Re: Noobie over here!
All of these engines are gas powered except for the electric. They all operate in slightly different ways. Chain drive describes the transmission consisting of a chain and toothed sprockets, and are usually mounted in-frame, but some are mounted on a rack over the rear wheel. Friction drive consists of a roller attached to the engine which turns the rear wheel. A 2 stroke engine mixes oil and gas, while a 4 stroke keeps the oil separate. There are several different types of GAS powered engines which can be mounted to a bicycle and used to propel it. What 2door is saying is to do your research and find out which TYPE of GAS powered engine you want to use on your bike.