Stretching the Hybrid's Electric Capabilities

Conventional U.S.-market Prius

The motor and battery pack in the Toyota Prius deliver bursts of power to the wheels, so that the gasoline engine operates in its most efficient revolution-per-minute range. The 30-kilowatt motor can accelerate the car from a dead stop to about 65 kilometers per hour. At that point the engine kicks in (if it hasn't done so already) to keep the motor from running faster than 6000 r/min and to prevent the 1.3 kilowatthour nickel-metal hydride battery pack from going below a state of 40 percent charged. Even if a driver putters along at 40 km/h or slower, the car's computerized control system will activate the engine before this threshold is reached. When running, the engine shunts 30 percent of its torque back to the vehicle's 15-kW generator, which keeps the battery pack topped up. Regenerative braking captures additional energy that otherwise would be lost as heat.

Hacked U.S. Prius with Plug-in Capability

Instead of having the motor mainly deliver peak power to complement the gasoline engine, the aim is to get as much distance as possible from the electric drive train without the engine's kicking in. To keep the motor going for more than 30 km rather than just the 1 or 2 km of the conventional Prius, the hacked versions [like the Tour de Sol model, above] boast battery packs with storage capacities on the order of 9 kWh. The battery pack can be charged directly from an ordinary wall socket, with inverters rectifying the ac current. The vehicle's control system is modified to prevent the transfer of propulsion from motor to engine at the usual speed. But with this tinkering come compromises: in its all-electric mode, when the motor and battery pack are doing the heavy lifting, the control system will not let the vehicle reach highway speeds.

European/Asian Prius

With air pollution a strong factor in Asia and Europe, the Prius version sold in those markets allows the driver to shut off the engine and run just on electricity, though the car is not a plug-in. The electric-only capability is desirable when creeping along in the central districts of cities where internal combustion engines have been banned or strictly limited at certain times. (Milan, Italy, recently ordered cars, motorcycles, and trucks off the roads on alternate days, and other cities, including London, have also taken steps to curtail automotive traffic in their business districts.)

Press a button, and the Prius runs just on its battery at speeds less than 55 km/h for 1 or 2 km. The electric-only setting is not locked in, however. At any speed greater than 55 km/h or with aggressive acceleration, the vehicle's control system will override the driver's selection and start the engine.

The button exists in U.S. models but is not connected to the car's control system.