Experience for going down hills and back up another

Discussion in 'Fuel Economy' started by EspElement, Feb 2, 2014.

  1. EspElement

    EspElement Well-Known Member

    Hello all,

    I have been fighting a question for a bit and have tried a few different methods however its hard to know with changing conditions. If I have a situation where I am at the top of a hill. I have to go down and back up to the same elevation. Is it best to coast down the hill and bog back up, or gain more speed with the engine down to give help getting back up the next. I know the DWL method however this is kind of the reverse. Having an ATX I can't do an EOC. Just so you all know.

    Thanks!
    J
     
  2. Unleaded

    Unleaded Well-Known Member

    I'd say gain some speed going down the hill (but don't go over the speed limit) so you can gain some momentum that can be used to go up.

    Without knowing your situation, I will say the speed limit is 65. As you are comming down the hill press on the gas just far down enough so that you do not go into fuel cut but not so much that you are accelorating too quick. Adjust the rate of acceloration to the point where you will get to the bottom just shortly after getting up to 65. Maintain 65 until you start climbing the next hill, then let the speed drop slowly, 65, 64, 63...etc. If you drop to 50, hold speed until reaching the top. Unless you are close enough to the top, you can drop the speed further. Just dont let it drop to the point where the transmission downshifts.
     
  3. RedylC94

    RedylC94 Well-Known Member

    That's a difficult question to answer simply, because there are so many variables that affect optimum strategy, including the transmission, speed limits, how long and how steep the hills are, how sharp the valley is between the downhill and the uphill, how well the car coasts (i.e., its weight-to-drag ratio), traffic, how slow you're comfortable going in that traffic, etc., etc. Sometimes your tactic of "gain more speed with the engine down to give help getting back up the next" might work best with one car, but not on the same hill with another car (mine for example, or a motorcycle) that has higher aero drag in proportion to its weight. That's because in the second case so much energy is wasted in air resistance during the high speed section at the bottom of the valley. Generally in such situations, I don't think you want to burn fuel to speed down much faster than you'd be going on a level stretch.
     
    Last edited: Feb 8, 2014
  4. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I will gain speed going downhill with very light throttle ,trying to hold a certain target MPG on the SG ( usually 60). Then I can let this excess bleed off slowly while holding 50-ish MPG. Summertime numbers ARE higher. When you have a commute you are very familiar with , you know exactly where the hills are, even the almost "invisible" hills we have here.
     
  5. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    Dang it, I'm struggling with the above. Have mercy on the acronym challenged!

    EOC equals Engine Off Coast?

    ATX: completely stumped. Is it your vehicle? Anyway, what is your vehicle? Is it a hybrid? That could be a factor, if your car can do regen braking on downhills.

    Rant: why do we talk like this? For example, I like the old days when cars had engines, not "ICE". One terse, succinct word, instead of acronym for 3 excessively descriptive words.
     
  6. some_other_dave

    some_other_dave Well-Known Member

    ATX, from context, means "Automatic Trans(X)mission". Or maybe "Automatic TransaXle".

    I think that we, as a society, have been influenced by the military and tech industry to the point where we love our TLAs! (Three-Letter Acronym.) They're easier to type, marginally, and can help to include those "in the know" and exclude "outsiders". Learning the jargon seems to be part of joining a community...

    -soD
     
  7. RedylC94

    RedylC94 Well-Known Member

    I definitely agree with that rant! It's just a pretentious way of showing off jargon---trying to come across as an expert, but actually coming across as dumb. Practically every vehicle that has an engine has an internal combustion engine, unless it's a Stanley Steamer, and those are pretty rare on the road these days, so what's the point? Even Wankel engines and Chrysler gas turbines are internal combustion engines.

    Besides, that silly habit confuses attempts to refer to frozen water by its ordinary name.

    My Internal Combustion Engines professor tended to refer to the topic as "I.C. Engines," but never as ICE.
     
    Last edited: Feb 8, 2014
  8. EspElement

    EspElement Well-Known Member

    Thanks guys for the replies. I didn't even think I was going to get any after it sat for a while. I suppose mostly because of the unique circumstances it has and also maybe my acronyms which you are right may make us look smarter even if it doesn't. I am sorry for that.

    Anyways, I do have a question for you all. With this how would I determine for sure what works best for me? I have a hard time knowing if what I did on a particular hill or something changed my entire drive. So for instance if I raced down the hill to go back up verses rolling down the hill and slowly drive back up. It seems to just mix in with my average mpg (miles per gallon....). Can I do a snapshot mpg with the scanguage? How do you guys go about this?

    Cheers
    J
     
  9. lxmike

    lxmike Well-Known Member

    esp: you can reset your trip average and it won't affect you tank trip. hit home button (with red cycle) then hit trip buttonand go to current and reset it. then you can try each method and see what work best for you.
     
  10. EspElement

    EspElement Well-Known Member

    Oh yes. I see what you are saying. That's a great idea! Is this how you test out your techniques?
     
  11. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    If an acronym can save me a few typing keystrokes , I will use it. But in normal conversation , I don't say ICE. I will admit to not knowing what all the acronyms mean and I have consulted the Glossary a few times.

    I have a Scangauge , but I don't reset my TRIP in the middle of a "trip". If you want to see a huge (unrealistic) number , you can do that. I want to see my instant , current trip and today's trip. That's all.
     
  12. EspElement

    EspElement Well-Known Member

    If I approach the scenario I was talking about there you could reset at the top of the hill. Approach it how you think it should be and remember the avg mpg at the end of the hill. Issue is we need to have similar ambient temps and engine temps to really get an accurate number....
     
  13. waltermlee

    waltermlee Well-Known Member


    WARNING: Engine off while coasting (EOC) should only be done using a Flat Towable Vehicle. Hybrids using stop start idle tech can do EOC automatically. In many USA jurisdiction, manually inducing EOC going downhill is illegal. In most USA jurisdictions, manually inducing EOC going uphill is still legal. :rolleyes:

    Under perfect DWL conditions, you are on rolling hills with a general trend of each successive hilltop being lower than the next one - like a giant roller coaster - In this case one slowly climb up the first hill in the lowest possible speed (because lower speeds are more fuel efficient) that is practical then at the top of the hill, one accelerate just enough to get the the max speed (as the vehicle goes over 45 mph aerodynamic drag will eat up more and more of your fuel efficiency) and coast downhill- before you get to the bottom of each down hill segment accelerate just get enough momentum enough to overcome the next uphill segment. Repeat. Why? It takes less energy to gain momentum (mass*speed) going down hill than uphill -SO it is more fuel efficient for your ICE to build up speed on the downhill segments and for the vehicle to rely on more on its mechanically stored momentum as it to coasts uphill.

    In a less than perfect world, the uphill road segments can become extended - like in superhighway section where uphill segments may be 1/2 to 2 miles long - and the downhill road segments can be very short - less than 300 feet. A vehicle's momentum will peter out long before the uphill segment does - your fuel efficiency can increase dramatically if a hypermiling route is designed planned to have short uphill climbs and long downhill drops - but sometimes it cannot be avoided. In cases where you have long uphill climbs and short downhill drops - the vehicle needs to have the power on all the time uphill even after adding momentum and you need to *bleed* off the speed-momentum gradually as you go uphill until you get to the top of the hill again. The Problem with short downhills that are too steep is that the top speed at the bottom of the hills is often higher than 65 mph and much of the momentum gain by the *gravity* is lost via aerodynamic drag - For high aerodynamic drag vehicles ( e.g the Honda Element, Ford F-150 pickup, Hummer H2) aerodynamic drag starts to eats up more than 50% of the vehicles energy at a much lower speed ... 25 to 45 mph (some SUV driver draft intentionally to save gas - but that can be really dangerous when combined with DWL). It pays to have a topographical map of the route ( hint: bicyclist often use topographical trip route planners mappers to do this)

    All Automatic Transmission (ATX) vehicles have a sweet point where they operate most effectively (and most fuel efficiently) - hook up a ScangaugeII to your car and find out where that sweet point is for ScangaugeII Xgauges Load/RPM/GPH and try to keep your ICE running at those speeds. Make sure your vehicle is as light as possible - extra weight makes acceleration going uphill even more expensive. Gasoline weights about 6 pounds per gallon. Beyond a ScangaugeII - the next best modification is over inflated tires or using low rolling resistant tire like the Bridgestone Ecopia or Michelin's Energy Savers or Goodyear's Fuel Max.

    The Secret Science for HR of Texas Congressmen Smith types :) = DWL is basically an earth bound version to a rocket science technique called gravity assisted acceleration ( aka planetary swing by) with the road acting as an orbital pathway. The downhill segment can be thought of as the gravitational pull of a planet (SW fans: that's no moon... I have a bad feeling about this...). the bottom of the hill is the planet's surface and the uphill segment is the planet's escape path for the earth bound vehicle. If a space craft flies too fast on re-entry to a planet it will *skip* off the planet's surface and escape the planet's gravitational pull and go back into outer space. Similarly - If your car goes too fast downhill the vehicle's momentum will cause your car not only to go up hill but over the next hill while the transmission is in Neutral and the engine is turned off. Another way of looking at DWL is to think of your car as a Matchbox or Hot Wheels die casted car on a plastic track. Design a Hot Wheels track that copies the hilly terrain of your route and launch a Matchbox version of your vehicle from the start of your track and see how far it can go under momentum alone.... BTW: I did this with my Prius.... I didn't learn that much but... it was really fun to do :)
     
    Last edited: Feb 13, 2014
  14. EspElement

    EspElement Well-Known Member

    Thank you so much waltermlee. This is exactly what I was looking for!!

    When you say automatics having a sweet point are you saying a point in which it shifts to the next gear and I can drop down 5 mph in the same gear and shoot my mpg up by lowering my gph? I noticed that if I can accelerate to around 40-45mph and drop it back down to 35mph I can get about 10-12 more mpg at that speed than if I never accelerated enough to shift to the next gear. It is a bummer because I can't control this...

    I will be re-reading your entire post again to make sure I understand more and will practice them. In general I see that speeding down a hill takes less fuel than going up the other side. Thus if I go down a hill at 50 mpg to then roll mostly up the next, I would save more than if I were to roll down the hill at 2000 mpg til I hit the bottom then had to work my way back up. Conditions like air drag at certain speeds may very this idea but I understand.

    Thanks again!!!
     
  15. NeilBlanchard

    NeilBlanchard Well-Known Member

    I agree - I call this "swooping". Free-wheel coast at the top of the downhill, then at a point that seems good for your car and situation, accelerate down the last part of the descent. Carry the speed up the ascent with a light throttle and lose a little speed to try and keep the MPG up as high as you can.

    Kinetic energy is your friend - and it is a very efficient energy storage medium. :)
     
  16. RedylC94

    RedylC94 Well-Known Member

    Indeed it is, but trying to carry a lot of it from a downhill to a following uphill implies high speed, which can mean a lot of energy lost to aerodynamic drag. That's especially true with a high-drag vehicle, or if the transition from downhill to uphill is gradual and lengthy, as opposed to an abrupt "roller-coaster" valley. The best strategy will vary with the length and steepness of the hills, as well as the sharpness of the valley between them.

    Thank you for referring to kinetic energy, which is a lot more relevant than momentum (one of the most commonly misused basic science terms).
     
    Last edited: Feb 13, 2014
  17. waltermlee

    waltermlee Well-Known Member

    Hopefully - I have steered you in the direction you want to go but just in case I goofed - don't just re-read my posting but verify and validate the terms and concepts via other references/sources. The FAQ on Cleanmpg * at-sign * com is a great starting point.

    Hypermiling results depends several factors: the power plant, the transmission, the tires, and the driving conditions-roads.

    The fuel efficiency sweet point for a conventional gasoline engine vehicle varies with each vehicle. FWIW a Honda Element FE sweet point is some where 2200 to 2800 RPM ( depending on the speed/gear and driving conditions).

    The most fuel efficient gear on an automatic is the highest gear assuming you can get there giving your driving conditions. The highest gear has the highest RPM for the lowest amount of gasoline consumed but has the lowest torque/power per revolution. In a manual transmission - the driver shifts up or down to gain more efficiency but an automatic transmission does that for the driver. Once you get to the highest gear - you want to stay there as long as possible because it takes energy to switch gears. if you have to go uphill and you know the engine doesn't have enough reserve power to get you uphill without shifting your gears down - it might be worth it to add 15 mph to your speed at the bottom of the hill to help you make it uphill.

    A Continuously Variable Transmission (CVT) automatically searches for the most efficient RPM transfer ratio for a given the torque requirements of engine

    When you accelerate to 40-45 mph on a very smooth flat road and then ease up on the throttle - your mpg temporarily increases because your speed is due more to the momentum created by the energy you used previously when you had accelerated to 40-45 mph than power being currently used - As you ease up on the throttle , road friction and aerodynamic drag will slows down your vehicle if you don't apply the accelerator again. If your car is aerodynamic and has a small front surface area like a 1998 Honda insight or a GM EV1 and your vehicle is using low rolling resistant tires like Michelin's Energy Savers all season tires THEN you'll coast a bit longer ... if your car is not aerodynamic and has a large front surface like a Hummer H1 or a Dodge Ram Pickup and it has snow tires on with chains THEN the coasting distance will be much shorter.

    In the old days - a hypermiling driver would have vacuum gauge display on his dashboard with the highest compression ratio equaling the most fuel efficient operating level given the driving conditions and the transmission gear. You emulate this by having a ScangaugeII Xgauge Load at about 60 to 80% or by knowing the RPM range for the Sweet point for a particular gear.

    HTH
    Walter
     
    Last edited: Feb 15, 2014
  18. yaris12

    yaris12 Member

    I think you will have to determine what is your optimal gas mileage. Keep at constant speed and see what your mpg is. Come back and go over the hill again but at a different speed. It is easier to coast at 50 mph than 70 mph.
    I change my fuel meter to give me instaneous reading and not go below 40 mpg at any one reading. For me going 5 mph faster than the speed limit before climbing the first hill gives me the inertia and carries onto the next couple hills. Sometimes a lower gear is better. Try to keep your test conditions the same, for ex wind in the dame direction
     

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