How might a bike starts and improve speed when gear added and raised.whts Basic?

wow, you want the whole process from start to finish! As a real brief intro, a bike having an internal combustion engine starts by a manual kick start or electric motor begining the combustion cycle mechanically until the process begins and maintains operation. Rpm and gear ratios effect speed and torque of the bike. The internal combustion process uses controlled burning of petroleum to force via pressure a piston up and down. This piston is connected to a rod which converts the vertical motion into a turning motion of the drive shaft. Via a transmission this turning motion is controlled via gear ratios to supply controlled power to the drive wheel via a chain, shaft or belt. Chain drive is cheap however the most inefficient of the three. This is a quick paragraph on the info you want, I'll let others chime in with further details.

engg4world=>
How might a bike starts and improve speed when gear added and raised.whts might be the basic phenomenon of petrol converting to mechanical energy?

you want to be know the basic working phenomenon of IC engine
you can get ir from Joshi-tumanes book. In this book the simple sentence & not in deep.