A single-shaft dual expansion internal combustion engine is described, and includes an engine block including first and second power cylinders and an expander cylinder fluidly coupled through a cylinder head, and first and second power pistons that connect to respective first and second crankpins of the crankshaft. A multi-link connecting rod assembly includes a rigid main arm extending orthogonally to a longitudinal axis of the crankshaft and supporting a first pivot pin, a second pivot pin and a third pivot pin. The first pivot pin connects via a connecting rod to an expander piston reciprocating in the third cylinder. A third crankpin of the crankshaft serves as the second pivot pin and has a throw that is located 180 degrees of rotation of the crankshaft from a throw of the first crankpin. The third pivot pin connects to a swing arm connected to the engine block.