[ams6110]

Yep, it's a completely mechanical way to recover the energy expended in braking and apply it to getting the vehicle rolling again. In this way the gasoline engine is mainly used to maintain speed and climb hills.

Other experimental vehicles have used flywheels to capture braking energy, but the advantage of compressed gas is that it can store the energy indefinitely (assuming no leaks).

Sounds like it could be more efficient and reliable than the electrochemical process that electric hybrids use, with less weight and environmental concerns related to battery manufacture and disposal.

[pkulak]

Well, there's always a risk when you're storing energy. In this case it would be explosion. Could be a very small risk, though.

[pilap82]

I had the same "concern" but they say: The energy stored in the nitrogen tank is small — equivalent to only about five teaspoons, or a couple dozen cubic centimeters, of gasoline

So I guess that the explosion of a tank full of "about five teaspoon" should not wreak too much damage :)

[DeepDuh]

What I gather from the article it's not the stored energy that's of some concern, but the 250 bar nominal pressure in the tanks - a rupture might lead to metal splinters flying around. However, they claim that the car's underfloor is enough shielding. I'd say let some independent safety tests figure that one out.

So far I think this is a very promising concept - I was always weary about electrical hybrids because of the use of rare earth metals and the environmental impact in some regions because of that. Putting some nitrogen tanks and mechanical/electronic bits in a car to achieve the same effect for city driving seems like a big winner - mass produced I'd think you could make those cars only minimally more expensive than normal gas powered ones. Electrical Hybrid car prices, meanwhile, don't seem to scale down much.

[gus_massa]

Here in Argentina ~1/5 of the cars use compressed natural gas (mostly methane) tanks at 200 bars, because it is cheaper than gasoline. I don't remember any horror story of a crash, and these cars are generally considered almost as safe as the usual cars. The main drawback is that the tank is very big and uses half of the back trunks.

More information: http://en.wikipedia.org/wiki/Compressed_natural_gas

[StavrosK]

We do the same in Greece, the tank is 50-60 liters (not sure if that volume is when the gas is at STP or compressed), though, and takes around the room of the spare. That's also where they put it.

[mistercow]

> "about five teaspoon" should not wreak too much damage

Well, using a little WolframAlpha magic[1][2], it looks like the energy would be equivalent to about 200 grams of TNT. The internet tells me that that's about equivalent to a hand grenade. The internet also tells me that this video is only 50 grams of TNT, but take that with a grain of salt: https://www.youtube.com/watch?v=7rE-8o9NDZk

Point is, be wary of scoffing at small amounts of gasoline. That stuff is potent.

[1] http://www.wolframalpha.com/input/?i=%28%28energy+density+of...

[2] http://www.wolframalpha.com/input/?i=851+kJ+to+%CE%BCt