A mono-block reciprocating piston engine power plant comprising: two or more cylinders each having a piston housed therein; an internal combustion IC section controlling the displacement of at least one of the pistons and; an Organic Rankine Cycle ORC section controlling the displacement of at least one of the pistons; wherein the IC and ORC pistons connect to and drive a common crankshaft of the mono-block engine power plant ; and wherein the Organic Rankine Cycle operates by the heat generated by the combustion in the internal combustion section, and the displacement of the pistons in the ORC section is achieved by injecting heated and pressurised ORC fluid; characterised in that the heated and pressurised ORC fluid is injected from a high pressure tank into a first ORC section cylinder causing downward displacement of a first piston therein; on the upward displacement of the first piston the ORC fluid is ejected from the first cylinder: wherein if the ORC fluid pressure and heat have decreased below predefined first thresholds, the ORC fluid is deemed a spent-fluid and is ejected into a condenser to be recirculated in the ORC cycle; if not, then the ORC fluid is transferred and injected into a second ORC section cylinder causing downward displacement of a second piston therein; wherein the ORC fluid is thus continuously circulated between the ORC section cylinders until deemed spent; and wherein when the spent ORC fluid is ejected into the condenser it is replaced with heated and pressurised ORC fluid injected into one of the cylinders from the high pressure tank. two or more cylinders each having a piston housed therein; an internal combustion IC section controlling the displacement of at least one of the pistons and; an Organic Rankine Cycle ORC section controlling the displacement of at least one of the pistons; wherein the IC and ORC pistons connect to and drive a common crankshaft of the mono-block engine power plant ; and wherein the Organic Rankine Cycle operates by the heat generated by the combustion in the internal combustion section, and the displacement of the pistons in the ORC section is achieved by injecting heated and pressurised ORC fluid; characterised in that the heated and pressurised ORC fluid is injected from a high pressure tank into a first ORC section cylinder causing downward displacement of a first piston therein; on the upward displacement of the first piston the ORC fluid is ejected from the first cylinder: wherein if the ORC fluid pressure and heat have decreased below predefined first thresholds, the ORC fluid is deemed a spent-fluid and is ejected into a condenser to be recirculated in the ORC cycle; if not, then the ORC fluid is transferred and injected into a second ORC section cylinder causing downward displacement of a second piston therein; wherein the ORC fluid is thus continuously circulated between the ORC section cylinders until deemed spent; and wherein if the ORC fluid pressure and heat have decreased below predefined first thresholds, the ORC fluid is deemed a spent-fluid and is ejected into a condenser to be recirculated in the ORC cycle; if not, then the ORC fluid is transferred and injected into a second ORC section cylinder causing downward displacement of a second piston therein; wherein the ORC fluid is thus continuously circulated between the ORC section cylinders until deemed spent; and wherein when the spent ORC fluid is ejected into the condenser it is replaced with heated and pressurised ORC fluid injected into one of the cylinders from the high pressure tank. <EOS>