ICE turns back on while braking at 70km/h

Sometimes the hybrid system can't harness all of the energy available for regenerative braking. In order to keep harvesting some, it needs to dump the excess somewhere. It does this by spinning the engine. It won't burn fuel, it just needs someplace to dump energy.

It's happening at a specific speed because it relates to the reduction ratio in the transmission - specifically the maximum speed of the generator when configured to harvest during braking.

 

Just above 70 kmh is the 3rd gen limit for electric-only. Maybe some connection?

 

2009 avatar, gen3 forum. is this the 2010 that is 2009 outside n/a?

 

Yeah not sure where BE is either.

Maybe the OP will educate us.

 

Let’s start a guessing game lol? I will start... Belgium?

 

No fair guessing it on the first try .

But plenty of hills there so understandable why the behavior is witnessed .

 

It is a Gen 3 as is obvious from the instrumentation shown in the video. That means it's a 2010 model year; they went on sale in 2009. If it was a 2009 model year, the instrumentation would look like this.



As to the actual question, @Mendel Leisk is on to something. But unless there's something different about that model year, the engine should not be using fuel in that situation. My PiP has the same speed limitations on electric only, but it just lets the engine spin when coasting at high speed and doesn't use fuel.

 

Doesn’t the motor have to be on “hot standby” over a given speed so the ICE can bring power on demand?

My guess is that the system is starting the motor in anticipation but shuts off once it realizes it’s not needed.

 

That does happen in some situations, and it might be difficult to differentiate between that case and the case of energy dumping during regenerative braking unless you looked at the data stream coming off the ECU. (You'd look at fuel flow to distinguish)

A quick dump of energy while regen-braking through that specific speed is a fairly well known phenomenon, so it's a reasonable guess in the absence of such data.

 

It's just a matter of gearing. The MGs can only spin up to a certain rpm. Above that, the ICE has to spin to reduce the rpm of the MG. Right around the 20 minute mark in this video, you can see an explanation of why. This guy works without a script, so he hems and haws a bit, but it's really good material.

As to the brief use of fuel, that's a mystery to me.

 

It's difficult to give you a concrete answer until you understand the actual gearing relationship in the transmission.

At higher speeds, regeneration is best achieved by harnessing the power from MG2. At lower speeds, best regeneration is achieved by MG1. MG1 is driven through taller gears, so it is spun faster than MG2.

As you might expect, each of these parts has a maximum rotational speed, not to be exceeded.

When you do this regen braking through that speed, you are transitioning from harnessing energy from MG2 to harnessing it from MG1. MG1 will be going to its top safe speed and then slowing down along with the car to make this work. The safest, smoothest way to do that without risking an overspeed on MG1 is to also have the engine spin a little bit.

 

This completely normal (and has been asked before, I'm sure).

First: the assumption that the ICE turns off at 80 km/h is false. It does not turn off but keeps spinning with no fuel injected. That's how the HSD works, above 67 km/h (real km/h, European speedometers always show a little less than 10% too much, so it's about 70 km/h on the speedo) or 42 mph the ICE will always spin to protect MG1 from turning too fast.
Second, what actually happens when you transition from a higher speed to a speed below this threshold is that the car will inject some fuel to make the transition smooth. And that's what you see as a little blip in consumption.
This may not be so noticable in US models as in European models no consumption means no bars displayed and in US models no consumption means all bars displayed.

 

Oh yes, there should be plenty to watch in the app. Post screenshots for those of us without!

 

It's little details like this that make all the difference. Thanks for adding this!

 

Makes perfect sense! Thanks.

When coasting, the metric display shows a zero and anything above that will register. In the US and certain other countries, when coasting, the car would show infinite miles per gallon, but the display won't go that high, so it stops at 100 mpg (at least on my PiP) So, if the engine sips a tiny bit of fuel to help it get spinning, it's still well over 100 mpg, so it doesn't show. However, the metric gauge is above zero, so it will show. Love it! Thanks, @Maarten28! You solved my mystery with the info about the engine needing a tiny sip of dino juice to get going without inducing a jolt.

 

I'm not 100% certain I've got this right, but I'm taking a stab at an even more detailed explanation. Corrections invited!

First, it is important to understand that the Prius power split device has three mechanical input/outputs. Three things that turn. They are permanently geared to each other. None can disconnect without actually taking the thing apart.

Therefore, rotational energy going into any one of these i/os must come out on the other, the other-other, or a combination of the two. Zero slippage.

I like this page for its animated depictions of the power split device. See the section about decelerating and hill descent.

The speed threshold you've identified is just about where the ICE no longer needs to be rotated for MG1 overspeed protection.

This is the part I'm not so sure about: I'm curious to learn if at some point Toyota changed the program to add a tiny bit of fuel to reduce the perceived 'lurch' in this reconfiguration, or maybe as an extra control for overspeed protection.

Try it on the simulator, and as you decelerate, you'll see where the engine stops and a big spike in MG1 activity. It makes sense to me that actually fueling the engine a tiny bit right there would smooth out the transition