In this paper, the authors analyze the combustion mechanism of the methane mode water-oil blended ICE in detail and the optimum water/oil ratio M-0, where M-0 = -4n-m/2 (mol(H2O)/mol(oil)) = 9(4n-m)/12n+m+16y (kg(H2O)/kg(oil)) (n refers to the number of carbon atoms of the oil molecule, m refers to the number of hydrogen atoms of the oil molecule, y refers to the number of oxygen atoms of the oil molecule,which is a methane module). The authors recommend two separate injection systems, one of which is the water injection system and the other is the oil injection system. As for the four stroke ICE, the water injection system begins to inject water at the moment when the air inlet valve is closed. As for the two stroke ICE, when the scavenge port closes after the air outlet valve, the water injection starts at the moment when the air inlet valve is closed. When the air outlet valve closes after the scavenge port closes, the water injection starts as soon as the scavenge port is closed. During water injection, its angular duration should be maintained within 30degrees CA, which will lead to higher combustion efficiency and relatively lower pressure of the water injection system, so that the reliability of the water injection system may be better guaranteed. (C) 2004 Elsevier Ltd. All rights reserved.