5 Modes of operation
The prius actually has 5 modes of operation as far as I can tell:
1. For fast accel its uses the gas and electric for more torque
2. highway it uses gas only mode and charges the battery at the same time
3. electric only mode you can cruise and enjoy great gas mileage
4. regen mode is crusing down a slope with the gas pedal released
5. is a special mode, when you release the gas pedal and the arrows on the energy screen go black. In this mode you will not use gas or electric.
Great vehicle, fun to drive, can't wait for the 3rd generation vehicle with the special batteries that will help to get 100mpg. Coming spring 2009!!
47-48 mpg around town
5 Modes revisited
Good observations. If you examine the reverse engineering of the Prius, the ECU programming has been to found to operate in the following priorities:
1) Take energy as necessary from the Engine and Battery when accelerator demand is asking for more acceleration and full engine power is not providing requested acceleration.
2) Keep the Battery SOC at ~60%. This is the real inside story of Prius operation. Doing this keeps the battery available to store the braking regeneration power, but otherwise extends the life of the battery as long as possible. (Specifically, the battery is not continually discharged and charged, it is kept at 60% unless regenerative braking or acceleration needs the battery.)
3) If the battery is above the ~60% State of Charge (SOC), then use the battery to supplement the engine till the SOC returns to 60%.
4) If the speed is lower than ~35 mph, then use the battery only till the SOC drops to ??? (anyone know the specific value?)
5) When slowing down or coasting, lower engine power to minimum and take all the incoming regenerative braking and put it into the battery.
This actually matches up with your mode description very well. There are some other aspects that are known.
1) Battery only operation is not possible above ~35 mph since the electric motor turning the wheels is at it's maximum rpm, not because the engine power is needed.
2) Plug-In hybrid modification by non-toyota organizations have found out that just by subsituting the actual battery SOC signal to the hybrid ECU is all that is necessary to make use of the added "plug-in" battery.