To enable use of an improved method, and an improved two-stroke and large-sized diesel engine scavenging air in the longitudinal direction.

In an operating method for a two-stroke and large-sized diesel engine 1 scavenging air in a longitudinal direction, scavenging slots 4 are disposed on the inlet area 2 of a cylinder 3 in order to supply a predetermined amount of scavenging air, and an outlet valve 7 is disposed on the cylinder cover 5 of the cylinder 3 in order to discharge combustion gas 6. In the operation method, fresh air available at an ambient pressure P₀ is sucked by an exhaust gas supercharger 9 and then is supplied at a predetermined supply air pressure P_L as scavenging air 10 to the cylinder 3 via the scavenging slots 4, and ignition gas mixed air 11 is generated in the cylinder 3 from the scavenging air 10 and the combustion gas 6. Pressure increasing means 12, 121 for increasing the supply air pressure P_L are disposed, and a volume V_L with the scavenging air reduced is supplied to the cylinder 3 at an increased supply air pressure P_L.