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Discussion Starter · #1 · (Edited)
I have a non-premium US-spec XSE - so, I just discovered something today - there is a certain road nearby, with a certain hill that, when I'm going 50 mph (cruise enabled - I'm not sure what other speeds it may do it at but I go 50 on that road), the ICE will turn on every time I go down that specific hill. I have been able to reproduce it 3 times in a row various times over the last 48 hours. Each time I was in EV mode and had between 10-20 miles of EV range. I have about 1/2 tank of gas, which I got about a week ago, so the majority of gas in the tank is new/fresh. I, like many, have noticed the ICE kick on a seemingly random times when in EV mode, but this is the first I've been able to consistently reproduce it. Any ideas? Are those particular conditions just matching Toyota's "turn the ICE on" criteria? Anyone else been able to REPEATABLY have the ICE come on (other than by things like pressing the front defrost button or when the battery is full and there is enough regen to kick it on)?
 

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Is the hill that this happens on near your start point with a full traction battery?
-seems no since 10-20 miles of EV.

How hard (how much energy) is the AC (heat or cool) working?

What was the ambient temperature?
 

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Discussion Starter · #3 ·
Is the hill that this happens on near your start point with a full traction battery?
Nope - Each time I was in EV mode and had between 10-20 miles of EV range, so battery around 50% or less.
 

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Discussion Starter · #4 ·
Is the hill that this happens on near your start point with a full traction battery?
-seems no since 10-20 miles of EV.

How hard (how much energy) is the AC (heat or cool) working?

What was the ambient temperature?
It has happened with HVAC both on (AC) and off, with ambient temps between the low to high 70s. This is the first time in my 7 months of ownership that I have been able to notice a hill where the ICE kicks on consistently (with traction battery not full). It's not a particularly steep hill either, BTW. Very interesting...
 

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So not the high AC or temps I was suspecting. Very interesting.

Were you braking or foot-off-the-gas coasting?

What is at the bottom of the hill?

Can you show us the hill?
 

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Make sure vents on side/corner of rear seats are not obstructed.
 

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My guess (and it is just a guess) is that the ICE is kicking on to provide some engine braking, perhaps because the regen motor’s capacity is exceeded on your particular hill at 50mph.

Try this: since you have an XSE you have the paddle shifters. Try using the left paddle shifter to turn on max regen braking while going down that hill, and see if that makes any difference. But either way it should give you an indication of how much regen the system can handle on its “max” setting.
 

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Discussion Starter · #8 ·
So not the high AC or temps I was suspecting. Very interesting.

Were you braking or foot-off-the-gas coasting?

What is at the bottom of the hill?

Can you show us the hill?
I'll get a video of it when I can. Cruise was on each time, set to 50 mph, so no foot on pedal. The hill is only maybe 100 yards long, and it flattens briefly at the bottom, then back up 100 yards (maybe 20% grade or so). The ICE does NOT kick in when I'm going south, only north (the dynamics of the road are a bit different on each side, and again, the ICE only kicks in when I'm going north). The road is pretty much straight. Again, I'll get a video of it. The ICE kicks in about 2/5 of the way down the slope.
 

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Discussion Starter · #10 ·
My guess (and it is just a guess) is that the ICE is kicking on to provide some engine braking, perhaps because the regen motor’s capacity is exceeded on your particular hill at 50mph.

Try this: since you have an XSE you have the paddle shifters. Try using the left paddle shifter to turn on max regen braking while going down that hill, and see if that makes any difference. But either way it should give you an indication of how much regen the system can handle on its “max” setting.
Ya, I thought about that too, however, I'm quite confident it's not maxing regen on the hill, as I've watched the gauge. It's almost like the dynamics of the vehicle are just right under those particular conditions to trip some pre-defined "turn the ICE on" parameters. If I have time, I'll try various things on the hill to see if I can determine anything.
 

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I have a non-premium US-spec XSE - so, I just discovered something today - there is a certain road nearby, with a certain hill that, when I'm going 50 mph (I'm not sure what other speeds it may do it at but I go 50 on that road), the ICE will turn on every time I go down that specific hill. I have been able to reproduce it 3 times in a row various times over the last 48 hours. Each time I was in EV mode and had between 10-20 miles of EV range. I have about 1/2 tank of gas, which I got about a week ago, so the majority of gas in the tank is new/fresh. I, like many, have noticed the ICE kick on a seemingly random times when in EV mode, but this is the first I've been able to consistently reproduce it. Any ideas? Are those particular conditions just matching Toyota's "turn the ICE on" criteria? Anyone else been able to REPEATABLY have the ICE come on (other than by things like pressing the front defrost button or when the battery is full and there is enough regen to kick it on)?
Are you using cruise control?
 

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It's the cruise control. There is another recent thread that talks about this. No one knows why it does that, but one idea is that there is only one algorithm for the cruise control. (vs one for ICE and one for EV mode)
Maybe that is why this never happens to me. I use EV for commutes on country roads up to 50mph. I only use cruise on highways out of town, and in HV mode.


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Cruise control has been identified as a trigger of unexpected ICE-on events. I bet you the ICE will never turn on driving your target hill without the cruse control activated (not hitting max regen).

Paddle shifters can turn on the ICE as well.

I think those systems (Cruise Control & Paddle Shifter) make it easier to hit some total energy requested/regenerated trigger to turn the ICE on. Maybe those system reserve the right to access x amount of power.
 
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Discussion Starter · #16 ·
It's the cruise control. There is another recent thread that talks about this. No one knows why it does that, but one idea is that there is only one algorithm for the cruise control. (vs one for ICE and one for EV mode)
Sounds like a good possibility - I use cruise a LOT.
 

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Discussion Starter · #17 ·
Cruise control has been identified as a trigger of unexpected ICE-on events. I bet you the ICE will never turn on driving your target hill without the cruse control activated (not hitting max regen).

Paddle shifters can turn on the ICE as well.

I think those systems (Cruise Control & Paddle Shifter) make it easier to hit some total energy requested/regenerated trigger to turn the ICE on. Maybe those system reserve the right to access x amount of power.
I tend to agree with this based on things I've noticed (sudden slow downs in EV with cruise enabled due to various conditions or using paddle to suddenly slow the vehicle while in EV). Interesting observation.
 

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It appears that Toyota cruise control uses friction braking as a final alternative. And it does not fully max out the regen breaking capability of the Prime. So the logic follows the following steps in order when it determines that speed needs to be reduced:
1) reduce/eliminate throttle
2) apply regen braking
3) use engine compression
4) use friction braking

Copy/paste from old Rav4 hybrid system. So for the Prime, it under utilizes #2 and doesn't turn off #3 when in EV mode. Should be a simple software fix.
 

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It appears that Toyota cruise control uses friction braking as a final alternative. And it does not fully max out the regen breaking capability of the Prime. So the logic follows the following steps in order when it determines that speed needs to be reduced:
1) reduce/eliminate throttle
2) apply regen braking
3) use engine compression
4) use friction braking

Copy/paste from old Rav4 hybrid system. So for the Prime, it under utilizes #2 and doesn't turn off #3 when in EV mode. Should be a simple software fix.
Ya Toyota got lazy and didn’t rewrite the software for the Prime vs other RAV4s.
 

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It appears that Toyota cruise control uses friction braking as a final alternative. And it does not fully max out the regen breaking capability of the Prime. So the logic follows the following steps in order when it determines that speed needs to be reduced:
1) reduce/eliminate throttle
2) apply regen braking
3) use engine compression
4) use friction braking

Copy/paste from old Rav4 hybrid system. So for the Prime, it under utilizes #2 and doesn't turn off #3 when in EV mode. Should be a simple software fix.
This is exactly right. Mine regularly uses engine braking on flat roads, when using cruise control in EV mode, and sudden deceleration is required. Bums me out, since then I want a full ICE warm-up to clear the exhaust of condensation, but never wanted the ICE on in the first place.
 
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