To improve the actual performance of internal combustion engine (ICE), the transient behaviors of in cylinder combustion and heat-work conversion processes of ICE were investigated and an optimization method was proposed. Based on an advanced turbocharged gasoline direct injection (TGDI) engine, the steady-state bench test, load-step test at constant-speed and vehicle road test were carried out. On this basis, the in-cylinder combustion and heat-work conversion processes of vehicle engine under load-step and vehicle driving conditions were compared with the steady-state results. By this means, the deviations of ICE transient performance from their steady-state values were demonstrated and also their impacts were revealed. The research results show that there is a satisfactory consistency of ICE performance especially the ignition advance angle under load-step and steady-state conditions. However, under vehicle driving conditions, the operating and control parameters gravely deviate from the steady-state values with large fluctuations, e.g., ignition advance angle is retarded largely under the sharp deceleration conditions. When the IMEP is below 4 bar, the ignition advance angle seriously deviates from the steady-state values; which results in large fluctuation of combustion characteristic parameters and finally leads to the decrease of heat-work conversion efficiency. Moreover, the fluctuation of excess air coefficient is one of the main reasons for the instability of ICE transient performance. To accurately control the ignition timing under low load and decrease the fluctuation of excess air coefficient is an effective way to improve the ICE performance under vehicle driving conditions. (C) 2016 Elsevier Ltd. All rights reserved.