Let’s say we are driving from A to B. The beginning of the drive starts with a steep uphill, but the remaining drive is long and downhill. Points A and B are the same elevation. Which direction (A to B or B to A) is more energy efficient (less fuel consumption)? Or is it maybe equal?

Question

EDIT: I am discussing real world scenario in a car. I will definitely check the fuel consumption next time I drive from Reykjavík (B) to Hveragerði (A) (and back) here in Iceland. Thanks for great answers 🙂

In: 1

Answer 1

Real life or ideal physics model?

In the ideal physics model, they would be equal.

In real life, it depends on the vehicle concerned. It will depend on how much braking is involved and how efficient the engine is at different loads and speed. With braking, there is no longer a closed system for energy as brake dissipate energy as heat. So one of the question is, how much braking is needed?

Answer 2

Going down hill with an appropriate cruise control is pretty fuel efficient. When you gotta pump the gas going up hill you’re gonna guzzle just trying to maintain speed

Answer 3

Assuming a petrol/diesel car then the short uphill and long downhill would use less fuel if you were to turn off the engine and roll down the downhill part, as you would be running the engine for a shorter time and just turning over an engine uses fuel even when not providing force to the car.

In reality if you’re driving normally or using an electric car, probably close to even.

Answer 4

Under ideal conditions you can just coast downhill without using any fuel.

If you have an EV and the conditions are right you can actually recharge on the way down.

In practice we don’t built many roads were this would work.

Answer 5

I wonder how the weight of the fuel, which would drop as you drove, would affect this. For instance A->B you carry almost all of the weight up the hill, but B->A A smaller percentage of the fuel would be transported to the top.

Answer 6

Steep uphill first is your most efficient if the downhill is enough to overcome friction. With engine running, the shorter distance of higher load means less total power loss (the engine is moving with a certain efficiency this is separate from the car moving) so basically you get to the top and cut off the engine, gravity does the rest, the forces of friction mean less braking (loss of work energy to heat) which is energy from the engine.