Abstract: A controlled auto-ignition or spark ignition two stroke piston engine, fed by homogeneous air-fuel mixture, with crankcase scavenging, in which the amount of air-fuel mixture transferred from the crankcase to the cylinder is not greater than the 50% of the cylinder volume; said engine has reed valves in the transfer ports that connect the crankcase to the cylinder to control the internal dynamics of the air-fuel mixture, limiting the instantaneous maximum flow that is transferred to the cylinder in order to achieve the best stratification level among the air-fuel mixture and the exhaust residual gases, getting this way the thermo chemical activation of part of the fuel contained in the mixture.