Hybrid Yaris - Why does the ICE quicks in when the battery is full?

[Pacproduct]

Hi everyone 🙂

Following an old conversation there:

Like some other users, I've been wondering for quite a while why my Hybrid Yaris (from 2017) does use the ICE when the Battery is full.

I really see no reason why the car would need to start the combustion engine for draining Battery power.
If that's because the car cannot use the Battery anymore for braking, then it could simply break mechanically so I guess that's not it.

Typically in my case it only happens when I drive downhill for more than let's say 2km: Once the battery is full, the car regularly starts the ICU for no apparent reason, when speed of the car is too low (roughly below 30km/h), randomly or when hitting the Go pedal... which puzzles me hard: why would the car accelerate with the ICU when the battery is literally at full capacity?

Does anyone know what's going on and why the car does that? :)

Apart from that, I find the hybrid system pretty clever and does a great job at managing/merging the EV and the ICE.
It's just when the battery is full that it really behaves in a strange way...

Cheers!

[MikeSh]

I'd guess it's using engine braking. When the Battery has space the engine is stationary and the small MG is spinning and probably generating, but the large (wheel connected) MG would give most of the regenerative braking.

If the Battery is full then the car can 'freeze' the small MG forcing the ICE to turn, thus giving engine braking. This can be done without fuel, so the engine isn't really started or running, just turning.

[Roy124]

Mike, makes sense.  I must admit to seeing this but not thinking through to your explanation.  The clue was the 2 km descent. 

All I did when the revs built up was not the mpg bar was on max deflaction ie MG rotation. 

Of course no rev counter on the new Yaris x

[TonyHSD]

Absolutely normal behaviour and also more prominent in hot weather, both are related too. Happens to me every day and sometimes engine goes to high rpm 3000+ , Battery full (80%) because of the long downhill drive . The car does that to maintain more appropriate Battery soc in case you stop and leave the car without use. It’s a cleaver system indeed and we should not worry about it, just let it does it’s own business. ⚡️🔋👌

[Pacproduct]

1 hour ago, MikeSh said:

I'd guess it's using engine braking

Once the Battery is full (visually on the screen - I get it's only around 80% in reality to go easy on the battery), the car does switch to some kind of a soft "break" mode and does use the ICE as an engine break indeed (and the EV light turns off). When it does that, it does not inject any petrol.

However in that mode, time to time, it does inject petrol here and there (visible on the dashboard, as real time consumption jumps up from zero to 2 to 5 l/100km briefly), for no apparent reason, for variable length of time (1 to 10 seconds). Even if I'm not accelerating. Even if the engine is hot. Most of the time when the car is rather going slow (or is it because I just braked?).

The result is that on some long descents, the average consumption of the Yaris is not what one would expect (0 l/100km), but a bit more than that (depends on many factors, roughly 1 l/100 I'd say). Don't get me wrong, it's a pretty low consumption for a very specific if not rare situation, overall the car does a decent job at being petrol-efficient 🙂

49 minutes ago, TonyHSD said:

It’s a cleaver system indeed and we should not worry about it, just let it does it’s own business.

The rest of the time, it does seem pretty clever I agree. That's why there must be a good reason for the car to use petrol downhill when the Battery is full, but so far I did not find any satisfactory explanation 🤔

 

1 hour ago, Roy124 said:

All I did when the revs built up was not the mpg bar was on max deflaction ie MG rotation.

I'll be honest, I don't really get what this phrase means.
Non-native here, sorry ^^'

[Roy124]

Deflection not deflation, is maximum mph/lpk

[RickyC]

3 hours ago, Pacproduct said:

Once the battery is full (visually on the screen - I get it's only around 80% in reality to go easy on the battery), the car does switch to some kind of a soft "break" mode and does use the ICE as an engine break indeed (and the EV light turns off). When it does that, it does not inject any petrol.

However in that mode, time to time, it does inject petrol here and there (visible on the dashboard, as real time consumption jumps up from zero to 2 to 5 l/100km briefly), for no apparent reason, for variable length of time (1 to 10 seconds). Even if I'm not accelerating. Even if the engine is hot. Most of the time when the car is rather going slow (or is it because I just braked?).

Probably it does a fuel injection to maintain the correct lubrification of engine valves ( gasoline behaves also as a lubrificant ) and ICE temperature.

[Pacproduct]

6 hours ago, RickyC said:

Probably it does a fuel injection to maintain the correct lubrification of engine valves ( gasoline behaves also as a lubrificant ) and ICE temperature.

Does this mean that when engine breaking, the engine needs some petrol/gasoline time to time to prevent it from taking damage?

Never thought of that, that's a possibility.
Does that mean that all standard non-hybrid vehicles damage their engine whenever we engine break with them on long downhill slopes?

[MikeSh]

2 hours ago, Pacproduct said:

Does this mean that when engine breaking, the engine needs some petrol/gasoline time to time to prevent it from taking damage?

Seems unlikely. Petrol/gasoline is generally expected to wash oils off - diesel on the other hand is a lubricant.

It could be done to burn off excess oil pulled up past the rings, but that seems unlikely to me with modern engines.

[Pacproduct]

2 hours ago, Gerg said:

Maybe this idea has already been offered in the OP's other thread:

Is the engine eventually consuming fuel in this 'braking' situation, as a means to maintaining the exhaust catalyst at a normal functioning temperature ( ~ 600 degrees) ?

With the engine pumping air through the catalyst with this particular type of engine braking mentioned, I would expect its cooling off to occur faster than when the ICE is off/stationary during normal EV-type operation.  If the cat. isn't hot, then when conventional ICE operation resumes there will be  an emissions spike until it is up to temperature, which is bad! 

In this Toyota hybrid case, keeping the catalyst hot is a 'keeping the plate spinning' game for the ECU, isn't it. 

 

That's a very probable explanation indeed. There one thing that makes me stay skeptical though: I'm pretty sure the car usually consumes fuel when going below a certain speed, but doesn't react this way if the speed is kept above say roughly 50km/h. No sure what the deal with that is, and to be completely honest this would need further testing, but I don't have any downhill long enough without turns at hand to be fully sure because the only 2 roads I take regularly that trigger this behaviour are small mountain roads that keep turning and turning...