Thoughts and Questions on EV mode and 2010 Prius Operation

Hello all,
This website has been very helpful for learning about how the Prius functions. For a few months now, I have been using an OBD VEEPEAK adapter with the Dr. Prius app. Given the age of my car, I was not surprised to learn that (at 8 bars in the battery display, after coming down from the highway efficiently -- maximizing the use of the motor to brake regeneratively without pressing too hard to engage the friction brakes) the maximum capacity of the battery in my vehicle is about 75% (again according to the Dr. Prius app).

So, when driving in the upper ECO area on the Hybrid System Indicator, the ICE engages and charges the battery...usually. Unless the charge is over 58% for my vehicle, in which case it will use the charge. More aggressively for a higher SoC.

Except, after a large number of trips and trying to control for various variables, I cannot see a noticeable MPG difference. It seems to be "wasting" the charge, by depleting the battery for absolutely no reason with the engine on. I'm sure this is normal operation, after all it does want space for the regenerative brakes later; I'm just curious if anyone could explain this oddity. Where is it putting this charge? Is it actually helping MPG somehow?

There's a thread from the time of my car's release way back in 2009 talking about an EV mode hack, and while engaging EV mode willy-nilly without a plug is obviously going to decrease MPG given thermodynamics, one poster talked about using it to get up to 25 MPH after a stop. When the battery has more than 58% and the ICE will not charge the battery when engaged, I do this as well, and have seen a minor but noticeable difference in MPG. The poster was reprimanded for that, given that the engine will work more when the battery is below that % in order to charge it; that's not how I'm using it. Although granted, in spite of the thermodynamics, I have tried that, and assuming it's not too far below 58% (56% at a minimum at EV take-off) it does not use enough charge to significantly change MPG, but it is noticeable once the ICE starts back up again.

Given that:
A) the Prius is "wasting" charge over 58% with the engine on (I cannot imagine that it is actually wasting it, and I hope for an answer regarding that)
and B) EV mode (post-warmup) allows more use of that 58%+ charge without "wasting" it
It would be mildly advantageous to "hack" EV mode for use of the higher acceleration (using potentially the full 80 HP of the motors instead of the 36 it usually lets you) under the specific scenario where the battery SoC, at least in my Prius as-is now, is over 58%. Of course, if A or B aren't true, which is what I expect, then this is not the case.

I also, as a sophomore (2nd year at university) software engineering student, cannot imagine it'd be much more than changing a single line of code -- the line that defines the speed limit of EV mode. Sometimes people assume computers to be far more complicated than they are, but at the same time, I've never seen inside a Prius computer, so if that poster has any experience in that, this point of mine may be mislead. I'm aware of that. But given that these speed limits are not based on environmental factors beyond engine warmup, it seems they would be hard-coded as const variables in the code.

So...anyone have any ideas?

 

That's exactly what I thought - it makes sense that the engine would be putting out less power, thereby saving fuel, and that the electric motors are using their charge to take up that power demand - I know that.

But that's not what the MPG is indicating. Which is where my confusion is coming from. In all my trips, it does not seem to have a noticeable difference on MPG, or at the very least not nearly as notable as simply having the engine truly off as in EV mode.

Thank you for your response!

Edit: I should note that when I say "That's not what the MPG is indicating", I'm referring to both the number of bars on the instantaneous MPG display when the engine is on in the ECO area, compared to the number of bars on the instantaneous MPG display when the engine is on in the ECO area, with either >58% SoC or <58% SoC. Also, each time I accelerate to 25 MPH using EV mode from a stoplight, I add one to a number in my head which I then record at the end of the trip. On trips where this number is higher, in general, the MPG at the end-of the trip is higher. Of course, it could be that the factors that allow that to happen (given <58% charge through regenerative braking, for example) also contribute to the higher MPG.

 

I appreciate the response!

My experience is limited to basic java and mostly C++. I cannot find anything about how the Prius is coded nor what language it is coded in nor how to actually access the computer's code, because any search, anywhere, along the lines of "Prius computer code" pulls up results about error codes, which is obviously not what I'm looking for.

Also, if it could be more advantageous to have a higher EV speed limit on a non-plug-in, one would think Toyota would have done that, which is what makes me think that may not even be the case. That using it for SoC <58% only up to 25 MPH from a light could easily be the most efficient use of it. After all, we are not trying to kill the battery outright, just use the excess charge efficiently.

It could be more advantageous to rewrite it such that it simply does not strongly use the excess charge with the engine engaged but again, if that's the case, why wouldn't Toyota have done that?

Lastly, the engine automatically idles at <46 MPH. While you're basically in "EV mode" when you first start the car (accelerating past the middle line will stay on the motors - you can feel it. Despite the idling engine, the drive is super-smooth - Until the engine is a certain warmth. Before then, it will do everything it can to stay on the motors), and that "EV mode" has the engine idling to warm up...
Even assuming hacking it would be as simple as changing a line of code - which again, is only my assumption, I've never coded anything as complex as an internal computer unit for a hybrid - simply changing that number to <46 MPH could have terrible, terrible consequences on the highway as a result of it potentially forcing the engine to be off. Which is why if I do decide to do the hack, after somehow finding out how to access the code of the computer (and back it up on something to ensure I can undo anything that I do), and at least being able to read the language its coded in, if I can't find any line as simple as:

if (engine_temp >= 70) //assuming C
{
EV_mode_limit = 25; //change this number
}

Then I would wait. But I'd imagine it'd at least look something like that. May take a few years for me to finish my degree before I really try anything, though it would be nice to actually see and begin to dissect the code.
I still feel it might not actually do anything good for MPG, at least over battery life or just the car being able to function properly, and that EV mode cutting out at 25 MPH and only using it when the battery is over the SoC where the engine would charge it is best.

I'm trying my best to take a scientific approach. In any case, thank you for your response!