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The heat of a lightning bolt causes the surrounding air to rapidly expand causing thunder in the process.

A reciprocating combustion engine ignites an air fuel mix to use the rapid expansion created in the process to build up pressure and use that to move pistons in order to create rotating motion.

Would a reciprocating engine that uses only air heated by a very high powered electric arc work?

It surely would be very ineffective, hard to supply with enough power without cables and would probably melt the engine block, but could it function in principle?

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  • $\begingroup$ I don't really understand the purpose of the question. You could probably create something that generated an electric arc and ran off of the heat generated, but obviously it's not going to be efficient (and one thing we could use less of are inefficient engines). $\endgroup$
    – JMac
    Apr 3 '17 at 18:40
  • $\begingroup$ @JMac this is only for my curiosity $\endgroup$
    – maklemenz
    Apr 3 '17 at 22:10
  • $\begingroup$ As it stands it seems way too broad to answer. You might be able to figure something out that can get incredibly slow velocity or something, but it's not a practical engineering problem. I don't think there's much reason to really try and come up with decent approximations. $\endgroup$
    – JMac
    Apr 3 '17 at 22:14
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    $\begingroup$ I think the "move a car for a few seconds" constraint is the stumbling block here, and unnecessarily so (define "car"?) - as a matter of principle, this is by no means too broad. There is a similar question on the askscience subreddit from some time ago with several reasonably concise answers. $\endgroup$
    – Air
    Apr 3 '17 at 22:50
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    $\begingroup$ Welcome to Engineering.SE! If this is simply for curiosity, or for writing up a science-fiction story with some basis in reality, and without strict design parameters, I'd recommend moving the question as a hard-science question on WorldBuilding.SE. That said, if they give you some ideas and you have some better specifications and definitions, come back here with those and we can definitely answer the questions. $\endgroup$
    – Mark
    Apr 6 '17 at 18:03
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Yes, but it would probably be a very poor engine.

As you noted in your question, there are huge practical issues with using the arc alone as your power source, compared to using it to ignite the fuel mixture in a conventional spark-ignition engine. But you are correct to observe that both the electric arc and the combustion process cause a rapid expansion inside the chamber, and that this rapid expansion could drive a piston.

What you propose is to convert electrical energy to thermal energy to kinetic energy and useful work (you want to move the car). Implicit in this is some form of energy storage, which we can assume to be chemical for the sake of comparison with conventional technologies. So, chemical -> electrical -> thermal -> mechanical. You have to go through at least three conversions of energy from one form to another in order to make this technology work.

Contrast that with electric motor technology, which stores energy in batteries (or fuel cells) and produces work relatively efficiently. Chemical -> electrical -> mechanical.

Contrast it also with the conventional internal combustion engine, which stores energy relatively densely in hydrocarbon fuels and produces relatively more work per refueling cycle. Chemical -> thermal -> mechanical.

Your proposed solution adds another conversion step to the electric motor solution. It seems to combine the worst characteristics of both established solutions, since you have the same storage challenges to work with as the electric car does. So I wouldn't call it a practical technology; but you might be able to produce a working prototype of this sort of engine, for use as a novelty or teaching aid.

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    $\begingroup$ A prototype (working or not) with an electric arc inside a metal "combustion chamber" might have some "shocking" unwanted side effects, though! Not to mention problems like "oops, I just arc-welded the piston to the cylinder." $\endgroup$
    – alephzero
    Apr 4 '17 at 5:51
  • $\begingroup$ It is interesting to note the inherent problem with internal combustion engines. As you say it is better to use the electricity directly to power a motor; the method above is using electricity to make heat to make expansion. Modern combustion engines use chemical energy to make heat to expand air and use that kinetic energy to drive the pistons. To me, the modern internal combustion engine is almost criminally inefficient! I have often said if Stanley Steamer had out manufactured Ford we would live in a different world today. $\endgroup$ Apr 4 '17 at 13:08

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