Do I understand it correctly, they basically built a 2-stroke engine that makes double use of the eingine's displacement? Like, a 125 ccm engine with the power output of 250 ccm? That would certainly be great news for tiny airplaines and similar niche products, where engine weight is premium. But I somehow fail to see how the fuel efficiency of a 2-stroke eingine is supposed to compete with a fuel cell for non-niche products...

Im hoping that soon i will be able to upgrade my loveable 1997 Mazda MX 5 to a green engine at a reasonable cost.

@ppeter, yes each half works like a 2 stroke in the way they combine intake and exhaust into the same cycle. The main difference seems to be that instead of having 2 separate pistons attached to a crank they have connected them back to back doing away with the crank case. It would mean using a linear generator instead of a rotary generator (and I don't know how they compare in efficiency) and it would have few moving parts.

Perfectly daft. By the time you have paid for the H2 tank and associated gubbins to make this work, you could have just added twice the battery capacity - and still saved at least 50% of the cost. Weight might swing it for some applications but really, for those sorts of applications a full H2 fuel cell system would make much more sense... except that it doesn't and never will as battery technology will progress much too quickly for it to keep up - technically and financially - and H2 will simply get left behind and (finally) forgotten.

We can see a piston moving freely to and fro, but how does it power anything

@ppeter I think the benefit is that fuel cells are ludicrously expensive while this engine is cheap and easy to both manufacture and maintain

Linear free piston 2-stroke engine. Potentially interesting, but facts are very hard to find. On the website there is a hint that the output is 16kW but no indication whether this is the 10kg unit. Only other hint I could find is that it has 1000 hr maintenance periods, so OK for occasional use backup power, but less useful for continuous generation or CHP duty. Why not tell the world what conversion efficiency they can achieve? Lack of information makes me suspicious.

@ppeter Its not about efficiency more about overall running costs. Its true that internal combustion engines aren't particularly efficient, but the big advantage over fuel cells is the cost saving of the fuel cells.

Given that hydrogen should be relatively cheap and green to produce in the near future, the efficiency could be viewed as less important than the cost of replacement hydrogen fuel cells or rechargeable batteries. Hence the advantage of this engine.

Of course technology advances can throw any argument of which is best out very quickly, but generally hydrogen power has a lot of things in its favour!

Brian M : Hydrogen combustion in air produces nitrous oxides,potent green house gases,so it is not as clean as using hydrogen in a fuel cell,which only has water as the waste product.

I am curious how this compares to the Liquid Piston engine. Liquid piston is (I believe) much lighter. And I am not convinced of the efficacy of H2 as a fuel. If the oxidizer is air, running a lean mixture gives you LOTS of NOx emissions.