I start telling people how to learn more about engines the way I learned. Go read as much as you can about engines, tranmissions, etc at howstuffworks.
Then, you actually apply that knowledge and take apart an engine to understand how it functions according to that knownledge. Thus, you have abstract education applied directly to understand a physical object.
The rest is up to reading about technical aspects of engines, like how they make power versus rpm, various ways to kill them, how intake and exhaust geometry affect torque across rpm, cam topography, etc.
Others have their backgrounds from working on engines in a more anecdotal way. They can tell you how to overhaul one, I can tell you how to build one.
Of course, it also helps to have an interest in the study to help solidify that knowledge. For example, I had drawn a schematic diagram for a gasoline powered jack hammer sometime around 4-5 years of age. (At that time, I actually thought thats what they ran on!

) Funny that up until a few month ago I was unaware that there actually are gasoline powered jack hammers.