An irreversible simplified model for the air standard Diesel cycle is established in this paper. This model takes into account the finite-time evolution of the cycle’s compression and power strokes and it considers global losses lumped in a friction like term. The relations between the power output and the compression ratio, and between the thermal efficiency and the compression ratio are derived. Numerical example leads to loop-shaped power versus efficiency cures as is common to almost all real heat engines. An irreversible simplified model for the air standard Diesel cycle is established in this paper. This model takes into account the finite-time evolution of the cycle’s compression and power strokes and it considers global losses lumped in a friction like term. The relations between the power output and the compression ratio, and between the thermal efficiency and the compression ratio are derived. Numerical example leads to loop-shaped power versus efficiency cures as is common to almost all real heat engines.