Better MPG on rolling hills than in the flats?

I've found the same thing, too. I've heard that there is an optimal rpm for the engine. I don't have a scangauge, so I don't know if maybe I'm hitting that optimal spot on the throttle going uphill, and then coasting downhill brings me above the flat-terrain average. I generally get 5-10% better on rolling hills (as long as the cruise control is off).

 

I'm not a "Pulse and Glide" kind of guy, but my guess is that rolling hills effectively put you into a pulse and glide kind of pattern and you would see similar MPG gains that those who purposely pulse and glide report.

 

I'm also sure you're in automatic P&G mode.
Hope your uphill is more steep than the downhill, something like...

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Ken@Japan

 

I have been experimenting with different routes in my daily commute. I only go about 11 miles each way. I have found that the route with more hills gives slightly better mileage, especially when the car is fully warmed up. Climbing the hills warms the engine more quickly and it is easier to glide on the downhill slopes compared to trying to pulse and glide in the flats.

 

I bet road surface also has a lot to do with it too.
Here in Portland, OR, I stuggle to stay close to 50mpg average, but our roads all have troughs worn into them by the use of winter studed tires. I bet running in these troughs creates more rolling resistance than running on a Southern CA concrete slab.

 

I have started to see similar things in my ride. I have several options for about a 25 ml ride to work one down a state road that is relatively flat with speeds around 45-50 and others down hilly back roads with speeds from 25-40. I consistently get about 3-5 better MPG down the back roads. I thought it might be do to the lower speeds but after reading things here it may also be do to the hills. I have started to notice this phenomenon in other aspects of my driving but it's only impressions since I can't do the experiment like going to work every day.

 

yes, that is what i am kind of noticing as well, and i only had prius for some 500 miles.


yes, sometimes the fuel economy going up the hill seem really poor, like 12 to 20 MPg when accelerating, but then rolling down the hill is mostly battery and regenerative braking that keep engine off for miles, if i do not hit red lights.

i think that accelerating "improperly" at traffic light is what kills the mileage as well, it sure make sense to accelerate briskly, then ease off the gas pedal, this probably has to do with optimal RPM/power ratio of engine/generator/traction motor.
basically, it is 3 major components (IVE, generator, traction motor) that we are talking about + hybrid battery, so the relationship of those to each other is very important as they are all interdependent/interconnected via power split device/planetary gears

also, when you accelerate, engine drives the generator mechanically and generator drives traction motor and generator also charges the battery on and off.

so, 12 MPg when accelerating may not be only 12 MPg as your car is
giving the energy to traction motor or/and battery, so the fuel ecconomy that is displayed on MFD is just the "engine fuel economy"

i ordered the scan gauge, as without it and without the tach I can't tell
what is optimal load and RPM for the engine, what not.
i am willing to bet that there is optimal ratio of engine speed that if you discover your fuel economy will improve quite a bit and maybe even the performance/acceleration.


interestingly, even on level road of a CA freeway you sometimes get less than 50 MPg, sometimes close to 60 MPg.

can't wait for the scan gauge to arrive

 

Most definitely. I still do see how some of these poster get the mileage they are getting. I'll bet it's hilly roads. That being said the Prius still gets better gas mileage than most cars.
The new EPA standard of 45mpg are the true estimate of what this car will get. Remember the 60mpg estimate not many people get that .
FishHawk

 

This is also the case with conventional cars, rolling hills give better mileage because the engine runs more efficiently at higher throttle (more fuel but even better power) and most EFI cars use almost no fuel on a trailing throttle. Some shut the fuel down altogether.

 

I was going to write my own post about hills giving better MPG. But I see that's been covered.

The best experience I had so far was going up the steep side of a small mountain, at a modest speed. Then going down the other side, I was coasting nearly the whole time.

I've found that an MPG killer is using the brakes. Adding a good long space between your car and the car ahead of you allows you to do a long coast instead of jamming on the brakes when there's trouble. Even though it's regenerative braking, you probably only get about half of that energy back.

 

There is no doubt that proper Prius operation in flats yield much better mpg than hilly terrain. There are people who get about 70 mpg on flat terrains using techniques such as Wayne's super highway mode and about 60-65 mpg using Hobbit's high speed mode for higher speeds.

Essentially they require you to pulse the car to a speed let's say about 55 mph for SHM and 68 mph for HSM. Then either try and hold the car at around 1280 RPM (or IGN 14) for SHM or glide to your lower declared speed (say 60) using less power or the "yellow arrow" condition (warp stealth) when you're going down hill for HSM. SHM will work even at higher speeds provided that the terrain is flat. These are more extensively discussed in these fora so go ahead and do a search.

Now if you're talking about speeds lower than 41 MPH where you can glide extensively then it might be possible that some hills are better, but all the hypermilers manage their records at flat terrain not hills.

 

Living in the hills, I can promise no fuel records are being set in my area. Now if I leave the Sierra Nevada and head to San Francisco, I can get 70 MPG all the way down. It is coming back home that kills me.

 

I have a doubt. I think I do get slightly better MPG on undulating terrain.

Definitely not on very hilly steep terrain, no. But on gentle hills with a fairly low speed limit of around 35 to 40 MPH, that's where I personally get my best MPG.

The best hills are where the gradient is gentle enough that you can mostly still climb them with the instantaneous fuel consumption up around 35 MPG. Then going down this gentle gradient is just enough to let you glide all the way without using battery charge (you may use a bit here and then regen a bit there, but overall you don't lose charge). So the result is I can get 70 MPG average in terrain like this without any special techniques. I don't have a scan gauge and I haven't been able to figure out how to get it quite this good on a pure flat run. So while it may be possible to get it as good on the flats I do believe it's actually easier to get good MPG on gentle hills (slightly undulating terrain).

Now really steep hills and mountain are another matter all together. Yeah I get worse (sometimes much worse) MPG under those conditions.

 

I think ystasino is correct, but our perception that hills are better is only because of typical driving styles. So practically, light rolling hills are better, but theoretically, I would imagine flat is better. To go with the flow of traffic, and for ease of driving, we'll probably just stay one speed on flat ground, often operating outside of most efficient engine output. With a Prius and warp stealth capability, it's conceivably just as efficient as Wayne using SHM to accelerate under load and decelerate in warp stealth, even on flat ground - but of course almost no one will do that because a. it's annoying to everyone else and b. it's annoying to the driver her/himself!

Like others have said, up hills you're driving under load, and as long as you're not going up an insanely steep hill, you're likely giving a very efficient output right on top of the specific fuel consumption mound, somewhere around 15 to 18 kW. On the way down warp stealth might happen, or else SHM in an almost loafing mode. That's why it appears more efficient in our everyday driving. On flat ground you could use the same technique, but it wouldn't be practical with anyone else on the road.

Oh, and, I know I don't have any figures here, but I'm not going to go out and test that stuff myself

EDIT: To clarify, I meant all of this for speeds over the glide threshold.

 

^^^ Yep the hills just provide a natural pulse (going up) and glide (going down). That why it's easier to get good MPG on very gentle hills. I'm not the type of person that wants to constantly pulse and glide and vary my speed on flat terrain in traffic.

Now really steep hills, on the other hand, are like a case of ridiculously over aggressive pulse and glide. It would be like nearly flooring the go pedal for the "pulse" and then moderately braking for the "glide". Obviously that type of "pulse and glide" is going to give you crap fuel economy. That's why gentle hills rule and steep hills suck.

 

Could you point me in the right direction of where to find out more about this "High Speed Mode".

Thanks.

 

You will need a SG to monitor kW. Here's the original paper and theoretical basis described by Hobbit

Sweet spot refinement

Mikewithaprius and myself have been trying to make it empirically relevant for our terrains with decent results and discuss it here.

http://priuschat.com/forums/gen-ii-...super-highway-mode-lower-speeds-50-55mph.html

My current practice is described in my last thread above. The term High-speed mode is something I use to describe Hobbit's hypothesis.

 

I recall part of a big road trip years ago where the mileage was surprisingly high. Most of the day's drive was up and down low, rolling hills. I found if I used just enough throttle, and let the speed build going downhill, I could coast most of the way up the next one. I remember that drive being a lot of fun, and thought the mileage sounded too good to be true. But, if the hills work like pulse and glide, maybe not.

Around here, it's up one big hill - or mountain - and then one long glide down the other side. 'Coasting up' isn't an option, and I'd need a trailer full of batteries to capture the excess on the way down, so low rolling hills are definitely easier on the gas.