Power unit attachable to a bicycle

1. A power unit for engaging a transmission band on a bicycle comprising: a drive unit including a fuel cell; a casing forming an opening for receiving a hydrogen fuel canister; a power sprocket that is mounted on the casing wherein the casing is mounted behind a portion of a frame of the bicycle supporting a seat and engageable by the transmission band and drivable by the drive unit; a first free-wheeling sprocket mounted on the casing behind the portion of the frame of the bicycle supporting the seat and arranged on a first side of the power sprocket; and a second free-wheeling sprocket mounted on the casing behind the portion of the frame of the bicycle supporting the seat on a second side of the power sprocket, the second side being opposite the first side; wherein the power sprocket is configured to receive the transmission band from the first free-wheeling sprocket on the first side and pass the transmission band to the second free-wheeling sprocket on the second side.

2. The power unit of claim 1 , wherein the power sprocket is configured to engage one surface of the transmission band and the first and second free-wheeling sprockets are configured to engage another surface of the transmission band.

3. The power unit of claim 2 , wherein each of the power sprocket, the first free-wheeling sprocket, and the second free-wheeling sprocket defines a respective center, the centers of the first and second free-wheeling sprockets define a base line, and the center of the power sprocket is offset from the base line in a direction that increases a wrap of the transmission band around the power sprocket.

4. The power unit of claim 2 , wherein each of the power sprocket, the first free-wheeling sprocket, and the second free-wheeling sprocket defines a respective center, the centers of the first and second free-wheeling sprockets define a base line, and the center of the power sprocket is aligned with the center of the first and second free-wheeling sprockets.

5. The power unit of claim 2 , wherein the surface of the transmission band engaged by the power sprocket is an inside surface.

6. The power unit of claim 2 , wherein the surface of the transmission band engaged by the power sprocket is an outside surface.

7. The power unit of claim 1 , wherein the drive unit includes an electric motor.

8. The power unit of claim 1 , wherein the fuel cell includes at least one hydrogen fuel canister incorporating metal-hydride hydrogen storage.

9. The power unit of claim 8 , wherein the hydrogen fuel canister is removable from the fuel cell.

10. A bicycle comprising: a frame having a rear portion, a portion supporting a seat, and a middle portion; a pedal sprocket drivable by a pedal assembly, the pedal sprocket being rotatably mounted on the middle portion of the frame; a rear wheel sprocket configured to drive a rear wheel, the rear-wheel sprocket being spaced from the pedal sprocket and rotatably mounted on the rear portion of the frame; a transmission band configured to transmit power from the pedal sprocket to the rear-wheel sprocket; a power unit including: a fuel cell; a casing forming an opening for receiving a hydrogen canister; a power sprocket mounted on the casing wherein the casing is mounted behind the portion of the frame supporting the seat and configured to engage the transmission band; a drive unit configured to drive the power sprocket; a first free-wheeling sprocket mounted on the casing behind the portion of the frame supporting the seat and arranged on a first side of the power sprocket; and a second free-wheeling sprocket mounted on the casing behind the portion of the frame supporting the seat on a second side of the power sprocket, the second side being opposite the first side; wherein the power sprocket is configured to receive the transmission band from the first free-wheeling sprocket on the first side and pass the transmission band to the second free-wheeling sprocket on the second side.

11. The bicycle of claim 10 , wherein one surface of the transmission band engages the first and second free-wheeling sprockets and another surface of the transmission band engages the power sprocket.

12. The bicycle of claim 11 , wherein each of the power sprocket, the first free-wheeling sprocket and the second free-wheeling sprocket define a respective center, the centers of the first free-wheeling sprocket and second free-wheeling sprocket define a base line, and the center of power sprocket is offset from the base line in a direction that increases the wrap of the other side of the transmission band around the power sprocket.

13. The bicycle of claim 11 , wherein each of the power sprocket, the first free-wheeling sprocket, and the second free-wheeling sprocket defines a respective center, the centers of the first and second free-wheeling sprockets define a base line, and the center of the power sprocket is arranged on the base line between the center of the first free-wheeling sprocket and the center of the second free-wheeling sprocket.

14. The bicycle of 11 , wherein the pedal sprocket, the rear wheel sprocket, and the power sprocket engage a common surface of the transmission band.

15. The bicycle of claim 11 , wherein the drive unit includes an electric motor.

16. The bicycle of claim 10 , wherein the fuel cell includes at least one hydrogen fuel canister incorporating metal-hydride hydrogen storage.

17. The bicycle of claim 16 , wherein the hydrogen fuel canister is removable from the fuel cell.

18. The bicycle of claim 11 , wherein the transmission band includes a chain.