I have read that the Prius gets better MPG in rolling hilly terrains. I would think that my be the same in the volt. Traditional car burns extra gas to go up a hill, then continues burning gas (less gas - esentially at idle) on the way down. Hybrids and EV's Burn energy on the way up, but gain energy on the way back down.

 

I bantered this about several months ago. A few years ago, I read that people see MORE return with a hybrid in hilly areas. I have since tried to find links to this article, but have been unable to. I have 2 homes, 1 in NY (with hills) and 1 in FL (completely flat). Both hybrid SUVs (Highlander and Tahoe) get slightly better mileage with some terrain (a few mpg). DonC argued that this was a 'pulse and glide' concept, that hypermilers use. I can tell you that I do not try to maintain speed up hills; I prefer to choose an engine rpm not to exceed and let speed fall accordingly. One thing that cannot be argued- if the downside slope of the hill enables the engine to shut off, you are ahead of the game. Your question involved a hypothetical constant speed scenario (and the old conservation of energy argument). However, I suspect that people often ease up on the gas going uphill and let it coast going downhill. This complicates the discussion, but would explain why hybrid owners reported faster payback in hilly terrain. If you drive on a flat surface for hours and hours at constant speed (the FL scenario) you find that your hybrid hardward is just dead weight. There may be energy in the battery, but it ain't goin nowhere. This doesn't resolve your debate, but it does help the discussion to take on a few more facets. [Go Steelers!]

 

I think it comes down to the fact that it takes more energy to push the car up an incline than to drive on flat ground. The benefits of rolling down a hill with regen braking can never outpace the initial power draw.

 

It takes energy to move a car up a hill because you're moving it from one potential energy state to a higher potential energy state. Unless you can recover all of that energy when coming down you'll end up with a net loss.

As an example, the amount of energy needed to effect this change of states is given by mass X gravitational constant X height. That means getting 2000 kg up 1000 meters would take 196M watts or 5.44 kWh. You recover some but not all of this energy through regen. Mickey Bly has said that regen is about 30% - 35% efficient on the Volt, which means that you would recover 1.77 kWh of that energy, leaving you with a net loss of 3.67 kWh. But you're right in thinking the EV will have fewer losses since it can at least recover 35% of the losses. With an ICE vehicle you have no ability to recapture any of the energy needed to move the car uphill so you'd lose all the the 5.44 kWh.

Note that for both an ICE and an EV, if you could just let the car coast without breaking, and let gravity turn the higher potential energy into kinetic energy, you'd avoid these losses. But as a practical matter this usually isn't possible.

 

Totolos,

The answer is your scenario is impossible (barring one uncommon exception). It’s OK until the hill car reaches the top of the grade. At that point the only way a regular car has of recouping the energy used to climb the hill is by increasing its kinetic energy, that is, by speeding up on the way down. But your scenario has the car’s speed clamped at 65 mph!

Of course, you could relax that condition and let the car accelerate but then the wind resistance will be more than that of the car on the flat maintaining a constant 65 mph. In short, the hill car is going to lose the energy game! In practice it will be much worse because you’ll probably have to brake to avoid other traffic or obey speed limits on the downhill run.

The one exception I can think of for your scenario to be valid is, if the mechanical friction of the cars (or perhaps wind resistance) is so high that you have to actually use the gas pedal on the way down the hill to maintain 65 mph - unlikely unless you are on a very shallow grade!

One way around this is to provide another mechanism for converting the potential energy gained by climbing the hill - like regenerative braking! It’s not perfectly efficient but at least it makes the game closer.

Graham

 

From my experience I think CD miles would would vary considerably depending on the sequence of hills. For a 100.1 mile trip starting at 3450ft elevation and traveling the following elevation profile (3450ft to 2140 to 2550 to 2445 to 1270 to 546 to 1530 to 934 to 2600 and reverse back to 3450). I got 58.1 miles on CD and 42 miles on CS using 1.59 gallons of gas. I don't think it would have been nearly as good if I ran the same round trip starting from the other end. But both directions start out with about 20 miles of downhill. In both cases after 20 miles the estimated CD miles was greater than when I started. Even on the end where I started with a full charge.(must have used history to calculate the estimated miles as I don't think it would regen charge above the full charge level).

 

2004 Civic Hybrid, about 15 miles, from 800' to 3000', then back down. Problem is with regenerative forces the small battery would recharge quickly then offer little or no braking, just no way to store the energy on the way back down. Volt should be better with a larger storage capacity.

 

Maybe combustion engines become much less efficient under the load of climbing a hill....

US car combustion engines are generally more efficient under the load of hill climbing. The problem comes on the downslope when the engine operates in an inefficient near-idle regime. Or, even worse, operates in an engine-braking regime which is the worst of all worlds because you burn gas to create drag. Some hybrids can shut off the engine while descending, and thus can deliver better MPG in certain hilly conditions than on flat ground.

Anecdotal hill-vs-flat MPG evidence is almost worthless. I've seen people make extraordinary claims, but when pressed for details admit that they let their speed bleed off going uphill and gradually accelerate back toward the speed limit on the downhill. Their average speed is thus lower in hills than on flat ground and their data is useless. Others collect hill data at higher elevations than flat data without compensating for lower air density and the resulting reduced aero drag.

Your constant 65 mph thought experiment, with air density and other variables are also held constant, will depend a lot on grade. At the maximum 6% grade for the US interstate system, a typical car will need some sort of downhill braking to maintain 65 mph. Whether the brake force is applied by relatively inefficient electric regen or atrociously inefficient friction or engine braking, energy will be lost and your overall MPG will suffer. A more benign 2-3% grade which permits 65mph while coasting should produce the following results:

Conventional car - lower MPG in hilly terrain as higher uphill ICE efficiency is more than offset by poor downhill efficiency
Convention car, shifted to neutral with engine off on downhills - higher MPG in hilly terrain
Full hybrid which can shut engine off on downhills - higher MPG in hilly terrain
EV - similar kWh usage for flat and hills

Electric motor efficiency does vary with load, but not nearly as much as a combustion engine, so in this controlled hill vs. flat experiment I'd expect similar energy consumption. The real world is not nearly as controlled, though. You'll always have sections with more severe grades that require regen as well as very inconvenient things like stop lights at the bottoms of hills. That's why they alway say "your mileage will vary (usually downward)".

 

The problem comes on the downslope when the engine operates in an inefficient near-idle regime. Or, even worse, operates in an engine-braking regime which is the worst of all worlds because you burn gas to create drag. Some hybrids can shut off the engine while descending, and thus can deliver better MPG in certain hilly conditions than on flat ground.

As far as I am aware most if not all modern cars will turn off the fuel injectors under engine braking conditions. With something like a ScanGauge you can see the fuel consumption go to 0.0 GPH.

 

I just checked some efficiency curves, and you are 100% correct. Here are references:

http://texasiof.ces.utexas.edu/texasshowcase/pdfs/presentations/d5/rschiferl.pdf (page 13)

http://www.tpub.com/content/altfuels08/5366/53660078.htm

The electrical motor has a flat curve that is 90+% efficient under load. The gasoline engine is much less efficient, particularly at low loads. Wikipedia claims, "Modern gasoline engines have an average efficiency of about 18% to 20% when used to power a car."

Of course fossil-fuel power plants have relatively low efficiency as well, so it's not Nirvana. Again relying on Wikipedia, "Typical thermal efficiency for electrical generators in the industry is around 33% for coal and oil-fired plants, and up to 50% for combined-cycle gas-fired plants."

 

Great post doggydogworld. Thanks!

 

As far as I am aware most if not all modern cars will turn off the fuel injectors under engine braking conditions.

True, though there are lots of exceptions (e.g. fuel injectors turn back on below 1500 rpm, turn on occasionally to keep the cat converter hot, etc.). Anyway, it's true that when the fuel injectors cut off under engine braking you are no long "burn gas to create drag", you merely create drag.

I don't see how you can burn gas to create drag.

RPMs are higher under engine braking. My 2002 Chrysler minivan shows higher indicated MPG coasting downhill in neutral than coasting down the same hill at the same speed in gear.

I think the engine braking drag is a much bigger deal than the fuel flow, though. EVs and those hybrids which can reduce or avoid engine drag have a big advantage if driven correctly.

 

Without reading through all the threads to see if this has been said already: It's the same basic reason that a car burns more energy going 100mph instead of 50mph over the same distance. Even though it gets there quicker, there are losses in efficiency through all these mechanical and electrical processes. Even if you took out all friction losses, there's no way to put all the energy back into the battery that was used to lift the car up the hill. And you are going to lose more getting of that energy on the way out too.