System for producing mechanical energy comprising at least: - a compressor; - an expansion valve; - a heat exchanger; said system having a motor operating mode in which said system further comprises: - means for intake of pressurised liquid nitrogen into an inlet for intake of liquid nitrogen of said exchanger, means for intake of air or of gaseous nitrogen into an inlet for intake of air or of gaseous nitrogen of said exchanger, means for escape of vapourised nitrogen to a vapourised nitrogen outlet of said exchanger and means for escape of cooled air or nitrogen to another outlet for cooled air or gaseous nitrogen of said exchanger; - means for intake into said expansion valve of said vapourised nitrogen to expand it therein; - means for intake into said compressor of said cooled air or gaseous nitrogen to produce therein compressed air or gaseous nitrogen; - means for expanding said compressed air or gaseous nitrogen; - means for recovering energy coming from the expansion of said vapourised nitrogen and from the expansion of said compressed air or gaseous nitrogen; characterised in that it further comprises: - means for reheating (500, 313) the compressed air or gaseous nitrogen before intake into said expansion means or inside said expansion means; - means for reheating (500, 313) said vapourised nitrogen before intake into said expansion valve or inside said expansion valve. - a compressor; - an expansion valve; - a heat exchanger; said system having a motor operating mode in which said system further comprises: - means for intake of pressurised liquid nitrogen into an inlet for intake of liquid nitrogen of said exchanger, means for intake of air or of gaseous nitrogen into an inlet for intake of air or of gaseous nitrogen of said exchanger, means for escape of vapourised nitrogen to a vapourised nitrogen outlet of said exchanger and means for escape of cooled air or nitrogen to another outlet for cooled air or gaseous nitrogen of said exchanger; - means for intake into said expansion valve of said vapourised nitrogen to expand it therein; - means for intake into said compressor of said cooled air or gaseous nitrogen to produce therein compressed air or gaseous nitrogen; - means for expanding said compressed air or gaseous nitrogen; - means for recovering energy coming from the expansion of said vapourised nitrogen and from the expansion of said compressed air or gaseous nitrogen; - means for intake of pressurised liquid nitrogen into an inlet for intake of liquid nitrogen of said exchanger, means for intake of air or of gaseous nitrogen into an inlet for intake of air or of gaseous nitrogen of said exchanger, means for escape of vapourised nitrogen to a vapourised nitrogen outlet of said exchanger and means for escape of cooled air or nitrogen to another outlet for cooled air or gaseous nitrogen of said exchanger; - means for intake into said expansion valve of said vapourised nitrogen to expand it therein; - means for intake into said compressor of said cooled air or gaseous nitrogen to produce therein compressed air or gaseous nitrogen; - means for expanding said compressed air or gaseous nitrogen; - means for recovering energy coming from the expansion of said vapourised nitrogen and from the expansion of said compressed air or gaseous nitrogen; characterised in that it further comprises: - means for reheating (500, 313) the compressed air or gaseous nitrogen before intake into said expansion means or inside said expansion means; - means for reheating (500, 313) said vapourised nitrogen before intake into said expansion valve or inside said expansion valve. - means for reheating (500, 313) the compressed air or gaseous nitrogen before intake into said expansion means or inside said expansion means; - means for reheating (500, 313) said vapourised nitrogen before intake into said expansion valve or inside said expansion valve.