Daniel, you don't say where you are located but in most of the world you own the copyright in a literary or artistic work as of right with no need to register it. So if you have a written description of a concept you can prevent other people publishing that description but not from using the concept as such. So for example, a company could read your engine description, make an engine based on it and sell it, without you being able to stop them. But you could stop them using your description, or a work derived from it, in their product documentation for example. IIRC it is only in the USA that you register copyright.
The way to protect ideas or concepts is through patents. An idea has to be new, not described in the open literature anywhere in the world, not obvious to "one skilled in the art", and capable of industrial application. To take what you wrote elsewhere on this forum about your engine for example, a patent examiner might judge that flash steam engines were prior art. Given that copyright only protects the expression of an idea not the idea itself, you clearly need to be very careful about what you publish even though you can assert your ownership of copyright on the document itself.
You can (in the UK) still (I think) file a patent specification free of charge. I have certainly filed a couple where i went along to the patent office, handed over a couple of envelopes and signed forms, and was given a stamped receipt showing date and time of filing. It does start to cost you when you file claims (which describe what you actually want to protect) since these need to be examined and the patent office starts to incur costs of an expert examiner. Costs mount up when you get a patent agent involved, when it gets closer to grant, and especially if you start filing internationally. Unless you have the funds to cover these patents are of little value, so it is very important to carefully review all the published work that could affect your priority and prevent a patent being granted. It saves a lot of time, bother and money. I used to run patent schemes in 4 different companies, and lose count of the number of times an engineer comes up with a great idea only to find that someone else has patented it, or it is described in the open literature. Google is your friend here – try searching on short succinct descriptions of your invention in different terms and see what comes up in the first page of results – often one finds links that are related and following those you find others. After chasing a few you can get a good idea of how novel your idea is.