Although Honda's Integrated Motor Assist gasoline-electric hybrid system was originally dual-mode, purely internal combustion and internal combustion with electric motor assist (best thought of as an electric turbocharger), it has been further developed to operate in electric-only mode in some conditions.
That's interesting, but during a recent week with a 2009 Civic Hybrid, I never did figure out just exactly what those conditions were. Unlike most competitors, Honda is very low-key about hybrid power train dynamics. There is, as of now anyway, no real-time power diagram on the dash to show passengers how green they are. And because the engine never stops rotating--when it's decommissioned the VTEC and engine management hardware and software merely stops fuel flow to the injectors, stops the ignition system, and holds the valves open to reduce pumping losses--it's extremely hard to tell when the engine is not running. Because it's really always running, just not using fuel or producing power when "shut off".
Earlier this week, I attended a Honda event, and a Civic Hybrid was part of the program. What better circumstance to find the answers to my questions, given that there were plenty of Honda engineers and public relations people to ask. Not only did I get my question answered, I got a demonstration, and then was able to try for EV-ness myself.
Here's how: Select the instantaneous mileage display on the readout next to the speedometer. At a steady and light throttle opening, on a level or slightly downhill road, when instantaneous mileage is 100 mpg--the maximum number displayed--and there is a light amount of assist, say two or three bars on that display in the main instrument cluster, and the battery is charged, you're in an electric vehicle. There is no sudden bump when the engine shuts off, and almost no change in the interior noise level, which is low anyway.
Too much throttle, and the car will be in gas-only or gas+IMA assist mode. Too little throttle, and it will go into coasting or regenerative braking mode. The Civic won't go too far in EV mode, although both the Honda representative and I got it to go for over a mile on one stretch of appropriate road. He was surprised. Honda is not making a big fuss over this development, as it's only an incremental development of a simple, elegant system. However, expect to see more of the IMA system in the future.