I read in New Scientist magazine 15-20 years ago, about a car running on metal. The only drawback according to the designer was the weight of the metal in the return tank. The metal itself had a similar energy per kg as petrol. I don't how the cylinders avoided being coated with the fuel metal. He used different metals as fuel. He kept the metal oxides and they could be reprocessed into fuel using electricity so the system could be renewable.

carbon isn't all bad, it is very light by comparison to iron or aluminum and collected can also be transformed

'recycling' metal oxides would take as much energy as the oxidation originally gave off, just sayin...

Burning metals in an ICE is a bad, bad idea! You still get NOx and other toxins including particulate matter that will no doubt go out the tail pipe. A much better way to convert the energy in a metal is in a metal-air fuel cell. www.phinergy.com and www.alcoa.com are doing it with about 2 or 3 times (200% ~ 300%) the efficiency of burning and zero emissions (the metal oxide sludge is collected and recycled). All batteries use metals as fuels but they also contain all the reactants within the cell. It's like having an oxygen tank for the ICE and then an exhaust collection tank within the vehicle chassis. The only difference for a fuel cell is the reactants are external to the reactor. In an ICE the reactants are also external but the toxic byproducts are emitted out the tail pipe. It's those emissions that are causing enormous problems. Please, let's stop this 'burning' mentality and stop treating the air we all have to breathe as a dump!

Two issues here:
First, "Another plus is that metal powders are recyclable." Let's see how well that's accepted when having to take the sack of fine ash to the recycle centre, and heaven help the potential lawsuit because someone is diagnosed with a rare cancer owing to exposure to it!
Second, "The main difficulty would be to ensure that iron refining is as carbon-neutral as possible". You don't say? Apart from the fact that iron isn't refined at all, it is extracted from iron oxide using vast amounts of energy, usually gas, which I suspect would be better spent being piped to homes to heat them!

This is pretty fascinating. I'd love to see something like this become mainstream.

I read about a system that used aluminium as an energy storage medium. Electricity is used to create the metal from the oxide. The engine then used Al metal wire plus water and a small current to split water and create hydrogen gas and Al oxide. This is done just prior to the carby or fuel injection system. The hydrogen is burned using oxygen from the air creating water in the exhaust which is then (mostly) recovered. This is probably where the "car that runs on water" BS came from. I'm not sure how efficient it is (Oxide must travel back to be stripped of oxygen so 2 way transport) but apparently a 600Km range was possible with only a few Kg of Al wire spool. The devil is always in the details but it sounded pretty cool and more viable than hydrogen as a storage medium.

A large proportion of coal fired power stations output was absorbed grinding the coal to dust, how much more enrgy would be used in grinding iron, after and including the mining and refining process? My guess is that it wouldn't be cost effective, when coal can be reduced to gas, and coke, much more easily.

Where does the energy come from to refine the metals in the first place? Unicorn flatulence?

The obvious difference is that fossil fuels are an energy source (although non-renewable) and metals are an energy storage technology. Many other synthetic fuels are possible.