Harley would like to have you to believe they have the best engines however they have some of the most out of date engine of almost any company. Even there newest engine fall short of engine other companies used. They are just lazy, bulky, loud, and harsh vibrating engines.
It's an internal combustion engine, so the principles are no different than any other gasoline operated engine. Though they have changed over the years, little has changed as far as internal workings, aside from the move to fuel injection, and the V-rod "revolution" engine which is the only liquid cooled Harley currently in production.