A cylindrical chamber in an engine in which a piston is driven by the pressure or expansion of the working fluid. A similar, but non-cylindrical, part of a particular type of engine.
Under the same cylinder diameter, the more the number of cylinders, the bigger the displacement, the higher the power; under the same displacement, the more the number of cylinders, the smaller the diameter, the higher the speed can be, thus obtaining a larger power.
In general, the more valves you have at the same volume, the better your intake and exhaust efficiency, just like a person running with their mouth open when they're Gasp. The traditional engine is more than a cylinder intake valve and an exhaust valve, this two-valve valve relatively simple, low manufacturing costs, maintenance is relatively easy. For the ordinary engine with low power requirement, two valves can obtain satisfactory power output and torque performance of the engine.