of heading to school or work, stopping by the market, and handling a variety of other errands, there's that occasional need to drive up a steep grade or get out of a tight spot with a quick burst of acceleration. To design a vehicle to do anything less is to sacrifice driver confidence and satisfaction, let alone safety.

That's why cars are equipped with engines much more powerful than needed most of the time. Though necessary, covering all possible needs is a dynamic that's also inefficient since, all things being equal, smaller engines are generally more fuel efficient than larger ones, sometimes significantly so.

Honda addressed this with the integrated motor assist (IMA) powerplant that debuted in its 2001 Insight hybrid electric vehicle. This two-seater uses a smaller-than-normal 1.0-liter, three-cylinder VTEC-E internal combustion engine that provides the power needed for most, but not all, everyday driving needs. On those occasions when heavy acceleration or climbing is needed, this hybrid turns to an ultra-thin, 10-kilowatt (13 horsepower) pancake-shaped electric motor located between the engine and transmission for supplemental boosts of power. Together, the two powerplants produce a combined 67 horsepower. Not a high-performance package, but one that confidently handles all driving needs.

Honda's follow-up hybrid, an iteration of its popular Civic, uses a larger 1.3-liter, 4-cylinder, dual port sequential ignition engine with two spark plugs per cylinder and the same supplemental electric motor. This IMA powerplant puts out a combined 110 horsepower to provide a very satisfying driving experience.

Toyota takes a different approach to hybrid propulsion. The Toyota Prius' hybrid system allows a driver to accelerate from a stop solely on electric power, generating absolutely zero emissions. Then, at a certain threshold, the 67 horsepower electric motor turns propulsion duties over to the vehicle's 76 horsepower, 1.5-liter four-cylinder internal combustion engine, which starts and takes over seamlessly. Both internal combustion and electric propulsion systems are used when driving demands warrant. The Prius' forward momentum is recycled into electrical energy through the car's motor-generator during braking and, like Honda's hybrids, this sedan's powerplant turns off completely when stopped to save energy.

As might be expected, some pretty powerful onboard computing power makes all this happen. In a carefully choreographed electronic dance, a hybrid's power control system determines when the car is powered with electric power only (in the case of the Toyota Prius), internal combustion power only, at what point electric boost is needed, when the engine-generator should provide power to the electric motor and battery pack, and when the regenerative braking system should recycle kinetic power during braking and coasting to recharge the hybrid vehicle's nickel-metal-hydride batteries.

While these three currently available models do illustrate the use of highly efficient, moderate horsepower powerplants, this doesn't mean that hybrids must by necessity be at the lower end of the horsepower chart. In fact, the Acura DN-X hybrid concept shows just what automakers might have in mind for hybrids, as this performance-oriented sport sedan incorporates a 400+ horsepower IMA powerplant to provide some significant highway motivation.

Other hybrid configurations are in development and production. A pure series hybrid uses an internal combustion or compression ignition engine-generator solely for providing electricity to a vehicle's drive motor, or motors. This type of powerplant has been used in diesel-electric locomotives for decades. A parallel hybrid allows a driver to select whether to drive via a car's traditional engine or an electric motor.

While generally not thought of in this way, the fuel cell vehicles now being field tested by various automakers are actually hybrids since they use an electrochemical engine (the fuel cell) and batteries to power electric motors. Other types of hybrids in development use energy generated during a vehicle's deceleration to compress air or hydraulic fluid in a high-pressure accumulator, then release this energy as a power assist while accelerating from a stop.

What the Future Holds

We will see other hybrid adaptations come to the fore, including the possibility of plug-in hybrids that allow driving a hybrid vehicle solely on electric power most of the time. This hybrid's internal combustion or compression ignition engine would only be used for more extended travel. Of course, such a configuration would require a much larger and heavier battery pack than the hybrids currently in showrooms and they could require lengthy recharging times, potentially presenting some of the same cost, packaging and operating challenges as the battery electric vehicles test marketed by some automakers in the late 1990s.

Hybrid electric power may be viewed as complex and a less-than-intuitive powerplant evolution by some, but then, major innovations in motor vehicle technology always seem that way until they blend into the mainstream. (Hoseless carriage, anyone?) By all indications, that's exactly the course that hybrid power now seems to be taking in the auto realm.

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