I don't know all the details but I'm told there are a bunch of batteries in those humps you see on the top and an electric motor for flat ground travel in the city. They make a low whine and no smoke unless they're pulling a grade. To be honest I'm usually too busy looking at the coeds to pay the buses much attention :D
When we cleared the ground to build the first cell site there back around 1989 the huggers put up a big banner on our fence: "Centel Cellular: Stinking Tree Killers"