I just traded in my 2017 Prime for a 2024 Prime, and it's doing the same annoying thing that the 2017 did. There's a ~300 foot hill about 2 miles from my house. When I descend it and try to keep the speed down with normal braking, the engine starts after about 10 seconds. This happens when the battery is (almost) fully charged and when it's discharged; it does this at any state of charge. I can keep this from happening by braking for 8 seconds, shifting to neutral, then braking the rest of the way with the normal (non regen) brakes. So question is - why is it doing that? I can see some rationale for doing it when the car is close to full charge; too much regen at that point could overvoltage the battery and/or controller, so it starts the engine to "waste" the additional energy in engine friction. But why does it do it at lower states of charge? (Other than that, though, it's been great.)
To avoid overcharging the traction battery, the hybrid system is designed to use excess energy on long downhills to spin the engine with MG1 to use the electricity. Since the engine has no fuel, it does not actually run, but just dissipates the excess regenerated electricity. My 2017 Prius did that and, occasionally, my 2025 Camry. All Toyota Hybrids i have ridden since 2005 do that, by design.
The degree of the slope could be a factor. The amount of regen braking is limited, and a steep slope could generate more force than it can cancel. With light braking applied, the system could be going to engine braking instead of friction brakes in order to avoid brake overheating or even battery overheating. Coasting with hard braking to check speed might prevent this, as the system won't see it as a call for constant light braking. Did you happen to try using B? I suspect it won't keep the engine off if my guess is right, but it would be the first thing I'd do on a descent that I don't want to speed up on. If this is the first time the engine spins with a PHEV, it will then go into the warm up cycle, and burn gas on a trip that could otherwise be EV only.
Interestingly enough, my 2025 Camry has no selectable B mode. I does have fake gear shifting through likely by using foxerd MG1 speeds in the transaxle.
Yes, this is annoying and my Prime does the same thing. Once the engine starts it stays running for quite a while until warmed up - so it does use gas. Are you sure it does this at low states of charge? I have figured that mine will do this at over 80% charge (more than 40 miles EV range remaining) when regen is limited to protect the battery. I haven’t had it happen at lower states of charge.
Pretty sure. I generally don't start out driving with low states of charge so I didn't see that often, but at least twice with the 2017 Prime I started out below 50% and just used the brakes normally during the descent - and the engine started,
That would be great. But then it idles (using gas) for a few minutes. Which both uses gas (bad) and runs the engine for a short time - also bad since it doesn't warm up fully.
You have a B mode, just under a different name. To save on costs, Toyota just used the more greatly produced automatic ICE shifter, labeling and all, for the hybrid models. It is just an electronic switch. Putting it into S gear activates the B mode software routine. As a bonus, you can now choose a degree of B-ness with the numbered 'gears' of the shifter. Battery temperature will also be a factor in addition to charge level. Too hot or too cold will reduce regen effectiveness and increase damage risk. It was probably the former when you saw it in the 2017.
The reason the engine keeps running is that the Toyota hybrid was NOT designed to be fuel efficient, but low polluting. The engine is lowest polluting when warmed up so, when it needs to first run, it runs until it is at that optimum temperature. Fuel efficiency was just a side benefit of being low polluting.
My response the OPa question is the same today as it was 7 years ago to the same type of question of the engine starting consistently while braking on a downhill. Plus one subtle hint about another Prime programmed threshold that can also play a part in the programmed (engine on) set of scenarios. Don't use the ( brake pedal ) or ( B mode ) while going downhill on the one hill your engine consistently starts up. Instead, learn the speed you need to hold at the top of your downhill, so that when you get to the bottom you are proceeding that your desired speed (without needing to brake on the way down). If there is a STOP at the bottom of your hill, that is going to make the engine on issue VERY hard to overcome. A STOP at the bottom of you hill is where shifting to neutral than braking with the pedal should still work to help keep the engine for starting while in EV mode. Don't forget to experiment as temperatures begin to raise up above overnight lows of 60 F. to find at the overnight low temp. the engine stops starting up on your downhill while in EV mode and braking on the downhill, if the car does at any temp. speed and temp are the easiest ( engine on ) thresholds to discover. Keeping in mind that there are several ambient temps the Prius and Prius Prime computers track in the cars programming, that effect how these car behave.
Tried that. There's a light at the top of the hill. Starting from a red light with no power input, I hit a speed of 80mph before I chickened out and started braking. Given that the speed limit is 50mph - not practical. So I use your other approach (neutral then brake.) That works but of course both wastes energy and wears the brakes more.
So which do you choose? Obviously, neither is optimal ! Adding how steep, how long or your mph at the bottom would have been good to have added to your OP. thanks
The description of S for the hybrid in the 2016 manual sounds just like B. This is what the 2025 manual says, "By selecting shift ranges using S mode, you can control accelerating force and engine braking force...A lower shift range will provide greater accelerating force and engine braking force than a higher shift range, and the engine revolutions will also increase." Higher rpms and greater braking force is what B 'gear' does to the car's operation. The difference is just in the shifter switch the driver is given to activate it.
B is a single setting and S shifting has 6 different pseudo-gears. ■S mode ●You can choose from 6 levels of accelerating force and engine braking force. ●A lower shift range will provide greater accelerating force and engine braking force than a higher shift range, and the engine revolutions will also increase. ●When the shift range is S4 or lower, holding the shift lever or paddle shift switch (if equipped) toward “+” sets the shift range to S6. ●To prevent the engine from over-revving, upshifting may automatically occur when the shift range is 4 or lower.
B is a psuedo gear that will provide greater accelerating and braking force with higher rpms. The car's operation changes when shifting to B is the same as using S to downshift. The difference is in the name and the ability to have different degrees of B with the Camry shifter.
If you do not want the heat engine to start by decanding a hill, put yourself on the neutral and use the mechanical braking. This is excellent for removing the rust from the brake discs that work often.
