Are there any plans out there for those little CO2 motors? They look fairly simple on the outside, and from what I've read their operating principles seem simple enough too.
Depends on what you are going to use it for.
What I recall as far as free. There are a few books that are out there too. Check your local library?
Learn cause and effect through experience. Mastering those relationships is the "Common Sense" ability within the art of any trade.