Is my hill climb theory sound?

[gone-ot]

...if you're driving with the cruise-control "on" never attempt to bump-up your speed while climbing a hill--it'll eat more gas and probably automatically kick you back a gear (automatic).

...delay the speed bump-up until you've crested the hill, and THEN do it as you're heading down the slope...much, much better FE wize!

[redpoint5]

Originally Posted by fidalgoman

The more work you do the more energy is needed. Pulling a hill at fifty takes a great deal more energy than at thirty five.

More energy expended for a shorter duration though. What you have stated does not appear to be in conflict with my theory.

I acknowledge that there are practical limits to my theory, such as running an engine outside of the efficient range, and wind resistance. However, I don't see any evidence to support the theory that going as slow as possible up a hill is best.

Originally Posted by Old Tele man

...if you're driving with the cruise-control "on" never attempt to bump-up your speed while climbing a hill--it'll eat more gas and probably automatically kick you back a gear (automatic).

...delay the speed bump-up until you've crested the hill, and THEN do it as you're heading down the slope...much, much better FE wize!

Thanks for throwing out the tip. I always buy manual transmissions and never run a cruise control up a hill. Generally I build up some speed before beginning a hill climb, and maintain that speed to the top, depending on what the conditions are like on the other side. For example, I will coast towards the top of a hill and let my speed bleed off if I know the back side will carry me back up to speed.

[Clev]

Originally Posted by redpoint5

I acknowledge that there are practical limits to my theory, such as running an engine outside of the efficient range, and wind resistance. However, I don't see any evidence to support the theory that going as slow as possible up a hill is best.

I don't think "as slow as possible" is the best, but "as fast as possible" is certainly not best. I climb a hill nightly to my home (gaining 4,675 feet over about 13.7 miles), and I can tell you that my FE average is better when I take that hill at 40 mph than at 50 mph or 55 mph. (I don't do less than 40 mph because I'd have to drop a gear to maintain my speed.)

This is in both of my cars, which are similar in size and horsepower, but built 14 years apart.

[fidalgoman]

Originally Posted by redpoint5

More energy expended for a shorter duration though. What you have stated does not appear to be in conflict with my theory.

I acknowledge that there are practical limits to my theory, such as running an engine outside of the efficient range, and wind resistance. However, I don't see any evidence to support the theory that going as slow as possible up a hill is best.

No not arguing just that from experience if you drive a car up a long mountain pass at maximum engine output it will take much more fuel than if you take your time. Even with the time difference the overall fuel consumption will be higher at full burn.

I once owned an 85 Jetta diesel that would pull the local mountain pass all out (in high gear) at 65+ MPH. Slowing down to 50 and dropping a gear resulted in about two thirds throttle and much better efficiency. Driving an underpowered car like the Jetta (52 HP) teaches you a lot about shift points, energy conservation etc.

[Olympiadis]

Originally Posted by redpoint5

When I started pondering the question of how to maximize fuel economy on hills, I used the most extreme example I could think of; vertical travel. Obviously, it is least efficient for a rocket to gain altitude as slowly as possible.

While a rocket launch is a close analogy for a car taking off from a stop, it is not applicable to a car hypermiling up and down hills. The rocket has no previously gained momentum to utilize, or any opportune times to use it. Think of the car hypermiling up and down hills as a type of pendulum.

Maintaining speed or accelerating while going up a hill allows you to benefit very little from your previously stored energy.

Maintaining speed or accelerating up-hill takes significantly more energy input than the same action would take on level ground, and significantly more again than the same action going down-hill. Gaining elevation within a given time period requires more "work" to be done.

It is most efficient to use as much stored energy as possible when climbing the hill, which means necessarily losing some of your speed - the rate of loss depending on your starting speed, and the distance you need to climb. You then want to find the new equilibrium where your vehicle speed is most fuel efficient in order to complete the rest of your climb, - up to the point that you may begin your coast (often before the apex of the hill).

Originally Posted by redpoint5

If we consider that it takes energy to simply hold position on a hill (brakes off and in gear obviously), it follows that getting to the top of the hill quickly is most efficient.

While basically true, it is not an absolute. You could say it is most efficient to get to the top of the hill as quickly as possible while using the least fuel.

Originally Posted by redpoint5

Interestingly, I have observed the greatest MPG on trips involving very steep mountain climbs. In my old Subaru AWD, I consistently averaged 27mpg with about 75% freeway and 25% city driving. However my trips into the mountains and hills returned 30-32MPG. I applied my theory of getting to the top of the hill quickly, and coasted down either idling or engine off.

I would say that in this case the benefit of increased coasting and engine-off time outweighed your loss in MPG while climbing. It's not really surprising since hilly areas offer more and better opportunities to utilize momentum.

Originally Posted by redpoint5

So it seems to me that it is most efficient to climb a hill as quickly as possible, even if that means opening the throttle further and watching the MPGs momentarily plummet. By doing this, you are minimizing the amount of time that gravity is working against you. Of course, there would be a speed at which aero drag negates any benefit of cresting the hill quickly.

Gravity is always working against you, and exponentially at that. The more you fight it, the more you lose.

I have seen the engine BSFC argument that suggests that engine fuel efficiency translates directly into vehicle fuel efficiency. This generally supposes that opening the throttle more can equate to better vehicle fuel efficiency, since engine BSFC will be increased.

I disagree with this. There are a great many non-constant factors that determine vehicle fuel efficiency, which makes a direct link to BSFC impossible. BSFC is normally measured at wide-open-throttle, which is not applicable to hypermiling. Even the BSFC measurements taken at part throttle are only using one type of load (cylinder filling) at a steady state, and so ignore other load factors (mass & gearing) that have such a significant effect on piston speed (not engine speed) during transitions in-vehicle.

Thus, when you find the optimum vehicle speed for vehicle fuel efficiency you may find that your engine is operating well outside of the engine's best BSFC.

I'm leading into the fact that your most efficient vehicle speed will change as the inclination of the road changes, and be for all practical purposes independent of engine fuel efficiency (BSFC).

Your best vehicle speed for fuel efficiency going down 4% grade may be 40 MPH, but then only 35 MPH when you are on flat ground, and 30 MPH when climbing a 4% grade. This is just an example to illustrate the concept.

You should find that you can accelerate going down hill and use less gas than you would have holding a steady speed on flat ground. A Scangauge will show you this in real-time. You will also see that just trying to maintain a speed going up hill will cost you a lot of fuel, - much more than if you slowly let your velocity fall.

My explanation assumes up and down hill driving. If you are stopped at the bottom of the hill and must climb it, then you have no momentum to utilize. It is normally most efficient to get up to your most efficient vehicle speed that you can maintain on the hill, fairly quickly, but short of invoking Power-Enrichment (PE) mode in your fueling logic. You would see this as 15% - 20% enrichment on a wide-band O2 gauge.

Also note that trying to maintain a fast speed while climbing a hill may also invoke PE mode.

[kgwedi]

IMHO...When doing Engine Off Coasting, the speed going down a hill doesn't matter, only the distance you travel without burning any fuel. The speed can give you more distance, but on a long downhill that becomes less influential on total distance traveled while EOC.
I usually go up hill at about 75-80% LOD, and traffic permitting don't concern myself with the speed at the crest.
Different hills require different strategies.

[euromodder]

Originally Posted by Old Tele man

...if you're driving with the cruise-control "on" never attempt to bump-up your speed while climbing a hill

If the climb is not too long, I've started to reduce the CC's preset speed slightly whenever FC goes up - then regaining the speed on the downhill rather than let the CC use engine braking to keep the speed.

Seems to work

[aerohead]

Originally Posted by redpoint5

I just read the 108 hypermiling tips and questioned the logic behind DWL. It suggests that the most efficient way to travel an incline is to let speed drop off, and to build it back up on the decline.

When I started pondering the question of how to maximize fuel economy on hills, I used the most extreme example I could think of; vertical travel. Obviously, it is least efficient for a rocket to gain altitude as slowly as possible. It takes an enormous amount of energy just for the rocket to sit in place at it's current altitude (1G of thrust). So if the rocket is producing 1.01G of thrust, it is expending 99% of the energy just to maintain the current altitude, and a measly 1% on gaining. Ignoring friction such as wind resistance for a moment, it follows that the more thrust (throttle) that is produced, the greater percentage that is used to gain altitude. At 100G of thrust, 99% is being spent on acceleration and altitude gain, and 1% is "wasted" on overcoming gravity.

If we consider that it takes energy to simply hold position on a hill (brakes off and in gear obviously), it follows that getting to the top of the hill quickly is most efficient.

Interestingly, I have observed the greatest MPG on trips involving very steep mountain climbs. In my old Subaru AWD, I consistently averaged 27mpg with about 75% freeway and 25% city driving. However my trips into the mountains and hills returned 30-32MPG. I applied my theory of getting to the top of the hill quickly, and coasted down either idling or engine off.

So it seems to me that it is most efficient to climb a hill as quickly as possible, even if that means opening the throttle further and watching the MPGs momentarily plummet. By doing this, you are minimizing the amount of time that gravity is working against you. Of course, there would be a speed at which aero drag negates any benefit of cresting the hill quickly.

Comments, criticisms?

If you'll do a comparison of the work-per-unit time that the engine is accomplishing at any given velocity on a level roadway,and then throw in the amount of additional work the engine must compensate for in order to raise the elevation of that car driving up a grade,you'll find that the velocity MUST fall off at increasing grade to maintain the same amount of engine work ( i.e. BSFC ) and mpg.
Chrysler ran the numbers for such a thing and continued to win the Mobil Economy Run.
I ran the numbers,it's quite insightful,with repeating number sets running diagonally across the page.

[Bicycle Bob]

At low speeds, the efficiency of rockets is abysmal because of low Froude numbers. The rocket is pushing on stuff that is running away, so it is in a higher "gear" than an elevator on the same run, but getting nothing for the "reduced torque."
However, I also disagree with DWL, as long as one stays away from throttle openings that go to a rich mixture for power, abandoning economy. Bike racers work hard going uphill, but rest when they would only be fighting the air.
Perhaps someone with a scan gauge will do a comparison between total consumption going over a hill using DWL vs constant speed to the same average speed.

[CapriRacer]

I am surprised no one has really focused in on the kinetic energy vs potential energy thing yet.

Obviously at the bottom of the hill, you only have kinetic energy. If you were to turn the engine off and coast up the hill, at the top you would have exchanged that kinetic energy for potential energy. One of three things would have occurred:

1) You came to a stop at the top of the hill, That is, the kinetic energy (speed) exactly matched the gain in potential energy (height)

2) You came to a stop BEFORE you reached the top (or you had to turn the engine on to reach the top). That is, the kinetic energy (speed) was not enough or you had to add energy.

3) You reached the top at a reduced speed. That is, the kinetic energy was greater than potential energy needed to reach the top of the hill.

In all three cases, kinetic energy is exchanged for potential energy in exactly the same amounts.

But what isn't accounted for is rolling resistance - which is strictly a function of distance - and aerodynamic losses - which are strictly a function of speed (the square of the speed to be exact!). Since the lowest average speed is when the vehicle comes to a stop BEFORE it reaches the top of the hill, anything else consumes more fuel.

But aerodynamic losses at slow speeds are pretty small - which probably why people are arguing the point (the principle doesn't manifest itself clearly). I think you'll find that it becomes easier to understand if you think about going DOWN the same hill. Clearly, using the brake to limit the speed ALWAYS results in energy loss.