Two-stroke otto cycle engines

A two-stroke engine comprising a cylinder and a piston accommodated in the said cylinder, said cylinder having an inlet port and an exhaust port, said engine further comprising an exhaust system communicating with the said exhaust port and including a first exhaust flow path, a second exhaust flow path in parallel with said first exhaust flow path, a reduction catalyst and an oxidation catalyst, the first exhaust flow path including said reduction catalyst and said second exhaust flow path bypassing said reduction catalyst, said first and second flow paths having respective downstream ends, said downstream ends being connected together upstream of said oxidation catalyst, said exhaust port being so controlled, in operation of said engine, that as said piston performs its downstroke the initial-flow of exhaust gas is substantially through said first flow path and the subsequent flow of exhaust gas is at least partly through said second flow path.