23' Prime - Gas engine starting in EV mode

I don’t know an exact number but it is not that low. I don’t have data readily available to support this but am pretty certain that when I am driving on a highway using cruise control lets say set to 75 mph and then approach my exit and quickly reduce the cruise control to say 30 mph I see a regenerative power close to -75 kW.

My bottom line for now that is in the current Gen 5 Prius Prime the data suggest that the engine starts after regenerative braking most likely due to drops in engine oil temperature. The reasoning is that the car is traveling at high speed with air flowing through the radiator and causing a drop in engine oil temperature. The gas engine starts to heat up the oil.

This is different from previous non plug-in Prii with smaller batteries where shifting to B when the battery was full resulted in a negative torque value from the motor thus slowing the car via engine braking.

 

What green marker?

 

Maybe we could also check what happens after the ICE has already run earlier in the trip.
I wonder if, in that case, the engine would stay off during regen since the oil should already be warm enough.
That could help confirm your hypothesis even more.

 

This one:



That's the battery charge limit. The blue marker above it is the battery discharge limit.

You can see how it moves up and down based on recent activity - it edges up while the battery's charging, and edges down when it's not, showing that it has a short term average charge rate limit.

At least earlier in that trip video - it settles down later, presumably once the system has warmed up.

That sort of variable limit probably occurs when the battery is at extreme temperatures, either colder or warmer than ideal, and also when nearer full capacity.

At ideal temperature and not near full capacity, the limit probably comes from the system, not the battery, so you won't see it move, it'll be pegged low. (Unless maybe you do have a really high continuous charge for a prolonged period).

 

Yesterday I had a chance to do some cross-testing on the downhill route I drive through frequently.
Sorry, I don’t have an OBD tool to monitor data directly — I can only share what’s shown on the MID.
(And please forget about my speeding — it was just for testing! )

Test route: continuous downhill about 2.7 km, descending roughly 182 m in elevation.

Scenario A: Cold engine, no prior ICE activation (low oil temperature)
• 3:54 — start of test section
• 5:35 — regen reached maximum level
• 5:40 — ICE was forced to start (and did not stop immediately)

Then I climbed back up the hill using ICE and switched back to EV mode at the top.

Scenario B: Hot engine (after climbing with ICE), switched back to EV mode before descending
• 10:58 — start of test section
• 12:32 — regen reached maximum level
• 12:36 — EV mode dropped out (ICE started)
• 12:51 — switched back to EV mode automatically

Video link:

Observations:
1. Every time the ICE starts, the regen intensity immediately decreases — even though the road slope remains constant.
2. Oil temperature doesn’t seem to affect whether the ICE is triggered, but when the oil is already warm, the system switches back to EV mode almost immediately. When it’s cold, the ICE keeps running for a while before returning to pure EV mode.

 

Good work on the test..

It makes sense to me that engine oil temperature has nothing to do with it.

What was the initial logic for suggesting that? Sure, oil temperature could be a reason to start the engine, but why would it only show up when going downhill? And why would the car care about the engine oil temperature if not using the engine?

The car can happily cruise at -5°C indefinitely in EV with no random engine starts, so it's not plausible to say "the car is traveling at high speed with air flowing through the radiator" is the reason for a downhill engine start at +10°C.

It seems like a case of just trying to find any related variable among those being displayed on the OBD, when the OBD wasn't showing the relevant variable at all.

I've watched a few more of john1701a's videos, and the behaviour of the charge rate limit is fascinating. It has the SoC component, so creeps up slowly as it discharging from full to maybe 75% (shown as 65% on john's OBD), at which point it stays stationary. But it also has the short-term component - it moves rapidly up and down based on recent charge/discharge bursts, and can move all the way to zero.

That short-term component is almost certainly the trigger here. Watching Mccoy's video, I can visualise the marker from john1701a's video sliding up and down until it finally gets too high.

Now, it looks like there is a battery temperature input to that charge rate limit - I can clearly see the short-term limit applying in john1701a's low-temperature videos, with the battery below 60°F (15°C). But I don't see it in the summer videos with the battery above 80°F (25°C).

Which would suggest that the downhill engine starts with low SoC should occur far less or not at all on warm summer days. Is that consistent with observation?

 

I can see your charge meter at 5:35 is maxed out but it raises a question. Is there a threshold where the charge meter tops out at for the display or are you actually reaching the peak of the amount of regeneration current when the charge meter is maxed out? I suspect it is the former but I will have to check next time I go for a drive.


Than you say that ICE was forced to start. Your flow meter does not indicate that the ICE is on.



So how are you determining that your ICE is running? It wasn't until 5:57 that your flow meter indicates your ICE is on.

Also is there a reason you are driving with in 'B' set to maximum level while presumably ascending a hill at 80 km/h.

In addition I think you are confusing two things. You have the EV mode indicator light which stays lit and the EV indicator. The EV indicator it looks like goes out when you are not using electricity to drive the car so if all the power for the car is being driven off of regenerative energy than the EV indicator goes out. Pretty certain when the EV indicator light goes out that it does not mean that the ICE is running.



You are also saying on your video that EV mode changes when it is the EV indicator that is going on and off. Pretty sure this correlates with where the energy is coming from to run the car. If it is coming from the battery the EV indicator light goes on and if you are supplying enough regenerative energy to run all of the electronics than the EV indicator light goes off. For the most part EV mode only changes when you change the mode with the EV mode button or you run out of charge in the battery.

I have gained a lot of insight about what the Prius is doing using the OBD dongle. From a quick look on Amazon it looks like you can get one for $32.

There is also a bunch of apps that will give you information that can help in understanding what all the graphics are indicating and how much information is available that is not visible on the MID.

 

Here is one answer.

From the people at cartalk.pandahub. Never heard of them but it sounds pretty reasonable to me.

Isn't he driving a 4th generation Prius Prime? There has been significant changes in the Prime powertrain between generations. Not exactly the best source when trying to figure out what is going on in a 5th generation Prius.

My understanding is that there has also been a significant change in battery cooling between generations. The 4th Gen Prius Prime was air cooled whereas the 5th generation Prius Prime is liquid-based refrigerant cooled. There must be some significant performance changes in the two systems like maximum discharge and recharge rate.

 

That's exactly what it means, as far as I've been able to determine, driving one for 2 years - the EV indicator is a simple "engine not running" light. It is not affected by the motive forces in play.

But it's utterly irrelevant to a non-running engine. The non-running engine in a Prius in EV mode has as much need of oil temperature as a non-running engine in a parked car.

Now, once it decides it's going to start spinning the engine, such as to provide drag, at that point the car has to worry about oil temperature, which is why it goes through a warm-up cycle.

But spinning up an engine just to warm it up makes no sense for a car in EV mode that isn't planning to run the engine at all.

Yes, that's 4th gen. It is the best source I've found though - if you can find or provide live OBD data from a G5 Prius including the instantaneous charge rate indicator, then we'll be able to do better. But it's the best so far.

Sure, there are changes between generations. But the G4->G5 transaxle changes are pretty minor - I dug into them here where I described them as being on a scale similar to the G1->G2 changes.

Battery capacity will likely have shifted the values for short-term charge rate limits, but it seems unlikely that the rate limit will have been totally eliminated.

In the absence of a strong reason to believe otherwise, it's wisest to assume that generations of Priuses will do roughly the same things for the same logical reasons.

So if we know the G4 has a short-term charge rate limit that triggers engine bleed-off regardless of SoC, and we also see G5s doing engine bleed-off at low SoC, it's not very logical to start from an assumption that it isn't doing it for the same reason as the G4, and trying to find a new different reason.

Don't become Paul Gregory, and get incredibly attached to a wild guess, and actively avoid any steps that would help establish the verity of that guess.

If you want to prove that the engine starts are due to some other factor than short-term charge rate limits, you need to find the instantaneous charge rate limit in your OBD, like john1701a's videos show, and demonstrate that it's not moving down so that it's breached.

 

That is what the video shows if you look at the flow meter and the indicator light.

 

If you are driving in a mode or have a habit of requesting ICE power when you drive that seems to me a good reason to keep the engine oil wam enough to keep the engine lubricated so it can abruptly start and provide power.

I have been driving and looking at OBD - II data for a month or two. I am not guessing. I am basing my judgement on data I have observed. Where is your data to back up your claims? I still haven't seen any solid evidence that the gas engine is providing drag through engine braking like cited in previous posts to car care nut video where he talks about that for Prii with much smaller traction batteries ie a tenth the capacity.

The idea that the engine is running to burn off extra electricity when the battery is not full is absurd. In a car that is organized mainly around the idea of efficiency where the goal is to recoup as much energy as possible to say that it uses electricity to drive the gas motor is ridiculous.

You do have a 5th generation Prius yourself right? Wouldn't it be better to gather you own data than to cite irrelevant videos on youtube. The Prius has doubled in power to say that because the transaxle hasn't changed much and use that as a reason to assume that 4th generation energy flows and limits are still a good source for determining what is going on in the 5th Generation Prius is lazy. Do they not sell bluetooth OBD dongles in Finland? Is there some sanction on Finland bluetooth devices?

 

I understand you’re trying to analyze all the clues and find alternative explanations for why the ICE kicks in, but so far none of them seem to fit very well.

About 1 hour later, on my way home, I drove down the same hill again. When the regen bar hit max for about 3 seconds, I gently pressed the brake pedal to lower the regen level — and this time, it stayed in pure EV mode without being forced out. That completely matches my previous experience.

5:45-5:48

From my experience, the most reasonable explanation still seems to be a short-term charge-rate limit protecting the hybrid battery or inverter system.

I’d definitely suggest you try it in your own driving conditions. After all, the whole point of this thread is to understand why the ICE kicks in. Since Toyota didn’t given us a clear answer, finding practical ways to manage it feels even more important, right?