Look at this video of cars on a waving bridge. Would you use more gas driving uphill on one of the waves?
Yes, if you actually drive uphill on the wave. But the cars in the video mostly don’t seem to be doing that: They seem to be driving at the same speed as the waves, so they stay in the troughs. That might even let them use less gas, if they can “surf” and take energy from the waves.
@Chronos made a good point that what any vehicle experiences depends on how they get into or out of phase with the undulations.
Continuing from there …
If we transmute this situation to driving on rolling terrain with fixed uphills and downhills, then yes, you’ll burn more fuel on the uphills, assuming constant speed. And burn less on the downhills, assuming constant speed.
For annoying reasons that amount to “The Universe hates humans & works diligently to frustrate their puny progress”, the net result is worse than on flat terrain. The uphill cost exceeds the downhill benefit.
In surfing, the surfer is being raised up by the incoming wave shoaling of the ocean; at the breaker zone (where the floor of the ocean suddenly comes up to a shelf or sandbar) that kinetic energy becomes concentrated into a smaller volume of more shallow water, raising the surface up into pronounced waves and in extreme cases even forming tubular waves, and by timing correctly the surfer can balance just below the crest of the wave, falling diagonally across it as it rises underneath them. This occurs even though the water isn’t moving forward at the same rate because the momentum of the wave is being transferred through the medium of water. See the figure below for more detail:
For the cars driving across the floating bridge in the link, they are actually creating the waves themselves by changing the buoyancy of the bridge in the location where the car is driving over. Because the cars are expending energy to move the water as the move forward they are losing some efficiency, although if all cars are moving at the same rate and spaced in sequence with the nodes of the wave (as the car just forward of the camera is certainly doing) they aren’t losing much energy once they get up to speed. If the cars were moving at different speeds or there was traffic going in opposing directions the wave motion would be more complex and it would take much more energy to drive along the undulating bridge surface (and would be more difficult to control).
Stranger
I saw this video today and wondered the same - although the cars are creating the waves, there is some periodicity and resonance to it all, so assuming there is regularly spaced traffic ahead of you (and perhaps behind), and that their spacing is roughly a multiple of the wavelength, it ought to be possible for a savvy driver to just back off a little bit and get a boost from riding the downhill side of a wave.
Then again, if everyone did that, then the whole thing would slow down and stop. You’d have to keep everyone at a pretty controlled speed for it to work, I’d think.
Yeah, it also might be disruptive in some way, since you’d essentially be stealing power from the other vehicles in the system, so they either slow down or they have to compensate by increasing throttle - it could end up adding some weird chaos or other effect to the system
I wouldn’t be surprised if a driver can feel the sweet spot, the exact amount of accelerator to give it to keep themselves in tune with the rest of the convoy.
I think you’re right - It will be a little bit self-reinforcing - if you’re too far forward, you’re driving uphill and the return value of your accelerator use will be lesser, so you’ll tend not to move forward (for the same reason your car tends not to roll uphill); if you’re too far back, the return value of your accelerator use will be greater (just like it is if you are driving downhill).
As someone who rides a bicycle for exercise, I can attest to this. Fortunately, where I live the only “hills” are bridges over the various waterways. They are enough to make my legs scream by the time I get to the top. The downhill just doesn’t make up for the uphill. The wind at a beach town can make up for the lack of hills. I HATE the wind.
When I used to trips in the PA mountains I find this to be the opposite. I got great mileage in my BMW 535i with a real time mpg gauge and 5 speed. Modern engines shut off the fuel on the downhill and get infinite mileage. Careful attention on the uphills keeps the increased fuel use to a minimum. You have to find the sweet spot for your car. In my Rav4 with an 8 speed automatic it’s above 65 mph. If I drop below that the engine does not have enough torque and downshifts.
Good point. Thank you.
I should have qualified my comment with “in pre-computerized cars”. Which darn few people have any more. I sure don’t.