The Toyota Prius and other hybrid models from the Big-T use that company's Hybrid Synergy drive. It's a system that's become familiar over the years, but even hybrid fans may not be aware that the system includes not one, but two electric motors. It also has two clutches and, depending on the implementation, up to three sets of planetary gears. Similar setups are used in models from Ford like the Fusion Hybrid, in GM's two-mode hybrid models, the Honda Insight and other hybrids.
The two-motor system provides a good deal of flexibility. Both motors can drive the car when a boost is needed, both can charge the batteries, or one can actually charge the batteries while the other drives. The system also increases smoothness. If you've ever driven one of these hybrids you've probably experienced the near-seemless transition between gas and electric operation. The infinitely variable ratios this setup allows help to keep the gas engine and electric motors working at their optimum speeds.
However, the two-motor system is also complex, costly, and potentially more difficult to maintain. For that reason, many hybrid systems entering the market in the last couple of years have only one electric motor. This is true for offerings from VW, Hyundai, and Nissan. Squeezing the single electric motor in between the gas engine and the transmission means that there are many fewer new components and the hybrid is much more similar to non-hybrid models. Theoretically, this should make the hybrid system less complex, less expensive, and easier to maintain. Which could be enough to offset not-quite-optimum operation.
Which system will win out? Some two-motor makers, such as GM, are looking into moving to one-motor systems. So it may be that simplicity will ultimately edge flexibility. However, the best selling hybrids – and the most fuel sipping – are based on a two-motor arrangement. Who knows, maybe we'll end up with four.