Engine braking in a split-cycle engine

A method of operating a split cycle internal combustion engine, wherein the split cycle internal combustion engine comprises:

• a compression cylinder accommodating a compression piston configured to provide compressed working fluid;
• and
• (ii) a combustion cylinder accommodating a combustion piston, wherein the combustion cylinder is coupled to the compression cylinder to receive compressed working fluid therefrom, and wherein the combustion cylinder comprises:
• (i) an inlet valve configured to control intake of compressed working fluid into the combustion cylinder, and (ii) an outlet valve configured to control exhausting of fluid from the combustion cylinder;
• a recuperator positioned between and coupled to the compression cylinder and combustion cylinder;
• and
• wherein the method comprises changing the position during the engine cycle at which the inlet and/or outlet valves open to switch operation of the engine between an active mode and an engine braking mode, and controlling at least one of:
• the inlet valve to open at a position which is closer to a bottom dead centre, BDC, position when operating in the engine braking mode than when operating in the active mode;
• and
• the outlet valve to open at a position which is closer to a top dead centre, TDC, position when operating in the engine braking mode than when operating in the active mode,
• wherein the recuperator is arranged to provide a heat exchange between fluid which has been exhausted from the combustion cylinder and compressed working fluid travelling from the compression cylinder to the combustion cylinder, and
• wherein the controller is configured to receive a signal that indicates a temperature of the recuperator and to control operation at least related to a proportional flow of the compressed working fluid of the engine through the recuperator based on the received signal so as to keep the temperature of the recuperator within a selected range.