A piece in New Scientist has caused a bit of a stir in fuel circles – apparently it’s possible to run combustion engines on metal nanoparticles. The principal drawback is the weight of the fuel required – while energy per unit volume is high, energy per unit mass is low (as one might expect from a high-density fuel such as iron). The technology may be useful for shipping and possibly rail (where weight is less of an issue), but for personal transport electricity is really the only option. It would be interesting to see it incorporated into domestic CHP (where Stirling engines are already used) as a backup fuel instead of diesel.