[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.