Coming back a long mountain descent (about 2500+ feet over about 13 miles) on my 350H, it seem my NX went into auto engine braking during a really steep 7% grade for a few miles.

I had noticed an unusual engine noise and the engine was revving 4k +rpm with no throttle input. When I did accelerate, the engine rpm dropped quickly back to it normal rpm range. I remove the throttle input and the rpm went back up again. Did it again, same operation. As expected, the battery gauge was showing full bars. It sure seems to be an auto engine braking function.

I know some older hybrid systems had a B (braking) mode and I had wonder why it seemed to have been removed on the new systems. I had not heard of this operation from any source. It might be in the manual somewhere, but it took me by surprise. This stretch of road is between Woodland Park and Colorado Springs. It is heavily travel and I don't know of any one locals that uses engine braking.

So was it auto engine braking? If yes, I assume the engine wear is not excessive. Otherwise Toyota wouldn't do it. But in this stretch, I would also have preferred to do on normal braking as most of the folks do.

 

Regenerative braking possibly

 

Our 350h does the same thing. I believe it is to help bleed off excess energy once the battery is fully charged

 

I suspect it turned on the engine to assist in braking. Even in EV mode our 450s decide to 'help' out sometimes both downhill and uphill on our local pass, which isn't particularly high (but decently steep) by turning on the engine. Its fairly annoying, particularly uphill, as sometimes my engine is cold and all of a sudden it kicks in and revs pretty high. Downhill is more consistent, tends to turn on just near the 2/3 mark. I do appreciate the engine braking in some cases, but would rather decide myself.

I believe downshifting in EV mode increases regenerative braking as (unless the ICE kicks in) there's definitely an increase in rolling resistance as I shift down. I'm thinking the 'B' mode is equivalent to downshifting when the ICE is off, which is why Toyota removed it. I'm also presuming your 350h works similarly in EV mode.

One other consideration is that if the batteries in our 450s are full, the ICE will come on even on very minor grades. Is it possible the same thing is happening on your 350h? When I was trying to figure out what was triggering the ICE, I changed my display to always show the tachometer, that me see when it was actually turning on.

 

I sorry to say but, I think you have a misunderstanding. Regenerative braking charges the battery; it doesn't bleed off excess energy but instead ADDS energy to the battery. If the battery is fully charged then braking is performed only through normally disc braking and not by engaging a MG in the hybrid system.

YMMV,
MidCow3

 

One reason the ICE comes on with a fully charged battery is because the hybrid system determines you are requesting faster acceleration than can be provided the the MG electric motors by themselves.

YMMV,
MidCow3

 

MidCow3, on my 450h and my wife's 450h, the engine definitely comes on when our batteries are fully charged on a downhill drive in EV mode. I've confirmed it several times....you can also google the various RAV4 Prime forums, they've seen the same thing. We live at about 1,000 feet up, so we're going downhill for about a mile from our driveway. The ICE comes on during braking, my foot isn't on the accelerator.

I used to charge our cars to 100%, but then noticed the ICE coming on just before the same stop sign almost every time. At first I wasn't sure, but then switched my display to show the tach even in EV mode, started paying attention and saw it happen, then found confirmation in the RAV4 forums. After this I switched to limiting the maximum charge to below 100% it doesn't happen anymore. I've tested both my wife and my 450s, same thing.

The ICE also comes on on downhills in EV mode when it decides to 'help' and even on very gentle driving uphill if the grade is steep enough, no hard acceleration needed. Our local mountain pass seems to steep enough to trigger both situations, I usually try to avoid it happening as I hate running the ICE for short distances and cold, but it is hard to avoid.

You can also probably find the thread in this forum where I and other owners experience the same thing. I can't remember the title, sorry. If you find it, I posted one or two links to RAV4 forums on unexpected ICE engagement.

 

Diesel - Electric Locomotive engines have enormous electric grids (think toaster wires!) on the
roof to dissipate the heat when excess regenerative braking current is created by long downhill runs.

 

Sorry you are sorry.... but you misunderstood. I guess I was not definitive in my statement ("I believe..."). I fully understand how a regenerative system works (Credentials: I am an Electrical and Aerospace Engineer). What I am saying is, once the battery is fully charged there is nowhere for the excess energy to "go", so the engine is engaged with the generator to essentially be used as an energy sink (like a compressor). No fuel is used during the energy dissipation, just compression. Unless you put your foot on the throttle and the engine is needed to provide propulsion. Toyota knows what they are doing and this is all designed into the system.

 

So my credentials BSEE, MBA, For others here is a simple explanation of exactly how the Toyota/Lexus hybrid system works with MG1 and MG2.
https://en.wikipedia.org/wiki/Hybrid_Synergy_Drive

specifically they should look at B braking as opposed to Regenerative Braking:

• B-mode braking
  • Wheels -> MG2 ->HVB
  • Wheels -> MG1 -> ICE (ECU - Electronic Control Unit - uses MG1 to spin ICE which drains battery – allowing more charge from MG2, and also links ICE to wheels causing "engine braking"; ICE RPM increases when charge level of HVB is too much to accept regen electricity from MG2, or increasing effort from driver pushing the brake pedal)
• Regenerative braking
  • wheels -> MG2 -> HVB

Peace and Good Will ... maybe we were actually saying the same thing

YMMV,
MidCow3

P.S. - In addition I had CISSP certification and was Senior Security Architect and an Archer Software Developer, but who cares about degrees , certifications and experience or knowledge..

 

In the end... engineering is hard. Communication is harder...

 

Funny, I was thinking the NX could have a big resistor to dissipate heat, when the battery is full, and can not allow regen braking. But as some have noted, I would rather just have the choice to use my brakes (along with a manual engine breaking option) for its well-know, tested, restive function (i.e. friction) to slow the car- heat those discs! LOL

Seriously, I not sure of the long term issues of using the engine as an air brake. Maybe nothing, but engines are expensive to repair and brakes are relatively cheap. When you add the hybrid consideration of a possible cold engine (and oil) and suddenly gong 4k+ under some air load and a heck of lot of movement- I kinda cringe a little.

 

In such situations when coasting down a hill and the battery is full, does it go into DFCO?

 

I'm sure it is turning on the ICE in DFCO mode to bleed off the car's kinetic energy as CaptAngry said previously. Up until the point the battery is completely full it can use regenerative braking, but once the battery is full, I'm not sure where that generated electricity can go, so I suspect the motors have to disengage.

I'm sure the car (I'm assuming it has DFCO vs. just leaning out) also uses DFCO downhill when the battery isn't full and the ICE is on. We have a mountain pass we take regularly where the ICE comes on (unwanted) often at a certain point on the way down, by when I've already used part of the battery going up the pass and have capacity, but I guess the car decides it doesn't trust me to brake properly.

Our previous non-hybrids did the same thing on downhills, even down-shifting sometimes. I believe DFCO is pretty common these days, it can save fuel, so mfgs gain by implementing it.

 

My understanding is the NX engine has an electric oil pump. I assume it is in operation for at least some time (maybe always) for any engine braking function.


What I am wondering, are there any disadvantages to engine breaking over its oblivious saving of pads/fading of typical breaking? Not that we have any control over it, it seems. Just me being my typical ****/overthinking disposition. LOL