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Comment by Jebdm | Hacker News Reader

It did say "up to 10%", which I think is fairly believable since they were testing with large vehicles.

Only problem is hydraulics aren't very efficient and waste much heat. They claimed a recovery of 1 kilowatt, which is just over one horsepower (746 watts by definition.)

Now if that vehicle has a 200 horsepower gas engine and is boosted by a 1 horsepower, a 10% claim might be stretching it. An older generation Toyota Prius has a 10 horsepower electric motor, which can propel it over 30 mph. One horsepower might be good for about 5 mph. I suppose there might be unusual driving conditions that might realize a 10% increase in mpg, but not in our world.

The average car needs ~25HP to maintain highway speeds. So 1hp could be ~4% of the car's energy needs. It takes a lot of energy and HP to accelerate quickly but you don't spend a lot of time accelerating like that. At lower speeds over bumpy roads I guess 10% might be possible.

You'd think there would be some back of the envelope physics we could do. Anyone want to jump in?

Here's a start .. maybe

A bumpy road only uses extra energy because the bumps cause the vehicle to move upwards at the expense of its forward momentum. This invention would in the best case be able to recapture that energy and thus simulate being on a perfectly smooth road. So the most power it could produce would be the potential energy gained by raising a vehicle N cm off the ground every M seconds. N, and M would be affected by speed, and road bumpiness.

Continuing on this line of though, say there's a 0 incline fairly bumpy road where on average there's 3 3cm bumps every meter and the vehicle is traveling 60km/h (~17m/s). So the car has to provide energy to raise the car 9cm for every meter traveled. That's 1.5m/s vertically. So for an average car of 2000kg, we have to provide 2000x9.8x1.5 J per second, which is ~29KW. (That's gravitational potential energy mass x grav. x h)

Wow, so maybe it's right. Or my math is wrong somewhere, or my hypothetical road is way to bumpy?

Shock absorbers prevent you from lifting the full weight of the car for each bump. So replace your 2000kg with the un sprung weight of the car say ~200kg. (Your tires also act as mini shock absorbers so you might need to subtract 1mm from the size of each of those bumps.)

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