|MadSci Network: Engineering|
I'm not specialist on engines and engineering (I'm just a mad physicist!), but, although the idea of using microwaves and water for running cars seems attractive, I have to say that it wouldn't be a good business to build an engine in the way you suggest, Thermodynamics gives you the answer.
Such an engine could actually work but running it would result in an enormous waste of energy. You need electric power (very high indeed) to produce microwaves intense enough to run liquid water into steam. On producing microwaves some electric energy is lost due to unavoidable inefficiency of transducers, circuits etc. Microwaves themselves are neither 100% efficient on heating water, so some energy is lost in this way. Being very optimistic I think maybe a 20%-30% of input electric energy would result in water heating. Now, steam must be used to transform heat in usable work. Again, this proccess results in loosing energy, probably as much as 50% of heat. Finally you have that, in the best case, only a 10-15% of input electric energy would result in effective work for moving the car. As you can see, it would be more efficient to use electric energy in an electric engine, and if the electricity you are using has been produced in a Thermal Power Plant (i.e. using petrol), you better use fossil fuels instead, as electricity generation and transport also produces energy lost.
In a fossil fuel engine we use chemical energy produced by combustion, but the sparkplug does only begins the fire, it doesn't give any power to the engine. Energy is obtained from breaking atomic bondings in petrol hydrocarbon compounds (that's what means combustion), and was put there by prehistoric vegetables and grass using solar energy. Prehistoric plants made the work for us, so we need no extra energy just need to extract petrol and burn it (dirty, but cheap).
Apart from that, I think building such a small and powerful microwave emitter would be quite difficult and running it would be extremely dangerous (you would need over 100 kW for that engine, and the car driver and the koala sleeping by the road are made of up to 60% of water which could be also heated and boiled by such powerful microwaves).
Finally, I'm mot sure you can break up water molecules using microwaves. I think you need ultraviolet light to break the bondings (it is not a matter of how much power you put on the water, but what's the frequency of the radiation you use).
But there's a way water can be used as fuel. Actually is not water but Hydrogen that is under investigation to be used as fuel, burning it with atmosferic Oxygen with water as final by-product. Hydrogen can be obtained from water by means of ''hydrolisis'' (dividing water into Oxygen and Hydrogen with an small electric current) or from organic substances by using bateriae which produce Hydrogen as by-product of their metabolism.
If you're still interested, you better look for more information on how effective (in terms of lost energy) of producing microwaves with an electric circuit and of heating water with microwaves. You can also look for information on the effect of microwaves and other radiations on water and living beings.
Here you have some links to sites devoted to explain how microwaves interact with matter (mainly in microwave ovens):
Enjoy,... and keep on thinking.
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