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 the compressed air or gaseous nitrogen before intake into said expansion means or inside said expansion means; - means for reheating 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 the compressed air or gaseous nitrogen before intake into said expansion means or inside said expansion means; - means for reheating said vapourised nitrogen before intake into said expansion valve or inside said expansion valve. - means for reheating the compressed air or gaseous nitrogen before intake into said expansion means or inside said expansion means; - means for reheating said vapourised nitrogen before intake into said expansion valve or inside said expansion valve. <EOS>