So sometimes in the middle of completely normal driving conditions (e.g. stopped at a red light after driving 30 mph on a flat street), with a fully charged battery, HV charge mode will just pop on. When this happens it usually cannot be turned off unless the car is shut off completely and restarted. At these times, trying to manually engage HV charge mode will generate an error message that says "HV Charge Mode at Maximum Limit, Plug In to Complete Charge"--but the car nonetheless stays in HV Charge mode until being shut down. I tried to get the dealer to fix it but of course it didn't happen when they test drove it (this happens say once every two weeks randomly), and no one at the dealer had ever heard of the problem. Ideas?
HV charge mode getting stuck on
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HV charge mode is likely not involved in any of this. When this happens do you see an indicator on the instrument panel that says HV Charge? If not then this has nothing to do with HV Charge and is probably just normal engine startup which can happen in certain situations while driving in EV mode.
One common situation is the engine will start up if you briefly press the accelerator hard enough while driving in EV mode. It's easy to not immediately notice that this has happened because in most cases after the engine starts it just idles for a couple of minutes, and so you may not hear it right away above the normal wind and road noise and you don't realize the the engine is on until a minute or two later when you notice on the display that you are no longer in EV. This creates the impression that it started randomly because you don't realize that the engine actually started a few minutes earlier when you pressed the accelerator. Also you are more likely to hear the engine only after you have come to a stop, then when you hear the engine running it creates the impression that it just came on when it has actually been running for the past couple of miles. Usually after a few minutes of idling the engine will shut off on it's on.
Another situation is when you have a full battery and you do any regen within a few miles from your house, like stopping at a stop sign near your house. Since the battery is full there is no place to store the extra electricity produced by regen so it starts the engine as a way to dissipate the energy. Again once the engine has started it will idle for a few minutes then shut off.
The third and less common situation is something called system check, where the engine comes on and remains on for a much longer time, like ten minutes or so. A few people have said they have experienced that when it goes into system check the engine never shuts off their entire drive. But this is less common than the first two situations.
The main difference in the three modes is the accelerator pedal mapping. There is a physical detent, sometimes called the "click" which you can feel when you press the pedal nearly all the way to the floor. In Eco mode the accelerator pedal is mapped so that you have to press the pedal past the detent to get the engine to come on. With a little practice you can press the pedal all the way up to where you feel the detent, and as long as you don't press the pedal past this point you can keep the engine from coming on.
In normal mode the pedal is mapped so that it takes less movement of the pedal to get the engine to come on. In other words the engine will come on before you get to the detent. This makes it easier to accidentally turn on the engine by pressing the accelerator pedal too far. The only way to avoid turning on the engine in normal mode is to keep an eye on the power gauge and avoid putting the needle into the gray area.
In Sport mode the accelerator pedal is mapped so that very little movement is needed. This gives the illusion that the car is producing more power, because you just barely press the pedal and the car quickly accelerates. But it also makes it even easier to accidentally turn on the engine.
In reality the same amount of power is available in all three modes, it's just a difference in how much pedal movement is required.
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Sport mode will also trigger the ICE when resuming ACC speed while the other two will not. It's not just a pedal mapping change.
That's why I said pedal mapping was the main difference, not the only one. Aggressiveness in ACC is one of them which seems to somewhat mirror pedal mapping, although I didn't realize it could start ICE in Sport mode which is interesting. How often does that happen? I don't use Sport mode very often but I always use ACC and I never had ICE come on while in Sport mode. Maybe if I used Sport mode more often I would see it happen.
Supposedly ECO mode does something different with climate but I have not been able to notice a difference.
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I tested Sport mode while resuming ACC after a stop light. There was a slight uphill climb as the freeway had passed over the road I was turning from.
Somewhere I read that ECO mode reduces how much cooling the AC compressor can do, but like you I haven't noticed the difference. I will admit however that I tend to drive my wife's clarity in normal (no ECO or Sport) mode since that seems to work best with the ACC for most driving. I'm a heavy cruise control user.
In my Prius, eco mode limits AC blower fan speed and slows the compressor. Very noticeable in hot, humid weather.
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