A three-pulse fuel injection mode has been studied by implementing two-input-two-output (2I2O) control of both peak combustion pressure (Pmax) and indicated mean effective pressure (IMEP). The engine test results show that at low engine speed, the first main injection duration and the second main injection duration are able to be used to control Pmax and IMEP respectively. This control is exercised within a limited but promising area of the engine map. However, at high engine speed, Pmax and IMEP are strongly coupled together and then can not be separately controlled by the two control variables: the first and the second main injection duration. A simple zero-dimensional (0D) combustion model together with correlation analysis method was used to find out why the coupling strength of Pmax and IMEP increases with engine speed increased.It was found that the closer coupling of Pmax and IMEP at higher engine speeds is due to the following reasons: (i) combustion occupied a larger crank angle; (ii) there was lower maximum heat release rate and (iii) there was a stronger correlation between IMEP and manifold air pressure (MAP) and promoted by the turbocharger.