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Discussion Starter · #1 ·
Sorry if this has been covered before, but I can’t seem to find a straight answer about this on the forums. Most threads talk about the “manual shift” S mode for general downhill engine braking, but I’m interested in how S mode specifically affects regenerative braking in a hybrid system, and I’ve seen completely conflicting responses.

For example, when approaching a stop sign and I bump it over from D to S (foot off the gas), there is a noticeable deceleration. From the point of view of regenerative braking, is this exactly the same as if I left it in D and lightly pressed the brake pedal to achieve the same level of deceleration? Or is S mode providing more actual “engine braking” than regenerative braking (i.e. comparatively less efficient)? I ask because, assuming no one is right behind me, I’d like to get into the habit of lazily coasting to a stop and maximizing regeneration, and I’m wondering if “downshifting” is a good way to do that without lightly holding the brake pedal for a quarter mile or if doing so wastes energy.
 

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I’m not by any means an expert, but I had a 2011 optima hybrid that I sold when I bought my RAV4 hybrid and the only regenerative charging going on was with brakes. I only used the shifting when coming down the mountains and it didn’t seem to do much. I still had to use some brakes do that worked. Bottom line, press your brakes and get charging.
 

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For example, when approaching a stop sign and I bump it over from D to S (foot off the gas), there is a noticeable deceleration. From the point of view of regenerative braking, is this exactly the same as if I left it in D and lightly pressed the brake pedal to achieve the same level of deceleration? Or is S mode providing more actual “engine braking” than regenerative braking (i.e. comparatively less efficient)? I ask because, assuming no one is right behind me, I’d like to get into the habit of lazily coasting to a stop and maximizing regeneration, and I’m wondering if “downshifting” is a good way to do that without lightly holding the brake pedal for a quarter mile or if doing so wastes energy.
Does your engine start up? (use the pretty display to see the status of the hybrid system) If so, it's not charging. Also, charging should show up in the display that tries to help you drive economically. If you don't see it charging, it's almost certainly not charging.

You may also find that it's not consistent. My car, (prius prime, so not directly comparable), will start the engine and use it for engine braking if the battery is full. Otherwise, it does put it in the battery. At first this seems odd, but it makes sense if you think about it. You asked for engine braking, but there is no room in the battery. What to do? Doing nothing would be bad, so the car starts the engine and gives you engine braking. If there is room in the battery, it gives you what I guess you could call "simulated" engine braking. Make sense?
 

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Discussion Starter · #4 ·
Does your engine start up? (use the pretty display to see the status of the hybrid system) If so, it's not charging. Also, charging should show up in the display that tries to help you drive economically. If you don't see it charging, it's almost certainly not charging.

You may also find that it's not consistent. My car, (prius prime, so not directly comparable), will start the engine and use it for engine braking if the battery is full. Otherwise, it does put it in the battery. At first this seems odd, but it makes sense if you think about it. You asked for engine braking, but there is no room in the battery. What to do? Doing nothing would be bad, so the car starts the engine and gives you engine braking. If there is room in the battery, it gives you what I guess you could call "simulated" engine braking. Make sense?
For this scenario, I'm assuming there is plenty of room for charge in the battery (let's say 50%). In both situations, it's definitely charging on the deceleration (according to the dash), I'm just wondering if one is better or worse from an efficiency standpoint for slowly coming to a stop.
 

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Well, I don't know, but I don't see how it could be any different. It's the same MG charging the battery. I suppose you save the power used to run the brake lights if you use the S mode. :) (one thing about the S mode is that your are certain that you are not using your actual brakes.)
 

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Sorry if this has been covered before, but I can’t seem to find a straight answer about this on the forums. Most threads talk about the “manual shift” S mode for general downhill engine braking, but I’m interested in how S mode specifically affects regenerative braking in a hybrid system, and I’ve seen completely conflicting responses..
The S-Mode shifter is just engine braking. It reduces regenerative braking by varying degrees and replaces it with engine braking. So - less regenerative braking. Regen braking will be reduced by as much as 60%. Its mainly just a toy for drivers to play with.
 

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For this scenario, I'm assuming there is plenty of room for charge in the battery (let's say 50%). In both situations, it's definitely charging on the deceleration (according to the dash), I'm just wondering if one is better or worse from an efficiency standpoint for slowly coming to a stop.
All braking is bad for efficiency. Regenerative braking is less bad, engine braking is more bad. Putting it another way: engine braking wastes energy, regenerative braking stores energy.
 

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Discussion Starter · #8 ·
The S-Mode shifter is just engine braking. It reduces regenerative braking by varying degrees and replaces it with engine braking. So - less regenerative braking. Regen braking will be reduced by as much as 60%. Its mainly just a toy for drivers to play with.
I guess my question should have been: are downshifting and lightly applying the brake pedal to slow the vehicle doing the same thing to the ICE and electric motors/generators?
So @rdgrimes is suggesting that downshifting is doing traditional engine braking and not just activating the same generator behind the scenes. Is this stated somewhere/are you sure? Not that I doubt you, there is just a lot of speculation on the matter and I can’t find any actual documentation stating one way or the other. But I’m inclined to believe you because you seem pretty confident and even quoted a figure of 60%; where did that figure come from?
 

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I do a lot of mountain driving in the winter to go skiing. S mode or manual mode does a blip of regen and then does engine braking. I've compared the regen meter and on S mode, the charge meter blips up for a sec when you engage it then lets the "gears" engage for engine braking. Pressing the brake pedal charges it way more. On steeper grades, the charge meter will be full while it will only be about 1/4 on s mode. I was kinda disappointed we didn't get the B mode like the prius but engine brake makes more sense on mountains. The battery will recharge fully after like a mile of downhill.


What I do is recharge fully with brakes, then engine brake all the way down the mountain.
 

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Every now and then there's a post with someone trying to replicate the one-pedal driving in other cars on their RAV4 using the S mode.

No, that doesn't work. I'm actually not sure if S mode increases regenerative braking (I might connect my ODB II and see), but in the end that's not what S mode is for and it won't do what you want.

The main problem is it's going to make your driving less efficient, and for two reasons. First, from my own observations, S mode keeps the engine on when it otherwise wouldn't be. Part of the point of a hybrid is to turn off the engine and just run on electricity, which saves gas. And second, it forces your engine to run in a lower gear ratio (effectively) than it otherwise would. Normally it would pick the best ratio to optimize efficiency, and by using S mode you're preventing that.

Toyota seems to have designed this vehicle to feel and operate like a traditional gas car, not like an EV. Coasting will do a tiny bit of regenerative braking, just like a gas car has a bit of engine braking when coasting. S mode simulates the downshifting you can do in automatics, it's not just additional braking. And so you're supposed to drive it like a gas car as well.

From one of the early RAV4 Prime reviews it does seem like the paddle shifters can add some additional regenerative braking while in EV mode. But it's still too early to know what's going on there for now.
 

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Discussion Starter · #11 ·
Thanks guys. I feel fairly confident that downshifting is not the way to go for my goal. Now that I’ve started this thread, the forum has suggested several related threads that I see have already answered my question (e.g. About those regenerative brakes). I guess I wasn’t searching the right keywords (and probably shouldn’t have limited myself to the 4.5 forums). Sorry for repeat discussion.
 

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I guess my question should have been: are downshifting and lightly applying the brake pedal to slow the vehicle doing the same thing to the ICE and electric motors/generators?
No, absolutely not. You cannot have ICE braking without regen braking being reduced. The synergy drive is a direct "power-split" connection between ICE and MG, so if one speeds up the other must slow. (imagine a differential planetary gear) Regen braking is a function of the RPM of the MG unit. In normal drive modes, the ONLY time the ICE is involved with braking is when the traction battery is at max charge state. In that case the ICE is used instead of regen when you press the brake pedal.
 

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No, absolutely not. You cannot have ICE braking without regen braking being reduced. The synergy drive is a direct "power-split" connection between ICE and MG, so if one speeds up the other must slow. (imagine a differential planetary gear) Regen braking is a function of the RPM of the MG unit. In normal drive modes, the ONLY time the ICE is involved with braking is when the traction battery is at max charge state. In that case the ICE is used instead of regen when you press the brake pedal.
I disagree. MG2 is connected to the wheels via a fixed gear ratio and does not play any role in the engine braking, and that's the moter that does the regen braking (along with MGR, I think). MG1 would act like you said (since it's what controls the effective gear ratio in the planetary gearset), but I'm not sure it's capable of doing any regen braking anyway. Maybe on the RAV4 Prime, assuming it has a sprag clutch or similar, but not in the RAV4h.
 

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