The present paper is an application of the second law of thermodynamics to the air standard diesel cycle. The study investigates the effects of cycle parameters (compression ratio, dimensionless initial and maximum temperatures and initial pressure) on the cycle performance (the first law and second law efficiencies, dimensionless heat added, work done, and available and unavailable energies along the cycle). The study shows that about half of the energy lost must under any case be given to the surroungings, while some portion of the other half may be regained as work.