Method for a diesel engine and diesel engine

A Method to minimize NOx during varying engine loads in a 4-stroke diesel engine, said engine comprising:

• at least one cylinder with a cylinder head,
• a reciprocating first piston mounted on a connecting rod,
• an actuator mounted on the cylinder-head, and
• a second piston operated by the actuator, said second piston being configured to be locked via a hydraulic circuit in various positions in a combustion chamber,
• wherein the cylinder head is provided with at least one outlet valve for exhaust gas evacuation, and at least one freely controllable inlet valve for the supply of combustion air,
• wherein at least one injector is connected to the combustion chamber for injection of fuel in said chamber,
• said method comprising:
• actuating the second piston, at the latest during the current compression stroke using the actuator, and locking the second piston using the hydraulic circuit at a position in the combustion chamber such that combustion air introduced by the piston is compressed at a predetermined compression ratio to meet a desired engine load;
• ending the inlet stroke by closing the freely controllable inlet valve at a piston position where a volume of the combustion air as introduced at the end of the compression stroke gives the predetermined compression ratio;
• and
• injecting an amount of fuel using the injector, the amount of fuel corresponding to the desired engine load.