Hybrid Architectures: Series vs. Parallel
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. Hybrid vehicles are commonly categorized as being arranged in series and parallel configurations.
Series Configuration
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.
Key Limitation: 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.
Parallel Configuration
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.
Major Hybrid Systems in the Market
Comparison of Leading Hybrid Systems
| System / Manufacturer | Type | Key Technology | Example Models | Efficiency Gain vs. Conventional |
|---|---|---|---|---|
| Honda IMA (Integrated Motor Assist) | Mild Parallel Hybrid | Electric motor-generator replaces flywheel; cannot power vehicle alone | Insight, Civic Hybrid, Accord Hybrid | 25-45% (Civic Hybrid) |
| Toyota HSD (Hybrid Synergy Drive) | Full Series/Parallel Hybrid | Exhibits both series and parallel characteristics; each power source can drive independently | Prius, Highlander Hybrid | ~72% (Prius vs. Corolla) |
| Ford Hybrid System | Full Series/Parallel Hybrid | Licensed Toyota technology; similar to Prius powertrain | Escape Hybrid (2005) | ~50% city driving |
| GM Belt-Alternator-Starter | Idle-Stop System (Not classified as hybrid by CARB) | Motor-generator replaces starter/alternator; shuts off engine at stops | Chevrolet Silverado/GMC Sierra (Fleet) | ~10% city driving |
Honda’s Integrated Motor Assist (IMA)
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.
Emerging Concepts and Future Directions
Idle-Stop Strategies
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 it when power is required.
- GM’s system uses an electric motor-generator device that replaces the standard starter and alternator
- Charges batteries during deceleration (regenerative braking)
- Modest efficiency payoff: ~10% in city driving (1-2 mpg gain)
- California Air Resources Board (CARB) does not classify such systems as “hybrids”
Diesel/Electric Hybrids
At some automakers such as DaimlerChrysler, research so far favors diesel engines over hybrids as the next step in saving fuel.
- Chrysler developing mild hybrid Dodge Ram with turbo diesel and motor-generator
- Initial fleet of 100 units for commercial customers only
- Mercedes-Benz developing diesel hybrids (Vision Grand Sports Tourer concept)
Industry Collaboration and Licensing
The hybrid technology landscape features significant cross-company collaboration:
- Ford Escape Hybrid uses technology licensed from Toyota (though no parts are directly supplied)
- Nissan has signed an agreement with Toyota to use HSD in future models
- Nissan Altima hybrid in development combining Nissan engine with Toyota HSD components
- More than 250,000 Toyota Prius vehicles (HSD and earlier THS) sold globally over seven years
