Why does EV mode kick out but the car continues in EV?

It is unfortunate that two different things are marked with the same 'EV' label, causing this confusion. Similarly, 'ECO' is used for two different things.

 

Sorry to tell you starting out in EV mode is NOT saving you any gas.

 

You have less HV battery to help the ICE at normal speeds plus you may be accelerating age on the battery itself. Perhaps if your trip drops elevation by 500 ft overall, you might be doing ok in that direction by having more battery capacity for regeneration. I go from 1300 ft to 800 ft inbound but have to climb that much coming back.

 

Because once the ICE comes on, the drained battery loads down the ICE while its trying to charge. Unless you are able to immediately go from EV to charging and fully charge the battery, all you've done is increase the load on the ICE.

 

Not necessarily. Unless you have a plug-in car, all electric energy drawn from the battery ultimately comes from burning fuel in the ICE, and must be replaced the same way. Laundering energy through the MGs and inverters and battery involves some fraction of loss at each stage, and is less efficient than sending it directly through the mechanical path to the wheels. So is best used under light loads when those extra conversion losses are smaller than the overhead of having the ICE run at all.

The overall tradeoff is a bit complex, but the ECUs handle it quite well without any operator intervention. While drivers can do better by gaming the system, it is extremely easy to get it wrong and do worse. Many newbies get caught in this trap.

Our hypermiling guru, Wayne Gerdes, suggests using EV at startup only to about 15-ish mph (on level ground), then let the ICE fire up. By accelerating to considerably higher speeds in EV, you are intruding into the range where ICE is more efficient than EV.

 

This topic has always fascinated me....and frustrated me. I hate that EV is set at a default action tripped off by speed. I have found that as an ordinary driver (not a hypermiler) I am successful at improving my mileage when I start using EV and using it until it trips off or when I know I can be in most optimum/balanced ECO Mode range when I shut EV off. For me it's most important that I stay out off Power Mode as much as possible. There is no point in using EV just to listen and watch the gas engine needing to recharge the battery. My approach doesn't always save lots of gas, but I've been successful by +20 mpg on a flat test range of around 30 miles. (+20 more than my ordinary mpg).

 

There are 2 electric-only modes.

STEALTH mode is what we got way back when Prius was first rolled out, it's a fundamental part of the hybrid system. The engine automatically shuts off when the power from it is not needed, relying exclusively on the battery & motor from that point.

EV mode is what came later, but instead you get a button to engage it. The difference is it trades off speed for power. You'll discover climbing a hill is much easier; however, you cannot go as fast.

With the plug-in model, since more battery is available, the EV mode offers both more power and faster speed. You still have STEALTH mode too, same behavior as with the other Prius models.

 

I endorse fuzzy1's post #7. The only time accelerating above 15 mph on battery power might make sense would be if you're about to descend a large hill that would recharge the battery, replacing all the charge depleted by the acceleration. Otherwise the engine has to supply that energy. I even have doubts about Wayne's 15 mph rule.

 

The energy stored in the battery is the most efficient (cheapest) energy in the car. When the engine is charging the battery, the Synergy firmware uses the bits and pieces of engine hp that are excess to the current needs of the car. If you have an OBD reader, you can watch the charging current varying as the engine operating conditions change. The energy put into the battery by the various reclaim systems is, essentially, free. That being said, using the battery energy to accelerate is the least efficient way to utilize it. It is much better to use it to maximize the glide. I consistently get 60+mpg tanks by pushing into the ECO half of the system indicator to turn the engine on, and then accelerating as slowly as traffic will allow using the engine. I then use pulse and glide as much as I and not impede traffic.