Question about engine braking

[jimr]

I live in a mountainous area so often need engine braking on steep downhills in my '25 off-road hybrid. Configuring via Tow/Haul mode or choosing a max gear in s+/-, the instantaneous mpg meter drops significantly from 60 (max) down to something like 30 or maybe 40. This indicates to me that unwanted power is being added. I'd think going downhill it should stay pegged at 60. Does anyone know what's going on here?

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[jimr]

I live in a mountainous area so often need engine braking on steep downhills in my '25 off-road hybrid. Configuring via Tow/Haul mode or choosing a max gear in s+/-, the instantaneous mpg meter drops significantly from 60 (max) down to something like 30 or maybe 40. This indicates to me that unwanted power is being added. I'd think going downhill it should stay pegged at 60. Does anyone know what's going on here?

No replies. Please delete thread.

 

[BadTrainDriver]

Replied!

 

[Nodak]

I live in a mountainous area so often need engine braking on steep downhills in my '25 off-road hybrid. Configuring via Tow/Haul mode or choosing a max gear in s+/-, the instantaneous mpg meter drops significantly from 60 (max) down to something like 30 or maybe 40. This indicates to me that unwanted power is being added. I'd think going downhill it should stay pegged at 60. Does anyone know what's going on here?

only thing is can think off ...

most likely if you watch the engine will downshift automatically.

especially if you watch it while in drive coming to a stop, you can see the tach blip as you reach zero mph

 

[kbstone13]

I live in a mountainous area so often need engine braking on steep downhills in my '25 off-road hybrid. Configuring via Tow/Haul mode or choosing a max gear in s+/-, the instantaneous mpg meter drops significantly from 60 (max) down to something like 30 or maybe 40. This indicates to me that unwanted power is being added. I'd think going downhill it should stay pegged at 60. Does anyone know what's going on here?

I'm interested in this too. I live in western Colorado and drive in the mountains regularly. The engine braking results in pulsing, which seems odd. It seems to have less pulsing when in Sport mode than in ECO or Normal Mode, but it still pulsates even in Sport mode and I have to use more brakes than I'd like. I shift down into 3 and sometimes 2nd to keep the truck slow (30 - 40 mph) on the steep sections, but I find that braking is still needed and pulsing still happens.

 

[jimr]

I'm interested in this too. I live in western Colorado and drive in the mountains regularly. The engine braking results in pulsing, which seems odd. It seems to have less pulsing when in Sport mode than in ECO or Normal Mode, but it still pulsates even in Sport mode and I have to use more brakes than I'd like. I shift down into 3 and sometimes 2nd to keep the truck slow (30 - 40 mph) on the steep sections, but I find that braking is still needed and pulsing still happens.

Yes I see that pulsing too. It seems particularly pronounced when traction the battery is full (8 lit-up bars on the MAX meter). Annoying. Definitely prefer the behavior of my 3rd gen 4runner when manually downshifting immediately slows the car down, and no electronics adding unwanted power!

 

[MikeD]

I'm not sure exactly what is happening overall (although I have not noticed the pulsing and will pay more attention), but here is what I have experienced: I use the "standard mode" engine breaking on long downhills and I think the engine really does need to be engaged to obtain that braking through the transmission. And at the slowish speeds and highish RPM (I tend to aim for around 4K RPM for controlled engine braking) gas is being consumed to keep the pistons firing - but lots of the energy from that gas is going toward charging the hybrid battery - standard mode on a long downhill is the way I have repeatedly gotten it charged up to 8 bars. If I want to maximize fuel efficiency - and its not too steep so I don't NEED the engine braking - I can switch back to drive and the engine will shutoff and coast (it will turn back on seemingly to implement more engine braking sometimes.) I don't know the algorithm for the engine shutoff/turnon and it seems inconsistent but having the instantaneous MPG at 35-40 instead of max (60) seems like a good trade when I'm going to get back the energy (and the gas milage) with lots more traction motor-only operation.

 

[outdoorCO]

I am seeing the same pulsing coming down Monarch Pass on a consistent basis. Was going to report to Toyota at next oil change.

 

[Gumpus]

My Civic Hybrid will run the engine to absorb excess electric power when the battery is 100% charged and regen braking is trying to feed more power to the battery. On long downhill grades you might get best fuel economy with minimum regen.

 

[jimr]

I'm not sure exactly what is happening overall (although I have not noticed the pulsing and will pay more attention)

If the pulsing is not noticeable by feel, try turning on the instantaneous MPG meter while going down a steep hill. The pulsing can be seen when the needle bounces back and forth rapidly between maybe 25 and 45 MPG. I would think to just keep the engine running the MPG meter should be pegged at 60 going downhill with no accelerator pedal input.

As to regenerative braking, wouldn't this be disbled if the MAX bars are pegged at 8 lit-up?

 

[MikeD]

If the pulsing is not noticeable by feel, try turning on the instantaneous MPG meter while going down a steep hill. The pulsing can be seen when the needle bounces back and forth rapidly between maybe 25 and 45 MPG. I would think to just keep the engine running the MPG meter should be pegged at 60 going downhill with no accelerator pedal input.

As to regenerative braking, wouldn't this be disbled if the MAX bars are pegged at 8 lit-up?

Hmm.... I think you mean by "instantaneous MPG" the meter I have showing on the right - correct?

This is a photo of an explicit example of hitting 8 bars after Hi4 on Ridge Road in Michaux State Forest in Central PA and then some more downhill on Rt. 233 after getting back into 2WD where this photo was taken.

Unfortunately for this discussion I was not working to get engine braking and you can see that I'm in drive and engine is running at 2100 RPM and MPGs are pegged at 60.

I have this view most of the time and am generally glancing at that MPG meter every few seconds and I have not seen that pulsing you describe. But as I said, I will pay more attention - especially when I use standard mode for engine braking.

On your last point, after the battery is fully charged I would guess that the algorithm dumps the excess power somehow, while keeping the battery at max. I don't think anything is "disabled", just less current running in that direction into the battery. But I am guessing (and maybe splitting hairs).

 

[jimr]

Hmm.... I think you mean by "instantaneous MPG" the meter I have showing on the right - correct?

This is a photo of an explicit example of hitting 8 bars after Hi4 on Ridge Road in Michaux State Forest in Central PA and then some more downhill on Rt. 233 after getting back into 2WD where this photo was taken.

Unfortunately for this discussion I was not working to get engine braking and you can see that I'm in drive and engine is running at 2100 RPM and MPGs are pegged at 60.

I have this view most of the time and am generally glancing at that MPG meter every few seconds and I have not seen that pulsing you describe. But as I said, I will pay more attention - especially when I use standard mode for engine braking.

On your last point, after the battery is fully charged I would guess that the algorithm dumps the excess power somehow, while keeping the battery at max. I don't think anything is "disabled", just less current running in that direction into the battery. But I am guessing (and maybe splitting hairs).

Yes by "instantaneous MPG meter" I meant the meter that's pegged at 60 in your example.

I wonder if the "pulsing" requires a steeper downhill for a longer range than you've dealt with? I will say the pulsing isn't always consistent, sometimes it's there and sometimes it isn't, on the same hill.

On your last point, I sure hope you're not correct. Adding excess power forcing the driver to burn up brake leather faster, isn't the best algorithm I can think of.

 

[MikeD]

On your last point, I sure hope you're not correct. Adding excess power forcing the driver to burn up brake leather faster, isn't the best algorithm I can think of.

That is NOT what I meant. The extra electrical energy that is generated by the engine braking is "dumped" in my lingo into some circuit where it dissipates as heat.

My point was that nothing is "disabled" - the engine braking is still happening and the charging is still happening, but the circuit and algorithm do not let the battery get over charged - However, I'm going to back off on that statement as I've thought about it some more:

The traction motor does not engage to put energy into driving the wheels forward unless the driver pedal action (or cruise control) are asking for that energy - i.e., no traction motor engagement (blue MAX meter) and no extra energy for brakes to deal with.

The traction motor clutch assembly is arranged to either put energy into the battery (rotor turning under force of inertia/gravity - charging mode), or taking it out and using the traction motor to drive the wheels - not both at the same time.

And this thought process leads me to the likely scenario - if the battery is fully charged (and inertia would otherwise keep the system in charging mode) the algorithm simply disengages the clutch that turns the rotor to generate current into the battery. (Edit: OK... so in this case engine braking only comes from the inertia in the lower gears - like a traditional ICE and not also from the traction motor in charge mode. I think.)

Bottom line: your use of "disabled" may be apt and I'm glad you pushed back so I could think through it.
AND I really welcome any experts out there to correct me and explain further.

And MAYBE, this traction motor clutch engagement/disengagement is related to the pulsing you describe. Any sense whether it tends to happen when battery fully charged?

 

[outdoorCO]

Has anyone brought this up with Toyota?
I have also seen this pulsing on steep declines in crawl control mode, which really sucks when you are trying to "crawl" under a controlled speed over rocks.

 

[jimr]

That [electronics adding unwanted excess power causing needless brake leather to be burned] is NOT what I meant. The extra electrical energy that is generated by the engine braking is "dumped" in my lingo into some circuit where it dissipates as heat.
...
And MAYBE, this traction motor clutch engagement/disengagement is related to the pulsing you describe. Any sense whether it tends to happen when battery fully charged?

Yes I would say if anything, the "pulsing" is more pronounced when the traction battery is fully charged.

Possibly tangential to your comments, but absolutely the truck IS causing unnecessary burning of brake leather whenever going downhill and downshifting. The electronics apply unwanted power, seemingly an amount necessary to maintain current speed. But I don't want that! I downshifted to slow down using something other than brake leather. When doing this (downshifting) the MPG meter immediately drops to a much smaller number (maybe 15 MPG) and then very quickly recovers and indeed some engine braking becomes noticeable. But the MPG recovery isn't to 60 MPG, but rather to something in the range of maybe 25 to 45 MPG. This behavior (unwanted power added when downshifting going downhill), unlike the "pulsing", is not a "sometimes" thing. In my experience it always happens.

One other comment that may be relevant to your response: I've rented Teslas locally a few times. I found that going downhill from home into the city in a Tesla, if the battery is fully charged, there is no question but what applying actual brake leather is necessary to slow down. But if not fully charged leaving home, generally zero brake pressure is needed even going down a fairly steep hill, due to regenerative braking. Of course a Tesla battery and motor are much bigger than in the 4R hybrid, but the point is that at least in a Tesla, there's no "dumping" of regenerative energy going downhill when the battery is full, you gotta use brake leather. Whether 4R hybrid is the same I can't say.

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