Method for operating a piston internal combustion engine

The engine cylinder (22) has an outlet valve (3) that supplies exhaust gas (8) to drive a turbocharger (9) this drives a compressor (10) to raise the pressure of air (11) that is fed through inlets (12) that open when the piston (5) is around bottom dead centre. The engine operates as a 4 stroke mode over one speed range and as a two stroke over a second range