Can Hills Save Gas?

[SnowStorm]

The basic idea behind a hybrid is that you can store energy in the battery and then use it when the load is light with the ICE shut down.  An ICE becomes increasingly inefficient as the load drops towards zero.  Hills also store energy - potential energy.  You get it back when you go down.  So, in effect, you now have a much bigger "battery" to work with.  My test run that gave 72.7 mpg (see the 50+ MPG thread) included a fairly long stretch where the road gently climbed a couple of hundred feet.  Going back down it was amazing to see the car just go on and on with the kW meter hardly even registering at all.  It seems counterintuitive but I do think you really want to charge the battery and climb hills at the same time.  When the ICE is running, it needs to work fairly hard for best efficiency.

 

Has anyone been able to compare flat runs to those with gentle hills where you can have the ICE run only when going up hill?

[valkraider]

Physics dictates that no, hills will not save gas.

 

Due to conversion losses - it will always cost more energy to climb one foot than you can recoup by descending one foot.

 

However, having a battery means hills in a hybrid will consume much less fuel than hills in a standard ICE. In the normal ICE all of the gravitational energy is completely lost while our batteries allow us to recover at least some of it.

[plus 3 golfer]

On steep hills where the decent would require braking you would want to deplete the battery to the extent practicable going up hill so that at the top of the hill the battery is very low.  So now the potential energy at the top of the hill will be converted to kinetic energy as you descend.  Any excess kinetic energy can then be converted to electrical energy to recharge the depleted battery instead of otherwise being expended via the friction brakes had the battery been full at the top. Unfortunately, without a way to tell the PCM not to charge the battery going uphill this is probably not attainable except perhaps in the Energi with it's larger storage capacity. 

 

On gradual hills, you'd want to likely fully charge the battery going up hill when ICE is running most efficiently (we really need the BSFC curve but I assume Ford takes that into account when deciding where to run ICE).  Then like you say, one can go a long way on a gradual downhill slope in EV mode.

 

Of course in either case, there are conversion losses to consider but one should be able to figure out the best way to attack hills especially if one travels the route routinely and monitors the data to "save gas."

 

But to answer your question I doubt one would see much difference between hills and flat if the hills are gradual and if speed was the same.  As the hills get steeper, flat is better than hills. 

Edited May 30, 2013 by Plus 3 Golfer

[fotomoto]

Snow,  one fellow here likened what you're thinking to surfing.  The hills in his scenario were highway/expressway overpasses.  His thinking was to power up and over them while also charging the battery and then surf one's way back down with EV assist and gravity.  This, of course, has to be done at speeds around or below the 62mph threshold.  I wish I could recall that thread.

[Tdefny]

They would help a lot if you find a route that is downhill in both directions, but light rolling hills do help a hybrid compared to a conventional car. As stated above, it forces the ICE to run under load, where it is more efficient. I think of it as an enforced version of P&G.

 

There is a hilly stretch nearby where my '02 Prius does 75 mpg. I cant wait to take the Max there and see what she will do.

[Jus-A-CMax]

Gentle hills are the best, you still have a chance with some P&G if it levels out. Short hills for me, I just punch it up (short power bursts spaced out so you can utilize the momentum to climb the short hill) especially if your SOC is high and alot of that mojo will come from the batt.

 

When climbing steep & really long grades ala Sherwin Grade between Bishop and Toms Place, keep it at 2 bars power and stay right, you'll be going 50-55mph as its the most economical way up, anymore power you'll go faster but you're just trading a lot of gas for lil time. This is at FULL load by the way so without the load, the climb should be more like 55mph at 2 bars.

Edited May 31, 2013 by Jus-A-CMax

[SnowStorm]

Storing energy in a hill still seems more efficient than storing it in a battery.  The battery storage cycle might not exceed 80% efficient (losses in the generator, charging circuit, battery charge/discharge cycle, invertor, motor).   whereas the hill is 100% (the only loss I can think of in the hill is the tiny amount of extra distance traveled as compared to flat).

 

Yes, I am thinking about gradual hills where you need a small (or zero) amount of energy out of the battery when going down.

 

I remember reading the surfing post too - oh yeah - "Surfin' U.S.A."

 

"...downhill in both directions" lol,  where is Mr Escher when we need him?

Tdefny:  What speed gave you 75 mpg?  What did you get at the same speed on a flat?

[ScubaDadMiami]

When climbing steep & really long grades ala Sherwin Grade between Bishop and Toms Place, keep it at 2 bars power and stay right, you'll be going 50-55mph as its the most economical way up, anymore power you'll go faster but you're just trading a lot of gas for lil time.

I think I understand, but I am not sure.  Do you mean two segments of ICE on the graph? 

 

Miami Beach is an island.  So, I have to deal with crossing a causeway with arching bridges, daily.  I haven't really developed a good feel for ICE efficiency (though I think I am getting pretty good at EV).  Not sure if higher RPMs equals faster charging per fuel spent, how much I should let speed drop per incline grade and distance of rise, etc..  I've been mostly playing it easy, but when I am going to climb the bridge with a low battery, and ICE will be on anyway, I am trying to figure out whether I should be a little more or less aggressive during the climbing phase. 

 

So far, no matter what I do, it all seems bad; it's just a question of which is worse.  About the only thing that is working is for me to get into EV before reaching the top of the hill, allowing me to stretch out EV on the downside.

 

Thanks for passing this along.

[Jus-A-CMax]

See pic below. Keep the white bar within the red box.

 

Those arches are not that steep, just P&G, you should be fine. Mind you, I have not been there but I can't see them putting up steep grades on bridges, usually they're pretty flat or just slight.

 

Without hitting the brakes and no ICE, you have a natural regen so the faster the wheel spin the quicker the batt will charge up. However, when your ICE is on, I don't have any stats here since I have no scangauge but others do ala +3 golfer, to me the regen is the same whether you stick it in eco-cruise at 60mph with low SOC and let the eco run the ICE to recharge, or if you do the same condition and procedure at 68 mph - it appears to charge at the same rate. Others will chime in tomorrow with their stats, I am sure.

 

 

By the way, do you own experimentation as my roads and conditions is different than you other guys. Plus people have different expectations as well such as getting there asap.

Edited May 31, 2013 by Jus-A-CMax

[plus 3 golfer]

Two things: 1) Valkraider is correct in that physics dictates and that is all things being equal, flat will be better than hills (algorithms should find the best combination of ICE and EV on flat ground) and 2) one can perhaps find ways to attack hills to get better FE than one would otherwise get going up and down the hill.  

 

So, like Jus-A-Cmax says "do your own experimentation".  But if one could duplicate the experiment on flat ground (same changes in speed, battery SOC and so forth as one goes up and down the hill), flat would likely win (the best one could hope for would be a tie).  The goal would be to find the best way to attack a hill to maximize FE.  I've said this before the PCM can't anticipate conditions.  The algorithms run the C-Max on what is known in real time.  But, we can anticipate conditions (grade, length of grade, max. and min speeds and so forth) and can perhaps find ways to achieve higher FE than otherwise would be achieved by letting the PCM run the car (eg, P&G vs eco-cruise, perhaps accelerate up some hills and slow down up others and so forth).

 

Bottom line: if there is a choice between travelling hilly terrain vs flat terrain, I'd always pick the flat terrain all other things being the same. IMHO, the best one could hope for is a tie in FE.  So, I see no reason to try to compare hilly FE with flat FE.  If one routinely has hills to negotiate,  I'd experiment and see what course of actions maximizes FE for a given hill and given charge level. If one had the BSFC map of ICE, one could see what power level and rpm would minimize fuel consumption and then attempt to run ICE at that level by varying vehicle speed to charge the battery while going up hill . 

[Jus-A-CMax]

Absolutely, gentle slope is my preferred course if I have choices...even if I have to go a little out of ways where I maximize FE and burn least gas. Thats the diff btwn 38 or 47 when u see your Trip data.

[SnitGTS]

I'm aware of the psychics involved and what not, but I've been thinking about this for a couple days and in a few specific situations I think a hybrid can take advantage of a hill to maybe get a little better overall FE than you would otherwise.  (I say overall because obviously the hill itself is going to hurt your FE)

 

Now I've only had my C-Max for 3 weeks so I have no real world data to back me up, but here's what I'm thinking.  Before I got my car, I drove a route to work that took me up and over a steep hill that I'm guessing is about 250 feet high, as soon as you peak you come right back down to about the same level you started at, I went this way because it is the shortest & most direct route to get to a local highway.  After going over it for the first time in my C-Max, I realized how bad my FE was going over it and that I should never go over it again!  I now drive a route that is maybe a half mile longer, but the hills are much more gentle.  Once winter comes around our cars will have to run the ICE initially to warm up anyway, so I might drive up the hill a few times to see if the extra strain early in the trip gets the car fully warmed up fast enough that I get better overall mileage.  Not to mention that by the time I get down the other side my battery's SOC should be pretty high!

Edited May 31, 2013 by SnitGTS

[MtnMarty]

I think it's the Law of Conservation of Energy...but I'm only a biologist, not a physicist..

[Tdefny]

Tdefny:  What speed gave you 75 mpg?  What did you get at the same speed on a flat?

It is about a 7 mile stretch that starts mostly uphill for the first mile or two, then rolling hills. Speed around 40. I filled up the Prius and reset the odos at the bottom of the hill and checked the MPGs when I reached my destination. Looking forward to doing something similar in the Max next time I go there.

[Noah Harbinger]

Adding hills can only increase the amount of energy needed to drive a given distance. That is law of conservation of energy. If you were driving EV-only, that would be a given. 

 

BUT… The ICE has different levels of efficiency at different speeds - the amount of energy you can extract from a given amount of gasoline varies depending on driving conditions. Whether there is a possible hill that allows a given car to operate more efficiently than flat ground - perhaps because a slight incline allows it to operate at a higher energy-per-gallon condition and then use regen + potential energy to glide downhill at extremely high MPG - is a very tricky thing to work out.

 

The main efficiency of a hybrid as I understand it is to allow the engine to spend more time operating in its most efficient ranges, and use the electrical motor to stretch that efficiency range to a wider percentage of real-world conditions. (That plus regen braking doesn't hurt).  

[ScubaDadMiami]

I am at less than a week of ownership, but I have been playing with the challenge from the start.

 

Although South Florida is mostly flat, I live on an island that is connected to the mainland via causeways that can have fairly high arching bridges with flat spots between, and then this ultimately dumps onto a series of ramps and inclines for most of the rest of my almost daily drive.  I made the drive twice, once via a more flat but traffic intensive route, and once via the route that goes through the higher bridges and ramps..

 

Now, it could be due to the fact that I don't yet know the proper technique for bigger hills, but I had a significant decrease in FE going the more hilly but continuous movement route versus the longer but flatter route that had tail end rush hour stop and go.  I still have some other combinations that I will check.  My vote would be that flatter is better, for sure.

[salsaguy]

there's some cool articles here:

http://www.ansys.com/Industries/Automotive/Application+Highlights/Electric+Powertrain

[mipmapped]

Late to this thread, but from my experience rolling hills definitely can increase MPG. My Energi (rated 42mpg - lower than the Hybrid) gave me 47mpg on a 65mph rolling hill run from San Francisco to San Jose on the I-280 (running in hybrid mode, little wind, hot day). I only get about 40mpg on the alternative flat CA 101 route at similar speeds. My EV miles were about half the journey - far more than I would normally get on a flat run at 65mph. I figure this is like pulse and glide, but more efficient as you aren't having to accelerate to speeds higher than 65 (with the assiciated increase in drag) to get a 65 average speed.

 

FYI, I used grade assist the whole way.

Edited June 30, 2013 by mipmapped

[MacGyver]

I think you're right- I reset my average when starting up the 405 hill just south of the 101, which is a fairly steep grade, getting like 20mpg going up, then downhill and flats to Torrance (about 25 miles) and it was one of my best runs ever, 63mpg AVERAGE.

 

There was stop and go traffic, which helps.