A working process model for turbocharged direct injection (DI) diesel engines was developed by intensively analysing the in-cylinder spray and combustion process. The model can be used to prdeict the comprchensive performanace and Nox emission of turbocharged DI diesel engines under transient conditions. A transient test-bed was constructed, which was composed of a six-cylinde turbocharged DI diesel engine. a console, a measuring and control PC (personal computer) system for data collecting and exhaust gas sampling control in transient conditions and an exhaust gas sampling and measuring syatem. Tests of three types of transient operation conditons were carried out. These tests and model simulations show that the NOx emission laws are different during transient conditions and in some acceleration and load acceptance conditons NOx emissions are better than the corresponding steady state conditions.