A software program has been developed for calculating the thermodynamic cycle, in particular the entropy changes, in a turbocharged direct-injection diesel engine, based on the measured cylinder pressure and the output of a shaft encoder. Assumptions of homogeneous mixture and equilibrium thermodynamic properties are made for the products of combustion, and the temporal variation in the fluid thermodynamic state is followed in a quasi-steady manner through a series of adjacent equilibrium states, separated by finite intervals of 1 degrees crank angle. Calculations of the thermodynamic properties and equilibrium chemical products of fuel-air reaction are based on the method of minimising Gibbs free energy with a flexibly defined number of product species. The effect of the engine operating conditions on the thermodynamic cycle is studied. Results show that the dynamic fuel-injection timing, and hence ignition delay, are strongly influenced by the operating conditions, and this explains the reasons for incorporating a fuel-injection control system in modern vehicular engines for optimising the engine combustion cycle. The effect of fuel-injection timing on the thermodynamic cycle is also studied with 8 degrees CA retard and advance reference to the base-line fuel-injection setting.