HILL DESCENT MODE?

[BIG ROCCO]

If I'm going down a long or steep grade, where the vehicle speed needs to reduced/controlled, what is the most fuel efficient method?  In other words, what recaptures the most energy and charges the battery the most - light application of the brakes, hill descent mode, shifting to L, or some combination of the above?

[fotomoto]

Being a flat-lander, I'm the last person to ask but the way I understand it is this:  the order you have the options listed is the correct order for the increasing steepness and/or length of the grade.  Bottom line:  once the battery is full from regeneration it's best to be in L to give the traditional friction brakes the easiest possible load.

[plus 3 golfer]

For the hybrid, if the engine is spinning when shifting to L, engine rpm will increase significantly just like a vehicle with a conventional transmission when one drops into a lower gear. If ICE was off (not spinning) when shifting to L, then ICE will start spinning. Thus, the increase in rpm acts as a load on the drivetrain (engine braking). There would be no additional regeneration. So, L for the hybrid should be the last choice to use to slow down.

 

I would use grade assist first and then if not slowing down enough, apply the brakes to maintain desired speed. This should maximize the use of regeneration.

[BIG ROCCO]

OK - I think you're saying grade assist is basically a more aggressive engagement of the generator mode, compared with regular coast mode, right?  That's what I thought, and I like the idea of hill descent mode, if it's charging the battery like crazy, and I don't have to ride the brake pedal, but if the (lightly applied) brake pedal charges the battery more, I would rather do that

[plus 3 golfer]

The way I see it (but I haven't gone down any long steep hills) is the use of grade assist will provide regeneration (assuming battery is not full) where use of brakes may apply the friction brakes if pressed to hard. Supposedly, grade assist attempts to maintain speed going downhill. So, if speed is still to fast, one can also apply the brakes. If speed is too slow, one can disengage grade assist and coast to speed up or apply throttle. If one can feather the brakes so as not to apply the friction brakes, then it likely makes no difference whether one uses grade assist or brakes to slow down.

 

We need someone to try such out. Eventually, I'll drive US 60 through the Salt River gorge or the Bee Line highway to Payson and give grade assist a try. Problem is that I will probably only have to go down a fraction of the grade before the battery is full and then will have to use L.

[BIG ROCCO]

The way I see it (but I haven't gone down any long steep hills) is the use of grade assist will provide regeneration (assuming battery is not full) where use of brakes may apply the friction brakes if pressed to hard. Supposedly, grade assist attempts to maintain speed going downhill. So, if speed is still to fast, one can also apply the brakes. If speed is too slow, one can disengage grade assist and coast to speed up or apply throttle. If one can feather the brakes so as not to apply the friction brakes, then it likely makes no difference whether one uses grade assist or brakes to slow down.

 

We need someone to try such out. Eventually, I'll drive US 60 through the Salt River gorge or the Bee Line highway to Payson and give grade assist a try. Problem is that I will probably only have to go down a fraction of the grade before the battery is full and then will have to use L.

 

LOL - that's funny - we live in Fountain Hills and have a weekend place in Pinetop-Lakeside, so we will be taking those 2 routes eventually as well!  For now, I am using my 2012 LaCrosse, until the C-Max is fully broken in.  Those long ups and down and sharp turns have got to be tough on the drivetrain and brakes, so I would rather take it easy on the new car until everything settles in.  The LaCrosse is a mild hybrid, and yes, the battery is usually filled up before the bottom of most hills.  The C-Max is my wife's car, but I did drive it this morning and played around with it - first I tried tried hill descent - it did feel like a light application of brakes...no discernible change in engine sound or feel.  Then I tried shifting to L while coasting for the first time - that definitely raised the engine revs a lot.

[plus 3 golfer]

I had a feeling you were thinking about heading North to the rim country. :)  

 

With my Jetta TDI DSG (direct-shift gearbox), I could descend the Salt River Canyon with very little application of the brakes by appropriately dropping down from 6th to as low as 3rd on some of the hairpin turns. 

[BIG ROCCO]

Yeah - headed North again tomorrow AM.  I've been manually shifting into 4th gear on the LaCrosse going down the long grades - leave it in D, but usually end up in 4th (selected by the PCM) going up the long/steep hills, too.  We usually go Beeline/87N and then 260E, but sometimes go 87S to Gilbert Rd S to 202E to 60E all the way to Show Low/Pinetop/Lakeside - that way is so beautiful!

 

The LaCrosse is good, but feels a little underpowered on the longest/steepest grades, especially once the battery is depleted.  I been getting about 37 MPG round trip with no AC use.  It will be interesting to see what the driving is like and the MPG in the C-Max, in a couple months.

[HannahWCU]

The way I see it (but I haven't gone down any long steep hills) is the use of grade assist will provide regeneration (assuming battery is not full) where use of brakes may apply the friction brakes if pressed to hard. Supposedly, grade assist attempts to maintain speed going downhill. So, if speed is still to fast, one can also apply the brakes. If speed is too slow, one can disengage grade assist and coast to speed up or apply throttle. If one can feather the brakes so as not to apply the friction brakes, then it likely makes no difference whether one uses grade assist or brakes to slow down.

 

We need someone to try such out. Eventually, I'll drive US 60 through the Salt River gorge or the Bee Line highway to Payson and give grade assist a try. Problem is that I will probably only have to go down a fraction of the grade before the battery is full and then will have to use L.

 

I have actually done this.  There is a grade near Asheville, NC that is a 6% grade for 6 miles.  In my experience, Hill Decent Mode is just like pressing the brake petal lightly, it increases the regeneration effect from the electric motor.  When using Hill Decent Mode, if I lightly press on the brake petal, I don't feel an increase in regen (i.e. slowing down) as i would with the Hill Decent off. 

 

Once the battery gets close to full capacity (about 95%) the ICE will automatically engage (but use no fuel) to retard your downhill run.  This would be the same as placing the gear selector in "L" as the engine RPM was fairly high (it was loud).  Although I didn't try this as I was traveling in excess of 55mph and didn't want anything bad to happen (LOL).  I assume the engine engages due to the fact that once the battery is full, the motor can no longer provide regeneration as there is no where for the energy to go.

 

It is all automatic once in Hill Decent Mode and pretty cool in my opinon.  I hope this helps.

[BIG ROCCO]

Yes - this is perfect - Hill Descent Mode is similar to light application of the brakes - both increase the slowing effect on the car when coasting by increasing the generator drag/battery charge rate.  L engages the ICE and the equivalent of a low gear in a non hybrid car, also slowing the car, but by wasting and not by storing the energy.  That's what I thought.