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Recent experiments of diesel injection processes have demonstrated that mixing accelerates after the end of injection (EOI). This finding has significant implications for low-temperature combustion (LTC) diesel engines. Previous simulations using a one-dimensional model of a single-pulsed air jet, which is analogous in many aspects to diesel jets, suggest that the rapid mixing after EOI in diesel jets is due to a temporary increase in the entrainment rate as the jet decelerates. In the present study, we performed a high-fidelity large eddy simulation (LES) of an unsteady air jet identical to that used for the one-dimensional model. The LES calculation agrees well with available experimental data and provides both spatially and temporally resolved details of the three-dimensional transient mixing field. Results show that entrainment increases during deceleration.