Quote:
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.