Is accelerating down hills fuel efficient?

[JockoT]

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.

[D15r]

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.

[Panther140]

Originally Posted by D15r

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.

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.

[Panther140]

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.

[Ecky]

Originally Posted by Panther140

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.

[Panther140]

Originally Posted by Ecky

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

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

[puddleglum]

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.

[Panther140]

Originally Posted by puddleglum

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

[JockoT]

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.

[cr45]

Originally Posted by Panther140

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.