There are many ways to blend the internal combustion (IC) engine and electric power. These technologies have much in common-and many points of differentiation. In addition to Toyota's Hybrid Synergy Drive (HSD) system, other manufacturers have devised alternative means of propelling cars and trucks with a blend of fossil fuel and electricity.
Hybrid vehicles are commonly categorized as being arranged in series and parallel configurations. WW II-era diesel submarines are a good example of the series arrangement. In this layout, an internal combustion (IC) engine spins an electric generator. Power from that generator is routed to storage batteries and to one or more electric motors. The electric motor(s) alone drive the vehicle. There is no direct mechanical link between the IC engine and the propulsion of the vehicle. So far, all vehicle makers seem to agree that the series configuration is impractical for automotive use due to its lower efficiency.
The parallel layout provides a means of linking two power sources-typically a gasoline engine and one or more electric motors-to the vehicle's drive wheels. Each source may or may not be able to drive the wheels independently of the other, and therein lays another point of distinction.
The most common parallel hybrid system is currently offered in the two-passenger Honda Insight, five-passenger Civic Hybrid, and the new Accord Hybrid. Honda positions an electric motor-generator (MG) in place of its IC engine's flywheel; this system is called Integrated Motor Assist. The MG bears some propulsion responsibility, it starts the internal combustion (IC) engine, and it charges the batteries, but it doesn't power the vehicle independently of the IC engine. Since it doesn't function on electric power alone, Honda's Integrated Motor Assist is best described as a mild hybrid system.
Efficiency gains in the Honda IMA system are less than those achieved by Toyota's HSD. Honda's Civic Hybrid offers 25-45-percent better fuel mileage than a conventional Civic. There is no other Toyota model comparable in size to the Prius, but as a point of reference, it beats the combined EPA estimated fuel economy rating of the smaller Corolla LE sedan with automatic transmission by 72 percent.
To date, Honda's hybrids have used a three- or four-cylinder gasoline engine aided by an electric motor. For 2005, the concept will be extended to a larger Accord sedan that combines a 3.0-liter V-6 with electric assist. The Accord Hybrid V-6 engine is programmed to use only three cylinders while cruising and to shut off entirely when the vehicle is at a stop.
According to Toyota's Dave Hermance, "Toyota's Hybrid Synergy Drive (HSD) differs substantially from Honda's system in that it is a full hybrid system that, depending upon driving circumstances, exhibits both series and parallel characteristics. Like a series hybrid, the Prius gasoline engine powers a generator, which supplies electrical current to an electric drive motor. Like a parallel hybrid, both the IC engine and the electric motor are capable of providing power to the wheels." HSD allows each power source to drive the vehicle independently (see "Inside the HSD" for a complete explanation), which is something competitive mild hybrids can't do.
More than 250,000 (with both HSD and the earlier Toyota Hybrid System (THS)) Toyota Prius have been built and sold during the past seven years, all over the globe. Following that success, two other makers have devised their own variations of the series/ parallel theme. The new-for-2005 Ford Escape Hybrid has a powertrain that is similar to that of the Toyota Prius. Although Toyota does not directly supply any parts or software for the Escape Hybrid, a licensing agreement permits Ford to use technology patented by Toyota. Nissan has also signed an agreement with Toyota to use HSD in the future. Efforts are well underway to develop a Nissan Altima hybrid, a car that will combine a Nissan engine with HSD components supplied by Toyota.
Another emerging concept doesn't use electric power to propel the vehicle at all, but instead shuts off the IC engine when it's not needed, then quickly restarts the IC engine when power is required. (This same strategy is found in true hybrid systems, as well). In some GM fleet and future consumer trucks, an electric motor-generator device replaces the standard starter and alternator. It's wired to batteries that store energy. The batteries are charged by two means: During deceleration, the motor-generator switches to generator mode to convert unwanted vehicle momentum into an electrical charge that can be used later. If this charging means proves insufficient, the generator draws power from the engine. Since the motor-generator device does not help propel the vehicle, a conventional gasoline engine handles that job unaided.
Shutting off the gasoline engine at stops saves some fuel. Since the motor-generator is far more powerful than a conventional starter, it quickly and easily wakes up the sleeping engine when the accelerator is tapped after the stoplight turns green. Operation is automatic and relatively seamless, but the efficiency payoff is modest. According to EPA estimates, the benefit is about 10% in city driving, which is at most, about a 1-2 mpg gain. Also, it is worth noting that the California Air Resources Board (CARB) does not classify such systems as "hybrids" at all. To qualify as a hybrid, CARB requires that the electric motor assist in propelling the vehicle.
GM began offering Chevrolet Silverado and GMC Sierra pickup trucks with this type of system to fleet operators last year. Expected to begin with the 2005 model year, they will be sold to regular customers living in California, Oregon, Washington, Alaska, Nevada, and Florida. The remainder of the U.S. is expected to be able to purchase these pickups sometime in the future.
The 5.3-liter V-8 engine fitted to these pickups is the same engine that powers regular Silverados and Sierras. Though a specific timetable for their introduction has not been released, GM has also announced plans for similar automobile systems that will use a four-cylinder or V-6 engine teamed with a belt-driven motor-generator.
At some automakers such as DaimlerChrysler, research so far favors diesel engines over hybrids as the next step in saving fuel. Chrysler will offer a mild hybrid Dodge Ram pickup truck combining a turbocharged diesel engine with a crankshaft-driven motor-generator. A demonstration fleet of 100 units will be offered to business and commercial customers, but Chrysler has announced no plans to sell hybrids to regular car and truck buyers. Mercedes-Benz is developing diesel hybrids as well, and showed the diesel-hybrid Vision Grand Sports Tourer at the January 2004 North American International Auto Show.