Internal combustion engine

In an internal combustion engine operating on a two-stroke cycle and including a first piston reciprocable in a first cylinder to drive a crank shaft and also to inhale and compress a volume of air and a second piston working in a separate cylinder and synchronised with the first piston for drawing in a charge of mixture of air and fuel through an inlet valve, compressing the charge and forcing it through a non-return valve into a relatively small ignition chamber including a sparking plug and communicating with a combustion chamber in the cylinder head, the improvement wherein said separate cylinder is co-axial with and opposed to said first cylinder, said inlet valve is a reed valve located at the crank case end of said cylinder, and said second piston is formed with axial passages, said second piston including at least one non-return valve operating in such a way as to allow the passage of mixture through said second piston during part at least of the outward stroke, whereby on each inward stroke of said second piston a charge of mixture is compressed and forced through said non-return valve and a further charge is drawn into said separate cylinder through said reed valve from a carburettor and on each outward stroke the further charge is subjected to light compression and passes to the other side of said second piston in readiness for compression on the next inward stroke.