Is accelerating down hills fuel efficient?
 

I have always accelerated when going down hills to maximize the effect of gravity, but I wonder if I am doing the right thing for mileage now. What do you Ecomodding pros do on hills? Thanks, I am pretty new to hypermiling.

If there is a climb ahead I will gently accelerate down a hill, to build up some momentum for the climb ahead. A bit of light throttle there, uses a lot less fuel than starting the hill at a lower speed and having to burn fuel to get up it.
If there is no climb ahead I would probably coast down the hill, or take advantage of DFCO. The last thing you want to do is accelerate down the hill, then have to brake because of a bend or other hazard to be negotiated.

With my four-ton pickup (and more so with 35' travel trailer in tow) I enter a highway with the intention of of using terrain to help the last 10-15/mph up to set cruising speed.

That is, I'll accelerate up past the 45-mph legal minimum, but I wish to use any sort of downgrade to get it up to the usual 59-mph and set the cruise control.

Terrain and traffic don't always cooperate in this, but it does lessen the engine load when those other factors allow for it.

I prefer that commercial traffic be a quarter-mile back, and cars less so.

Alternately,

One uses the brakes, not the throttle, to merge onto an Interstate. Ideally one is at or above the legal maximum well before the end of the ramp (most entrances are downhill for this reason). In this use of a downgrade the penalty is minimal compared to the safety advantage.

Choosing the point of merge means more in my experience.

As to rolling terrain I find that setting cruise control back by two mph obviates the penalty of its use.

With hills, I let it run out some. Maybe five mph downgrade. On an upgrade, more of a change in speed is acceptable. But not to the point of having traffic jam behind me (relative to flatland steady-state).

As I run 10k miles monthly as a pro driver, spacing with other vehicles is paramount. I can't do a thing about tailgaters (less than 100' behind me at speed), but there's no way I'll allow things to jam up ahead of me (less than 200').

So, yeah, aceelerate when there's less penalty, and don't exceed 80% Engine Load in a climb.

Frankly, we most of us run so few highway miles annually (this based on engine hours) that what we do out there past a sensible set speed (60 is The Wall) doesn't mean much to the calendar average.

IOW, with no lane changes and no use of brakes, 58 or 62 or 66 don't mean much. It's tire wear and other factors pertaining to longevity that factor higher.

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Here in Scotland most on ramps are uphill. Add to that short slip roads and trucks struggle.

People confuse 'speed' with 'energy'.

Unless the hill is so steep that you have to drop back gears or go into fuel enrichment, gaining momentum has the same cost on the flat, up hill and down hill. What goes up, must come down, essentially.

Ideally, you want to crest the hill at such a speed that you end up at the speed limit (or your target speed) at the bottom.

Only accelerate on descents if the next hill you face requires that extra momentum. Otherwise, climb slow and steady, in top gear at BSFC (80% load).

*All assuming you have a NA, throttled spark ignition engine.

I've run a test on hills and indeed you get better economy with hills than without. I have a great hill that I use regularly. It's a steep climb, followed by a shallow decent. BSFC up, EOC down. Crazy mileage.

"It depends". As a general rule, I'll engine-off coast down hills and keep high load up hills - unless the hill is steep enough that I'd need to downshift, in which case I will accelerate (partway) down the hill to make sure I can get up the next one.

In simple terms It's always more efficient to accelerate downhill than it is on the flat or up hill. But it needs to be applied with situational context and that's the tricky part which the driver has to compute :-P

Its actually the other way around.

If you use your engine to accelerate down hills until the vehicle has XXX amount of energy, you will encounter higher wind speed than if you accelerate up a hill to get that same amount of energy.

That is because when you accelerate down hills, all of the energy is momentum.

When you accelerate up hills, lets say half of that energy is potential energy.

With some of the energy being in the form of potential energy, you don't encounter the same drastic wind drag losses that you would encounter if all of the energy were in the form of momentum (aka, high speeds).

Wind drag increases exponentially as speed increase, and it becomes quite costly as you go above about 60mph.

I calculated it once. It takes multiple times more energy to overcome wind drag at 80 than it does to overcome wind drag at 55.

So if you accelerate up a big hill, but hit a peak speed of 55, and then coast down the other side for 3 miles, one gallon of gas goes farther than if you spent that gallon of gas accelerating down the hill and plowing wind at 80mph.

Coast down one hill, you'll coast down to 45mph by the next hill. then accelerate up that hill, coast down the other side etc etc.

If you accelerate down that hill, you have to also overcome wind drag and still burn a lot of gas climbing the next hill.

More economical, not more efficient.

Your adding in a lot of varibles, who said anything about what speeds are involved? It takes more fuel to accelerate up hill than it does down hill, that can't be argued. Adding varibles it can be argued as you have done so.

A lot depends on the vehicle too. If you have a 5L V8, with torque to die for, then hill climbing is easy. If you have a tiny 1.2L 4 pot like me, then hill climbing is a task, made easier with a bit of momentum.

Not losing speed up hill in the first place seems to work for me in my car, which is a relative pos economy wise ...again situational context applied here. Highest gear stable speed.

It takes more fuel, because the vehicle is gaining potential energy in addition to momentum. More of the fuel gets turned into energy, and the vehicle goes farther with that amount of energy because it is not encountering as much wind drag.

Per unit of fuel burned, the vehicle gains more energy when accelerating up hill.

There are also fewer parasitic losses (due to less wind drag). So, the vehicles potential energy is used to move the vehicle forward (while coasting down the hill) rather than plow through wind at the higher velocities that would be encountered if you use that same amount of fuel to accelerate down the hill.

As far as the amount of energy it takes to climb the hill, it doesn't matter how fast or slow you do it. It takes X amount of energy to move your vehicle up to the top of that hill, whether you're coasting or not.

Energy consumed while climbing the hill is a recoverable loss. Energy consumed to overcome wind drag (which increases exponentially with speed) is NOT recoverable.

Not only is more of the energy recoverable when you use it to accelerate up the hill, but the vehicle operates closer to WOT. That means you are operating in a part of the engine's power band that has a more efficient brake specific fuel consumption. Translation: Your car gains more energy per unit of fuel that is burned.

Its not even about context. The engine does more work per unit of fuel when accelerating up hill. The vehicle has a lower peak speed, so there is less non-recoverable energy lost to overcome wind drag.

If two identical cars are each filled with one gallon of gas, and driven on the same hilly road, the guy who accelerates up hills will travel farther before running out of fuel than the guy who accelerates down the hills.

Have you guys ever heard of the pulse-and-glide method of hypermiling? Accelerating up hills and coasting down them should be thought of the same as the pulse and glide method.

However, it is even more efficient than the standard pulse and glide method on flat ground. That is because it leverages potential energy. It allows you to use the pulse and glide method without as much wind drag.

Exactly. Also, air resistance goes up exponentially, and this method of pulse and glide allows you to maintain a nearly fixed speed.

Glad I am not alone in here :thumbup:

And yeah, wind drag is a key component in the pulse-glide method when going up and down hills.

I think I understand what you guys are getting at, but I'm sorry, I respectfully disagree. I would have to see some empirical proof to believe that. It goes against the whole concept of driving with load which is a well accepted practice. "As far as the amount of energy it takes to climb the hill, it doesn't matter how fast or slow you do it." That's wrong, speed is a primary component of acceleration. Have you ever ridden a bicycle up a hill? Go try riding riding up a steep hill in high gear at the same speed you can ride down hill. You'll be exhausted. If you slow down and use a lower gear, it's not that tiring.
Granted, speeding up to 80 going down hill is a waste because of wind drag. But, say you want your avg. speed to be 55 on a road with lots of hills. If I understand you, your saying to start the climb at 50, accelerate (pulse) up the hill to 60 and coast back down the hill, dropping back to 50 before the next hill. Driving with load technique, says start up at 60, dropping to 50 by the top, then pulse back up to 60 on the way down. I'm pretty sure DWL will use less fuel than your method. If you can show me some data that proves me wrong, I'll gladly recant.

Now everything is situational as has been said already. In some situations I will accelerate up hill and coast down as well, but, it really depends on the specifics. I agree it may be more efficient in some situations but not as a general rule.

It is literally not situational. At all. As has been said already.

I work in powertrain engineering, and I can give you data like it is coming from a firehose. It will benefit you to first understand the concepts that cause this phenomena. The efficiency curve of a spark-ignited gasoline engine is important to understand. Its also vital to understand that wind drag plays a major role.

https://en.wikipedia.org/wiki/Brake_...el_consumption

https://en.wikipedia.org/wiki/Potent...tential_energy

https://en.wikipedia.org/wiki/Conservation_of_energy

http://phors.locost7.info/phors06.htm

With my small engine I find it impossible to keep in BSFC ideal zone when climbing a steep hill, unless I drop at least two gears. Then my instantaneous mpg drops to about 10 mpg. I can easily accelerate downhill in the BSFC zone and then climb the hill in 5th, at BSFC zone, at 35 mpg.
I might add, I don't pick up a huge increase in speed on the downhill sections, seldom EVER exceeding 55 mph in all my driving.

Small engine cars.

So, to put it a little differently, on low-grade hills I will generally leave my car in 5th and climb it under high load, near peak BSFC, then shut the engine off and coast down the other side. This allows me to maintain a constant speed, which is more efficient (in this particular case) than shutting off the engine at the bottom of the hill, coasting up and arriving at the top with near zero speed, then accelerating rapidly under power down the other side.

It takes the same amount of energy to push a car up the same hill (minus wind resistance) at 25mph as at 50mph, the difference being that at 50mph, you're using up that energy twice as fast. That's why you perceive it to be harder on a bicycle - you're applying twice as much force, but for only half the time, for the same net energy used.

Let's say the hill is shallow enough that you can keep the engine in an efficient BSFC range, and you want to average 40mph:

1) You can climb at 40mph under power, then shut off the engine and coast at 40mph down the other side, and average 40mph.

2) Alternately, you could come at a hill at 80mph, shut off the engine at the bottom and coast up, hitting ~0mph at the top, then accelerate under power down the other side of the hill to reach the bottom again at 80mph, ready for the next hill. In this case you would also be averaging 40mph.

In both cases you use the same amount of energy to lift the mass of the car to the top of the hill, but in the second case you have to overcome a lot more total wind resistance, because it goes up exponentially with speed. For this reason, pulse and glide over gently rolling hills can often be even more efficient than pulse and glide on level ground.

Problems do arise when a hill is steep enough that you need to shift your engine into lower BSFC ranges. Although maintaining a constant speed is more efficient in terms of total energy needed, you're right in that you often more than offset those savings if you have to downshift several times. I'd say in a majority of those cases, it's best to leave the car in its efficient range while accelerating up the hill and allow yourself to bleed speed, rather than rev it up and lose efficiency. For a hill this steep, you're almost certainly going to need brakes going down the other side to avoid speeding though, and any time you have to use your brakes, you're turning potentially useful energy into brake dust.

You should almost never be accelerating under power down a hill.

If a hill was this steep on the descent I would take advantage of no throttle and DFCO. Very seldom round these parts that hills are equally steep both directions though. As I have said elsewhere, what I would do depends on the terrain as well as the traffic at the time.

I get what you guys are saying, but I still think it is situational in the real world to get the best fe numbers.

As an example sometimes i coast down a hill on to a flat then reach another down hill, could be a tiny gradient, at which point ive dropped below an acceptable speed so i accelerate to a speed which will suit the next coming climb...speed limits, traffic etc. SO your saying i should be waiting for the up hill to gain speed? But Many hill where I am will barely allow me to accelerate from my efficient climb speed without a downshift or 2.

Please note that I am saying that I prefer to maintain my speed up a 'hill' this would mean I reach the top without losing momenteum.

I know that in rolling country side, uphill down hill I can get better mpg in my car, and by that theory given the inefficiencies of the car engine then there should be a wieght which is optimum for the car which won't be the lowest right?

Please note I get what is being said about efficiencies although obviously my so i'm not arguing the theory just discussing the practicality.

What I said was that climbing the hill requires the same amount of energy regardless of how fast you do it. Overcoming wind drag is a separate topic. That wind drag would exist regardless of the hill. Wind resistance and grade resistance are different factors and I was focusing on the topic of overcoming grade resistance.

Obviously if you encounter wind resistence while climbing the hill, you are going to burn more fuel. But the hill itself did not take more energy from the vehicle. That was the wind. Not the hill.

Brake specific fuel consumption is still more efficient under heavy load, even with fuel enrichment. My "argument" is actually a fact that applies to all quantity-regulated spark-ignited gasoline engines. Do you know what BSFC is? It basically indicates how much work the engine will get done if you give it a certain amount of fuel.

If you supply an engine with one liter of gasoline, and set the engine to peak torque near WOT, it will do more work before running out of fuel than if you give the same engine the same amount of fuel and run it with low load.

For a while, I worked as a fuel systems engineer for a well known company that manufactures internal combustion engines. I'm still very much involved in engineering projects which have the sole intent of increasing the thermal efficiency of engines. Not everything regarding efficient operation is intuitive. I'm not saying to blindly accept what I'm saying, but I recommend you withhold conclusion on this topic and research more.

Let it downshift on the way up the hill. Thats okay. The point is to get as much of the work done with the engine near WOT as possible. If you have a manual transmission, pick a gear that keeps you near peak torque, then open the throttle all of the way. If you have an automatic, it will still run more efficiently with the throttle at WOT. If the RPMs rise too high, let off of the throttle quickly to let it upshift. Then resume WOT operation.

If there is no hill around, you have to accelerate on the flat ground. If the grade isn't steep enough to have a significant impact on the amount of potential energy instantaneously aiding or being acquired by the vehicle, then you will most likely only see negligible gains from leveraging them with strategic pulsing and gliding.

So, although I've been arguing for accelerating up hills, my real-world experience shows (after a certain point) a huge FE penalty from downshifting. My Insight has peak torque (from gasoline alone) at 5000rpm, but accelerating in low gear, high RPM reliably murders economy with this engine, despite BSFC charts not showing a cliff.

http://ecomodder.com/wiki/images/e/e...nsight_5mt.jpg


I drive the same routes frequently (hundreds of times over the last few years) so I've had a chance to do a lot of apples-to-apples comparisons. Let's say I have a 10 mile trip on the highway. I reset the fuel economy gauge before taking the on ramp, and check it again after 10 miles.

Pedal to the floor during acceleration but shifting at ~2000rpm and taking my time getting up to speed, I may arrive at my destination with 100-110mpg on the gauge, after 11-12 minutes of steady-state cruising at 50-55mph.

Shifting at ~3500rpm, I might arrive around 90mpg.

Taking 2nd gear up to highway speeds and shifting at 5000rpm, I'll be lucky to get my average up to 75mpg.

Redline it and it will take a long time to climb out of even the 30's with a result of the end of trip reading being in the low 60's.

I figure 10 miles is probably long enough that the greater rate of acceleration probably makes very little impact on average trip speed, so all I can think of to account for this is a serious drop in efficiency, despite what I can make of BSFC charts.

YMMV

This is very similar to my findings, though I can only wish my numbers were as good as yours!

Small engine cars are already near full throttle whilst climbing hills at highway speed. Several posters have indicated as much to you yet you keep repeating what is only applicable to large engine cars.

That makes sense. 5000RPMs is too high of an RPM for that car. Gotta upshift, we're on the same page there.

Good. Keep them at WOT and near/at peak torque while climbing. Coast down the other side of the hill.

The sizing of the engine has nothing to do with it. Regardless of which car you are in, drive like Ecky and I are saying. It is the most efficient way. Get the work done with the engine at peak torque and WOT. Coast down the other side. Avoid wind drag.



I see. You assumed extra conditions when I specifically addressed the factor of grade resistance. Grade resistance, rolling resistance, wind resistance, and vehicle parasitic losses are all separate factors.

We are talking about grade resistance and how to leverage it most efficiently.

Peak torque on many small 4 (and 3) cylinders is above 4000rpm, but peak efficiency is usually below 3k. In the case of my Insight, it's 4800 peak torque, 1750 peak efficiency, with terrible BSFC at 4800rpm.

I did an 80 mile trip today, utilising Ecky and Pather's methods and I must say, I got good mpg. My 1.2 litre 4 pot Jazz develops maximum torque at just over 2500 rpm. so I used WOT to get there, then minimum MAP to keep it there.
Averaged 67.8 mpg (UK)/56.5 mpg (US).

Shown below is the calculated engine power requirement for a European spec Mitsubishi Mirage 1.0 for 2 different operating conditions.

1/ flat road, constant 80 km/hr

2/ 5% gradient hill, constant 80 km/hr

It is evident from the BMEP of 9.21 that this engine is almost at full throttle climbing the hill.

http://ecomodder.com/forum/attachmen...1&d=1508600177

Shown below is the 2 bmep values added to a bsfc map obtained from a similar engine.

http://ecomodder.com/forum/attachmen...1&d=1508600997

It is evident that the engine is operating in the island of maximum efficiency whilst climbing the hill at constant velocity.

It makes no sense to go to WOT and have the sfc driven down from 240 g/kWhr to 280 g/kWhr.

Hence the concept that all cars should accelerate up hills is flawed!

Wouldnt it be very cool if you had the bsfc chart and where you are on it displayed in the car?

I'm surprised to see WOT being advocated here when I thought Ecomodder rule #1 was do not use WOT.
How do you guys justify/explain using the fuel enrichment mode of WOT in preference to the closed loop control of say 80%-90% WOT in this uphill situation?
Not wanting to sound confrontational, I have an open mind about this, as I have yet to nail the art of efficient uphill, and on some occasions I have been surprised when flooring it uphill that the overall fuel economy was not as bad as I feared, so there could be something in it!

In my Insight, my AFR meter shows no measurable fuel enrichment with high throttle percentages. Under most driving conditions (where I'm running 1500-2000RPM), "25% throttle" via pedal position is effectively the same as "wide open", because pressing it further doesn't provide any more torque and doesn't burn any more fuel. It's a manual transmission, so pressing the pedal further also doesn't tell a computer to shift.

Generally speaking, having the throttle "wide open" up a shallow hill (without downshifting) will just about maintain a constant speed. So, for me, WOT is maintaining a constant speed up a hill, and it's also keeping me at peak BSFC.

So, I suppose it's important to qualify what we mean when we say "WOT".

As you can see from the charts, ~80% load is a good rule of thumb for efficient hill climbing. WOT is less efficient either due to fuel enrichment (which is pretty rare these days on naturally aspirated engines), or due to less than ideal ignition timing.

When I drove a delivery truck , with light traffic & the hope the " Law " was not around , I would gain as much speed as possible in order to help make it up the next hill . But that was a much different situation . And I was not paying for the fuel .

With a car & light traffic , I let off of the accelerator to the point where I think there is no engine braking . Then increase accelerator pressure to make it up the other side .

Excessive speed only adds additional wind resistance , consuming additional fuel .

Happy thanksgiving to you all , :-)
Wyr
God bless

In my truck I would do the same as wyr.. just enough throttle downhills so I can hear the whine of that light loading on the rear end gears. Out of boost, but negating any engine braking.

But I'm talking a box on 65-85mph highways. Air is the major factor for my driving of hills. You don't really want to be throttling hard downhill no matter the situation. Air resistance would eat up your fuel. I'm not sure what kinds of speeds the other posts are talking about, that would make a big difference.

My POV (nothing unique I suspect).

If we simplify to remove all other variations. I think that's the 1st (ideal) case (Peak Energy Efficiency BSFC up hill (which not always the same as WOT) , EOC down hill, same average speed/conditions,etc).

The YMMV comes in (I suspect) from when some of the other variables start changing as well. For example.

• If to get the power to accelerate up hill you moved the engine/transmission into a less energy efficient condition. This is a negative reducing possible gains.
  • The worse BSFC for the ICE is one part, and can be seen on a BSFC if you have such.
  • Worse for transmission is not mapped as often as a ICE BSFC.
    Although transmissions are usually very efficient (upper 90s %). All gears are usually not exactly equal in their throughput energy efficiency. Some will be a little bit more efficient than others. When the simple case of two gears being otherwise equally efficient, higher transmission RPMs are usually less energy efficient than lower transmission RPMs.
    • Also worth noting here, from 98% to 99% is a 1% difference in an absolute sense, but it is also double the losses (1% x2 = 2%). As such it might not always equal to a 1% difference in observed overall fuel efficiency.
• Any air speed variation is always less aerodynamically energy efficient than an equal average wind speed. This is due to the exponential nature of wind resistance, and does not apply to any types of linear resistance.
  • For example:
    Person A's speed is steady relative to the air.
    Person B's speed varies relative to the air.
    If both average the same overall speed, and if all else were equal.
    The speed variations of person B would require more energy.. Thus less efficient.
  This is a negative acting to reduce potential gains.
• Speed creep.
  • Many (not all) people get desensitized to faster speeds. The slower speeds seem slower than they really are. This can lead to the person who is accelerating having a subconscious leaning toward less time at slower speeds. Increasing the overall net average speed. If than one averages a faster speed they will have more net wind resistance energy losses, and thus lower combined MPG.
  • This also happens with some people with a positive psychological feedback to the psychic effects of acceleration itself. If they perceive the acceleration to be a positive thing, some people will then subconsciously do more of it. To people of that type the gravitation effect of going up hill adds to the feeling of road acceleration.
• For any newbie who might has asked:
  Why not maintain peak BSFC energy efficiency on both up and down hill .. because then you'd be going much faster and have much higher wind resistance losses. The faster you go the more energy per mile you are spending. If ICE and transmission is already at peak energy efficiency, any speed faster than that is only a negative.
  • There would be a potential overall breaking even point if there is another hill to climb after the down hill. Exact break even point would vary. But, as such , under some conditions, staying at Peak Energy Efficiency ICE & Transmission might be a net overall benefit. Although the continued running of ICE on the down hill, will (almost always) be a negative for that one hill.
  • Worth noting that higher fuel efficiency is possible at lower energy efficiency. If you go slower your energy losses/(required consumption) per mile go down. As long as those energy consumers go down faster than one's fuel to wheel energy efficiency you are getting higher Fuel Efficiency , even with lower Energy Efficiency.
• Changes in the air (Up-hill vs down-hill)
  Uphill and down hill wind speeds are almost never the same. Nor is the air pressure. Nor is the air density. etc. One's wind resistance is not about vehicle to ground, but vehicle to air.
  • Although of course not true in all individual cases/conditions. I suspect overall for a large enough sample size. That statistically one side has higher wind speeds , higher air density, etc than the other side of the hill. I further suspect that (in a large enough sample size) it is the side that is up-hill from the POV of the wind that is most often the one with faster,denser air speeds.
  • So, going faster up hill statistically most often faces a relative air benefit. Although likely to be tiny.
    For Example:
    Say The Wind is moving West to East.
    • Those going up-hill (driving west to east) have the wind at their backs and their relative air speed is lower, thus less wind resistance , thus less energy to travel the same speed. Both Uphill and down hill. But the Air itself as slowed a little and provides less help, thus more wind resistance on the down hill side.
    • Same hill same wind direction. The air/wind (statically large enough sample size) has lost a tiny bit of density/pressure/speed as it climbed the hill, and now requires slightly less power/energy for the car against it driving up hill. Those heading East to west are driving into the wind (both uphill and down hill). Up hill face slightly less dense air that is slightly slower. While they will face slightly faster and slightly more dense air on the down hill.