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It was pasta....
If you could make a transportation engine that ran on solid fuel in a convenient way, that would be cool. In general they would have to be external combustion types, so the fuel doesn't have to flow through pipes or bounce around in a cylinder, etc. etc.
Well it's been done before, actually, via the steam engine. Steam locomotives burned solid fuels. And there were even steam autos, though they tended to use liquid fuels.
A liquid fueled external combustion engine is by nature less efficient than a liquid fueled internal combustion engine because the energy contained in the fuel's heating power is more easily lost outside of an engine than inside it. If you heat a boiler much of the heat escapes as hot gasses up the chimney rather than manages to transfer through the boiler shell.
Those same hot gasses are all contained in an internal combustion engine until they do motive work, and are then released at a much lower energy content.
Energy efficiency aside, however there are other efficiencies that an external combustion engine might be able to capitalize on, which an internal combustion engine can't. One such efficiency is the ability to use unrefined solid bio-fuels.
While internal combustion engines can also use renewable biofuel fluids, like methane, and alcohol, both of those must be produced and refined from primary sources that are perfectly legitimate solid fuels to begin with.
In every fermentation, distillation, and refinement process to produce a fluid, you use lots of energy, and effort and infrastructure. You also create a waste stream that has to be disposed of. This actually can limit the production of liquid biofuels. Biodiesel produces a serious glycerine waste stream. Methane production can create excessive nitrogenous waste. Alcohol production similarly produces a waste streams which would be hard to deal with if the bulk of automotive fuel was made from it. Most of the liquid fuels we manufacture are also based upon monocultural food crops, which require farm quality land and massive amounts of fertilizers and pesticides.
External combustion engines are far less finicky. They can use basically anything that can be burned to produce heat. They are really heat engines. In fact a steam engine can use solar heat from a reflector. Or geothermal heat. The heat source can be from just about anything.
Steam isn't the only medium a heat engine can use. Hot air engines are heat engines that use air as a medium. In fact some don't even use that as the working fluid. Some more esoteric types use hydrogen, not as a fuel, but as a transfer medium.
bedtime....
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