Well, Shan, if you're going to go that route, why have engines? Just make the handlebars work like the old railroad handcars used to work. Move the handlebars up and down and they would move a lever attached to a flywheel that would in turn be geared or chained to the rear wheel. Start pedaling then go to pumping the handlebars up and down. Wheeeee! Transportaion plus a good cardiovascular workout combined.