This work presents a thermodynamic analysis of a naturally aspirated, four-stroke, diesel engine with a swirl prechamber, under firing conditions during the open and closed part of the cycle. For calculating the heat exchange between gas and walls in both the main chamber and (swirl) prechamber, the relevant characteristic velocities and lengths are calculated by setting up a zero-dimensional energy cascade turbulence model. One-dimensional, quasi-steady, compressible flow with heat transfer inside the throat passageway connecting the two chambers is used. Combustion in both the main chamber and the swirl prechamber is attacked by proposing a two-zone combustion model, and following the movement of the spray plume inside an air solid body rotation environment in the prechamber and its later progression into the main chamber through the connecting throat. To validate the analysis, an extensive experimental investigation is undertaken at the laboratory of the authors on a flexible Ricardo, single-cylinder, swirl chamber diesel engine, and evaluating its performance in a wide range of operating conditions. The experimental results are found to be in good agreement with the theoretical results obtained from the computer program implementing the analysis.