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Old 10-19-2017, 12:40 PM   #20 (permalink)
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Originally Posted by Panther140 View Post
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.

This is incorrect.
There are 3 energy components involved in climbing a hill at constant speed.
1/ Gain in Potential Energy.
2/ Parasitic loss due to tyre rolling resistance.
3/ Parasitic loss due to aerodynamic drag.

As the aerodynamic drag is proportional to the square of the velocity then as the velocity increases the total energy required to overcome aerodynamic drag whilst climbing the hill also increases.

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.

Small engine cars often have very little excess power when travelling at highway speeds up a hill. Any acceleration will cause fuel enrichment beyond stoich AFR and lead to poor mpg. Your argument is only applicable to vehicles with large engines relative to their mass.

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.
Small engine cars.
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