Today I drove my Prius Plug-in from my home almost to the highway in EV mode. When about one minute from the highway, I pushed the PWR mode button twice, on and off, and the ice came on for warmup. All the green symbols above the speedometer were cleared. I then again pressed ECO to bring back that mode. I believe the vehicle was in what I think of as blended HV mode. This mode blends the extra battery capacity of the Plug in with the ICE. After a 40 mile trip, the available EV miles dropped from 13.9 to 1.2, and the trip meter registered 76 MPG. I went up and down a serious hill which probably reduced the mileage. I believe there are two HV modes. The blended HV mode I used today, and a normal HV mode that operates like a standard Prius and attempts to not deplete available EV miles, saving them for later city driving. I think the normal HV mode is entered by toggling the EV/HV button until the green HV symbol appears above the speedometer next to the ECO symbol. I haven't used the PWR mode yet, but I am guessing it will blend because it removed the green HV symbol.
I saw the same thing on my way into work this morning. I had about a 1 mile drive to the freeway during which my EV range dropped from 13.1 to 10.6. Once on the highway, I was driving between 60 and 70 mph with the ICE on. At about 18 miles, my EV range had dropped to nothing. I think the car was treating the battery as a big power-assist to supplement the ICE. As such, I may not have used as much gas during the initial stage, but I lost the EV miles I was hoping to save for the ride home. I will try your suggestion in the morning and see if it works.
EV mode is basically the charge depletion (CD) mode. Even if the gas engine assists, the car will continue to deplete the battery without hurting gas MPG. Ex: If the gas engine is making minimum power needed for max efficiency, blending battery power would lower the gas mileage. In that scenario, the battery will not be depleted at the moment. HV mode is charge sustain (CS) mode. There is a target battery state of charge, probably soc% when you pressed the EV/HV button. Depending on acceleration or braking, the battery charge will go up and down. The car will discharge or recharge the battery to maintain the target.
If you want to save the battery charge, hit the EV/HV button when you are on the highway. The PHV starts in EV mode by default and it'll deplete the battery any chance it gets. To avoid that you go into HV mode where it'll hold the charge.
I've noticed a similar mode on my non-plugin prius if the SOC is at least 7/8 bars; I get 75+mpg while going 65+mph. Obviously with me having a non-plugin it doesn't last very long. It is interesting to see how long this charge does last for plugin Priuses, and makes me wonder if perhaps this "blended HV" mode may make the plug-in Prius more viable for longer commutes like I have (35 miles/direction, mostly highway)
26 miles. (13 "free" miles, 13 miles at 50mpg. 13mi/50=0.26 gallons, 26 miles/0.26 gallons=100mpg). The trick of course is to try and do better than that, by using the electrons for ICE assist, rather than pure propulsion. You and I know that can work -very- well with practice. If the PiP used what the user has entered into the Nav system (or "knows" I am going to work etc) then it could easily beat that by choosing when to use each power source more carefully.
