I'm trying to figure it out too. It seems like it's related to 'perceived engine load' - possibly linked to CVT measure/monitoring? I drive with the "instant fuel economy" indicator on screen; it seems EV is most likely to kick in once engine load is light enough (i.e. instant fuel economy is at max and/or CVT has downshifted). Not entirely sure, but trying to monitor for related things that aren't listed as limiting conditions for EV.
I took a little drive and am still stuck and wondering why on certain sections of road pushing the white EV button will not engage EV Mode. Even though the owners manual explanations for it not engaging do not apply.
The car automatically engages EV Mode. Runs in for awhile then disengages. Engine runs, charging up the battery, then EV re-engages. During a period with automatic engagement of EV Mode I may push the white button to "lock in" EV mode. This allows EV to remain engaged a little longer, digging deeper into the battery power reserve, before automatic dis-engagement. Also with EV locked in, EV remains engaged to a higher acceleration load point (the blue /gray line on the power meter).
During normal driving conditions (eg the bottom of a hill) and strong battery (4 bars+) I may push the white EV button. Most of the time EV Mode engages. If not I'll get a "battery too low" or "engine too cold" notification. ...Understandable.
The problem is "EV Mode Unavailable" and the owners manual explanations do not apply. This happens repeatedly on certain sections of roadway, under various temperatures, battery states of charge, engine warm up, engine acceleration load conditions (light, very light, and zero), and vehicles speeds (all under 45 mph). Why? What are the limiting conditions operating here?
A CVT / transmission downshifting situation is certainly a very reasonable possibility. So I went to the Honda Press Kit for some help and found the following, which leaves the question unanswered.
"....Unlike many competing hybrid systems, Honda's operates without the use of a conventional stepped or continuously variable transmission, utilizing instead a high-capacity lock-up clutch that connects the engine/generator motor combo to the propulsion motor. This allows power to be supplied directly from the engine to the front wheels, supplementing the propulsion motor under certain conditions for either maximum power or efficiency.
...
Drive Force Transfer
Like the latest Accord Hybrid, Insight is not equipped with a conventional stepped or continuously variable transmission. Instead, motive force transfer is accomplished through the interaction of Insight's gasoline engine and two electric motors. Coordinated by the ECU, this form of drive force transfer offers smooth and predictable acceleration matched with efficient low-rpm highway cruising when the gasoline engine is in operation. Gasoline engine shutdown is seamlessly integrated into the operation of the vehicle when appropriate.
The drive force transfer system operates without the need for a torque converter, mechanical pulley or belt. It instead uses two motors for driving and generating power. The system is optimally and rapidly able to control both engine and electric motor rotation in order to deliver higher fuel efficiency and quicker engine response in each driving mode.
When cruising at mid- or high-speeds in the high-efficiency range of the engine, a lock-up clutch is engaged, connecting the drive motor to the generator motor to transmit engine torque directly to the drive wheels as efficiently as possible (Engine Drive operation). In EV Drive operation, when the battery-powered drive motor is used for either acceleration or regenerative braking, a clutch disengages the stopped gasoline engine from the drivetrain to eliminate efficiency loss from mechanical friction in the engine."