This paper presents the results for liquid water injection (WI) into a cylinder during the compression and expansion strokes of an internal combustion engine (ICE), with the aim of achieving an optimal in-cylinder pressure and improving power output using CFD simulation. Employing WI during the compression stroke at 80 • of crank angle (CA) before top dead centre (bTDC) resulted in the reduction of compression work due to a reduction in peak compression pressure by a margin of about 2%. The decreased peak compression pressure also yielded the benefit of a decrease in NOx emission by a margin of 34% as well as the prevention of detonation. Using WI during the expansion stroke (after top dead centre-aTDC) revealed two stages of the in-cylinder pressure: the first stage involved a decrease in pressure by heat absorption, and the second stage involved an increase in the pressure as a result of an increase in the steam volume via expansion. For the case of water addition (WA 3.0%) and a water temperature of 100 • C, the percentage decrease of in-cylinder pressure was 2.7% during the first stage and a 2.5% pressure increase during the second stage. Water injection helped in reducing the energy losses resulting from the transfer of heat to the walls and exhaust gases. At 180 • CA aTDC, the exhaust gas temperature decreased by 42 K, 89 K, and 136 K for WA 1.0, WA 2.0, and WA 3.0, respectively. Increasing the WI temperature to 200 • C resulted in a decrease of the in-cylinder pressure by 1.0% during the first stage, with an increase of approximately 4.0% in the second stage. The use of WI in both compression and expansion strokes resulted in a maximum increase of in-cylinder pressure of about 7%, demonstrating the potential of higher power output.