We just ran a story about some new type of 2 stroke engine oil which is made using a byproduct of biodiesel production. While we all debate the merits of the oil and its inclusion on a website devoted to green automotive technology, I thought I'd show you a potential use for said oil.
Soon, you will be able to purchase this new device called the RevoPower wheel. The wheel replaces the standard from bike rim on your bicycle, allowing powered assist to your pedaling. The system uses a series of sprockets and chains to rotate the outer rim of the wheel using a 2 stroke engine. The engine turns off when you stop, and has no idling capability. I read their site where they explain why a 4 stroke engine was not chosen, you can find it here. I did note that the engine does meet CARB and EPA emissions requirements. However, I am pretty sure that the requirements for a non-road vehicle are less restrictive than for road-going cars and motorcycles.