23' Prime - Gas engine starting in EV mode

I believe it’s more about the battery not being able to handle much regen when it has lots of charge and particularly when it is cold. The car can’t switch to mechanical brakes so it starts the engine and the engine then needs to run long enough to warm up.

Try coasting down the hill in neutral and the engine will not start.

Also pre-warming the car for 20 minutes, using the app, helps prevent the engine from starting.

 

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I don't think that is right. I will pay more attention to the contributions of both battery power, battery pack remaining charge and engine power next time I take that route. If I remember correctly even when the engine cycles on it is still not making a significant contribution to driving the car down the road. I am driving in EV mode and using Predictive Efficient Drive when I have made these observations. Unfortunately there isn't a PID I have found in OBD Fusion for battery temperature.

Do you have a reference for your theory regarding battery not being able to handle regeneration current from MG2? The gauges I have in OBD Fusion are capable of showing regenerative power and I have never seen any indication in regards to the hybrid system not being able to handle regenerative power when going down hill.

Here is an excerpt from ChatGPT regarding what Toyota has referred to as "Engine Exercise."



I believe the algorithms on the hybrid control ECU are trade secrets. That partially explains why it is hard to find information in regards to what is going on with these short run engine cycles.

 

Hi Otatrant,

Since I’m just a Prius Plug-in user myself, I should probably correct my wording: I “suspect” the system’s purpose is to burn off the excess energy from regeneration. This idea came from a discussion I saw in another Facebook forum, and so far it’s the most logical explanation that fits what I’ve observed. I’m not sure about the background of the person who originally gave this explanation, but here’s what he wrote:

“The ICE is kicking in when in B or D because the traction battery is full (around 80%), and therefore it has to expend the extra incoming energy by letting MG1 spin the ICE at a faster RPM than the usual B-mode engine-braking range (when the battery isn’t full yet). At that point, it’s sending the surplus energy through the spinning pistons—so they act as an air pump to slow the car down while releasing the extra energy without using fuel. But when you press the accelerator, MG1 has no choice but to stop using that extra energy to spin the ICE and instead use it to drive the wheels. As a result, the ICE drops to 0 RPM, B-mode temporarily disengages, and you no longer get engine braking—causing you to ‘free-fall’ or speed up downhill. To counter this and keep slowing down, you can use left-foot braking to engage the mechanical brakes, dissipating the energy as heat through friction at the rotors. Once you’re on flat ground (in B or D), the car will try to use as much stored energy as possible and delay starting the ICE until the state of charge drops back into the normal operating range. Of course, you also need to switch back to D if you’re still in B.”


Few months ago, I actually recorded the same phenomenon—when the gas engine was forced to start on a downhill:

In the video, you can see that after about 1:45 of regeneration, then about three seconds of peak charging caused the ICE to start. So, at this point, I’m quite convinced that this “burn-off-the-energy-from-regeneration” explanation fits the system logic best.

You might try this on the downhill section where your ICE usually kicks in: occasionally press the accelerator slightly to interrupt continuous regeneration and see if that prevents the ICE from starting. Based on my own commute experience, avoiding long, uninterrupted regen almost always keeps the gas engine from being triggered.

In addition, based on my current driving experience, whether or not the car is pre-warmed doesn’t have a direct correlation with preventing the gas engine from starting.

 

How do you know that was the cause? The charge meter on the MID still shows the car is charging the battery as the car slows down coming to a stop and the ICE is running.

Even if that worked at preventing the ICE from starting on its own that doesn't mean the rest of the theory about the battery turning the cylinders to burn off energy is correct. In addition I have never come across any information indicating that the battery rotates the ICE which is the reverse of what every explanation of how the hybrid system works. The consensus I am referring too is that the ICE can generate electricity to charge the battery but the battery does not drive the ICE. Do you have any references supporting this theory beyond facebook?

In addition if the car was using electricity to rotate the cylinders of the ICE wouldn't you expect the EV range to drop. In the video it remains at 41 km as the engine runs for 5-10 seconds.

 

Burning of energy wouldn’t be the reason, as the engine produces energy, does not burn it.

When the battery is full, the motors are freewheeling. For some reason, Toyota wants some engine motive power is used when that happens so that the car is not barrelling down in neutral gear.

 

Engine doesn’t need to be combusting for engine braking. Also, combustion produces energy, doesn't burn it.

Try the same at 50% SOC and see what happens.

Why is your battery SOC not showing? The Gen 5 display is worse than the Gen 4 display. Gen 5 seems to be actually a Gen 4 refresh with some upgrades to power but then some downgrades to other things. Gen 6 will be a major redesign.

Eco score only 23%?? It is supposed to be close to 100%. Going well over the speed limit? LOL

 

I was also confused at first by the idea that the ICE might start up just to burn off excess regen energy.

Even now, I can’t say for sure that’s the exact reason.

But — I’m 100% certain that regeneration directly triggers the ICE to start.


I’ve become very familiar with this behavior（skill?) after driving the same two long downhill routes near my home many times.

As long as I carefully control the level of regeneration — especially by avoiding long, continuous maximum regen — the ICE never starts.

I’m also quite sure that the ICE activation has nothing to do with the traction battery’s charge level, warm-up status, or even ambient temperature.


To me, the strangest part of this logic is this:

If there’s too much regen energy, why can’t the system simply reduce engine braking and let the mechanical brakes handle the excess instead?

Starting the ICE — and even potentially burning some fuel — just to deal with over-regeneration feels counterintuitive.

From a driver’s point of view, it doesn’t make sense that the system would rather start the gas engine than use the friction brakes.

 

Haha, I knew someone would have something to say about the speed and that Eco score

No worries — next time I’ll make sure not to upload any clips unless they hit 100%!

Anyway, my main feedback here is still about the ICE kicking in during regen — that’s the part I’m trying to figure out.

 

Beyond a post on facebook and a correlation between long periods of regeneration the ICE cycling on how do you jump to causation?

 

I have some evidence that the amount of charge effects the engine startup:

The only time I experience the ICE starting with EV range remaining has been leaving home with a full battery and going downhill right away. It doesn’t start during summer months and doesn’t typically start if I prewarm the car.

I have also experienced the engine starting on a long, extended, steep hill after driving 13 km and with about 60 km of EV range remaining. Going down this same hill with a near empty battery with about 10 km of EV range never causes the engine to start regardless of how much regeneration I apply.

 

If you are really curious you could get a bluetooth OBD adapter and one of the many OBD cell phone apps or even just look at you charge meter on the MID display.

First I am pretty certain that the car is not starting the ICE engine to burn off excess regeneration energy because the battery is too full or too hot.

For the last couple months I have not been driving one route where I observe similar behavior i.e. descend a couple hundred feet of elevation followed by an ascent of a couple hundred feet of elevation. When I do travel this route the ICE engine cycles on in between the short segment in between the descent and ascent almost like clockwork.

In the last couple of months I have also been experiment with an old OBD iOS app OBD Fusion and the Carista bluetooth dongle. Here are some of the PIDs I selected to display and what I would expect to see on them during one of these brief ICE run cycles.

First. Hybrid battery remaining charge. The car completes charging at home somewhere around 89-90%. After regenerative braking during the couple hundred feet of descent I have noticed maybe a ~1-2% increase in remaining charge. Here is a screenshot to give you an idea of what I would see. Since I am currently not connected to the dongle it reads 0% but when I unplug from the charger it will read something like 88-89%. At the bottom of the hill it will read like 89-90%.


A couple of other PIDs are traction battery or hybrid/EV battery system current, hybrid/ev battery power and trip fuel. I use predictive efficient drive which means that for the most part on short trips the only gas I use is when the ICE cycles on to do whatever it has to do. During these brief run cycles I have noticed trip fuel consumed to be somewhere around a tenth of a gallon.



On the hybrid/ev battery power I see a positive number indicating that I am driving primarily on electricity and similarly a positive number of the gauge for hybrid/ev battery current. Here are two screenshots to give an idea of what it looks like. Negative numbers indicate energy or current going into the battery and conversely positive numbers indicate energy/current leaving the battery.





So if the theory that these short run cycles burn off excess electricity what should I expect to see? In my experience the car is already using electricity when the engine cycles on for a brief period. I also don't think I have ever seen a jump in battery current or battery power during one of these brief run cycles.